Study On The Heat Transfer Of Spiral-Tube Heat Exchangers In Energy Piles

Ground source heat pump technology is a major form for the utilization of shallow geothermal energy.It is known as the most energy-saving and environmentally-friendly for space heating and cooling in the 20th century.Energy piles are an attractive and cost-effective solution to partly replace borehole heat exchangers for buildings with pile foundations and ground-coupled heat pump systems,which is a trend for the engineering applications of shallow geothermal energy.As a new type of ground heat exchangers,spiral-tube heat exchangers in energy pile have been used in energy pile projects in recent years.It is one of the hot spots for energy pile ground source heat pump system.In this work,the research method of combining theoretical analysis,numerical simulation and experimental research is used to systematically study spiral-tube heat exchangers in energy pile.Firstly,a laboratory experimental bench for the heat transfer characteristics of a spiral-tube heat exchanger in energy pile with was built.Based on the thermal response tests by using the constant heating power method,heat flow,fluid temperature,tube wall temperatures,pile wall temperatures and soil temperatures were obtained.Thetemperature response of an energy pile heat exchanger was acquired by the arrangement of a large number of high-precision temperature probes.The total equivalent thermal resistance of energy piles and equivalent sub thermal resistances were obtained through data processing and investigated deeply.At the constant heating power,the heat flow of energy pile first rapidly rises,gradually reaches a relatively stable value,and then fluctuates with the voltage.With the pulsed heating power,the heat flow increases rapidly in the start-up phase of heater;when the heater is stopped,the heat flow reaches a maximum value;after the heater stops,the heat flow decreases rapidly and then gradually decreases to the minimum value.When the fluid-pipe thermal resistance is at quasi-steady state,it is 1-2 orders of magnitude smaller than the total thermal resistance;the pile thermal resistance is 0.5-1 orders of magnitude smaller than the total thermal resistance.When operating is lasting for a short time,the pile thermal resistance occupies the largest share of the total thermal resistance.When operating is lasting for a long time,the soil thermal resistance occupies the largest share of the total thermal resistance.Increasing the thermal conductivity and volumetric specific heat of grout medium can effectively reduce pile thermal resistance,soil thermal resistance and the total thermal resistance.Therefore,improving the thermal physical properties of grout medium can effectively improve the heat transfer efficiency of energy pile heat exchangers.The soil with high thermal conductivity and high volumetric specific heat can effectively reduce pile thermal resistance,soil thermal resistance and total thermal resistance.Secondly,a two-dimensional axisymmetric model(the ring-tube heat source model),which could replace 3D numerical heat transfer model under certain conditions,was developed with the coupled boundary conditions between fluid and pipe and solved by using the finite element method.The simulation by means of the ring-tube heat source model could provide the temperature responses of working fluid,pipe,pile and soil.The fluid temperature comparison between the simulation by using the proposed model and the test data indicates that the ring-tube heat source model is valid and feasible.The comparison between the proposed model and 3D numerical model indicates that the simulation with the ring-tube model maintains the basic heat transfer characteristics of 3D heat transfer model.The equivalent sub thermal resistances were obtained,including fluid thermal resistance,pipe thermal resistance,pile thermal resistance and soil thermal resistance.The soil thermal resistance and the pile thermal resistances increase with time when energy pile is undertaking a heat flux.The pipe thermal resistance and fluid thermal resistances can be considered as stable when Fo>0.002.The stable fluid thermal resistance is 1-2 orders of magnitude smaller than the total thermal resistance when Fo>0.01.The stable pipe thermal resistance is one orders of magnitude smaller than the total thermal resistance when Fo>1.Furthermore,the parametric analysis on the short-term thermal resistances of spiral-rube heat exchangers in energy piles was performed,including the relative height of spiral-tube,the curvature of spiral-tube,Reynolds number of working fluid and the relative thermal diffusivity.Both pile thermal resistance and pipe thermal resistance increases with an increase of the relative height of piles.The large radial scale heat source and small length-to-radius ratio helps to improve the heat transfer performance of energy piles.The pile,pipe and fluid thermal resistance decrease with an increase of the curvature of spiral-tube.It should be due to that an increase of curvature lead to an increase of both the pipe heat transfer area and heat capacity amount of working fluid.The pile thermal resistance decreases with an increase of grout thermal diffusivity.Improving the thermal diffusivity of backfilled materials can significantly reduce the pile thermal resistance and total thermal resistance for energy piles,especially when the thermal diffusivity of backfilled materials is less than that of soil.The total thermal resistance only decreases slightly with an increase of Reynolds numbers of working fluid.Enhancing the convective heat transfer of working fluid in pipes helps little for improving heat transfer performance of energy pile.Thirdly,based on the accurate temperature response of energy pile heat exchangers by using transient 3D heat transfer model,the influence of the type or the pitch of heat exchange tube on the heat transfer charactiritics of energy piles.G-functions of average fuid temperagure are fitted from the simulation and used to calculate COPs of heat pump.The influence of spiral pitch was analyzed under different design conditions.A transient 3D heat transfer model was established in combination with heat conduction model in solids and non-isothermal pipe flow model.The long-term temperature response of the 3D model is verificated with the ring-coil heat source model while the short-term temperature of the 3D model is verificated with thermal response test.An in-situ test has been performed so as to obtain temperature response profiles of a spiral-tube energy pile heat exchanger.The thermal behavior in the temperature response test have been predicted by using the established model.The simulated results are in good agreement with the experimental results.A comparative analysis of heat transfer processes for three types of heat exchangers(U-tube,W-tube and spiral-tube)in foundation piles was conducted.The changes in temperature and equivalent thermal