Equation Turbulence Model Research Articles

Effect of turbulence modeling on hydrodynamics of a turbulent contact absorber

A computational fluid dynamics (CFD) study is conducted to find a suitable two equation turbulence model for accurate prediction of hydrodynamics of an inhouse turbulence contact absorber (TCA) at high gas and liquid velocities. Based on the multi-fluid Eulerian approach, hydrodynamics of TCA is simulated by incorporating three turbulence models i.e. standard k–ε model, RNG k–ε model and SST k–ω model in ANSYS Fluent®. The solid phase stresses were closed by using the kinetic theory of granular flows (KTGF). TCA hydrodynamics parameters; expanded bed height and bed pressure drop were used to compare the results of this study with experimental data and also with earlier numerical study published with laminar viscous model. It was found that the RNG k–ε model predicted the bed height and pressure drop better than its counterparts. To accurately find the effects of secondary phase turbulence, two RNG k–ε model options i-e. per phase and dispersed were also evaluated. The results show that the “per phase” option of RNG k–ε model produced the expanded bed height and pressure drop in close agreement with available experimental data at similar operating conditions.

Computational study on the performance improvement of axial-flow pump by inlet guide vanes at part loads

In order to investigate the influence of adjustable inlet guide vanes on the hydraulic performance of axial-flow pump and impeller, the axial-flow pump with different inlet guide vane adjustable angles was simulated based on the RNG k-e turbulent model and Reynolds time-averaged equations. The multivariate non-linear regression prediction model was established about the effect of adjustable inlet guide vane on the hydraulic performance of axial-flow pump based on the numerical simulation. The performance curves of axial-flow pump with different inlet guide vane adjustable angles were obtained by calculation, showing that the performance curve shifts towards larger flow volume in case the guide vanes are turned from positive to negative angle. With the increase of inlet guide vane positive angle, the optimum operating of pump is closer to smaller operating condition, but the hydraulic efficiency of pump is lower. The larger the flow is, the larger the decreasing amplitude of pump efficiency is. With the decreases of inlet guide vane negative angle, the higher hydraulic efficiency of pump increases first then decreases. The inlet guide vane angle affects the axial velocity distribution of the blade trailing edge.

Characterization of liquid sloshing in U-shaped containers as dampers in high-rise buildings

The focus of the present study is on the efficacy of U-shaped containers used as liquid dampers in high-rise building system. An experimental study was undertaken with a U-shaped container to understand the liquid sloshing effect on the response control of tall buildings by conducting free, as well as forced vibration tests. The time history of the free surface motion was recorded using a video recording camera, and relevant data were captured. As carrying out experiments for different scenarios all the time is a difficult proposition, it was felt necessary to develop appropriate numerical models to understand the sloshing behaviour of the liquid contained in U-shaped containers. As an initial step towards it, both two-dimensional (2D) and three-dimensional (3D) numerical models are developed to observe the sloshing phenomenon of liquid stored in U-shaped containers. Using the volume of fluid (VOF) method, simulations are carried out on ANSYS (FLUENT) platform, which provides computational fluid dynamics (CFD) framework while solving the well-known Reynolds-Averaged Navier-Stokes (RANS) equations. Two equation turbulence model (k-epsilon) is used specially to examine the effects of turbulence on the numerical results. The numerical results thus obtained are matched with the experimental ones to demonstrate the applicability of the developed numerical model.

Turbulence Quantification in Supercritical Nitrogen Injection: An Analysis of Turbulence Models

In Liquid Rocket Engines, higher combustion efficiencies come at the cost of the propellants exceeding their critical point conditions and entering the supercritical domain. The term fluid is used because, under these conditions, there is no longer a clear distinction between a liquid and a gas phase. The non-conventional behavior of thermophysical properties makes the modeling of supercritical fluid flows a most challenging task. In the present work, a RANS computational method following an incompressible but variable density approach is devised on which the performance of several turbulence models is compared in conjunction with a high accuracy multi-parameter equation of state. Also, a suitable methodology to describe transport properties accounting for dense fluid corrections is applied. The results are validated against experimental data, becoming clear that there is no trend between turbulence model complexity and the quality of the produced results. For several instances, one- and two- equation turbulence models produce similar and better results than those of Large Eddy Simulation (LES). Finally, considerations about the applicability of the tested turbulence models in supercritical simulations are given based on the results and the structural nature of each model.
 Keywords: Supercritial fluids, RANS turbulence modeling, Liquid rocket engines

