Analysis of the influence of venturi structural parameters on the performance of hydraulic impactors

The venturi effect can be used to control the fluid pressure at different locations in the pipe, and the pressure difference generated based on this principle can push the piston inside the hydraulic impactor up and down to impact the drilling tool, thus expanding the use of coiled tubing and improving the efficiency of drilling. In this paper, the effects of several different necking sizes, tapering angles and diffusion angles on the flow field state and pressure control performance inside the venturi are studied, and the influence laws are analyzed by using Fluent finite element software, and the effects are verified by numerical simulation. The related research have a great significance to the selection of venturi structure size, and it also provides the theoretical basis for the optimization of the structure parameters of the hydraulic impactor.


Introduction
The Venturi Effect, also known as the Vinzel Effect, is the adsorption effect that occurs when low pressures are generated in the vicinity of a fluid flowing at high speeds.Venturi tubes can be made by utilizing this effect.It can play a role in throttling and pressure reduction, so the venturi tube is widely used in hydraulic cavitation [1,2], fluid metering [3], pressure drop detection [4], pressure control [5][6][7] and other occasions.Yin Junlian [8] et al. investigated the effect of different diffusion angles on turbulence in a venture tube at high Reynolds number, and obtained a model of the mean velocity distribution and turbulent kinetic energy distribution of the fluid in the diffusion area.Liu Ping [9] et al. studied the influence of three types of tapering section structure on the internal flow field state and flowlimiting performance of the flow-limiting venturi, and conducted tests on the fluid performance of the venturi with three different tapering sections under normal recharge and critical operating conditions to study its pressure loss and flow-limiting characteristics.SPARROW [10] numerical simulations using the k-ωSST turbulence model for diffusion tubes with diffusion angles of 5°, 10°, and 30° at different Reynolds numbers were performed to analyze the dilatation tubes with three different diffusion angles by dividing the region of the velocity field where the flow velocity is negative into the backflow area.Coiled tubing is a flexible pipe that has been widely used in the oil and gas extraction industry since its introduction.As a new drilling technology with low cost, high efficiency, safety and reliability, coiled tubing drilling has the features of continuous drill column, pressure operation, uninterrupted circulation, and easy pre-setting of optical fibers and cables.However, its limitations, such as the inability to rotate the tubular column, single sliding drilling, lower strength and fatigue life than conventional drilling rods, limited application of large drilling pressure, poor performance in coping with hard formations, and insufficient ability to release the jammed tubulars after encountering the jammed tubulars, have seriously affected the progress and efficiency of its drilling operations in horizontal wells [11].In order to improve the operational efficiency, impact or pressurized tools must be utilized to pressurize the drill bit.In downhole systems with fluid pressure circulation, hydraulic energy is the most direct and easily converted energy.In summary, this paper is based on a coiled tubing drilling pressurizing tool, and through the study of the dimensions of the tapering and diffusion sections of the venturi, the fluid pressure at different positions is controlled, and then the piston displacement is controlled to achieve the purpose of pressurizing the tool at the bottom of the tubular column.

Geometry of the venturi tube
Based on the classical venturi structure for numerical analysis, the geometric parameters of the venturi studied in this paper are shown in Figure 1.Among them, the length of inlet section is 20, the length of outlet straight section is 200, the diameter of straight section is D, the diameter of throat is d, the angle of asymptotic tapering section is α, the angle of asymptotic broadening section is β, the length of contraction section is L1, the length of throat is L2, the length of contraction section is L3, the inlet pressure of straight section is P1, the pressure of vacuum section is P2, and the outlet pressure of straight section is P3.Because the control of pressure drop by venturi is related to the angle of asymptotic section and asymptotic broad section, therefore, the values of α are taken as 30° to 90° and β is taken as 1° to 10° to study the value of pressure and the trend of change in the vacuum section.

