Modified Cavitation Model Research Articles

Identification of incipient cavitation in control valve

Control valves as important elements in hydraulic systems are used to transport liquid and gaseous media, for example, water, water vapor, and hydrogen. They are mainly used in the power engineering, chemical industry, and so forth. Due to their large dimensions, the verification of hydraulic parameters is problematic. In the case of liquid flow, cavitation can occur, which is mainly characterized by noise, vibrations, or changes of hydraulic parameters. During the incipient cavitation in the valve, there are no noticeable changes in the hydraulic characteristics. The article, therefore, deals with the methodology of determining the cavitation by measuring and evaluating the characteristics of the valve, spectral analysis of noise, and vibrations during water flow. Mathematical modeling is also a suitable tool for determining the extent of cavitation in valve design. Newly created modified cavitation model, where the flowing medium is defined as a mixture of incompressible water and compressible gases (vapor and air), better corresponds to physical principles than the two-phase approach (water and vapor) with experimentally modified density and viscosity. Measured noise and vibration frequency characteristics and mathematically modeled pressure frequency characteristics identify cavitation in the (1 to 10) kHz range. The article proves that when identifying cavitation, the method of measuring hydraulic characteristics fails, but the method of measuring noise and vibrations is suitable in practice, i.e., in real industrial equipment, and the modified cavitation model is suitable for identifying cavitation regions in structural designs of the hydraulic elements.

Investigation of the enhanced cavitation model considering turbulence effects of fuel flow in the injector nozzle holes

The cavitation model is essential in simulating cavitation flow characteristics of injector nozzles, with its accuracy directly impacting simulation precision. This paper developed a quantitative relationship between critical cavitation pressure, turbulent kinetic energy, and shear stress, and introduced a modified cavitation model that considers turbulence effects. A simulation study was conducted on the fuel flow characteristics inside the nozzle of a high-pressure common rail diesel injector with seven nozzle holes. The results indicate that compared to the original model, the cavitation development calculated by the modified model is enhanced at different injection pressures. In terms of axial distribution, cavitation inside the nozzle hole increases, but the impact on cavitation of different intensities varies, with the development of moderate to low-intensity cavitation being promoted, while strong cavitation is inhibited. In terms of radial distribution, the intensity of cavitation on the upper wall inside the nozzle hole increases, which is caused by the higher mass transfer rate near the upper wall at the nozzle hole inlet. As the injection pressure increases from 120 to 240 MPa, the change rate of cavitation volume ratio inside the nozzle holes calculated by the original and modified models increases from 5.1% to 9.7%. It is necessary to use the modified model for the calculations of gas/liquid two-phase flow within the nozzle hole, especially at high injection pressure.

Cavitating flows in microchannel with rough wall using a modified microscale cavitation model

Cavitation can induce drastic disruption and significantly enhance heat transfer, especially in microchannels. However, due to the scale effect, the cavitation characteristics in micro-scale can be significantly different from those in conventional scale. A modified cavitation model according to the Rayleigh-Plesset bubble kinetic equation is formulated by including the effects of surface tension, and temperature difference between the local environment and the far-field environment. The impacts of latent heat of vaporization and condensation are introduced into the energy equation. The micro-scale cavitation model proposed in the paper is verified through comparison with experimental data. The influence of surface roughness on cavitation flow in microchannel with restrictor is then investigated. The results demonstrate that bottom surface roughness in microchannels significantly increases flow resistance, suppresses cavitation flow and causes high frequency pressure fluctuations.

Effects of in-nozzle liquid fuel vortex cavitation on characteristics of flow and spray: Numerical research

The current work studies the dynamics of vortex cavitation under various conditions, including the pressure and orifice number, and their effects on the nozzle flow and spray characteristics using a modified cavitation model considering the swirling flow. Its validation against experimental data has demonstrated a high precision of the code in terms of vortex cavitation inside the nozzles. The higher the inlet pressure, the higher the mass flow rate and discharge coefficient, but the weaker the vortex cavitation; while the vortex cavitation intensity decreases, the discharge coefficient increases and the mass flow rate is almost constant with increasing back pressure. Moreover, under the influence of vortex cavitation, the primary jet liquid spreading angle, spray penetration, and spray volume coefficient increase first and then decrease with increasing inlet pressure; with an increase of back pressure, however, the primary jet liquid spreading angle increases, the spray penetration vice versa. Additionally, once the inner well-organized large-scale longitudinal vortices are destroyed, the vortex cavitation will not incept, which induces a larger discharge coefficient and spray penetration, but a smaller primary jet liquid spreading angle as a result. The swirling flow in the multi-hole nozzle is inhibited; hence, vortex cavitation is difficult.

