Performance Analysis And Research Of Buried Tube Heat Exchanger And Heat Pump System Based On Seepage Conditions

Ground source heat pumps use the low-grade shallow heat contained in underground rock and soil to realize heating in winter and cooling in summer.It has many advantages such as stability,high efficiency,environmental friendliness and wide adaptability.It is one of the effective energy-saving measures to alleviate the shortage of fossil energy and solve environmental governance problems.It has received increasing attention and application in the world.The ground source heat pump system realizes the heat transfer between the fluid and the soil through the buried pipe heat exchanger,and its heat transfer performance directly affects the utilization efficiency of geothermal energy.Under actual working conditions,the geological conditions around the buried tube heat exchanger are complex and changeable,and there are generally obvious stratifications,accompanied by groundwater seepage or flow,which affects the heat transfer capacity of the buried tube heat exchanger.Therefore,it is of great significance for the scientific design and optimization of buried tube heat exchanger to carry out systematic research on buried tube heat exchanger under soil stratification and seepage conditions.In this paper,based on the similarity theory,an equiratio reduction similar experimental bench based on the actual buried pipe heat transfer is constructed,and the influence of seepage field on the working characteristics of buried pipe heat exchanger in actual operation is studied.Studies have shown that the existence of seepage will strengthen the heat exchange between the buried tube heat exchanger and the soil,and accelerate the recovery of the surrounding soil temperature.And with the increase of seepage velocity and water flow velocity in the U-shaped tube,the heat exchange capacity of the buried tube heat exchanger becomes stronger and stronger;Among them,the heat transfer per unit linear meter of the buried pipe under the seepage velocity condition of 2.5×10^-3m/s is 13.7%higher than that under the condition of 2.5×10^-6m/s.Under the condition of water velocity in U-shaped pipe of1m/s,the heat transfer per unit linear meter of buried pipe is 16.70%higher than that of U-shaped pipe with water velocity of 0.6m/s.And increasing the temperature of the water inlet of the U-shaped tube in summer will further enhance the heat transfer capacity of the buried tube heat exchanger.Among them,the water temperature at the inlet of the U-shaped pipe is 35℃,and the heat transfer per unit linear meter of the buried pipe is 13.27%higher than that of the U-shaped pipe when the water temperature at the inlet of the U-shaped pipe is 33℃,and vice versa in winter.The heat transfer characteristics of buried tube heat exchanger inside the borehole of soil and buried tube heat exchanger were analyzed,and a theoretical model of buried tube heat exchanger suitable for complex seepage conditions was derived.Based on this model,a three-dimensional unsteady heat transfer model of the buried tube heat exchanger under seepage conditions was established in Feflow software,and the heat transfer characteristics and changing laws of the buried tube heat exchanger under different seepage velocities and seepage layer thickness conditions were analyzed.Studies have shown that seepage strengthens the heat exchange between the buried pipe heat exchanger and the soil,and the outlet water temperature of the buried pipe under the seepage condition of 2.5x10^-6m/s is up to6.3%higher than that under the condition of no seepage,and the effect is obvious.And with the increase of seepage velocity,the heat transfer capacity will be stronger.As the thickness of the seepage layer increases,the heat transfer capacity of the buried pipe will be stronger.For every 10 m increase in the thickness of the seepage layer,the outlet water temperature of the buried pipe increases by 0.126℃,and the heat transfer capacity becomes stronger and stronger.Based on the TRNSYS dynamic simulation software,a ground source heat pump system coupling model under seepage conditions was established,including the heat transfer model of buried pipes under seepage conditions,the heat pump unit model and the building load model,and the influence of seepage on the long-term operation performance of the ground source heat pump system was analyzed.Studies have shown that the COP of ground source heat pump systems is higher under seepage conditions.And as the seepage velocity increases,the COP of the ground source heat pump system will gradually increase.Among them,the system COP of the seepage velocity condition of 2.5×10^-3m/s is 1.45%higher than that of the 2.5×10^-6m/s condition in the 60-day period at most;it is 10.63%higher in the 30-year period.