limitation design constraints, system designers can optimize for lower heat rejection radiator subsystem mass. Another example for the use of the high heat flux capability of the heat pipe is in electronics cooling. High heat flux evaporators of the new breed of heat pipes are capable of handling the power densities occurring on semiconductor chips. This, coupled with the ability to fabricate heat pipe envelope and wick structures from silicon and silicon powder, respectively, presents the possibility of direct low mass, low temperature-loss cooling of power electronics with a potentially important gain in reliability as well. Silicon heat pipes have been tested to evaporator heat fluxes of in the tens of watts per square centimeter range using methanol and ammonia as working fluids at 20°C. As a general conclusion, recent advances in heat pipes are giving spacecraft designers a much broader spectrum of options. High Axial Heat Transport Rates One immediate application of the new-found increased evaporator heat flux capability can be realized within the heat pipe itself to make improvements in the axial heat transport capability. The new understanding and acceptance of stable two-phase flow within wick structures leads one to want to use and take advantage of this knowledge. The higher local power densities now achievable also imply a requirement for higher liquid flow-rates to feed these thirsty evaporators. Beginning with open grooves, various low-drag liquid condensate return passages have been devised. When covered to prevent adverse vapor shear effects and increase performance, they are usually referred to as arteries or tunnels. When arteries are allowed to penetrate into the evaporator of a heat pipe and vapor generation begins within the wick, the arteries are prone to vapor penetration and become vapor locked, effectively shutting down heat pipe operation. Many practitioners have abandoned arteries for this reason. However, recent work has shown that there exist several techniques for minimizing vapor effects in arteries. First, the arteries are located out of the immediate heat flow zone. Second, the artery wall is formed from material with smaller pores than the balance of the wick [3]. Thus, vapor tends to flow around the artery in the larger pores, rather than penetrate it. Third, the artery can be lined with a smooth surface with small nucleation sites requiring a large superheat to initiate boiling [4]. Fourth, boiling in the artery can be suppressed by subcooling of the returning condensate. Subcooling will also condense vapor which may enter the artery anywhere along its length when an isolated ‘pore’ is overstressed and breached. This subcooling can be achieved passively and simply by designing the heat pipe with the artery located in an area where it can reject heat by radiation, or, in higher performance applications, the artery may be coupled to a small radiator assembly. A small amount of subcooling (by a degree or two) can be sufficient to double power throughput in some cases, depending on the range of pore sizes within the wick structure. Using a combination of these techniques, large increases in power throughput have been achieved. As an example, the data seen in Fig. 3 is from tests performed on a sintered aluminum powder metal wick heat pipe of dimensions 48 ft long by 1.25 in. diameter, using ammonia as the working fluid at 300 K [5]. The lower performance curve was generated without artery subcooling, the higher performance curve was with artery subcooling. The subcooling in this case was done on an ‘artery extension’, which was a 15 cm long fill-tube which extended from the evaporator artery. Accordingly, as
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