Analytical And Numerical Investigations Of Flow And Heat Transfer Characteristics Of Rectangular Microchannels In The Entrance Region

With the development of microchannel heat sinks in the direction of miniaturization and integration,different phenomena have emerged from the conventional scale,and the scale effect cannot be ignored.For short channels,due to the shorter total length,the proportion of the entrance length to the overall length increases,and the entrance effect has a greater impact.At present,the transport phenomenon of the entrance area of the circular microchannel has been extensively studied,but the analysis of the transport phenomenon of the entrance section under the other microchannel cross-section is very rare.Whether it is micromachining or conventional processing,rectangular channels are easier to manufacture than circular channels.Therefore,using Fluent software,this paper studies the transport characteristics of the laminar entrance region of the rectangular microchannels to gain a deep understanding of the velocity distributions,temperature distributions and flow heat transfer characteristics of the entrance region and the fully developed region.This paper first introduces the relevant theories about pressure drop and heat transfer characteristics of microchannels in the entrance region,and obtains the scale analysis of the entrance region and the fully developed region,which will be applied for the results of the numerical simulation.Next,this paper analyzes the friction factor characteristics of the flow entrance section,mainly studying the variation of pressure drop and flow development length of the rectangular microchannels with different aspect ratios?and Reynolds number Re.The study focuses on velocity distribution characteristics and the product of apparent friction factor and Reynolds number f_appRe in the entrance region.It is found that the different Reynolds numbers under the same?has a great effect on the friction factor of the rectangular microchannels in the entrance region,and the friction factors are basically the same when the flow reaches the fully developed region.Under the same Re numbers,different?have little effect on the friction factor of the rectangular microchannels in the entrance region,but the friction factor changes significantly when the flow reaches the fully developed region.When Re is less than 20,the hydrodynamic development length gradually decreases with increasing?.When Re is more than 20,with the increase of the aspect ratio,the maximum hydrodynamic development length occurred at?=0.3-0.4,and then gradually decreases.Based on the results,the correlations of the dimensionless flow development length with respect to Re and?are developed.Finally,under the circumferential constant wall temperature and axial constant heat flux boundary condition,the Nusselt number Nu of the thermal entrance section of the rectangular microchannels is analyzed.Comparing the Nusselt number of the numerical simulation with the existing data in the literature,the validity of the model is verified.The paper mainly studies the variation of heat transfer characteristics of the entrance section of the rectangular microchannels with different?and Reynolds numbers,and focuses on the temperature distribution characteristics and Nusselt numbers in the entrance section.It is found that in the entrance region,when the Re number is low,the Nusselt number changes significantly.As the Re number decreases,the influence of the aspect ratio?on Nu_x gradually disappears.As the aspect ratio increases and Re decreases,the dimensionless thermal development length L^*_th gradually increases.Based on the above results,new Nusselt number correlations and dimensionless thermal development length correlations are developed,which will provide the basis for the optimization and practical engineering applications of rectangular microchannels.