Investigation on the operation and gas-liquid two phase flow characteristics of vane type deep-sea pump

The development of oil and gas resources towards the deep sea is an inevitable trend in international energy development. Due to its advantages of high production and transportation efficiency, good flow safety and low investment cost, oil and gas mixed transportation technology has become the preferred technology for deep-sea oil and gas production and transportation system. The core device of the oil-gas mixed transportation system is the gas-liquid two-phase flow pump. Under the gas-liquid two-phase condition, the pressure surging will occur with the increase of the inlet gas volume fraction (IGVF) in the pump. At this time, the pressurization capacity of the pump decreases significantly, accompanied by violent vibration, resulting in extremely unstable operation. To solve this problem, the numerical simulation and experiment of the two-phase operation characteristics of a centrifugal pump and a semi-open mixed-flow pump were carried out in this study. The influence of operating parameters on its pressurization capacity was clarified, and the internal gas-liquid distribution was visually tested, revealing the distribution and its spatiotemporal evolution of gas-phase in the impeller. Finally, the unsteady characteristics of pressure surging were analyzed in detail.


Introduction
There are two processes for deep-sea oil and gas exploration, one is traditional oil and gas multiphase gathering and transportation, and the other is multiphase mixed gathering and transportation [1], as shown in Figure 1.The multiphase mixed transportation technology (red line in Figure 1) is superior to traditional separated phase transportation technology (blue line in Figure 1) in overcoming longdistance resistance losses, improving flow safety, saving separation equipment costs, improving production and transportation efficiency, and improving marginal oilfield economy [2,3].Therefore, the mixed oil and gas transportation process has become the preferred solution for deep-sea oil and gas production and transportation systems [4].This technology uses a gas-liquid two-phase flow pump to directly extract well fluid, and a pressurized system to transport oil and gas over long distances.Since the 1980s, the research and development of oil and gas mixed transportation technology has been a hot research topic in developed countries.Especially in the past decade, the United States, the United Kingdom, Germany, Japan, Russia and other countries have invested heavily in research and development, and have successfully applied it to offshore oil recovery projects, achieving very good benefits [5].
The mixed transportation pump is the core equipment of the oil and gas mixed transportation system, with advantages of compact structure, simple mechanical structure, and good resistance to wear and corrosion, etc.There are two kinds of gas-liquid two phase flow pumps commonly used in deep-sea oil field: the vane type and the displacement type.The vane type pump is a speed type pump using blades to transfer momentum to the working fluid to make it flow out of the pump.The commonly used vane type pumps include the centrifugal, the mixed flow, and the helico-axial flow.The screw pump is a displacement type machine which increases the pressure of the working fluid by reducing its volume.It operates well under the condition of high gas volume fraction (GVF), with strong pressurization capacity, hard characteristic curve, low speed, suitable for viscous working medium and so on.However, it is sensitive to solid particles and can only be used in small flow conditions.At the same time, due to its strict sealing requirements, its safe service life is short [6].In contrast, vane type pumps have the advantages of large flow rate, simple structure, light weight, high resistance to solid particles, and can be used in a wide flow rate range.Therefore they are widely used in electric submersible pump systems.The electric submersible pump system, an efficient underground mixed transport pump system that converts kinetic energy into pressure energy, has made significant progerss since its invention by Russian engineer Arutunoff in the 2010s, and has become a widely used artificial lifting method in oil production [7,8].
Under low GVF conditions, centrifugal pumps have strong pressurization ability, but when the GVF is greater than 10%, their performance deteriorates due to the generation of gas pocket flow in the impeller channel.The low pressurization capacity of helico-axial multiphase pumps has become the main factor restricting their development.The performance and structure of mixed flow pumps are between that of centrifugal pumps and helico-axial flow pumps.They have outstanding advantages of large flow rate, good variable speed performance, compact structure, ability to operate under high sand content conditions, easy to start, and high efficiency [9].At the same time, due to the influence of the impeller structure, the centrifugal force and Coriolis force on the fluid in the mixed flow pump are mutually offset, greatly reducing the trend of gas-liquid two-phase separation [10], so that conditions of high IGVF can handled [11][12].However, the phase separation phenomenon still exists in the pump, and pressure surging under high GVF conditions.Therefore, semi open pumps are favored in various fields due to their advantages such as easy maintenance, ability to transport viscous fluids and multiphase media.Due to the presence of blade tip clearance, the internal two-phase flow and pressure surging mechanism in a semi-open mixed flow gas-liquid two-phase flow pump are more complicated.However, some studies have pointed out that at high IGVF conditions, the leakage flow in the blade tip clearance can help improve its pressurization ability.
In gas-liquid two-phase operation, the internal flow of the pump during gas-liquid two-phase operation is extremely unstable, leading to the decrease of its head and efficiency, which affects the normal operation and service life of the pump [13][14][15].Scholars have found that the flow pattern transition caused by the evolution of bubbles in the flow field is closely related to the operating characteristics of mixed transport pumps, and the gas pockets in the flow field are the direct cause of the onset of surging.However, the internal flow field of gas-liquid two-phase flow pumps is relatively complex, and many factors affect the formation of gas pockets, which makes it more difficult to study the mechanism of pressure surging characteristics.Therefore, in this study, the centrifugal and semi-open mixed flow pump were taken as the research object, and the theoretical analysis, experimental verification, and numerical simulation were combined to explore the operating characteristics of gasliquid mixed flow pumps and the flow evolution law of the internal flow field, and several flow field control strategies were proposed.This study has important scientific significance and engineering value for improving the operational stability of mixed transport pumps, prolonging the service life, and optimizing mixed transport pumps.It provides technical support for promoting the development of deep-sea oil and gas resources.

