Variable Conductance Thermal Management System for Balloon Payload

摘要

While continuously increasing in complexity, the payloads of terrestrial high altitude balloons need a thermal management system to reject their waste heat and to maintain a stable temperature as the air (heat sink) temperature swings from as cold as -90°C to as hot as +40°C. The current solution consists of copper-methanol Constant Conductance Heat Pipes (CCHPs). The problem with these devices is that the conductance cannot effectively be reduced under cold operating or cold survival environment conditions, so an active heater requiring significant energy is required to maintain the instruments in their normal operating range. This paper presents the development of a low cost Variable Conductance Heat Pipe (VCHP) that allows the thermal resistance to increase passively under cold operating or cold survival environment conditions, keeping the instrument section warm with minimal electric heating. This VCHP is based on smooth-bore, thin-wall stainless steel tubing, with methanol, toluene or pentane as working fluids, and is capable of passively maintaining a relatively constant evaporator (payload) temperature while the sink temperature varies between -90°C and +40°C. Two configurations were developed, a cold reservoir one (reservoir is attached to the condenser) and a hot reservoir one (reservoir is attached to the evaporator). Both configurations were tested with the above mentioned working fluids and the experimental results were consistent with the modeling results. In all experimental cases, the evaporator temperature was maintained within the required interval of -10°C...+50°C while the sink temperature varied between -90°C and +40°C. The hot reservoir configuration showed the tightest temperature control. For example, the pentane based hot reservoir VCHP allowed the evaporator temperature to change only 3.7°C from the coldest to hottest heat sinks. The largest temperature variation observed was 32.6°C for the pentane based cold reservoir VCHP, still meeting the design requirements. Survival tests were also carried out but only for the toluene based cold reservoir VCHP. A duration of 13,000 seconds was needed by the evaporator to cool from 49°C down to 20°C while the power was shut down and the sink (condenser) was continuously as cold as -90°C.