The Characteristics Of Flow And Pressure Drop In Rectangular Microchannel

With the development of technology,micro-devices and micro-systems become a research hotpot.The micro-devices have many advantages due to the reduction of size,but it also brings the increase of the heat flux.Then the microchannel technology has become the research direction and trend for its efficient heat dissipation.The microchannel heat exchangers have heat transfer enhancement,compact structure,high efficiency,and have important research significance in various disciplines.However the mechanism of the miniaturized system to change the physical process has not been fully understood.When the scale is small enough,it will appear different phenomena with the conventional scale.Some basic problems are still not clear,including the flow and pressure drop characteristics in microchannels,especially for the entrance region.The flow and heat transfer coefficient of entrance region is high,and the overall heat transfer efficiency can be improved by utilizing the entrance region.Under continuous flow and slip laminar flow,pressure drop and friction factor of three-dimensional rectangular microchannels have been analyzed using theoretical analysis and numerical simulation methods.In this work,the friction characteristics of the entrance region are studied,and the influence of the aspect ratio of the cross sections,the dimensionless axial distance and the slip parameter on the flow and pressure drop are mainly analyzed.Based on the semi-theoretical analysis model used in this work,the friction factor of the entrance region of three-dimensional rectangular microchannels are quantitatively studied.For the continuous flow,the numerical simulation results of fappRe are in good agreement with the available classic data and experimental data.Then,we expand the available literature data in the classical literature,and the expended data are consistent with the semi-theoretical analysis model data.On the basis of the extended data,a new correlation of Hagenbach coefficient is developed.Secondly for the slip flow,the numerical simulation data of fappRe are compared with the semi-theoretical analysis model.Within the error permissibility,the preliminary study on the friction distribution characteristics of slip flow is implemented.The numerical simulations of continuous flow and slip flow are carried out,and obtain the expected results.In three-dimensional rectangular microchannels,fappRe in the entrance region is large,and rapidly decreases with the increase of axial dimensionless distance,and then the trend gets slow,and eventually reaches a stable value which presents the full development flow stage.In addition,fappRe decreases with the increase of the aspect ratio.For the slip flow,the larger Kn,the greater the slip flow and the larger slip velocity.Compared with the corresponding no slip flow,fappRe gets smaller for the existence of velocity slip.In this work,we extend the data of fappRe in the classical literature,under continuous flow and slip flow conditions,the fappRe of three-dimensional rectangular microchannels are quantitatively studied.Besides,the expended data and the semi-theoretical model used can predict the friction and pressure drop distribution characteristics for continuous flow and slip flow.They are the basis for further research and convenient for the engineering practice and application.Only by deeply understanding the distribution of pressure drop and friction factor,can people better design the microchannel heat exchanger.