| During the operation of the subway,a large amount of waste heat will be generated,resulting in a large amount of heat stored in the surrounding rock of the tunnel,which will cause the temperature of the surrounding rock to increase year by year,and the temperature field of the underground rock and soil will also change greatly,eventually destroying the ecological environment around the tunnel and causing underground Space heat pollution.Commonly used solutions include tunnel ventilation,air conditioning system cooling,spray,phase change cooling,and thermal insulation.Although the above method can achieve the purpose of tunnel cooling,it fails to utilize the waste heat generated in the tunnel,which causes energy waste to some extent.The subway waste heat source heat pump system is one of the technologies to solve the tunnel thermal pollution and effectively utilize its waste heat.The design of the front end heat exchanger is decisive for the performance of the entire system.This study intends to conduct a practical measurement and numerical analysis of the heat transfer performance of a heat exchanger front-end heat exchanger of a subway waste heat source,a thin-shell heat exchanger,in order to provide design basis for engineering practice.Firstly,based on the structure of the thin-shell heat exchanger and its heat transfer process,the corresponding theoretical heat transfer model is established;Secondly,a thin-shell heat exchanger experimental platform was built inside the tunnel,and the heat transfer characteristics under different working conditions were measured and analyzed;Thirdly,based on the theoretical heat transfer model of the thin-shell heat exchanger,the corresponding numerical model is built,and the model is compared and verified by the measured data;Finally,based on the verified numerical model,the heat transfer characteristics and influencing factors of the thin-shell heat exchanger are further analyzed.The main conclusions based on the above studies are as follows:1)The measured results show that the heat transfer effect of the thin-shell heat exchanger in the subway tunnel is good.When the return water temperature of the system is 35°C,the heat exchange per unit area is 120 W/m~2.2)The simulation results show that in winter conditions,the heat exchange per unit area is reduced by 8%for every 1°C increase in the inlet temperature of the heat exchanger.In summer,the inlet temperature of the heat exchanger is increased by 1°C.The heat increased by 9%.It indicates that the inlet temperature of the heat exchanger has a great influence on the heat exchange effect.3)The simulation results show that the heat transfer rate per unit area of the heat exchanger changes by less than 1%per revolution of the flow rate in the heat exchanger tube of 0.01 m/s.It indicates that the flow velocity inside the pipe has little influence on the heat exchange amount,and the increase of the flow velocity in the pipe will significantly increase the flow resistance of the pipe network system.Therefore,it is recommended that the flow velocity in the pipe should not exceed 0.2 m/s in practical applications.4)The simulation results show that for summer conditions,the heat exchange per unit area of the heat exchanger is reduced by 3%for every 1°C increase in the air temperature of the tunnel;under the winter conditions,the heat exchange per unit area of the heat exchanger increases by 2%for every 1°C increase in the air temperature of the tunnel;It indicates that the air temperature of the tunnel has a great influence on the heat transfer of the thin-shell heat exchanger.It is suggested that the actual impact of the piston wind generated during the operation of the subway train on the air temperature inside the tunnel should be effectively controlled in the actual project.5)The simulation results show that the closer the distance between the heat exchanger laying position and the wall surface of the tunnel is,the more favorable it is to extract the heat of the surrounding rock of the tunnel.It is recommended that the thin-shell heat exchanger be placed close to the inner wall of the subway tunnel as far as the construction conditions permit.6)The results of the saliency analysis show that the significant influences of the influencing factors on the heat transfer performance of the heat exchanger are:inlet temperature,heat exchanger buried depth,tunnel air temperature,and pipe flow velocity.This study can provide a theoretical basis for the application of thin-shell heat exchangers in the waste heat source heat pump system of the subway.The research results can provide reference for practical engineering design. |
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