Experimental Study On Flow Boiling In Open Microchannels With Porous Material

With the development of microelectronics,computers,missiles,satellites,military radars and other complicated equipment,smaller volume and faster operation speed are required.The power density of devices increases significantly,which proposes a great challenge for the heat dissipation unit of high heat flux devices.Flow boiling in microchannels dissipates the heat by latent heat of vaporization which has the larger heat transfer coefficient,the better temperature uniformity and the lower working medium consumption.Based on this background,four kinds of open microchannel heat sinks were fabricated using porous materials sintered by copper powder,which had a high permeability and large specific surface area.The visualization of bubble behaviors and flow pattern transitions during flow boiling in porous microchannels were carried out.Besides,the characteristics of heat transfer and pressure drop were experimentally studied.The experimental results were discussed based on the phase change heat transfer mechanism,porous medium heat transfer mechanism and microscale effects.The effects of porous structure parameters,open gap structure and operating conditions on flow boiling characteristics(i.e.,the bubble dynamic behaviors,two-phase flow pattern and its conversion mechanism,heat transfer,pressure drop and instability)in open porous microchannels were analyzed.So it could guide the optimization design of microchannel heat exchanger with high efficiency.The large number of pores in porous materials provided two kinds of nucleation sites,namely the internal cavity and the surface cavity.The pore size distribution and the capillary performance factor of porous material were the main factors affecting the flow boiling heat transfer in microchannels.A wide range of vaporization core radius and strong capillary suction could be achieved by controlling the copper powder shape and sintering environment,leading to a large nucleation site density and a good liquid replenishment capability during boiling.The boiling curve for the porous material microchannel shifted leftward and upward compared with that for the copper microchannels made of compact material.In addition,the ONB point was obviously advanced in the porous material microchannels,which demonstrated the obvious advantages in reducing the wall superheat.The theoretical model showed that there were three heat transfer mechanisms during flow boiling in porous structure: heat conduction,micro convection and micro liquid film evaporation.Four flow patterns formed in the open microchannels,namely the bubbly flow,the slug flow,the type I stratified flow and the type II stratified flow.The large heat transfer coefficient was obtained during type I stratified flow.When stratified flow was formed in the porous material microchannel,the liquid in the pore could nucleate continuously.The convective evaporation caused by the high-speed movement of the steam block and the nucleate boiling dominated the heat transfer jointly.The heat transfer performance of the porous open microchannel was more than two times larger than that of the compact material microchannel.The pressure drop was closely related to the transition of flow pattern.The sudden change of pressure drop occurred when the flow pattern transformed from bubbly flow to type I stratified flow.Moreover,the larger the wall roughness of the porous material microchannel was,the greater the pressure drop of two-phase flow was.It was also found the increasing the height of the open gap could reduce the pressure drop and improve the heat transfer coefficient.Meanwhile,the existence of open gap greatly reduced the flow instability,which was beneficial to the wall temperature uniformity of equipment with large heat dissipation requirements.