Evaporative Cooling And Heat Extraction System

Abstract

Heat transfer is accomplished from electronic components by inserting simaneously both a liquid coolant and a gas into a chamber having a high heat flux surface forming one side of the chamber. A porous membrane section forms on opposite side of the chamber and permits both the gas and a hot vapor formed from the water to escape. In operation the gaseous flow is used to break up the liquid coolant film that forms on the high heat flux surface.

Description

STATEMENT OF GOVERNMENT INTEREST

The invention described herein may be manufactured and used by or for the Government of the United States of America for governmental purposes without the payment of any royalties thereon or therefor.

BACKGROUND OF THE INVENTION

The present invention generally relates to heat transfer systems and more particularly to systems suitable for cooling of avionics equipment by the enhancement of enthalpy and heat of vaporization of suitable coolants.

Many prior known systems for cooling avionic equipment use variations of the well known heat pipe. The heat pipe is a sealed thermodynamic system relaying on internal evaporation and condensation cycles. It comprises an enclosure, a wicking material lining the internal walls of the enclosure, and a working fluid for saturating the wick. One end of the heat pipe is called the evaporator and serves to absorb heat energy. Vapor formed in the evaporator is then transported to the other end of the heat pipe called the condenser. Here the heat is released from the heat pipe by means of the internal condensation of the working fluid to the internal walls of the heat pipe. The working fluid is then recirculated to the evaporator end by the capillary action of the wick where the cycle is repeated.

Certain performance characteristics of the heat pipe including heat flux handling capacity vary depending on the vertical orientation of the evaporator end with respect to the condenser. During aircraft maneuvers this is apt to continually change. Other limitations of the device include heat extraction dependence upon operating temperature level, sonic vapor flow velocities, fluid entrainment flow, wick dryout, internal generation of non-condensible gases, and shock and vibration problems. In addition a high heat dissipating component must be mounted to the heat pipe by suitable means. This introduces thermal impedances that are detrimental to any heat transfer system.

SUMMARY OF THE INVENTION

It is therefore a general object of the present invention to provide an improved system for the cooling of avionics equipment. It is a further object to provide a system that is independent of attitude or gravity so as to eliminate variation in operational characteristics during aircraft maneuvers. Another object is to provide a system whereby the enthalpy and heat of vaporization of coolants are more effectively utilized and controlled. A further object is to provide a lightweight system in which all coolant must become high quality steam before being removed from the system. Additional objects are to improve the operating temperature level and to obviate other known limitations on effectiveness of prior systems.

This is accomplished in accordance with the present invention by providing a heat transfer system operable as either a closed or open loop system in which both a liquid coolant and gas are inserted simultaneously into an area having a heat exchange surface to be cooled and enclosed so that only the gas and vapor formed from the liquid coolant may escape through a porous material provided for such purpose. The porous material is separated from the heat exchange surface from which heat is to be extracted by a finite distance. The physical characteristics of the porous material are such that the material will readily permit the passage of gas or vapor molecules but will restrict the liquid coolant from passing quickly through the porous material by its surface tension properties. The gas is injected under pressure as pulsating or steady flow through the coolant fluid and against the heat exchange surface to reduce the thickness of a liquid evaporating film or thin liquid boundary layer and to break up insulating vapor bubbles that form adjacent the surface to be cooled thus improving the heat transfer capability of the device.

BRIEF DESCRIPTION OF THE DRAWING

FIG. 1 is an embodiment of the present invention partially in cross section;

FIGS. 2a and 2b are views of a spacer of FIG. 1;

FIG. 3 is a top view of a transistor of FIG. 1;

FIG. 4 is an alternate embodiment of the invention partially in cross section to show the operation more precisely; and

FIG. 5 is a top view of the alternate embodiment of FIG. 4.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Referring to FIGS. 1-3, inclusive, there is shown a system 10 having a housing 11 with a first opening 12 receiving a flow of liquid coolant through a hose 14 or a comparable substitute such as piping. The liquid coolant can be water or other coolant with good heat transfer properties. A second opening 15 receives cabling 16 having electrical wires 17 connected to a transistor 18. The connection of these wires 17 to transistor 18 is not shown but may be introduced into the housing of transistor 18 using any suitable watertight and heat resistant means such as epoxy for a potting material. The transistor 18 is inserted in the housing 11 and has a flanged section 19 that abuts shoulder 21. As shown more clearly in FIG. 3 this flanged section 19 permits water to flow across the sides of transistor 18. A spacer 25 is threaded to housing 11 and has teeth 26 symmetrically spaced around it to permit gas injectors 27 to supply gas such as air to the hot surface 28 of transistor 18. The spacer 25 holds transistor 18 in place and spaces it from a porous membrane 31 forming a cavity 33. The porous membrane 31 can be a material such as brass and can be matched to the coolant used, to restrict it by its surface tension from passing through the porous material until it has boiled and vaporized. This permits the vapors to readily pass through the porous membrane while inhibiting the flow of the coolant in liquid form. In this manner, the heat of vaporization, btu's/lb., of the coolant liquid is utilized to extract additional heat. The heat transferred as a result of vaporization will be many times that extracted by an enthalpy change alone. These porous membranes 31 are well known to those of skill in the art and are readily available commercially. A retaining ring 32 is then screwed into housing 11 to hold porous membrane 31 in place against spacer 25. A cap 37 impervious to the vapor and gas is then secured over housing 11. An outlet line 38 removes the vapor and gas from housing 11 and may discharge it to a condenser and a vent for the gas (not shown) for recirculation, or the vapor and gas can be wasted in an open loop system.

