| The waste heat generated during the operation of the subway will cause the temperature rise of tunnel and surrounding rock,which will affect the thermal environment of the subway tunnel and destroy the thermal balance of the underground rock.In order to solve this problem,the utilization of ground source heat pump technology in underground tunnels has attracted more and more attention.Compared with the traditional buried tube heat exchanger,the application of capillary in tunnel has the advantages including convenient construction,small occupation of area and large heat transfer area.At present,there are many studies on the heat transfer performance and design methods of traditional buried pipe,while the capillary heat exchanger,although it is widely used as the end of building indoor radiation,there are few studies on its heat transfer performance and design method when it is used as the front-end heat exchanger in tunnel.This paper is dedicated to study the heat transfer performance and design method of capillary heat exchanger for the ground source heat pump system used in subway tunnel with the capillary as the front-end heat exchanger.The capillary heat exchanger is simplified as a surface heat source,then the unsteady heat transfer model is established by TRNSYS,and it is verified by the experimental data of the demonstration ground source heat pump system in winter.The heat transfer process between capillary tube and surrounding rock mass and tunnel air is simulated and calculated,and the influence of external disturbance factors on the capillary heat transfer performance is analyzed.According to the capillary heat transfer performance under different geological conditions,the design method of capillary heat exchanger is given.Based on the analysis,it discovers:In summer conditions,the heat exchange per unit area of the capillary increases linearly with the increase of the inlet water temperature,the decrease of the initial temperature of the surrounding rock and the tunnel air temperature.In the total heat released by the capillary,most of the heat enters the surrounding rock,just a small part of the heat enters the tunnel air through conducting.With the decrease of the initial temperature of surrounding rock and the increase of the tunnel air temperature,the proportion of heat taken from the surrounding rock of the tunnel presents a gradually increasing trend;In winter conditions,the heat exchange per unit area of the capillary increases linearly with the decrease of the inlet water temperature,the increase of the initial temperature of the surrounding rock and the tunnel air temperature;In the total heat obtained by capillary,most of the heat is taken from tunnel air and a small part is from surrounding rock.With the decrease of surrounding rock temperature and the increase of tunnel air temperature,the proportion of heat taken from the tunnel air presents a gradually increasing trend;The heat transfer performance of capillary increased with the increase of the flow rate in the tube,when the flow rate is greater than 0.1 m/s,the effect of increasing the flow rate on the heat transfer performance of capillary was not obvious,When the flow rate increases by 0.02 m/s,the increase of heat exchange per unit area was less than or equal to 4.6%;The greater the thermal conductivity of surrounding rock mass,the better the heat transfer effect of capillary front-end heat exchanger;the greater the thermal diffusivity,the worse the heat transfer effect;under different geological and temperature conditions,the heat transfer performance of capillary is quite different,from the point of view of heat transfer performance,When the thermal conductivity and thermal diffusion coefficient of surrounding rock are relatively small,the difference between the initial temperature of surrounding rock and the inlet water temperature and the air temperature of tunnel is small,the heat transfer of capillary heat exchanger is small,it is not suitable to use capillary heat exchanger;The long-term operation of the system is not conducive to the heat exchange between capillary and surrounding rock mass,the shorter the daily operation time of the system,the higher the heat exchange per unit area of the heat exchanger,the heat transfer efficiency of capillary heat exchanger during operation can be improved by adjusting the daily running time ratio.The work of this paper is a more in-depth study on the heat transfer performance of the Metro ground source heat pump system with capillary as the front-end heat exchanger,which provides a theoretical basis and design ideas for the popularization and application of capillary as the front-end heat exchanger in the metro tunnel ground source heat pump system. |
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