Study On Heat Transfer Characteristics Of Ground-source Heat Pump System In Utility Tunnel

In order to conform to the sustainable development strategy,reduce energy consumption and environmental pollution,the application of renewable energy plays an important role at home and abroad.Among them,the ground source heat pump(GSHP)technology has been widely used.It can use the soil to store heat,achieve cross-season use of heat effectively,energy saving and pollution-free.In recent years,a new type of ground source heat pump system called energy underground structure is booming.The principle is to arrange buried pipes in the underground building structure to form the underground heat exchanger together.Energy underground structure not only inherits all the advantages of traditional ground source heat pump system,but also has the advantages of saving underground space and system cost.At present,there are four kinds of energy underground structures: energy pile,energy diaphragm wall,energy tunnel and the energy utility tunnel.Among them,there are many studies on the first three structures,while the research on the energy utility tunnel is still limited to the composition of the system.In this article,the heat transfer performance of buried pipe heat exchanger is studied and analyzed.The results of research can provide reference value for the practical engineering application of energy utility tunnel in the future.In this paper,the two-dimensional mathematical and physical unsteady heat transfer model for buried pipe heat exchanger in utility tunnel is established.In order to verify the feasibility of the ground-source heat pump system in utility tunnel,select five representative cities from five climate divisions in China,respectively,Urumqi in the severe cold area,Xi'an in the cold area,Chengdu in the hot summer and cold winter area,Guangnan in the hot summer and warm winter area and Kunming in the warm area.The outlet water temperature of buried pipes in different cities was obtained by simulation calculation with using CFD software FLUENT,and the heat transfer per unit length of buried pipes was calculated.When the buried pipes absorb or release heat in the concrete structure of the utility tunnel,it is calculated that the temperature of the air in the utility tunnel increases by 1~2 ?,which does not exceed the maximum air temperature to open the exhaust fan in utility tunnel in ?Technical code for urban utility tunnel engineering?.This shows that the heat transfer of the buried pipe has little effect on the temperature of air in the utility tunnel,and also confirms the feasibility of the ground-source heat pump system in utility tunnel.Secondly,taking the city-Xi'an in cold area as an example,the influence factors of heat transfer performance of buried pipes are analyzed by single factor analysis method.Including thermal conductivity of concrete and soil,length of buried pipes,pipe spacing,inlet temperature of fluid and flow velocity,etc.The effects of various factors on heat transfer performance of buried pipes and temperature in utility tunnel are calculated.Meanwhile,the optimum of the fluid velocity in buried pipes and the combination of length and spacing of buried pipes are given,that is,the flow velocity is 0.6 m/s,the pipe spacing is 0.75 m and the pipe length is 67 m,it provides basic reference data for subsequent research and calculation.Finally,taking Xi'an as an example as well,a year-long simulation of the buried pipe heat exchanger in utility tunnel is carried out.The year-round operation characteristics of buried pipes are calculated.And the changes of temperature field of surrounding rock in different seasons can be seen intuitively from the changes of temperature cloud chart and temperature values of soil measurement points.In addition,the influence of long-term operation of buried pipe in concrete structure on air temperature in the cabin of utility tunnel is judged by heat transfer between air and inner wall of utility tunnel.It is concluded that the long-term operation of ground-source heat pump system in utility tunnel has no effect on ventilation system in utility tunnel.All in all,this paper confirms the feasibility of the ground-source heat pump system in utility tunnel,then further studies and analyses the heat transfer performance of the buried pipe heat exchanger,which has certain theoretical significance and engineering reference value.