Study On Thermal Properties Of Lateritic Clay And Its Influence On Heat Transfer Performance Of Vertical Buried Pipe In Karst Area

Guangxi province is an important strategic hub of the "one belt and road" in China,and future development and construction will have great demand for energy.It is of great realistic and strategic significance to actively promote the popularization and application of ground source heat pump technology in this area.Karst topography is widely distributed in Guangxi province,and thus it forms many typical karst areas.Lateritic clay is the most widely distributed cohesive soil in karst area of Guangxi province.It contains special mineral composition,cementitious material and microstructure,which makes its hydraulic,mechanical and chemical properties more complicated than ordinary clay.Moreover,the karst groundwater in this area is very rich,the groundwater level is shallow,and there is usually obvious groundwater flow in rainy season.Due to the large buried depth of the vertical heat exchangers of ground source heat pump system,there are many challenges in the promotion and application of the vertical buried pipe under the complex stratum and hydrological environment.Therefore,it is necessary to study the influence of karst groundwater seepage and lateritic clay layer on the heat transfer performance of ground source heat pump system.In this paper,measurements of soil thermal property,mercury intrusion porosimetry(MIP)tests,scanning electron microscope(SEM)tests,model tests,numerical calculation and simulation were conducted to study four basic problems of "thermal properties of lateritic clay","predicting models for thermal property of lateritic clay","effects of karst groundwater seepage" and "coupling effects of heat and moisture transfer in lateritic clay".Thermal properties of lateritic clay in karst area and its influence on the heat transfer performance of vertical buried pipe and its mechanism were comprehensively revealed.The main contents and conclusions are as follows:1.A series of measurements on thermal properties of undisturbed and compacted lateritic clay were carried out by KD2 Pro thermal characteristic analyzer.It was found that the thermal conductivity of undisturbed lateritic clay was significantly affected by spatial variability,while that of compacted lateritic clay was more homogeneous.At the same volumetric water content,the thermal conductivity and diffusivity of undisturbed specimens were higher than those of compacted specimens,while the volumetric specific heat capacity of undisturbed and compacted specimens was basically the same value.Thermal property of lateritic clay is also related to the preparation method of specimens.Thermal conductivity of specimens in drying was higher than that in wetting due to hysteresis effects,and the deformation of the undisturbed specimen was obviously higher than that of the compacted specimen.The micro morphology and pore structure of undisturbed and compacted specimens were tested by SEM and MIP techniques.It was found that the microstructure units of Guilin lateritic clay were mainly composed of debris,particles and flakes,and some of the stacks were connected by cementation materials,which led to the blurring of the structure units.Results from SEM and MIP tests show that compacted Guilin lateritic clay exhibits bimodal pore size distribution(PSD),and the pore sizes are mainly distributed in the range of 0.01-1 ?m and 1-100 ?m.The undisturbed specimens display essentially unimodal PSD,which mainly distributed in the range of 0.01-1 ?m.For undisturbed and compacted lateritic clay,the shrinkage and expansion caused by dehydration and moisture absorption mainly occur in small pores in the range of 0.01-1 ?m.2.The difference in microstructure between undisturbed and compacted lateritic clay specimens directly affects the heat transfer paths,such as contact and liquid bridge between soil particles.Because the pore size distribution of undisturbed lateritic clay specimens is relatively single,and the soil pore size is mostly small.This is more beneficial for heat transfer and liquid bridge formation between soil particles and aggregates,which explained why the thermal conductivity and diffusivity of undisturbed specimens were larger than those of compacted ones.The difference in thermal conductivity between the wetting and drying processes is caused by the following: the effects of irregularities in soil pores(i.e.,the “ink-bottle” effect),leading to the difference in the pore water distribution,which is a significant factor affecting soil thermal conductivity;irrecoverable deformation induced by drying and wetting,resulting in differences in pore size distributions,that eventually affects the heat transfer paths through soil particles and water bridges and thus varying soil thermal conductivity;and the difference in the volume of air brought in(i.e.,entrapped air)between drying and wetting,which has a great influence on soil thermal conductivity.3.The cementitious material(free iron oxide)of undisturbed lateritic clay specimens was removed by the selective chemical dissolution method,and then the thermal conductivity,SEM and MIP tests were conducted on specimens with and without the free iron oxide.Results show that at the same volumetric water content,the