U.S. patent application number 14/663537 was filed with the patent office on 2015-10-01 for porous body, honeycomb filter, and method for producing porous body.
The applicant listed for this patent is NGK Insulators, Ltd.. Invention is credited to Hiroyuki NAGAOKA, Satoshi SAKASHITA, Takehide SHIMODA.
Application Number | 20150278409 14/663537 |
Document ID | / |
Family ID | 53199778 |
Filed Date | 2015-10-01 |
United States Patent
Application |
20150278409 |
Kind Code |
A1 |
SAKASHITA; Satoshi ; et
al. |
October 1, 2015 |
POROUS BODY, HONEYCOMB FILTER, AND METHOD FOR PRODUCING POROUS
BODY
Abstract
A porous body has a flow-rate-weighted mean diameter Ru of 10
.mu.l or more and 24 .mu.m or less, which is obtained as follows:
with reference to porous-body data obtained by a CT scan in which
positional information is associated with voxel-type information, a
plurality of virtual curved surface solids, which are each a curved
surface solid made up of a plurality of virtual spheres, are placed
to fill space voxels (Step S100); fluid analysis is carried out to
obtain information regarding the flow rates of a fluid in
individual space voxels during passing of the fluid through the
porous body (Step S110); and, the flow-rate-weighted mean diameter
Ru is obtained, which is a weighted mean obtained by weighting the
equivalent diameter R'.sub.i of each virtual curved surface solid
with the volume V.sub.i and average flow rate U.sub.i of each
virtual curved surface solid (Step S120).
Inventors: |
SAKASHITA; Satoshi;
(Yokkaichi-City, JP) ; NAGAOKA; Hiroyuki;
(Kakamigahara-City, JP) ; SHIMODA; Takehide;
(Nagoya-City, JP) |
|
Applicant: |
Name |
City |
State |
Country |
Type |
NGK Insulators, Ltd. |
Nagoya-City |
|
JP |
|
|
Family ID: |
53199778 |
Appl. No.: |
14/663537 |
Filed: |
March 20, 2015 |
Current U.S.
Class: |
55/418 ; 264/628;
703/2 |
Current CPC
Class: |
C04B 35/6365 20130101;
C04B 38/0006 20130101; B01D 2046/2496 20130101; C04B 2235/6021
20130101; G06F 30/20 20200101; C04B 2235/5436 20130101; C04B 28/24
20130101; C04B 2235/3229 20130101; B01D 46/2451 20130101; C04B
2111/0081 20130101; C04B 2235/3217 20130101; C04B 2235/606
20130101; C04B 35/195 20130101; C04B 2235/349 20130101; C04B 28/24
20130101; C04B 2235/3225 20130101; C04B 14/324 20130101; C04B
38/0006 20130101; C04B 38/0645 20130101; C04B 14/4656 20130101;
C04B 38/0074 20130101; C04B 38/0054 20130101; C04B 35/195 20130101;
C04B 2235/3244 20130101; B28B 11/243 20130101; C04B 38/0006
20130101; C04B 2111/00793 20130101; C04B 24/02 20130101 |
International
Class: |
G06F 17/50 20060101
G06F017/50; B01D 46/24 20060101 B01D046/24; B28B 11/24 20060101
B28B011/24 |
Foreign Application Data
Date |
Code |
Application Number |
Mar 31, 2014 |
JP |
2014-072362 |
Claims
1. A porous body having a flow-rate-weighted mean diameter Ru of 10
.mu.m or more and 24 .mu.m or less, wherein the flow-rate-weighted
mean diameter Ru is obtained as follows: based on an image obtained
by three-dimensionally scanning the porous body, porous-body data
is created in which positional information indicating position of a
voxel in the image is associated with voxel-type information
indicating whether the voxel is a space voxel representing space or
a matter voxel representing object; a process is carried out in
which a single parent virtual sphere is placed in the porous-body
data so as to have as large a diameter as possible so that the
parent virtual sphere fills the space voxels without overlapping
the matter voxel, at least one child virtual sphere whose center
overlaps the parent virtual sphere that has been placed is placed
such that voxels occupied by the at least one child virtual sphere
do not overlap the matter voxel and fill space voxels, and a single
virtual curved surface solid made up of the parent virtual sphere
and the at least one child virtual sphere is placed such that
curved surface solid voxels, which are voxels occupied by the
virtual curved surface solid, fill space voxels; this process is
repeated to place a plurality of the virtual curved surface solids
such that voxels occupied by different virtual curved surface
solids do not overlap each other; based on the porous-body data,
fluid analysis is carried out by the lattice Boltzmann method in
terms of a fluid flowing through a predetermined inflow plane into
the porous body, to obtain flow-rate vectors of the fluid in
individual space voxels during passing of the fluid through the
porous body; and based on information regarding the virtual curved
surface solids that have been placed and information regarding the
flow-rate vectors in individual space voxels, the
flow-rate-weighted mean diameter Ru is obtained by an expression
(1) below [ Math . 1 ] Ru = i = 1 n ( R i ' .times. V i .times. U i
) i = 1 n ( V i .times. U i ) ( 1 ) ##EQU00003## where Ru:
flow-rate-weighted mean diameter [.mu.m] n: number of virtual
curved surface solids that have been placed [number] R'.sub.i:
equivalent diameter of each virtual curved surface solid [.mu.m]
(i=1, 2, . . . , n) V.sub.i: volume of each virtual curved surface
solid [cc] (i=1, 2, . . . , n) U.sub.i: average flow rate of fluid
passing through each virtual curved surface solid [mm/s] (i=1, 2, .
. . , n).
2. The porous body according to claim 1, wherein, based on the
information regarding the virtual curved surface solids that have
been placed, an arithmetic mean diameter Rc=(R'.sub.1+R'.sub.2+ . .
. +R'.sub.n)/n is obtained, and a difference .DELTA.R (=|Ru-Rc|) is
2 .mu.m or less.
3. A honeycomb filter comprising a partition that include the
porous body according to claim 1 and form a plurality of cells of
which one end is open and the other end is sealed and serving as a
fluid channel.
4. A method for producing a porous body, comprising: a raw-material
mixing step of mixing talc having an average particle size of 1
.mu.m or more and 18 .mu.m or less, alumina, an auxiliary raw
material containing a material that undergoes a eutectic reaction
with talc and being prepared in an amount so as to satisfy a weight
ratio of 0.5% or more and 1.5% or less by weight relative to the
talc, and a pore-forming agent, to provide green body; and a
molding and firing step of molding the green body to provide a
compact and firing this compact at a firing temperature of
1350.degree. C. to 1440.degree. C.
5. The method for producing a porous body according to claim 4,
wherein the talc has an average particle size of 5 .mu.m or more
and 12 .mu.m or less.
6. The method for producing a porous body according to claim 4,
wherein the weight ratio of the auxiliary raw material relative to
the talc is 0.5% or more and 1.0% or less by weight.
7. The method for producing a porous body according to claim 4,
wherein the auxiliary raw material is at least one selected from
zirconium oxide, cerium oxide, and yttrium oxide.
8. The method for producing a porous body according to claim 4,
wherein the firing temperature is 1410.degree. C. to 1430.degree.
C.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates to a porous body, a honeycomb
filter, and a method for producing a porous body.
[0003] 2. Description of the Related Art
[0004] It is known that porous bodies are employed in exhaust-gas
cleaning structures such as honeycomb filters. For example, Patent
Literature 1 describes a method for producing a porous body in
which a base material composed of an inorganic material and a
pore-forming agent are mixed to provide green body; the green body
is shaped into a compact; and the compact is fired at a
predetermined firing temperature. According to this method for
producing a porous body, a porous body having a high trapping
capability can be produced by setting the base material and the
pore-forming agent so as to satisfy a predetermined range of
(D90-D10)/D50 where D10 represents the particle size at 10 vol %,
D50 represents the particle size at 50 vol %, and D90 represents
the particle size at 90 vol %.
CITATION LIST
Patent Literature
[0005] [PTL 1] International Publication No. 2013/146499
SUMMARY OF INVENTION
[0006] The higher capability of trapping particulate matter (PM) in
exhaust gas porous bodies have, the more preferable they are. Thus,
there has been a demand for a porous body having a higher trapping
capability.
[0007] The present invention has been made in order to meet such a
demand. A main object of the present invention is to provide a
porous body and a honeycomb filter that have a higher trapping
capability.