Inlet Swirl and Dump Gap Effect on Static Pressure Recov ery in Dump Diffuser Combustor u sing Two Equation Turbulence Model

Cold flow through an axi-symmetric dump diffuser, provided with a swirl effect at inlet is studied by using various two equation k-ε turbulent models such as; standard k-ε, RNG k- ε and Realizable k-ε model. There is a liner with a liner dome head of hemispherical in shape is used as a bluff body. From the comparison in between the three k-e turbulent models, it has been observed that the overall prediction of flow variables with Realizable k-e model are much better than others. The effects of dump gap and inlet swirl on velocity distribution and pressure variation on liner and casing walls have been investigated. The static pressure recovery within the diffuser is evaluated from area weighted average pressure at inlet and exit plane. It is noticed that an optimum inlet swirl results in the most effective pressure recovery due to the minimum irrecoverable energy dissipation in the vortices formed in the domain. The optimum swirl number is found to be 0.38 and it occurs when DG (non-dimensional dump gap) is kept as 1.0. The variation in dump gap changes the flow pattern and the possible pressure recovery. As the dump gap is increased, the static pressure is recovered more effectively with moderate swirl level in the inlet flow.

Unsteady Transverse Gas Injection in a Supersonic Nozzle Flow

The gasdynamic processes accompanying the injection of a pulsed gas jet into the diffuser part of a nozzle are considered in relation to the formation of control forces in rocket engines. The gas flow through the injection nozzle is controlled by a rotary damper. The finite volume method and moving mesh technology are used to discretize the Reynolds-averaged Navier–Stokes equations and the equations of the SST turbulence model. The effect of the opening time of the damper on the dynamics of the formation of the control force, as well as the dependence of the traction and energy characteristics of the nozzle on the flow rate of the injected working fluid, are studied. The results of calculations within the non-steady-state and quasi-steady-state problem statement are compared. The formation of the shock wave and vortex structure of the flow is discussed. Conclusions are drawn about the effect of the input parameters of the problem on the coefficient of the traction change.

Numerical Investigation on the Effect of Radiant floor panels configuration on Thermal comfort condition in Occupied zone

Radiant floor heating panels of a room with different locations is modeled by using CFD. All cases is simulate by employing k-ε equation turbulence model. After validation of code, average room air temperature, vertical air temperature difference and velocity of room air are used to predict the thermal comfort conditions in the occupied zone. Effect of indoor room air temperature and radiant floor heating panel locations on thermal conditions in occupied zone is studied. The results present that both the indoor room air temperature and radiant floor heating panels locations have effect on thermal comfort conditions and at low indoor temperature the radiant floor heating panel is insufficient to achieve thermal comfort conditions but can be reduced heating load.

Experimental validation of wind energy estimation

Wind power assessment in complex terrain is a very demanding task. Modeling wind conditions with standard linear models does not sufficiently reproduce wind conditions in complex terrains, especially on leeward sides of terrain slopes, primarily due to the vorticity. A more complex non-linear model, based on Reynolds averaged Navier-Stokes equations has been used. Turbulence was modeled by modified two-equations k-? model for neutral atmospheric boundary-layer conditions, written in general curvelinear non-orthogonal co-ordinate system. The full set of mass and momentum conservation equations as well as turbulence model equations are numerically solved, using the as CFD technique. A comparison of the application of linear model and non-linear model is presented. Considerable discrepancies of estimated wind speed have been obtained using linear and non-linear models. Statistics of annual electricity production vary up to 30% of the model site. Even anemometer measurements directly at a wind turbine?s site do not necessarily deliver the results needed for prediction calculations, as extrapolations of wind speed to hub height is tricky. The results of the simulation are compared by means of the turbine type, quality and quantity of the wind data and capacity factor. Finally, the comparison of the estimated results with the measured data at 10, 30, and 50 m is shown.

