The Study On Heat Transfer And Flow Characteristics Of Multi-directional Turbulent Heat Transfer Tubes

In large thermal power plants,the reheater heat exchange elements are mostly smooth tubes with low heat transfer efficiency,and the heat in the flue gas cannot be taken away by the working medium in time,which makes the thermal efficiency lower.In this paper,from the perspective of enhanced heat transfer,the Multi-directional turbulent heat exchanger tube is applied to the reheater of an ultra-supercritical unit.Numerical simulation is used to analyze the steam side and gas side of the Multi-directional turbulent tube reheater heat exchanger tube.The heat transfer and flow characteristics of the gas side are studied,the principle and effect of enhanced heat transfer are analyzed,and the results are optimized to obtain the optimal structural parameters,tube spacing and inlet velocity for the Multi-directional turbulent exchange.The result can provides theoretical guidance for application of heat tubes in reheaters.Firstly,the steam side characteristics of the reheater heat exchange tube are studied by three-dimensional numerical simulation.The results show that the pressure gradient at the tube wall increases and the Turbulence Kinetic Energy increases due to the presence of ribs,and the disturbance to the thermal boundary layer is enhanced.The steam with different temperatures at the tube wall is quickly mixed,and the heat transfer effect is enhanced.It was calculated that Nu_s and f_s on the steam side of the multi-directional turbulent heat transfer tube increased 1.37 to 1.94 times and 1.75 to 4.7 times,respectively,of the smooth tube.Using the Response Surface Methodology,the factors with the highest significance in response to Nu_s,f_s and PEC_s are u,h and u respectively.The genetic algorithm is used to optimize the multi-objective convection heat transfer tube to obtain the best comprehensive heat transfer.The combination of properties is:n=6,h=0.56 mm,p=30 mm,u=15.856 m/s.Corresponding PEC_s=1.2955,Nu_s=2137.78,f_s=0.0558.Then,based on the above optimization results,the multi-directional turbulent heat transfer tube bundle model is established to study the characteristics of the gas side.The results show that the comprehensive heat transfer performance of the cross row tube bundle is better than that of the straight tube bundle;the Nu_g and f_g on the gas side in the straight tube bundle are proportional to the transverse pitch of the tube bundle.The decrease of s₁/d_o increases,and Nu_g decreases with the decrease of the longitudinal pitch of the tube bundle s₂/d_o,but f_g decreases with the decrease of the longitudinal pitch ratio s₂/d_o.The flue gas side Nu_g and f_g in the cross row tube bundle increase with the decrease of s₁/d_o and s₂/d_o.According to the analysis of the response surface,the factors with the highest significance of Nu_g,f_g and PEC_g in the influencing factor are v,s₁/d_o and s₂/d_o,respectively,and the Nu_g,f_g and PEC_g are the most significant for the cross tube bundle.The factors are v,v,s₁/d_o;multi-objective optimization is used to obtain the optimal pitch ratio and gas flow rate of the smoothing tube bundle:s₁/d_o=2.0,s₂/d_o=1.6,v=11.909 m/s,the corresponding PEC_g=1.2913,Nu_g=48.642,f_g=0.08838;the optimum pitch ratio and flue gas flow rate of the fork tube bundle are:s₁/d_o=3.0,s₂/d_o=2.4,v=11.838 m/s,which corresponds to PEC_g=1.3627,Nu_g=63.8301,and f_g=0.3918.Finally,the multivariate linear regression method is used to fit the Nusselt number of the steam side and the gas side of the Multi-directional turbulent heat exchange tube and the criterion correlation of the drag coefficient,and the applicable range of the above correlation formula is also given.The error is within the engineering tolerance.