To predict liquid-gas two-phase flow, accurate tracking of the evolving liquid-gas interface is required. Volume-of-Fluid (VoF) method has been used for computationally modeling such flows where a single set of governing equations are solved for both phases along with an advection equation for the volume fraction. Properties in each cell are determined by a linear weighted average of the properties of the two fluids based on the phase fraction. While the method predicts water-air flows well, the predictions tend to deviate significantly from experiments for liquids with high viscosity. A new property averaging technique is proposed, which is shown to provide accurate results for high viscosity liquids. Computational predictions using open source VoF solver interFoam and the proposed method are compared with experimental data for multiple two-phase applications. Four different problems, viz., suspended drop, jet breakup, drop impact on thin films, and on liquid pools, are considered to extensively validate the new method. Data for aqueous solutions of propylene and ethylene glycol are used to cover a range of surface tension (72 - 36 mN/m) and viscosities (1 - 40 mPa.s). For all cases, the modified VoF solver is observed to perform significantly better than original VoF method. It reduces spurious currents in simulations of drop suspended in air. For the cases of drop impacting on a pool and during drop generation from liquid jets, the time progression of the surface tension governed dynamics is improved from the slower estimate of interFoam solver.

Single-phase and two-phase frictional pressure drop in horizontally oriented double helically coiled tubes confined in a cylindrical shell are experimentally studied using an instrumented test loop that represents a prototypical liquified natural gas (LNG) fuel delivery system for internal combustion (IC) engines. Adiabatic experimental data addressing liquid (water) and gas (nitrogen) single-phase flows, as well as two-phase flows (air–water) in the helicoidally coiled tubes are presented. The range of Reynold numbers for single-phase flow experiments is 2600–4800. In two-phase flow experiments, the only liquid and only gas Reynolds numbers varied in 1030–6600 and 1700–17700 ranges, respectively. In a laminar single-phase flow regime, the measured friction factors are in relatively good agreement with well-established correlations. In the turbulent flow regime, the measured friction factors are moderately higher than the prediction of well-established published correlations. Two-phase flow frictional pressure drops are compared with some relevant correlations, with the poor agreement. The generated experimental data are empirically correlated based on the two-phase flow multiplier concept.

A combined volume of fluid and immersed boundary method is developed to simulate two-phase flows with high density ratio. The problems of discontinuity of density and momentum flux are known to be challenging in simulations. In order to overcome the numerical instabilities, an extra velocity field is designed to extend velocity of the heavier phase into the lighter phase and to enforce a new boundary condition near the interface, which is similar to nonslip boundary conditions in fluid–structure interaction problems. The interface is captured using a volume of fluid method, and a new boundary layer is built on the lighter phase side by an immersed boundary method. The designed boundary layer helps to reduce the spurious velocity caused by the imbalance of dynamic pressure gradient and density gradient and to prevent tearing of the interface due to the tangential velocity across the interface. The influence of time-step, density ratio, and spatial resolution is studied in detail for two set of cases, steady stratified flow and convection of a high-density droplet, where direct comparison is possible to potential flow analysis (i.e., infinite Reynold s number). An initial study for a droplet splashing on a thin liquid film demonstrates applicability of the new solver to real-life applications. However, detailed comparisons should be performed in the future for finite Reynold s number cases to fully demonstrate the improvements in accuracy and stability of high-density ratio two-phase flow simulations offered by the new method.

As a propellant for hybrid rocket engines using liquid oxidizer and solid fuel and for liquid rocket engines, the use of self-pressurized fluids such as nitrous oxide has become widespread. Since these fluids can be self-pressurized by their high saturated vapor pressure, the propulsion system becomes smaller and simpler. However, this self-pressurization generally forms a gas–liquid two-phase flow by flashing or cavitation. This flow is considered highly unsteady because the temperature and pressure greatly change with the discharge process. In this study, unsteady flow characteristics due to self-pressurization were experimentally obtained by conducting many cold flow tests with carbon dioxide as self-pressurizing fluids. As a result, it was clarified that the fluid temperature dropped about 10–15 K with the pressure drop due to feedline pressure loss during the discharge process. From these experimental results, we estimated the bubble growth and void fraction change that would satisfy the temperature drop. In this paper, the obtained test results and estimated temperature drop are reported.

