Experimental Study On Heat Transfer Performance Of Buried Pipe Heat Exchanger With Twisted Tape Inserts In The Backfill Materials Of Sand/graphite

Shallow geothermal energy as renewable energy plays a crucial role in buildings,and the ground source heat pump system is one of the key technologies in the field of heating and air conditioning systems.The main purpose in this current work is to examine the characteristics of flow and heat transfer of the U shaped twisted tube and smooth tube based on a lab-scale setup,respectivly,which is considered the influence of the inlet temperature,Reynolds number,variety of backfill materials and the twisted tape inserts.In addition,the moisture and thermal migrations in sand/graphite along with polyacrylamide(PAM)additive are addressed through another setup.The thermophysical properties of sand/graphite are also determined.The main conclusions are shown as follows.The thermal conductivity and the specific heat capacity of the samples increased with increasing the moisture content,while with the increase of the moisture content,the thermal diffusivity increased first and then decreased.Adding 5%graphite to the sand,the average thermal conductivity and the thermal diffusivity of sand increased 42.58%and 46.03%,respectively.The relationships between the three thermophysical properties and moisture contents of sand and sand/graphite blend were obtained.The experiment of heat and moisture transfer shows that the final temperature distribution and the range of temperature transfer increased with the increase of the temperature of heat source and initial moisture content.Adding 5%graphite to the sand,the temperature of the sample increased 2.88%and the range of temperature transfer increased about 20%.Adding 0.3%PAM to the sand/5%graphite blend,the average moisture of the sample increased 1.04%.The final temperature of the sample increased 0.61%after Adding 0.3%PAM to the sand/5%graphite because of the water retention ability of the PAM weakened the moisture transfer of the sample.The experiment of the characteristics of buried pipe heat transfer shows that when inserted the twisted tape in smooth tube,the heat transfer coefficient of the twisted tube ranged from 442 W m-2K-i to 897 W m-2K-1,and the average heat transfer and heat transfer coefficient increased 8.2%and 15.6%,respectively.When the inlet temperature is 70?,the average pressure drop between the inlet and outlet of twisted tube increased 135.26%compared to the smooth tube with the Reynolds number increased from 3000 to 9000.And the heat transfer enhancement index of the twisted tube decreased 9%compared to the smooth tube and the smooth tube had a better performance.Adding 5%graphite to the sand as the backfill materials,the temperature of the soil increased 7.59%,the average heat transfer increased 28%.The main thermal resistance of the buried pipe heat exchanger system was consist in the outside of the buried pipe heat exchanger.Improving the thermal performance of the backfill materials can greatly improve the heat transfer efficiency.This paper is of significance to evaluate the key influence factors of the heat teansfer performance of the buried pipe.