Heat Rejection System for Thermal Management in Space Utilizing a Planar Variable-Conductance Heat Pipe

The project objective is to develop an Integral Variable Conductance Planar Heat Pipe (VCPHP) for use as a potential means of heat rejection in spacecraft systems. The primary objective is to combine three heat rejection concepts (heat absorption, heat transport, and heat rejection)into a compact and effective thermal system. That is, to expand the surface of a traditional heat pipe into a planar (rather than cylindrical) shape to act as a radiative surface, ultimately reducing system complexity, parts count, and weight. Additionally, the concept of a variable conductance heat pipe (VCHP) which adds an inert gas into the heat pipe was investigated. The inert gas in the VCHP can expand or shrink depending on the operating condition, effectively changing the heat dissipating area of the condenser and the saturation temperature the VCHP. The objectives of this work are:(i) to study concept feasibility via analytical modeling; and (ii) to validate analytical models by producing and testing VCHP prototypes and collecting and correlating experimental data. In terms of the NASA’s Technology Area Breakdown Structure (TABS) this project addresses the subject of Heat Rejection in Thermal Management Systems (TA14).
During the grant award period, the following milestones have been achieved: Design and fabrication of a prototype variable conductance planar heat pipe radiator In order to prove the feasibility of the basic concept, two kinds of prototype variable conductance planar heat pipe radiator were carefully designed and fabricated. The first heat pipe was made from brass, which has high thermal conductivity and outstanding machinability. The second one was a liquid crystal polymer (LCP) based heat pipe, which has acceptable thermal conductivity (~20W/mK) and low density. For simplicity, all heat pipes have similar sandwich structure designs (two base plates with axial grooves and a spacer between with charging taps and a pressure gauge). On the base plates, triangular or rectangular grooves were machined by a computer numerical control (CNC)mill cutter. To minimize the air leakage from the side, two rubber gaskets machined by a laser cutter were placed between the plates and the spacer. The heat pipes were evacuated and charged with a pre-determined amount of ethanol.