Numerical Study On Heat Transfer Characteristics Of Entrance Region Of Concentric Annular Microchannel

With the rapid development of MEMS technology,the heat exchanger went to compact and highly integrated direction,microchannel heat exchanger has more and more important engineering application value.Different from the conventional scale problem,the length of the entrance region accounts for a large proportion of the total length of the microchannel.The research on the thermal transfer performance of the entrance section of microchannels mainly focuses on rectangular and elliptical microchannels,while the research on the entrance section of annular channels is less,the annular channel is irreplaceable in many fields.In this thesis,the finite volume method is used to numerically study the heat transfer characteristics of the entrance section of concentric annular microchannel.The change rule of temperature distribution,Nusselt number and the thermal entrance length are analyzed.In this thesis,the heat transfer characteristics of the entrance section of concentric annular microchannel are systematically studied under four typical thermal boundary conditions:constant heat flux on inner wall and outer wall adiabatic,constant heat flux on outer wall and inner wall adiabatic,constant wall temperature on inner wall and outer wall adiabatic,constant wall temperature on outer wall and inner wall adiabatic.The results show that under different thermal boundary conditions,the local Nusselt number is always the largest at the entrance of the annular microchannels and decreases along the axial direction,and is constant at the fully developed section.The local Nusselt number at the entrance of the annular microchannels is much higher than that at the fully developed section,indicating the excellent heat transfer performance at the entrance of the annular microchannels.When the inner to outer diameter ratio r~*is constant,the local Nusselt number at the fully developed section does not change with the change of Reynolds number,while the increase of Reynolds number will lead to the decrease of local Nusselt number in the entrance region.The sensitivity of local Nusselt number to the change of Reynolds number is due to the influence of axial thermal conductivity.It is found that when the Reynolds number Re is constant,when the inner wall is used as the heating wall,the increase of r~*will lead to the decrease of the local Nusselt number in the entrance region and the fully developed section,while when the outer wall is used as the heating wall,the increase of r~*will lead to the increase of the local Nusselt number in the entrance region and the fully developed section.Secondly,this thesis explores the variation rule of the thermal entrance length of the annular microchannel with the inner to outer diameter ratio and Reynolds number under different thermal boundary conditions.It is found that when the Reynolds number Re is constant,the thermal entrance length increases with the increase of the inner to outer diameter ratio r~*.When r~*>0.25,the effect of r~*on the thermal entrance length is no longer obvious.When the inner to outer diameter ratio r~*is constant,the thermal entrance length decreases with the increase of Reynolds number Re.When Re>250,the effect of Reynolds number Re on the thermal entrance length is not obvious.Based on the numerical results,the local Nusselt number correlation for the entrance region of the annular microchannel under different thermal boundary conditions is obtained by using a gradual method,it can predict the heat transfer situation at any dimensionless axial distance under different Reynolds number and inner to outer diameter ratio,and then put forward the correlation formula between the thermal entrance length and the inner to outer diameter ratio and Reynolds number,with the maximum error no more than 5.28%,which provides a reference for research and practical engineering applications.Finally,considering the roughness of the practical channels,the rough surface is approximated as sinusoidal rough infinitesimal,the numerical model of concentric annular microchannels with sinusoidal rough infinitesimal surface is established,and the heat transfer characteristics of its entrance region are studied.By comparing with the heat transfer performance of smooth surface annular microchannels,it is found that the heat transfer characteristic of rough annular microchannels is enhanced.This is because the rough surface causes disturbance to the flow,resulting in the thinning of boundary layer and reducing the heat transfer resistance of the channel surface.In the range of Reynolds number studied in this thesis,the annular microchannels with small wavelength and large amplitude sinusoidal rough infinitesimal has the best comprehensive performance.