Experimental And Stimulation Study On The Unsteady Heat Transfer Of Vertical Casing-type Buried Pipes

The massive consumption of fossil fuels has caused great pollution to current environment.As one of the renewable energy,shallow geothermal energy has been widely used.As an effective technical means,ground source heat pump plays an important role in the utilization of shallow geothermal energy.Among them,the heat transfer performance of the ground heat exchanger directly determines the efficiency and stability of the ground source heat pump operation.At present,most researches on ground heat exchangers are focused on U-tube ground heat exchanger.In contrast,the casing-type buried tube heat exchangers has the advantages of simple structure,full use of drilling resources and large heat transfer rate.It is gradually being recognized and applied.However,there are relatively few experimental and numerical simulation studies on the heat transfer performance of the casing-type buried tube heat exchangers at present.To this end,this paper builds a sandbox experimental platform and establishes the three-dimensional mathematical model of casing-type buried tube.The unsteady heat transfer characteristics of the casing buried pipe type ground heat exchangers and the surrounding soil are studied through experiments and numerical simulations.It is expected to provide theoretical basis and specific guidance for the application and cognition of the casing-type buried tube heat exchangers.In this paper,a sand box is built for the heat transfer characteristics of vertical casing-type buried tube.Unsteady heat transfer characteristics of vertical casing-type buried tube were experimentally studied.The influence of factors such as inlet water temperature,inlet flow rate and operation mode of heat storage on the heat transfer characteristics of vertical casing-type buried tube and its surrounding soil was analyzed.The variation laws of soil temperature,average quantity of heat pet meter and average heat transfer coefficient around the casing-type buried tube were obtained.The experimental results show that the heat transfer per unit depth can be increased by increasing the inletwater temperature within the deformation range of buried pipe material temperature;The higher the water temperature at the fluid inlet,the more obvious the temperature change of the soil closer to the buried pipe;The increase of fluid inlet temperature and flow had no significant effect on the average soil heat transfer coefficient at the borehole;In the process of soil heat storage,the higher the flow,the higher the soil temperature;Alternate mode operation with 1:1 and 1:2,The interval running time is smaller,the greater the fluctuation of soil temperature around the buried pipe;In the continuous operation mode,the average heat transfer coefficient decreases first and then remains unchanged;Intermittent operation mode,the average heat transfer coefficient in 0 ~ 36(W/m ·℃)swings between,presents a from high to low gradually tending to zero fluctuate of cyclical changes;In the process of heat extraction,the soil temperature fluctuation in the radial distance is less affected by intermittent operation;Heat transfer per well depth in intermittent operation mode is higher than that in continuous operation mode;Compared with single heat storage or heat extraction,In the process of running alternately hot and cold,the average quantity of heat pet meter is larger and the average heat transfer coefficient is larger.Then,the mathematical model of unsteady heat transfer of three-dimensional casing buried pipe type ground heat exchanger is established,which is verified by the experimental data.On this basis,numerical simulation study of unsteady heat transfer process of vertical casing-type buried tube heat exchanger.The influence of factors such as diameter of pipe,methods of inlet and outlet,circulating medium,backfill material,initial temperature of soil and bottom spacing on the unsteady heat transfer characteristics of vertical casing-type buried tube heat exchanger is analyzed.The average quantity of heat pet meter,energy efficiency coefficient and soil temperature around the buried pipe are obtained.,and the thermal short circuiting of the casing buried pipe type ground heat exchanger is discussed.The numerical simulation results show that the outer pipe diameter remains unchanged,the larger the inner pipe diameter,the greater average quantity of heat pet meter.The unit well depth amount is larger under the mode of in out flow.The greaterthermal conductivity of the cycle medium is,the greater average quantity of heat pet meter.The lower the initial soil temperature,the lower average quantity of heat pet mete.The larger the inner diameter,the higher the initial soil temperature and the great the energy efficiency coefficient of the buried pipe heat exchanger.When the diameter combination is90/54,the thermal short circuiting is the most serious.Using calcium chloride solution as circulating medium can effectively slow down the thermal short circuiting.The larger the thermal conductivity of backfill material,the higher the soil temperature at the same radial distance.In the flow mode of outside in and inside out,the temperature has a large influence range in the radial distance direction.Under the calculation conditions in this paper,when the distance between the bottom of the inner pipe and the bottom of the outer pipe is 1.5m,the heat transfer effect of the fluid in the buried pipe and the surrounding soil is better.The greater the seepage velocity,the greater the heat transfer capacity of the buried pipe,and the phenomenon of heat accumulation along the seepage direction.