Interphase Mass Transfer Model Research Articles

Cavitation dynamics and flow aggressiveness in ultrasonic cavitation erosion

The mechanisms of cavitation erosion have been the subject of numerous studies, but the cavitation dynamics and flow aggressiveness thereby produced are less known. We develop a new numerical model to capture the cavity evolution and pressure pulsation, which are related to cavitation erosion. The source term in the interphase mass transfer model is obtained from Rayleigh–Plesset equation under the acoustic field. Mathematical closure is achieved by the temperature-dependent Tait equations of state to apply to the cavitation. The comparison with the measurement results of ultrasonic cavitation shows that the model has a good predictive ability in cavity evolution, cavity volume and pressure pulsation. The surface erosion pattern is attributed to concentrated pressure. Given the shielding effect of the central bubble, the pressure is concentrated on the edge, resulting in an annular region with the greatest depth. The cavity characteristics are gap-dependent. When the gap height is 1.0 mm, cavitation erosion is the most serious. The size of the erosion area is determined by the maximum bubble number density on the sample surface.

A novel interphase mass transfer model toward the VOF simulation of subcooled flow boiling

In this study, a novel interphase mass transfer model has been developed to predict mass and energy exchange in the vicinity of the phase interface. The proposed model involves the new time relaxation parameter which takes into account the effect of the subcooling, the wall superheat, and Reynolds number, and is obtained from simulation results by the method of trial and error. Transient simulations of subcooled flow boiling in Hua’s experimental horizontal rectangular channel are performed. The Volume of Fluid (VOF) method is used to track the vapor–liquid interface using OpenFOAM software package. The constant coefficients of new time relaxation parameter are determined using fitted method. The predicted results show that the deviation between the predicted and experimental wall superheats using the proposed model, which is within 20%, is smaller than that of Lee model. As a further step, the novel interphase mass transfer model has been validated in other experimental data under similar condition. These results indicate that the novel interphase mass transfer model is quite accurate and robust.

CFD Simulation of the Particle Dispersion Behavior and Mass Transfer–Reaction Kinetics in non-Newton Fluid with High Viscosity

Abstract Solid particle dispersion and chemical reactions in high-viscosity non-Newtonian fluid are commonly encountered in polymerization systems. In this study, an interphase mass transfer model and a finite-rate/eddy-dissipation formulation were integrated into a computational fluid dynamics model to simulate the dispersion behavior of particles and the mass transfer–reaction kinetics in a condensation polymerization-stirred tank reactor. Turbulence fields were obtained using the standard k–ε model and employed to calculate the mixing rate. Cross model was used to characterize the rheological property of the non-Newton fluid. The proposed model was first validated by experimental data in terms of input power. Then, several key operating variables (i.e. agitation speed, viscosity, and particle size) were investigated to evaluate the dispersive mixing performance of the stirred vessel. Simulation showed that a high agitation speed and a low fluid viscosity favored particle dispersions. This study provided useful guidelines for industrial-scale high-viscosity polymerization reactors.

Modelling of biohydrogen production in stirred fermenters by Computational Fluid Dynamics

A bioreactor for the production of hydrogen from the dark fermentation of organics is studied by a comprehensive modelling strategy. The bioreactor is a dual impeller vortex ingesting stirred tank working under batch and attached-growth conditions. Two geometrical configurations of the reactor are investigated: one devised to ensure an effective fluid dynamics behaviour and the other proposed to increase the hydrogen productivity. The turbulent gas–liquid fluid dynamics, the production and the recovery of H2 from the liquid phase are predicted by the numerical solution of the two-phase Reynolds averaged Navier–Stokes equations and the species mass transport equations, including a simplified kinetic model for the fermentative hydrogen production found in literature and a local interphase mass transfer model for the hydrogen stripping from the aqueous to the gas phase. A simplified model for the description of the interfacial area in the context of the two-fluid model is also proposed. This work suggests a method for the predictive simulations of a complex biological process via numerical modelling based on Computational Fluid Dynamics. The main outcome of the proposed investigation method is a detailed estimation of the different relevant variables and their interaction on a local basis, providing a viable tool for the optimization and the scale-up of bioreactors.

Simulation of subcooled flow boiling with an SVR based interphase mass transfer model

Numerical simulation has been becoming more and more important for the investigation of subcooled flow boiling in practical engineering. A precise description of interphase mass transfer between the liquid phase and the vapor phase is quite crucial for simulations regarding phase change. In this paper, Lee’s interphase mass transfer model was improved to make the simulation of subcooled flow boiling more accurate and robust. With the experimental data collected by Liu et al., the trial and error method was adopted to develop a support vector regression (SVR) based prediction model of time relaxation parameter. The proposed model could successfully predict the time relaxation parameter with a MSE of 0.56% on the training data and 2.0% on the test data. Numerical validation suggests that current SVR based prediction model could accurately give the value of time relaxation parameter in different flow conditions. Besides, simulations performed with the improved interphase mass transfer model indicate that the inlet velocity has much more powerful influence on transferred heat flux than the inlet liquid subcooling and the system pressure in subcooled flow boiling.

