High heat flux loop heat pipes

Loop Heat Pipes (LHPs) can transport very large thermal power loads, over long distances, through flexible, small diameter tubes and gravitational heads. While recent transported as much as 1500 W, the peak heat flux through a LHP's evaporator has been limited to about 0.07 MW/m(2). This limitation is due to the arrangement of vapor passages next to the heat load which is one of the conditions necessary to ensure self priming of the device. This paper describes work aimed at raising this limit by threefold to tenfold. Two approaches were pursued. One optimized the vapor passage geometry for the high heat flux conditions. The geometry improved the heat flow into the wick and working fluid. This approach also employed a finer pored wick to support higher vapor flow losses. The second approach used a bidisperse wick material within the circumferential vapor passages. The bidisperse material increased the thermal conductivity and the evaporative surface area in the region of highest heat flux, while providing a flow path for the vapor. Proof-of-concept devices were fabricated and tested for each approach. Both devices operated as designed and both demonstrated operation at a heat flux of 0.70 MW/m(2) This performance exceeded the known state of the art by a factor of more than six for both conventional heat pipes and for loop heat pipes using ammonia. In addition, the bidisperse-wick device demonstrated boiling heat transfer coefficients up to 100,000 W/m(2).K, and the fine pored device demonstrated an orientation independence with its performance essentially unaffected by whether its evaporator was positioned above, below or level with the condenser.