Numerical and experimental analysis of cavitation characteristics in safety valves of the nuclear power second circuit using a modified cavitation model

Cavitation frequently arises in the safety valve of nuclear power plants’ secondary circuits operating under high pressure conditions. This study integrates valve flow characteristics and velocity strain rate corrections into the Zwart-Gerber-Belamri model to accurately simulate cavitation inside the valve, reducing the impact of physical empirical coefficient variations on cavitation length prediction. Subsequently, a visualisation test rig is developed to validate the accuracy of the numerical model, and experimental cavitation results are obtained using the grayscale detection method. The evaporation/condensation coefficients are optimised using the AES-MSI model and GA based on the experimental results. The accuracy of the constructed model is validated by comparing it with experimental results obtained under various operating conditions. Finally, the high-fidelity numerical model is employed to investigate the effects of pressure drop and valve openings on cavitation, elucidating the underlying mechanisms governing cavitation variations resulting from pressure drops. Furthermore, a comprehensive equation is derived to determine the effective flow area, aiding in the identification of cavitation locations and offering insights into the relationship between cavitation behaviour and valve openings. The modified cavitation model proposed in this study can be readily extended to investigate cavitation prediction in other valves or throttle elements.

LES investigation of the tip vortex cavitating flow with special emphasis on the interaction between cavitation and vorticity by a modified cavitation model

LES investigation of the tip vortex cavitating flow with special emphasis on the interaction between cavitation and vorticity by a modified cavitation model

Numerical and experimental analysis of the cavitation characteristics of orifice plates under high-pressure conditions based on a modified cavitation model

The flow systems in nuclear power plants and aircraft engine fuel pipelines are often subjected to extreme high-pressure conditions, which can induce cavitation and severely affect essential system components. Orifice plates are the most typical infrastructures representing the flow principle of the aboved flow systems. In this paper, a modified cavitation model combining local flow characteristics, supervised learning, and genetic algorithms is proposed to investigate the cavitation flow characteristics of orifice plates under high-pressure conditions. The modified cavitation model eliminates the influence of the bubble diameter and nucleation site volume fraction on mass flow rate and achieves dimensionality reduction at the physical level. The relationship between evaporation/condensation coefficients and mass flow rate is constructed by supervised learning, and the two coefficients are determined using the genetic algorithm. The mass flow rates are calculated by the modified cavitation model within a 5% experiment error, proving the accuracy of the modified cavitation model. The effect of the diameter ratio (the diameter of the pipe to the orifice plate) and pressure drop on the mass flow rate are obtained based on the validated cavitation model. Finally, an empirical formula for calculating the mass flow rate based on the diameter ratio and pressure drop is derived. The modified cavitation model shows great potential for cavitation prediction applications for throttling devices such as nuclear power safety valves and aircraft engine nozzles.

Merging theory-based cavitation model adaptable with non-condensable gas effects in prediction of compressible three-phase cavitating flow

The phenomenon of turbulent multiphase flow, such as cavitating flow, with phase change, becomes more complicated taking into account the non-condensable gas as an additional phase. The water, which is the operating fluid, contains a specified amount of dissolved air; as a result, the cavitating flow needs to be considered as a three-phase (i.e. water, vapor and air) flow. Although the existence of dissolved air is well known, most numerical models neglect it; as such, its effect is usually underestimated so far. To this end, the present work is devoted to developing a modified cavitation model based on the merging theory, taking into account the dissolved air in an Eulerian approach. The diffusion process is modeled to constitute the new bubble of the mixture; as a result, the bubble pressure is corrected based on the local air level. Also, the pressure fluctuation effect is applied in the calculation of the pressure of the mixture bubble. To have a more accurate estimation of the density of phases in the situation with a high-pressure difference, the liquid and gas phases are assumed as compressible fluids and modeled by the Tait equation and ideal gas law, respectively. To avoid overestimation of the turbulent viscosity while the standard turbulence model is used, the Density Corrected-Based model is employed to modify the turbulent viscosity employed in k−ε turbulence model. The dynamic characteristics of cavitating flow are analyzed using Power Spectral Density (PSD) and Continuous Wavelet Transform (CWT) techniques. In addition, the cavitating flow is visualized experimentally, and the captured frames are analyzed using image processing to provide temporal-spatial gray level distribution. The present work declares the major role of gas content on the cavitating flow and assesses the efficiency of the proposed numerical model in the prediction of three-phase cavitating flow.