Experimental platform
In view of the on-site demand of deepwater oil and gas development, the theoretical and experimental research on multiphase pump pressurization was carried out based on the research and development platform for deepwater oil and gas production and transmission pipeline multiphase flow experiment built by the multiphase flow State Key Laboratory of Xi'an Jiaotong University.The low pressure visualization test was carried out in the State Key Laboratory of Eco-hydraulics in Northwest Arid Region of Xi'an University of Technology.The experimental pipeline system mainly consists of a liquid phase pipeline, a gas phase pipeline, and a gas-liquid mixture pipeline.The liquid phase pipeline is divided into two branches of large and small size, and the gas phase pipeline is divided into three branches of large, medium, and small size.The physical diagram of the experimental section is shown in Figure 2.

Mathematical model
The gas-liquid two-phase flow in the centrifugal pump is simulated by using the Euler-Euler inhomogeneous flow model [16].The parameter settings in CFX are shown in Table 1.
Table 1.Parameter setting for the numerical simulation.

Setting item Two-phase flow parameters
Working fluid Liquid phase: water Gas phase: air ideal gas    3.

Operating characteristics of the pump
Figure 4 shows the comparison curve of average pressure increase between mixed flow and centrifugal gas-liquid two phase flow pumps.It can be seen that the pressurization capacity of both centrifugal and mixed flow gas-liquid two phase flow pumps decreases with the increase of IGVF.Under the condition of small IGVF, the centrifugal and mixed flow pressurization capabilities are relatively similar.With the increase of IGVF, the turbocharging capability of the mixed flow decreases slowly, and the centrifugal decreases sharply.Especially, when the IGVF exceeds 20%, the centrifugal basically loses its pressurization capacity.

