The Research On Fluid Condensation And Heat Transfer Enhancement In Microchannels

During the 1980s and 1990s,the proposal of the microchannel heat exchanger has motivated the rapid escalation of the high heat dissipation electronic equipment cooling and microelectromechanical system.Microchannel heat exchanger provide several performance benefits,including miniature and light weight,compact packaging,intense heat removal,homogeneous distribution of axial temperature etc.has shown potential applications in areas such as aerospace thermal control systems,microelectronic component cooling,air conditioning heat exchanger.Owning to the scale effect in microchannels,surface tension rather than gravity force is main heat mechanism in non-circular microchannels during liquid-gas two phase flow and heat transfer process,which proposes effective solutions for limited research on microgravity enviroment.Furthermore,microchannel heat exchanger features as good pressure resistance and high enenrgy efficiency,and creates possibilities for environmentally friendly and sustainable carbon dioxide(CO₂)in applications like supercritical heat pump.There is huge potential in solving energy shortage and environmental problems caused by global economic development and energy consumption,which has received extensive attention from industrial and academic circles in all over the world.,how to enhance the thermal performance of heat exchanger in order to save energy,improve the economical and social benefits,maintain the sustainable development,have been enduring research hotspot.Therefore,the flow condensation and supercritical carbon dioxide cooling in microchannels are computationally and experimentally investigated in this paper,with the aim to explore heat mechanism and influncing law.Key findings from the study are as following:(1)The Volume of Fluid(VOF)method combines with surface tension model and the User Defined Routine(UDF)of phase change model and supplementary paprameters in turbulent model are adopted to build the transient three dimenssional computational model for FC-72 flow condensation in microchannel.The spatial and temporal liquid-gas interface is tracked,and the one or multiple flow patterns(pure vapor,smooth-annular,wavy-annular,transition,injection,slug,bubbly,and pure liquid)at different operating conditions(wall heat flux,inlet mass velocity and inlet saturated temperature),as well as flow pattern transitions maps are captured.Spatial variation rules of vapor quality,wall temperature and fluid temperature at different operating conditions are provided.The computed results show that the captured flow patterns and bottom wall temperatures match fairly well with previously experimental data,which varify the effectiveness of this model.As the mass velocity increased,the uniform fluid temperature gradient gradually covers all the condensation length instead of concentrated along the bottom wall.These variations show close correspondence with axial spans of dominant flow patterns.(2)The film annular flow and condensation heat transfer in curved microchannel(CM)are computationally studied.Included are identified liquid-gas interface and condensation film distribution at different axial locations along the channel,as well as axial variations of heat transfer coefficient in curved microchannel and straight microchannel(SM).The dominate position between surface tension and gravity force in non-circular microchannel is determined.The effects of different operating conditions(wall heat flux and inlet mass velocity)and geometric structure(CM and SM)paprameters(hydraulic diameter,curving of curved microchannel)on axial heat transfer coefficient are discussed.The computed results present that the surface tension force is dominate heat transfer mechanism in non-circular microchannel during condenstion compared with the gravity force.The axial heat transfer coefficient increases with the increase of the mass velocity,and show independence with the wall heat flux.The curved microchannel performs higher heat transfer coefficient than straight microchannel,and the heat transfer coefficient could be further improved with increased curving of the curved microchannel.(3)The condensation flow patterns and transitions mechanisms in wavy microchannel(WM)are computationally studied.The spatial and time-dependent variations of condensation flow patterns(droplet flow,annular-wavy flow,injection flow and slug-bubbly flow)are tracked,and the condensation flow regime maps for different operating conditions are plotted.Followed by thorough discussion on the effects of different operating conditions(wall heat flux and inlet mass velocity)and geometric structure(WM and SM)paprameters(amplitude and wavelength of WM)on flow regime transition characteristics(the annular length,the occurrence frequency of injection flow,the initial slug volume and the bubble detachment frequency).The computed results indicate that increased inlet velocity or decreased wall heat flux could lead to extended annular flow length,increased injection flow frequency,increased slug department frequency with larger initial bubble volume.(4)The experimental apparatus for supercritical carbon dioxide cooling heat transfer is designed and constructed for direct measurement of wall temperature,inlet temperature,inlet mass velocity,inlet and outlet temperature on one side of carbon dioxide,and inlet mass velocity,inlet and outlet temperature on another side of cooling water.And then the wall heat flux and inner wall temperature on one side of carbon dioxide as well as heat transfer coefficient,Richardson number and Nusselt number are computed based on direct measurement.This study examines the effects of different operating conditions(wall heat flux,inlet mass velocity,inlet pressure on the side of carbon dioxide)and flow orientations(horizontal,vertical upflow,and vertical downflow)on bouyancy effect due to various thermal properties,and further on heat transfer performance.The adaptability of exsiting heat transfer correlations are analyzed by comparing with experimental data.The experimental results reveal that increased mass flow rate could strengthen the heat transfer.Wall heat flux exhibites relatively mild influence on heat transfer coefficient.The peak value of heat transfer coefficient decreases with increased inlet pressure,and the corresponding critical temperature moves to higher value.A significant heat transfer augmentation in downward flow is discerned compared with horizontal flow.The heat transfer enhancemnet is observed in concentrated forced flow,by contrast with mixed convection and even free convection charged flow.(5)Three dimenssional computational model for supercritical carbon dioxide cooling heat transfer in microchannel is built based on three conservation laws,adaptive turbulent moldel for given conditions and physical model,as well as additional temperature dependent thermal properties.After validation of model effectiveness by comparing the computed results with theempritical correlations,the cooling heat transfer and pressure drop of supercritical carbon dioxide in wavy microchannels with consistent crests and troughs(WMCCT)and wavy microchannels with opposite crests and troughs(WMOCT)are computationally studied.The influence of different operating conditions(wall heat flux,inlet mass velocity,inlet temperature and inlet pressure)on Richardson number,axial variations of heat transfer coefficient and pressure drop are examined,Furthermore,the effects of different geometric structure(wavy microchannels with consistent and opposite crests and troughs and straight microchannel)paprameters(amplitude and wavelength of WM)on flow and turbulence fields including second flow,axial variations of heat transfer coefficient and heat transfer augmentation,axial variations of pressure drop and pressure drop augmentation,overall thermal performance are predicted,and the relationship between bouyancy effect due to various thermal properties and heat transfer performance is ascertained.The computed results show that the secondary flow are captured in both the velocity and the turbulence kinetic energy contours with vectors.The wavy microchannels(WMCCT and WMOCT),particularly evident in WMOCT,perform higher heat transfer coefficient as well as pressure drop than straight microchannel,and could be further improved with increased wave amplitude or decreased wavelength with limits,while continuously increasing wave amplitude and decreasing wavelength will lead to overall thermal performance deterioration.This paper provides computational and experimental treatment of the mechanisms of fluids flow condensation in the microchannels,and the supercritical carbon dioxide cooling heat transfer in the microchannels,associated with influence law of heat transfer enhancemnet.The ultimate objectives of this paper is to provide theoratical reference for design and optimization in microchannel heat exchangers.