The aim of the present study is to investigate fluid dynamics and pressure drop across sudden contractions in a two-dimensional, axisymmetric pipe carrying a two-phase mixture of air (secondary phase) and water (primary phase), using the Eulerian–Eulerian model of the multiphase flow physics to solve the mass, momentum, volume fraction and turbulent quantities with relevant boundary conditions in a finite volume framework. The realizable per-phase k-ε and Reynolds stress models have been used as the closure for turbulent quantities along with enhanced wall function for the near-wall treatment. The effects of various parameters such as mass flux, mass flow quality, area ratio (0.056–0.619), flow directions (horizontal/vertical), and system pressure on the two-phase pressure drops due to a contraction in the pipe have been quantified. For both the single and two-phase flows, it has been observed that the pressure drop decreases with area ratio, and increases with mass flux and mass flow quality of two-phase flow. The vena contracta for a single-phase flow was found. But for two-phase flow, neither the vena contracta nor the recirculation zone has been observed, as the mass quality exceeds above 50%. A higher pressure drop has been observed for vertical pipes as compared to horizontal pipes. The present numerical results have also been validated with published experimental results, believed to be one of the alternatives to the costly experimental methods for predicting the flow dynamics and pressure drop.

New criteria for fully resolving two-phase flow regime transitions using direct numerical simulation with the level set method for interface capturing are proposed. A series of flows chosen to capture small scale interface phenomena are simulated at different grid refinements. These cases include droplet deformation and breakup in a simple shear field, the thin film around a Taylor bubble, and the rise of a bubble toward a free surface. These cases cover the major small scale phenomena observed in two-phase flows: internal recirculation, interface curvature, interface snapping, flow of liquid in thin films, and drainage/snapping of thin films. The results from these simulations and their associated grid studies were used to develop new meshing requirements for simulation of two-phase flow using interface capturing methods, in particular the level set method. When applicable, the code used in this work, PHASTA, was compared to experiments in order to contribute to the ongoing validation process of the code. Results show that when the solver meets these criteria, with the exception of resolving the nanometer scale liquid film between coalescing bubbles, the code is capable of accurately simulating interface topology changes.

Nitrous oxide is a suitable propellant for rocket engines and has been widely used in various countries, given its high saturated vapor pressure, which enables self-pressurization. Because nitrous oxide is in a state of vapor–liquid equilibrium in tanks, it is easy to form a gas–liquid two-phase flow by cavitation in feed line. Since accurately estimating the performance of rocket engines requires evaluating the characteristics of propellant flows, tests reported in this paper were conducted using hybrid rocket engines under three conditions: cold flow test, hot firing test at low back pressure, and hot firing test at high back pressure. With consideration to the subcooling degrees, nitrous oxide may be in an unsteady superheated state in the upstream flow of the injector. In a comparison of the pressure ratios between the injector in each test condition, it is observed that a critical two-phase flow was formed in the injector in the cold flow test and in the low backpressure firing test. In the high backpressure hot firing test, the injector flow may be choked, but the large oscillations were observed in chamber pressure and thrust. According to the FFT analysis results, these oscillations were caused by chugging and acoustic oscillation. In light of these experimental results, it is suggested that when the chamber pressure fluctuates due to combustion instability such as chugging and acoustic oscillation, it may affect the injector flow characteristics and the critical two-phase flow.

In this study, a numerical simulation method based on Eulerian–Eulerian model and population balance model (PBM) (i.e., computational fluid dynamics (CFD)–PBM coupling model) was developed to investigate the gas–liquid two-phase performance of centrifugal pump under bubble inflow. The realizable k– ε model turbulence model was implemented in ansys fluent solver. The air and water were employed as the working fluids, which was consistent with the experiment. The water head and pressure increment obtained by the experiment were used to validate the numerical method. The results show that the CFD–PBM coupling model is superior to the Eulerian–Eulerian model, particularly in the “surging” conditions. Using the CFD–PBM coupling model, the influences of parameters, such as inlet gas volume fraction, liquid phase flowrate, and rotational speed, on the head and efficiency of the centrifugal pump were investigated. Under the design condition, when the inlet gas volume fraction increases from 3% to 5%, the bubbles form air mass and stagnate in the impeller channel. The stagnated air mass can hardly be discharged with the liquid phase. Thus, the pump head drops suddenly, i.e., the surging occurs. The two-phase performance of centrifugal pump can be improved under the surging condition by increasing the liquid flowrate and the rotational speed to a certain value. The results contribute to an alternative simulation method to investigate the characteristics of bubble flow in pump and shed new lights on the understanding of the performance of centrifugal pumps under two-phase flow conditions.

