Gas Volume Fraction Research Articles

Transient simulation of cavitation evolution process in a diesel engine injector

In this study, a method for simulating the whole transient process of the cavitation evolution in a diesel injector was proposed. Diesel is taken as the fuel. The basic characteristics of partial, local, and super cavitation phenomena inside the injector were reproduced and discussed at different time steps. On the basis of movement features of the injector needle, Firstly, the moving process of the diesel injection such as the upward, suspension, and downward movements were simulated under different injector geometric parameters. The time-dependent movement process of the injector needle were controlled by a user-defined program developed by ourselves. Thereafter, a dynamic grid system and a two-phase flow cavitation numerical model were constructed and adopted to numerically reproduce the two-phase flow and the cavitation inside the injector. It was found that the volume fraction of the gas diesel during the partial cavitation is greater than that during the super-cavitation. The faster the needle valve opens, the earlier super-cavitation forms, but the gas volume fraction of diesel decreases faster in the injection orifice. Increasing the drain orifice diameter is helpful for earlier formation of the super-cavitation.

Numerical simulation of particle erosion in the vertical-upward-horizontal elbow under multiphase bubble flow

Pipe erosion is one of the main reasons for oil leakage and safety accidents in many engineering fields. The purpose of this study is to investigate the impact of gas volume fraction, elbow angle and rib installation position on pipe erosion under multiphase bubble flow. The computational fluid dynamics (CFD) simulation approach based on volume of fluid (VOF) model and discrete phase model (DPM) was adopted to analyze the effect of gas-liquid distribution and particle movement on pipe erosion under different gas volume fractions. A good agreement of pipe erosion distribution was observed between the simulation results and measurement values. Furthermore, the CFD simulation results show that the pipe erosion rate increases rapidly as the gas volume fraction increases. The erosion rate also increases as the elbow angle increases from 15° to 90°. The rib at small installation position angles can protect the pipe from the direct impact of particles.

Numerical simulation of multi-size bubbly flow in a continuous casting mold using an inhomogeneous multiple size group model

Multi-size bubbly flow in a one-quarter-scale water model of a continuous casting mold was simulated using the inhomogeneous multiple size group (IMUSIG) model. The Sauter mean bubble diameter, gas volume fraction, interfacial area density, turbulence kinetic energy, and velocity field predicted by the IMUSIG and multiple size group (MUSIG) models were compared. The IMUSIG model separated the motions of the small and large bubbles on the basis of two velocity groups, and both models adopted 15 size groups. The results showed that the IMUSIG model better reflected the bubble size distribution near the nozzle. Small bubbles were further transported by the jet flow, which enlarged the bubble cluster range and decreased the gas volume fraction and interfacial area density in the upper part of the submerged entry nozzle, likely diminishing the coalescence rate in this region; consequently, the overestimation of Sauter mean bubble diameter outside the nozzle was corrected. The liquid-phase flow fields predicted by the IMUSIG and MUSIG models were very similar, and the IMUSIG-predicted magnitudes of velocity and turbulence kinetic energy were only slightly lower than the MUSIG predictions; this slight difference is attributable to the expanded bubble cluster range and higher interfacial area density, which weakened the upper recirculation in the IMUSIG model.

Investigation on the Influence of Entrained Air to Internal Flow of Centrifugal Pumps under Cavitation Condition

The introduction of air is often used in engineering to delay the occurrence of cavitation. A steady simulation on a single-stage and single-suction centrifugal pump was used to study the influence of the air on the internal flow under cavitation condition at 1% inlet void fraction. Steady simulation based on SST k-ω turbulence model and Rayleigh-Plesset cavitation model included in Ansys CFX 17.0 were processed to obtain vapor volume fraction, gas volume fraction and the turbulent energy of the impeller under the condition of natural cavitation and cavitation with entrained air. A homogeneous two-phase flow model was adopted to describe the air-water mixed flow. The experimental cavitation performance curve provides boundary condition support for the numerical simulation. In conclusion, it shows that the head of the pump keeps steady after 1% air entrained like natural cavitation. Then it goes worse when the cavitation number decreases but with lower NPSHr which means cavitation performance is improved by 1% air entrained. It indicated that the entrained air changes the distribution of the vapor in the passage of the impeller. However, the descent of the pressure over a certain value will also do harm to the internal flow of the model pump whether it is ventilating or not. The internal flow of the pump will be seriously damaged and even be blocked completely due to more and more vapor generated causing a dramatic decline of the head.

