access deny [1301]
Research field:
Expert:
Phone:
Address:
access deny [1026]
In this paper, a theoretical model is proposed for Leidenfrost droplet evaporation by solving the mass, momentum, and energy conservation equations. This model involves a set of four equations, of which the values of vapor layer thickness, evaporation rate on the lower surface of the drop, the volume of evaporating droplet, and temperature distribution in vapor layer are obtained. This set of equation is solved with Fortran code by the predictor-corrector method. The main unknown value in these equations is the vapor layer thickness, which is predicted in every step of simulation and corrected by the balance of forces that act on the drop. In this study, the upper surface of the drop, where contacts with air and the lower surface of droplet
Two-phase flow was simulated within the effervescent atomizer by the volume of fluid interface tracing model. Different gas-to-liquid mass ratios (GLRs) of 0.08, 0.32, 1.24 and 4.9% at 0.38?L?min−1 liquid flow rate have been studied. The purpose of this simulation is to study two-phase flow regimes within the effervescent atomizer and their effects on the atomization quality. The compressibility effect of the gas phase was considered in the present study, whereas it was ignored in the previous works. However, in this paper, the compressibility of gas phase has been included in the governing equations for GLR values of 1.24 and 4.9%, due to the high gas velocity. The effect of the gravity has also been considered through the
The present study was conducted to investigate the elimination of Reactive Black 5 (RB5) through the photocatalytic process alone and a combination of photocatalysis and cavitation using nanotitanium dioxide as a photocatalyst. To use a new form of cavitation, hydrodynamic cavitation (HC) was generated by an orifice plate with a 7 mm hole diameter at the inlet pressure of 4 bars. First, the photocatalytic process was optimized by changing the parameters of pH, nanotitanium dioxide concentration, irradiation power and dye concentration; then, by adding cavitation and producing combined photocatalysis-cavitation and with the same optimal conditions obtained in the previous stage, the amount of nanophotocatalyst was adjusted, and the amount of
In this research, the behaviour of a single droplet of the dielectric field under the effect of the applied external uniform magnetic field has been investigated. Previously, it was thought that no force is applied to the dielectric fluids when exposed to the uniform magnetic field. A stagnant droplet in a quiescent fluid column was considered in order to determination of the net effect of the applied uniform magnetic field. Considering that the droplet behaviour has been investigated in the horizontal plane, the net effect of the gravity on the droplet and the surrounding fluid is also zero. Therefore, any change in the condition of the considered droplet will be due to the effect of the applied magnetic field. Numerical analysis has been
In this article, two-phase slug regime in a duct with rectangular cross-section is investigated numerically, using the volume of fluid (VOF) method. Equations of mass, momentum and advection of volume fraction are solved accompanying k-∈ realizable turbulence equations. To ensure the creditability, numerical results have been compared with experimental results using same geometry. With occurrence of instability in the entrance of duct, Kelvin-Helmholtz condition satisfies and with increasing instability, slug phenomenon occurs. With closing the cross-section of duct, slug causes pressure gradient in it. Trapped air behind a slug transfers the momentum and increases the kinetic energy of slug. In this research the kinetic energy of a slug
In this research, interaction and oblique coalescence of bubbles under buoyancy force was simulated, numerically. The governing equations are continuity and momentum equations which have been discretized using the finite volume method and the SIMPLE algorithm. For simulating the interface of two phases, the level set method has been incorporated. Level Set method suffers from poor mass conservation of dispersed phase especially in the case of severe deformation of interface. In order to control of mass conservation of the level set method, re-initialization equations and a geometric mass control loop are used which this loop is implemented in the level set method for the first time in this research. Using proposed geometric mass control loo
Hydrodynamics of multiple rising bubbles as a fundamental two-phase phenomenon is studied numerically by lattice Boltzmann method and using Lee two-phase model. Lee model based on Cahn-Hilliard diffuse interface approach uses potential form of intermolecular forces and isotropic finite difference discretization. This approach is able to avoid parasitic currents and leads to a stable procedure to simulate two-phase flows. Deformation and coalescence of bubbles depend on a balance between surface tension forces, gravity forces, inertia forces and viscous forces. A simulation code is developed and validated by analysis of some basic problems such as bubble relaxation, merging bubbles, merging droplets and single rising bubble. Also, the result
In present study, the rising of superheated vapor bubble in saturated liquid is simulated using volume of fluid method in OpenFOAM cfd package. The surface tension between vapor–liquid phases is considered using continuous surface force method. In order to reduce spurious current near interface, Lafaurie smoothing filter is applied to improve curvature calculation. Phase change is considered using Tanasawa mass transfer model. The variation of saturation temperature in vapor bubble with local pressure is considered with simplified Clausius–Clapeyron relation. The couple velocity–pressure equation is solved using PISO algorithm. The numerical model is validated with: (1) isothermal bubble rising and (2) one-dimensional h
In the present study, flow regimes of co-current, air-water two-phase flow in a vertical tube with 70 mm internal diameter were investigated. Simulation was accomplished by open source software, OpenFOAM, and One Fluid model has been used to simulate two-phase flow, which in this model, the interface of two-phase flow has been followed by Volume of the Fluid model. Hitherto, most of the researchers conducted experimentally and the researchers in many of the numerical studies just investigated the small tubes. The simulation was investigated according to boundary conditions of the vertical tube. Air and water superficial velocities in inlet and pressure in outlet were constant. Moreover, a no-slip condition in the internal tube walls has bee
In this research, counter-current gas-liquid two phase flow is investigated adiabatically in vertical Plexiglas tubes with internal diameters of 60 mm, 80 mm and 110 mm. All of the tubes have the same height of 2 m. So far, there have been few studies on counter-current flows in large diameter tubes.
