| With the development of the strategy of"Powerful Transportation",the construction of transportation infrastructure in the north cold region develops rapidly.As the main form to overcome the terrain obstacles,the number of tunnel projects increases dramatically.The underground tunnel is directly connected with the external atmospheric environment at the entrance and air shaft.In winter,a large amount of outdoor low temperature air intrudes into the tunnel,which is easy to cause tunnel lining cracking and ice covering,resulting in freezing and cracking of facilities and equipment such as fire pipes and drainage pipes.Therefore,the thermal environment and anti-freezing of facilities and equipment at the entrance of underground tunnels in cold region have been paid much attention and need to be studied and solved urgently.In this study,the thermal environment and electric heat tracing insulation of underground tunnel openings in cold region through theoretical analysis,field test and laboratory test.Firstly,the heat exchange mode at the entrance of underground tunnel is analyzed,and the boundary conditions of the heat exchange process are simplified,and the differential equation of temperature distribution at the entrance of underground tunnel is derived.The thermal environment of the underground tunnel in the cold area of north China was tested,and the variation law of the air temperature at the entrance of the underground tunnel with distance and time was obtained.The test results were fitted to solve the air temperature distribution equation Tf at the entrance of the tunnel.On the basis of the daily average temperature,the hourly air temperature distribution equation Tfzs was established,and the two equations were verified.The variation trend of the predicted value is consistent with that of the tested value,which has a certain guiding significance for the freezing prevention of the facilities and equipment in the tunnel entrance section.The test results of the underground tunnel show that the lowest temperature in the tunnel every day occurs from 0 to 6 at night,when there is no train running.In addition,the thermal environment of different subway tunnel openings in the same area is obviously different.When there is a protective structure at the tunnel entrance,the direct connection between the tunnel entrance and the outside atmosphere is avoided,and the radiation heat dissipation to the sky at winter night is weakened.The thermal environment in the tunnel is better than that of the tunnel entrance without protective structure.In order to further reduce the operating energy consumption of the electric tracing heat preservation system in the tunnel,laboratory tests were conducted to study the time when the water temperature of the fire pipe dropped from 5℃to0℃under different ambient temperature conditions,and the cooling time of all experimental conditions was within the theoretical calculation time range.In addition,when the water temperature is greater than 3.6℃,the wall temperature of the fire hose is close to the water temperature;when the water temperature is lower than 3.6℃,the wall temperature is about 0.3℃lower than the water temperature.The higher the water temperature in the fire hose is,the more heat it releases to the external environment.Therefore,reducing the start and stop temperature of the electric tracing heat insulation system helps reduce the system heat loss.It is recommended that the start and stop temperature of the electric tracing heat insulation system be set to 0°C to 5°C.It can not only reduce the power consumption of single operation,but also reduce the start-stop frequency of the system.The research can provide technical guidance for the control of the thermal environment at the entrance of underground tunnels and the design of the operation of the electric tracing thermal insulation system,so as to achieve the goals of safe operation,reducing operation risks,energy consumption and reducing operating costs. |
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