Showing papers in "Progress in Computational Fluid Dynamics in 2011"

Parallel solution-adaptive method for two dimensional non-premixed combusting flows

Abstract: A parallel Adaptive Mesh Refinement (AMR) algorithm is proposed and applied to the predictions of both laminar and turbulent steady non-premixed compressible combusting flows. The parallel solution-adaptive algorithm solves the system of partial-differential equations governing two-dimensional axisymmetric laminar and turbulent compressible flows for reactive thermally perfect gaseous mixtures using a fully coupled finite-volume formulation on body-fitted multi-block quadrilateral mesh. Numerical results for co-flow laminar and turbulent diffusion flames are described and compared to available experimental data. The numerical results demonstrate the validity and potential of the parallel AMR approach for predicting both fine-scale features of laminar and complex turbulent non-premixed flames.

The Blasius and Sakiadis flow along a Riga-plate

Abstract: In a previous paper (Pantokratoras and Magyari, 2009), the Electro-Magnetohydro dynamic (EMHD) free convection flow of a weakly conducting fluid from an electromagnetic actuator was considered. In the present note, we study the cases where the Riga-plate either moves with constant velocity (Sakiadis flow) or the Riga-plate is situated in a constant free stream (Blasius flow). The results are obtained with the numerical simulation of the governing equations. Velocity profiles are presented, as well as values of the skin friction. Results are presented for both impermeable and permeable Riga-plate with uniform suction or uniform injection.

POD study on the mechanism of turbulent drag reduction and heat transfer reduction based on Direct Numerical Simulation

Abstract: Direct Numerical Simulations (DNSs) have been performed for turbulent channel flows of Newtonian fluid and drag-reducing fluid with heat transfer. Main coherent structures in the drag-reducing flow and heat transfer were extracted from the DNS database by Proper Orthogonal Decomposition (POD) method. The mechanisms of turbulent drag reduction and heat transfer reduction were studied through the analyses of the coherent structures and energy distribution reflected by different POD eigenmodes.

A numerical study of a torque converter with various methods for the accuracy improvement of performance prediction

Abstract: A comparative study was carried out on numerical methods for simulating a flow inside a torque converter. To investigate the effect of different methods for handling the relative motion of the parts, three methods were considered – the frozen rotor, sliding mesh and mixing plane methods. To improve the accuracy of performance prediction, the influence of viscosity variation with the temperature was studied by a thermo-fluid analysis. From parametric studies on the numerical scheme and the mesh resolution, it is observed that the results with the frozen rotor and sliding mesh methods agree well with the experimental data, whereas the mixing plane method induces a larger difference. The effect of viscosity variation on the accuracy of simulation is also investigated.

Heat transfer characteristics of parallel and counter flow micro-channel heat exchangers with varying wall resistance

Abstract: Thermal behaviours of parallel and counter flow micro-heat exchanger are investigated numerically, with emphasis on the role of wall resistance and thermal entry effects. The influence of different parameters, such as, thermal resistance (R), Knudsen number (Kn), effectiveness (e) and Number of Transfer Units (NTU) are investigated. It is found that the temperature jump at the walls increases with increasing R and Kn. On the other hand, the NTU increases with decreasing R and Kn. Furthermore, for counter flow, thermally fully developed flow may not exist over a large section of the heat exchanger due to the heat conduction in the separating wall and the entrance region.

Non-linear transient hydromagnetic partially ionised dissipative Couette flow in a non-Darcian porous medium channel with Hall, ionslip and Joule heating effects

Abstract: We study the transient magnetohydrodynamic (MHD) viscous laminar flow and heat transfer in a channel containing a Darcy-Forchheimer porous medium, under a constant pressure gradient with Hall current, ionslip, transpiration, viscous and Joule heating. The dimensionless momentum and heat conservation equations are solved using the Network Simulation Method (NSM). The effects of a number of thermophysical parameters on the transport phenomena are studied including Darcy number (Da), Forchheimer quadratic drag number (Fs), Hartmann number (Ha), Hall current parameter (βe), ionslip parameter (βi) and Eckert number (Ec). The model finds applications in geophysics and MHD energy generators.