The operation of the device will now be described with reference to FIGS. 1-3, inclusive. Electric power to high power transistor 18 is received from leads 17. A coolant is then supplied to the inlet of housing 11 and flows over the transistor surface. The water passes along the sides of transistor 18 into the cavity 33 where the liquid flow is blocked by porous membrane 31. The liquid in cavity 33 vaporizes from the heat of transistor 18. The evaporization rate of the liquid coolant is further enhanced by the use of gas impacters 27 directed at the hot surface of transistor 18. The gas impacters dissipate the surface resulting in a more efficient use of the liquid coolant. The vapor and gas passage through porous membrane 31 is shown by arrows. Afterwards, the vapor and gas are discharged through outlet 33 which may have a pump connected to it (not shown) for the removal of water vapor and gas for closed or open cycle operation.

Referring now to FIGS. 4 and 5 an alternate embodiment is shown having a casing 50 with a flange 51 connected to a plate 52 by means of screws 53. An integrated circuit chip 54 receives power from wires 55 through cable 56 and electrical connector 57. A flanged portion 63 of a spacing ring 61 is connected to plate 52 by screws 62. The plate 52 must have good heat transfer properties and could be made of silver or copper. A liquid coolant inlet 64 for fluid such as water has a line 65 connected to it for supplying the coolant to a cavity 67. Gas impacters 68 are inserted into the cavity 67 through retaining ring 61. The gas impacters 68 have a line 69 for supplying a gas such as air to a hot surface 71 of plate 52.

In operation the electric power is supplied to chip 54 on lines 55. The surface 71 becomes heated and a liquid coolant is supplied to cavity 67 by line 65.

A gas such as air is simultaneously inserted by pulsating or steady flow into cavity 67 through line 69 and gas jet impacters 68. This aids in dissipating the liquid film on hot surface 71. The vapors formed pass through porous membrane 72 which is connected to ring 61. The membrane 72 passes only gas and water vapor and is impervious to water. The vapor and gas are passed through membrane 72 as shown by the arrows are then wasted to the atmosphere.

An alternative system available to those of skill in the art would be to trap the vapor and gas passed through membrane 72 and to recirculate and form a closed loop cycle from the embodiment of FIGS. 4 and 5.

There has therefore been shown a superior heat extraction and cooling system that is applicable to high heat flux devices, components, parts, mounting boards, printed circuit boards, and equipment. The system provides a method whereby the enthalpy and heat evaporization of coolants are more effectively utilized and controlled than in previously used devices. It provides an external pumping source to provide liquid coolant to the hot surface from which heat is to be extracted without relying on the capillary action of any material to provide liquid pumping. Consequently, this invention is independent of position orientation and is not affected by gravity or other acceleration effects. The invention can operate as a closed loop or open loop system. In one embodiment the coolant is brought into direct and intimate contact with the surface or surfaces from which heat is to be extracted thus eliminating intermediate metal, mounting, and bonding thermal resistances. It operates at a relatively low temperature level, consistent with component part material temperature limits for a given heat flux density. The entire surface area is utilized for heat extraction. This invention will eliminate the adverse effect of localized film boiling by the constant supply of liquid coolant as it is evaporated and by gas jet impaction. It will enhance the rate of vaporization and hence the heat extraction rate by incorporating gas jet impaction directed through the coolant liquid and at the surface from which heat is to be extracted.

It will be understood that various changes in the details, materials, steps and arrangements of parts, which have been herein described and illustrated in order to explain the nature of the invention, may be made by those skilled in the art within the principle and scope of the invention as expressed in the appended claims.

Claims

What is claimed is:

1. A heat exchange system comprising:

A gas porous material spaced a finite distance from said heat source; connecting means between said heat source

and gas porous material said connecting means connecting means comprising a spacer between said heat source and said gas porous material, a first inlet receiving a gas and positioned for conducting the gas to impinge upon said heat source, and a second inlet receiving a liquid so that said liquid comes in contact with said heat source;

and a housing enclosing said heat source, said gas porous material and said spacer.

2. A heat exchange system according to claim 1 further comprising:

said gas porous material adapted to be penetrable by gases and liquid vapor, and impervious to liquid.

3. A heat exchange system according to claim 2 wherein said heat source is a transistor.

4. A heat exchange system according to claim 1 wherein said housing and said spacer have adjacent passageways comprising said first inlet and adjacent passageways comprising said second inlet.

5. A heat exchange system according to claim 4 wherein said housing further comprises:

a third inlet adapted to provide a passageway for connecting electrical wiring to said heat source; and

an outlet adapted to discharge liquid, liquid vapor and gas.

6. A heat exchange system according to claim 5 further comprising:

said housing comprises inner threads;

said spacer being secured on said threads; and

a retaining ring adjacent to said porous material secured on said threads.

7. A heat exchange system according to claim 6 further comprising:

an end of said housing being substantially cylindrical in shape; and

a cap covering said end of said housing.

8. A heat exchange system according to claim 2 wherein said connecting means further comprises:

a spacer between said heat source and porous material; and

a heat conducting plate between said spacer and said heat source.

9. A heat exchange system according to claim 8 further comprising:

a cover connected to said heat conductor plate for forming an enclosure for said heat source.

10. A heat exchange system according to claim 9 further comprising:

an electrical connector connected to said cover; and

electrical wiring connected to said electrical connector and said heat source for providing electrical power to said heat source.