Failure analysis of ammonium chloride salt coagulation corrosion of U-tube heat exchanger in diesel hydrogenation unit

Diesel hydrogenation unit is an important equipment for processing inferior oil in petroleum refining enterprises. Among them, the U-tube heat exchanger has been in high temperature and corrosive environment containing chlorine for a long time. Serious problem of tube bundle corrosion failure caused by ammonium chloride salt deposit. Anatomy of the diesel hydrogenation heat exchanger showed that the leakage part mainly appeared in the outermost bundle of the heat exchanger tube near the exit section, and the local area showed a large area of gully damage. Metallographic and XRD results show that the failure tube bundle material does not have manufacturing defects, and the corrosion location scale contains NH4Cl and iron oxides. The process flow analysis based on PR-NRTL model showed that the heat exchanger was in the NH4Cl crystallization temperature range, and the crystallization temperature increased with the increase of N and Cl contents in raw materials. Fluid simulation analysis of inlet tube box and single U-tube is carried out by using component transport equation and condensation model. Computational fluid dynamics (CFD) results indicate that there is a low velocity zone with high volume fraction of gas phase in the tube box. The higher gas volume fraction results in the formation of more NH4Cl particles into the U-tube bundle during heat transfer cooling. With the decrease of temperature, liquid water content in U-tube increases gradually and mainly distributes at both sides and bottom of tube bundle. The gas phase gradually fills the top of the tube bundle, and the oil and gas stratification becomes more obvious. The mass transfer rate and corrosion rate were the highest at the bottom and both sides of the tube bundle. With the temperature decreases, the NH4Cl particles generated in the tube continue to increase. The particles were deposited at 2000–3000 mm near the outlet section of the U-tube tube. NH4Cl particles absorbed liquid water, resulting in under-deposit corrosion of the tube bundle, and eventually led to the thinning and leakage of the heat exchanger tube bundle.

Estimation of the local convective heat transfer coefficients of low frequency two-phase pulsating impingement jets using the IHCP

ABSTRACT In this paper, a numerical-experimental investigation was performed for estimation of the local convective heat transfer coefficients of a two-phase pulsating heated jet impinging a cold disk-shaped mass. By solving the energy equation, temperature history of thermocouples was obtained and then using the “temperatures measured” at locations of the sensors and the “temperatures calculated” from the numerical solution of heat equation and then minimizing the sum of squared errors by the conjugate gradient method (CGM), the convective heat transfer coefficients are estimated using the nonlinear inverse heat conduction procedure (IHCP). To compare the heat transfer characteristics of the pulsating, steady, bubbly, and liquid-only jets, the heat transfer coefficients were estimated with respect to the square pulse frequency produced by a solenoid valve in the range of and the gas volume fraction in the range of . In the case of single-phase jet, results show that flow pulsation increases Nusselt number up to at the stagnation point and for farther radial distances as compared to the steady jet. Also, for steady jet, increasing Reynolds number resulted in a increase in the stagnation point Nusselt number and a increase at farther radial distances.

A Nodal Analysis Based Monitoring of an Electric Submersible Pump Operation in Multiphase Flow

Electrical submersible pump (ESP) operation is compromised by free gas, resulting in premature pump failure and production losses in new wells. It is essential to detect the onset of abnormal operations. We develop a model that predicts abnormal ESP operation when the free gas level increases in the pump. The model compares operation parameters with the parameters of normal operating ranges; it shuts down the ESP when necessary. We used a Schlumberger PIPESIM software (version 2017.01) to perform nodal analysis technique; we tested the model using the other multiphase correlation model and field case studies (where the gas problem in ESP was reported). We employ a homogenous model to calculate the differential pump pressures at various gas volume fractions. Nodal analysis of the intake and discharge point predicted the commencement of abnormal ESP conditions and the associated parameters (critical gas fraction, minimum operating pump intake pressure, and pump discharge pressure). The model results were similar to other surging correlation models (e.g., Romero, Dunbar, Turpin, Cirilo, and Zhou models); they were also identical to field case studies. We identify three performance stability phases when an ESP is exposed to free gas. These are the normal and abnormal operating ranges, as well as the ESP shutdown condition. Modeling permits careful monitoring of ESP operations that can be compromised by free gas.