Computational fluid dynamics model 2.2.1. Reynolds equation
Venturi tubes are theoretically based on Bernoulli's law and utilize the differential pressure generated by fluid flow to regulate pressure.It is therefore based on steady flow.In this paper, the fluid domain inside the venturi tube is analyzed using Siemens Star CCM+, and the fluid domain is modeled in three dimensions because of the pressure transfer holes (vacuum segments) in the throat section of the venture tube.In order to accurately describe the turbulence characteristics of the flow field inside the venturi when the piston of the pressurized tool is moving under negative pressure, the averaging can be considered as time averaging of the steady state case as well as the overall averaging of the repeatable transient case, i.e., numerical simulations are carried out using Reynolds Averaged Navier-Stokes Turbulence Models (RANS).RANS turbulence models provide closure relations for the Reynolds-Averaged Navier-Stokes equations, that govern the transport of the mean flow quantities.The mean mass, momentum and energy transfer equations are shown in equations ( 1), ( 2) and (3) .The selected K-Omega turbulence model is a two-equation model that solves transport equations for the turbulent kinetic energy k and the specific dissipation rate ω.One reported advantage of the K-Omega model is its improved performance for boundary layers under adverse pressure gradients.Perhaps the most significant advantage, however, is that it may be applied throughout the boundary layer, including the viscous-dominated region, without further modification.Furthermore, the standard K-Omega model can be used in this mode without requiring the computation of wall distance.
Mean Mass Equation: where:ρ is the density. is the mean velocity.   + 2/3 ρk is the modified mean pressure, where  is the mean pressure and k is the turbulent kinetic energy.I is the identity tensor. is the mean viscous stress tensor.  is the resultant of the body forces. is the mean total energy per unit mass. is the mean heat flux.

Mesh settings
Mesh is the basis of finite element analysis, and the mesh setting has a great influence on the accuracy of the analysis results.In this study, a polygonal mesh is used, and in order to obtain a high computational accuracy, the mesh encryption is set for the throat section, as shown in Figure 2. Through the meshindependence verification, the base mesh size of 1mm is finalized according to the calculation results and calculation volume, and the mesh of the throat section is set to 25% of the base mesh size, i.e., 0.25mm.The effect of encryption can be seen very clearly by visual inspection.Local encryption of the mesh is a common method to improve the calculation accuracy and reduce the resource consumption.
Since the vacuum section is short in length, the fluid cannot be fully developed in the simulation, so it is stretched, and the stretched length is ten times of the pipe diameter of the vacuum adsorption section. ( (3)

Boundary conditions and solutions
In the calculation of finite element model, the boundary conditions need to be determined.In this paper, the fluid inlet position is defined as the inlet boundary, the outlet position is defined as the outlet boundary, and the wall surface is defined as the wall during the numerical analysis of the steady-state performance of Venturi pressure control device the fluid domain.The SIMPLE algorithm is adopted to solve the control equation of the pressure-velocity.Finally, the convergence of the residuals can be used to determine whether the system has reached a stable state, and the monitoring of key parameters can also be set to determine whether the system has reached a stable state.According to the parameters required for the design, set the overall pressure drop of the venturi and the pressure monitor at the inlet of the vacuum tube to judge the convergence.Set the variable residual monitor parameters and initialize them for iterative solution calculation.4 shows the velocity distribution of the fluid domain inside the venturi at different β when the asymptotic tapering angle α 90°.As seen through the velocity diagram, when β 1°, the fluid flow rate is stable under this separation angle as the distribution of the fluid is almost in a straight line form due to the small setting of β, as seen through the velocity diagram.Similar to α 30 °, with the increase of β, it can be seen that due to the existence of the separation angle, the upper part of the flow field appeared in a low-speed core area, resulting in the fluid flow bias to the other side of the bias of the fluid is easy to cause the vibration of the system.And with the increasing angle of β, the degree of fluid flow bias increases and moves to the exit direction.It can be seen that the change of β has less effect on the velocity in the flow field.angle increases to 10°, the pressure of the whole broadening section produces pressure drop and the pressure drop value is maximum.Compared with α 30°, range of the low-pressure zone corresponding to the same angle of the broadening section is enlarged, and the system pressure drop value also increases significantly.

Figure 7.
Negative pressure in the vacuum section and system pressure drop of the venturi tube at different angles of the taper section and the taper section.

