Progress in Computational Fluid Dynamics, An International Journal (30 papers in press)
DNS of wake from perforated plates: aspect ratio effects
by Abhinav Singh, Vagesh Narasimhamurthy
Abstract: Direct numerical simulations of flow over a thin perforated plate with square holes placed normal to the free-stream is carried out at Reynolds number 250 (based on plate width, d, and inflow velocity, Uo). The effect of the plate aspect ratio L/d (where, L is the plate length) is studied by varying the spanwise length L as 1d, 3d, 6d and 12d. The hydrodynamic forces acting on the plate and the shedding frequency match for all the cases, though the wake dynamics is clearly distinct between the lower and upper L/d cases. In the low L/d cases, the wake is fairly coherent along the span and has reduced three-dimensionality. The flow here is undergoing wake-transition, where the wavelength of the secondary instability is about 1d. Further, the jet flow emanating from the perforation holes exhibit in-phase oscillation along the span. Another interesting feature here is the presence of energetic subharmonics, where the wake is experiencing period-doubling bifurcations. In the high L/d cases, the wake exhibits higher incoherence in the form of intermittent vortex-dislocations and incoherent oscillations of the perforation jets, which promotes flow three-dimensionality. The present study further indicates that a critical aspect ratio of about 6 is required for the simulations to be free from numerical effects and to predict the flow physics accurately.
Keywords: DNS; spanwise aspect ratio; bluff-body; period-doubling cascade.
A simple and efficient two-dimensional hydrodynamic model for unsteady flow simulation in undulating bathymetry
by Anupal Baruah
Abstract: Two-dimensional modeling in a river having undulating bathymetry is challenging in terms of numerical stability, accuracy, and robustness. The main reason behind the model stability is the oscillations in the solutions due to the nonlinear and hyperbolic form of the flow equations. Sometimes it is observed that using primitive variables as temporal derivatives in conservation equation is a source of model instability, especially in a braided river. In this work, the surface form of two-dimensional hydrodynamic equations of shallow water is solved using a five-point TVD Mac-Cormack predictor-corrector scheme. The water surface gradient is used as a gravitational force component in the source term to overcome the possible error that may occur during bed slope discretization in the case of undulating bathymetry. Model is then applied over a wide range of problems including irregular bed, dam breach flow, and mix flow with a hydraulic jump. Model outputs are validated with the available experimental data.
Keywords: TVD; Complex topography; mix flow; water surface slope; two-dimensional shallow water equation; unsteady flow.
Parametric study of nanofluid jet impingement cooling of protruding heaters with porous medium
by Chinmayee Behera, Shantanu Pramanik
Abstract: Abstract: In the present study fluid flow and thermal behavior of a nanofluid jet in connection with forced convection cooling of an array of discrete protruded heaters fixed on an impingement wall and embedded in a porous layer has been investigated for various channel heights (H/L), Darcy Numbers (Da), porosities (ε) and nanoparticle volume fraction (ϕ). The numerical simulations are performed using Al2O3-water based nanofluids with nanoparticles having a diameter of 30 nm. The numerical investigation has been performed using commercial CFD software ANSYS-FLUENT (17.0) implementing LTE model simulating flow through the porous layer. The value of ϕ has been varied at 0%, 2%, 4% and 6% for each case of H/L ratios (0.5 and 1), Da (10-3, 10-4 and 10-5) and porosities (0.5, 0.7 and 0.9), respectively. Increase in percentage of ϕ, with other parameter fixed, has always resulted in improved heat transfer. Maximum value of Nuavg is observed at ϕ=6%. For H/L=0.5 and 1.0, maximum increment in average Nusselt number (Nuavg) has been found to be 33.3% and 15.38%, respectively, along the fourth heater for Da=10-2, ϕ=6% and ε=0.5. Maximum increment in Nuavg has been found to be 15.7% along the first heater for Da=10-4, on the contrary, the minimum value of the increment of 0.3% is observed along the second heater at Da=10-5 in both case for a fixed value of ϕ=6%. However, the effect of variation in porosity variation (0.5, 0.7 and 0.9) on enhancement of heat transfer is not discernible except on the fourth heater where increment of Nuavg is 33.3% at ε=0.5 and ϕ=6%.
Keywords: Nanoparticle; impinging jet; porous layer; discrete heaters,LTE model; single phase.
