| Antarctic ice sheet is the largest reservoir and cold source on the earth,of which the dynamic changing process will play an important role in regulating sea level and global climate.The dynamic change of the ice sheet is largely determined by the mass balance of the ice shelf.Due to the influence of ocean heat flux,the bottom of the ice shelf thaws and collapses,resulting in great changes in the mass flux of the ice sheet.Therefore,the research on the interaction process between the bottom of the ice shelf and the ocean is a scientific issue in the field of polar research.Hot-water drilling technology is regarded as the most effective technical means to study the process.In the hot-water coring drilling,a coring drill head or ring with multi nozzles is used to obtain ice core or annular channel releasing frozen drills and instruments for ocean observation.At present,the quality of ice core obtained by the coring drill head is not stable and the rate of penetration(ROP)of coring ring is slow.In order to optimize the hot-water coring drill to meet the needs of obtaining ice core with high quality or high ROP,this paper systematically studied the influence mechanism of annular multi-nozzle on the rate of penetration and the quality of ice core.The main research results were as follows:1.From the perspective of heat transfer theory,this paper analyzed the process of hot-water coring drilling and expounded that the occurrence of coring drilling was due to the uneven convection heat transfer of annular jet to ice,causing melting with varying degrees,which resulted in coring drilling.It was confirmed that the heat transfer of annular jet to the bottom ice belongs to jet impingement heat transfer,while to the ice core belongs to chase type.The main factors affecting the ROP and quality of ice core were finally determined to be nozzle height,nozzle number,nozzle diameter,nozzle angle,nozzle flow rate and water temperature;2.Theories of convective heat transfer were used to analyze the heat transfer efficiency between hot water and ice.The influence of different factors on the heat transfer between water and ice was determined: the closer the distance between nozzle and ice was,the higher the heat transfer efficiency to the bottom ice.The results showed that the influence range was close to the stagnation point;The increase of water flow rate and the decrease of bit thickness improved the heat transfer efficiency between hot water and the bottom ice;The increase of nozzle diameter and quantity was not conducive to the heat transfer efficiency;When nozzle angle increased,the heat transfer efficiency between water and the bottom ice reduced,but appropriately increasing the nozzle angle will reduce the impact on the ice core,the optimal angle of the nozzle was about 15°;Increased length of the ice-core barrel will reduce the heat transfer efficiency between water and the ice core,yet the impact will weaken after a certain length.For actual drilling,the design length of core barrel should be at least 1.5 m;3.The influence of different factors on the heat transfer was simulated by using ANSYS Fluent,`The influence rules of different factors on the heat transfer were summarized as follows: the heat transfer to bottom ice increased with the augment of flow rate and decreased with the increase of nozzle diameter,nozzle quantity and nozzle angle;The heat transfer to ice core increased with the increase of nozzle diameter,flow rate,but decreased with the increase of nozzle quantity and nozzle angle;4.Nine kinds of drill heads with multi nozzles were processed and the test platform of hot-water ice-core drilling was built,which was used to carry out 27 tests of coring drilling.The ROP and ice-core diameter of different drill heads were analyzed.It was found that the diameter of nozzle was negatively related to the ROP and core quality.When the nozzle diameter increased,the ROP decreased from 14.1m/h to 10.5 m/h and the diameter decreased from 70.5 mm to 65.8 mm;Although the increasing nozzle number reduced the ROP,it can effectively improve the quality of ice core.When the nozzle number increased from 24 to 60,the ROP decreased from11.2 m/h to 7.6 m/h and the diameter increased from 69 mm to 74 mm;The increase of nozzle angle significantly reduced the ROP.When the nozzle angle increased from0° to 30°,the ROP decreased from 20 m/h to 5 m/h and the core diameter appeared peak value when it was near 15°;The increase of flow rate increased the ROP,but the core quality reduced.When the flow rate increased from 40 L/min to 100 L/min,the ROP increased from 7.6 m/h to 11.8 m/h and the core diameter decreased from 72 mm to 67.5 mm.Among all factors,the influence of nozzle angle on the ROP and ice-core quality was significant.5.Based on the influence mechanism of multi-nozzle on ROP and ice-core quality,the hot-water coring system was designed and developed.The system is mainly composed of power system,surface winch and drilling tools.The power system includes high-pressure pump and heater,which can provide hot water with flow rate of 25 L/min,maximum pressure of 22 MPa and temperature of 90?.The drill included a single nozzle and coring ring.The inner and outer diameter of coring ring was 210/222 mm,the nozzle angle was 0°,the nozzle diameter was 1 mm and the nozzle quantity was 45,they were all determined by the obtained influence mechanism;the preliminary test was carried out in laboratory of Jilin University,which verified the normal operation of the system and operation experience for the later application in the Antarctic field were accumulated. |
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