[0008] A porous body of the present invention has a
flow-rate-weighted mean diameter Ru of 10 .mu.m or more and 24
.mu.m or less, wherein the flow-rate-weighted mean diameter Ru is
obtained as follows:
[0009] based on an image obtained by three-dimensionally scanning
the porous body, porous body data is created in which positional
information indicating position of a voxel in the image is
associated with voxel-type information indicating whether the voxel
is space voxel representing space or matter voxel representing
object;
[0010] a process is carried out in which a single parent virtual
sphere is placed in the porous-body data so as to have as large a
diameter as possible so that the parent virtual sphere fills space
voxels without overlapping the matter voxel, at least one child
virtual sphere whose center overlaps the parent virtual sphere that
has been placed is placed such that voxels occupied by the at least
one child virtual sphere do not overlap the matter voxel and fill
space voxels, and a single virtual curved surface solid made up of
the parent virtual sphere and the at least one child virtual sphere
is placed such that curved surface solid voxels, which are voxels
occupied by the virtual curved surface solid, fill space voxels;
this process is repeated to place a plurality of the virtual curved
surface solids such that voxels occupied by different virtual
curved surface solids do not overlap each other;
[0011] based on the porous-body data, fluid analysis is carried out
by the lattice Boltzmann method in terms of a fluid flowing through
a predetermined inflow plane into the porous body, to obtain
flow-rate vectors of the fluid in individual space voxels during
passing of the fluid through the porous body; and
[0012] based on information regarding the virtual curved surface
solids that have been placed and information regarding the
flow-rate vectors in individual space voxels, the
flow-rate-weighted mean diameter Ru is obtained by an expression
(1) below
[ Math . 1 ] Ru = i = 1 n ( R i ' .times. V i .times. U i ) i = 1 n
( V i .times. U i ) ( 1 ) ##EQU00001##
[0013] where
[0014] Ru: flow-rate-weighted mean diameter [.mu.m]
[0015] n: number of virtual curved surface solids that have been
placed [number]
[0016] R'.sub.i: equivalent diameter of each virtual curved surface
solid [.mu.m] (i=1, 2, . . . , n)
[0017] V.sub.i: volume of each virtual curved surface solid [cc]
(i=1, 2, . . . , n)
[0018] U.sub.i: average flow rate of fluid passing through each
virtual curved surface solid [mm/s] (i=1, 2, . . . , n).
[0019] The inventors of the present invention have found that a
filter including a porous body having a flow-rate-weighted mean
diameter that is not excessively large or small tends to have a
high trapping capability. The inventors have found that a
flow-rate-weighted mean diameter Ru of 10 .mu.m or more and 24
.mu.m or less results in a sufficiently high trapping capability. A
porous body according to the present invention satisfies this
condition and hence achieves a sufficiently high trapping
capability. This is probably achieved for the following reason. The
larger the extent of variation in volume or average flow rate among
a plurality of virtual curved surface solids that have been placed,
the larger the tendency for the flow-rate-weighted mean diameter Ru
to become excessively large or small. Regarding pores of a porous
body that are simulated with virtual curved surface solids having a
large volume, the probability that a fluid passing through the
pores comes into contact with the wall surfaces of the porous body
tends to decrease. Regarding pores of a porous body that are
simulated with virtual curved surface solids having a small volume,
a fluid tends not to pass through the pores or a catalyst tends not
to be appropriately applied to the wall surfaces of the pores, the
catalyst being used for the porous body to be used as a filter.
Regarding pores of a porous body that are simulated with virtual
curved surface solids having an average flow rate higher than the
average flow rate (simple average flow rate) of the whole pores of
the porous body, a fluid passes through these pores in a short time
and hence these pores tend not to contribute to the trapping
capability. Regarding pores of a porous body that are simulated
with virtual curved surface solids having an average flow rate
lower than the average flow rate (simple average flow rate) of the
whole pores of the porous body, the amount of a fluid entering
these pores is small and hence these pores tend not to contribute
to the trapping capability. As has been described, pores of a
porous body that are simulated with virtual curved surface solids
having an excessively large or small volume or an excessively high
or low average flow rate tend not to contribute to the trapping
capability. This is probably the reason why, in porous bodies
having a large portion that tends not to contribute to the trapping
capability, the flow-rate-weighted mean diameter Ru is found to be
excessively large or small; and a flow-rate-weighted mean diameter
Ru of 10 .mu.m or more and 24 .mu.m or less results in a
sufficiently high trapping capability.
[0020] In such a case where a plurality of child virtual spheres
are placed for placing a single virtual curved surface solid, the
plurality of child virtual spheres are allowed to overlap each
other. The fluid analysis is carried out in terms of a fluid
flowing from a predetermined inflow plane to a predetermined
outflow plane of the porous body.
[0021] In a porous body according to the present invention, based
on the information regarding the virtual curved surface solids that
have been placed, an arithmetic mean diameter
Rc=(R'.sub.1+R'.sub.2+ . . . +R'.sub.n)/n is obtained, and a
difference .DELTA.R (=|Ru-Rc|) is preferably 2 .mu.m or less. When
the difference .DELTA.R satisfies this range, a higher trapping
capability tends to be achieved.
[0022] A honeycomb filter according to the present invention
includes
[0023] a partition that include the porous body according to any
one of the above-described embodiments of the present invention and
form a plurality of cells of which one end is open and the other
end is sealed and serving as a fluid channel.
[0024] In this honeycomb filter, since the porous body forming the
partitions satisfies, for example, the above-described range of the
flow-rate-weighted mean diameter Ru, a sufficiently high trapping
capability is achieved during passing of a fluid through the
honeycomb filter.
[0025] A method for producing a porous body of the present
invention includes:
[0026] a raw-material mixing step of mixing talc having an average
particle size of 1 .mu.m or more and 18 .mu.m or less, alumina, an
auxiliary raw material containing a material that undergoes a
eutectic reaction with talc and being prepared in an amount so as
to satisfy a weight ratio of 0.5% or more and 1.5% or less by
weight relative to the talc, and a pore-forming agent, to provide
green body; and
[0027] a molding and firing step of molding the green body to
provide a compact and firing this compact at a firing temperature
of 1350.degree. C. to 1440.degree. C.
[0028] The inventors of the present invention have found that
production of a porous body by the above-described production
method including the steps provides a porous body having a higher
trapping capability. The reason for this is probably as follows.
Use of talc having an average particle size of 18 .mu.m or less
probably results in suppression of generation of excessively large
pores. By adding an auxiliary raw material containing a material
that undergoes a eutectic reaction with talc, excessively small
pores through which a fluid tends not to pass are probably filled.
In addition, the firing temperature of 1350.degree. C. to
1440.degree. C. probably results in a sufficiently high strength of
the porous body. As a result, generation of excessively large or
small pores is suppressed and generation of pores causing an
excessively high or low flow rate is suppressed; and this probably
results in a high trapping capability. Note that the production
method allows production of the above-described porous body
according to the present invention, which has a flow-rate-weighted
mean diameter Ru of 10 .mu.m or more and 24 .mu.m or less.
[0029] In a method for producing a porous body according to the
present invention, the talc may have an average particle size of 5
.mu.m or more and 12 .mu.m or less. The talc having an average
particle size of 12 .mu.m or less suppresses more effectively the
above-described generation of excessively large pores.
[0030] In a method for producing a porous body according to the
present invention, the weight ratio of the auxiliary raw material
relative to the talc is preferably 0.5% or more and 1.0% or less by
weight. The auxiliary raw material may be at least one selected
from zirconium oxide, cerium oxide, and yttrium oxide. In addition,
the firing temperature is preferably 1410.degree. C. to
1430.degree. C.
BRIEF DESCRIPTION OF THE DRAWINGS
[0031] FIG. 1 is a front view of a honeycomb filter 30 including
porous partitions 44.
[0032] FIG. 2 is a sectional view taken along line A-A in FIG.
1.
[0033] FIG. 3 illustrates the configuration of a user personal
computer 20 serving as a microstructure analyzer.
[0034] FIGS. 4A and 4B illustrate conceptual views of porous-body
data 60.
[0035] FIG. 5 is an explanatory view of porous-body data 60.
[0036] FIG. 6 is a flow chart illustrating an example of an
analysis processing routine.
[0037] FIG. 7 is a flow chart illustrating an example of an
virtual-curved-surface-solid placement process.
[0038] FIG. 8 is an explanatory view illustrating an example of an
virtual-curved-surface-solid table 83.
[0039] FIGS. 9A and 9B illustrate explanatory views of placement of
a parent virtual sphere.
[0040] FIGS. 10A and 10B illustrates explanatory views of placement
of child virtual spheres and a virtual curved surface solid.
[0041] FIG. 11 is a graph illustrating the relationship between
flow-rate-weighted mean diameter Ru and the number of leaked
particles in Experimental examples 1 to 4.
DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0042] Hereinafter, embodiments for carrying out the present
invention will be described with reference to the drawings.