Figure 4.
Comparison of external characteristics between centrifugal and mixed flow pump.Figure 5 shows the average static pressure distribution from the inlet to the outlet of the pump at different GVF conditions.With the increase of GVF, the pressurization capacity of the mixed transport pump gradually decreases.The higher the GVF is, the greater the impact on the pressurization capacity will be.Under different GVF, the change of the pressurization capacity of the final stage mixed transport pump is greater than that of the first stage.At the same time, due the influence of dynamic and static interference and the limited number of blades, the pressure at the inlet of the diffuser fluctuates.Because the guide vane of the mixed pump diffuser has a certain effect of rectification and pressurization, the diffuser pressure also rises slightly in each stage.Figure 8 shows the time-domain distribution of radial force on the impeller after one revolution, which can reflect the magnitude of radial force on the impeller after one revolution.It can be seen that the radial force on the impeller is the highest under pressure surge conditions, mainly due to the turbulent flow field and uneven distribution of fluid inside the impeller, resulting in uneven force on the impeller.The radial force acting on the impeller under pure liquid phase conditions is the smallest followed by the 30% IGVF working condition.Each time the impeller rotates, and the radial force fluctuates for seven cycles, corresponding to the number of guide vanes.It can be seen that the radial force is greatly affected by the dynamic and static interference between the guide vanes and the impeller.Figure 9 shows the radial force vector distribution when the impeller rotates once.The horizontal axis represents the component of the radial force on the X-axis in the Cartesian coordinate system, and the vertical axis represents the component of the radial force on the Y-axis.The (0,0) point represents the center of the main axis, which can reflect the motion trajectory of the impeller rotating around the main axis.From Figure 10, it can be seen that under pressure surge conditions, the radial force of the impeller moves roughly around the center of (0.5, 2.5), and the radial force trajectory undergoes eccentricity.This is mainly due to the uneven distribution of gas phase caused by the generation of airbags in the impeller under this condition, resulting in uneven force distribution on the impeller.The radial force trajectory of pure liquid phase, 3.27% IGVF, and 30.16%IGVF conditions fluctuates roughly around the center point of the spindle (0,0).From this, it can be seen that the gas phase will have an impact on the direction and magnitude of the radial force of the pump, and the unsteady radial force experienced by the pump under gas containing conditions will be intensified.Figure 10 shows the time-domain diagram of the axial force of the second stage impeller after three rotations of the pump.It can be seen that under pressure surge conditions, the axial force decreases faster compared to pure liquid phase conditions and low void fraction conditions, especially in high void fraction conditions (IGVF=30%) where the axial force is the smallest.

Flow field characteristics of the mixed flow pump
When the dimensionless liquid phase flow coefficients Qld are 0.5, 1.0, and 1.2, the velocity vector distribution of the leakage flow at the middle height of the blade tip gap was studied, under conditions of liquid phase and IGVF of 3.27%, 8.62%, and 30.16% respectively, as shown in Figure 11.When the direction of leakage flow points from the blade pressure surface to the suction surface, the angle between the leakage flow velocity vector and the blade bone line is recorded as a positive value.On the contrary, when the direction of leakage flow points from the suction surface of the blade to the pressure surface, the angle between its velocity vector and the blade bone line is recorded as a negative value.
It can be seen from Figure 11 that both the flow rate of liquid phase and the inlet void fraction have an impact on the magnitude and direction of the tip leakage flow velocity.Under low liquid flow conditions, the direction of the leakage flow vector is directed from the blade pressure surface to the blade suction surface, and the angle between the leakage flow vector and the blade bone line is about 90 degrees positive.Under optimal liquid flow conditions, the angle between the leakage flow vector and the bone line on the front chord of the blade is small, and even 0 degrees positive.On the back chord of the blade, the vector is still directed from the blade pressure surface to the blade suction surface, and the angle is about 90 degrees.Under high liquid flow conditions, the angle between the leakage flow vector on the chord length of the front half of the blade and the bone line is 0 or even negative (from the blade suction surface to the blade pressure surface).This is mainly because under low flow conditions, the angle of attack between the fluid velocity direction and the blade inlet tangent is positive.At this time, vortices will be generated on the blade suction surface, while under high liquid flow conditions, the angle of attack between the direction of fluid velocity and the tangent line at the inlet of the blade is negative, and at this time, vortices will be generated on the pressure surface of the blade.12.The numbers in the first row represent the average vorticity of the cross-section, in units of s^-2, and the numbers in the second row represent the inlet void fraction IGVF.It can be seen from Figure 12 that at 0.89 times the blade height, the high vorticity area is mainly concentrated near the blade suction surface.At 0.93 times the blade height, the high vorticity area is mainly concentrated at the blade tip gap.The vorticity is the highest under low liquid flow conditions, followed by the highest efficiency conditions, and the lowest under high flow conditions, mainly caused by the turbulence of flow patterns under low flow conditions.Under low liquid flow conditions, the high vorticity area at 0.93 times blade height section is much larger than that at 0.89 times blade height section.As the liquid flow rate increases, the high vorticity area at 0.93 times blade height section continuously decreases.Under high liquid flow conditions, the area at 0.89 times blade height section is amaller than that of the high vorticity area.The surface average vorticity at the tip gap (Sp=0.93)decreases with the increase of IGVF.Figure12.The vorticity contour of the impeller section.