Experimental Study on the Effect of Gas Volume Fraction on the Cavitation Performance of a Low-Specific-Speed Centrifugal Pump

In order to study the cavitation performance of centrifugal pumps with low specific speeds under the condition of gas–liquid two-phase flow, a cavitation test rig for pumping gas–liquid two-phase flow was set up. The cavitation performance of the pump with a specific speed of 32 was studied. The variation of the head, pressure pulsation intensity, and vibration intensity with the cavitation allowance NPSHa (Net Positive Suction Head available) of the centrifugal pump were obtained at different inlet gas volume fraction (IGVF) conditions of 0, 1%, 2%, and 3%. The results show that the cavitation performance of a low-specific-speed centrifugal pump can be improved obviously in a certain liquid flow range when the IGVF is 1%, especially at a low liquid flow rate. When cavitation did not occur or the degree of cavitation was low, a lower IGVF can reduce pressure pulsation intensity at the pump outlet and the vibration intensity at the pump inlet under design flow rate and high flow rate conditions. Additionally, all performances of the low-specific-speed pump are more sensitive to gas when the liquid flow rate is low. The results can provide a reference for improving the cavitation performance of low-specific-speed pumps for transporting gas–liquid two-phase flow and single-phase liquids.

Effect of pressure fluctuation in oil-gas multiphase pump on cavitation and performance of sealing liquid film

During the operation of the oil-gas multiphase pump, due to the variable gas content of the incoming flow, the medium pressure at the high-pressure end fluctuates widely, which is easy to cause the stress change at both ends of the mechanical seal compensation ring and “instability” phenomenon. In this paper, a three-dimensional spiral groove seal film model is established. Considering the cavitation effect of liquid film gap, the variation law of end face cavitation and sealing performance of liquid film seal is explored according to the effect of pressure pulsation in the pump on the inner diameter side (Static mechanical seal, PLAN74 flushing scheme in API 682) and outer diameter side (Rotary mechanical seal, PLAN53A flushing scheme in API 682). The results show that the local pressure drop in the sealing liquid film is the main cause result to cavitation of liquid film. The inner diameter side pressure fluctuation has a great impact on the end face cavitation, and the wave peak is about 100 times that of the outer diameter side pressure. The inner diameter side pressure fluctuation should be avoided in engineering application. There is hysteresis between gas volume fraction (GVF) and boundary pressure fluctuation. The synchronization between sealing performance parameters and outlet pressure fluctuation is good, but there is phase difference with inlet pressure fluctuation. The inner diameter side pressure fluctuation has a great change on the sealing performance. The peak is about 5–20 times that of the outer diameter side pressure, which is easy to cause vibration of the mechanical seal. The amplitude and period of pressure fluctuation have a great impact on the GVF and sealing performance. The larger the fluctuation period is, the less obvious the hysteresis of GVF is, and the closer to the variation law of pressure fluctuation. During the actual operation of the oil-gas multiphase pump, the mechanical seal connected with the medium in the pump is rotary, and the flushing scheme ought to be PLAN53A to ensure the safe and stable operation of the oil-gas multiphase pump.

An experimental study on the multiscale properties of turbulence in bubble-laden flows

The properties of bubble-laden turbulent flows at different scales are investigated experimentally, focusing on the flow kinetic energy, energy transfer and extreme events. The experiments employed particle shadow velocimetry measurements to measure the flow in a column generated by a homogeneous bubble swarm rising in water, for two different bubble diameters ($2.7$mm and$3.9$mm) and moderate gas volume fractions ($0.26\,\%\sim 1.31\,\%$). The two velocity components were measured at high resolution, and used to construct structure functions up to twelfth order for separations spanning the small to large scales in the flow. Concerning the flow anisotropy, the velocity structure functions are found to differ for separations in the vertical and horizontal directions of the flow, and the cases with smaller bubbles are the most anisotropic, with a dependence on void fraction. The degree of anisotropy is shown to increase as the order of the structure functions is increased, showing that extreme events in the flow are the most anisotropic. Our results show that the average energy transfer with the horizontal velocity component is downscale, just as for the three-dimensional single-phase turbulence. However, the energy transfer associated with the vertical component of the fluid velocity is upscale. The probability density functions of the velocity increments reveal that extreme values become more probable with decreasing Reynolds number, the opposite of the behaviour in single-phase turbulence. We visualize those extreme events and find that regions of intense small-scale velocity increments occur near the turbulent/non-turbulent interface at the boundary of the bubble wake.