CFD-based Prediction of Hydrogen Deflagration in Semi-open Ducts
by Eilidh Seville, Helen Scott, Iain Waddell, Eero Immonen, Bingzhi Li
Abstract: Explosion of hydrogen-air mixture in an enclosure produces high local pressure levels, potentially resulting in personnel and property losses. Design of explosion venting systems for mitigating this explosion risk typically involves the use of computational methods. The purpose of this article is to present an unsteady RANS CFD model for simulating hydrogen deflagration in the high flame speed regime in semi-open ducts. This is a relevant scenario from the practical point of view, but with only few previously reported successful modeling attempts. The model proposed herein has been implemented in ANSYS Fluent 19 R2, and is compared to a well-known experiment reported in the literature. The results show a good agreement of the flame speed prediction up to almost 500 m/s, and improved accuracy, relative to existing models, for higher flame speeds. A validation study presented in this paper shows that the model is also robust with respect to mesh resolution as long as the time step in the simulations is small enough (order of 10^-5 s). As a relatively lightweight alternative to computationally intensive Large-Eddy Simulation models, the proposed modeling approach is expected to be beneficial for practical power system design concerns.
Keywords: CFD; Hydrogen; Deflagration; Semi-open ducts; Flame speed; Explosion; Safety.
Numerical Investigation of Atomization Using a Hybrid Eulerian-Lagrangian Solver
by Botond Pál, Dirk Roekaerts, Barry Zandbergen
Abstract: This study investigates the potential of a newly released multi-phase solver to simulate atomization in an air-blast type atomizer. The "VOF-to-DPM" solver was used to simulate primary and secondary atomization in an air-blast atomizer with a coaxial injector-like geometry. The solver uses a hybrid Eulerian/Eulerian-Lagrangian formulation with geometric transition criteria between the two models. The conducted study assumed isothermal, non-reacting flow at room temperature. The primary focus was predicting Sauter Mean Diameter and droplet velocity data at a sampling plane downstream of the injection site. The results showed that the solver is able to produce the expected data and to predict trends similar to those found in experimental measurements. The accuracy of the produced droplet diameters was roughly a factor 2 off compared to experiment. This is attributed primarily to mesh resolution. It was concluded that the solver has the potential to predict atomization at a reasonable computational cost, but further study is needed to confirm its full capabilities.
Keywords: Atomization; Ansys Fluent; Multi-phase flow; Spray Formation; Volume of Fluid.
Simplified Level Set Method Coupled to Stabilized Finite Element Flow Solver for Moving Boundaries
by Ahmed Mohamed, Mohamed Abdulrahman, Amr Guaily
Abstract: A simplified version of the level set method (SLSM) is proposed for interfaces moving with prescribed motion e.g., deployment of a spoiler. The core of the proposed method is the signed distance function which is constructed by direct computation for all vertices in the mesh. The main advantage of the proposed algorithm is that it avoids the need for re-meshing and its associated problems as well as it could be used equally with all types of grids. The SLSM is successfully coupled to the finite element method flow solver, and the resulting hybrid solver is used to study the flow field during the spoiler deployment of a NACA0012 airfoil. Finally, two algorithms are proposed for force calculations over moving interfaces with new definitions for the smeared Dirac-Delta and Heaviside functions. At the moment of spoiler deployment, an abrupt rise in the lift force is observed and interpreted. The results assure the robustness of the proposed hybrid solver.
Keywords: Simplified Level Set Method; Fluid-Solid Interaction; Spoiler Deployment; Lift and Drag Computation.
Computational Analysis of Mixing Characteristics of Port-Injected BioCNG with Air for Different Designs of Intake Manifold
by Akash Chandrabhan Chandekar, Biplab Kumar Debnath
Abstract: The bioCNG with 90% of methane makes a potential renewable alternative of CNG for the application in dual-fuel diesel engine. The bioCNG is to be inducted in the manifold through port injection. The work is aimed to find the optimum port length and port injector diameter. The objective is to mix bioCNG homogenously with air, before entering the engine. This would create an opportunity for major replacement of diesel to reduce the running cost of the vehicle. The two injector diameters (7 and 10 mm) and four port lengths (0, 100, 150 and 200 mm) are investigated. The manifold has a diameter of 28.5 mm and the port injector is inclined at 35? to the manifold axis. The geometries are simulated in ANSYS Workbench 19.2, followed by analysis in Tecplot. The results show that the port length of 200 mm with an injector diameter of 7 mm delivered homogeneous mixture for the given dual-fuel engine.
