Research On Heat Transfer Enhancement In Tube With Punched Vortex Generator

Heat exchanger is the main equipment for heat transfer.The performance of heat exchanger significantly affects the energy efficiency.Enhanced heat transfer technology is one of the effective ways to improve the efficiency of heat exchanger.Vortex generator is a common enhanced heat transfer element.Vortex generators in the flow field can induce vortices,thin or destroy the fluid boundary layer,and improve the momentum exchange between hot and cold fluids.In this paper,the influence of the structure parameters of the punched vortex generator on the heat transfer in the pipe was studied by numerical simulation.The function prediction correlations of Nusselt number(Nu),friction factor(f)and comprehensive heat transfer factor(PEC)for the structural parameters of the punched vortex generator were obtained,and the heat transfer enhancement mechanism of the punched vortex generator was analyzed.Firstly,the numerical model was verified.On the basis of grid independence verification,the correctness of the selected numerical model was verified by using literature data(including DNS direct numerical simulation,PIV Experimental data,RSM Reynolds stress model).Meanwhile,the empirical formula of smooth tube heat transfer was compared with the simulation results.The maximum error between them was 9.3%(Nu)and 10.12%(f).The results show that the numerical simulation method can accurately simulate the flow and heat transfer characteristics of the punched vortex generator.Then,the main structural parameters affecting the comprehensive heat transfer factor were studied.In order to study the influence of six structural parameters of trapezoidal punched vortex generator on the heat transfer performance and friction resistance of rectangular pipe,including bottom width(w),height(h),the ratio of upper and lower bottom(r),vortex generator angle(α),the position of the hole(p)and the ratio of the hole(φ).Using Taguchi optimization method,orthogonal experiments were designed and 25 groups of numerical simulation experiments were carried out.The results show that: The influence of bottom width(w),height(h),the ratio of the hole(φ)and the position of the hole(p)on the comprehensive heat transfer factor PEC was dominant,and the contribution rates were 37.26%,31.92%,9.88% and 8.07% respectively,and the total contribution rate was 87.13%.Based on the orthogonal experiment,the optimal combination of structural parameters was determined,and the corresponding numerical simulation was carried out.The results show that the comprehensive heat transfer factor was increased by 4% compared with the optimal combination in the orthogonal experiment.Then,the influence of the main structural parameters on the comprehensive heat transfer factor was studied.Taking the bottom width(w),height(h),the position of the hole(p)and the ratio of the hole(φ)as the research objects,a comprehensive experiment was carried out,with 5 different levels for each factor.Based on 625 groups of numerical simulation,the function prediction correlation of the comprehensive heat transfer factor PEC to the main structural parameters was established by using the numerical simulation results.96% of the experimental data are within 12% of the predicted values,which can better predicted the simulation results.Finally,the mechanism of heat transfer enhancement was analyzed.Taking the flow field of the vortex generator with the best and the worst comprehensive heat transfer effect as the research object,the flow field characteristics such as velocity,vorticity,vortex structure,turbulent dissipation rate and wall shear stress were analyzed and compared.The results show that,The increase of wall shear stress can increase the turbulent dissipation rate of the fluid boundary layer,reduce or destroy the fluid boundary layer,so as to strengthen the momentum exchange between hot and cold fluids and enhance the heat transfer.