Gas-liquid two-phase flow of liquid film breaking process under shock wave

The gas-liquid two-phase flow of liquid dispersing and breaking under the action of shock wave includes complex physical phenomena, such as turbulent mixing of gas-liquid two-phase, instability and breakage of liquid interface, and formation of internal cavity structure after atomization. In order to investigate the shock-wave-caused breaking process of the liquid film, a three-dimensional numerical simulation of the gas-liquid two-phase flow process is performed by using the computational fluid dynamics method. In the simulation, the Mach number of shock wave is 1.5 and the thickness of liquid film is 2 mm. The finite volume method is used to solve the three-dimensional Navier-Stokes equation. The volume of fluid model is applied to the gas-liquid two-phase flow. The <i>k-ε</i> double equation turbulence model is selected for the turbulence calculation. The evolution process of the wave system structure of the shock wave and the deformation, breakage and atomization characteristics of the liquid film are obtained, and compared with the experimental results. The results show that the incidence, reflection, and transmission phenomena occur during the interaction between the shock wave and the liquid film, and the intensity of the transmitted shock wave and the liquid surface tension have an important effect on the breaking process of the liquid film. The transmitted shock wave affects the shape of the broken cloud cluster on the left of the liquid film, while the incident shock wave and reflected shock wave affect the shape of the broken cloud cluster on the right side of the liquid film. The volume of the atomized cloud formed in the breaking process of the liquid film increases rapidly, first reaching 6.7 dm<sup>3</sup> within 2.5 ms, then keeping stable basically. After the shock wave exits from the tube, a long narrow jet is formed. The maximum velocity reaches 519 m/s and appears in the interior of the jet, and then decreases continuously. Under the action of the jet, an expanding three-dimensional cavity structure is formed inside the atomizing cloud, and an annular vortex with negative pressure in the core area occurs in the cavity structure. Finally, the annular vortex continuously entrains the surrounding fluid in the process of forward movement, the strength of the vortex decreases and gradually dissipates in the space. This work is conducive to further understanding the interaction process of gas-liquid two-phase flow.

Modifying a meshless method to solving κ−ε turbulent natural convection heat transfer

The conventional meshless local Petrov–Galerkin method is modified to enable the method to solve turbulent convection heat transfer problems. The modifications include developing a new computer code which empowers the method to adopt nonlinear equations. A source term expressed in terms of turbulent viscosity gradients is appended to the code to optimize the accuracy for turbulent flow domains. The standard [Formula: see text] transport equations, one of the most applicable two equation turbulent viscosity models, is incorporated, appropriately, into the developed code to bring about both versibility and stability for turbulent natural heat transfer applications. The amenability of the new developed technique is tested by applying the modified method to two conventional turbulent fluid flow test cases. Upon the obtained acceptable results, the modified technique is, next, applied to two conventional natural heat transfer test cases for their turbulent domain. Based on comparing the results of the new technique with those of the available experimental or conventional numerical methods, the proposed method shows good adaptability and accuracy for both the fluid flow and convection heat transfer applications in turbulent domains. The new technique, now, furthers the applicability of the mesh-free local Petrov-Galerkin (MLPG) method to turbulent flow and heat transfer problems and provides much closer results to those of the available experimental or conventional numerical methods.

