Heat exchange – Intermediate fluent heat exchange material receiving and... – Liquid fluent heat exchange material
Reexamination Certificate
2002-05-29
2004-02-03
McKinnon, Terrell (Department: 3743)
Heat exchange
Intermediate fluent heat exchange material receiving and...
Liquid fluent heat exchange material
C165S185000, C361S700000, C257S715000
Reexamination Certificate
active
06684940
ABSTRACT:
BACKGROUND OF THE INVENTION
1. Field of the Invention
present invention relates to the field of heat transfer. More particularly, it relates to the use of new heat transfer fluids in a heat transfer system, particularly for heat pipe systems both in terrestrial and microgravity environments.
2. Description of the Relevant Art
Heat pipes can be described as devices employing closed evaporating-condensing cycles for transporting heat from a locale of heat generation to a location of heat rejection, using a capillary structure or wick for return of the condensate. These devices often have the shape of a pipe or tube that is closed on both ends. For the purpose of the present invention, the term “heat pipe” is used in a more general sense to refer to devices of any type of geometry that are designed to function as described.
The heat pipe is a highly efficient heat transfer system and has been broadly used in spacecraft, energy recuperation, power generation, chemical engineering, electronics cooling, air conditioning, engine cooling and other applications. Recently, thermal management has become one of the most critical technologies in electronic product development and directly influences cost reliability, and performance of the finished products. Heat pipes are excellent heat transfer devices, but a serious constraint on conventional heat pipes is the reduction of transport capabilities in which the condenser is located below the evaporator section in a gravitational field, or when the heat pipes are used at low-gravity conditions.
All of the heat pipes, including conventional heat pipes, capillary pumped loops (CPLs), loop heat pipes (LHPs), and micro heat pipes, have a common concern, namely the heat transfer limits. These limits determine the maximum heat transfer rate that a particular heat pipe can achieve under certain working conditions. Among them the capillary limit and the boiling limit are the restrictive factors at normal operating temperatures. Both of them are caused by the characteristics of the surface tension.
The boiling limitation is closely related to the bubble formation and detachment from the wall and/or wick at the evaporator section of heat pipes. It is well known that surface tension effects, including temperature-driven surface tension gradients, are dominant when the buoyancy force is diminished in microgravity conditions. Most previous studies have shown that, rather than assisting in the detachment process, surface tension unfortunately tends to keep the bubbles on the wall and in that way to impede bubble detachment. The surface tension gradient driven by temperature has been also considered as a force holding the bubbles attached to the wall surface. This, of course, is quite detrimental to boiling heat transfer in nucleate boiling regime.
Recently, many efforts have been made to try to enhance boiling heat transfer through Marangoni effects in fluid mixtures at normal gravity, as well as in microgravity. Marangoni effect represents the flow resulting from gradients in surface tension giving rise to the transfer of heat and mass. It is particularly relevant to microgravity conditions wherein gravity-induced convection is absent. It is found that a small amount of a surface-active additive considerably increases the nucleate boiling heat transfer coefficient of water at normal gravity. However, the effect under microgravity conditions is not known. A serious problem with using a surfactant is its foaming in the vapor. McGillis and Carey reported in their article “On the Role of Marangoni Effects on the Critical Heat Flux for Pool Boiling of Binary Mixtures”,
Journal of Heat Transfer
, Vol.118, No. 1, 1996, pp. 103-118, that small additions of alcohol to water increased the critical heat flux (CHF) above that of the pure water, and higher concentrations of the alcohol began decreasing the CHF to near that of the pure alcohol. On the other hand, for water ethylene glycol mixtures, addition of the glycol decreased the CHF relative to that of pure water.
Abe et al tested water-ethanol mixtures of 11.3 and 27.3 wt % of ethanol and reported in the article “Pool Boiling of a Non-Azeotropic Binary Mixture under Microgravity”,
International Journal of Heat and Mass Transfer
, Vol. 37, No. 16, 1994, pp. 2405-2413 that heat transfer is enhanced by reductions in gravity over the major portion of the nucleate boiling regime, but the CHF decreases 20-40% from the terrestrial level. The boiling heat transfer performance of the mixtures at normal gravity is much worse than that of pure water and, although enhanced under microgravity, it still cannot reach the level of pure water at normal gravity. Therefore, the water-ethanol mixtures are unacceptable for space applications.
Ahmed and Carey in their article entitled “Effects of Gravity on the Boiling of Binary Fluid Mixtures” appearing in
International Journal of Heat and Mass Transfer
, Vol. 41, No.,16, 1998, pp. 2469-2483, conducted an experiment with water-2-propanol mixtures under reduced gravity. They concluded that the Marangoni effect arising from the surface tension gradients due to concentration gradients is an active mechanism in the boiling of binary mixtures, and that the boiling mechanism in these mixtures is nearly independent of gravity.
The experimental results obtained by Abe et al and by Ahmed and Carey clearly show that for so-called positive mixtures, in which the more volatile component has a lower value of surface tension, the Marangoni mechanism is strong enough in the mixtures to sustain stable nucleate boiling under microgravity conditions.
Besides the surface tension gradients due to concentration gradients, Marangoni effects are also induced by temperature gradients, which are more common and more important in heat transfer devices. Unfortunately, all working fluids used in existing heat transfer devices, including heat pipes, have a negative gradient of surface tension against temperature which is quite detrimental to boiling heat transfer, as mentioned above. In addition to the Marangoni flow around bubbles induced by the negative surface-tension-temperature gradient that presses the bubbles onto the heating surface resulting in an unfavorable situation for boiling performance, another Marangoni effect induced by the surface-tension-temperature gradient is the moving of a liquid body towards the region of lower temperature, thus preventing liquid spreading on a heated portion of the heating surface, such as the evaporator section of heat pipes.
All heat pipes have a boiling limit, which is directly related to bubble formation in the liquid. If the number and size of vapor bubbles generated at the wall and/or the fin-wick interface are small, these bubbles may migrate from the solid surfaces to the liquid-vapor interface and vent into the vapor groove without destroying the capillary menisci. However, as the heat flux is increased further, bubbles may coalesce, form a vapor blanket at the wall and/or the fin-wick interface, and eliminate the capillary force that circulates the liquid condensate. Vapor bubbles that are coalesced at the evaporator section may block the liquid return from the condenser section and the boiling limit can be reached. For the heat pipes with a wick structure, the critical temperature difference across the liquid layer at the evaporator section, which reflects the boiling limit, is given as:
Δ
T
crit
=
T
w
-
T
v
=
2
σ
T
w
h
fg
ρ
v
(
1
R
b
-
1
r
ef
)
where T
w
and T
v
are the wall temperature and the vapor temperature at the evaporator section, respectively; &sgr; is the surface tension of the working fluid; h
fg
is the enthalpy of vaporization of the working fluid; &rgr;
v
is the vapor density; R
b
is the radius of vapor bubble at the liquid-wall interface, and r
ef
is the effective pore radius of the wick or the effective curvature radius of the liquid film on the wall. It is obvious that the critical temperature difference closely relates to the characteristics of the sur
Chao David F.
Zhang Nengli
McKinnon Terrell
Stone Kent N.
The United States of America as represented by the Administrator
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