Keywords: bioCNG; dual-fuel; intake manifold; port fuel injector; helicity; mass fraction of CH4; homogeneous mixing.
Computational study of sloshing in spherical tanks and the effect of using annular baffle over slosh force frequency and damping
by Habib Ahmad, Ammar Ahmed, Ismail Baila, Ajmal Shah
Abstract: Partially filled liquid containers are subjected to sloshing when acted upon by an external force or acceleration. The sloshing liquid applies varying forces on walls of the container that have to be included in its design. In this study, the CFD method was validated against experimental data from existing literature. Afterward, the impact of the liquid fill level in the container, initial perturbation velocity, and variation in time step size over the frequency and damping of sloshing force was studied. Moreover, a novel idea of introducing an annular baffle in spherical tanks has been proposed to minimize the effects of sloshing on the container. The damping effect of the annular baffle in a spherical container was analyzed using a validated CFD method. The result comparison of the two cases shows a significant increase in the damping of slosh force by the introduction of an annular baffle.
Keywords: Sloshing; Spherical tanks; Slosh damping; CFD; Annular baffle.
A Computational and Experimental Study on Aerodynamics of Motor-driven Propellers Using Thrust Stand and Rotating Cup Anemometer
by Zaid Siddiqi, Jin Wook Lee
Abstract: The aims of this study are: (1) Develop a cost-effective experimental set up that incorporates a commercially available thrust-stand and rotating cup anemometer; (2) Analyze rotor performance at rotational speeds ranging from 6,000 PRM to 14,000 RPM; (3) Use the Multiple Reference Frame (MRF) approach to develop Computational Fluid Dynamics (CFD) simulations that provide a good agreement with experimental results; (4) to assess the efficacy of using a rotating cup anemometer to measure propeller downwash velocity. A 6-inch propeller is paired with a 2600 KV motor. Overall rotor efficiency peaks near 7,200 RPM and beyond 9,300 RPM; it declines rapidly as the mechanical and electrical power output increases. To assess the thrust and downwash velocity, two turbulence models are used in steady state: k-? Realizable and the k-? SST. The k-? SST shows good overall agreements for thrust, while the k-? Realizable provides a more accurate modeling of the downwash velocity. Based on the CFD results, the rotating cup anemometer is found to be unsuitable for measuring propeller downwash velocities.
Keywords: Unmanned Aerial Vehicle; Computational Fluid Dynamics; Multiple Reference Frame; Thrust; Thrust-stand; Anemometer.
The behavior of time-independent non-Newtonian fluids in an impact problem of a wedge using smoothed particle hydrodynamics method
by Jafar Gerdabi, Amir Hossein Nikseresht, Mohammad A. Esmaeili-Sikarudi
Abstract: The impact of solid objects on non-Newtonian fluids may occur in many industrial applications such as liquid armors or moving speedboats on muddy water. So far wedge impact on a Newtonian fluid surface has been simulated through various Eulerian and Lagrangian numerical methods while there are few studies on how non-Newtonian fluids may behave under such impacts. This paper aims to investigate the behavior of time-independent non-Newtonian fluids in a 2DOF (2 Degrees Of Freedom) wedge impact problem using a numerical Lagrangian in-house code which is based on the Weakly Compressible Smoothed Particle Hydrodynamics (WCSPH) method. At first, a dam break test case is simulated using non-Newtonian fluid models to validate both the generated code and the method. Afterward, wedge impact on a Newtonian fluid is simulated and its results are compared with Boundary Element Method (BEM) and experimental results. Finally, wedge impact on various non-Newtonian fluids is simulated and the resulted force exerted on the wedge, pressure coefficient, wedge velocity, and fluid free surface profile are compared with each other and with the Newtonian fluid results. The results show that in general, the vertical force exerted on the wedge by the non-Newtonian dilatant and Herschel-Bulkley fluids is bigger than Newtonian and other non-Newtonian fluids. The percentage increase in the maximum vertical force when non-Newtonian fluids instead of Newtonian fluids are used is greater in the case of the lighter wedge versus the heavier one. Also, the dilatant fluids behave differently after wedge impact and it is interesting to know that after the impact there is no formation of the fluid jet.
Keywords: WCSPH; Non-Newtonian fluids; Artificial viscosity; Density correction; Repulsive force; Impact.
