Venturi Geometry Research Articles

Transient effects by pintle motion in cavitating venturi tube for throttled bipropellant thruster

An experimental study was conducted to investigate the behavior of an H2O2/kerosene bipropellant thruster during throttling. The thruster used 90 wt% hydrogen peroxide, and thrust was controlled using a variable area cavitating venturi valve equipped with a linear motor translating a pintle in a venturi geometry. In particular, transient effects were compared for three settings of pintle motion. Transient effects impacting pressure, temperature, thrust, efficiency, and instability data were analyzed. The results show that even at low chamber pressure, ignition occurred due to throttling, and no instabilities were created by changing the throttling speed. On the contrary, transient movements affected the recorded pressure data, impacting the characteristic velocity and thrust. Finally, this study provided new insight into H2O2/kerosene bipropellant throttling for deep throttling missions. Moreover, using the results from this study, the precision of future control algorithms will be improved.

Experimental study on the effect of throat length in the dynamics of internal unsteady cavitating flow

Cloud cavitation, both in external and internal flow fields, has been an active field of research because of its different harmful effects, such as noise, vibration, and material damage, in several applications. In the present work, the same is studied experimentally using venturi geometries. Venturi geometry was selected because of its diverse applications. The two venturi geometries chosen are nearly identical in all respect except the throat length. The influence of throat length is examined in this study because previously, these two venturi geometries (with and without throat) produced contradictory results in terms of the underlying mechanisms of cavity shedding, namely, re-entrant jets and condensation shocks observed at different cavitation numbers. Different diagnostic strategies were adopted to characterize cavitation events, viz., sound pressure level, dynamic pressure fluctuations, and high-speed imaging. High-speed images were studied to obtain mean cavity length. Proper orthogonal decomposition along with wavelet analysis was also employed. From these analyses, it was shown that for the venturi with 23 mm throat length, the condensation shock is followed by the re-entrant jet as cavitation number is reduced, while reverse is seen for venturi with zero throat length. Simulations of unsteady, non-cavitating, turbulent flow through these venturis show that this difference in the order of predominance of the two mechanisms can be explained by the product of cavity thickness (approximated by boundary layer height) and average pressure gradient value.

Numerical investigation of three-dimensional partial cavitation in a Venturi geometry

Sheet cavitation appears in many hydraulic applications and can lead to technical issues. Some fundamental outcomes, such as, the complex topology of 3-dimensional cavitation pockets and their associated dynamics need to be carefully visited. In the paper, the dynamics of partial cavitation developing in a 3D Venturi geometry and the interaction with sidewalls are numerically investigated. The simulations are performed using a one-fluid compressible Reynolds-averaged Navier–Stokes solver associated with a nonlinear turbulence model and a void ratio transport equation model. A detailed analysis of this cavitating flow is carried out using innovative tools, such as, spectral proper orthogonal decompositions. Particular attention is paid in the study of 3D effects by comparing the numerical results obtained with sidewalls and periodic conditions. A three-dimensional dynamics of the sheet cavitation, unrelated to the presence of sidewalls, is identified and discussed.

Experimental investigation of a cavitating Venturi and its application to flow metering

An experimental investigation has been carried out to evaluate the performance of a cavitating Venturi flow. For that purpose, a closed loop circuit with a centrifugal pump and a transparent asymmetric converging-diverging test section has been built which allows to set the pressure level and the flow rate. The system is instrumented with several pressure sensors and temperature probes that are continuously monitored during the tests. The experiments have consisted in generating non-cavitating and cavitating flows inside the Venturi under controlled conditions. The obtained results, which have been characterized as a function of the Venturi's discharge coefficient, pressure ratio and pressure loss coefficient, are in good agreement with previous studies carried out with standard Venturi geometries, specially under non-cavitating flows. The Venturi's performance under cavitation flows has been found to be dependent on the Venturi's inlet pressure and similar to a chocked flow condition with constant volumetric flow rate. On the basis of these observations and the analogous behaviour with compressible gas nozzles, a new flow coefficient has been derived which remains constant at any cavitating regime. Thus, this coefficient permits to use a Venturi as a flow meter on cavitation conditions.

Development of Attached Cavitation at Very Low Reynolds Numbers from Partial to Super-Cavitation

The present study focuses on the inception, the growth, and the potential unsteady dynamics of attached vapor cavities into laminar separation bubbles. A viscous silicon oil has been used in a Venturi geometry to explore the flow for Reynolds numbers ranging from Re=800 to Re=2000. Special care has been taken to extract the maximum amount of dissolved air. At the lowest Reynolds numbers the cavities are steady and grow regularly with decreasing ambient pressure. A transition takes place between Re=1200 and Re=1400 for which different dynamical regimes are identified: a steady regime for tiny cavities, a periodical regime of attached cavity shrinking characterized by a very small Strouhal number for cavities of intermediate sizes, the bursting of aperiodical cavitational vortices which further lower the pressure, and finally steady super-cavitating sheets observed at the lowest of pressures. The growth of the cavity with the decrease of the cavitation number also becomes steeper. This scenario is then well established and similar for Reynolds numbers between Re=1400 and Re=2000.