Augmentation of Nanofluids Heat Transfer in a Circular Pipe with Coiled Tube Insert as Heat Source and Swirl Generator

This research present steady state heat transfer and fluid flow characteristics in concentric pipe with coiled tube insert for turbulent flow regime with metal oxide nanofluid using ANSYS-FLUENT 18.0 where the governing equations of mass, momentum and heat transfer were solved simultaneously, using the k-e two equations turbulence model. Copper was chosen as the as metal for the construction of pipe and the helical tube insert. Coiled tube with curvature to pitch ratio as 1 and 2.5 mm in diameter with 1% volume fractions of TiO2 and CuO Nanofluid with Reynolds number ranged from 4000-16000 were considered in this research. The heat generated from constant water temperature (80 °C) with constant flow rates in helical coil (Re=4000). The Result shows that the heat and friction coefficients conducted by vortex generator raised with Reynolds number and accretion of nanoparticle presence. Furthermore, the maximum rate of heat transfer with significant intension in friction coefficient has been produced TiO2 nanofluid by as compared with CuO and water.

Effects of Coil Pitch Spacing on Heat Transfer Performance of Nanofluid Turbulent Flow through Helical Microtube Heat Exchanger

This article provides Numerical simulation on forced convective heat transfer performance of Nanofluid flowing through copper helical microtube of inner diameter of 1.5 mm with different pitch using ANSYS-FLUENT 18.0. The simulation was performed for water, CuO/water, Al2O3/water Nanofluid with 1-2% volume concentration and different pitch of microtube (10, 14 and 18 mm) for turbulent flow regime of Reynolds number varied 5000 to 20000 and governing equations of mass, momentum and heat transfer were solved simultaneously, using the k-e two equations turbulence model. Based on the obtained results, regardless of the concentrations used, the nanofluids exhibited a higher transfer rate than water. This is mainly attributed to the nanoparticles that are in the used nanofluids. The friction factor and the heat transfer rate were enhanced considerably due to the shape and size of the tube, which in this case is a helical microtube. Moreover, the maximum heat transfer performance has been conducted by Al2O3/water Nanofluid with 2% volume concentration and microtube pitch of 18 mm.

PIV measurements on a Kaplan turbine and comparison with scale-adaptive numerical analysis

Kaplan turbines build a mainstay of the hydro power generation as they offer best efficiency even for high flow conditions. A double regulating approach facilitates a high degree of flexibility but yields to be a sophisticated turbine type. Typical phenomena like gap flow and flow separation are challenges in simulation and complicate reliable predictions. To get a better understanding of flow physics elaborate numerical simulations by means of Computational fluid dynamics (CFD) are conducted as well as experimental flow investigation using the non-intrusive optical measurement technique Particle Image Velocimetry (PIV). Measurements are taken at the spiral case inlet, the draft tube cone and the diffuser giving instantaneous or phase-locked time averaged vector fields of flow velocity in a plane. Depending on the optical access it is possible to gather the in-plane velocity components or all three velocity components using Stereoscopic PIV. The obtained measurement results are then taken as reference on the one hand side for a definition of realistic inlet boundary condition and on the other hand side for assessment of the chosen numerical approach for predicting flow phenomena in Kaplan turbines. In this particular case the focus is on the interaction of runner and draft tube. Results of unsteady simulations with standard two equation and advanced hybrid RANS-LES turbulence models are compared to measurement data.

Lift/Drag Characteristics of Different Ice Accretion Airfoil

Aircraft icing has influence on the preliminary safety and performance evaluation of airplane. In this paper, based on different ice shapes (none, rime, mixed), the aerodynamic models of airfoil are established to analyze. Various lift/drag characteristics are obtained by numerical simulation using computational fluid dynamics (CFD) under different icing conditions. Navier-Stokes (N-S) equation and Spalart-Allmaras (S-A) turbulence model are adopted. The results show: 1. It’s obvious that lift-drag characteristics are different under various ice shapes. The lift coefficient (α=0° ∼7°): CL none>CL rime>CL mixed, the drag coefficient (α=0° ∼7°): CD none<CD rime<CD mixed. 2. At the same angle of attack (α=6°), lift/drag characteristics in the condition of different ice shapes are different. The pressure gradient at the leading edge is ▽p none>Vp rime>▽p mixed, which has important effect on the speed of airstream flow and the flow separation at the front zone of airfoil.