Computational Study of Combined Effects of Tip Leakage Jets and Wetness on the Performance of Steam Turbine Exhaust System using Actuator Disc Model
by Sreeja Sadasivan, Senthil Kumar Arumugam, Mahesh C. Aggarwal
Abstract: A numerical model to study the individual and combined effects of the tip clearance jets and the presence of wetness on the performance of the steam turbine exhaust hood has been developed. The Eulerian- Eulerian wet steam flow equations along with mass, momentum, energy, and k-epsilon model equations are solved in the present study. An increase in inlet wetness level improves the pressure recovery capacity of the steam turbine exhaust hood by reducing the total pressure loss. The presence of moisture causes a turbulence attenuation which in turn reduces the vortex strength. The combined effect of the tip leakage jets and the presence of inlet wetness improves the performance of the exhaust system further. However, the presence of a non-uniform condenser pressure gradient caused by the temperature variation of the cooling water flow increases the level of complexity of the flow in the exhaust system deteriorating the performance.
Keywords: Actuator-disc model; Wetness; Steam turbine exhaust hood; Turbulence damping; Rotor-tip leakage.
Numerical investigation on shear flow and boiling heat transfer on shell-side of LNG spiral wound heat exchanger
by Zhiyong Wu, Ying Fan, Weihua Cai, Yiqiang Jiang
Abstract: A numerical model was developed to investigate the shear flow and boiling heat transfer on shell-side of spiral wound heat exchanger used for natural gas liquefaction. Thermal ratio of phase transition (?) as a important parameter in simulation was studied, the error on defining?proposed in reference was illuminated. After?was anew defined, the value of ? was appropriately selected under different working condition, and the major factors governing?were analyzed. Simulation results were consistent with experimental data, the simulation deviations of both frictional pressure drop and heat transfer were within
Keywords: Spiral wound heat exchanger; shell side; shear flow; boiling.
Wake interaction studies of flow past tandem circular cylinders for different diameter and gap ratios
by R. Rajita Shenoi, Neeraj Paul Manelil, Thirumalachari Sundararajan, Shaligram Tiwari
Abstract: Three-dimensional computations are carried out to study the flow past tandem stationary cylinders at moderate Reynolds number (Re = 2000). Wake structures are analyzed with the help of instantaneous vorticity contours and hydrodynamic characteristics are reported using drag and lift coefficient plots. Effects of diameter ratio and gap ratio on wake structure as well as its dynamic characteristics are presented. Cross recurrence analysis is performed to study the coupled interactions between cylinder wakes. In particular, conditions for the in-phase synchronization, out-of-phase coupling or non-synchronous coupling of the wakes behind the tandem cylinders are identified. Qualitative results obtained from cross recurrencernplots are quantified using parameters such as recurrence rate, determinism and averaged diagonal length. These results give deep insight into wake synchronizations between the cylinders. In-phase or out-of-phase wake oscillations can lead to catastrophic structural failures and predicting such conditions can be a useful design tool.
Keywords: Tandem circular cylinders; diameter ratio; inter-cylinder spacing; dynamic wake characteristics; recurrence plots; synchronization.
Coupled natural convection and radiation in a cubic cavity filled with an air - H_2O mixture in the presence of a heated obstacle
by Xuan Bach NGUYEN, Didier SAURY, Denis LEMONNIER
Abstract: Natural convection in a cubical cavity with a hot obstacle located on its floor is investigated. The inner fluid is a semi transparent mixture of dry air and water vapor, which creates a coupled convective and radiative transport within the fluid. The conservation equations are solved by a finite volume method and the radiative transfer equation by using the discrete ordinates method. The radiative properties of the mixture are accounted for by a spectral line weighted sum of gray gases model associated to the rank correlated approach. It was observed that the volume radiation has a strong influence on the thermal and dynamic fields. The nearly vertical stratification of the temperature field around the plume is broken. Radiation also accelerates the boundary layers near the lateral surfaces and the ceiling and the floor of the enclosure. The total heat transfer is decreased due to both the reduction in convective process near the vertical walls and the attenuation by radiation.
Keywords: Heat transfer; Convection; Radiation; Numerical method.