[0043] FIG. 1 is a front view of a honeycomb filter 30 including
porous partitions 44 formed of a porous body according to an
embodiment of the present invention. FIG. 2 is a sectional view
taken along line A-A in FIG. 1. The honeycomb filter 30 is a diesel
particulate filter (DPF) having a function of filtering off
particulate matter (PM) in exhaust gas emitted from a diesel
engine. The honeycomb filter 30 includes a large number of cells 34
(refer to FIG. 2) defined by the porous partitions 44. A peripheral
protective part 32 is formed around the cells 34. The material of
the porous partitions 44 is preferably a ceramic material formed of
inorganic particles of Si--SiC, cordierite, or the like from the
standpoint of strength and heat resistance. The porous partitions
44 preferably have a thickness of 200 .mu.m or more and less than
600 .mu.m. This thickness is 300 .mu.m in this embodiment. The
porous partitions 44 have an average pore size (measured by mercury
porosimetry) of 10 .mu.m or more and less than 60 .mu.m and a
porosity (void ratio) of 40% or more and less than 65%, for
example. As illustrated in FIG. 2, a large number of cells 34
formed in the honeycomb filter 30 are divided into open-inlet cells
36 having an open inlet 36a and an outlet 36b sealed with an outlet
sealing material 38, and open-outlet cells 40 having an inlet 40a
sealed with an inlet sealing material 42 and an open outlet 40b.
Such open-inlet cells 36 and open-outlet cells 40 are alternately
disposed so as to be adjacent to one another. The density of the
cells is 15 cells/cm.sup.2 or more and less than 65 cells/cm.sup.2,
for example. The peripheral protective part 32 is a layer for
protecting the periphery of the honeycomb filter 30. The peripheral
protective part 32 may contain, for example, the above-described
inorganic particles, inorganic fibers of aluminosilicate, alumina,
silica, zirconia, ceria, mullite, or the like, and a binder such as
colloidal silica or clay.
[0044] The honeycomb filter 30 is placed downstream of a diesel
engine (not shown) and used to clean PM-containing exhaust gas to
be released into the air, for example. Arrows in FIG. 2 indicate
flows of exhaust gas in this occasion. The exhaust gas containing
PM emitted from the diesel engine flows through the inlets 36a of
the honeycomb filter 30 into the open-inlet cells 36, then passes
through the porous partitions 44 into the adjacent open-outlet
cells 40, and is released through the outlets 40b of the
open-outlet cells 40 into the air. While the exhaust gas containing
PM flows from the open-inlet cells 36 through the porous partitions
44 into the open-outlet cells 40, PM is trapped; and hence the
exhaust gas having flowed into the open-outlet cells 40 is a clean
exhaust gas not containing PM. The interior of the pores of the
porous partitions 44 is coated with an oxidation catalyst such as
platinum (not shown). The oxidation catalyst oxidizes trapped PM to
thereby suppress a decrease in the porosity of the porous
partitions 44 or a sharp increase in pressure loss.
[0045] The porous partitions 44 of this embodiment subjected to an
analysis (described below) of the microstructure of the porous body
forming the porous partitions 44, are found to have a
flow-rate-weighted mean diameter Ru of 10 .mu.m or more and 24
.mu.m or less. It is more preferable that a difference .DELTA.R is
2 .mu.m or less. Hereinafter, the method of analyzing the
microstructure will be described.
[0046] FIG. 3 schematically illustrates the configuration of a user
personal computer (PC) 20 designed as a microstructure analyzer for
analyzing the microstructure of the porous partitions 44. The user
PC 20 includes a controller 21 that includes, for example, a CPU 22
that executes various processes, a ROM 23 that stores various
processing programs or the like, and a RAM 24 that temporarily
stores data; a HDD 25 that is a mass storage memory storing various
processing programs such as analysis processing programs and
various data such as porous-body data 60 that is three-dimensional
voxel data of a porous body. The user PC 20 is equipped with a
display 26 that displays various information on the screen and
input devices 27 with which the user input various commands, such
as a keyboard. As will be described in detail below, the
porous-body data 60 stored in the HDD 25 includes a porous-body
table 71 and an inflow-outflow table 72. The user PC 20 functions
to analyze the microstructure of a porous body on the basis of the
porous-body data 60 stored in the HDD 25. During this analysis of
the microstructure, porous-body data 80 is stored in the RAM 24. As
will be described in detail below, the porous-body data 80 includes
a porous-body table 81, an inflow-outflow table 82, and a
virtual-curved-surface solid-table 83.
[0047] The HDD 25 of the user PC 20 stores, as the porous-body data
60, the three-dimensional voxel data of the porous partitions 44
obtained by subjecting the honeycomb filter 30 to a CT scan. In
this embodiment, an XY plane defined by the X direction and the Y
direction illustrated in FIG. 2 is selected as an imaging section;
and a plurality of such imaging sections are produced in the Z
direction illustrated in FIG. 1. In this way, the CT scan is
carried out to obtain voxel data. In this embodiment, the
resolution in each of the X, Y, and Z directions is 1.2 .mu.m,
which provides a cube having 1.2 .mu.m sides and serving as the
minimum unit of three-dimensional voxel data, that is, a voxel. The
resolution in each of the X, Y, and Z directions can be
appropriately set in accordance with, for example, the performance
of the CT scanner or the size of particles to be analyzed. The
resolutions in the directions may be set to be different from each
other. The resolutions in the X, Y, and Z directions may be, but
not limited to, set to values within a range of 0.5 to 3.0 .mu.m,
for example. Each voxel is identified in terms of its position by
X, Y, and Z coordinates (a coordinate value of 1 corresponds to a
length of a side of a voxel, 1.2 .mu.m). The coordinates are
associated with type information indicating whether the voxel
represents a space (pore) or an object (material of the porous
partitions 44) and stored in the HDD 25. In this embodiment, voxels
representing spaces (space voxels) are tagged with a
type-information value of 0; and voxels representing objects
(matter voxels) are tagged with a type-information value of 9.
Actually, the data obtained by a CT scan is luminance data at
individual X, Y, and Z coordinates, for example. The porous-body
data 60 used in this embodiment can be obtained by converting this
luminance data into a binary representation with respect to a
predetermined threshold so that voxels are determined as to whether
each voxel at coordinates is a space voxel or a matter voxel. The
predetermined threshold is set as a value that allows appropriate
determination as to whether the voxels are space voxels or matter
voxels. This threshold may be empirically set in advance such that
the measured porosity of the porous partitions 44 is substantially
equal to the porosity of the binarized voxel data, for example.
Such CT scans can be carried out with SMX-160CT-SV3 manufactured by
SHIMADZU CORPORATION, for example.
[0048] FIGS. 4A and 4B illustrate conceptual views of the
porous-body data 60. FIG. 4A is a conceptual view of the
porous-body data 60 obtained as voxel data by subjecting, to a CT
scan, the porous partition 44 in a region 50 in FIG. 2. This
porous-body data 60 in this embodiment is extracted from the voxel
data of the porous partition 44. The porous-body data 60 is voxel
data of a rectangular parallelepiped portion having a length of 300
.mu.m (=1.2 .mu.m.times.250 voxels) in the X direction, which is
the same as the thickness of the porous partition 44 in the passing
direction of exhaust gas, having a length of 480 .mu.m (=1.2
.mu.m.times.400 voxels) in the Y direction, and having a length of
480 .mu.m (=1.2 .mu.m.times.400 voxels) in the Z direction. This
porous-body data 60 is subjected to the analysis processing
described below. Note that the dimensions of the porous-body data
60 can be appropriately set in accordance with, for example, the
thickness or size of the porous partition 44 or an acceptable
calculation load. For example, the length in the X direction is not
limited to 300 .mu.m and may be another value that is the same as
the thickness of the porous partition 44 in the passing direction
of exhaust gas. The length in the X direction is preferably the
same value as the thickness of the porous partition 44 in the
passing direction of exhaust gas; however, the length and the
thickness may be different values. The lengths in the Y and Z
directions are also not limited to 480 .mu.m and may be other
values. The lengths in the Y and Z directions may be different from
each other. Among the six planes of the rectangular parallelepiped
of the porous-body data 60, two planes (planes parallel to the Y-Z
plane) are an inflow plane 61 (refer to FIG. 2) that is a boundary
surface between the porous partition 44 and the open-inlet cell 36,
and an outflow plane 62 (refer to FIG. 2) that is a boundary
surface between the porous partition 44 and the open-outlet cell 40
in the region 50; and the other four planes are sections of the
porous partition 44. FIG. 4B illustrates an XY plane (imaging
section) 63 at a position where the Z coordinate is 3 in the
porous-body data 60 and an enlarged view 64 of a portion of the XY
plane 63. As illustrated in the enlarged view 64, the XY plane 63
is made up of arranged voxels having 1.2 .mu.m sides. Each voxel is
expressed as a space voxel or a matter voxel. Note that the imaging
section obtained by a CT scan is a planar data having no thickness
in the Z direction as illustrated in FIG. 4B. However, each imaging
section is handled as having a thickness of the gap (1.2 .mu.m)
between imaging sections in the Z direction. In other words, each
voxel is handled as a cube having 1.2 .mu.m sides as described
above. Note that, as illustrated in FIG. 5, the porous-body data 60
is stored, in the HDD 25, as data including the porous-body table
71 in which the XYZ coordinates of each voxel serving as positional
information are associated with type information, and the
inflow-outflow table 72 indicating the inflow plane 61 and the
outflow plane 62. In FIG. 5, "X=1" in the inflow-outflow table 72
denotes the plane at X=1 of the XYZ coordinate system, that is, the
inflow plane 61 illustrated in FIG. 4A. Similarly, "X=251" denotes
the outflow plane 62. The HDD 25 stores, in addition to the
porous-body data 60, a plurality pieces of other porous-body data
representing the voxel data of the porous partitions 44 in terms of
regions other than the region 50.