Cavitation Flow Characteristics of Water and Liquid Oxygen in the Inducer Considering Thermodynamic Effect

Clean energy liquid oxygen is more and more widely used in aerospace, but the research on the thermodynamic characteristics of liquid oxygen is still less. In this paper, a method for correcting the local saturated vapor pressure by thermodynamic effect is proposed, so we establish the Zwart modified cavitation model considering the thermodynamic characteristics. We choose the turbine pump inducer as the research object, and select liquid water and liquid oxygen at different temperatures as the fluid medium. Based on two different Zwart cavitation models, the cavitation flow characteristics of liquid water at different temperatures of 298 K, 320 K and 350 K are numerically simulated and analyzed, and the cavitation flow characteristics of liquid oxygen at different temperatures of 85 K, 90 K and 95 K are also numerically simulated and analyzed. According to the analysis of the simulation results, for liquid water at the temperature of 298–350 K, since the change range of its thermophysical parameters is very small, the inhibition of cavitation is not obvious, and the thermodynamic characteristics are not significant. For liquid oxygen, the cavitation effect is obvious at different temperatures. When the temperature increases gradually, the thermodynamic effect of liquid oxygen becomes more obvious, which can effectively inhibit the cavitation phenomenon of the inducer in the steady-state cavitation flow.

Numerical Study on the Cavitation Characteristics of Micro Automotive Electronic Pumps under Thermodynamic Effect.

In order to study the influence of thermodynamic effects on the cavitation performance of hydromechanics, the Singhal cavitation model was modified considering the influence of the thermo-dynamic effects, and the modified cavitation model was written into CFX using the CEL language. Numerical simulation of the cavitation full flow field at different temperatures (25 °C, 50 °C and 70 °C) was carried out with the automotive electronic water pump as the research object. The results show that the variation trend of the external characteristic simulation and experimental values is the same at all flow rates, and the calculation accuracy meets the subsequent cavitation demand. With the increase in temperature, the low-pressure area inside the automotive electronic pump’s impeller decreases. NPSHr decreases and the cavitation resistance is enhanced. During the process of no cavitation to cavitation, the maximum pressure pulsation amplitude in the impeller channel gradually increases. The generation and collapse of cavitations cause the change of pressure pulsation in the internal flow field, causing pump vibration.

Flow in fuel nozzles under cavitation and flash-boiling conditions

Cavitation plays a crucial role in various fuel systems and spray applications. Due to the limitation in experimentally measuring the cavitation flow in fuel nozzles, numerical simulations can be used as an alternative in exploring the underlying physics. Most of the previous simulations of cavitation flow in nozzles were carried out under isothermal conditions, which become invalid when the thermal effect is strong. In this study, we use OpenFOAM and a modified cavitation model to investigate the influence of thermal effect on fuel nozzle flow characteristics under cavitation and flash boiling conditions. The comparison with experimental data shows that the modified cavitation model can predict well the cavitation flow in nozzles. Vaporization caused by cavitation at the nozzle throat and vaporization caused by the flash boiling near the outlet are simulated under different conditions. Phenomena such as mass flow reduction, outlet velocity blockage, and the transition from cavitation flow to flash boiling flow are well predicted. Further results show that with the increase in the inlet fuel temperature, the cavitation at the throat and flash boiling at the outlet occur gradually and then mix under high-temperature conditions. With the decrease in the ambient pressure, the flow in the nozzle gradually transits from single-phase flow to cavitation flow and then to flash-boiling flow. Increasing the injection pressure can inhibit the generation and the growth of superheated vapor near the nozzle outlet.

Numerical study of the entropy production characteristics in cryogenic liquid nitrogen cavitating flow

This study applies the entropy production theory to the cryogenic liquid Nitrogen cavitation flow to explore the inducement mechanism of the entropy production and to reveal the cavitation effect on it. This research uses the modified cavitation model which considered the thermodynamic effect of the liquid nitrogen cavitation flow and uses the k-ω SST turbulence model to solve the viscosity coefficient of the flow field. The detailed entropy production characteristics are analyzed based on the entropy production theory. The results show that the entropy in the cryogenic cavitation flow is mainly caused by the direct dissipation term (EPDD) and wall shear stress term (EPWS), while the EPMT and EPFT can be ignored. The velocity gradient and the process of cavitation are the crucial causes of the entropy generation. Regions with higher velocity gradients will be accompanied by higher entropy production rates compared with other regions, resulting in the irreversible energy loss. The irregular flow of fluid causes energy to be lost in the form of heat, while the occurrence of cavitation will produce mass and momentum exchange at the cavity boundary, which causes energy loss.