Conclusions
The role of the venturi in the hydrodynamic impact tool analyzed in this paper is to generate negative pressure in the vacuum section of the throat to reset the piston upward tool and repeat the impact process.The greater the negative pressure, the faster reset speed, which can improve the working frequency of the impact tool.The following conclusions are drawn by finite element analysis of the structure of the tapering and broadening segments at different angles: (1) The velocity distribution of the venturi is greatly affected by the angle of the gradual broad section.The smaller angle, the more stable fluid flow, and the larger angle, the fluid will be biased toward the side without pressure transfer holes, which is easy to cause system vibration.
(2) As can be seen from the previous sets of data, the negative pressure value of the vacuum section is greatly affected by the tapering section and the broadening section, the larger angle of broadening section, the smaller angle of the tapering section, the higher negative pressure value.
(3) When the angle of tapering section α 90° and the angle of broadening section β 1°, the negative pressure of the vacuum section is the largest and reaches 522kPa, and at this time, the total pressure drop of the system is 3.46MPa, and the pressure loss is small, which is favorable for the upward movement

Figure 1 .
Figure 1.Geometrical diagram of the venturi tube.

Figure 3 .
Figure 3. Distribution of fluid velocity in Venturi tube at different Angle of taper section for α 30°.

Figure 3
Figure 3 shows the velocity distribution of the fluid domain inside the venturi at different β when α 30°.

Figure 4 .
Figure 4. Distribution of fluid velocity in Venturi tube at different Angle of taper section for α 90°.Figure4shows the velocity distribution of the fluid domain inside the venturi at different β when the asymptotic tapering angle α 90°.As seen through the velocity diagram, when β 1°, the fluid flow rate is stable under this separation angle as the distribution of the fluid is almost in a straight line form due to the small setting of β, as seen through the velocity diagram.Similar to α 30 °, with the increase of β, it can be seen that due to the existence of the separation angle, the upper part of the flow field appeared in a low-speed core area, resulting in the fluid flow bias to the other side of the bias of the fluid is easy to cause the vibration of the system.And with the increasing angle of β, the degree of fluid flow bias increases and moves to the exit direction.It can be seen that the change of β has less effect on the velocity in the flow field.

Figure
Figure 4. Distribution of fluid velocity in Venturi tube at different Angle of taper section for α 90°.Figure4shows the velocity distribution of the fluid domain inside the venturi at different β when the asymptotic tapering angle α 90°.As seen through the velocity diagram, when β 1°, the fluid flow rate is stable under this separation angle as the distribution of the fluid is almost in a straight line form due to the small setting of β, as seen through the velocity diagram.Similar to α 30 °, with the increase of β, it can be seen that due to the existence of the separation angle, the upper part of the flow field appeared in a low-speed core area, resulting in the fluid flow bias to the other side of the bias of the fluid is easy to cause the vibration of the system.And with the increasing angle of β, the degree of fluid flow bias increases and moves to the exit direction.It can be seen that the change of β has less effect on the velocity in the flow field.

Figure 5 .
Figure 5. Distribution of fluid pressure in Venturi tube at different Angle of tapersection for α 30°.Figure5 shows the pressure distribution in the vacuum section of the venturi at different asymptotic broadening section angles when the asymptotic tapering section angle α 30°.The velocity diagram that when β 1 °, a low-pressure core area appears near the vacuum section through the pressure diagram, and with the increase of the range of the low-pressure area gradually increases.When the angle is increased to 10°, the pressure of the whole broadening section produces pressure drop and the pressure drop value is maximum.

Figure 6 .
Figure 6.Distribution of fluid pressure in Venturi tube at different Angle of taper section for α 90°.Figure6shows the pressure distribution of the vacuum section of the venturi at different asymptotic broadening section angles when the asymptotic tapering section angle α 90°.The velocity diagram shows that when β 1 °, a low-pressure core area appears near the vacuum section through the pressure diagram, and with the increase of β, the range of the low-pressure area gradually increases.When the

Figure 6
Figure 6.Distribution of fluid pressure in Venturi tube at different Angle of taper section for α 90°.Figure6shows the pressure distribution of the vacuum section of the venturi at different asymptotic broadening section angles when the asymptotic tapering section angle α 90°.The velocity diagram shows that when β 1 °, a low-pressure core area appears near the vacuum section through the pressure diagram, and with the increase of β, the range of the low-pressure area gradually increases.When the tapered segment β Vacuum section negative pressure of α 30°V acuum section negative pressure of α 90°S ystem pressure drop of α 30°S ystem pressure drop of α 90°o f the piston.