[0049] Hereinafter, analysis processing carried out by the user PC
20 for the porous-body data 60 will be described. FIG. 6 is a flow
chart of an analysis processing routine. This analysis processing
routine is carried out as follows: according to a command of
carrying out analysis processing input by the user via the input
devices 27, the CPU 22 executes the analysis processing programs
stored in the HDD 25. Hereinafter, the case of carrying out
analysis processing of the porous-body data 60 will be described;
however, analysis processing of other porous-body data can be
similarly carried out. The porous-body data that is to be analyzed
may be set in advance or may be selected by the user.
[0050] Upon start of the analysis processing routine, the CPU 22
executes an virtual-curved-surface-solid placement process (Step
S100) of placing virtual curved surface solids so as to fill space
voxels of the porous-body data 60.
[0051] Here, the description of the analysis processing routine is
interrupted and the virtual-curved-surface-solid placement process
will be described. FIG. 7 is a flow chart of the
virtual-curved-surface-solid placement process. This
virtual-curved-surface-solid placement process is executed by the
CPU 22. Upon execution of the virtual-curved-surface solid
placement process, the CPU 22 reads the porous-body data 60 stored
in the HDD 25 and stores it in the RAM 24 (Step S200). As a result,
the data the same as the porous-body data 60 including the
porous-body table 71 and the inflow-outflow table 72 and stored in
the HDD 25 is stored in the RAM 24 as the porous-body data 80
including a porous-body table 81 and an inflow-outflow table 82. In
the porous-body data 80 that have been read, virtual wall surfaces
are defined (Step S210). Specifically, the user inputs via the
input devices 27 the distance from the porous-body data 80 that is
a 300 .mu.m.times.480 .mu.m.times.480 .mu.m rectangular
parallelepiped to the surrounding virtual wall surfaces; and the
CPU 22 receives this input and stores it in the RAM 24. For
example, in a case where the distance to the virtual wall surfaces
is defined as 1 .mu.m, the CPU 22 recognizes that the virtual wall
surfaces are present 1 .mu.m outside the planes of the porous-body
data 80 in the X, Y, and Z directions, and that regions outside the
virtual wall surfaces are fully occupied by matter voxels.
Specifically, since the porous-body data 80 is a 300
.mu.m.times.480 .mu.m.times.480 .mu.m rectangular parallelepiped,
it is recognized as being covered by the virtual wall surfaces that
form a 302 .mu.m.times.482 .mu.m.times.482 .mu.m rectangular
parallelepiped. These virtual wall surfaces are set in order to
define the region within which virtual curved surface solids
(parent virtual spheres and child virtual spheres) described below
can be placed.
[0052] Subsequently, the CPU 22 sets the diameter Ra of a parent
virtual sphere to a maximum value Ramax (Step S220) and determines
as to whether or not the parent virtual sphere having the diameter
Ra can be placed within space voxels inside the virtual wall
surfaces defined in Step S210 (Step S230). The parent virtual
sphere having the diameter Ra is a virtual sphere that has a size
with a diameter of Ra (.mu.m) and whose center is positioned at the
center of one of voxels. Whether or not the parent virtual sphere
having the diameter Ra can be placed is determined in the following
manner, for example. One voxel is selected from space voxels
(voxels having a type-information value of 0) at the time. If the
parent virtual sphere that has the diameter Ra and whose center is
positioned at the selected voxel is placed and the parent virtual
sphere overlaps the matter voxel, another space voxel is selected
again as the center. In this way, space voxels are selected one
after another; and, if the parent virtual sphere at a voxel does
not overlap the matter voxel, it is determined that the parent
virtual sphere that has the diameter Ra can be placed at the
position. If the parent virtual spheres centered at all space
voxels at the time overlap the matter voxel, it is determined that
the parent virtual sphere that has the diameter Ra cannot be
placed. Note that the voxel serving as the center may be selected
in random order; alternatively, the voxel may be selected in the
order of voxels from the voxels on the inflow plane 61 to the
voxels on the outflow plane 62. The maximum value Ramax is equal to
or more than the maximum diameter of normal pores in the porous
partition 44 and can be set with reference to a value determined in
an experiment, for example. If Step S230 determines that no parent
virtual sphere can be placed, the diameter Ra is decremented by 1
(Step S240) and Step S230 and processes subsequent thereto are
carried out. In this embodiment, the decrement is set to 1;
however, the decrement can be appropriately set in accordance with
an acceptable calculation load or the like.
[0053] If Step S230 determines that a parent virtual sphere can be
placed, this single parent virtual sphere having the diameter Ra is
placed at the position (Step S250). Specifically, in the
porous-body table 81 of the porous-body data 80 stored in the RAM
24 in Step S200, the type information of voxels occupied by the
parent virtual sphere having the diameter Ra is changed to another
value of 3, which indicates that the voxels are occupied by the
parent virtual sphere. In this embodiment, when the centers of
voxels are contained within a parent virtual sphere, the type
information of the voxels is changed to another value of 3.
Alternatively, when a predetermined percentage (for example, 50%)
or more of the volume of voxels are occupied by a parent virtual
sphere, the type information of the voxels may be changed to
another value of 3; the type information of only the voxels
completely contained within a parent virtual sphere may be changed
to another value of 3; or when at least portions of voxels are
occupied by a parent virtual sphere, the type information of the
voxels may be changed to another value of 3. This is also the case
for voxels occupied by child virtual spheres described below.
[0054] Subsequently, the CPU 22 sets the diameter Rb of a child
virtual sphere to the same value as that of the diameter Ra (Step
S260) and determines as to whether or not the child virtual sphere
having the diameter Rb can be placed within space voxels inside the
virtual wall surfaces defined in Step S210 (Step S270). The child
virtual sphere having the diameter Rb is a virtual sphere that has
a size with a diameter of Rb (.mu.m), whose center is positioned at
the center of one of voxels, and shares some occupied voxels with
the parent virtual sphere. The child virtual sphere is placed such
that the center of the child virtual sphere overlaps the parent
virtual sphere that has been placed in Step S250. Whether or not
the child virtual sphere having the diameter Rb can be placed is
determined in the following manner, for example. One voxel is
selected from voxels occupied by the parent virtual sphere (voxels
having a type-information value of 3) at the time. If the child
virtual sphere that has the diameter Rb and whose center is
positioned at the selected voxel is placed and the child virtual
sphere overlaps the matter voxel, another voxel occupied by the
parent virtual sphere is selected as the center. In this way, the
voxels are selected one after another; and, if the child virtual
sphere at a voxel does not overlap the matter voxel, it is
determined that the child virtual sphere that has the diameter Rb
can be placed at the position. If the child virtual spheres
centered at all space voxels occupied by the parent virtual sphere
at the time overlap the matter voxel, it is determined that the
child virtual sphere that has the diameter Rb cannot be placed.
[0055] If Step S270 determines that a child virtual sphere can be
placed, this single child virtual sphere having the diameter Rb is
placed at the position (Step S280). Specifically, in the
porous-body table 81 of the porous-body data 80 stored in the RAM
24 in Step S200, the type information of voxels occupied by the
child virtual sphere having the diameter Rb is changed to another
value of 4, which indicates that the voxels are occupied by the
child virtual sphere. Note that this change in type information is
not carried out for voxels having a type-information value of 3,
which are occupied by the parent virtual sphere. In other words,
the voxels that are occupied by both the parent virtual sphere and
the child virtual sphere are tagged with the type information of
the parent virtual sphere. After the single child virtual sphere is
placed, Step S270 and processes subsequent thereto are carried out
and Step S280 is repeated to place child virtual spheres having the
diameter Rb until it is determined that another child virtual
sphere having the diameter Rb can be no longer placed. Note that
child virtual spheres are allowed to overlap each other. In other
words, voxels occupied by a child virtual sphere are allowed to
overlap voxels occupied by another child virtual sphere.
[0056] If Step S270 determines that no child virtual sphere can be
placed, the diameter Rb is decremented by 1 (Step S290); it is
determined as to whether or not the diameter Rb is less than a
minimum value Rbmin (Step S300); if the diameter Rb is equal to or
more than the minimum value Rbmin, Step S270 and processes
subsequent thereto are carried out. The minimum value Rbmin is the
lower limit of the diameter Rb of child virtual spheres. The
minimum value Rbmin is a threshold that is defined so as not to
place child virtual spheres that have a relatively small diameter
and do not considerably influence the analysis result, for example.