In this study, upward air–water two-phase flow tests were carried out in a 3 cm diameter pipe under atmospheric pressure, and over 3000 data points were collected from a wide range of superficial gas and liquid velocities (⟨j g ⟩ ≈ 0.02–30 m/s and ⟨j f ⟩ ≈ 0.02–2 m/s) for the investigation of flow regime identification. The probability density function (PDF) of transient void fraction signals and its full-width at half-maximum (FWHM) were obtained and used for analysis and data classification. Considering the features of PDF profiles, the flow conditions can be classified into four regions, which show a fair agreement with the existing flow regime maps in general trends. Furthermore, by examining the FWHM distributions, two more regions with high-FWHM (HF) values were identified as the transitions of higher-flow bubbly-to-slug and slug-to-churn flows as well as most portion of churn flow, and a valley region next to the HF regions can express the transition of churn-to-annular flows. Overall, six groups of flow conditions can be classified based on the present methodology, and each group can be corresponding to specific flow regimes or transition regions. This study can provide a simple and efficient way for flow regime identification.

Gas entrainment into centrifugal pumps decreases pump performance and may raise safety issues, e.g., through insufficient cooling. Although there is some phenomenological knowledge in the form of correlations between operating parameters and pump performance, a further understanding via direct observation of the gas–liquid mixture was so far not possible. In this paper, we demonstrate the capability of ultrafast X-ray computed tomography (UFXCT) to disclose gas–liquid two-phase flow dynamics in the impeller region of a centrifugal pump mockup. Experiments were performed for gas injection at impeller speeds between 1300 rpm and 1600 rpm. We analyzed the X-ray image sequences with respect to characteristics of the gas distribution and compared them with time-averaged image data of a real pump obtained earlier with gamma-ray computed tomography (CT).

Inducers show generally a positive influence on the performance of centrifugal pumps in the two-phase regime, since they produce more uniform mixtures and increase the pressure before the impeller. However, the effect is much more pronounced in part-load compared to overload conditions. In this study, the air–water two-phase flow behavior in a pump inducer was numerically investigated. The main objectives were to clarify the effect of the inducer, the effective operating range, and to examine flow mixing. Several flow conditions were studied, covering part-load, optimal, and overload pumping conditions, together with different relevant gas volume fractions (1%, 3%, and 5%). The simulations were performed using a transient setup and a moving-mesh approach. Two-phase air–water interactions were modeled by the volume of fluid (VOF) method. After checking the proper discretization in space and time, the model was validated against experimental results, revealing excellent agreement. The numerical analysis was able to explain different effects of inducers in part-load and overload conditions. Under overload conditions, the flow separates, leading to the generation of axial vortices and to a negative pressure change across the inducer; additionally, the residence time is reduced, hindering mixing. These vortices are intensified as the gas volume fraction increases, reducing further the pressure downstream of the inducer. This is the reason why inducers can mainly be used in part-load and near optimal conditions in order to improve pumping of two-phase flows.

This paper presents an experimental investigation of the pressure drop (DP) through valves in vertical upward flows. Experiments were carried out using a 1¼″ (DN 32) ball and gate valve. Five opening areas have been investigated from fully open to the nearly fully closed valve, using air with a superficial velocity of 0–3.5 m/s and water 0.05–0.91 m/s. These ranges cover single-phase and the bubbly, slug and churn two-phase flow regimes. It was found that for the single-phase flow experiments, the valve coefficient increases with the valve opening and is the same, in both valves, for the openings smaller than 40%. The single-phase pressure drop increases with the liquid flowrate and decreases with the opening area. The two-phase flow pressure drop was found considerably increased by reducing the opening area for both valves. It reaches its maximum values at 20% opening for the ball valve and 19% opening for the gate valve. It was also inferred that at fully opening condition, the two-phase flow multiplier, for both valves, has been found close to unity for most of the tested flow conditions. For 40 and 20% valve openings the two-phase multiplier decreases in the power-law with liquid holdup for the studied flow conditions. Models proposed originally for evaluating the pressure drop through an orifice in single-phase and two-phase flows were also applied and assessed in the present experimental data.

