Mathematical Modeling of Variable Conductance Heat Pipes for Steady State and Transient Operation

Abstract

Variable Conductance Heat Pipes (VCHPs) have been used in spacecraft thermal control systems (TCS) for more than 25 years. Being heat-driven capillary-pumped heat transport devices, they are extremely reliable, durable and, more importantly, maintenance-free for long unmanned space missions. Today, the technology reaches the highest level of readiness. The VCHP heat transfer mechanism is simple and, therefore, can be easily formulated into a set of mathematical equations. Indeed, a number of analytical models are available for the thermal engineers to simulate the VCHP performance. Nonetheless, all of these models were derived from the same basic assumption regarding the thermal equilibrium between the fluid in the pipe (including the non-condensable gases or NCGs) and the VCHP metal casing. For simulated cases in which the temperatures of the reservoir and/or the vapor do not change quickly with time, the model predictions seem to be quite satisfactory. On the other hand, as the utilization of VCHPs in the TCS becomes increasingly sophisticated, a higherfidelity model is hence needed. As an example, when the VCHPs are employed to provide tight temperature control for the spacecraft Central Thermal Bus, the reservoir heaters are cycled ON/OFF at a very fast rate, shuttling the NCGs in and out of the reservoir in the process. The gas flow induces a heat exchange between the reservoir and the condenser that is not accounted for in the current models. The U.S. Naval Research Laboratory funded a research effort that began in 2009 to incorporate the effects of thermal non-equilibrium and mass/heat transfer of the fluid flow into the VCHP model. The resulting model underwent the initial phase of the verification process. The results indicated that the model predicted well the transient behaviors of a system of Aluminum-Ammonia VCHPs.