NUMERICAL SIMULATION OF THE INFLUENCE OF FURNACE GEOMETRY ON ITS PERFORMANCE IN A TANGENTIALLY FIRED PULVERISED-COAL SUPERCRITICAL BOILER
by Sankar G, Balasubramanian K R, Chandra Sekhar Dhannina, Santhosh Kumar D
Abstract: The influence of furnace geometry on furnace performance was numerically investigated and validated for a tangentially fired pulverised coal boiler. Six simulation cases were studied by varying the plan area of the furnace; three cases were based on a rectangular furnace and three cases were based on a square furnace. Performance characteristics that were investigated include variation of wall heat flux, Furnace Exit Gas Temperature (FEGT) and NOx concentrations at the outlet of the furnace. The simulations revealed that, with a 20% increase in plan area, a decrease in FEGT was observed, which corresponded to a drop of approximately 30
Keywords: Heat flux distribution; Tangentially fired boiler; NOx emission levels; Furnace geometry; Supercritical boiler; Computational fluid dynamics (CFD).
Solid Rocket Motor Interior Ballistics Fluid-Solid Interaction Simulation Using Level Set Method for 2D Grains
by Hossam Alqaleiby, Aly Hashem, Mohamed Tosson, Amr Guaily
Abstract: The flow inside a solid rocket motor is considered taking into account the fluid-solid interaction between the combustion gases and the burning grain surface. An unsteady flow model is presented for the flow variables while the time-dependent burning surface is captured using the level set method. Then a hybrid model is presented by coupling the flow model with the interface capturing technique. The proposed hybrid model is successfully tested against previously published experimental and numerical data. The use of the level set method enables the flow model to consider complex grain shapes like the star shapes. The proposed hybrid model is used to produce the pressure-time curves for cross-shaped grain and a star grain with five points.
Keywords: Fluid-solid interaction; Level set method; Finite element method; Internal ballistics.
Chebyshev spectral collocation method for MHD duct flow under slip condition
by Canan Bozkaya, Önder Türk
Abstract: The magnetohydrodynamic problem of a fully developed flow of an incompressible and electrically conducting fluid is solved numerically by a Chebyshev spectral collocation method in a square duct with walls of variable electric conductivities under a slip condition for velocity. The flow is driven by a constant pressure gradient under the effect of an externally applied oblique magnetic field. The efficiency of the method that is implemented in the physical space on preassigned collocation points is exploited to discretise the governing equations. The corroboration and validation of the proposed technique are carried out by means of a case study with published results substantiating that its implementation results in satisfactorily good agreements. Novel results are presented graphically, and the combined effects of the most characteristic magnetohydrodynamic flow parameters such as the slip length, conductivity parameter, and Hartmann number on the velocity and induced magnetic field are investigated.
Keywords: MHD flow; rectangular duct; slip condition; variable conductivity; Chebyshev spectral collocation.
CFD based Optimization of Base Pressure Behavior on Suddenly Expanded Flows at Supersonic Mach numbers
by Jaimon Dennis Quadros, Sher Afghan Khan, Prashanth T.
Abstract: In this work, a suddenly expanded flow process is modeled for determining base pressure characteristics. The base pressure developed in a suddenly expanded flow process majorly depends on the pertinent selection of Mach number (M), nozzle pressure ratio (NPR), area ratio (AR), and length to diameter ratio (L/D). Numerical analysis of the flow process was carried out using the computational fluid dynamics (CFD) technique. The input-output test cases for CFD analysis were developed as per two statistical methods, namely central composite design (CCD) and Box-Behnken design (BBD). The CFD results were validated by experiments that were conducted by using a nozzle and expanded duct. The input-output relationship obtained from the BBD was found to be statistically adequate and yielded better prediction accuracy. The BBD response model was used for generating data that trained the recurrent and backpropagation neural networks. The recurrent neural network outperformed both the backpropagation neural network and Box-Behnken design.
Furthermore, to assess the right range of conditions for maximizing base pressure, the genetic algorithm (GA), desirability function approach (DFA), and particle swarm optimization (PSO) techniques were implemented. The performance of PSO and GA techniques was found to be better, as they carried out search operations in many directions at multi-dimensional space simultaneously. All the models and optimization techniques were compared, and final comments are drawn.
Keywords: base pressure; CFD; Response surface methodology (RSM); neural networks; optimization.
