The characteristics of the flow field associated with a multi-hole combined external rotary bit have been studied by means of numerical simulation in the framework of an RNG

Hydraulic jet radial drilling technology finds its roots in ultra-short radius radial horizontal wells in oil and gas drilling industry [

The hydraulic jet drilling nozzle has undergone the development from the initial single-hole and multi-hole direct-injection type to the single-hole internal rotation type and multi-hole internal rotation type, and the rock breaking efficiency is gradually improved. The underground working conditions in coal mine entails small size of nozzle and large spray distance, wide range of action, high rock breaking efficiency and good hole-forming effect. In addition, the gradual increase of mining depth has posed higher demands for drilling hole-forming effect, so the structural parameters of hydraulic jet drilling nozzle and rock-breaking hole-forming effect need further optimization and improvement.

Liao et al. [

Therefore, based on the requirements of hole-forming depth and diameter in the radial drilling process, this paper studies the flow field characteristics of the multi-hole combined external rotation nozzle using numerical simulation and laboratory test methods, RNG

Hydraulic jet drilling nozzle is a key component of radial drilling technology. It requires large jet impact area, high rock-breaking hole-forming efficiency and low power consumption, and requires that the size of the nozzle is reduced as much as possible to achieve steering on the basis of ensuring the jet performance. I recent years, the improvement of manufacturing technology and the research progress of clearance seal mode has made the miniaturization of external rotation nozzle possible. The multi-hole combined external rotation nozzle is mainly composed of a shell, a rotating body, a rotating shaft, a forward nozzle and a reverse nozzle [

High pressure water jet is a complex unsteady flow. In jet flow, various physical parameters of fluid, such as velocity and pressure change randomly with time and space. However, it is generally believed that the unsteady N-S equation is still applicable to the instantaneous motion of turbulent flow no matter how complex the turbulent motion is. The governing equation of incompressible flow is as follows:

(1) Conservation equation of mass

where,

(2) Momentum conservation equation

where,

(3) Turbulence model

In this paper, the drilling bit is composed of multiple nozzles. The internal watershed structure is relatively complicated. The fluid is easy to form a vortex at the corner of the nozzle. It is more complicated than the common jet. Therefore, the reflow group

Standard

RNG

where, _{k} is the reciprocal of the effective turbulent Prandtl number of the kinetic energy k; _{ε} is the reciprocal of the effective turbulent Prandtl number of the dissipation rate _{1ε} = 1.44, _{2ε} = 1.92, _{3ε} = 0.09, _{k} = 1.0, _{ε} = 1.3, _{μ} = 0.09.

(4) Equation discretization

A finite volume method (controlled volume method) is used to establish a discrete equation for fluid motion. For incompressible fluid, the governing equation of the three-dimensional jet is as follows:

where _{Φ} is the viscous dissipation term.

The general conservation equation is as follows:

where

(5) Algorithm

The SIMPLE algorithm used in this paper is mainly applied to the numerical calculation of incompressible flow fields. For a given pressure field, the discrete momentum equation is solved to obtain the velocity field. The second order upwind scheme is adopted in this paper [

As shown in

Establish a physical model of the research problem, and then abstract it into a mathematical and mechanical model. Then determine the spatial influence area of the geometry to be analyzed.

Establish the entire geometric body and its spatial influence area, and divide the outer surface of the geometric body and the entire calculation area into a spatial mesh. The sparseness of the grid and the shape of the grid cells will have a great impact on future calculations. In order to ensure the stability and efficiency of calculation, different algorithm formats generally have different requirements on the grid.

Add the initial conditions required for the solution, and the boundary conditions at the inlet and outlet are generally speed and pressure conditions.

Select the appropriate algorithm, set the specific control solution process and accuracy conditions, solve the problem to be analyzed, and save the data file results.

Select the appropriate post processor (Post Processor) to read the calculation result file, analyze and display it.

Based on the study of jet characteristics under submerged conditions during the drilling process of the multi-hole combined external rotation nozzle, the three-dimensional model of the external rotation nozzle is simplified, and the structural models of the nozzle rotating body, internal flow channel and nozzle are extracted, as shown in

Due to the complexity of most calculation areas in practical engineering calculation, the final accuracy of CFD calculation results and the efficiency of calculation process mainly depend on the grid generated and the algorithm adopted [

It can discretion areas with complex shapes, because unstructured grid cells can completely fill the entire space in any calculation area, and can represent the boundary of the object fairly accurately, thereby ensuring the initial accuracy of the boundary.

It can quickly add and delete nodes in the grid, and it is more convenient to cope with moving boundary problems.

The adaptive grid method can be easily adopted to improve the quality of the solution.

In this numerical simulation, the physical model is divided into two parts: tetrahedron mesh generation method is adopted in the calculation domain, and mesh densification measures are taken for the parts with severe velocity gradient change, so as to ensure the stability of the calculation process and the correctness of the calculation results. In order to simulate the process boundary to have better simulation effect, the boundary layer is set, the thickness of boundary layer is 0.00002. The number of grids reached 880000, As shown in

In this paper, steady state calculations are used first, and when convergence is reached, transient calculations are performed. The steady state convergence condition is that the Equation Residual is below 0.001. Set the number of iterations to 10000 in the steady state. The transient calculation time step size is 0.001, and the number of time steps 3000. The main numerical simulation settings and boundary conditions as shown

Item | Configuration |
---|---|

Solver | Pressure-based |

Turbulence model | RNG |

Pressure-velocity coupling equation | SIMPLE algorithm |

Initialization method | Hybrid |

Mesh division method | Tetrahedral mesh |

Grid computing form | Dynamic meshing |

Inlet boundary | Pressure inlet |

Outlet boundary | Pressure outlet |

Spatial Discretization | Second-Order Upwind |

Wall condition | fixed without sliding |

The rock breaking experiment system using high-pressure water jet is shown in ^{4} MPa, and density is 2.711 × 10^{3} kg/m^{3}.

Compared with the dynamic rock-breaking hole-forming process, the superimposed scanning rock-breaking along their respective tracks is realized through the combination arrangement of different nozzles under the action of rotation, showing a “progressive” dynamic rock-breaking hole-forming process, and finally achieving the regular drilling effect. The simulation results of flow field characteristics are basically consistent with the actual dynamic rock-breaking process.

We are very grateful to the editor and anonymous reviewers for their valuable advices.

Density of fluid

Time

Components of velocity

Directions

Dynamic viscosity coefficient

_{k}:

Reciprocal of the effective turbulent Prandtl number

Kinetic energy

_{ε}:

Reciprocal of the effective turbulent Prandtl number

Dissipation rate

Transient energy under controlled volume conditions

Convection item

Diffusion item

Original item