Numerical and Experimental Analysis on Cavitation Characteristics of Orifice Plate Under High-Pressure Conditions Based on Modified Cavitation Model

Numerical and Experimental Analysis on Cavitation Characteristics of Orifice Plate Under High-Pressure Conditions Based on Modified Cavitation Model

Numerical Simulation of Flow Characteristics in Fuel Nozzles Under Cavitation and Flash-Boiling Conditions

Cavitation plays a crucial role in various fuel systems and sprays applications. Due to the limitation in measuring the cavitation flow in fuel nozzles experimentally, numerical simulations can be used as an alternative in exploring the underlying physics. Most of the previous simulations of cavitation flow in nozzles were carried out under isothermal conditions, which becomes invalid when the thermal effect is strong. To consider the thermal effect on the flow characteristics in fuel nozzles, a modified cavitation model is adopted in this study to simulate the fuel flow in a plain orifice nozzle under cavitation and flash-boiling conditions. The thermal effect is considered by modifying the source/sink terms representing condensation/evaporation based on energy balance. The comparison with experimental data shows that the traditional cavitation model overpredicts the vapor generation, while the modified cavitation model can predict better the relationship of the flow coefficient and the fuel temperature, including the cavitation at the throat and the flash boiling at the outlet. The effects of key parameters are also analyzed. The results show that with increasing the inlet fuel temperature, the cavitation at the throat and the flash boiling at the outlet occur gradually and then mix at high-temperature conditions. With the decrease in the ambient pressure, the flow in the nozzle gradually transits from single-phase flow to cavitation flow and then to flash-boiling flow. Increasing the injection pressure can inhibit the generation and the growth of superheated vapor near the nozzle outlet.

Numerical analysis of an unsteady natural cavitating flow around an axisymmetric projectile under various free-stream temperature conditions

Here, an unsteady natural cavitating flow around an axisymmetric projectile is computationally studied using a homogeneous multiphase approach. The numerical models used are based on a dual-time preconditioning method, an interface-capturing scheme, and a modified cavitation model, to capture unsteady cavitation structure behaviors. Full compressibility of two phases and an energy equation are used to determine the effects of temperature on a cavitating flow. First, the experimental data were validated at a cavitation number of σ = 0.435. Both quantitatively and qualitatively good agreements were achieved, including the evolution of the cavity shape, cavity length, and cavity thickness. Then, the key characteristics of an unsteady cavitation structure regarding the cavity growth, re-entrant jet, cavity shedding, and collapse were analyzed. In particular, the mechanism of the re-entrant jet inside the cavity, causing the periodic cavity shedding, was explored. Furthermore, detailed mechanisms of cavity shedding with periodic cavity–vortex–pressure interaction behaviors were analyzed. Finally, the influences of free-stream temperature on the cavitation structure behaviors were carefully investigated. It was found that both the cavity length and thickness increased with the free-stream temperature under the atmospheric pressure condition. The most sensitive effects occurred when the free-stream temperature approached its boiling point. In case of the same reference cavitation number and Reynolds number, the cavity shape tended to be mushier with increasing water temperature.

Numerical Investigation of the Cryogenic Cavitating Flow Characteristics in Bi-Directional Cone Flowmeters: A Systematic Parameter Study

As a new type of cone flowmeter, the bi-directional cone flowmeter is a promising device for metering the cryogenic fluid due to its excellent advantages. This work presents a numerical study on the cavitating flow characteristics of liquid nitrogen in the bi-directional cone flowmeters based on a modified cavitation model with thermal effects. The influences of some factors, including operating conditions and structural parameters on the cavitation process and flow characteristics, are clarified. The results show that with the increase in the inlet velocity, the cavitation degree in region III increases, and when the outlet pressure increases, the cavitation degree in region II decreases apparently. Reducing the angle of the front cone can significantly decrease the degree of cavitation in the region II; however, it will strengthen the effects of cavitation on pressure and temperature fluctuation in the region III. As the angle of the rear cone decreases, the cavitation degree in the region III decreases, while the cavitation degree in region II barely changes. The results of this work could present important references for the design and optimization of the bi-directional cone flowmeter for cryogenic engineering.

