by Dr. Franklin Miller, Cryogenic Engineering Laboratory, University of Wisconsin – Madison, SCW Co-Chairman, fkmiller@wisc.edu

As space flight missions move away from tanks filled with cryogenic liquids towards cryocoolers to provide cooling for cryogenic payloads, new heat transport technologies are needed to cool distributed systems.

High purity metals such as copper and aluminum provide high thermal conductivity at cryogenic temperatures but have overall system conductance that scales inversely with length and linearly with cross-sectional area. This means that the mass of thermal buses made of these materials becomes prohibitive for thermal transport of significant heat loads over long distances.

Other options for distributing cryogenic cooling include pumped fluid systems or heat pipes utilizing two-phase heat transfer. Heat transfer by circulating fluids is a better option than solid conductors because the overall conductance of fluid systems does not drop off over the thermal length as it does with solids. Several flight-ready technologies exist, including pumped fluid loops and heat pipes. More recently, researchers have investigated pulsating (or oscillating) heat pipes (PHPs) at laboratories and universities in several countries.

A pulsating heat pipe, shown schematically in Figure 1, consists of a closed tube that is bent into multiple loops and divided into three main sections: condenser, evaporator and adiabatic section. The tube is sufficiently small in diameter such that surface tension forces dominate and a PHP partially filled with vapor and liquid has regions that are filled with liquid or vapor slugs. It is important to note that in the case where the PHP has many turns the effect of gravity becomes negligible, which is especially important for microgravity applications.

PHPs have a self-induced oscillatory movement due to the volume contraction and expansion of the liquid and vapor slugs and therefore they require no pump or wick to induce fluid motion.
Possible applications for PHPs include distributed cooling for optics/detectors for IR systems and distributed cooling for thermal shields on cryofluid tanks.

Engineers at several research institutions have conducted recent experimental work on cryogenic PHPs with promising results. Experimental characterization of 4 K helium PHPs is ongoing, for example, at the Chinese Academy of Sciences, Zhejiang University, CAE Grenoble, and the Cryogenics Engineering Laboratory (CEL) at UW-Madison.

Researchers have studied PHPs with adiabatic lengths up to 1 m that use hydrogen as a working fluid and have reported effective thermal conductivities as high as 100,000 W/mK for these systems. Effective conductivity is defined as: where Q is the heat load applied at the evaporator section, Tevap and Tcond are the evaporator and condenser sections’ temperatures respectively. Leff is the adiabatic length and Ac is the total cross-sectional area of the fluid inside of the capillary tubing.

The author recognizes that effective thermal conductivity does not entirely capture the conductance per mass for PHPs but it is the standard number that is reported in the literature for these devices and does provide some indication of the relative performance between PHP designs.

Most recently at UW-Madison, engineers used PHPs with three evaporators and one condenser to evaluate the performance of PHPs operating with distributed non-uniform heat loading. Tests were conducted with helium at load temperatures equal to 4 K and with nitrogen at load temperatures equal to 80 K, and show that the performance with non-uniform heat loads is similar to performance for PHPs with uniform heat loading.

The low mass, high conductance, simplicity of construction, insensitivity to the magnitude of gravity and tolerance of non-uniform distributed heat loads make PHPs a potentially promising cryogenic technology for future space flight missions. ■