| When the subway runs in a tunnel,a large amount of waste heat is generated.Long term heat accumulation leads to the temperature in the tunnel too high and the underground heat balance is destroyed.Therefore,it is a widely concerned topic to solve the heat accumulation in the tunnel and make reasonable use of it.The subway source heat pump can not only recover the waste heat of tunnel,but also transmit the heat to the ground building for heating.Many application researches have been made at home and abroad.Compared with the traditional buried pipe heat exchanger,the subway source heat pump with capillary as the front-end heat exchanger proposed by this research group has the advantages of convenient construction and large heat exchange area.At present,the theoretical research of heat exchanger and heat pump system under unsteady heat transfer condition is generally to solve analytical solution and simplify modeling.When the heat transfer process is complex,a series of problems such as large calculation amount,difficult solution and long time are not suitable for practical engineering with changeable conditions,and can not be adjusted flexibly according to timely boundary conditions.Therefore,in this paper,according to the idea of thermal power analogy,a model of resistance capacity of unsteady capillary heat exchanger is proposed.Under the condition that the heat transfer process and boundary change law have been defined,the heat flux density in different positions in the tunnel is calculated quickly,and the heat attenuation and delay prediction are predicted,which greatly enhances the practicability of the existing simplified model,and can be feedback in time in engineering.It is more conducive to energy conservation by regulating and controlling.Specifically,in this paper,for the subway source heat pump system with capillary as the front-end heat exchanger,because the capillary heat exchanger is laid in the circumferential part of the tunnel in the actual project,the arc length is small,the tunnel part of the embedded heat exchanger can be approximately simplified as a semi infinite flat wall.Based on the theory of resistance capacitance model of unsteady heat transfer,combined with the thermal parameters of each layer in the tunnel,the star resistance capacitance model of capillary heat exchanger is established to study the heat transfer performance of capillary.Matlab is used to program the calculation process of the model,and the model is verified by the measured data of the demonstration project.The results show that the maximum error of capillary outlet temperature is 4.6%,and the accuracy is good.According to the measured and simulated boundary conditions,the star resistance capacity model of the capillary heat exchanger is substituted to simulate the heat transfer process between the capillary heat exchanger,tunnel air,the materials and surrounding rock in the tunnel.The change law of the capillary heat transfer performance under the existing conditions is studied.The capillary laying area is 2900m~2 and the total flow in the front heat exchanger is 25 m~3/h.The maximum heat exchange per unit area in capillary cooling season is 55.27 W/m~2 and the maximum heat release per unit time is 160.28 k W;the maximum heat exchange per unit area in heating season is 38.33 W/m~2 and the maximum heat absorption per unit time is 111.15 k W.Compared with the heat flux density of different depth,it can be seen that when the depth of surrounding rock increases,the thermal resistance heat capacity increases,the smaller the heat flux density value,the smaller the amplitude of fluctuation,the more gentle the trend and the longer the delay time.The influence of the two factors on the heat transfer is analyzed.The comparison condition of the tunnel air temperature with the same temperature difference and the capillary water inlet temperature is set.According to the calculation results of the heat flux density,it can be concluded that the influence of capillary water temperature on the surrounding rock temperature is greater than that of the tunnel air temperature on the surrounding rock temperature. |
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