On the use of adaptive relaxation times in lattice Boltzmann methods
by Simon Marié
Abstract: The lattice Boltzmann collision model with multiple relaxation times is modified to make the relaxation rates dependent on the shear stress. With these adaptive relaxation times, the optimal choice made by the theoretical development of MRT does not hold anymore. However, it is shown that the numerical properties of the collision model remains close to the MRT model. In particular, it is shown that the adaptive model recover the BGK properties in terms of acoustical dissipation but keeps the numerical stability of MRT when shear stress becomes high. Then the adaptive model is studied in terms of stability and accuracy on the Taylor-Green vortex case, and the acoustic properties are tested on the sound radiated by a square cylinder.
Keywords: Lattice Boltzmann method; multiple relaxation times; Adaptive relaxationrntimes; Taylor Green Vortex; aerodynamic sound.
Experimental and Numerical Investigation of Co-Axial Rotor Interaction to Thrust
by Ahmet Soydan, Hurkan Sahin, Baris Bicer, Sebnem Sariozkan, Mehmet Sahin
Abstract: The experimental and numerical computational investigation of co-axial rotor performance has been increased over the past decade in order to understand complex interactions in coaxial-rotor flows to improve design of unmanned-aerial vehicles. Nevertheless, the issues related rotor aerodynamic performance, wake interactions, etc. are not well understood. In the current work, aerodynamic interactions in co-axial rotor have been investigated with both experimental and numerical methods in hover flight by varying tip diameters, rpm, axial distance etc. In order to calculate the co-axial thrust efficiency, in-house test bench has been created. On the numerical side, the three-dimensional unsteady Navier-Stokes equation is solved using a pressure based, segregated, compressible and time-accurate solver of OpenFOAM. A sliding mesh interface procedure is utilized to link rotating regions and SST k-$omega$ model is employed for the turbulence modelling. The computational results indicate relatively good agreement with in-house experimental data.
Keywords: Multi-rotor; Coaxial rotor; OpenFOAM; RANS; Sliding mesh; Hover flight.
Numerical Study of Characteristics of Confined Diffusion Flames of Synthetic Gases in Coflow and Inverse Coflow Configurations
by Mohd. Ibrahim, S. Muthu Kumaran, Vasudevan Raghavan
Abstract: Comprehensive numerical predictions of diffusion flame characteristics of practical synthetic gas (syngas) fuels in coflow (CDF) and inverse coflow (IDF) configurations with stoichiometric amount of coflow air have been compared. A numerical model with variable thermo-physical properties, multi-component diffusion, Soret diffusion, detailed chemical kinetics mechanism and an optically thin radiation model has been used. Effect of hydrogen content in fuel, which controls the reactivity, on flame characteristics has been revealed systematically. Higher peak flame temperature is observed in IDF than in CDF. However, its radially averaged temperature peak is lower in IDF than that in CDF within the flame zone. Higher rate of radial oxygen transport is seen in IDF, resulting in lesser flame radius. Combustion efficiency is higher in IDF than in CDF, for syngas fuels with intermediate reactivity. Understanding the characteristics of these flames will lead to better design of multi-slot burners using mixture of syngas fuels.
Keywords: Synthetic gas; hydrogen content; inverse coflow diffusion flames; detailed chemical kinetics; multi-component diffusion; flame structure; numerical model; combustion efficiency; thermo-physical properties; optically-thin radiation model.
Numerical prediction of solid-liquid slurry flows and derivation of simulation-based correlations for local solid concentration
by Manoj Kumar Gopaliya, Deo Raj Kaushal
Abstract: This paper presents correlations for predicting local solid concentration for pseudo-homogeneous and mildly heterogeneous solid-liquid slurry flows through horizontal pipelines using curve fitting techniques applied on exhaustive simulation data. These data are obtained using a duly validated Eulerian multiphase model whose applicability is ascertained for the slurry flow cases under consideration. The ranges of geometric and working input parameters of solid-liquid slurry flow analyzed during the present research work are quite comprehensive and wide which increases the applicability realm of the derived correlations.
Keywords: Local solid concentration; Slurry flows; Simulation; CFD.