Numerical simulation and experimental study on gas–liquid two-phase unsteady flow in a centrifugal pump during the transition process

In order to study the transition process of the gas-liquid two-phase flow in the centrifugal pump, the model pump was studied experimentally. At the same time, the Eulerian-Eulerian heterogeneous flow model and the SST k- ω turbulence model were used to simulate the transition process of the gas-liquid two-phase flow in the pump. The two-phase flow transition process in the pump was studied in detail by using the numerical model verified by experimental results. The flow law of the transition process in the pump was revealed. The judgment criterion for the end of the gas-liquid two-phase flow transition process in the pump was determined. The results show that the gas volume fraction at the pump outlet is more accurate than the outlet pressure. After 0.45 s, the gas volume fraction at the pump outlet does not change with time. It can be regarded as the end of the transition process. The average gas volume fraction at each monitoring point increases rapidly with time, then decreases slowly, and finally reaches stability. The average pressure at each monitoring point first drops rapidly, then rises slightly, and finally reaches a stable value. Along the radial direction of the section VIII in the volute, the amplitude of the gas velocity fluctuation gradually decreases, and the gas velocity gradually changes from greater than the liquid velocity to less than the liquid velocity since the driving effect of the impeller becomes weak. This study is of great significance to further improve the structural design of the gas-liquid two-phase flow pump and improve the stability of the pump operation.

The rise of bubbles in shear thinning viscoelastic fluids

Bubbles in a liquid rise under gravity and separate to the top. Bubbly liquids exist commonly in nature and play a significant role in energy-conversion, oil and chemical industries. Therefore, understanding how bubbles rise is of great importance. Rheological properties of the fluid have a strong impact on single bubble rise and have been shown to change collective bubble rise at low gas volume fractions significantly. We expect that a viscoelastic fluid can strongly modify the rise of bubbles in more concentrated suspensions.We generate bubbly liquids up to gas fractions of 30 %. We measure the bubble size and the rise velocity in micellar solutions made of cetyltrimethylammonium bromide (CTAB) and sodium salicylate (NaSal), a common system to create shear-thinning solutions.We show that when the NaSal concentration is small and the solutions are Newtonian, the bubble rise velocity decreases with increasing volume fraction of bubbles and the relationship between the two follows the Richardson-Zaki prediction. For the shear thinning viscoelastic solutions, the Richardson-Zaki relation no longer applies. Bubble clustering leads to faster rise velocities and a weaker dependence on the bubble volume fraction. At the largest concentration two rise regimes are observed. A fast one similar to that in the other shear thinning samples, followed by a very slow bubble rise. The slow rise velocity is attributed to the smallest bubbles rising so slowly, that at the shear rates around them, the fluid behaves as a Newtonian fluid. Therefore, bubble rise becomes again comparable to Stokes expectations. We also show that the peculiar dependence of the rise velocity with volume fraction of bubbles in the shear thinning viscoelastic solutions can have important implications in flotation as the area flux changes strongly with bubble volume fraction.

Study on a new method for generating a gas curtain in the gas-curtain launching

To decrease the launching resistance of underwater guns, a new gas-curtain formation method is proposed based on the gas-curtain launching principle, which is setting up several rectangular grooves on the internal wall of the barrel to guide the gas flow to the front of the projectile and form a gas curtain. A three-dimensional unsteady two-phase flow model of the drainage is established. The formation and drainage characteristics of the gas curtain are studied under the different number of grooves with a constant injection area and 10 MPa injection pressure. The results indicate that the circumferential and radial converging effects of multi-gas jets are enhanced, but the axial expansion is enhanced first and then weakened when the number of grooves increases from 4 to 12. The vortex scale in the Taylor cavity increases with the increase in the number of grooves, but the distance moving downstream is shortened. The gas volume fraction in the tube increases with the increase in the number of grooves. However, the drainage efficiency of the gas curtain on the warhead increases first and then decreases with the increase in the number of grooves, while the change of the gas efficiency–cost ratio is just the opposite.

