Heat Transfer Characteristics And Engineering Application Of Rock Mass At Source Side Of Buried Pipe In Cold Zone

Using clean energy instead of traditional coal for winter heating in northern China can greatly reduce CO₂emissions,thus helping to achieve the carbon peaking and carbon neutrality goals.Ground source heat pump technology is considered to be the most clean and environmentally friendly green energy utilization technology.In northeast China,the winter is long and the climate is cold.The heat load required by the building is much greater than the cold load.The cold and heat load extracted from the underground is unbalanced.With the continuous progress of drilling technology,the development and utilization of middle and deep geothermal has application value.The middle and deep ground source heat pump system can not only supplement the shallow ground source heat pump system to become the auxiliary heat source of the building in winter heating,but also can carry on the heat storage to the shallow stratum in the cross-season period.The combined application of deep/shallow ground source heat pump is of great significance to improve the efficiency of ground source heat pump system,and can provide a new idea for the use of geothermal clean heating for buildings in cold zones.In this paper,by means of theoretical analysis,numerical simulation,in-situ experiment and laboratory test,the heat transfer characteristics of shallow/mid-deep buried pipe and rock mass at source side under cold weather conditions are studied.By establishing a fluid-solid coupled unsteady numerical heat transfer model of buried pipe that can accurately reflect the actual geological conditions on site.The heat transfer model of buried pipe is coupled with heat pump unit model and building load model.The heat transfer law and influencing factors of buried pipes under different structures and operating conditions were studied.The management and control strategy of mid-deep/shallow joint application is developed.The main research contents are as follows:（1）A field test platform for shallow rock and soil thermal response is built.It is found that the 150m formation in the north of Changchun is dominated by clay layer and mudstone,and the surface clay gradually thinned from west to east.The average temperature of the formation ranges from 9.5℃to11.5℃.The temperature is low,showing a trend of high in the east and low in the west,high in the south and low in the north.Based on the linear heat source heat transfer model,the comprehensive thermal conductivity and specific heat capacity of shallow strata rocks were calculated by fitting the inlet and outlet temperature of buried pipes with the simulated temperature.（2）Based on the geometric average model of thermal conductivity,the cuttings thermal conductivity,cuttings powder porosity and in-situ formation porosity were organically combined.The saturated cuttings powder was tested by Transient Plane Source（TPS）method,so as to realize the prediction of in-situ formation rock thermal conductivity through drilling cuttings.The in-situ thermal conductivity of rocks at different depths in the northern part of Changchun was predicted by processing and testing 24 cuttings samples from different depths.（3）Include building load model is established,buried in heat and heat pump units dynamic performance simulation model of ground source heat pump system coupling model,the winter and summer dominance of winter heat load is studied under the action of unbalanced load,layered rock and soil in the ground source heat pump system under the long-term operational efficiency for soil temperature,in heat exchanger,such as change rule.It was found that the heat transfer rate of buried pipe and outlet temperature decreased significantly in the first week of heat pump operation,and then gradually stabilized.In the absence of adjustment measures,the temperature of rock mass at the source side of the heat pump keeps decreasing.At the end of 2010,the temperature of rock mass at the source side of the buried pipe is 4.71℃lower than the initial temperature.（4）Based on the analysis of the super-large axial diameter ratio structure of the mid-deep buried pipe and the characteristics of stratum stratification and geothermal gradient,a full-size unsteady numerical calculation model of fluid-solid-thermal coupling between the mid-deep buried pipe and the source-side rock mass under ultra-long operation conditions in cold climate was established.Systematically analyzed the well depth,flow rate,cementing cement thermal conductivity,internal and external pipe diameter ratio（casing type）,water outlet insulation depth（U type butt type）,start-stop ratio and other influencing factors on the heat transfer of buried pipe and rock mass temperature change.In the first week of heat pump operation,the heat transfer rate of buried pipe and ground temperature decreased the most,accounting for 80%.After one month of operation,the heat transfer tends to be stable,but the heat transfer radius continues to increase.At the end of the heating season,the heat transfer radius is about 11m.（5）In order to explore and establish a combined application model of deep/shallow buried pipes suitable for cold climate conditions.In this paper,the calculation model of cold and heat unbalance rate is given,and the combined ratio model of middle and deep buried pipes is established by simulating the heat storage of shallow buried pipes and the heat uptake of middle and deep buried pipes across seasons.The middle and deep buried pipes can be used as supplementary heat source and cross-season heat storage heat source for shallow buried pipes in heating season.（6）Based on the guarantee of underground temperature field heat balance and the principle of compound energy system efficiency and using the deep ground source heat pump heating conditions,air conditioning for working condition were in cooling priority strategy,working condition of the summer and transition seasons with deep ground source in vulcanizing hot,cold regional public building of heat sources joint operation,deep and shallow ground source heat pump in winter use with the boiler heating;In the transitional season,heat transfer in the middle and deep layer is used,and in summer,the combined cooling strategy of shallow ground source heat pump and plate exchange is used.And in summer,shallow ground source heat pump and plate exchange are used for combined cooling operation strategy.Based on this control strategy,a combined application control and monitoring software is developed in the cold zone.In this paper,the regional geology,buried pipe and heat transfer in geotechnical engineering,building energy efficiency and heating intelligent control operation technology integration,system integration technology,put forward in the cold area of shallow/mid-deep buried pipe joint application mode,realized the energy-efficient building and green building,exchanger of ground source heat pump technology in cold area has an important guiding significance to the popularization and application.This paper has 92 figures,43 tables and 150 references.