Conclusion
In this study, the centrifugal and semi-open mixed flow pump was taken as the research object, and a combination of theoretical analysis, experimental verification, and numerical simulation research was used explore the operating characteristics of gas-liquid mixed flow pumps and the flow evolution law of the internal flow field.We established a design method for high-pressure and high IGVF gas-liquid two phase flow pumps, preliminarily created a design technology for high gas content mixed transport pumps, and successfully developed a multiphase mixed transport pump with independent intellectual property rights.The influence of operating parameters on its pressurization capacity was clarified, and the internal gas-liquid distribution was visually tested, revealing the distribution and its spatiotemporal evolution of gas-phase in the impeller.

Figure 1 .
Figure 1.Two ways of oil and gas gathering and transportation.The screw pump is a displacement type machine which increases the pressure of the working fluid by reducing its volume.It operates well under the condition of high gas volume fraction (GVF), with strong pressurization capacity, hard characteristic curve, low speed, suitable for viscous working medium and so on.However, it is sensitive to solid particles and can only be used in small flow conditions.At the same time, due to its strict sealing requirements, its safe service life is short[6].In contrast, vane type pumps have the advantages of large flow rate, simple structure, light weight, high resistance to solid particles, and can be used in a wide flow rate range.Therefore they are widely used in electric submersible pump systems.The electric submersible pump system, an efficient underground mixed transport pump system that converts kinetic energy into pressure energy, has made significant progerss since its invention by Russian engineer Arutunoff in the 2010s, and has become a widely used artificial lifting method in oil production[7,8].Under low GVF conditions, centrifugal pumps have strong pressurization ability, but when the GVF is greater than 10%, their performance deteriorates due to the generation of gas pocket flow in the impeller channel.The low pressurization capacity of helico-axial multiphase pumps has become the main factor restricting their development.The performance and structure of mixed flow pumps are between that of centrifugal pumps and helico-axial flow pumps.They have outstanding advantages of large flow rate, good variable speed performance, compact structure, ability to operate under high sand content conditions, easy to start, and high efficiency[9].At the same time, due to the influence of the impeller structure, the centrifugal force and Coriolis force on the fluid in the mixed flow pump are mutually offset, greatly reducing the trend of gas-liquid two-phase separation[10], so that conditions of high IGVF can handled[11][12].However, the phase separation phenomenon still exists in the pump, and pressure surging under high GVF conditions.Therefore, semi open pumps are favored in various fields due to their advantages such as easy maintenance, ability to transport viscous fluids and multiphase media.Due to the presence of blade tip clearance, the internal two-phase flow and pressure surging mechanism in a semi-open mixed flow gas-liquid two-phase flow pump are more complicated.However, some studies have pointed out that at high IGVF conditions, the leakage flow in the blade tip clearance can help improve its pressurization ability.In gas-liquid two-phase operation, the internal flow of the pump during gas-liquid two-phase operation is extremely unstable, leading to the decrease of its head and efficiency, which affects the normal operation and service life of the pump[13][14][15].Scholars have found that the flow pattern transition caused by the evolution of bubbles in the flow field is closely related to the operating characteristics of mixed transport pumps, and the gas pockets in the flow field are the direct cause of the onset of surging.However, the internal flow field of gas-liquid two-phase flow pumps is relatively complex, and many factors affect the formation of gas pockets, which makes it more difficult to study the mechanism of pressure surging characteristics.Therefore, in this study, the centrifugal and semi-

Figure 2 .
Figure 2. Gas liquid mixed transport pump test section platform.

2. 3 .
Fluid domain and grid generation In this study, the numerical simulation of three stage centrifugal mixed flow pumps and three stage semi-open mixed flow pumps is carried out.The geometric modeling and mesh partitioning are shown in Figure 3. (a) Calculation domain and grid division of three stage centrifugal pump.(b) Calculation domain and grid division of three stage semi-open pump.

Figure 3 . 2 .
Figure 3. Geometric modeling and mesh generation.For the three-stage centrifugal test pump, the design volume flow rate Qd is 26 m3/h, the design head Hd is 25 m, and the design rotational speed nd is 3500 r/min.The fluid domain of the pump is shown in Figure 3(a).The geometric parameters of the impeller are show in Table2.Table2.Geometric parameters of the three-stage centrifugal test pump impeller.

Figure 5 .
Figure 5. Average static pressure distribution from the inlet to the outlet.Based on the characteristics of centrifugal and mixed flow pump, this study has created a new multiphase pressurization technology scheme with the first 15 stages of mixed flow preliminary turbocharging to reduce GVF, and the last 25 stages of centrifugal structure to improve the pressurization efficiency.The experimental results are shown in Figure 6 and Figure 7.The singlephase total pressure of the pump prototype is 11.5MPa.Under the condition of 40% IGVF, the twophase pressure increases by 7.6MPa.

