Fabrication And Performance Of Boiling Enhanced Microchannels With Segmented And Reentrant Structures

With the increasing development of technology,the miniature components in various areas are facing more and more severe problems of high heat-flux dissipation,which puts forward great challenges to the traditional heat transfer enhancement.Microchannels combined with boiling heat transfer,possessing many advantages such as simple structure,small volume,large heat exchange area and excellent heat transfer capability,have been recognized as an effective way for the heat dissipation of the micron-grade heat sources in miniature devices.In view of the urgency and great development potential to improve the boiling heat transfer performance with the miniaturization,the design and optimization of novel structured microchannels,as well as the application in the experimental and theoretical researches on the enhancement of boiling heat transfer,have become imminent.Aiming to design high-performance microchannels with boiling heat transfer enhancement and solve the defects of high cost and deterioration in traditional ones,this thesis systematically studied two kinds of microchannels,i.e.,solid and porous microchannels with segmented and reentrant structures.The main contents in this thesis were as follows:1.Fabrication of microchannels with segmented and reentrant structuresThe features of segmented and reentrant structures in the microchannels were analyzed,and the corresponding mechanism of enhanced boiling heat transfer was discussed.Porous and solid matrix based microchannels were fabricated by solid-state sintering and wire electric discharge machining?WEDM?,and the surface quality of the two forming processes was analyzed.The effect of particle size in sintered substrate on the capillary performance was tested and analyzed.The results showed that although there was a melt layer formed on the porous substrate after WEDM,it was very thin?50-100?m?and still remained good porous properties,so the heat transfer performance would not be significantly affected.2.Pool boiling heat transfer enhancement?1?With the setup of pool boiling experimental systems and the deionized water as the working fluid,the performance of pool boiling heat transfer enhancement from porous microchannels were reported in details.Under different degrees of subcooling,the porous microchannes were tested and compared with solid ones,what's more,the structural parameters,including particle type,channel width,and particle size,were optimized.Porous microchannels showed superior pool boiling performance with the maximum heat transfer coefficient of 27.28kW·m^-2·k^-1 under saturated condition,which was 20%higher than that of the solid microchannels.With the same particle size range,powder shape had a little influence on the boiling performance.The microchannels with medium widths?0.4 and 0.55 mm?can keep the best heat transfer rate in a relatively wide heat flux range and the ones with medium particle size?50-75?m?have the great capability to balance the effect of nucleation site density and the capillary performance.?2?Both in saturated and subcooled boiling conditions,high-speed visualization was used to capture the bubble dynamic characteristics including bubble emergence,growth and departure.The bubble govern mechanisms under different heat fluxes and subcooling were discussed.With the subcooling of 13°C,the bubble detachment diameters of the porous microchannels were smaller than that of the solid ones at any given heat flux.When the heat fluxes qa?475 kW/m²,the bubbles increased from 1.17mm to 1.96mm,but when the heat fluxes were higher than 475 kW/m²,the bubbles decreased with increasing the heat flux.The bubble growth mechanisms were different in low and high heat flux regions,the bubble diameters were reduced as the subcooling increased.3.Flow boiling heat transfer enhancement?1?With the setup of flow boiling experimental systems and comparison to the conventional rectangular microchannels,the solid microchannels with segmented and reentrant structures were systematically studied from the point of flow boiling characteristics.Under different mass fluxes and subcooling,the performance of flow boiling heat transfer enhancement were mainly evaluated,as well as the transition of flow patterns and heat transfer mechanisms.The effect of channel width was analyzed to optimize the structural parameter.?2?Porous microchannels with segmented and reentrant structures were comprehensively investigated by using the same experimental setup.With comparison to the reentrant porous microchannels in the literature,this kind of porous microchannels proposed by this thesis showed further advantages in flow boiling enhancement.The operational parameters,including higher mass fluxes and subcooling,as well as the structural parameter?channel width?were studied to provide important guidelines for the optimization design.