Study On Ventilation Heat Transfer Characteristics And Application Of Underground Pipe Rack In Guangzhou City

With the development of social economy in China,the contradiction between energy demand and energy tension is becoming increasingly serious.In the context of the "dual carbon" policy,it is imperative to reduce the energy consumption of HVAC in buildings.At present,although the earth to air heat exchanger system that uses soil geothermal energy to precool and preheat the air has been developed more maturely,it also has some defects,such as poor thermal performance and single application mode.In order to make up for these shortcomings,this paper proposes to replace the existing earth to air heat exchanger system by using the urban underground pipe rack as the heat exchange carrier of soil and air,so as to effectively reduce the air conditioning energy consumption of buildings.In this paper,not only the thermal performance and thermal characteristics of underground pipe racks are discussed,but also the appropriate energy-saving application mode of pipe racks is proposed on this basis,and the energy-saving benefits of different modes are compared.Firstly,the thermal performance of the pipe rack in Guangzhou in summer and winter was tested.The pipe rack can effectively cool and warm the outdoor air in summer and winter respectively.In the experiment,the air temperature at the outlet of the 200 m pipe rack can be maintained at 29.5～30.6℃ and 19.1～20.9℃ in summer and winter,respectively.The temperature drop in summer and temperature rise in winter are 0.8～7.5℃ and 1.2～11.2℃respectively.The effective cooling time of pipe racks in summer is 9:00～18:00 every day,while in winter,when the outdoor temperature is low,the pipe rack can heat it all day.The energy supply of the pipe rack gradually increases with its length.The average daily cooling capacity of 150 m comprehensive trench and cable trench in summer is 55.1 k Wh and 62.9k Wh respectively,and the average daily heating capacity in winter is 514.6 k Wh and 532.1k Wh respectively.As the air temperature in the pipe rack in summer is too high to meet the thermal comfort requirements of the human body,it is recommended to use pipe rack by the method of "pipe rack fresh air+air conditioning system" or "pipe rack-air conditioning hybrid system".In winter,the air in the pipe rack can be directly used for building ventilation.On the basis of the thermal performance experiment of the pipe rack,a three-dimensional transient numerical model of the air heat transfer of the pipe rack is established by Fluent software,and the influencing factors of the thermal performance of the pipe rack in summer are analyzed by this model.The results show that the pipe rack length,the air velocity and the inlet air temperature have significant effects on the cooling performance of the pipe rack in summer.The longer the pipe gallery,the smaller the air velocity is,the higher the air temperature at the air inlet,and the more obvious the air temperature drop in the pipe rack.Because the temperature of the shallow soil in Guangzhou has little change,the burial depth has little influence on the thermal performance of the pipe rack.It is recommended that the length of the pipe rack for summer cooling in this area should not exceed 160 m,the buried depth should not exceed 4m,and the air velocity in the pipe rack should not exceed 3m/s.The24-hour dynamic simulation shows that the pipe rack can only cool the outdoor air at9:00～18:00.For different application modes of pipe rack energy saving,the refrigeration COP experiment was conducted on the "pipe rack-air conditioning hybrid system" in Guangzhou through the experimental test device built in the hot and humid climate wind tunnel.In this system,the precool air in pipe rack in summer as the cold source of air-cooled air conditioning can improve the energy efficiency of air conditioning and reduce the cooling energy consumption.The air in the 200 m comprehensive trench and cable trench of the pipe rack can increase the COP of air conditioner by 0.5～23.0% and 0.9～16.7% respectively,but the optimization effect only occurs when the air temperature in the pipe rack is lower than the outdoor air temperature(9:00～18:00),and the higher the outdoor air temperature,the more obvious the optimization effect.Since the energy saving period of pipe rack coincides with the peak period of cooling load and cooling energy consumption of urban public buildings,the system has the potential to reduce the installed capacity of building air-conditioning equipment and the cooling energy consumption of buildings in summer.Finally,the annual energy consumption of the application of "pipe rack fresh air+air conditioning system" and "pipe rack-air conditioning hybrid system" in small office buildings in Guangzhou is simulated and calculated by using Energyplus software.The results show that when the fresh air frequency of "pipe rack fresh air+air conditioning system" is 7 times an hour,the indoor air temperature of the office in summer can be reduced by 7.4℃ compared with that of natural ventilation,and the annual cooling load and cooling energy consumption can be reduced by 24.8% and 27.8% respectively.However,the room still needs air conditioning to further cool down,and the air in the pipe rack in winter can basically meet the indoor heating demand.The "pipe rack-air conditioning hybrid system" reduced the total cooling energy consumption of the office from May to September by only 182.0 k Wh,about7.9%,but the hourly energy consumption of the air conditioner can be reduced by 19.4%during the high outdoor temperature period(12:00～17:00).In contrast,when the ventilation times more than 3 times an hour,the overall energy saving of "pipe rack fresh air+air conditioning system" is more significant,and the "pipe rack-air conditioning hybrid system" has remarkable energy saving potential in high outdoor temperature periods.