Numerical Simulation of Hydrofoil Cavitation in Pitching Motion

Based on CFD, the NACA4412 pitching motion hydrofoil is compared and analyzed for the lift/drag coefficient, the pressure and the gas volume fraction at different attack angles. The result is that the lift/drag coefficient curve of the dynamic hydrofoil is a closed curve. When the hydrofoil is pitching up, the lift/drag coefficient will be generally greater than that in the static state, which first increases and then decreases as attack angles increases. When the hydrofoil is down, the lift/drag coefficient will be generally smaller than that in the static state, which first decreases and then increases as attack angles decreases. Moreover, when the hydrofoil is pitching upwards, there will be a sudden change in pressure on the subsurface, resulting in an extreme value. In addition, there is more comprehensive cavitation on the upper surface of the dynamic hydrofoil, and local cavitation on the subsurface.

One-step generation and characterisation of aerated emulsions using laser diffraction

Dispersions of bubbles and drops in water are widely encountered in many fields, from food industry to cosmetics and oil recovery. Their generation typically requires two steps: either the emulsion is generated first and then aerated, or an emulsion and a foam are generated separately and then mixed. While a variety of techniques to characterise single dispersions of oil or gas exist, the characterisation of composite systems is usually more tricky. In this work, we show that aerated emulsions can be generated by mixing the oil, gas and aqueous phases all together in one step using the double-syringe technique, without the need of two separate processes. We show that such one-step generation ensures a good scale separation between bubbles and drops, with a size ratio in first approximation constant over a very wide range of oil and gas volume fractions. Furthermore, we show that laser diffraction granulometry, so far used to measure only bubbles or drops separately, can be used to rapidly assess the bubble size in these composite dispersions for bubbles up to 100 μm in diameter. We show that the results are in excellent agreement with a well-established direct technique to measure bubble size in foams as optical microscopy, and we explore the main limits within which the light scattering measurements are robust.

Propagation characteristics and application effects of measurement-while-drilling pressure wave for early gas-kick detection

An important technical measure to ensure drilling safety is to use advanced well-control technology to detect the early gas-kick. This research proposed an early gas-kick detection method based on the propagation characteristics of measurement-while-drilling (MWD) pressure wave, which was completely different from the conventional detection method, to timely detect gas-kick. In this work, we simulated the propagation characteristics of the pressure wave in the annulus gas-liquid two-phase flow by establishing an unsteady mathematical model for the first time, breaking through the limitations of testing the propagation characteristics of the pressure wave through physical experiments and revealing the law of propagation and attenuation of the pressure wave under arbitrary variable values. Second, we conducted field tests on the application effects of this technology based on the time difference of the pressure wave propagation in the gas-liquid two-phase flow and liquid single-phase flow, which confirmed the feasibility and advancement of this technology. Research results showed that gas volume fraction, system pressure, angular frequency, virtual mass force, drag force, and shear stress would affect the propagation and attenuation of the pressure wave in the annulus gas-liquid two-phase flow to varying degrees. Moreover, the application effects showed that using this technology to detect the occurrence of early gas-kick could be approximately 7–9 min earlier than the conventional detection method.

Investigations on the effect of rotational speed on the transport of air-water two-phase flows by centrifugal pumps

This study presents experimental and numerical investigations on air-water flows in a centrifugal pump, extending our previous studies. The main objective of the present article is to examine the influence of rotational speed on pump performance. For that purpose, the pump performance was considered and extensively compared for two different rotational speeds, i.e., 650 rpm and 1000 rpm, covering different single and two-phase flow conditions. To allow flow visualization, the impeller and the pump body were manufactured from transparent materials, and a high-speed camera was employed for recording the two-phase flow regimes. The performance of the pump was illustrated and reported for either a constant gas volume fraction or a constant air flow rate at the pump inlet. The desired operating conditions at the inlet were achieved using various predetermined experimental procedures to study the performance hysteresis. Pump surging and breakdown conditions were also considered. Additionally, 3D URANS single-phase numerical simulations were performed to explain additional internal flow details. The moving-mesh approach combined with the k-ω SST turbulence model has been used for that purpose. The simulations agree well with the experiments in most cases. Overall, the results show that increasing the rpm enhances two-phase mixing, which leads to various and significant improvements regarding two-phase pumping performance.

