Structural Optimization And Heat Transfer Effect Prediction Of Heat Exchanger For Waste Heat Recovery In Subway

During the operation of Subway trains,a large amount of waste heat is released into the air of tunnels.Previous scholars try to recovery of soil around subway tunnel using ground heat exchanger of waste heat,or convection type air heat exchanger in the tunnel ventilation shaft setting recycling waste heat from the air tunnel,but there are soil thermal imbalance,recycling heat or less vulnerable to external environment influence,heat recovery efficiency is low,difficult to popularization and application.Release most of the waste heat generated in the subway train directly into the subway station section of tunnel in the air,so through the analysis of the subway tunnel air flow and temperature distribution characteristic,put forward a kind of used for air waste recycling metro station tunnel of the tropical longitudinal finned tube heat exchanger,the hanging on a subway tunnel wall tube heat exchanger,waste heat source near the subway tunnel subway train,rely on the refrigerant and heat transfer of air extraction tunnel in the waste heat in the air.Based on the investigation and analysis of the changing law of air velocity in subway tunnels,this paper simplified the working conditions of the complicated heat exchanger into two cases:the natural convection heat transfer when there is no vehicle passing through and the forced convection heat transfer when there is a train passing through under the action of piston wind.By analyzing the natural convection heat transfer process between the longitudinal finned tube heat exchanger and the air in the subway tunnel,the heat transfer model of the longitudinal finned tube heat exchanger is established,and the model is solved by means of numerical calculation.The numerical simulation method was used to analyze the influence of the structural parameters of the longitudinal fins on the performance of the heat exchanger under natural convection.In addition,the influence of the installation position of the heat exchanger tube row composed of a plurality of longitudinal fin tubes in the tunnel and the spacing of the tube on the heat exchange performance is calculated and analyzed.The theoretical analysis method was used to study the influence of structural parameters on the performance of heat exchanger under forced convection.Finally,combined with the actual operation of subway trains,the heat transfer effect of the optimized heat exchanger in the alternation of piston wind scour and natural convection is predicted.Through the analysis of simulation and calculation results,the following main conclusions are obtained:1)In the case of natural convection,it is found that the convection heat transfer coefficient of a single longitudinal finned tube decreases with the increase in the number of fins,decreases with the increase in the height of fins,decreases with the increase in the thickness of fins,and first increases and then decreases with the increase in the Angle between fins.The optimal heat transfer performance was achieved when the number of fins n=3,the thickness of fins delta?=1.0mm,and the Angle of fins delta?=55~o;2)For longitudinal finned tube row,found that the tunnel wall of finned tube row of the natural convection in the drainage effect,can effectively reduce the stagnation zone around the finned tube,at the same time reduce the wake of tube row above the effect of the under the influence of heat exchange tube row,make tube heat exchanger and the distance of the tunnel wall L is equal to 2 times the pipe diameter d,tube spacing between adjacent S equal to five times the base pipe diameter d row the average convection heat transfer coefficient of peak of 8.14W/(m~2·K);3)For longitudinal finned tubes,the convective heat transfer coefficient increases with the increase of the number of fins in forced convection,decreases with the increase of fin height,and increases with the increase of fin thickness.When the number of fins is n=4 and the fin thickness is?=1.0mm,the heat exchange and economy are optimal;4)The total heat transfer of the tube-row heat exchangers with different fin numbers under different operating conditions in a driving interval was calculated.It was found that the heat exchange capacity was the highest when the number of fins n=4,and six longitudinal finned tubes at the peak period.The heat exchanger can be recycled up to314.26W/m.Based on the optimization of the structural characteristics of the heat exchanger,this paper adapts to the special internal environment of the subway tunnel and improves the heat transfer performance of the heat exchanger,in order to provide theoretical support for optimizing the special heat exchanger structure of the subway tunnel,and lay a foundation for the popularization and application of the waste heat recovery system.