| Due to harsh polar conditions such as extreme low temperatures,storms,heavy snowfall,and the lack of infrastructure,drilling operations in polar ice sheets are highly complex.Conventional geological and oil drilling techniques are impractical for ice drilling due to heavy equipment,high power consumption,and operational difficulties.Cable-driven mechanical drilling rigs have been widely used in polar ice drilling due to their lightweight and ease of operation.However,the necessity for repetitive coring operations leads to prolonged auxiliary time,reducing drilling efficiency.In recent years,the technology of air reverse circulation continuous coring drilling has rapidly developed.As drilling progresses,ice cores and chips can be continuously transported to the surface through the central channel of double-wall drill pipes by compressed air,eliminating the need for frequent coring,thus saving auxiliary time.However,the main challenge of this technology lies in the continuous addition of drill pipes during the drilling process,which wastes time and reduces efficiency.To address this issue,this paper proposes a novel air reverse circulation drilling system that utilizes flexible pipes wound on a surface winch to transport ice cores and chips instead of drill pipes.This significantly reduces the system’s weight and eliminates the need for assembling and disassembling drill pipes,thus saving time.Clearly,the key to this technology lies in using compressed air to smoothly transport ice cores and chips through the wound pipe to the surface.Therefore,analyzing the flow characteristics of ice cores and chips inside the wound pipe is crucial for ensuring smooth transportation and improving the efficiency of air drilling operations.This paper employs a combination of visual experiments and numerical simulations to analyze the transport characteristics,force properties,and flow field distribution of ice cores,ice chips,and their combined movement inside the wound pipe under different conditions.A theoretical model for the pressure and air flow required to transport ice cores and chips inside the wound pipe is established,and the gas injection parameters required at different drilling depths are calculated.The main research contents and conclusions of this paper are as follows:(1)Visual experiments and numerical simulations were conducted to analyze the transport characteristics of ice chips inside the wound pipe.The results indicate that when gas-solid two-phase flow passes through the wound pipe,centrifugal force causes particles to concentrate mainly on the outer wall,forming a granular fluidized bed.The movement of ice chip particles can be divided into three regions: the mobile bed region(Region I),responsible for transporting most of the ice chip particles,the gas flow region(Region II)occupied by air,and the transitional region(Region III).Centrifugal force induces secondary flow in the flow field.In most cases,single secondary flow eddies can be observed in the gas flow region,which is disadvantageous for particle transport.However,when the gas flow velocity is high or the space occupied by the particle bed is small,a counter-rotating secondary flow vortex appears in the upper part of the particle bed,which has a dual effect on particle transport.The ice transport efficiency(ITR)decreases with increasing solid-to-gas ratio,bend ratio,and particle diameter.When the gas velocity is 20 m/s,increasing the solid-to-gas ratio from 0.5 to2.0 results in a decrease in ITR of approximately 75.1%,while increasing the bend ratio from 0.06 to 0.12 leads to a decrease in ITR of 34.6%.In contrast,increasing the particle diameter from 1 mm to 4 mm results in a relatively small decrease in ITR,only 7.8%.Additionally,the pressure drop for transport increases with increasing solid-to-gas ratio,bend ratio,and particle diameter.(2)The motion speed,pressure drop,and aerodynamic forces experienced by ice cores during their movement were investigated through experiments and Computational Fluid Dynamics(CFD)simulations using dynamic mesh technology.The results reveal that when ice cores pass through the wound pipe,they are subject to both centrifugal force and gravity.In most cases,ice cores move along the outer wall of the pipe,except when they reach the top of the pipe(θ=180°)and the centrifugal force acting on them is less than gravity,in which case the ice cores do not adhere to the wall.The transport velocity of ice cores increases with increasing Reynolds number and core-to-pipe diameter ratio but decreases with increasing bend ratio and ice core aspect ratio.Increasing the core-to-pipe diameter ratio from 0.80 to 0.92 results in a significant increase in ice core velocity by 62.7%,while increasing the Reynolds number from 85000 to 160000 leads to a 49.9% increase in ice core transport velocity.In contrast,the effects of bend ratio and ice core aspect ratio are relatively minor.Furthermore,the pressure drop increases with increasing Reynolds number,core-topipe diameter ratio,bend ratio,and ice core aspect ratio.The dynamics of ice core movement inside the wound pipe are mainly driven by aerodynamic forces,including pressure differential forces and lateral forces,with the pressure differential force being the primary driving force.As the ice cores occupy space within the pipe,compressed air forms a vortex downstream of the ice cores,and with increasing Reynolds number,core-to-pipe diameter ratio,ice core aspect ratio,and bend ratio,the size of the vortex region gradually increases and its distribution becomes increasingly uneven.