Research On The Temperature Field And Closure Rate Of Ice Hole Formed By Thermal Drill

With the study of cryoshere by the international community,thermal drilling technology is widely used in the drilling activities of ice caps,ice shelves,sea ice and mountain glaciers,as an efficient and environmentally friendly drilling method.Due to the special low temperature environment in the polar region,the borehole closure has brought great limitations to the development of thermal ice drilling technology.In order to ensure the safety of continuous drilling,it is necessary to estimate the closing rate of drilling.How to effectively prevent the freezing of drill is one of the key issues in the research of hot-point drilling technology.This paper introduces the research of borehole and ice temperature field at home and abroad,summarizes the research of hole closure in polar area,makes in-depth theoretical analysis and comprehensive experimental research on the ice temperature field and rate of borehole closure based on the hot-point drilling method,and puts forward the solution of lateral heating for the borehole closure problem.The main research results are as follows:1.Based on the knowledge of heat transfer,this paper analyzes the ice temperature field and the borehole closure rate in terms of the hot-point drilling method.The ice temperature field after the hot-point drilling was calculated using MATLAB software through two methods:?1?using the one-dimensional heat conduction equation of Cartesian system,ignoring the effect of curvature of cylindrical hole on heat transfer;?2?thermal diffusion equation of unsteady heat transfer in one-dimensional cylindrical coordinate system.The ice temperature field under different ice temperature and initial aperture is analyzed.The results show that for the case of borehole closure without internal heat source,at first temperature rises in the ice layer outside the borehole wall,and the closure the borehole wall is,the faster the temperature rise is and the greater the temperature rise amplitude is.Then the temperature drops.The initial value of borehole closure rate is large,but it will rapidly decrease to a stable value.However,the velocity suddenly increases a few minutes before the borehole is completely closed,and the overall curve is U-shaped.The lower the temperature is,the higher the stable closing rate is and the sharper the U-shaped bottom is.For the closure rate under the same temperature and different radius,the smaller the initial radius is,the greater the closure rate is.When the large-diameter borehole is reduced to the small-diameter borehole,the closure rate is smaller than the small-diameter borehole.In order to solve the problem of borehole closure,the diffusion equation in the ice hole with internal heat source was solved by MATLAB.By comparing the temperature field models under different temperature,borehole diameter and internal heat source power density,it is found that the borehole closure rate is inversely related to the ice layer temperature,the internal heat source power density and the initial borehole diameter.If we want to keep the hole opened or enlarge the ice hole,we need higher ice temperature,higher power density and diameter of the hole.For the lateral heating problem with ice temperature of-30?and initial drilling radius of 0.1 m,the power density of 0.06 W/cm² is the optimal solution to maintain the hole diameter for a long time.2.The temperature field of ice layer in and around the borehole and the whole process of borehole closure is simulated with the finite element analysis software COMSOL Multiphysics.The analysis of the ice temperature field shows that the water in the borehole cools uniformly at a fixed speed and freezes from the outside to the inside at a uniform speed.The maximum value of the ice thermal disturbance area is about 8.75 times of the borehole radius during the whole process.Through the analysis of the ice temperature field under the conditions of different temperatures and drilling diameters,the results show that the cooling and freezing rate of water increases with the decrease of temperature when the radius is the same.When the ice temperature is the same,the smaller the radius is,the faster the water cooling and freezing rate is.With the decrease of the radius,the faster the growth rate is.The ice temperature field with heat source was analyzed with the improved model.Through the calculation of the lateral heating body model,it was found that under the same temperature,the larger the borehole diameter is,the smaller the energy density is required by the lateral heating body to maintain the aperture is.Similarly,for the fixed power density,the larger the borehole diameter is,the stronger its ability to maintain the borehole size is.Under the same conditions,the hollow lateral heating pipe and the sealing lateral heating body were simulated.The analysis results show that the heating efficiency of the hollow lateral heating pipe is higher than that of the sealing lateral heating body,and the reaming effect is higher.Hollow lateral heating is an efficient and convenient treatment method for hot-point drilling reaming and reduces risks of the hole sticking in ice.3.A set of ice temperature field detection platform for hot-point drilling was established.The ice temperature field of hot-point drilling with different power and diameter at different initial ice temperature and the change of ice temperature field around the drilling hole after drilling are detected in all directions in a real time.The experiment shows that the ice temperature field has obvious hysteresis in the process of drilling,that is,when drilling to the bottom,the ice temperature around the middle of the borehole begins to change.At the end of drilling,the melting water in the ice hole will cause the ice temperature to rise to a certain extent,so that the area of thermal disturbance will gradually expand beyond the measurement range of the ice barrel.The radius of the thermal disturbance range of the ice layer is basically independent of factors such as the ice temperature and drilling speed,and its value is about 7.2-12.5 times of the drilling radius.The temperature gradient of the ice layer around the borehole is inversely related to the temperature of the ice layer and positively related to the diameter of the borehole.That is to say,the lower the ice layer temperature is,the greater the temperature gradient is.At the same temperature,the smaller the borehole diameter is and the lower the temperature gradient is.4.The basalt bucket and ice bucket were made to simulate ice drilling,and the closure rate of ice drilling was measured through experiments.The results show that there are four stages in the process of borehole closure:the first stage of liquid water cooling,the second stage of liquid water phase heat absorption,the third stage of borehole closure,and the fourth stage of solid ice cooling.For the ice hole with ice temperature of-30?and initial drilling radius of 0.1 m,the cooling rate of water in basalt bucket is about 5.9⁷.9?/h,the freezing rate is about 11.0¹2.9 mm h^-1,the freezing rate gradient is-0.68mm h^-1?^-1.The cooling rate of water in the ice bucket is about 8.5⁹.2?/h,the freezing rate is about 10.3¹1.5 mm h^-1,the freezing rate gradient is-0.60mm h^-1?^-1.In the first stage,the cooling speed of the water in the ice bucket was nearly twice as fast as that in the basalt bucket.In the third stage,the difference between the two studied cases was small.However,in the specific freezing process,the borehole closure experiment in the ice bucket can reflect the actual situation of the borehole after the hot-point drilling.The results in the ice bucket show that the freezing rate is exponentially related to the drilling radius,and the freezing rate increases gradually along the direction from the hole wall to the center of the drilling hole.The freezing rate near the hole center is 6-9 times higher than that at a distance of 81.3 mm.Based on the results of hole closure rate,the design drilling speed and length of the RECAS thermal probe were predicted.The results show that the 8-m-long RECAS probe would be stuck in the ice when the drilling speed is less than 2 m/h and the ice temperature is lower than-20?.5.In order to prevent sticking accidents caused by hole closure,a lateral heating solution is provided for RECAS thermal probe on the premise of not reducing the length of drilling tools and increasing the drilling speed.A lateral heating shell is designed,and the appropriate heating elements and layout are selected through comparative analysis.After that,the minimum power density required for thermal probe to stay in the ice was obtained through experiments.The 160-mm diameter lateral heating pipe maintains the power density required by the hole diameter at different temperatures in the borehole less than 220 mm,which provides certain safety factor for hot-point drilling.