Steady cavitating propeller performance by using OpenFOAM, StarCCM+and a boundary element method

Accurate and reliable numerical predictions of propeller performance are a fundamental aspect for any analysis and design of a modern propeller. Prediction of cavitation and of cavity extension is another important task, since cavitation is one of the crucial aspects that influences efficiency in addition to propagated noise and blade vibration and erosion. The validation of the numerical tools that support the design process, including open-source codes, is, consequently, essential. The public availability of measurements and observations which cover not only usual thrust and torque in open water conditions (including cavitation) but also unsteady functioning with pressure pulse measurements in the case of the Potsdam Propeller Test Case certainly represents an extremely useful source of information and an excellent chance for verification and validation purposes. In the present work, the prediction of the Potsdam Propeller Test Case propeller performance using the OpenFOAM computational fluid dynamics package is proposed. After a preliminary validation and calibration of the OpenFOAM native Schnerr–Sauer interphase mass transfer model for cavitating flow, based on the experimental results on a 2D NACA66Mod hydrofoil, open water propeller performance and cavitation predictions are carried out. The OpenFOAM results are finally compared both with the available experimental measurements and with calculations carried out with StarCCM+ and with a proprietary boundary element method code, in order to assess the accuracy and the overall capabilities of the open-source tools (from meshing to post-processing) available in the OpenFOAM package. The comparison, in addition to assessing the accuracy of the open-source approach, is aimed to verify its advantages and drawbacks with respect to widely used solvers and to further verify the reliability of traditional boundary element method approaches that are still widely adopted for design and optimization (thanks to their extremely higher computational efficiency) in a very demanding test case.

The discrete equation method (DEM) for fully compressible, two-phase flows in ducts of spatially varying cross-section

For the simulation of light water nuclear reactor coolant flows, general two-phase models (valid for all volume fractions) have been generally used which, while allowing for velocity disequilibrium, normally force pressure equilibrium between the phases (see, for example, the numerous models of this type described in (Städtke, 2006)). These equations are not hyperbolic, their physical wave dynamics are incorrect, and their solution algorithms rely on dubious truncation error induced artificial viscosity to render them numerically well-posed over a portion of the computational spectrum. The inherent problems of the traditional approach to multiphase modeling, which begins with an averaged system of (ill-posed) partial differential equations (PDEs) which are then discretized to form a numerical scheme, are avoided by employing a new homogenization method known as the discrete equation method (DEM) (Abgrall and Saurel, 2003). This method results in well-posed hyperbolic systems, this property being important for transient flows. This also allows a clear treatment of non-conservative terms (terms involving interfacial variables and volume fraction gradients) permitting the solution of interface problems without conservation errors, this feature being important for the direct numerical simulation of two-phase flows.Unlike conventional methods, the averaged system of PDEs for the mixture are not used, and the DEM method directly obtains a well-posed discrete equation system from the single-phase conservation laws, producing a numerical scheme which accurately computes fluxes for arbitrary number of phases. The method effectively uses a sequence of single-phase Riemann problem solutions. Phase interactions are accounted for by Riemann solvers at each interface. Non-conservative derivative product terms are correctly approximated. Some of the closure relations missing from the traditional approach are automatically obtained. Lastly, the continuous equation system resulting from the discrete equations can be identified by taking the continuous limit with weak-wave assumptions. In this work, this approach is tested by constructing a DEM model for the flow of two compressible phases in 1-D ducts of spatially varying cross-section with explicit time integration. An analytical equation of state is included for both water vapor and liquid phases, and a realistic interphase mass transfer model is developed based on interphase heat transfer. A robust compliment of boundary conditions is developed and discussed. Though originally conceived as a first step toward implict time integration of the DEM method (to relieve time step size restrictions due to stiffness and to achieve tighter coupling of equations) in multidimensions, this model offers some unique capabilities for incorporation into next generation light water reactor safety analysis codes. We demonstrate, on a converging-diverging two-phase nozzle, that this well-posed, 2-pressure, 2-velocity DEM model can be integrated to a realistic and meaningful steady state with both phases treated as compressible.

Numerical Simulation of Field‐Scale Contaminant Mass Transfer during Air Sparging

Air sparging is a remediation technology for removing volatile organic contaminants (VOCs) from below the water table using air injection. Contaminant mass removal can be controlled by the mass transfer between liquid phases (aqueous and nonaqueous phase liquid [NAPL]) and the injected gas phase. Traditional multiphase flow numerical models ignore subgridblock‐scale effects of gas channel flow on local mass transfer and assume local equilibrium between liquid and gas phases. This traditional, single‐domain modeling approach tends to overpredict contaminant mass removal during sparging. A dual‐domain multiphase flow approach for local interphase mass transfer may more accurately simulate contaminant removal, while still accounting for injected gas plume behavior. Mass transfer limitations are introduced through a numerical grid modification where each traditional single‐domain gridblock is split into two domains: one with very strong capillary pressures that remains mostly water saturated and one with weak capillary pressures that becomes mostly gas saturated during sparging. The two domains are coupled through a first‐order mass transfer expression. This simple model can then approximate the localized (subgridblock‐scale) gas channel effects on mass transfer that are limited by liquid phase diffusion. In this study, data from a two‐dimensional laboratory‐scale experiment and a field‐scale air sparging demonstration were used to test the capabilities of the dual‐ and single‐domain approaches. It is the first time mass transfer has been simulated for air sparging at either of these scales using a multiphase flow kinetic interphase mass transfer model. Both experiments involved air sparging remediation of tetrachloroethylene, a NAPL. The single‐domain local‐equilibrium model resulted in an overprediction of mass removal rates in both cases. The dual‐domain mass transfer approach provided a much better fit of the experimental data, and it is shown that the apparent interphase mass transfer coefficient becomes smaller with increasing scale.