Evaporator employing a liquid superheat tolerant wick

Details Evaporator employing a liquid superheat tolerant wick Evaporator employing a liquid superheat tolerant wick

2001-08-21

2003-05-20

Bennett, Henry (Department: 3743)

Heat exchange

Intermediate fluent heat exchange material receiving and...

Liquid fluent heat exchange material

C165S104330, C361S700000, C029S890032, C174S015200

Reexamination Certificate

active

06564860

ABSTRACT:

INTRODUCTION
The present invention relates generally to the field of heat transfer. More particularly, the present invention relates to wicks for use in loop heat pipe evaporators.
BACKGROUND OF THE INVENTION
There are numerous instances where it is desirable to transfer heat from a region of excess heat generation to a region where there is too little heat. The object is to keep the region of heat generation from getting too hot, or to keep the cooler region from getting too cold. This is a typical thermal engineering problem encountered in a wide range of applications including building environmental conditioning systems, spacecraft thermal control systems, the human body, and electronics.
A variety of techniques can be employed to achieve this heat sharing effect. These include heat straps (simple strips of high conductivity material), closed loops of pumped single-phase fluid, heat pipes, mechanically pumped two-phase loops, and capillary pumped two-phase loops.
The most advanced and efficient concept is the capillary pumped two-phase loop and the related loop heat pipe (LHP). LHP technology has recently been developed for spacecraft applications due to its very low weight to heat transferred ratio, high reliability, and inherent simplicity.
An LHP is a two-phase heat transfer system. The LHP is a continuous loop in which both the vapor and the liquid always flow in the same direction. Heat is absorbed by evaporation of a liquid-phase working fluid at the evaporator section, transported via the vaporized fluid in tubing to a condenser section to be removed by condensation at the condenser. This process makes use of a fluid's latent heat of vaporization/condensation, which permits the transfer of relatively large quantities of heat with small amounts of fluid and negligible temperature drops. A variety of fluids including ammonia, water, freons, liquid metals, and cryogenic fluids have been found to be suitable for LHP systems. The basic LHP consists of an evaporator section with a capillary wick structure, of a pair of tubes (one of the tubes is for supply of fluid in its liquid state, and the other is for vapor transport), and a condenser section. In many applications, the pressure head generated by the capillary wick structure provides sufficient force to circulate the working fluid throughout the loop, even against gravity. In other applications, however, the pressure differential due to fluid frictional losses, static height differentials, or other forces may be too great to allow for proper heat transfer. In these situations it is desirable to include a mechanical pump to assist in fluid movement. Systems employing such pumps are called hybrid capillary pumped loops.
In designing LHP evaporators, the art has long taught the use of cylindrical geometry, particularly for use in containing high-pressure working fluids, such as ammonia. Referring to
FIGS. 1-3
, prior art evaporators
10
,
30
,
50
are illustrated as having cylindrical geometry, where a wick
4
has a central flow channel
2
and is surrounded at its periphery by a plurality of peripheral flow channels
6
. Capillary evaporators having a central channel
2
in the wick
4
are sensitive to a problem called back-conduction.
Back-conduction in capillary evaporators refers to the heat transfer due to a temperature gradient across the wick structure, between the vapor grooves
6
in the evaporator and the liquid that is returning to the evaporator in the central channel
2
. This energy is normally balance by sub-cooled liquid return and/or heat exchange at the hydro-accumulator in the case of loop heat pipes. Refer to Ku, J., “Operational Characteristics of Loop Heat Pipes”, SAE paper 99-01-2007, 29th International Conference on Environmental Systems, Denver, Colo., Jul. 12-15, 1999, which is incorporated herein by reference in its entirety.
It would be beneficial to minimize back-conduction for several reasons. First, decreased back-conduction would permit minimization, or even elimination, of liquid return sub-cooling requirements. Second, decreased back-conduction would allow the evaporator operating temperature to approach sink temperature, particularly at low power. Third, decreased back-conduction would allow loop heat pipes to operate at low vapor pressure, where the low slope of the vapor pressure curve allows small pressure differences in the loop to result in large temperature gradients across the wick. Finally, decreased back-conduction would minimize sensitivity to adverse elevation.
Thus, what is needed is a wick for use in an LHP evaporator that has improved back-conduction performance.
Aside from any back-conduction considerations, another inherent disadvantage of the cylindrical evaporator is its cylindrical geometry, since many cooling applications call for transferring heat away from a heat source having a flat surface. This presents a challenge of how to provide for good heat transfer between the curved housing of a cylindrical evaporator and a flat surfaced heat source.
Typically, the evaporator housing is integrated with a flat saddle to match the footprint of the heat source and the surface temperature of the saddle is dependent upon the fin efficiency of the design.
FIG. 1
shows a prior art cylindrical evaporator
10
(cross section perspective view) integrated with a single saddle
20
for mounting to a single, flat-surface heat source (not shown). Heat energy is received via a single heat input surface
22
.
FIG. 3
shows an alternative design for a prior art cylindrical evaporator
30
(cross section perspective view) integrated with a single saddle
40
that has extended fins. Heat energy is received via a single heat input surface
42
.
FIG. 2
shows a prior art cylindrical evaporator
50
(cross section perspective view) integrated with two saddles
60
,
70
. Heat energy is received via two opposed heat input surfaces
62
,
72
.
For large heat sources, requiring isothermal surfaces, multiple evaporators are often required. The number of required evaporators would also increase as the thickness of the envelope available for integrating the evaporator (i.e., the distance between the heat input surface
22
and the bottom
24
of the evaporator of
FIG. 1
, or the distance between the opposed heat input surfaces
62
,
72
of the evaporator of
FIG. 2
) decreases. That is because the width of the cylindrical evaporator is a function of the evaporator diameter and the diameter is limited to integration thickness. Increasing the number of evaporators increases the cost and complexity of the heat transport system.
Capillary evaporators with flat geometry have been devised, which match a heat source having rectangular geometry. Flat geometry eliminates the need for a saddle and avoids the inherent thickness restraints currently imposed upon cylindrical capillary evaporators.
The art of flat capillary evaporators for use with high-pressure working fluids teaches use of structural supports for resisting any deformation forces exerted thereon due to the pressure of the working fluid. The plates are sealed together, which often requires use of bulky clamps or thick plates. Clamps, thick plates and added support mechanisms have the disadvantages of unnecessary weight, thickness and complexity.
U.S. Pat. No. 5,002,122 issued to Sarraf et al. for Tunnel Artery Wick for High Power Density Surfaces relates to the construction of an evaporator region of a heat pipe, having a flat surface
12
for absorbing high power densities. Control of thermally induced strain on the heated surface
12
is accomplished by an array of supports
14
protruding through the sintered wick layer
18
from the backside of the heated surface and abutting against a heavier supporting structure
16
. The sintered wicks
18
are taught as being made from silicon and glass. The supports
14
protruding through the wick
18
are bonded to the plate
12
to provide the necessary support.
U.S. Pat. No. 4,503,483 issued to Basiulis for Heat Pipe Cooling Module for High Power Circuit Boards is directed to a heat

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