Thermal non-equilibrium simulation of diffuse and constricted anode attachments of a high intensity transferred arc
by Chong Niu, Jin-Yue Geng, Su-Rong Sun, Yan Shen, Tao Zhu, Hai-Xing Wang
Abstract: The arc attachment modes on anode of a high intensity argon arc with water-cooled constrictor are numerically investigated by a two-temperature model. An inelastic electron energy loss factor, representing the energy loss due to non-equilibrium chemical kinetic processes, is introduced to the electron energy equation. Two different attachment modes, i.e., diffuse and constricted anode attachments, are obtained based on the experimental conditions. Results for both anode attachment modes indicate that the temperature discrepancy between electrons and heavy particles is very pronounced in the arc fringes and in the regions close to the anode. By comparing the axial and radial Lorentz forces between the diffuse and constricted anode attachments, it is found that the larger axial and radial Lorentz forces in a constricted anode attachment are the main cause for the formation and maintenance of anode jet. The effects of inelastic electron energy loss on different anode attachment modes are discussed and the results indicate that it is very important to consider the chemical non-equilibrium processes in an appropriate way for predicting arc attachment modes on anode reasonably.
Keywords: Constricted anode attachment; diffuse anode attachment; inelastic electron energy.
Effect of the Injector Flow Field on the Performance of a Model Scramjet Combustor
by Sagarika Iyyer, Viswanathan Babu
Abstract: The effect of injector flow field on the jet-freestream interaction and the mixingrnperformance, is investigated. Numerical simulations of the supersonic injection of hydrogenrnthrough a supersonic, diamond-shaped, wall mounted injector as well as an equivalent circularrninjector, into a Mach 2.4 supersonic crossflow in a model scramjet combustor have been carriedrnout. Two equivalence ratios, namely, ?=0.3 and 0.5, which result in pure scram mode of operation,rnare considered. Oil flow images and Schlieren images overlaid with contours of hydrogen massrnfraction are used to demonstrate that the fuel penetrates higher with the diamond injector and thatrnthe fuel issues out of a part of the injector only on account of the jet being over-expanded. Thernhigher penetration leads to better near-field mixing but the mixing slows down in the far field. Therncircular injector exhibits more lateral spreading as a result of flow separation ahead of the injector.rnAlthough the penetration and the near-field mixing are less, mixing continues farther downstreamrnresulting in almost the same level as that of the diamond injector. Fuel plume outline, contours ofrnlocal equivalence ratio and local mixing efficiency are used to obtain insights into the spreadingrnand the mixing of the fuel.
Keywords: scramjet; supersonic combustion; simulations; injection; mixing.
Three Dimensional Gas-Solid Riser Flow Simulation
by Is Bunyamin Suryo, Tri Yogi Yuwono, Uwe Schnell
Abstract: In this paper, numerical simulations of three dimensional gas-solid riser flow using in-house computational fluid dynamics code are carried out and evaluated. This study employs the Two-Fluid Model approach using the kinetic theory of granular flow in numerical simulations. This research utilized Wen-Yu, Syamlal, et al., and Gidaspow drag models to calculate the momentum interaction between gas and solid phase. The restitution coefficient of 0.7 and two different time-steps are run to analyze the hydrodynamics flow in the riser. The radial profile comparison between numerical and experimental results is examined regarding axial solid volume fraction, gas, and solid velocities. Using the time step of 0.00015 s, numerical simulation using Syamlal et al. drag model obtains a good agreement with the experimental result.
Keywords: Gas-Solid Riser Flow; Computational Fluid Dynamics; Two-Fluid Model; Kinetic Theory of Granular Flow; Drag Model.
Analysis Of Three-Dimensional Cavitating Flow Over Different Cavitators Using Boundary Element Method
by Ramin Fadaei Roodi, Mahmoud Pasandidehfard
Abstract: The most essential component in the cavitation flow is the pressure distribution on the streamline. Hydrodynamic forces, which are essential in projectile design, could be achieved through the simulation of cavitation flow and demonstration of the pressure distribution. In present paper, the three-dimensional boundary element method (BEM) is employed to simulate the partial cavitation (? = 0.1 - 0.5) around projectiles with different cavitators. Based on the assumption of potential flow and using integral expression of Green\'s theory and an iterative algorithm, the three-dimensional flow around the projectiles is simulated. To each surface boundary mesh element dipole and source potential components have been specified. The assessment of the effect of quadratic element size on both cavitation cloud shape and pressure distribution is investigated for spherical and conical cavitators with different head angles. It has been observed that in Boundary Element Method, using small element size is not generally a reasonable procedure to achieve better results. However, comparing with the experimental data a relation for the element size based on the cavitator shape and cavitation number variables is proposed. Further, the BEM predictions obtained by these relations have been evaluated by comparing them with the numerical simulation results using ANSYS Fluent software. It is obtained that using BEM and employing specific quadratic elements proposed in this paper, satisfactory values for the cavity dimensions and pressure distributions can be found in just a few minutes.
