Fabrication And Performance Of Grooved Porous Structures For Micro-scale Heat&Mass Transport Enhancement

With the rapid progress of MEMS technology, micro heat&mass transport devices, suchas micro heat pipes, microchannel heat sinks, micro fuel cells, biomedical microchips, microreactors and etc., has been widely used in the areas of energy, chemical engineering,electricity, aerospace, biomedicine, and so on. The critical requirement of heat&masstransport introduced in the micro-scale devices promoted the development of enhanced heat&mass transport structures, which has attracted wide research interests of the communities ofmanufacturing, heat transfer and etc. However, the difficulties of the fabrication method andprocess of enhanced heat&mass transport structures may delay the applications of thesedevices. In view of this, this thesis proposed two kind of grooved porous structures, i.e.,sintered-grooved composite porous structures and reentrant porous structures, and thefabrication, characterization, heat&mass transport performance were symmetrically studied.The main contents in this thesis were as follows:(1) Fabrication of grooved porous structuresSintered-grooved composite porous structures were fabricated by integrating the grooveploughing and the solid-state sintering method, which provides a novel kind of wick fortwo-phase heat transfer devices, e.g., heat pipes,vapor chambers, capillary pump loops andetc. Using the ploughing method, the micro V-grooves with the height-width aspect ratio of2can be processed. The mechanism of the ploughing process were investigated, and the effectsof fabrication parameters on the groove shape were studied. Besides, the sintering mechanismof copper particles on top of grooves was analyzed, and the effects of particle shapes and sizeson the composite porous structures were examined. On the other side, reentrant porousstructure was fabricated by the solid-state sintering method with a special wire cut graphitemodule. Unlike other microchannel shapes in the literature, it presents an anti-“”shape, withthe small exit slot and large reentrant cavity. The design, module fabrication and sinteringprocess were studied.(2) Characterization of physical parameters of grooved porous structuresIn this chapter, four main physical parameters, namely, surface profile and roughness, poresize, wetting characteristics and thermal conductibility, of the grooved porous structures were evaluated. Firstly, the surface profile and roughness of ploughing groove and sintered porousmatrix were tested using a surface profilometer, and the effects of fabrication parameters ontheir roughness were accessed. Secondly, by combing the scanning electron microscope andmercury intrusion method, the pore size and its distribution were examined. Furthermore, thewetting characteristics of sintered porous matrix were examined using the sessile drop method.The influences of particle shape and size, sintering temperature and time, reentrant grooves onthe contact angle were analyzed. At last, the thermal conductibility of sintered-groovedcomposite wicks and sintered porous wicks were tested and analyzed using the steady heatconduction method.(3) Liquid and gas permeability of grooved porous structuresThis chapter focused on the liquid and gas permeability of the above two kinds of groovedporous structures. Using deionized water and pressurized nitrogen, respectively, forced fluidflow method was utilized to access the permeability. For the sintered-grooved compositewicks, it shows that a largely enhancement of both liquid and gas permeability comparing tothe sintered porous wicks. The reentrant porous structure also showed a3magnitude ofenhancement in the liquid permeability to the sintered porous matrix, but it was smaller thanthe reentrant copper structure. The effects of particle shape and size on the liquid and gaspermeability were also investigated for the sintered-grooved composite wicks.(4)Capillary performance of sintered-grooved composite wicksThis chapter mainly reveals the capillary performance of sintered-grooved compositewicks, which integrates both two critical parameters of a wick, i.e., capillary pressure andpermeability. Capillary rate-of-rise method was used to characterize the capillary performance,and a novel infrared (IR) thermal imaging method were developed to identify and locate theliquid meniscus accurately. The above method was validated by different working liquid testsand the comparison with the current results with the literatures. Results shows that thecomposite wicks enhanced the capillary performance by over20%compared to the sinteredones. While comparing to the grooved wick, it enhanced the capillary pressure for about2–4times. Therefore, a good balance is achieved using the composite wicks. The capillaryperformance should be determined by integrating the gravity effect, and the well-knownWashburn’s equation should not be applied to characterize the capillary-rate-of-rise processes. Furthermore, the effects of fabrication parameters, namely, particle shape and size, groovewidth and height, sintering temperature and time on the capillary performance were reported.The optimal parameters were as follows: irregular copper powder with the size of75-110μm,the groove with the depth of0.85mm and pitch of0.45mm, sintering processes of950°Calong with30min.(5)The application of reentrant porous microchannels in the microchannel heat sinkWith the setup of the forced convection and flow boiling experiment systems, both singlephase and two-phase boiling performance of water in reentrant porous microchannels (RPM)were reported in details. During the tests with different inlet temperatures and flow velocities,It was found that compared with reentrant copper microchannels (RCM), RPM shows itsenhancement and advantages in the following aspects, i.e., increasing the temperatureuniformity of heat sink, lowering down the boiling superheat, enhanced two-phase heattransfer coefficient, and mitigation of two-phase flow instability. The local two-phase heattransfer coefficient of RPM were found to be2-5times that of RCM. Heat transfermechanism of flow boiling were access by the analysis of heat transfer plots and visualizationof high-speed camera. Nucleate boling heat transfer governed in RPM at low heat fluxes,while it transited to forced convective boiling mechanisms at moderate to high heat fluxes.The flow pattern also transited from the bubble flow to annular flow along with thin filmevaporation. Furthermore, the effects of hydraulic diameter, particle shape and size on thetwo-phase heat transfer performance were comprehensively investigated, which providedimportant guidelines for the optimal design of the porous microchannle heat sink.