Study On Unsteady Coupled Heat And Moisture Transfer Between Shaft Wall And Airflow In The Thick Artificial Frozen Alluvium Vertical Shaft

Many coal-rich areas in China are covered with thick alluvium.The construction of the shafts in these areas mostly uses artificial freezing method.The artificial frozen wall about 8-10 m in radius direction with the temperature less than-20°C around the pit shaft has formed after adopting the freezing method.Due to the large depth of the shaft built in the thick alluvium,the ground temperature rises,which will cause heat damage for the underground driving face.Based on the special situation that the frozen wall has large cooling capacity and heat damage has existed,this paper proposes the new method of cooling airflow supply by the frozen vertical shaft wall to realize cooling and studies the unsteady coupled heat and moisture transfer between the frozen wall and airflow in the artificial frozen shaft.Based on heat and mass transfer,thermodynamics,frozen soil physics and other theories,the similar physical experiment system and the theoretical model of coupled heat and moisture transfer between the frozen shaft wall and saturated moist air are established.The cooling effects of this method in the practical engineering case are analyzed.The research has practical significance for utilization of cooling capacity from the frozen wall and heat damage control through ventilation and cooling in the thick frozen alluvium vertical shaft.The main researches are as follows:(1)Establishing the numerical model for unsteady coupled heat and moisture transfer between the frozen shaft wall and airflow;According to the theories of heat and mass transfer,thermodynamics and frozen soil physics and other theories,the laws of heat and moisture transfer in the frozen shaft and the shaft wall are analyzed,and the concept of equivalent specific heat at constant pressure and comprehensive thermal conductivity coefficient are introduced.They are used to establish the axisymmetric theoretical model of unsteady coupled heat and moisture transfer between the frozen shaft wall and the supply air which is saturated moist air and dry air.In the model,the gravity compression heat is considered as the internal heat source and the moist air is regarded as the incompressible fluid,and unsteady coupled heat and moisture transfer between saturated moist air and the frozen shaft wall is directly converted into heat transfer calculation.The phase change heat transfer of frozen soil thawing of the frozen wall adopts equivalent heat capacity method.The finite element method is used in full field coupled analysis.(2)Establishing the physical experiment system to verify model for unsteady coupled heat and moisture transfer between the frozen shaft wall and airflow;Based on similarity theory,the physical experiment system for the frozen shaft wall and airflow is set up according to similarity criterion,which is used to research unsteady coupled heat and moisture transfer between the natural temperature airflow supply and the frozen shaft wall.The model of saturated moist air under the air supply working condition with relative humidity of moist air about 60% and 90% is verified by the physical experiment.The results show that the calculation error of dry bulb temperature in both working conditions are less than 5%,and the calculation error for moist air relative humidity of 90% is less than 1.6%.The maximum error of inlet and outlet enthalpy difference is 13.7%.It can be recognized that the calculation model is suitable for relative humidity of moist air above 90%.(3)Numerical simulation research on the laws of unsteady coupled heat and moisture transfer between airflow and the frozen shaft wall;In unsteady coupled heat and moisture transfer between saturated moist air and the frozen shaft wall,the influence of moisture transfer is greater than the compression heat and sensible heat from the perspective of energy transfer and transformation.The moisture transfer can greatly increase the total heat transfer capacity and in the majority of working conditions it can be increased by more than 50%,while the compression heat can only be increased by 10%.The natural convective heat transfer is weak when the air supply velocity is greater than 0.8m/s.The air supply velocity and inlet temperature are the main factors affecting the airflow heat and moisture parameters,the comprehensive heat transfer coefficient and the total heat transfer of the airflow,among which the influence of air supply velocity is the biggest and the moisture content of the frozen soil is the secondary.However,moisture content,air supply velocity and temperature are the main factors affecting the thawing of the frozen wall.From the energy consumption point,the energy saving effect by obtaining cooling capacity from the supply airflow in the frozen shaft to realize cooling is analyzed.(4)Research on the laws and cooling application of unsteady coupled heat and moisture transfer with seasonal airflow supply in the frozen vertical shaft;The temperature variation laws of the antifreeze supply air and the frozen vertical shaft wall in winter are researched and the minimum antifreeze supply air temperature in different air supply working conditions is obtained.The shaft bottom air temperature and temperature variation laws of the frozen wall are researched in the working conditions with the constant and natural temperature air supply in the shaft mouth.The shaft bottom air temperature interval frequency characteristics and its distribution laws with natural temperature and constant temperature air supply are obtained respectively.It can be decided that the air supply of the frozen shaft in the first year can control the driving face temperature that will be not higher than 26°C,and as for the characteristic that the frozen wall temperature is still lower than-3°C in the second year,this paper proposes the new cooling method for the driving face by utilizing cooling capacity of the frozen wall to cool airflow supply in the vertical shaft.And it has been further proved that this method is efficient and feasible within two years after the construction of the frozen shaft.The paper has 98 figures,26 tables,and 171 references.