thermal conductivity and diffusivity of undisturbed lateritic clay specimens after removal of free iron oxide increased by 29.3% and 27.7%,respectively.Moreover,the hysteresis characteristics of the relationship between thermal conductivity and volumetric water content of undisturbed lateritic clay after removal of free iron oxide are weakened compared with specimens with free iron oxide.Results from microstructure tests show that the amount of pores in undisturbed specimens without free iron oxide was significantly less than that with free iron oxide,and the relatively large pores in undisturbed specimens disappeared after removal of free iron oxide.The effects of free iron oxide on the thermal conductivity of undisturbed lateritic clay can be explained well by these changes in microstructures.4.Thermal properties of compacted lateritic clay specimens under different temperatures(i.e.,5??10??20??30??40??50??60??70??80? and90?)were measured.Results show that at the same water content and dry density,the thermal conductivity,thermal diffusivity and volumetric specific heat capacity of lateritic clay increased with the increase of temperature.At the same water content,the influence of temperature on the thermal conductivity and diffusivity of specimens with high dry density were greater than that of specimens with low dry density.At the same dry density,the thermal conductivity and diffusivity of fully dry and saturated specimens were negligibly changed with temperature.The effect of temperature on the thermal conductivity of lateritic clay is mainly related to the latent heat transfer(LHT)of water vapor in soils,the more water vapor and heat transfer paths can be used for LHT,the more obvious the temperature effect on the thermal conductivity.5.A series of experimental studies on the heat transfer characteristics of buried pipes were carried out by building a coupled seepage-heat transfer-mass transfer model test facility,which internal dimension is 1.6 × 1.2 × 1.6 m.Results show that the higher the initial water content and dry density of soil are,the better the heat transfer of buried pipe is.Compared with the effect of soil dry density,the heat transfer characteristics of buried pipe are more sensitive to the change of initial soil water content.Geological stratification has a certain influence on the heat transfer characteristics of buried pipes.Under the same heat load,the effect of heat released from buried pipe on the temperature field of sandy soil layer was significantly greater than that of lateritic clay layer,which is mainly related to the thermal conductivity of different subsurface materials.During the heat transfer process of buried pipe,the moisture transfer in unsaturated soil is affected by driving mechanism of both temperature and humidity gradients.When the driving mechanism of temperature gradient is dominant,the moisture content in the soil near the buried pipe is lower than that far from the pipe;when the driving mechanism of humidity gradient is dominant,the soil water content at the far-end(i.e.,the position far from the buried pipe)is also higher than that at the near-end.The effect of groundwater seepage on the heat transfer of buried pipes has obvious directionality.It will carry the heat from the upstream side to the downstream,which promotes the heat transfer from the buried pipe to the downstream,but the heat transfer to upstream is restrained.6.Based on the geometric average method and inter-particle contact heat transfer(IPCHT)model,a prediction model of thermal conductivity considering the influence of temperature was established.The predicted performance of the model was verified by the test results of this study and the literatures.On this basis,three-dimensional heat transfer models of coupled heat and moisture,heat and permeability for buried pipes considering the temperature effect of thermal conductivity were established,and the heat transfer models were verified by the results of model tests.Then,the influences of groundwater seepage,ground temperature gradient and initial ground parameters(e.g.,water content and dry density)on the heat transfer and storage characteristics of buried pipe were simulated and analyzed by using the heat transfer models of buried pipe.Results show that thermal flux of buried pipe increased with the increase of seepage velocity and temperature,and the direction of groundwater seepage mainly affected the direction of heat effect of buried pipe.The temperature and water content of the soil around the buried pipe increased with increasing the heat storage temperature,and the inlet flow rate of the buried pipe has little influence on the temperature and humidity field of the soil around the pipes.With the increase of initial volumetric water content,the heat flux of buried pipe increased,and the influence of heat released from buried pipe on soil temperature field at the far-end was more obvious,but the influence on soil temperature at the near-end was reduced.The moisture migration in soil around the buried pipe first increased and then decreased with increasing the volumetric water content,and there was a maximum value at low water content.However,when the soil layer was dry and nearly saturated,the heat released from buried pipe had little influence on the soil moisture field.