Effect of the working fluid transportation in the copper composite wick on the evaporation efficiency of a flat loop heat pipe

An excellent heat transfer performance of the loop heat pipe (LHP) was not only dependent on the efficient evaporation in the evaporating zone, but also on the working fluid transportation in the wick. A double-layered composite copper wick for both efficient evaporation and high permeability was proposed to eliminate the vapor trap in the wick at high heat loads. The transportation layer was 2 mm in thickness near the compensation chamber, and the evaporation layer was 3 mm in thickness facing the evaporating zone. It was experimentally found that vapor trap and dry-out in the wick at high heat loads were relieved, because the balance between high flow rate of working fluid supply and efficient evaporation was achieved. The optimized composite wick with the particle sizes ranging from 96-180 mu m and 48-96 mu m was ideal for the liquid transportation and evaporation in the wick, respectively. The LHP could start in 120 s stably at a low heat load of 20 W (2.83 W/cm(2)), and a maximum heat load of 140 W (19.80 W/cm(2)) was achieved at the allowable evaporator wall temperature of 90 degrees C. The highest heat transfer coefficient was 30,794 W/m(2) K, and the lowest evaporator thermal resistance and LHP thermal resistance were only 0.046 degrees C/W and 0.143 degrees C/W, respectively, at a heat load of 140 W.