Research On Thermal Deformation And Lose Efficiecy Of Underground Heat Exchanger

More and more attention were paid to energy of earth superficial layer which is theimportant form of renewable energy sources, among them, ground source heat pump(GSHP)and underground thermal energy storage(UTES) have been the most important appliedtechnology nowadays. Under use of earth energy, heat exchange pipe of underground heatexchanger and around soil pipe-soil structure are important parts of heat exchange system, itnot only involved in complicated heat exchange, mass exchange of underground structure,and also involved in thermal strain issues of heat exchanger pipe soil structure and therebygenerated heat exchange decay and lose efficiecy issues. The cold and hot change conditionunder long-time operation further intensify thermal deformation and pipe-soil gap, pipesqueezing and curve, especially possible the certain deformation impacts from local defectof pipe-soil structure. Especially, low temperature frost heave under0℃or severe thermalstain under high temperature, their worsen impacts for underground heat exchanger alwaysrestrict use efficiency and reliability of UTES, so the research on deformation and loseefficiency of underground heat Exchanger need to be done without delay.In view of the limited acquaintance of thermal deformation and lose efficiency ofunderground heat exchanger in domestic and overseas, this dissertation combined withNational Natural Science Foundation of China (NSFC) to carry out related basic applicationresearch. By the methods of experiment research, theoretical analysis and numericalcalculation, research on multiple factors effects and influences of underground heatexchanger thermal deformation was carried out.We can know more about mechanism andcharacteristic of deformation, gap and pipe-soil displacement, and characters of heatexchange and influencing roles. By further explore the issues of pipe structure frost heaveunder low temperature,the basic characters of frost heave was studied, it laid the foundation of engineering application.By analysis of mechanics and heat transfer theory, the basic control equation of thermaldeformation and heat transfer of pipe-soil structure was established; the relationship of heatconductance and contact surface gap value was defined. Elastic thermal deformationcalculation model of heat exchange pipe and elastic-plastic thermal deformation-heattransfer coupling for pipe-soil structure calculation model and of contact surface interactionfriction model were established, contact surface disperse, contact tracking method andcontact status algorithm were introduced in the process of structure deformation, implicitdeformation displace and unsteady heat exchange coupled algorithm was adopted, theanalytical and calculate method of elastic-plastic thermal deformation-heat transfer couplingfor pipe-soil structure was put forward, by experimental verification, the model andcomputing method got good inosculation.Under research on thermal deformation aberrance of underground heat exchangerpipe-soil structure, experimental analysis system was built innovatively, visuallymeasurement analysis method of underground heat exchanger thermal deformation, strainidentify of thermal deformation, identify method of gap and heat transfer impacts were putforward. The experiment systematically verify the effectiveness of computational analysismodel, and carry out sufficiently basic characters, influencing roles and effective controls ofheat deformation of underground heat exchanger pipe-soil structure, this research laid thefoundation for engineering application.Based on engineering practice, analysis examples choose two types initial forms of heatexchange pipe; they are plane bending and space bending which carried out basiccharacteristic analysis of pipe-soil thermal straining and thermal variation. This researchrespectively established plane and spatial continuous bending calculation model of pipe-soilstructure, carried out thermal deformation displacement of pipe-soil structure and unsteadyheat transfer numerical coupling calculation, proceeded influence and distribution characterof pipe-soil structure heat deformation for gap evolution, temperature change with time andtemperature difference analysis of contact surface, explained gap generation mechanism and heat transfer attenuation phenomenon. Calculated and analyzed the influence on gaps underdifferent temperature operation condition, center distance of pipe, elasticity modulus andheat expansion coefficient of heat exchange pipe, and friction coefficient of pipe-soilstructure, defined influence and roles characteristic of many complex factors. Preliminarymodel and method of frost heave deformation of pipe-soil structure were established. Bycalculation examples, basic characteristics of frost heave deformation was analyzed, it laidthe main foundation for further carrying out the research on frost heave deformation, gapsand heat transfer variation under low temperature environment of pipe-soil structure.Under the calculation and analysis determinate platform, combining with engineeringpractice, the research on basic characteristics of thermal strain, structure shape change,displacement, gap and heat transfer influence was carried out, the possible causes of twokinds typical structure defects which are pipe local ovalization and large camber bending ofheat exchange pipe were explored, failure issues causing by thermal deformation in theprocess of long-term large temperature change was elaborated. The research carried outsystematically the influence of initial ellipticity on pipe body oval area thermal deformationand circulation area, the influence oval area length and pipe-soil structure parameters onthermal deformation of pipe ovalization area. The influence of thermal deformation of largecamber bending pipe-soil structure on evolution and development of pipe-soil gap, variationcharacteristic of contact surface temperature time and temperature difference in the processof heat transfer were analyzed, the foundation for engineering application was laid and thetheoretical reference frame was supplied.