Keywords: 3D boundary element method; BEM; conical; spherical; cavitator; partial cavitation; element size; pressure distribution; cavity length.
Optimization of Francis turbine draft tube using response surface model
by Ali Abbas, Arun Kumar
Abstract: The draft tube design of low and medium head hydraulic turbines plays an important role in determining the efficiency and power output. Proposed Francis draft tube geometry was optimized with two different optimization goals i.e. (a) rehabilitation and (b) a new design. Using design of experiment approach, 81 design points were generated for both cases separately and response surface methodology was used for optimization. Two objective functions the i.e. pressure recovery (Cp) and loss factor (?) were considered in optimization process. Multi objective genetic algorithm coupled with RSM was adapted to optimize the proposed draft tube geometry for achieving the desired optimization goal. For case 1, optimum values of Cp and ? value were found as 0.8290 and 0.1158 respectively. Value of pressure recovery was enhanced 7.0% at best efficiency point. For case 2, optimum value of Cp was found as out 0.851 which is 10.0% higher compared with performance of existing draft tube.
Keywords: Francis turbine; Draft tube; CFD; Response surface methodology; Design of experiment; Multi objective genetic algorithm;.
Value of blade number in centrifugal flow pumps in both turbine and pump mode through experimental and numerical means
by Yang Sun-Sheng, Fang Tian, Wang Tao, Punit Singh
Abstract: The hydraulic significance of blade number of pump impellers is experimentally and numerically investigated in turbine mode and pump mode respectively for three different specific speeds and validated using the classical theoretical approach of streamline shape (angular momentum of fluid) and loss gradient. The results indicate that for both pump and turbine mode operation, the effect of streamline shape is dominating over the change in hydraulic loss with increase in blade number. The frictional component of losses for blade passage using an existing empirical model is compared with computational results. While the flow line shapes markedly improved, yet the losses could not be reduced. The study finds an optimum number of blades different for turbine (more blades) and pump (fewer blades) operation. It recommends systematic adaption of increased blade number particularly for pump as turbine users.
Keywords: Centrifugal pump; pump as turbine; blade number; angular momentum; streamline; hydraulic losses; Euler velocity vectors.
CFD modeling of the flow in zero-secondary flow ejectors: a sensitivity analysis to numerical parameters
by Ala Bouhanguel, Valérie Lepiller, Philippe Desevaux
Abstract: This paper proposes a sensitivity study of various CFD simulation parameters conducted in the case of a 2D axisymmetric ejector operating without induced flow. The influence of numerical parameters (solver, discretization scheme, mesh and turbulence model) is investigated by comparing the various CFD results obtained (pressure, velocity, Mach number, mass flow rate) with each other. The influence of the turbulence model is closely examined by comparing CFD results with velocity measurements obtained by Particle Image Velocimetry.rnRules to be respected to achieve correct CFD simulation of the flow in supersonic ejectors are proposed. The use of the pressure-based coupled solver associated with the SST k-omega turbulence model is recommended. The terms of energy and pressure can be solved using 1st order discretization while the other equations (momentum, turbulent quantities) require 2nd order discretization to correctly predict the supersonic flow with shocks.rn
Keywords: Ejector; Supersonic flow; CFD; Turbulence model; Discretization scheme.
Two-phase modeling of the nanofluid mixed convection in a porous open cavity
by Hadi Shaker, Majid Abbasalizadeh, Shahram Khalilarya, Saber Yekani Motlagh
Abstract: The aim of the current work is to study the mixed convection of Fe3O4-water magnetic nanofluid in a heated porous open cavity. Buongiornos two-phase model is utilized to consider the Brownian and thermophoresis of nanoparticles in the carrier fluid. Using the Darcy-Brinkman and Boussinesq approximations, the governing equations are solved by the finite volume technique, numerically. Numerical computations are performed for various Richardson numbers (Ri=0.01, 0.1, 1 and 10), Reynolds numbers (Re=10, 100, 300 and 600), volume fraction of nanoparticle (?=0 and 0.06), Porosity (?=0.5, 0.8 and 1) and Darcy numbers (0.0002 Keywords: Mixed convection; magnetic nanofluid; Buongiorno’s two-phase model; Porous Open cavity; Darcy-Brinkman.