In present study, subcooled boiling is simulated using color function volume of fluid (CF-VOF) method. For this purpose, energy equation and Tanasawa mass transfer model accompanied with some suitable source terms are implemented in OpenFOAM solver (interFoam). The surface tension between vapor–liquid phases is considered using continuous surface force (CSF) method. In order to reduce spurious current near interface, a smoothing filter is applied to improve curvature calculation. The variation of saturation temperature in vapor bubble with local pressure is considered with Clausius–Clapeyron relation. The numerical model is validated with one-dimensional Stefan problem.The shape and life time history of single vapor bubble condensation
In the present research, a two-phase flow system is designed, manufactured, assembled and adjusted to study two-phase flow behavior isothermally. Test sections are tubes standing in vertical position and are made of transparent acrylic with inner diameters of 40?mm and 70?mm. Two axial locations of 1.73?m and 3.22?m are chosen for data acquisition. Flow pattern maps are presented for both tubes. Effects of tube diameter and axial location on pattern transition boundaries are investigated. Air and water are chosen as working fluids. The range of air and water superficial velocities are 0.054–9.654?m/s and 0.015–0.877?m/s for the 40?mm diameter tube, but these values are 0.038–20.44?m/s and 0.036–1.530?m/s for 70?mm diameter tube. The
There are different approaches and methods to predict the interface changes in two-phase flows. Among these methods, volume-of-fluid (VOF) and level-set (LS) are some of the most famous ones. Common VOF schemes are mass conservative but cannot predict the surface tension with a good accuracy. In contrast, LS uses a continuous sign function which in turn computes surface tension more accurately than VOF. But, re-distancing the LS function causes mass gain/loss which violates the mass conservation. To have the advantages of both methods, a scheme called VOSET is introduced in the present research which couples the two methods fully geometrically. VOSET is used in the structure of the code developed to study the interface topological changes o
In ventilated cavitating flow structure, two parameters are very important, Fr number and gas entrainment coefficient. The objective of this paper is to investigate the ventilated cavitating flow structure by numerical methods and verify with experimental results. The numerical simulation is performed by ANSYS-FLUENT and homogenous mixture model with a free slip velocity and DES turbulence model, and the gravity effect is considered. The results show when the gas entrainment coefficient Qv is constant, two typical mechanisms of the gas leakage exist at different Fr numbers, namely toroidal vortex mode and two hollow tube vortex mode. With the increasing of Fr, the cavity would transfer from the two hollow tube vortices to the toroidal vorti
Cavity length estimation is important as supercavity condition is generated. The cavity length is function of cavity number and is calculated by relations deduced from experimental results which are different from each other and are not driven from analytical approaches. Literature survey shows that correlations based on cavity length in relation with Reynolds and cavity numbers have not been attempted. The present work purpose is to estimate analytical based relations for cavity length with respect to mass transfer, continuity and momentum conservation equations. This effort which has been conducted by order of magnitude method resulted in three relations. The first analytical based relation calculates cavity length versus cavity number. T
In the present attempt, in the first stage, the turbulent models differences, the initial assumptions to drive, privilages and shortcomings have been considered with details. Then, its consistency with the physics of slug flow was analysed with high accuracy. In the second stage, simulations using different turbulent models were conducted. The obtained results were compared to each other and with the experimental results of other investigators. Finally, the most consistent model with the physics of the slug flow was selected. The turbulent model of RNG k-ε showed more reliabilitycompared to other turbulent models. Thus, it was selected and used to obtain slug flow behavior with higher accuracy. The parameters as pressure distribution durin
Underwater explosion phenomena is a complicated problem, however, it has different and important applications. This type of explosion includes strong shock waves with generation of low pressure cavitation׳s zone and deformable interfaces. The main objective of this article is simulation of interaction of shock wave with interface of two-phase gas–liquid flow and capturing the complicated interface generated from explosion. For this reason, a five equations reduced model is considered with using a new cavitation model including gravity force effect. From the numerical point of view, a Godunov method was applied using HLLC solver. By using MUSCL-Hancock strategy, second order accuracy is achieved. To verify the developed computer code, a o
Abstract Development and application of high-speed underwater vehicle is the motivation for many researchers to consider super-captivating flows. Frictional drag decreases and vehicle's velocity increases due to cavity generation. The objective of the present research is to find the coefficients of a relation to estimate cavity length around a submersible vehicle equipped with a wedged-shaped cavitator-which has important practical applications. For this purpose, the super cavitation phenomena has been simulated numerically around three bodies with different geometry. In the first stage, to validate the results of numerical simulation of present work a well-established experimental result of a cylindrical body with hemispheric cap is used f
In present study, the impact of a single bubble on an inclined wall and its movement is investigated by applying volume of fluid method (VOF) in OpenFOAM open source cfd package using a solver called interFoam. Both phases are incompressible and surface tension between the two phases is estimated by CSF method. The effect of some parameters such as contact angle, wall slope and Bond and Morton dimensionless numbers on bubble shapes and velocity are studied. The numerical results show bubble velocity along wall increases with the increase of wall slope angle. Three bubble regimes are recognized and introduced in this study named: sliding, bouncing, and zigzagging based on wall slope. The bubble regime changes from sliding to bouncing when wa
In this research two-phase slug flow regime in a T-junction branching divider is examined in two regular and irregular groups. Simulation is accomplished by OpenFOAM™ open source software. Simulation uses single fluid with volume of fluid (VOF) method to follow gas-liquid two-phase flow interface. Constant velocity boundary condition for inlet, constant pressure for outlet boundaries and no slip boundary condition are considered for fixed walls. Since slug flow regimes are one of the most complex two-phase flow regimes whose behavior could result in serious damages to the downward equipment, the present research concentrates on the examination of slug flow behavior in the downstream of the T-junction. This study has concluded that using T