A novel method for optimization of slit Venturi dimensions through CFD simulation and RSM design.

This research presents a novel comprehensive method for optimizing the design of cavitating slit Venturi for a given cavitation intensity. This method is applicable to any cavitation number and can be used to provide the Venturi geometry that is suitable for a specific application. In this paper, cavitating Venturi design process is represented in seven steps. As an example, for the cavitation number of 0.2, geometrical and operational parameters of the Venturi were determined using the proposed seven steps. During the design process, the Venturi discharge coefficient was calculated using computational fluid dynamics (CFD) simulations. Furthermore, Venturi parameters such as inlet pressure, throat area, width, length, height and divergence angle, were optimized by the combination of CFD and Response Surface Methodology (RSM). In addition to calculating the mentioned optimum parameters, other hydraulic parameters of Venturi including discharge coefficient, flowrate, throat velocity, cavitation volume and length were also determined. Finally, the proposed design method in this study was verified by conducting sets of laboratory experiments.

Numerical Prediction of Various Cavitation Erosion Mechanisms

Abstract Numerical prediction of cavitation erosion is a great scientific and technological challenge. In the past, many attempts were made—many successful. One of the issues when a comparison between a simulation and erosion experiments is made, is the great difference in time scale. In this work, we do not attempt to obtain quantitatively accurate predictions of erosion process but concentrate qualitatively on cavitation mechanisms with quantitative prediction of pressure pulses which lead to erosion. This is possible, because of our recent experimental work on simultaneous observation of cavitating flow and cavitation erosion by high speed cameras. In this study, the numerical simulation was used to predict details of the cavitation process during the vapor collapse phase. The fully compressible, cavitating flow simulations were performed to resolve the formation of the pressure waves at cavitation collapse. We tried to visualize the mechanisms and dynamics of vapor structures during collapse phase at the Venturi geometry. The obtained results show that unsteady Reynolds-averaged Navier–Stokes (URANS) simulation of cavitation is capable of reproducing four out of five mechanisms of cavitation erosion, found during experimental work.

In silico screening of venturi designs and operational conditions for gas-liquid mass transfer applications

Venturi ejectors are commonly used to facilitate gas-liquid mass transfer, but their mass transfer characteristics depend on various operational and ejector design parameters, which make experimental identification of optimal conditions cumbersome. A CFD model using a two-phase Eulerian approach with a free surface model to describe local venturi hydrodynamics was developed and used to characterize the influence of gas-liquid flow ratio and venturi design on venturi-mediated gas-liquid mass transfer in bulk liquid. When standardized to gas injection rate, computed venturi-local gas-liquid interfacial area showed same dependency on gas injection rate as experimentally determined kLa during aeration of a 1 m3 tank. The model consequently enables in silico characterization of the venturi flow regime, which is critical to the venturi’s mass transfer performance. An experimental factorial design study investigated the influence of venturi geometry on gas-liquid mass transfer and found that the venturi throat diameter is a critical design parameter. The computational model revealed that this was due to its significant impact on generated gas-liquid interfacial area. The ability to characterize venturi flow regimes and qualitatively predict the relative effect of process parameters on kLa by modelling local venturi dynamics rather than the complete reactor system greatly reduces computational complexity, suggesting the model’s usage as an efficient screening methodology to increase venturi gas-liquid mass transfer performance.

Experimental and numerical study of cavitation flows in venturi tubes: From CFD to an empirical model

This work is devoted to experimental and numerical studies of cavitation phenomena in Venturi tubes with different geometries. A two-phase mixture model has been validated against experimental data. The numerical results showed good agreement with the experimental data. Experimental studies have been carried out for two different Venturi tubes with convergent angles of 19° and 45°, respectively. The effect of the convergent angle on the cavitation performance was investigated experimentally and numerically. Both the numerical and experimental studies reveal that the change in the convergent angle has significant effects on flow characteristics and the generation of cavitation. It was shown that a 45° convergent angle enhances cavitation in comparison with 19° angle. A scaled-up study of the Venturi geometry has been conducted using CFD-based numerical simulations. Finally, a semi-empirical model enabling the prediction of cavitation in Venturi tubes has been developed and validated.