Numerical investigation of modified cavitation model with thermodynamic effect in water and liquid nitrogen

In this paper, three different cavitation models were employed and coupled with thermodynamic effect to simulate the cavitation flow in water and liquid nitrogen. The water cavitating flows around a NACA0015 hydrofoil and liquid nitrogen cavitating flows around a tapered hydrofoil were investigated numerically. The three-dimensional computations were compared and validated with the cavitation experiments. The applicability of the numerical solutions of cavitating regarding to the cavitation model was studied. The change of saturated vapor pressure caused by latent heat of vaporization and the change of local vaporization pressure caused by turbulent kinetic energy were taken into consideration. The numerical result of the modified Merkle cavitation model shows good agreement with the experimental result, indicating that the modified Merkle cavitation model could be employed to simulation the cavitation flows in water and liquid nitrogen. Additionally, corresponding of the thermodynamic effect in water and liquid nitrogen cavitation were discussed.

Study of the flow in a cryogenic pump under different cavitation inducements by considering the thermodynamic effect

PurposeThe purpose of this paper is to accurately predict the cavitation performance of a cryogenic pump and reveal the influence of the inlet pressure, the surface roughness and the flow rate on the cavitation performance.Design/methodology/approachFirstly, the Zwart cavitation model was modified by considering the thermodynamic effect. Secondly, the feasibility of the modified model was validated by the cavitation test of a hydrofoil. Thirdly, the effects of the inlet pressure, the surface roughness and the flow rate on cavitation flow in the cryogenic pump were studied by using the modified cavitation model.FindingsThe modified cavitation model can predict the cavitation performance of the cryogenic pump more accurately than the Zwart cavitation model. The thermodynamic effect inhibits cavitation development to a certain extent. The higher the vapor volume fraction, the lower the pressure and the lower the temperature. At the initial stage of the cavitation, the head increases first and then decreases with the increase of the roughness. When the cavitation develops to a certain degree, the head decreases with the increase of the roughness. With the decrease of the flow rate, the hydraulic loss increases and the cavitation at the impeller intensifies.Originality/valueA cavitation model considering the thermodynamic effect is proposed. The mechanism of the influence of the roughness on the performance of the cryogenic pump is revealed from two aspects. Taking the hydraulic loss as a bridge, the relationships among flow rates, vapor volume fractions, streamlines, temperatures and pressures are established.

Numerical investigation of the blade tip leakage vortex cavitation in a waterjet pump

The blade tip leakage vortex (TLV) cavitation is a concern for hydraulic machineries. The underlying mechanism of the TLV cavitation is complex due to the interactions between the vortical flow and cavities, leading to the necessity of improving the numerical methods. In consideration of the influence of the angular momentum on the vortex cavitation, the Zwart's cavitation model is improved by adjusting the condensation coefficient based on the vortex intensity. The modified cavitation model is used to investigate the TLV cavitation from a waterjet pump. The energy characteristics of this pump and the cavitation phenomena near the blade tip are validated by the referenced experiment. The blade tip leakage flow and the TLV cavitation features are revealed by present simulation under different cavitation conditions. It can be found that both the TLV and its cavitation reduce the leakage flow rate through the rotor tip. According to the analysis of the pressure fluctuation characteristics, it shows that the TLV cavitation especially its terminal region enhances the pressure pulsation. With the decreasing of the cavitation coefficients, the amplitude of the pressure fluctuation is increasing and the corresponding positions with the maximum pulsation migrate to the downstream. • The blade tip leakage vortex cavitation characteristics for an axial waterjet pump are numerically studied. • An improved Zwart's cavitation model is used and evaluated. • The vortical flow features near the blade tip are investigated on different cavitation conditions. • The tip leakage vortex cavitation enhances the pressure fluctuation at the rotor casing.

Numerical Study on Cavitation Generation in a Water Jet Added with Turbulent Drag-Reducing Additives Based on Improved Cavitation Model and Cross Model

An improved cavitation model was proposed to simulate the cavitation phenomenon in water jet added with turbulent drag-reducing additives. The Cross model which expressed the shear-thinning properties of drag-reducer solution was also employed. The neglected viscous term and surface tension term in traditional cavitation models were reconsidered as viscosity and surface tension force were important factors affecting bubbles’ motion in drag-reducer solution. The numerical solutions of bubble radius in drag-reducer solutions of different concentrations were fitted and the change rates were added to the original mass transfer mechanism equations. The results showed the modified cavitation model described the cavitation characteristics in drag-reducer solutions more accurately and effectively reflected that the cavitation region’s wake became longer with the increase concentration of the drag reducing agent. With the concentration increasing, the distribution of cavitation cloud became more homogeneous and th...