Field tests of the thermal response of an energy utility tunnel
[Objective]Energy utility tunnel is a new type of energy underground structure based on a soil-source heat pump system buried-pipe heat exchanger and underground utility tunnel.Currently,the thermal response mechanism of an energy utility tunnel is unclear,specifically the heat transfer efficiency and thermal stress of the energy pipe corridor floor.Based on the comprehensive pipe corridor project of Longyuan Road located in Jiaozuo City,a thermal response test was conducted under various test conditions to discuss heat transfer and mechanical properties of the energy pipe corridor.[Methods]Heat exchange tubes were laid at the bottom of the pipe corridor to form an energy pipe corridor.The inlet and outlet water temperatures and the temperature and strain of the pipe corridor floor were measured.Subsequently,heat exchange performance and mechanical properties of the pipe corridor floor were discussed.[Results]The average initial temperature of the energy pipe gallery floor at a depth of 7 m was approximately 21.4℃ in summer and 12.4℃ in winter.Moreover,the initial average temperature of the soil layer under the bottom floor was approximately 20.2℃ in summer and 13℃ in winter.The maximum thermal compressive stress under the heat removal condition in summer is 1.35MPa,and the maximum thermal tensile stress under the heat extraction condition in winter is 0.89 MPa,both did not exceed the strength value of concrete in the pipe corridor bottom plate.Inlet water temperature increased from 30℃ to 35℃,and heat transfer power increased from 22 W/m to 28.7 W/m,resulting in a heat transfer power increase of approximately 30％.Moreover,when the flow rates were 300 L/h,600 L/h,and 900 L/h,heat transfer power were 14.6 W/m,29.3 W/m,and 28.7 W/m,respectively.Compared with a continuous operation,an intermittent operation increased the heat transfer power from 30.9 W/m to 36.9 W/m on the second day and from 30.6 W/m to 35.4 W/m on the third day.When the initial average temperatures were 21.3℃ and 12.5℃,the heat transfer power were 23.9 W/m and 51.6 W/m,respectively.The heat transfer power of winter heating conditions was 14.3 W/m,and that of summer heat removal conditions was 22 W/m.[Conclusions]Field test results show that the temperature at various locations of the base plate of the corridor is the same in the process of heat transfer;however,temperature stress is different.The transverse temperature stress is greater than the longitudinal temperature stress,and the transverse temperature stress gradually decreases on moving from north to south.Furthermore,the longitudinal temperature stress is greater in the center and lesser on both sides.The heat transfer power decreases with the test time extension and gradually stabilizes,and the heat transfer power fluctuates greatly in the first two days;therefore,the test duration should be more than 48 h.The heat transfer power increases with the increase in water inlet temperature.Increasing the flow rate can improve the heat transfer power;however,a large flow rate can make the heat transfer insufficient,resulting in a decrease in the heat transfer power.Thus,an intermittent operation can ensure higher heat transfer power compared with that during a continuous operation.However,even when the operation time was extended,heat transfer power continued to decline compared with that on previous day.Therefore,the energy pipe gallery floor is more suitable for summer cooling.
energy utility tunnelthermal responsefield testheat transfer power
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