In this embodiment, Rbmin is set to 2 .mu.m.
[0057] If Step S300 determines that the diameter Rb is less than
the minimum value Rbmin, a virtual curved surface solid is placed
(Step S310), the virtual curved surface solid being made up of the
parent virtual sphere that has been placed in Step S250 and the
child virtual spheres that have been placed in Step S280.
Specifically, in the porous-body table 81 of the porous-body data
80 stored in the RAM 24 in Step S200, the type information of
voxels occupied by the parent virtual sphere (voxels having a
type-information value of 3) and voxels occupied by the child
virtual spheres (voxels having a type-information value of 4) is
changed to another value of 5, which indicates that the voxels are
curved surface solid voxels occupied by the virtual curved surface
solid. In addition, the positional information of the curved
surface solid voxels whose type information has been changed to
another value of 5 this time is associated with the identification
code of the virtual curved surface solid. Such identification codes
of virtual curved surface solids are values individually allocated
in the order that the virtual curved surface solids are placed, for
example. The curved surface solid voxels of a single virtual curved
surface solid are associated with the same identification code. The
information regarding the virtual curved surface solid is stored in
the RAM 24 (Step S320) and it is determined as to whether or not
99% or more of space voxels are substituted with the curved surface
solid voxels (Step S330). Specifically, with reference to the type
information of voxels in the porous-body table 81 stored in the RAM
24, this determination is carried out as to whether or not the
ratio of the number of voxels having a type-information value of 5
to the total number of the voxels having a type-information value
of 0 and the voxels having a type-information value of 5 is 99% or
more. Note that the threshold used in this determination is not
limited to 99% and another value may be employed. If the ratio of
space voxels that have been substituted with curved surface solid
voxels is less than 99% in Step S330, Step S230 and processes
subsequent thereto are carried out to place another virtual curved
surface solid. If the ratio of space voxels that have been
substituted with curved surface solid voxels is 99% or more in Step
S330, the virtual-curved surface-solid placement process is
finished.
[0058] Note that, during the placement of virtual curved surface
solids one after another by repeating Step S230 and processes
subsequent thereto, in this embodiment, voxels occupied by an
virtual curved surface solid placed at a time are allowed to
overlap voxels occupied by another virtual curved surface solid
that has been placed. Specifically, in Step S230 in the
virtual-curved-surface-solid placement process, even when a parent
virtual sphere having a diameter Ra to be placed overlaps a virtual
curved surface solid that has been placed, this parent virtual
sphere is allowed to be placed. In other words, in Step S230, if a
parent virtual sphere having a diameter Ra does not overlap the
matter voxel, regardless of whether or not the parent virtual
sphere overlaps virtual curved surface solids that have been
placed, Step S230 determines that the parent virtual sphere having
a diameter Ra can be placed at the position. Similarly, in Step
S270, even when a child virtual sphere having a diameter Rb to be
placed overlaps a virtual curved surface solid that has been
placed, this child virtual sphere is allowed to be placed. In other
words, in Step S270, if a child virtual sphere having a diameter Rb
does not overlap the matter voxel, regardless of whether or not the
child virtual sphere overlaps virtual curved surface solids that
have been placed, Step S270 determines that the child virtual
sphere having a diameter Rb can be placed at the position. As a
result, compared with the cases where virtual curved surface solids
are placed so as not to overlap other virtual curved surface
solids, virtual curved surface solids having a relatively large
volume can be placed.
[0059] In Step S320, the virtual-curved-surface-solid table 83
serving as information regarding virtual curved surface solids is
stored as a part of the porous-body data 80 in the RAM 24. The
virtual-curved-surface-solid table 83 includes association among
the identification code of each virtual curved surface solid, the
central coordinates (X, Y, Z) and diameter of the parent virtual
sphere of the virtual curved surface solid, and the central
coordinates and diameter of at least one child virtual sphere of
the virtual curved surface solid. FIG. 8 describes an example of
the virtual-curved-surface-solid table 83. As described in this
drawing, the virtual-curved-surface-solid table 83 includes,
regarding each of a plurality of virtual curved surface solids that
have been placed as a result of repeating of Steps S230 to S320,
association among the identification code, the central coordinates
and diameter of the parent virtual sphere, and the central
coordinates and diameter of at least one child virtual sphere of
the virtual curved surface solid. Since some virtual curved surface
solids each include a plurality of child virtual spheres, such an
virtual curved surface solid is associated with information of the
plurality of child virtual spheres: the child virtual spheres can
be identified as the first child virtual sphere, the second child
virtual sphere, . . . , in the order that these spheres are placed,
for example. Note that a virtual curved surface solid that has no
child virtual sphere, that is, an virtual curved surface solid made
up of a parent virtual sphere alone, may be placed.
[0060] As a result of the virtual-curved-surface-solid placement
process, the virtual-curved-surface-solid table 83 is stored in the
RAM 24 and the placement of virtual curved surface solid results in
substitution of space voxels with curved surface solid voxels.
Hereinafter, the manner in which the virtual-curved-surface-solid
placement process is carried out to place a single virtual curved
surface solid made up of a parent virtual sphere and child virtual
spheres will be described. FIGS. 9A and 9B illustrate explanatory
views of placement of a parent virtual sphere. FIGS. 10A and 10B
illustrate explanatory views of placement of child virtual spheres
and a virtual curved surface solid. For convenience of explanation,
FIGS. 9A, 9B, 10A and 10B illustrate sections parallel to the X
direction in the porous-body data 80 and two-dimensionally
illustrate the placement of the virtual curved surface solid. FIG.
9A is an explanatory view of an example of the porous-body data 80
in which Step S210 has just been carried out and a virtual curved
surface solid is to be placed. FIG. 9B is an explanatory view in
which a single parent virtual sphere is placed. FIG. 10A is an
explanatory view in which a plurality of child virtual spheres are
placed for the parent virtual sphere that has been placed in FIG.
9B. FIG. 10B is an explanatory view in which a virtual curved
surface solid made up of the parent virtual sphere and child
virtual spheres is placed. As illustrated in FIG. 9A, the
porous-body data 80 is made up of matter voxels and space voxels;
and, the inflow plane 61, the outflow plane 62, and the virtual
wall surfaces 85 are defined. The virtual curved surface solid
(parent virtual sphere and child virtual spheres) is placed so as
not to extend beyond the virtual wall surfaces 85. In this state,
the processes of Steps S220 to S250 are carried out: in a case
where the diameter Ramax is set to a sufficiently large value, the
diameter Ra is decremented by 1 in a stepwise manner; and when the
diameter Ra is decremented to be equal to the maximum diameter of a
parent virtual sphere that can be placed in the porous-body data 80
so as not to overlap the matter voxel and so as not to extend
beyond the virtual wall surfaces 85, this single parent virtual
sphere is placed (FIG. 9B). Subsequently, Steps S270 to S300 are
repeated until Step S300 determines that the diameter Rb becomes
less than the minimum value Rbmin. As a result, a plurality of
child virtual spheres having various diameters are placed such that
the centers of the child virtual spheres overlap the parent virtual
sphere, and the voxels occupied by the child virtual spheres do not
overlap the matter voxel and fill space voxels (FIG. 10A). After
Step S300 determines that the diameter Rb is less than the minimum
value Rbmin, a single virtual curved surface solid made up of the
parent virtual sphere and the child virtual spheres that have been
placed is placed (FIG. 10B). In this way, processes of Steps S230
to S320 in which a single virtual curved surface solid is placed
are repeated until Step S330 determines that 99% or more of space
voxels have been substituted with the curved surface solid voxels.
Thus, virtual curved surface solids are placed one after another in
other space voxels in which virtual curved surface solids have not
been placed yet, to thereby fill space voxels with curved surface
solid voxels. As a result, spaces (pores) having complex shapes in
a porous body are substituted with virtual curved surface solids
each made up of a combination of a plurality of spheres, so that
the spaces within the porous body can be simulated with higher
accuracy as a combination of a plurality of virtual curved surface
solids.
[0061] Hereinafter, the description of the analysis processing
routine in FIG. 6 will be resumed. After the
virtual-curved-surface-solid placement process of Step S100 is
finished, the CPU 22 carries out a fluid analysis process (Step
S110) in which fluid analysis is carried out on the basis of the
porous-body data 80 stored in the RAM 24 to thereby obtain
information regarding flow rates in individual space voxels during
passing of a fluid through the porous body. This fluid analysis
process is carried out by the well-known lattice Boltzmann method.
Specifically, the fluid analysis is carried out by the lattice
Boltzmann method in the following manner: the central points of
voxels of the porous-body data 80 are defined as lattice points; a
predetermined relational expression regarding flows of a fluid
entering through the inflow plane 61 and flowing between each
lattice point and its adjacent lattice point is used. As
information regarding the flow rates in individual space voxels,
flow-rate vectors each made up of flow rate and flow direction in
individual space voxels of the porous-body data 80 are obtained.
