Study On Heat Transfer Characteristics Of Shell-and-tube Spiral Underground Heat Exchange

As a renewable energy technology,ground source heat pump can make full use of underground rock and soil for heat exchange,meet the needs of indoor refrigeration and heating,and more energy saving and environmental protection than traditional HVAC systems.In order to enhance the heat exchange capacity of the buried tube heat exchanger and make more reasonable use of soil heat transfer,this paper studies the heat exchange characteristics of the buried tube heat exchanger from two aspects: the comparison between the buried tube heat exchanger and the traditional single U and double U buried tube heat exchangers and the influence of nanofluids as heat exchange medium on the heat exchange capacity of the buried tube.(1)This study built a small vertical buried pipe heat exchanger sandbox experiment platform,studied the heat exchange characteristics of the casing spiral buried pipe and the single U and double U buried pipe heat exchangers,and the effects of different inlet temperature and inlet flow rate on the temperature difference between the inlet and outlet of the buried pipe heat exchanger,the heat exchange per unit well depth,the surrounding rock and soil body temperature and the average heat transfer coefficient.The results show that when the inlet temperature is 35℃ and the pipe flow rate is 0.5m/s in summer,the heat transfer per unit well depth of the casing spiral buried pipe is increased by 75.1% and 46.5% compared with the single U and double U buried pipe heat exchangers,respectively.The heat transfer capacity can be enhanced by changing the flow rate of the medium in the buried pipe heat exchanger.In a certain flow rate range,the heat transfer per unit well depth gradually increases with the increase of the flow rate.Increasing the water inlet temperature of the heat exchange medium in the buried tube heat exchanger can increase the temperature difference between the buried tube heat exchanger and the rock and soil mass,and improve the heat exchange capacity of the buried tube heat exchanger.In the process of buried tube heat transfer,the temperature change of the surrounding rock mass of the double-U and double-U type buried tube heat exchanger is higher than that of the single U type.The average heat transfer coefficient of the bushed spiral buried tube is higher than that of the single U and double U buried tube heat exchangers.(2)Through the establishment of the numerical model of the tubular spiral buried tube heat exchanger,the influence of the nanofluid as the circulating fluid in the tube on the heat exchange characteristics of the tubular spiral buried tube is studied,and the influence of the nanofluid on the heat exchange characteristics of the tubular spiral buried tube heat exchanger is analyzed from the three aspects of the type,flow rate and concentration of the nanofluid.The numerical simulation results show that the heat transfer per unit well depth of buried pipe increases gradually with the increase of flow rate,and the heat transfer per unit well depth is almost unchanged when the flow rate is higher than 0.75m/s.When the volume fraction of the nano-fluid increases,the heat transfer per unit well depth of the buried pipe increases gradually.When using Cu O,water and water based heat exchanger with volume fraction of 4vol.% as heat exchange medium,the unit well depth heat exchange is increased by 22.14%,14.18% and 15.36%,respectively,compared with water used as the casing spiral buried pipe heat exchanger.In this paper,through the sandbox experiment,the heat exchange characteristics of the casing spiral buried pipe and the traditional single U and double U buried pipe heat exchanger are compared and analyzed,and then the numerical simulation is further studied.The effect of circulating fluid in spiral tube on heat transfer capacity of tube tube heat exchanger with nano fluid provides a basis for further research on the heat transfer characteristics of tube tube.