Simulation Study On Heat Transfer Of Frozen Soil Drilling In The Lunar Polar Region Based On Discrete Element Theor

Water ice in the frozen soil found in the lunar polar region is the main form of water resources on the moon.The CE-7 mission plans to use drilling sampling to obtain in-situ lunar soil permafrost samples and analyze their composition.It is one of the important tasks in the future lunar exploration to drill and sample the frozen soil in the lunar polar region.The interaction between drilling tool and lunar soil is a complicated process during drilling.The drilling load and heat generation will cause disturbance to lunar soil samples and affect the reliability of probe data.Combined with the discrete element method,the drilling simulation of lunar soil frozen soil under atmospheric pressure and low temperature is carried out and verified by experiment.Then,the above model is used to predict the evaporation of water ice in the drilling process of simulated frozen lunar soil with different water content and different drilling conditions under vacuum and low temperature.The main research work is as follows:(1)In this paper,the theoretical model of drilling heat generation is established by using the discrete element method.On the premise of equivalent heat transfer of the particle system,the heat transfer parameters of the discrete element particles are matched and calibrated,and the simulated lunar soil particle system is established.On this basis,the heat transfer model of the discrete element drilling is established.The discrete element simulation results were verified by experiments under atmospheric pressure and low temperature.(2)On the basis of simulating the preparation of lunar frozen soil in the polar region,aiming at the condition of atmospheric pressure and low temperature,the simulation research and experimental verification are carried out under different environmental and drilling conditions.Under the reasonable drilling conditions in this paper,the maximum temperature rise of the bit is about 60 ℃,and the simulation results fit well with the test results.Through further analysis,it can be seen that the flow of lunar soil can effectively take away heat,and it is also one of the reasons why the simulated lunar soil particles at high temperature at the front of the drill tool.(3)Analyze the problem that the heat distribution ratio between the drill and the simulated lunar soil is not constant during drilling.When the heat distribution ratio between the drill and the simulated lunar soil is inconsistent with the given allocation ratio,they will reduce their influence through mutual heat transfer and reduce their influence on the results.The influence analysis of the main parameters in the simulation shows that the thermal conductivity and specific heat capacity of simulated lunar soil particles had more significant effects on the temperature variation of the system.The thermal conductivity and specific heat capacity of the drilling tool have less influence on the system temperature.(4)The volatility of frozen soil(dry soil,5wt%,and 10wt%)simulated with different water content in the process of drilling was predicted.When the vacuum and low temperature conditions of drilling tool were-60 ℃ and lunar soil was-240 ℃,most of the lunar soil temperature in the simulated lunar soil particle system with three water content was not higher than-60 ℃,and the volatilization risk was high.Under the vacuum low temperature conditions of-103.15 ℃ for drilling tool and-183 ℃ for lunar soil,dry soil conditions: most of the lunar soil temperature near drilling tool is not higher than-150 ℃,and the volatilization risk is low.Conditions with moisture content of 5wt%and 10wt%,the temperature of most lunar soil near the drilling tool is not higher than-90 ℃,with volatilization risk.