The flow-rate vectors in individual space voxels are associated
with the porous-body table 81 of the porous-body data 80 in the RAM
24 and stored. Note that this fluid analysis is carried out with
numerical values that are necessary for the analysis and are
defined in advance in, for example, the HDD 25 or the like, such as
a fluid average flow rate T.sub.in at the inflow plane 61, a fluid
viscosity .mu., and a fluid density .rho.. These numerical values
may be set by the user via the input devices 27. The average flow
rate T.sub.in is the average flow rate of the fluid immediately
before entry into the porous body and corresponds to the initial
value of the flow rate in the fluid analysis. In this embodiment,
the average flow rate T.sub.in is set to 0.01 m/s. The fluid is
assumed to be the air at 0.degree. C. and at 1 atm having a
viscosity .mu. of 1.73.times.10.sup.-5 [Pas] and a density .rho. of
1.25 [kg/m.sup.3]. Note that the fluid analysis process in Step
S110 is carried out such that the virtual curved surface solids
that have been placed in Step S100 are not considered and the
curved surface solid voxels are also assumed to be space voxels. In
this embodiment, the fluid analysis process in Step S110 is carried
out on the basis of the porous-body data 80 stored in the RAM 24.
Alternatively, the fluid analysis process in Step S110 may be
carried out on the basis of the porous-body data 60 stored in the
HDD 25.
[0062] Subsequently, the CPU 22 carries out a flow-rate-weighted
mean diameter evaluation process (Step S120) of obtaining the
flow-rate-weighted mean diameter Ru and determining the quality of
the porous body on the basis of this obtained value to thereby
evaluate the trapping capability of the porous body. This process
is carried out with information regarding the plurality of virtual
curved surface solids that have been placed in Step S100 and
information regarding the flow rates that have been obtained by the
fluid analysis in Step S110. The flow-rate-weighted mean diameter
Ru is obtained by an expression (1) below.
[ Math . 1 ] Ru = i = 1 n ( R i ' .times. V i .times. U i ) i = 1 n
( V i .times. U i ) ( 1 ) ##EQU00002##
[0063] where
[0064] Ru: flow-rate-weighted mean diameter [.mu.m]
[0065] n: number of virtual curved surface solids that have been
placed [number]
[0066] R'.sub.i: equivalent diameter of each virtual curved surface
solid [.mu.m] (i=1, 2, . . . , n)
[0067] V.sub.i: volume of each virtual curved surface solid [cc]
(i=1, 2, . . . , n)
[0068] U.sub.i: average flow rate of fluid passing through each
virtual curved surface solid [mm/s] (i=1, 2, . . . , n).
[0069] In this expression, the number n of the virtual curved
surface solids that have been placed is equal to the total number
of virtual curved surface solids that have been placed in the
virtual-curved-surface-solid placement process of Step S100. The
equivalent diameter R'.sub.i, volume V.sub.i, and average flow rate
U.sub.i of each virtual curved surface solid are obtained in the
following manner, for example. A single virtual curved surface
solid is selected and curved surface solid voxels corresponding to
the identification code of the selected virtual curved surface
solid are examined with reference to the porous-body table 81 in
the RAM 24. The number of voxels of curved surface solid voxels of
the selected virtual curved surface solid is obtained; and the
product of the number of voxels and the volume of a single curved
surface solid voxel (in this embodiment, 1.728 .mu.m.sup.3) is
defined as the volume V.sub.i. In addition, on the basis of
information (the central coordinates and diameters of the parent
virtual sphere and child virtual spheres) contained in the
virtual-curved-surface-solid table 83, the surface area S.sub.i of
the selected virtual curved surface solid is obtained. The
equivalent diameter R'.sub.i is obtained by equivalent diameter
R'.sub.i=6.times.(volume V.sub.i of virtual curved surface
solid)/(surface area S.sub.i of virtual curved surface solid). In
addition, the quantity Q.sub.i of the fluid passing through the
selected virtual curved surface solid per unit time is obtained;
and the average flow rate U.sub.i is obtained by average flow rate
U.sub.i=Q.sub.i/{.pi.(R'.sub.i).sup.2/4}.
[0070] In this embodiment, the voxels occupied by different virtual
curved surface solids are allowed to overlap. For this reason, the
volume V.sub.i used in the expression (1) is a value modified such
that voxels each occupied by a plurality of virtual curved surface
solids (that is, curved surface solid voxels each serving as
components of a plurality of virtual curved surface solids) are
each assumed to be occupied only by any one of the virtual curved
surface solids. Specifically, the volume V.sub.i is modified by
assuming that curved surface solid voxels each serving as
components of a plurality of virtual curved surface solids are each
a component that belongs only to a single virtual curved surface
solid having the largest equivalent diameter R'.sub.i among the
plurality of virtual curved surface solids; and the modified value
is used in the expression (1). This is carried out in the following
manner, for example. Without consideration as to whether or not the
curved surface solid voxels each serve as components of plurality
of virtual curved surface solids (that is, the curved surface solid
voxels are each assumed to serve as components of any corresponding
virtual curved surface solids), the volume V.sub.i is obtained.
Thus, the volume V.sub.i before modification is obtained. On the
basis of the volume V.sub.i before modification, the equivalent
diameter R'.sub.i is obtained. The volume V.sub.i of virtual curved
surface solids is obtained again in descending order of the
equivalent diameter R'.sub.i. At this time, curved surface solid
voxels that have been used once for obtaining volume V.sub.i are no
longer used for calculation of the volume V.sub.i of other virtual
curved surface solids (these curved surface solid voxels are no
longer counted for obtaining the number of voxels of the curved
surface solid voxels of the virtual curved surface solids). As a
result, the volume V.sub.i (modified volume V.sub.i) can be
obtained such that the curved surface solid voxels each serving as
components of a plurality of virtual curved surface solids are each
assumed to be a component that belongs only to the virtual curved
surface solid having the largest equivalent diameter R'.sub.i. This
modified volume V.sub.i is used as the volume V.sub.i in the
expression (1). Note that the equivalent diameter R'.sub.i used is
a value based on the volume V.sub.i before modification; and the
modified volume V.sub.i is not used to obtain again the equivalent
diameter R'.sub.i.
[0071] The quantity Q.sub.i of the passing fluid is obtained in the
following manner, for example. Among curved surface solid voxels of
the selected virtual curved surface solid, the curved surface solid
voxels forming the surface of the virtual curved surface solid are
identified on the basis of information contained in the
virtual-curved-surface-solid table 83. The curved surface solid
voxels forming the surface of the virtual curved surface solid can
also be identified by, for example, identifying, among curved
surface solid voxels of the selected virtual curved surface solid,
curved surface solid voxels adjacent to any one of space voxels,
matter voxels, and curved surface solid voxels of the other virtual
curved surface solids. Alternatively, the curved surface solid
voxels forming the surface of the virtual curved surface solid may
be identified on the basis of the central coordinates and diameters
of the parent virtual sphere and child virtual spheres contained in
the virtual-curved surface solid table 83. Subsequently, flow-rate
vectors associated with curved surface solid voxels forming the
surface are examined with the porous-body table 81 in the RAM 24 to
identify curved surface solid voxels having flow-rate vectors
pointing directions toward inside the virtual curved surface solid;
and the magnitude of the flow-rate vectors of the identified curved
surface solid voxels is obtained for each curved surface solid
voxel. The quantity Q.sub.i of the passing fluid is obtained as
quantity Q.sub.i of the fluid passing per unit time=(sum of
magnitude of flow-rate vectors).times.(number of curved surface
solid voxels having flow-rate vectors pointing directions toward
inside virtual curved surface solid among curved surface solid
voxels forming surface of virtual curved surface solid).times.(area
(=1.44 .mu.m.sup.2) of single surface of curved surface solid
voxel).
[0072] As described above, the CPU 22 provides the equivalent
diameter R'.sub.i, volume V.sub.i, and average flow rate U.sub.i of
individual n virtual curved surface solids and provides the
flow-rate-weighted mean diameter Ru by use of the expression (1).
In cases where the flow-rate-weighted mean diameter Ru obtained is
10 .mu.m or more and 24 .mu.m or less, the CPU 22 evaluates that
the porous body (region 50 in the porous partition 44) from which
the porous-body data 60 is derived is good in terms of trapping
capability; and, in the other cases, the CPU 22 evaluates that the
porous body is poor in terms of trapping capability. The CPU 22
stores, in the RAM 24, for example, the value of the
flow-rate-weighted mean diameter Ru and the evaluation result.