resistances as well as the heat transfer rate on the surface of energy piles were analyzed so as to conduct a thermal performance comparison.Numerical results show firstly that the average external thermal resistance almost the same and is independent of tube types of energy piles,and secondly that the spiral-tube heat exchanger gives the lowest temperatures in working fluid and the minimum internal thermal resistances under the same initial conditions and boundary conditions.In particular,the exhausted heat between the energy piles and the ground is not much difference for the three heat exchangers.In addition,the change distributions in temperature in pile for the spiral-tube heat exchanger are uniformly distributed compared with the other two heat exchangers.Therefore,the spiral-tube heat changer gives the better thermal performance than the other two heat exchangers in the context of long-term and short-term thermal load due to the longer pipeline,larger heat capacity of working fluid and geometrical arrangement form.Then the 3D numerical model was used to study the influence of spiral pitch on the thermal behaviors of a spiral-tube heat exchanger in energy piles.The heat transfer process of spiral-tube energy piles with four kinds of pitch(SP=0.25,0.5,1.0,2.0)was simulated and analyzed.The total thermal resistance and the sub thermal resistances were obtained for the four piles.The change in fluid temperature and internal thermal resistance diminishes but the change is narrowed down as spiral pitch decreases.The total heat transfer rate on the surfaces of piles is almost same for the four piles,and the remained heat in both the pile body and working fluid should be almost as much.However,the spiral-tube with a small pitch could provide a better uniform distribution for both the temperature and heat flux in pile.The G-functions of fluid temperature(gf)for the four piles were proposed,and the fluid temperature and COP were simulated based on superposition method and G-functions.As spiral pitch decreases,the difference between the extreme fluid temperatures diminishes and the mean COP increases for both heating and cooling.A spiral-tube energy pile with a small pitch is energy-efficient and has a better applicability.An increase of spiral pitch(SP=0.25,0.5,1.0,2.0 sequentially)results in a reduction of the mean COPs from 0.77%to 16.49%according to the influential factors(the undisturbed ground temperature and the different air conditioning loads).A decrease of spiral pitch could improve the energy efficiency and the applicability of a ground-coupled heat pump system by taking water or anti-freeze fluid with a low concentration to be the working fluid.Finally,based on a large-scale ground source heat pump system in Jinan,an on-site thermal response test bench for full-scale energy piles and borehole heat exchangers was established.The thermal behavior of ground heat exchangers under contant heating poower and operation is meaningful for system design and energy efficient of GCHP systems.Four thermal performance tests by means of the constant heating power method were conducted to compare the thermal behavior of spiral-tube energy piles and double U-tube borehole heat exchangers under constant heating power.An operation test for space heating were carried out based on the ground-coupled heat pump system to compare the operation thermal behavior of spiral-tube energy piles and double U-tube borehole heat exchangers.Equivalent thermal resistances,fluid temperatures,heat transfer rates,COP and concrete temperature were analyzed.Equivalent thermal resistances of spiral-tube energy piles and double U-tube borehole heat exchangers were derived from the test data of thermal performance tests.The equivalent thermal resistance of spiral-tube energy piles is much less than that of double U-tube borehole heat exchangers.The thermal response of spiral-tube energy piles is more rapid than that of double U-tube borehole heat exchangers.The heat transfer performance of spiral-tube energy pile is much superior to that of double U-tube borehole heat exchangers.A larger radial scale of heat source,a small aspect of length to diameter and a long heat exchange tube is helpful to improve the heat transfer efficiency of ground heat exchangers.A fluctuating heat transfer rate and fluid temperature with variable amplitude for ground heat exchangers were observed from the operation test.The amplitudes and operating frequency of fluid temperature and heat transfer rate should be determined by the heat capacity of heat pump and thermal loads of building.The amplitudes and operating frequency of fluid temperature and heat transfer rate should be affected by the control parameter of heat pump.For the case in this work,the average equivalent thermal resistances of double U-tube borehole heat exchangers are about 5.65 times as much as that of spiral-tube energy piles.The heat extracted by the spiral-tube energy piles over the average depth of 27 m is a little higher than that by the double U-type borehole heat exchangers over the average depth of 90 m.Between the periods of pseudo steady status for space heating,the peak heat transfer rate per unit meter length of energy piles is about 3.65 times as much as that of borehole heat exchangers,while the peak heat transfer rate per meter length of pipe for energy piles(dn25)is about 1.32 times as much as that for borehole heat exchanger s(dn32).The concrete temperature for energy piles increases gradually while thermal load becomes small.The minimum concrete temperature is 5.05 ℃ lower than the initial soil temperature;when heating is over,the concrete temperature is reduced of 3.24℃.If the energy piles do not work for a night,the concrete temperature could be recovered about 0.65 ℃.After space heating is over and after a 15-days recovery period,the concrete temperature is 1.7 ℃ lower than the initial soil temperature.In this work,the equivalent sub thermal resistance of spiral-tube heat exchangers in energy pile and its evolution law were obtained by experimental means,which provide a theoretical basis for the simplification of numerical heat transfer model and the analysis of heat transfer characteristics.The ring-tube heat source model is accurate and is a practical tool for the analysis of technical problems and engineering design of spiral-tube energy piles.Based on the simulation with 3D heat transfer model,G-function of average temperature of working fluid is proposed which optimize the design method of spiral-tube heat exchangers in energy pile.In addition,the analysis of influences of buried pipe type and spiral pitch and the analysis of influence of pitch on COP of heat pump are meaningful for the engineering design of energy pile heat exchangers.The different time scale thermal response experiments of full-scale energy piles and borehole heat exchangers is significant for the engineering design and operation strategy of energy pile heat exchangers.