Structure Optimization And Thermal-hydraulic Performance Analysis Of Finned Tube Oil Cooler

As an important component of vehicle hydraulic power steering system,oil cooler plays a role in controlling the temperature of hydraulic oil,which can ensure a safe and reliable operation of the steering system.Improving the thermal-hydraulic performance of finned tube oil cooler can promote the compactness of heat exchanger,reduce its metal consumption and improve the economy of vehicle operation.In the process of heat exchange between hydraulic oil(inside the tube)and air(outside the tube),the heat transfer resistance outside the finned tube is dominant.Therefore,improving the thermal-hydraulic performance of the oil cooler should focus on reducing the heat transfer resistance outside the finned tube.In this paper,the structure of heat transfer surface outside the finned tube is optimized to reduce the heat transfer resistance and improve the thermal-hydraulic performance of the oil cooler.Therefore,based on the original structure and production technology of the oil cooler,the elliptical tube with multiunit fin(louvered fin and vortex generator with the rectangular shaped winglet at the rear of tube)is proposed to replace the finned tube.Numerical analysis and discussion on the characteristics of air flow and heat transfer both outside the finned tube and the multiunit finned tube.The influences of the relevant factors of multiunit finned tube(including the main structural parameters,such as louvered fin inclinationθ,width L_h,number N,fin spacing F_p and winglet angleα,etc)on the air flow and heat transfer outside the tube have been studied in detail,and the recommended values of various structural parameters are obtained.On this basis,the orthogonal experimental design method is used to further explore and analyze the influence of the primary order of the structural parameters of the multiunit finned tube and the value of the parameters combination.The correlation formulas between the structural parameters of the multiunit finned tube and the heat transfer factor j and friction factor f are also obtained by multiple regression analysis.The main research contents and conclusions of this paper are as follows:(1)The differences of air flow and heat transfer between the finned tube and multiunit finned tube are compared.The multiunit fin can continuously deflect the flow direction of the airflow and enhance the convective heat transfer between the airflow and the wall.The combination of elliptic tube and multiunit fin can greatly reduce the wake area and improve the heat transfer effect at the rear of the tube.The results show that the heat transfer factor j and friction factor f of the multiunit finned tube are both significantly higher than those of the finned tube in the range of Reynolds number from 550 to 2100.The factor j is increased by 1.64~1.79 times,and f is increased by 1.69~1.71 times.Combined with the calculated values of the comprehensive evaluation factor JF,the thermal-hydraulic performance of the multiunit finned tube is significantly improved,which is 37.8~49.6%higher than that of the finned tube.(2)The effects of the main structural parameters of the multiunit finned tube on the air flow and heat transfer outside the tube are obtained.Under certain conditions,increasing the number N,inclinationθ,and width L_h of the louvered fins can increase the convective heat transfer coefficient outside the multiunit finned tube,and the flow resistance can increase accordingly.Reducing the fin spacing F_p is beneficial to enhancing the convection heat transfer outside the multiunit finned tube,but the flow resistance also increases.Increasing the winglet angleαcan enhance the convection heat transfer at the rear of the tube and increase the flow resistance.(3)The influence of different structural parameters and their combinations on the thermal-hydraulic performance outside the multiunit finned tube is discussed by means of orthogonal experimental design.The order of the influence of main structural parameters of the multiunit finned tube on the heat transfer factor j is:F_p>L_h>θ>N>N×F_p>α;the order of influence on the friction factor f is:F_p>N>L_h>θ>α>N×F_p;the order of influence on the factor JF is:F_p>L_h>θ>N×F_p>N>α.The combined values of the optimized structure parameters obtained by the orthogonal experimental design method are:θ=25°、L_h=12.8mm、N=12、F_p=1.4mm、α=30°,which are 52.9-86.3%higher than the finned tube.(4)Based on the obtained data,the correlations between the main structural parameters of the multiunit finned tube and the heat transfer factor j and the friction factor f are obtained by regression analysis.The relative error between the calculated value and the simulated value is within 15%.