Experimental study of heat transfer and start-up of loop heat pipe with multiscale porous wicks

A loop heat pipe (LHP) with composite multiscale porous wicks was designed and investigated. The focus was on heat transfer and start-up characteristics. Three layers of wick were used to form the composite wicks. The primary layer was sintered on the evaporator wall using copper powder with different average particle diameters (dp = 13, 37, 88, and 149 μm) to form a groove multiscale wick. The second layer was laid on top of the first one by a second sintering. The third layer was made of absorbent wool with excellent thermal insulation. A series of experiments were performed to study the effects of various parameters, including wick structures, tilt angles (θ = −90°, 0°, and 90°), liquid filling ratios (38.5-64.1%), liquid line lengths, and heating power. Compared with conventional monoporous wicks, the composite multiscale porous wicks shortened the start-up time, decreased the wall temperature, and suppressed the temperature instability of the LHP. At a heat load of 200 W, the LHP with composite wicks could reach a heat flux of 40 W/cm2 for the anti-gravity operation, under which the wall temperature was only 63 °C. Some reasons that accounted for performance improvement were as follows: the porous groove wall increased the surface area and multiscale structures realized a successful synergy between vapor release and liquid supply, large pores for vapor release, and small pores for liquid suction. In addition, a synergy between thermal conductivity and insulation was achieved, which ensured a high thermal conductivity for the primary layer wick and a good thermal insulation for the entire wick. This greatly reduced the heat leakage from the evaporator to the compensation chamber.