Figure 6 .
Figure 6.Pump performance curve with liquid phase flow rate.

Figure 8 .
Figure 8.Time domain distribution of radial force in a three-stage mixed flow pump.

Figure 9 .
Figure 9. Radial force vector distribution of three-stage mixed flow pump.Figure9shows the radial force vector distribution when the impeller rotates once.The horizontal axis represents the component of the radial force on the X-axis in the Cartesian coordinate system, and the vertical axis represents the component of the radial force on the Y-axis.The (0,0) point represents the center of the main axis, which can reflect the motion trajectory of the impeller rotating around the main axis.From Figure10, it can be seen that under pressure surge conditions, the radial force of the impeller moves roughly around the center of (0.5, 2.5), and the radial force trajectory undergoes eccentricity.This is mainly due to the uneven distribution of gas phase caused by the generation of airbags in the impeller under this condition, resulting in uneven force distribution on the impeller.The radial force trajectory of pure liquid phase, 3.27% IGVF, and 30.16%IGVF conditions fluctuates roughly around the center point of the spindle (0,0).From this, it can be seen that the gas phase will have an impact on the direction and magnitude of the radial force of the pump, and the unsteady radial force experienced by the pump under gas containing conditions will be intensified.

Figure 10 .
Figure 10.Time domain distribution of axial force in a three-stage mixed flow pump.
Vector distribution of tip leakage flow under single-phase water condition.(b) Vector distribution of tip leakage flow at IGVF = 3.27% condition.(c) Vector distribution of tip leakage flow at IGVF = 8.62% condition.(d) Vector distribution of tip leakage flow at IGVF = 30.16%condition.

Figure 11 .
Figure 11.Vector distribution of tip leakage flow.The vorticity distribution of each cross-section under different operating conditions is shown in Figure12.The numbers in the first row represent the average vorticity of the cross-section, in units of s^-2, and the numbers in the second row represent the inlet void fraction IGVF.It can be seen from Figure12that at 0.89 times the blade height, the high vorticity area is mainly concentrated near the blade suction surface.At 0.93 times the blade height, the high vorticity area is mainly concentrated at the blade tip gap.The vorticity is the highest under low liquid flow conditions, followed by the highest efficiency conditions, and the lowest under high flow conditions, mainly caused by the turbulence of flow patterns under low flow conditions.Under low liquid flow conditions, the high vorticity area at 0.93 times blade height section is much larger than that at 0.89 times blade height section.As the liquid flow rate increases, the high vorticity area at 0.93 times blade height section continuously decreases.Under high liquid flow conditions, the area at 0.89 times blade height section is amaller than that of the high vorticity area.The surface average vorticity at the tip gap (Sp=0.93)decreases with the increase of IGVF.Sp = 0.89 Sp = 0.93 0% 3.27% 8.62% 30.16% 0% 3.27% 8.62% 30.16% Figure 11.Vector distribution of tip leakage flow.The vorticity distribution of each cross-section under different operating conditions is shown in Figure12.The numbers in the first row represent the average vorticity of the cross-section, in units of s^-2, and the numbers in the second row represent the inlet void fraction IGVF.It can be seen from Figure12that at 0.89 times the blade height, the high vorticity area is mainly concentrated near the blade suction surface.At 0.93 times the blade height, the high vorticity area is mainly concentrated at the blade tip gap.The vorticity is the highest under low liquid flow conditions, followed by the highest efficiency conditions, and the lowest under high flow conditions, mainly caused by the turbulence of flow patterns under low flow conditions.Under low liquid flow conditions, the high vorticity area at 0.93 times blade height section is much larger than that at 0.89 times blade height section.As the liquid flow rate increases, the high vorticity area at 0.93 times blade height section continuously decreases.Under high liquid flow conditions, the area at 0.89 times blade height section is amaller than that of the high vorticity area.The surface average vorticity at the tip gap (Sp=0.93)decreases with the increase of IGVF.Sp = 0.89 Sp = 0.93 0% 3.27% 8.62% 30.16% 0% 3.27% 8.62% 30.16%

Table 3 .
Geometric parameters of the three-stage semi open mixed flow pump.