| The polar regions have unique climate environment,geographical value and potential resources,which are of great significance to the sustainable development of mankind and the construction of a community of shared future.There are more than700 subglacial lakes under the Antarctic ice sheet.Research on subglacial lakes can explore the subglacial environment and geological structure,invert the evolution history of the ice sheet,obtain Antarctic paleoclimate information,and discover new life forms.In addition,the development of subglacial lake detection technology can also provide technical support for humans to explore the subglacial environment of exoplanets such as Mars,Enceladus,and Europa in the future.The existing direct detection methods for subglacial lakes mainly include mechanical drilling,hot water drilling and hot point drilling.Both mechanical drilling and hot water drilling are potentially polluting.Hot point drilling can achieve pollution-free drilling,but the drilling efficiency is low and it is difficult to effectively penetrate the dusty ice layer.The ice drill with near-bottom hot water circulation adopts the heating jet drilling method at the bottom of the hole,which has the advantages of no pollution,low power consumption,high drilling efficiency,and the ability to penetrate the dusty ice layer.Four kinds of local circulation hot water drills have been developed abroad,which are used for hole formation of alpine glaciers and outer space exploration,which verifies the reliability of the drilling tools,but most of the technologies involved belong to the field of extraterrestrial planet detection.,test results,experimental data and related theories have not been published.Therefore,this paper aims to design a ice drill with near-bottom hot water circulation to solve the problem of low drilling efficiency of the recyclable fully automatic sub-ice environment detector RECAS when it encounters dusty ice.In this paper,the theoretical analysis and numerical simulation methods are used to establish the physical model of the drilling process of the ice drill with near-bottom hot water circulation and the influence of the structural parameters of the drilling tool and the drilling parameters on the drilling process is explored.The theoretical analysis and numerical simulation results are verified in combination with the experimental study,and the key structural parameters and drilling parameters of the drilling tool are determined.On this basis,a ice drill with near-bottom hot water circulation is developed.Through the experimental study of the drilling process of the whole drilling tool,it is verified that the drilling tool has the ability to drill in the dusty ice layer.The main research process and conclusions of the paper are as follows:(1)Based on the theoretical research of fluid mechanics and heat transfer,the mathematical model and physical model of the drilling process of the ice drill with near-bottom hot water circulation are established.The process has an impact and provides a basis for the selection of drilling parameters of the RECAS detector.Under the condition of ice layer temperature-30?: when the jet water temperature and flow rate are constant,reducing the rate of penetration can achieve effective hole reaming,the drilling speed is reduced by 16%?19%,and the borehole diameter is increased by10%;As the average ROP target,when the hole expansion coefficient is 1.1,the heat flow water temperature should reach 40°C and the flow rate should reach 2.0L/min.At this time,the corresponding hole bottom heating power is 5k W.This parameter can provide an important basis for selecting design parameters of drills.(2)Based on the ANSYS Fluent software platform,a physical model of hot water drilling at the bottom of the hole was established to simulate the melting process of the ice layer near the bottom of the hole.The influence of the drilling process,the variation law of the rate of penetration is clarified,and the basis for the design of drilling tools and the selection of drilling parameters is provided.Under singlevariable control conditions: the height of the drill bit is the same,the shape of the drill bit has little effect on the shape of the drill hole,and the 90° conical drill bit can obtain a higher rate of penetration;under the condition of different nozzle diameters,a nozzle with a diameter of 3.0mm has the fastest rate of penetration;the higher the jet hot water flow rate and temperature,the faster the drilling speed;the drilling speed is positively correlated with the temperature of the ice layer,the Antarctic ice layer generally increases with the drilling depth,so the drilling speed will increase with the increased as the drilling depth increased.The research on the ice melting process near the bottom of the hole can provide a reference for the design of the relevant drilling tools and the selection of drilling parameters,and improve the drilling efficiency.(3)The single-factor rotation method is used to select and test the hot water drill bit,and the influence of the drill bit shape,nozzle diameter,weight on bit,temperature,flow rate and other factors on the rate of penetration is studied.The test results show that the conical drill bit is better than the parabolic drill bit for different shapes of drill bits and the same height,among which the 90° conical drill bit has the highest drilling speed,and the shorter and flatter the drill bit,the smaller the weight on bit change;nozzles with different diameters,under the condition of constant flow,the smaller the nozzle,the faster the flow rate,the 3 mm diameter nozzle has the fastest drilling speed,and the borehole diameter has no obvious correlation with the nozzle diameter,and the hole diameter of the drilling tool can reach 165?180mm;The speed changes the weight-on-bit parameters.When the weight-on-bit is too large,the nozzle is blocked and the drilling speed is reduced.When the weight-on-bit is too small,the heat exchange efficiency and the drilling speed are reduced.The optimal weight-on-bit of the drilling tool designed in this paper is about 30% of its own weight,The flow rate is positively correlated with the drilling speed.The selection of drill bits and related tests can provide a reference for the subsequent overall design of drill tools and selection of drilling parameters.(4)Completed the design and test of the RECAS supporting ice drill with nearbottom hot water circulation,which confirmed that the number of injection orifices,injection orifice angle,drill bit structure,ice layer temperature and ice dust layer all have an impact on the rate of penetration,the optimal nozzle and drill bit structure are obtained,which verifies the feasibility of drilling in the dust-bearing ice layer.The test results show that the multi-orifice nozzle with nozzle diameter of 1.5 mm,the number of orifices is 9,and the orifice angle is 35°,and the single-orifice nozzle with orifice diameter of 3.0 mm has the fastest drilling speed,but the drilling pressure is unstable and periodic.After adding 6 rear orificecs behind the single-orifice nozzle,the drilling speed is reduced by 13%,and the thermal melting efficiency is reduced by9%,but the heat transfer at the bottom of the hole can be improved,and the drilling process is more stable;the hollow drill bit with a wall thickness of 5 mm can effectively eliminate the heat conduction cold spots outside the drill bit,increase the heat transfer efficiency at the bottom of the hole,and improve the rate of penetration;when the ice layer temperature rises from-30 °C to-10 °C,the drilling speed is increased from 1.36 m/ h rises to 1.60 m/h;the drill can drill into the dusty ice layer.When the thickness of the dust layer is 10?20 mm,the rate of penetration is reduced by 14%.To sum up,the structural design and drilling process of the ice drill with near-bottom hot water circulation are studied,which provides a reference for the future development of ice thermal drilling technology,and also promotes the future development of polar scientific research. |
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