NUMERICAL ANALYSIS OF SOLAR TOWER SYSTEM UTILIZED WITH FLAT PLATE AND POROUS ABSORBER

The performance of solar updraft tower investigates numerically by comparing between two solar collectors with and without porous absorber flat plate. In this study, copper metal foam (10 and 40) PPI at the same porosity (0.9) are used as an absorber plate. The effect of the absorbing porous medium is studied to increase the air flow towards the updraft tower by tilting of the porous absorber at an angle (2° and 6°) from the horizontal line for (10 and 40) PPI and compared with the horizontal absorber flat plate. To simulate the physical quantities inside the porous medium, at steady state, symmetry, three dimensional, Darcy model and energy numerical model with local thermal equilibrium (LTE) assumption are adopted and numerical models is approximated by a RNG (Re-Normalization Group) k- ϵ turbulent model and discrete ordinates (DO) radiation model equations. The numerical study is analyzed by Fluent software package (version 18.2) to solve the governing equations. The results showed that the tilting of a porous absorber plate at an angle (2° and 6°) from the horizontal line lead to increase in the mass flow rate inside the solar updraft tower and the maximum performance is found by using 40 PPI at tilt angle 2°.The performance of solar updraft tower investigates numerically by comparing between two solar collectors with and without porous absorber flat plate. In this study, copper metal foam (10 and 40) PPI at the same porosity (0.9) are used as an absorber plate. The effect of the absorbing porous medium is studied to increase the air flow towards the updraft tower by tilting of the porous absorber at an angle (2° and 6°) from the horizontal line for (10 and 40) PPI and compared with the horizontal absorber flat plate. To simulate the physical quantities inside the porous medium, at steady state, symmetry, three dimensional, Darcy model and energy numerical model with local thermal equilibrium (LTE) assumption are adopted and numerical models is approximated by a RNG (Re-Normalization Group) k- ϵ turbulent model and discrete ordinates (DO) radiation model equations. The numerical study is analyzed by Fluent software package (version 18.2) to solve the governing equations. The results showed that the tilting of a porous absorber plate at an angle (2° and 6°) from the horizontal line lead to increase in the mass flow rate inside the solar updraft tower and the maximum performance is found by using 40 PPI at tilt angle 2°.

Modeling and computational simulation of fluid flow, heat transfer and inclusions trajectories in a tundish of a steel continuous casting machine

Currently, the continuous casting process is the main route used in the integrated steelmaking process. The tundish is an essential component of the caster machine and plays an important role in the control of inclusion in the production of the clean steels. Thus, viable methodologies and predictive models to evaluate this metallurgical reactor efficiency are important technological allies to improve the clean steel production. The main goal of the present study is to develop a computational tool to analyze the turbulent fluid flow, temperature distribution and inclusions removal during the continuous operation of the tundish of an industrial facility to improve its internal configuration. New configurations using weirs and dams are proposed which improve the steel quality. We used the Ansys CFX® software based on the element-based finite-volume method (EbFVM) to solve the coupled turbulent flow and heat transfer model equations in an Eulerian flame. A random nucleation mechanism for the inclusions generation and a modified Lagrangian model with a random walking model (RWM) to account for the effect of turbulence on the inclusions trajectories were proposed. The flow pattern was successfully validated with experimental data. The results were presented in terms of velocity and temperature fields and residence time distribution (RTD) curves. Then, the model was used to investigate new scenarios and enhanced geometry configurations.

