Phase-Change Heat Transfer In Phase-Separation Porous-Wall Microchannels

Static and dynamic instablilities,running-parameter-dependent heat transfer,large two-phase pressure drops and difficulties in heat transfer enhancement are widely existed in microscale two-phase systems.Because at microscale,channel 3D effect is weak,leading to stagnancy of two-phase interface and resistance of fluid transfer.Neither two-phase distribution nor interface behavior benefits for stable and efficient heat and mass transfer.Therefor,“Microscale flow pattern moderation"concept was proposed regarding with flow boiling as well as flow condensation in microchannel:Phase-seperation porous-wall microchannels were designed based on the basic idea that surface tension dominants fluid beheaviors at microscale.Solid microstructers were served as vapor-liquid interface moderators.Stablized and efficient heat mass transfer process were achieved.The main part of my work presented applicaitons of the“Microscale flow pattern moderation" concept in both boiling and condensation,which is summarized as follows:The second chapter mainly solved the problem of dynamic instability of microchannels.Gredient porous wall microchannel evaporator was proposed,the surface energy gradient was used to regulate the interface behavior.Since the porous wall allows pressure to be exchanged between adjacent channels at any time and any place,the overpressure caused by bubble nucleation and growth was released in time,opening the fluid passage and eliminating bubble blockage.The definition of "bubble confinement ratio" is newly proposed.The bubble confinement ratios exhibit a sinusoidal function with time.Moreover,the bubble confinement ratios between adjacent channels behaved out-of-phase characteristics.The interface "blinking" flow pattern was found,which is physically a density wave oscillation propagating in the channel width direction.Since the width of the porous wall was much smaller than the length of the channel,the "blinking" frequency of the interface was 10-100 times higher than that of the axially propagated density wave.The relationship between the"blinking" oscillations and the wall temperatures were established using the "lumped parameter model".The amplitudes of the wall temperature oscillation were inversely proportional to the "blinking" frequencies,that is.the high-frequency interface oscillation results in ultra-stable operating wall temperatures.Gradient porous wall microchannels open up a new way to eliminate the flow instability of heat exchangers and thermal energy systems.The third chapter mainly solved the problems of two-phase static instabilities and the heat transfer coefficients being vulnerable to change of running parameters.A new heat transfer mechanism was discouved by examing the heat transfer proporties of the porous wall microchannel evaporator:the convective evaporation mechanism in the bare channels competed with the nuclate heat transfer mechanism in the porous wall areas,resulting in stable two-phase heat transfer coefficients which do not change with the heat fluxes,nor do them change with mass flow rates.This is because when the heat flux and/or mass flow rate change,one of the above two heat transfer mechanisms is strengthened and the other is attenuated,resulting in stable heat transfer coefficients.The hybrid heat transfer mechanism is different from any boiling heat transfer mechanisms reported in literatures.The "thermal driving effect"leading to the redistribution of the working fluid flow in the pin fin regions and the bare channel channel regions is important to understand the hybird mechanism.The redistribution of flow within channel also helped the pressure drop to be independent of the inlet mass flow rate.The unchangable heat transfer coefficients and pressure drop characteristics ensure flexible operating-parameter selections.The phase separation evaporator can be operated at very small mass flow rates as long as no drying occurs.In addition,any unexpected disturbance of the running parameters will not affect the heat transfer of the evaporator,which is good for operation and management of the heat exchange system.The fourth chapter mainly solves the problem that the micro-scale two-phase flow has large flow resistance and the heat transfer performance deteriorates at low vapor qualities.Two phase-separation silicon-based microcondensers are proposed.Lined pin-fin arrays alternately form liquid channels and vapor channels along channel width direction.It was concluded that the reduced Gibbs free energy was responsible for condensed liquid entering liquid passages when the gas-liquid interface advances throats of the two pin fins.Accroding to the energy dissipation model,the mechanism of pressure drop reduction by phase-separation micro-condenser was revealed:small droplets were merged into a continuous phase during phase separation process,interface area so as to the internal friction dissipation of the two-phases flow is hereby reduced.Three micro condensers were carefully designed:solid-wall microchannel condenser(SWM),parallel phase-separation condenser(PPS with constant cross-section of fluid channels)and conical phase-separation condenser(CPS with variable cross-section of fluid channels).The experiment results showd that the phase separation condensers had mass flow increased by 15%at the same pressure drop compared to that of solid wall condenser.PPS condenser enhanced heat transfer at moderate or small cooling intensities,but deteriorated heat transfer at large cooling intensities due to the large amount of condensate expanding from the liquid passage,overflowing all of the pin-fin sidewalls.The variable cross-section of vapor-liquid passages in the CPS condenser adapted to the change of the vapor and liquid flow rates.Pressures in vapor passages were maintained at high levels,locking the liquid in the liquid passage,eliminating the excessive expansion of the liquid.So the pin-fin side walls facing the steam passage were always covered by a thin liquid film which was good to heat transfer.The CPS condenser achieved heat transfer enhancment in all operating conditions at any locations.The condensation heat transfer coefficients were increased by 74%maximumly while two-phase resistances being reduced.The CPS condenser had the best performance among the three condensers.The fabrication of phase seperators did not involve any nanotechnology,the performance of the condenser was therefore very stable.