Study On Heat Transfer Performance And Shape Factor Of Micro Rectangular Cross Flow Heat Exchanger

The micro rectangular block heat exchanger has the characteristics of large unit heat transfer area,high heat transfer efficiency,small volume and high exchange rate,and can be used in many industrial fields such as automobiles,ships,aerospace and so on.In the micro rectangular block heat exchanger,the solid wall thickness and the hydraulic diameter of the fluid channel are in the same order of magnitude.The heat conduction in the solid will have a great impact on the performance of the heat exchanger,and the structural parameters are the main factors affecting the solid heat conduction of the heat exchanger.The correlation formula of thermal resistance is obtained,and a fitting formula for quickly calculating the thermal conduction thermal resistance of a miniature rectangular block cross-flow heat exchanger is obtained.Firstly,the heat transfer process of micro rectangular block cross flow heat exchanger is analyzed,the relationship between the calculation process of heat transfer and various parameters is obtained,and the calculation method of shape factor and affected structural parameters are deduced.Then,by changing different structural parameters,including the distance between the tubes of the same row of cold fluids,the distance of tubes of the same row of hot fluids,the distance between the upper and lower tubes,and the diameter of the tubes,the total heat transfer coefficient,heat transfer efficiency,heat flux density vector and average The Nusselt number is used as the evaluation index to analyze the influence of the above structural parameters on the thermal conductivity of the solid and the heat transfer performance of the heat exchanger.Then,the shape factor values under different structural parameters are obtained by fitting,and the corresponding shape factor correlation formula is fitted through nonlinear multiple regression analysis,and the thermal conductivity and thermal resistance correlation formula is further derived,and the accuracy is verified.The main conclusions of the paper are as follows:（1）Influence of structural parameters on solid heat conduction and heat transfer performance of rectangular block heat exchanger:（1）The influence of the center distance of the same row of cold and hot fluid on the heat conduction and heat transfer performance of the heat exchanger is basically the same,so it is symmetrically designed and analyzed as the same influencing factor.The increase of the distance between the pipes in the same row makes the distribution more uniform,and the temperature gradient between the respective pipes of the fluid is evenly distributed,reducing the influence of the axial heat conduction between the pipes.The strength of heat transfer has no effect,and the average Nusselt number on the cold and hot sides remains basically unchanged;the overall heat transfer coefficient increases,and the heat transfer efficiency also increases.Therefore,under the operating condition of small Reynolds number,the design of rectangular block cross flow heat exchanger should increase the center distance of the same row of pipes as much as possible,and the arrangement between the upper and lower pipes should be uniform.（2）When the distance between the upper and lower tubes increases from 2.6mm to 3.6mm,the total heat transfer coefficient changes little,and the heat transfer efficiency is also stable at about 0.41,when the distance between the upper and lower pipes increases from 3mm to8 mm regardless of the size range,the heat flow vector between the cold and hot fluid pipes gradually decreases,and the heat flow density vector between the respective pipes becomes dense,indicating that the heat exchange between the cold and hot pipes is weakened and the axial heat conduction is strengthened.The average Nusselt number decreases gradually,and the decreasing range increases when the distance between the upper and lower tubes exceeds 6mm;The effect of convective heat transfer is weakened.Therefore,in the design of rectangular block cross flow heat exchanger,on the premise of ensuring sufficient solid pressure and support strength,the spacing between upper and lower tubes should be reduced as far as possible to increase the total heat transfer coefficient.（3）The increase of the pipe diameter strengthens the heat transfer between the cold and hot pipes,weakens the influence of the axial heat conduction,and the average Nusselt number also increases,which strengthens the convective heat transfer.The heat transfer effect is enhanced,and the heat transfer efficiency decreases accordingly.Therefore,under the operating conditions of small Reynolds number,within the reasonable range of the distance between the tube centers in the same row,for the design of the micro-rectangular-block cross-flow heat exchanger,the large tube diameter can be selected as much as possible to enhance the heat transfer effect.（2）Fitting of heat transfer shape factor of rectangular block heat exchanger:（1）With the change of the center distance of the same row of pipes,the change of the numerical curve of the shape factor conforms to the form of power function curve.With the increase of the center distance of the same row of pipes,the value of the shape factor increases,and the trend is slower.（2）With the change of the distance between the upper and lower tubes,the value of the shape factor gradually decreases,but the range is very small and has little effect.（3）With the change of pipe diameter,the numerical curve of shape factor also conforms to the curve form of power function.As the pipe diameter increases gradually,the value of shape factor increases and the trend slows down.（4）The correlation between shape factors and structural parameters has 1 dependent variable and 3 independent variables,and there is a nonlinear relationship between the dependent variable and the independent variable.The nonlinear multiple regression analysis is used,and the power equation is used for fitting,and the shape factor is finally obtained.The correlation formula with the structural parameters,the thermal conductivity and thermal resistance correlation formula is derived,and the accuracy is verified.The maximum error is within 15%,which meets the accuracy requirements of engineering applications.