Application Of Refrigeration And Heat-collect Technology To Subgrade Engineering In Cold Regions

To subgrade engineering in cold regions,soil temperature variation and the corresponding various thermal and mechanical effects are the main adverse factors to subgrade stability.The imbalance between heat input and heat output,which means the excessive heat increment of permafrost in warm season and the excessive heat loss of seasonal frozen soil in winter respectively,is the essential cause of subgrade defects.At present,the active cooling measures aganist permafrost subgrade mainly include block stone layer,ventilator,heat pipe etc.There exsit the limitaitions of low cooling efficiency and bad seasonal matching among above measrues.The controlling measures against frost heave in seasonally frozen subgrade mainly focus on the soil property improvement,water content control and passive thermal insulation,but were lack of active control against temperature variation and heat transfer capacity.Based on the analysis of heat transfer process,the most effective way to maintain thermal stability of subgrade in cold regions is to control the excessive heat input of permafrost in warm season and the excessive heat loss of seasonally frozen soil in winter.The aims above can be realized by the refrigeration technology and heat-collect technology,respectively.The refrigeration technology and heat-collect technology have both already formed complete and mature discipline system in the feild of environmental regulation of human settlements and industrial applications.They both need energy compensation to drive the irreversible heat transfer process.In China,renewable energy sources such as solar energy,wind energy and geothermal energy are all well distributed in cold regions.Renewable energy is the focus of global energy strategy.The refrigeration technology and heat-collect technology driven by new energy have great potential in subgrade engineering in cold regions.This paper is mainly supported by the National Natural Science Fund of China,"Temperature-controlling heat pile technology for high-speed railway construction in permafrost regions and its environmental impacts(41731281).Based on the principles and methods of refrigeration technology and heat-collect technology,conventional vapor compression refrigeration technology,thermal driven refrigeration technology,solar heating technology and ground-source heat pump technology are compared at the contents of technical characteristics and application level.Combined with the renewable energy utilization technologies,four types of active temperature-control methods and corresponding apparatus for subgrade in cold regions were put forward,including a compression refrigeration tube and an adsorption refrigeration tube for permafrost subgrade,and a solar collector tube and a ground-souce heat pump tube for seasonallyfrozen subgrade.In order to establish a set of active temperature controlling technology system for subgrade in cold regions,the thermal characteristics and technical applicability of the above four measures were studied by the means of technical investigation,feasibility analysis,device design and processing,model test and so on.The main research results of this paper are as follows.(1)The reasons for bad seasonal matching and low thermal effeciency of the existed measures for subgrade in cold regions lie in the fact that they are all limited to the heat transfer driven by natural temperature difference.Through the interdisciplinary with the Building Environment and Energy Untilization Engineering field,the refrigeration technology and heat-collect technology were introduced to permafrost and seasonally frozen regions,respectively.Additional heat output and heat input process,which are reverse to natural heat transfer direction,were artificially made to strictly control the heat balance and temperatue change of subgrade.(2)Through a series of freeze-thaw cycling tests,three axial compression test and physical properties tests,the coupling effect of dry density variation and moisture redistribution on soil mechanical properties were analyzed.The inherent mechanism of strengthening and weakening effect of freeze-thaw cycling was revealled.Then,it was advised that more attention about subgrade stability in cold regions should be placed on the real active themal measures and more effective heat balance regulations.(3)The technical characteristics of different refrigeration technologies and heat-collect technologies were compared.Besides,the distribution conditions of renewable energies in China and the corresponding untilization technologies were introduced.Then,the special requirements of thermal maintance and design principles for subgrade engineering were induced.On the basis of the vapor compression refrigeration technology and heat driven adsorption refrigeration technology were selected for the permafrost subgrade,and the solar collecting technology and ground-source heat pump technology were selected for the seasonally frozen subgrade.These four methods all can be driven by the renewable energies.(4)Two types of refrigeration devices for permafrost subgrade,including vapor compression refrigerating tube and solar adsorption refrigerating tube,were proposed.The specific structure layout,the detailed selection of device components,the processing steps,and the realization of intelligent function were put forward.The proposed devices were made and the refrigeration performance test system were established.Through the monitoring data of refrigerating temperature and the soil temperature under different experimental conditions,the value ranges and influencing factors of performance indexes of the two devices,including refrigerating temperature level,refrigerating capacity,refrigerating efficiency,refrigerating coefficient and effective freezing radius etc.,were analyzed respectively.(5)Two types of heat-collect devices for seasonally frozen subgrade,including solar collecting tube and ground-source heat pump tube,were proposed.The basic components and overall structure layout of the devices were designed.The proposed devices were made and its heat collecting performance test system were established.Based on the monitoring data,the value ranges and influencing factors of performance indexes of the two devices,including the maximum heating temperature,average heating temperature,heating capacity,heating efficiency and average effective heating radius etc.,were analyzed respectively.(6)Based on the calculation of heat storage variation and the analysis of steady heat transfer process,the cooling load of permafrost subgrade in warm season and the heating load of seasonally frozen subgrade in winter were determined,respectively.According to the performance test results of above four types of thermal devices,the matching evaluation between the thermal performance of the proposed devices and the thermal load of subgrade were discussed,and then the corresponding application scheme were put forward.