Multi-scale Structure Surface Of A Loop Heat Pipe For Performance Improvement

A loop heat pipe, a two-phase heat transfer device, is applied in space industry and electric devices area with the advantage of high heat transfer efficience, low thermal resistance, long transfer distance and flexible tube. An experimental system of a flat loop heat pipe with separate bodies was designed and the relevant performance was investigated. The loop heat pipe mainly consists of evaporator, condenser, vapor line and liquid line, of which the porous wick in the evaporator is the key component in loop heat pipe where the phase change occurs.A multi-scale structure from micron to millimeter would be applied as the porous wick in the loop heat pipe. The surface of the wick would have pore sizes ranging from macro-scale to micro-scale simultaneously by the method of powder sintering. Pool boiling heat transfer performance and visualization of the3D porous coating surface were investigated experimentally at first to provide the theoretical basis of porous wick utilization in the heat transfer augmentation of heat loop. The conclusions revealed that the multi-scale surface separated the vapor and liquid phase effectively because the micro pores of particles provide the driving force of the liquid flow circulation while the macro structures act as the vapor flow channels. The critical heat flux reaches3.7times of that of the plain surface at the congruent conditions which provides the theoretical basis. Then the multi-scale porous wick was improved to be utilized in the flat loop heat pipe, the structure of which was compaosed of three layers. The first layer, providing the parallel vapor channel in the millimeter-scale, was made of copper powder, the diameter of which is88μm. The second layer was sintered on the first wick layer with an average particle diameter of149mm and a thickness of2.0mm. The third porous layer used the absorbent wool with an ultra-low thermal conductivity of0.05W/Mk to prevent the heat flew to the compensation chamber from evaporator.The start-up and operating characteristic was investigated experimentally using the deionized/distilled water as working fluid. The results showed the multi-scale porous wick we fabricated was able to shorten the start-up time, increase the critical heat fluex and reduce the wall temperature by20-30℃, comparied with the one microchannels manufactured on the evaporator wall conventionally, because the multi-scale wick could separate flow paths of vapor and liquid much more effectively. Secondly, structures near the evaporator wall influenced the operating characteristic significantly. The best geometric parameters were values the ratio, porous stack width to vapor channel width, was between1.5to2.The best liquid charge ratio appeared at51.3%. The less liquid charge caused the earlier appearance of dry-out and temperature oscillation. The over liquid charges sustain large heat loads, but increase the evaporator wall temperatures. And the liquid charge ratio. And the best inclination angle varied at different liquid charge.