Numerical simulations and optimizations for turbine-related configurations

In order to accelerate the numerical simulation and optimization of gas turbine-related configurations, a source based computational fluid dynamics (SCFD) approach is developed for flow and heat transfer simulations. Different sources de-pending on the fluid porosity at each grid node in the computational domain are introduced to the continuity, momentum, energy and turbulence model equations, so that both the fluid and solid regions can be solved as one region. In the present paper, test cases including a ribbed channel and a winglet shrouded turbine cascade with tip injection are investigated using the SCFD and CFD with body-fitted meshes. Impacts of grid clustering and turbulence model equation sources on the SCFD precision are examined. Numerical results show that the SCFD predicts consistent aero-thermal performance with the fluid dynamics with body-fitted meshes and experiments. The validated SCFD scheme is then employed in a response surface optimization of tip jet holes on the winglet shroud tip. A jet arrangement with the minimum energy loss and injection mass-flow rate is obtained, indicating that source based predictions can be applied to the preliminary aero-thermal design of turbine blades.

Effect of injection angle of primary and secondary holes on the mixing and combustion in a can type combustor using CFD

Combustion is one of the vital process in power generation system and so is the combustor in a Gas Turbine. The performance of combustor has greater impact on the efficiency of the cycle, as a result combustor has always been on the prime focus for researchers to improve the efficiency of gas turbine. The present study is focused on numerical analysis, using ANSYS FLUENT, of combustion performance using slotted swirler with swirl angle of 45°. In this study, considering the importance of primary and dilution injection on combustion performance, the injection angle is varied 60° to − 60° from normal to the liner wall. Furthermore, the curvature of primary and dilution hole is formed in the form of nozzle and diffuser having convergence and divergence angle of 30° and 45°. Because of high swirling effect (S = 1.32), RSM (7 equations) turbulence model is used along with Non-Premixed combustion mechanism by generating β-PDF table. The performance is determined by the mass fraction of CO2 at outlet. Higher the mass fraction of CO2 at outlet, signifies better combustion. From the study it is reflected that the slotted swirler have better combustion performance than non-slotted swirler whereas variation of injection angle suggests, injection at 30° clockwise is more suitable. Also, primary injection done through nozzle channel (convergence angle of 45°) and dilution injection through diffuser channel (divergence angle of 45°) results in most efficient combustion performance.

Impact of vegetation on dustiness produced by surface coal mine in North Bohemia

The contribution deals with the numerical simulation of the efficiency of the protective vegetative barriers close to an open coal mine. An assignment has come from the Czech mining company (Severočeské doly a.s.) requesting the assessment of the impact of a mine extension on the environment. The flow field and the concentrations are computed on 2D cuts with a real geometry of Bílina coal mine. System of the RANS equations for viscous incompressible flow with variable density in Boussinesq formulation is used for description of the flows. The two equations turbulence model is used for the closure of this set of equations. The transport equations for different diameters of pollutant are solved. The numerical solver is based on finite volume method and AUSM+up scheme. Advanced vegetation model which includes both influence of vegetation on the flow field as well as processes inside vegetation is adopted. For estimation of the efficiency of barrier several integral and point criteria are used.

An innovative augmentation technique of savonius wind turbine performance

This work presents an innovative technique to enhance the performance of the Savonius wind turbine. The new technique is based on introducing an upstream deflector and downstream baffle. The shape and location of both devices are optimized using a genetic algorithm. The performance of the turbine with the optimized devices is compared with the single Savonius turbine performance. The study employs the finite volume solver (ANSYS-FLUENT) to solve unsteady Reynolds Averaged Navier–Stokes equations and turbulence model equations. The optimized configuration results in much higher power coefficient than the Savonius turbine. The average peak power coefficient using both deflector and baffle is 0.47 compared to 0.24 of the Savonius turbine. The peak power coefficient of the turbine corresponds to a speed ratio close to unity. This improved performance is attributed to the favorable aerodynamic interaction between the turbine and the downstream baffle which accelerates the flow around the rotor and generates larger turning torque. The baffle generates a jet effect on the advancing bucket and accelerates the flow behind the bucket creating a large zone of negative pressure and thereby increases the driving torque. Furthermore, the upstream deflector (also called shield or curtain) produces a shield for the returning bucket of the turbine which diminishes the adverse effect associated with the returning bucket on the aerodynamic torque of the turbine. This remarkable improvement of turbine performance will encourage the future application of the Savonius wind turbine in small power applications of wind energy.