Study Of Performance Parameters Of Sintered Porous Metal Materials And Their Boiling Heat Transfer And Phase Separation Characteristics In Cryogenic Fluids

Sintered porous metal materials are regarded as one kind of the mostpromising function materials, they have special features totally different fromcompact metal due to internal pores, and have broad application prospects infields such as new energy, energy conservation, aerospace and highlyefficient cooling. The applications of cryogenic fluids are becoming more andmore broad with the development of science and technology. For cryogenicfluids such as liquid nitrogen and liquid hydrogen, the study of their poolboiling heat transfer performances on sintered porous metal surface havegreat importance for applications in areas of aerospace, electronic cooling,cryobiology and cooling of superconducting magnets and cables. For liquidhelium, the phase separation characteristics of sintered porous metal materialare the basis to prevent the leak of superfluid helium to outer space and tocomplete tasks of phase separation and effective management of space fluids.In order to characterize the sintered porous metal materials, porosity wastested; surface porosity, average pore size and pore size distribution wereobtained through image processing and analysis based on scanning electronmicroscope(SEM) images of nature fracture surfaces; permeability and airpermeability were measured by experimental apparatus based on Darcy’s law,and the effects of original power grain size, porosity, thickness, lowtemperature condition, average pore size and pore size distribution on permeability were also discussed; effective thermal conductivities weremeasured by a hot disk thermal constant analyzer and compared withpredicted results from empirical equations. The results demonstrate thatvolume average pore size and pore size distribution can effectively reflect thechange of air permeability, and thickness has an important influence onporosity and permeability. A stronger wall effect presents in the case of athinner thickness and a smaller original power grain size. Low temperaturecondition has no influence on permeability.The highly difficult pool boiling experiments of liquid nitrogen weresuccessfully conducted, the experimental apparatus was reasonably designedto overcome difficulities such as cryogenic seal and heat insulation, andresearch results of liquid nitrogen pool boiling heat transfer characteristics onsmooth cooper surface, particle sintered porous surface and sintered metalfoam surface were gained for the first time. Analysis was conducted againstvarious researchers’ contradictory conclusions on pool boiling heat transferon sintered porous surfaces and the heat transfer characterists weredetermined. Thermal resistance models were estabillished to describe the heattransfer mechanisms during different heat transfer stages, and the influencesof working fluid, thickness and pore size were analyzed. The hysteresisphenomenon was also studied and so as to the visualization images of bubleson boiling surface. Conclusions obtained were summarized below:(1)Due tolow value of surface tension, contact angle and saturation temperation ofliquid nitrogen, its nucleate boiling starts rather early on porous surfaces.Sintered metal foam surface can decrease the surface superheat the enhancethe heat transfer coefficient due to larger number of effective nucleas and theadditional convective heat transfer contribution at the beginning stage of poolboiling process.(2)The pool boiling heat transfer curve of liquid nitrogen onsmooth copper surface satisfies the typical law of nucleate boiling. The heattransfer coefficients of pool boiling on sintered porous surfaces are larger atthe beginning, then decrease, and keep stable at the end with the increase ofheat flux. The both two sintered porous surfaces show strong inhibition effect on pool boiling heat transfer in the larger heat flux field.(3)The pool boilingheat transfer on sintered porous surface undergoes nucleate boiling stage andsurface boiling stage with the increase of heat flux. The latent heat consumedby bubbles growing and convective heat transfer caused by vapor disturbanceare the main heat transfer mechanisms during the nucleat boiling stage andare governed by fluid’s properties mainly. The surface nucleate boiling andthe heat conduction of vapor film inside sintered porous layer are the mainheat transfer mechanisms during the surface boiling stage.(4)Owing to thecapillary suction force caused by the sintered porous structure, the surfacenucleate boiling can sustain for a long time due to the continual supply ofcooling fluid, so the CHF condition will not take place even with a ratherhigh heat flux.(5)The thicker the sintered porous layer is, the stronger theinhibition effects to vapors are. The vapor film will be easier to form insidethe sintered porous layer and the pool boiling heat transfer on porous surfacetransforms into the surface boiling stage earlier.(6)Hystersis phenomenonmainly takes place during the nucleate boiling process and will weaken theheat transfer coefficient obviously, but this phenomenon is seldom observedduring the surface boiling stage.The phase separation characteristics of porous plug made by sinteredporous metal materials in cryogenic fluids superfluid helium were studied.Theoretical analysis of flow characteristic for porous plug separator wascarried out by applying two-fluid model of superfluid helium and theHagen-Poiseuille law. The calculated and experimental results of porousplugs under different thickness and permeability were also compared anddiscussed. The variation of mass flow rate with temperature difference forporous plug was obtained, and the effects of permeability, thickness andhelium bath temperature on mass flow rate were discussed as well.