Attached cavitation in laminar separations within a transition to unsteadiness

Attached sheet cavitation is usually observed in turbulent water flows within small laminar separation bubbles which can provide favorable conditions for inception and attachment of cavities. In the present study, viscous silicone oils are used within a small scale Venturi geometry to investigate attached cavitation into laminar separated flows for Reynolds numbers from 346 to 2188. Numerical simulations about single phase flows are performed with steady simulations for a Reynolds number range Re ∈ [50; 1400] and with unsteady simulations for Re ∈ [1000; 2000]. They reveal the emergence of two large laminar boundary layer separations downstream of the Venturi throat in addition to low pressure zones which can possibly induce both degassing or cavitation features. Experiments are performed with high-speed photography, and several multiphase dynamics are observed in these viscous flows, which are considered as quasisteady flows at low Reynolds numbers Re ≤ 1400. Degassing phenomenon with air bubble recirculation has been first observed at pressures far above liquid vapor pressure whereas typical attached cavities have been identified for low pressure conditions as “band” and “tadpole” cavities into the different separations of the laminar flows. For higher Reynolds numbers, a flow regime transition can be noticed in the wake of well-developed gas structures, characterized by wake instabilities, causing vortex cavitation above a critical Reynolds number associated with the bubble width Recb≃616. This regime transition can possibly occur either quasicontinuously in the wake of an attached “band” vapor cavity or intermittently behind a recirculating air bubble generated with degassing. This last phenomenon is associated in our study to classical “patch” cavitation.

Bayesian optimisation of RANS simulation with ensemble-based variational method in convergent-divergent channel

ABSTRACTThis paper investigates the applicability of a hybrid data assimilation approach, namely ensemble-based variational method (EnVar), to optimise Reynolds Averaged Navier-Stokes (RANS) simulations in convergent-divergent channel from the perspective of Bayesian inference. Concretely, the ensemble-based variational method is applied to infer the inlet velocity and turbulence model corrections by assimilating Direct Numerical Simulation (DNS) results or limited experimental data. The approach is first adopted to infer the inlet velocity profile for the WallTurb Bump and Venturi geometry. The improvement can be achieved near the inlet region for the bump, but for Venturi in light of the view field limited in adverse pressure gradient region, the observation space is not sensitive to the perturbation of inlet condition. In a second step, the model corrections in SST model are investigated by assimilating the limited sparse experimental data. With the inferred model corrections, the predictions on both velocity and turbulent kinetic energy (TKE) get improved. The results indicate that the ensemble-based variational method is efficient in inferring unknown quantities of both low dimension (D=20) and high dimension (D=2400) with small ensemble size robustly and non-intrusively. This approach could prove very useful for Bayesian inference or optimisation in CFD problems.

Fundamental investigations for lowering emissions and improving operability

This paper first highlights recent developments in lean direct injection (LDI) combustion technology. In view of the needs and opportunities to lower emissions and expand the operability range of LDI, fundamental research has been undertaken to elucidate the effects of air swirler vane angle, air swirler rotation direction, and overall equivalence ratio on the LDI flow field and flame structure/response. Moreover, additional investigation has been conducted to understand the fundamental differences between representative LDI and airblast injectors in order to highlight the importance of the flare feature to LDI venturi geometry. The results of these fundamental studies are discussed to help identify possible areas for optimization to generic LDI designs that may improve individual swirler performance.

Numerical investigation of periodic cavitation shedding in a Venturi

Unsteady partial cavitation is mainly formed by an attached cavity which presents periodic oscillations. Under certain conditions, instabilities are characterized by the formation of vapour clouds convected downstream the cavity and collapsing in higher pressure region. Two main mechanisms have been identified for the break-off cycles. The development of a liquid re-entrant jet is the most common type of instabilities, but more recently, the role of pressure waves created by the cloud collapses has been highlighted. This paper presents one-fluid compressible simulations of a self-sustained oscillating cavitation pocket developing along a Venturi geometry. The mass transfer between phases is driven by a void ratio transport equation model. The importance of traveling pressure waves in the physical mechanism is put in evidence. Moreover, the importance of considering a non-equilibrium state for the vapour phase is exhibited.

Combined experimental and computational investigation of the cavitating flow in an orifice plate with special emphasis on surrogate-based optimization method

We investigated the influence of geometrical parameters of the orifice plate on the cavitation structures, and optimized these parameters by using a surrogate-based model with special emphasis on the concentration of hydroxyl radical released. The results show that for the orifice plate of the hydrodynamic cavitation system, the possible location of the inception of the cavity spreads to throat and divergent section of the venturi geometry. Based on the surrogate model and global sensitivity assessment, the diameter of throat Dt and diameter of inlet Din significantly influenced the size of the cavity, while the length of throat Lt had little effect on both cavitation intensity and flow rate. It should be noted that when Lt is decreased, the size of cavity would be slightly decreased but the flow rate increased clearly. The increase of the diverging section is in favor of the size of cavity. By comparing the experimental measurements on the concentration of Methylene blue, the optimum geometry of the orifice plate for best cavitational activity is proposed.

