Characterisation of a variable conductance heat pipe prototype for a photonic application

Thermoelectric modules (TEMs) consume a large amount of power when used for precision temperature control of high- power photonic devices, particularly when subjected to a wide range of ambient temperatures. The use of variable conductance heat pipes (VCHPs) as a lower power alternative to TEMs is investigated here. The performance of a methanol-argon VCHP with a non-wicked reservoir for both passive and active control is characterized. The concept of an "ideal" working fluid for a gas-loaded VCHP is introduced. It has a liquid-vapor saturation curve resulting in perfect passive evaporator temperature control in the limit of an infinitely-large reservoir when the VCHP is subjected to changes in heat load and/or ambient temperature. The saturation curve of this ideal fluid is compared to that of the fluid used here, i.e., methanol, showing why perfect passive control is unrealistic for varying ambient temperature. An experimental prototype was constructed and measurements obtained from it were compared to the predictions of the flat front model. It was found that, even with active control, the evaporator temperature could not be maintained sufficiently at low ambient temperatures due to axial conduction through the adiabatic section of the prototype VCHP. However, excluding these low ambient temperatures, the VCHP provides a significant reduction in power consumption compared to a TEM.