| With the development of microelectronic technique,more and more heat exchanger parts tend to be miniaturized,but the heat transfer amount per unit area is getting bigger and bigger.Due to the limitation of convective heat transfer,air cooling technique has been unable to meet this requirement.Thus research and development a high-performance micro-cooler has become an urgent issue.Because flow boiling can effectively utilize the latent heat of phase change,the heat transfer efficiency is improved by several times.Since 1981,the flow boiling heat transfer in multi-microchannels has been extensively studied.The relevant flow and heat transfer mechanism are further revealed.Metal foam is an ultra-light porous metal material.The unique structure can greatly improve the nucleation ration and increase the fluid disturbance.Therefore,the heat transfer is enhanced further.Some scholar has studied the heat transfer characteristics of flow boiling in convectional channels filled with metal foam.But the relevant research in micro/small channels has not been reported.Whether the flow boiling characteristics are consistent at different scales,whether the change regulation of heat transfer coefficient is the same for different scale channels,and how to build a new suitable heat transfer model,all the above problems are not resolved.In the paper,the parallel small channels with the hydraulic diameter of 2.5mm are adopted,which was filled with the meat foam,the PPI is 10 and 20.And then a series of experimental researches and theoretical analysis were conducted.The visualization test section and experimental test loop was designed and built.By analyzing the pressure signal and temperature signal,flow pattern maps and heat transfer mechanism under different working condition were obtained.Then the influence rules of heat transfer coefficient and inlet pressure are obtained by theoretical calculation.Finally,through polynomial data fitting method,a new heat transfer correlation was built.In the research of flow pattern and heat transfer mechanism,the flow patterns in empty channels and metal foam filled channels were photographed by means of microscope and high-speed camera.Simultaneously,the flow pattern maps under different vapor quality were drawn.The effects of mass flux,heat flux and metal foam structure on flow pattern transition were analyzed.It is found that the transition boundary between slug/plug flow and annular flow had a vapor quality between 0.04 and 0.05 under 20 PPI metal foam.In the analysis of heat transfer characteristics,through drawing the boiling curve,the relation between wall temperature and heat transfer mechanism was analyzed.Meanwhile,the heat transfer coefficient curves revealed the influences of mass flux,heat flux,vapor quality and metal foam on heat transfer coefficient,which may provide valuable advices for design and operation the micro/small heat exchanger.In the study of heat transfer correlation and inlet pressure,two previous heat transfer model was compared with this experimental data.Then a new heat transfer correlation was built according to the change trend of local heat transfer coefficient based on the basic form of the original heat transfer model.And it showed the pretty good accuracy.Finally,under different condition,the difference of inlet pressure was analyzed to study the influence factor and rules of flow resistance in empty channels and metal foam filled channels. |