Experimental Study of Gas–Liquid Pressurization Performance and Critical Gas Volume Fractions of a Multiphase Pump

Abstract Gas entrainment may cause pressurization deterioration and even failure of pumps under conditions of high inlet gas volume fraction (GVF). When the inlet GVF increases to a critical value, an obvious deterioration performance of pump occurs. Air–water pressurization performance and inlet critical GVFs of a centrifugal multiphase pump are investigated experimentally under different inlet pressures and gas–liquid flow rates. To determine the first and second critical GVFs, a new method is proposed by computing the local extreme points of the second derivative of performance curves. New prediction correlations for two critical GVFs are established with relative errors lower than ±10% and ±8%. Boundaries of three different flow patterns and the transition flow rates are determined and presented by critical GVFs on the flow pattern diagram. Moreover, boundaries of maximum pressurization are determined by performance curve clusters and a power function correlation of gas–liquid flow rates when reaching the maximum pressurization is established. With the increase of inlet pressure from 1 MPa to 5 MPa, two-phase pressurization performance is significantly increased; occurrences of pressurization deterioration are obviously delayed with the first and second critical GVFs increasing by maximums of 8.2% and 7.1%.

Effect of the inlet gas volume fraction on the turbulent dissipation characteristics in the multiphase pump

The internal flow of the multiphase pump is complicated owing to its specific structure. To reveal the effect of the inlet gas volume fraction (IGVF) on the turbulent dissipation characteristics, the method of combining numerical simulation based on k-ε turbulence model with experiment was adopted, and the turbulent dissipation of the multiphase pump was quantitatively and qualitatively analyzed in both the pure water and gas-liquid two phases condition. Results showed the vortexes were primarily distributed in the diffusers at different inlet gas volume fractions (IGVFs), near the middle of the first diffuser and the outlet of the next diffuser. At the same time, the larger value of the turbulent dissipation than that in the impellers was concentrated in the inlet and outlet of the impellers and diffusers. In addition, the effect of IGVFs on the turbulent dissipation increased gradually from the hub to the shroud at the inlet section of the first impeller. Moreover, the turbulent dissipation became increasingly unsymmetrical from the hub to the shroud at the outlet section of the first impeller.

Numerical Investigation of Turbulence Production Under Two-Phase Flow in a Simulation of Electrical Submersible Pump Performance

Abstract Investigating the characteristics of gas–liquid two-phase flow in a centrifugal pump is critical for evaluating pump performance. In this study, a numerical simulation was performed to understand the effects of gas–liquid turbulence formation in the impeller channels on the performance of a single-stage centrifugal pump. The focus was on the inlet gas volume fraction (IGVF) at constant flowrate and constant impeller speed on the flow structure in the impeller and the pressure surging characteristics. The correlation between Reynolds shear stress and turbulent kinetic energy on pump performance is shown by examining pressure, velocity, turbulent kinetic energy, and gas volume fraction (GVF). The model verification is verified using the theoretical pump pressure head determined by the manufacturer, which demonstrated adequate uncertainty (μ<5%) for a flowrate of 250 L/min. The results show a 9.41% decrease in wall shear stresses when the in-situ GVF varies between 1% and 10%. Wall shear stresses combined with circulation resulted in a 3.76% decrease in turbulent kinetic energy, while the contribution of turbulence to pump performance degradation is minimal. Other factors such as gas entrapment, impeller inlet blockage due to bubble coalescence, pressure gradient, and bubble size had a much greater impact on performance. The performance decreased dramatically by 26.53% when the GVF increased from 1% to 10%. The proposed flow structure can be further investigated together with the wall shear stress and circulation phenomena, such as vorticity and shear layers in the context of two-phase flow.