A three-dimensional (3D) gradient-augmented level set (GALS) two-phase flow model with a pretreated reinitialization procedure is developed to simulate violent sloshing in a cuboid tank. Based on a two-dimensional (2D) GALS method, 3D Hermite, and 3D Lagrange polynomial schemes are derived to interpolate the level set function and the velocity field at arbitrary positions over a cell, respectively. A reinitialization procedure is performed on a 3D narrow band to treat the strongly distorted interface and improve computational efficiency. In addition, an identification-correction technique is proposed and incorporated into the reinitialization procedure to treat the tiny droplet which can distort the free surface shape, even lead to computation failure. To validate the accuracy of the present GALS method and the effectiveness of the proposed identification-correction technique, a 3D velocity advection case is first simulated. The present method is validated to have better mass conservation property than the classical level set and original GALS methods. Also, distorted and thin interfaces are well captured on all grid resolutions by the present GALS method. Then, sloshing under coupled surge and sway excitation, sloshing under rotational excitation are simulated. Good agreements are obtained when the present wave and pressure results are compared with the experimental and numerical results. In addition, the highly nonlinear free surface is observed, and the relationship between the excitation frequency and the impulsive pressure is investigated.

Real-time monitoring of pressure and flow in multiphase flow applications is a critical problem given its economic and safety impacts. Using physics-based models has long been computationally expensive due to the spatial–temporal dependency of the variables and the nonlinear nature of the governing equations. This paper proposes a new reduced-order modeling approach for transient gas–liquid flow in pipes. In the proposed approach, artificial neural networks (ANNs) are considered to predict holdup and pressure drop at steady-state from which properties of the two-phase mixture are derived. The dynamic response of the mixture is then estimated using a dissipative distributed-parameter model. The proposed approach encompasses all pipe inclination angles and flow conditions, does not require a spatial discretization of the pipe, and is numerically stable. To validate our model, we compared its dynamic response to that of OLGA © , the leading multiphase flow dynamic simulator. The obtained results showed a good agreement between both models under different pipe inclinations and various levels of gas volume fractions (GVF). In addition, the proposed model reduced the computational time by four- to sixfolds compared to OLGA © . The above attribute makes it ideal for real-time monitoring and fluid flow control applications.

The key to effective magnetic drug targeting (MDT) is to improve the aggregation of magnetic drug carrier particles (MDCPs) at the target site. Compared to related theoretical models, the novelty of this investigation is mainly reflected in that the microvascular blood is considered as a two-phase fluid composed of a continuous phase (plasma) and a discrete phase (red blood cells (RBCs)). And plasma flow state is quantitatively described based on the Navier–Stokes equation of two-phase flow theory, the effect of momentum exchange between the two-phase interface is considered in the Navier–Stokes equation. Besides, the coupling effect between plasma pressure and tissue fluid pressure is considered. The random motion effects and the collision effects of MDCPs transported in the blood are quantitatively described using the Boltzmann equation. The results show that the capture efficiency (CE) presents a nonlinear increase with the increase of magnetic induction intensity and a nonlinear decrease with the increase of plasma velocity, but an approximately linear increase with the increase of the particle radius. Furthermore, greater permeability of the microvessel wall promotes the aggregation of MDCPs. The CE predicted by the model agrees well with the experimental results.

This study is focused on the development and validation of an analytical model to predict the performance characteristics of multiphase flow twin-screw pump for wide range of operating conditions. A 200 HP, 635 gpm capacity multiphase flow twin-screw pump was tested with inlet pressure varying from 15 psig to 100 psig at gas void fraction (GVF) varying from 0% to 100% to validate the model. A new model is proposed to study the leakage flow in the twin screw pump. Adiabatic compressible flow is assumed in the circumferential clearance. The acceleration of the two-phase flow is taken into account in the new model. The change of Mach number of the leakage flow in the clearance and the possibility of choked flow at the outlet of the clearance was studied. Model provided important information about pressure distribution across the screw length, volumetric efficiency of the pump, and chocked flow condition. Model verification using experimental data concluded the paper.