[0073] Subsequently, the CPU 22 carries out a difference .DELTA.R
evaluation process (Step S130) of providing a difference .DELTA.R
that is an absolute value of the difference between an arithmetic
mean diameter Rc and the flow-rate-weighted mean diameter Ru and
determining the quality of the porous body on the basis of the
provided value to thereby evaluate the trapping capability of the
porous body. This process is carried out with information regarding
the plurality of virtual curved surface solids that have been
placed in Step S100 and the flow-rate-weighted mean diameter Ru
that has been obtained in Step S120. The difference .DELTA.R is
obtained in the following manner. The arithmetic mean diameter
Rc=(R'.sub.1+R'.sub.2+ . . . +R'.sub.n)/n is obtained. The
arithmetic mean diameter Rc may be obtained through the calculation
of equivalent diameter R'.sub.i as in Step S120 or may be obtained
with the values of equivalent diameter R'.sub.i that have been
obtained in Step S120. Then, the difference .DELTA.R is obtained by
the difference .DELTA.R=|Ru-Rc|. In cases where the CPU 22
evaluates the porous body as being good in the flow-rate-weighted
mean diameter evaluation process in Step S120 and the difference
.DELTA.R obtained is 2 .mu.m or less, the CPU 22 evaluates that the
porous body (region 50 in the porous partition 44) from which the
porous-body data 60 is derived is better in terms of trapping
capability; and, in the other cases, the CPU 22 evaluates that the
porous body is not "better". In other words, in this embodiment, in
cases where the porous body is evaluated as being good in Step S120
(the flow-rate-weighted mean diameter Ru is 10 .mu.m or more and 24
.mu.m or less) and the porous body is evaluated as being good in
Step S130 (the difference .DELTA.R is 2 .mu.m or less), the porous
body is evaluated as being "better" in terms of trapping
capability. In cases where the evaluation result is good in Step
S120 but the evaluation result is not good in Step S130, the porous
body is evaluated as being "good" in terms of trapping capability.
In cases where the evaluation result is poor in Step S120,
regardless of the evaluation result in Step S130, the porous body
is evaluated as being "poor" in terms of trapping capability. The
CPU 22 stores, in the RAM 24, the values of the arithmetic mean
diameter Rc and difference .DELTA.R and the evaluation results.
[0074] After carrying out the evaluation processes of Steps S120 to
S130, the CPU 22 carries out an analysis-result output process
(Step S140) of outputting analysis result data such as information
stored in the RAM 24 during the processes and storing the analysis
result data in the HDD 25, and finishes the routine. The analysis
result data includes, for example, those stored in the RAM 24, such
as the porous-body data 80 including the porous-body table 81, the
inflow-outflow table 82, and the virtual-curved-surface-solid table
83; the value of the flow-rate-weighted mean diameter Ru and the
evaluation result obtained in Step S120; and the values of the
arithmetic mean diameter Rc and difference .DELTA.R and the
evaluation results obtained in Step S130. The analysis result data
may also include values used in processes of Steps S120 to S130,
such as the equivalent diameter R'.sub.i, the volume V.sub.i, and
the average flow rate U.sub.i; and values used in the fluid
analysis process in Step S110, such as the average flow rate
T.sub.in, the fluid viscosity .mu., and the fluid density
.rho..
[0075] The porous partition 44 of this embodiment is subjected to
the above-described analysis of the microstructure and, as a result
of the analysis, it is found to have a flow-rate-weighted mean
diameter Ru of 10 .mu.m or more and 24 .mu.m or less. It is more
preferable that the difference .DELTA.R is 2 .mu.m or less.
[0076] Hereinafter, a method for producing the honeycomb filter 30
including the porous partitions 44 according to this embodiment
will be described. Here, a method for producing the honeycomb
filter 30 including the porous partitions 44 mainly composed of
cordierite will be described as an example.
[0077] The porous partitions 44 of the honeycomb filter 30 can be
produced by a raw-material mixing step of mixing a base material
and a pore-forming agent, to provide green body; and a molding and
firing step of molding the green body to provide a compact and
firing this compact. The base material is a mixture of talc having
an average particle size of 1 .mu.m or more and 18 .mu.m or less,
alumina, and an auxiliary raw material containing a material that
undergoes a eutectic reaction with talc and being prepared in an
amount so as to satisfy a weight ratio of 0.5% or more and 1.5% or
less by weight relative to the talc. The talc preferably has an
average particle size of 5 .mu.m or more and 12 .mu.m or less. The
weight ratio of the alumina to the talc is 30% or more and 45% or
less by weight, for example. The base material may further contain
silica in an amount so as to satisfy a weight ratio of 42% or more
and 56% or less by weight relative to the talc, or magnesia in an
amount so as to satisfy a weight ratio of 12% or more and 16% or
less by weight relative to the talc. The auxiliary raw material is
at least one selected from zirconium oxide, cerium oxide, and
yttrium oxide, for example. The weight ratio of the auxiliary raw
material relative to the talc is more preferably 0.5% or more and
1.0% or less by weight. The pore-forming agent is preferably burnt
off by firing to be carried out: starch, coke, or porous resin, for
example. The raw-material mixing step may be carried out by adding,
for example, a binder such as methylcellulose or
hydroxypropoxylmethylcellulose, water, and a dispersing agent. The
dispersing agent may be a surfactant such as ethylene glycol. The
process of preparing green body is not particularly limited and may
be carried out by a method using a kneader or a vacuum clay
kneader, for example. The green body is, for example, extruded
through a die having a shape corresponding to the arrangement of
the cells 34, so as to have the shape illustrated in FIGS. 1 and 2.
In the extruded material, the cells 34 are sealed with the outlet
sealing material 38 and the inlet sealing material 42.
Subsequently, the extruded material is subjected to a drying
treatment, a calcination treatment, and a firing treatment to
thereby produce the honeycomb filter 30 including the porous
partitions 44. The outlet sealing material 38 and the inlet sealing
material 42 may be formed of the raw material forming the porous
partitions 44. The calcination treatment is carried out at a
temperature lower than the firing temperature, to burn off organic
components contained in the honeycomb filter 30. The firing
temperature is 1350.degree. C. to 1440.degree. C. and is preferably
1410.degree. C. to 1430.degree. C. By carrying out the
above-described steps, the honeycomb filter 30 including the porous
partitions 44 can be obtained.
[0078] According to the embodiment having been described in detail
so far, the porous partitions 44 formed of a porous body are found
to have a flow-rate-weighted mean diameter Ru of 10 .mu.m or more
and 24 .mu.m or less as a result of the microstructure analysis
using the user PC 20; and hence the porous partitions 44 have a
sufficiently high trapping capability. In addition, in cases where
the difference .DELTA.R is 2 .mu.m or less, the porous partitions
44 have a higher trapping capability.
[0079] The larger the extent of variation in the volume V.sub.i or
average flow rate U.sub.i among a plurality of virtual curved
surface solids that have been placed, the larger the tendency for
the flow-rate-weighted mean diameter Ru to become excessively large
or small. Regarding pores of a porous body that are simulated with
virtual curved surface solids having a large volume V.sub.i, the
probability that a fluid passing through the pores comes into
contact with the wall surfaces of the porous body tends to
decrease. Regarding pores of a porous body that are simulated with
virtual curved surface solids having a small volume V.sub.i, a
fluid tends not to pass through the pores or a catalyst tends not
to be appropriately applied to the wall surfaces of the pores, the
catalyst being used for the porous body to be used as a filter.
Regarding pores of a porous body that are simulated with virtual
curved surface solids having an average flow rate U.sub.i higher
than the average flow rate (simple average flow rate) of the whole
pores of the porous body, a fluid passes through these pores in a
short time and hence these pores tend not to contribute to the
trapping capability. Regarding pores of a porous body that are
simulated with virtual curved surface solids having an average flow
rate U.sub.i lower than the average flow rate (simple average flow
rate) of the whole pores of the porous body, the amount of a fluid
entering these pores is small and hence these pores tend not to
contribute to the trapping capability. As has been described, pores
of a porous body that are simulated with virtual curved surface
solids having an excessively large or small volume V.sub.i or an
excessively high or low average flow rate U.sub.i tend not to
contribute to the trapping capability. This is probably the reason
why, in porous bodies having a large portion that tends not to
contribute to the trapping capability, the flow-rate-weighted mean
diameter Ru is found to be excessively large or small; and the
trapping capability probably correlates with the flow-rate-weighted
mean diameter Ru. In cases where the flow-rate-weighted mean
diameter Ru is 10 .mu.m or more and 24 .mu.m or less, the
flow-rate-weighted mean diameter Ru is not excessively large or
small and a sufficiently high trapping capability is probably
achieved. Among cases where the flow-rate-weighted mean diameter Ru
is substantially the same, cases where the absolute value of the
difference between the arithmetic mean diameter Rc and the
flow-rate-weighted mean diameter Ru is small tend to achieve a
better trapping capability of the porous body. Accordingly, a
porous body that has a flow-rate-weighted mean diameter Ru of 10
.mu.m or more and 24 .mu.m or less and a difference .DELTA.R of 2
.mu.m or less tends to have a higher trapping capability.