RANS computations for identification of 1-D cavitation model parameters: application to full load cavitation vortex rope

Due to the massive penetration of alternative renewable energies, hydropower is a key energy conversion technology for stabilizing the electrical power network by using hydraulic machines at off design operating conditions. At full load, the axisymmetric cavitation vortex rope developing in Francis turbines acts as an internal source of energy, leading to an instability commonly referred to as selfexcited surge. 1-D models are developed to predict this phenomenon and to define the range of safe operating points for a hydropower plant. These models involve several parameters that have to be calibrated using experimental and numerical data. The present work aims to identify these parameters with URANS computations with a particular focus on the fluid damping rising when the cavitation volume oscillates. Two test cases have been investigated: a cavitation flow in a Venturi geometry without inlet swirl and a reduced scale model of a Francis turbine operating at full load conditions. The cavitation volume oscillation is forced by imposing an unsteady outlet pressure conditions. By varying the frequency of the outlet pressure, the resonance frequency is determined. Then, the pressure amplitude and the resonance frequency are used as two objectives functions for the optimization process aiming to derive the 1-D model parameters.

Experimental study of the thermodynamic effect in a cavitating flow on a simple Venturi geometry

The thermodynamic effects in cavitating flow are observed on a simple Venturi profile. A thorough experimental investigation of the temperature field on cavitating flow has been performed in water of 100°C at different operating conditions. Temperature measurements were performed with Infra-Red (IR) high-speed camera, while visualisation was made with conventional high-speed camera. Both, average temperature fields and temperature dynamics are presented at different operating conditions and compared with collected data in visual spectrum. In the vicinity of the throat a temperature depression up to 0.5 K was recorded.

Some Design Aspects for Venturi Gas Lift Valves

Summary The Venturi valve represents a significant advancement in the technology of gas lift in petroleum wells. Its use is expanding, and an understanding of the theoretical and practical aspects is fundamental to maximize the benefits of its application. The application-related aspects are reasonably well-covered in the literature; however, to achieve good performance and promote the expected benefits, some valve-design aspects have to be taken into account. This paper describes key points, including a recommended Venturi geometry, the influence of several Venturi design parameters on performance, and an analysis of the role of the check valve. The design guidelines provided in this work are supported by experimental tests performed at Petrobras’ Gas Lift Valves Test Unit and by sizable experience with Venturi valves after more than 600 valves run in wells or tested.

Modelling for isothermal cavitation with a four-equation model

In a recent study, an original formulation for the mass transfer between phases has been proposed to study one-dimensional inviscid cavitating tube problems. This mass transfer term appears explicitly as a source term of a void ratio transport-equation model in the framework of the homogenous mixture approach. Based on this generic form, a two-dimensional preconditioned Navier–Stokes one-fluid solver is developed to perform realistic cavitating flows. Numerical results are given for various inviscid cases (underwater explosion, bubble collapse) and unsteady sheet cavitation developing along Venturi geometries at high Reynolds number. Comparisons with experimental data (concerning void ratio and velocity profiles, pressure fluctuations) and with a 3-equation model are presented.

Investigation of three-dimensional effects on a cavitating Venturi flow

A numerical investigation of the behaviour of a cavitation pocket developing along a Venturi geometry has been performed using a compressible one-fluid hybrid RANS/LES solver. The interplay between turbulence and cavitation regarding the unsteadiness and structure of the flow is complex and not well understood. This constitutes a determinant point to accurately simulate the dynamic of sheet cavities. Various turbulent approaches are tested: a new Scale-Adaptive model and the Detached Eddy Simulation. 2D and 3D simulations are compared with the experimental data. An oblique mode of the sheet is put in evidence.

A CFD study of the effect of venturi geometry on high pressure wet gas metering

Venturi meter is increasingly being preferred as wet gas flow meter because of its simple construction and ease of operation. It has been found that the performance of venturi in single phase gas is very different from that of water. In this work, with a view to optimise the design, the effects of diameter, diameter ratio and convergent angle on the performance of a venturi meter for wet gas metering has been studied by CFD modelling of the high pressure wet gas flow. The performance of eight wet gas correlations for flow prediction has also been studied. Simulation results reveal that a convergent angle of 10.5 deg to be a better choice for wet gas metering. Homogeneous flow model, Steven’s and De Leeuw’s correlations are found to be better than the other correlations. While homogeneous flow model performs consistently, Steven’s and De Leeuw’s performance drops at 40 bar.