Pressure drop has been measured for upward single- and two-phase gas–liquid flow across an orifice in a vertical pipe. A conductance probe provided average void fraction upstream of the orifice. Six orifices with different apertures/thickness were mounted in turn in a 34 mm diameter transparent acrylic resin pipe. Gas and liquid superficial velocities of 0–4 m/s and 0.3–0.91 m/s, respectively, were studied. For single-phase flow, pressure drop, expressed as an Euler number, was seen to be independent of Reynolds number in turbulent region. The Euler number increased with decreasing the open area ratio/orifice thickness and increasing velocity. The pressure drop was well predicted by the correlation of Idel'chik et al. (1994, Handbook of Hydraulic Resistances, 3rd ed., CRC Press, Boca, Raton, FL.), which uses a form of Euler number. The corresponding two-phase flow pressure drop depends on the flow pattern. Decreasing open area ratio/orifice thickness increased the pressure drop. For a given liquid superficial velocity, the pressure drop increases with gas superficial velocity except for low open area ratio where this increase is followed by a decrease beyond a critical superficial gas velocity for the high liquid superficial velocities. Relevant correlations were assessed using the present data via a systematic statistical approach. The two-phase multiplier equations of Morris (1985, “Two-Phase Pressure Drop Across Valves and Orifice Plates,” European Two Phase Flow Group Meeting, Marchwood Engineering Laboratories, Southampton, UK.) and Simpson et al. (1983, “Two-Phase Flow Through Gate Valves and Orifice Plates,” International Conference on Physical Modelling of Multiphase Flow, Coventry, UK.) are the most reliable ones.

In order to study the two-phase flow behaviors of a horizontal bubble plume in a tank, experimental tests along with computational fluid dynamics (CFD) simulations were carried out in this paper. An experimental facility was designed and constructed which allows air–water bubble jet being injected horizontally into a water tank by three-parallel injector nozzles with different gas and liquid superficial velocities (〈j g 〉 in = 2.7–5.7 m/s and 〈j f 〉 in = 1.8–3.4 m/s). Two sizes of injector nozzles (D = 0.053 m and 0.035 m) were tested to examine the injector size effect. Important parameters including void fraction, fluid velocity, bubble Sauter mean diameter, and their distributions in the tank were measured and analyzed. In addition to the experimental work, selected flow conditions were simulated with ANSYS CFX 13.0. Compared with the experimental data, the present CFD simulation can predict the general trends of void and flow distributions and the recirculation fluid velocity with an accuracy of ±30%. The present CFD simulation methodology has been validated by the experimental results and can be applied to bubble plume analyses and design.

Fluctuating force induced by horizontal gas–liquid two-phase flow on 90 deg pipe bend at atmospheric pressure condition is considered. Analysis was conducted to develop a model which is capable of predicting the peak force fluctuation frequency and magnitudes, particularly at the stratified wavy two-phase flow regime. The proposed model was developed from the local instantaneous two-fluid model, and adopting guided acoustic theory and dynamic properties of one-dimensional (1D) waves to consider the collisional force due to the interaction between dynamic waves and structure. Comparing the developed model with experimental database, it was found that the main contribution of the force fluctuation due to stratified wavy flow is from the momentum and pressure fluctuations, and collisional effects. The collisional effect is due to the fluid–solid interaction of dynamic wave, which is named as the wave collision force. Newly developed model is capable of predicting the force fluctuations and dominant frequency range with satisfactory accuracy for the flow induced vibration (FIV) caused by stratified wavy two-phase flow in 52.5 mm inner diameter (ID) pipe bend.

In this work, we have studied how gas accumulates in an industrial centrifugal pump under various steady-state two-phase flow conditions. Thereby, we considered both horizontal and vertical pump installation positions. Phase fractions within the impeller region of the pump have been quantitatively disclosed using high-resolution gamma-ray computed tomography (HireCT) and applying time-averaged rotation-synchronized CT scanning technique. The study was made for inlet volumetric gas flow rates between 0% and 5%. To account for different inlet flow conditions, which are assumed to occur during unwanted gas entrainment by hollow vortices, we produced disperse and swirling gas–liquid inlet flows. In this way, the influence of inlet flow boundary conditions on the pump performance as well as gas fraction distributions and gas holdup within the impeller wheel region could be successfully analyzed and compared with respect to the impeller alignment. It was shown that the installation position offers only a minor effect on the pump performance in comparison to the inlet flow conditions. In addition, for the first time, thin gas films at the pressure side of the impeller wheel blades could be visualized in an industrial centrifugal pump.