(3)Through the coupling of dynamic mesh technology with a two-fluid model,the influence of different variables on ice core motion patterns,the distribution of ice chip particles in the flow field,and ice core pressure drop under coexistence conditions was investigated.The results indicate that when ice cores and ice chip particles move together,they are subjected to centrifugal force and gravity inside the wound pipe.In most cases,ice cores adhere closely to the outer wall of the pipe,while ice chip particles tend to aggregate near the tail of the ice cores,forming a wake region.The shape and size of the wake region are influenced by the relative velocity between the ice cores and particles.When the wind speed is less than 25 m/s,the core-to-pipe diameter ratio is less than 0.88,the bend ratio is greater than 0.06,and the solid-to-gas ratio is greater than 1,particles are more likely to aggregate at the tail of the ice cores,while the influence of particle diameter and ice core aspect ratio on the wake region is relatively small.The dynamics of ice core movement inside the wound pipe during shared motion are primarily driven by aerodynamic pressure differential forces,lateral forces,and particle thrust forces,with the pressure differential force being the main driving force.The magnitude of aerodynamic forces acting on ice cores increases with decreasing ice core velocity and increasing Reynolds number,core-to-pipe diameter ratio,ice core aspect ratio,bend ratio,solid-to-gas ratio,and particle diameter,with the core-to-pipe diameter ratio having the greatest impact.For example,at a wind speed of 20 m/s,increasing the core-to-pipe diameter ratio from 0.80 to 0.92 results in a total dynamic force acting on ice cores increasing from 233.8 N to 2413.2 N,an increase of 932.2%.In contrast,the influence of particle diameter is minimal.(4)When ice cores and ice chip particles move together,the transport velocity of ice cores inside the wound pipe increases with increasing Reynolds number,core-topipe diameter ratio,and solid-to-gas ratio,while it decreases with increasing bend ratio,ice core aspect ratio,and particle diameter.Increasing the core-to-pipe diameter ratio from 0.80 to 0.92 results in a significant increase in ice core velocity by 44.8%,while increasing the Reynolds number from 85000 to 160000 leads to a 41.1% increase in ice core transport velocity.In contrast,the effects of bend ratio,ice core aspect ratio,solidto-gas ratio,and particle diameter on ice core velocity are relatively minor.When ice cores and ice chip particles move together,the ice chip particles exert additional thrust force on the ice cores.Meanwhile,due to the occupation of compressed air flow space by ice chip particles,the aerodynamic forces acting on ice cores also increase.Therefore,the transport velocity of ice cores is slightly higher when moving together with ice chip particles compared to when there are no ice chip particles present.For example,at a wind speed of 20 m/s,a core-to-pipe diameter ratio of 0.84,an aspect ratio of 1.5,a bend ratio of 0.08,and a solid-to-gas ratio of 1.5,the ice core transport velocity without ice chip particles is 7.20 m/s,while under shared motion conditions,the ice core velocity increases to 7.65 m/s,representing a 6.3% increase.(5)Based on a combination of global optimization algorithms and the Macquart method,predictive formulas for the pressure drop and minimum air flow rate required for ice core transport in the presence of ice chips were established.A comprehensive analysis of the pressure drop and required air flow rate for ice core and ice chip transport inside the coiled tubing was conducted,and a mathematical equation relating injection parameters to borehole depth was established.The results indicate that with an increase in borehole depth,the injection pressure shows a linear increase.For instance,considering a core-to-tubing diameter ratio of 0.84,as the borehole depth increases from 0 m to 1000 m,the required injection pressure increases from 0 to 2.02 MPa.Combining these findings with practical drilling conditions,the minimum airflow rate required for ice core and ice chip transport inside the coiled tubing was calculated.With the core-to-tubing diameter ratio increasing from 0.80 to 0.92,the minimum airflow rate for ice core and ice chip transport inside the coiled tubing decreased from 3.60m~3/min to 1.49 m~3/min. |