[0080] In the production of the porous partitions 44, use of talc
having an average particle size of 18 .mu.m or less probably
results in suppression of generation of excessively large pores. By
adding an auxiliary raw material containing a material that
undergoes a eutectic reaction with talc, excessively small pores
through which a fluid tends not to pass are probably filled. In
addition, the firing temperature of 1350.degree. C. to 1440.degree.
C. probably results in a sufficiently high strength of the porous
body. As a result, generation of excessively large or small pores
is suppressed and generation of pores causing an excessively high
or low flow rate is suppressed; and this probably results in a high
trapping capability.
[0081] The present invention is not limited to the above-described
embodiment. It is clear that the present invention can be
implemented in a variety of embodiments without departing from the
technical scope thereof.
EXAMPLES
[0082] Hereinafter, an example in which a porous body according to
the present invention was actually produced will be described as an
Example. Note that Experimental examples 1 to 3 are Comparative
examples with respect to the present invention and Experimental
example 4 is an Example of the present invention. However, the
present invention is not limited to the Example described
below.
Experimental Example 1
[0083] A honeycomb filter of Experimental example 1 was produced in
the following manner. A talc powder A having an average particle
size of 12 .mu.m, alumina, and zirconium oxide (ZrO.sub.2) serving
as an auxiliary raw material were prepared and mixed together to
provide a base material. The weight ratio of the zirconium oxide
(ZrO.sub.2) to the talc powder A was set to 1.75%. The weight ratio
of the alumina to the talc powder A was set to 201. The
thus-obtained base material and a pore-forming agent (starch)
having an average particle size of 30 .mu.m were mixed together in
a mass ratio of 100:30. This mixture was mixed with methylcellulose
serving as an organic binder and an appropriate amount of water to
thereby provide green body. Subsequently, this green body was
extruded through a predetermined die to thereby provide a compact
including the porous partitions 44 having the shape illustrated in
FIGS. 1 and 2 (note that the exterior of the compact had a shape of
a quadrangular prism). Subsequently, the compact obtained was dried
with microwaves, further dried with hot gas, then subjected to
sealing of cell openings, calcined in an oxidizing atmosphere at
550.degree. C. for 3 hours, and then fired in an inert atmosphere
at 1430.degree. C. for 2 hours. The sealed portions were formed in
the following manner: masks were placed on alternate cell openings
at an end of the compact; and this masked end was immersed in a
sealing slurry (formed of the same material as the green body), so
that openings and sealed portions were arranged alternately.
Similarly, the other end was also masked and sealed portions were
formed such that cells having one open end and the other sealed end
and cells having one sealed end and the other open end were
alternately arranged. The compact having been fired was ground so
as to have a cylindrical shape. Subsequently, the periphery of this
cylinder was coated with a periphery coating slurry prepared by
kneading alumina silicate fibers, colloidal silica, polyvinyl
alcohol, SiC, and water. This slurry was cured by drying to thereby
form a peripheral protective part 32. Thus, the honeycomb filter of
Experimental example 1 was obtained. This honeycomb filter had a
diameter of 118.4 mm in cross section, a length of 127 mm, a cell
density of 360 cells/square inch, and a partition thickness of 5
mil.
Experimental Example 2
[0084] A honeycomb filter of Experimental example 2 was produced as
in Experimental example 1 except that the talc powder A was
replaced by a talc powder B having an average particle size of 25
.mu.m.
Experimental Example 3
[0085] A honeycomb filter of Experimental example 3 was produced as
in Experimental example 1 except that the talc powder A was
replaced by a talc powder C having an average particle size of 20
.mu.m, the weight ratio of the zirconium oxide (ZrO.sub.2) to the
talc powder C was set to 1%, and the firing temperature was set to
1420.degree. C.
Experimental Example 4
[0086] A honeycomb filter of Experimental example 4 was produced as
in Experimental example 1 except that the weight ratio of the
zirconium oxide (ZrO.sub.2) to the talc powder A was set to 1% and
the firing temperature was set to 1420.degree. C.
[0087] [Preparation of Microstructure Analyzer]
[0088] A microstructure analyzer for evaluating Experimental
examples 1 to 4 was prepared. An analysis processing program having
the function described in the embodiment above was created. This
program was stored in the HDD of a computer including a controller
including a CPU, a ROM, and a RAM, and the HDD. Thus, the
microstructure analyzer was prepared.
[0089] Analysis of Microstructure
[0090] The porous partitions (porous body) of the honeycomb filter
of Experimental example 1 were subjected to a CT scan. From the
resultant voxel data, a piece of data was extracted in which the
length in the X direction was 300 .mu.m (=1.2 .mu.m.times.250
voxels), which was the same as the partition thickness in the
passing direction of exhaust gas, the length in the Y direction was
480 .mu.m (=1.2 .mu.m.times.400 voxels), and the length in the Z
direction was 480 .mu.m (=1.2 .mu.m.times.400 voxels). This data
was defined as the above-described porous-body data 60 and stored
in the HDD in Experimental example 1. The above-described analysis
processing routine was executed for the porous-body data 60 in
Experimental example 1. This provided analysis result data
including, as described above, the porous-body table, the
virtual-curved surface solid table, values of the
flow-rate-weighted mean diameter Ru and difference .DELTA.R, and
values of the equivalent diameter R'.sub.i, volume V.sub.i, and
average flow rate U.sub.i of each virtual curved surface solid.
Similarly, analysis result data was obtained for Experimental
examples 2 to 4. As a result, the flow-rate-weighted mean diameters
Ru in Experimental examples 1 to 4 were respectively 26.3 .mu.m,
29.9 .mu.m, 24.5 .mu.m, and 23.7 .mu.m. The differences .DELTA.R in
Experimental examples 1 to 4 were respectively 2.0 .mu.m, 3.0
.mu.m, 2.1 .mu.m, and 2.1 .mu.m.
[0091] [Measurement of Number of Leaked Particles]
[0092] Regarding Experimental examples 1 to 4, the number of leaked
particles was measured as a value representing the actual trapping
capability. Specifically, the filters of Experimental examples 1 to
4 were attached to the exhaust pipe of a car through which exhaust
gas passes. The gasoline engine of the car was run on the basis of
NEDC (New European Driving Cycle)-mode driving to thereby pass a
fluid containing particulate matter (engine exhaust gas) through
the filters. In the fluid having passed through the filters, the
amount of remaining particulate matter was measured as the number
of leaked particles. This number of leaked particles was converted
with respect to 1 km of driving distance. Thus, the number of
leaked particles [number/km] was obtained as a value representing
the trapping capability.
[0093] Table 1 summarizes the average particle size of talc
powders, the weight ratio of the auxiliary raw material (zirconium
oxide) to talc, the firing temperature, the flow-rate-weighted mean
diameter Ru, the difference .DELTA.R, and the number of leaked
particles in Experimental examples 1 to 4. FIG. 11 is a graph
illustrating the relationship between the flow-rate-weighted mean
diameter Ru and the number of leaked particles in Experimental
examples 1 to 4.
TABLE-US-00001 TABLE 1 Weight ratio of Flow-rate- Number of
Particle size auxiliary raw Firing weighted mean Difference leaked
of talc powder material to talc temperature diameter Ru .DELTA.R
particles [.mu.m] (% by weight) [.degree. C.] [.mu.m] [.mu.m]
[Number/s] Experimental 12 1.75% 1430.degree. C. 26.3 2.0 8.67E+11
Example 1 Experimental 25 1.75% 1430.degree. C. 29.9 3.0 1.23E+12
Example 2 Experimental 20 1% 1410.degree. C. 24.5 2.1 6.54E+11
Example 3 Experimental 12 1% 1410.degree. C. 23.7 2.1 5.45E+11
Example 4
[0094] Table 1 and FIG. 11 indicate the following. Comparison among
Experimental examples 1 to 4 indicates that the smaller the
flow-rate-weighted mean diameter Ru, the smaller the number of
leaked particles (the higher the trapping capability). In
Experimental example 4 in which the flow-rate-weighted mean
diameter Ru is 24 .mu.m or less, the number of leaked particles was
smaller than that in Experimental examples 1 to 3. In Experimental
example 4, the condition that the number of leaked particles is
6.times.10.sup.11 or less [particles/km], which is the limit for
exhaust gas emitted from automobiles (Euro 6 (2017 and after)), was
satisfied. Thus, the trapping capability was sufficiently high.
Note that FIG. 11 illustrates both the limit of Euro 6 (2017 and
after) (number of leaked particles: 6.times.10.sup.11
[particles/km]) and the limit of Euro 6 (before 2017) (number of
leaked particles: 6.times.1012 [particles/km]) (broken lines in
FIG. 11).
[0095] The present application claims priority of Japanese Patent
Application No. 2014-072362 filed on Mar. 31, 2014, the entire
contents of which are incorporated herein by reference.
* * * * *