Modelling the powder-binder separation in injection stage of PIM

Abstract: In this paper, a non-isothermal multiphase flow numerical model has been developed based on Eulerian approach to simulate the powder-binder separation phenomenon in the injection stage of Powder Injection Moulding (PIM) and the simulated results are validated with experiments. The model takes into account of the non Newtonian viscosity of the powder and binder phases. Simulation as well as experiments have been carried out for a standard tensile test specimen with gating and ejection systems. A temperature correction method has been introduced during solidification taking into account of the latent heat release of binder. It can be seen from the simulated results that injection temperature has larger effect on phase segregation than injection speed due to larger effect of temperature on viscosity.

A lattice Boltzmann model for natural convection with a large temperature difference

Abstract: A Variable Property Thermal Lattice BGK (VPTLBGK) model is proposed for the natural convection with a large temperature difference. The basic idea is replacing constant relaxation times by spatially varying ones so that the variable transport coefficients in LBE are realised. Numerical results show that the VPTLBGK model is effective in studying the variable property effects. It is found that a Rayleigh-number-dependent power-law relation is satisfied between the Nusselt number and the Rayleigh number for the natural convection with variable transport coefficients.

A study on the characteristics of liquid interface and condensation process of superheated vapour near a solid wall by Molecular Dynamics method

Abstract: A vapour-liquid-solid three-phase model is constructed to simulate liquid interface characteristics and condensation process near a solid wall by Molecular Dynamics (MD) simulation method. The three-phase model shows laminated liquid density distribution and oscillating distribution of tangential stress in the near-wall region. The density and temperature changing with time in the condensation process of superheated vapour argon on difform platinum surfaces are investigated. It is found that heat transfer enhancement of roughened surfaces is no longer obvious after the formation of a stable liquid film on them.

Unsteady MHD stagnation-point flow with heat and mass transfer for a three-dimensional porous body in the presence of heat generation/absorption and chemical reaction

Abstract: The problem of unsteady Magnetohydrodynamic (MHD) flow with heat and mass transfer near the stagnation point of a three-dimensional (3D) porous body in the presence of heat source/sink and chemical reaction effect has been studied numerically using an efficient iterative implicit finite-difference method. The numerical results are validated by favourable comparisons with previously published work. Three forms for the free stream velocity distributions namely a constantly accelerating flow, a periodic fluctuating flow and an exponentially decelerating flow are considered. Numerical results for the velocity components in the x-and y-directions, temperature distribution and concentration distribution as well as the skin-friction coefficients and the Nusselt and Sherwood numbers are presented graphically for various parametric conditions and discussed.

The effect of variable viscosity in double diffusion problem of MHD from a porous boundary with Internal Heat Generation

Abstract: The steady, laminar boundary layer, two-dimensional Magnetohydrodynamics (MHD) flow past a continuously moving (with constant velocity) semi-infinite vertical porous plate is studied taking into account the Dufour and Soret effects (Double diffusion) on variation of fluid viscosity with temperature. The effect of an exponential form of Internal Heat Generation (IHG) is also considered. The fluid viscosity is assumed to vary as a linear function of temperature. The governing fundamental equations of the problem are obtained by using the usual similarity technique. The local similarity solutions of the transformed dimensionless equations are solved numerically. The effects of the governing parameters within the boundary layer of the flow field are studied and the corresponding set of numerical results for the non-dimensional velocity, temperature and concentration profiles as well as the skin friction parameter, Nusselt number and Sherwood number are illustrated and displayed with the aid of graphs and tables to show typical trends of the solutions considering with and without IHG. It has been found that the results of the present study are completely different from similar problems in the absence of double diffusion and IHG.

A CFD-applicable discrete combustion model for thermally large particles

Abstract: The objective of this paper is to present a comprehensive new single-particle combustion model that can be used effectively in Computational Fluid Dynamics (CFD) applications. As a major difference from standard particle sub-models used in CFD software packages, the new model is suitable for simulating thermally large particles, which means that it considers the intra-particle temperature gradients and overlapping stages of drying, pyrolysis and char conversion. The model was validated by comparison with experiments and also with a more detailed model presented in our earlier work. Although the model was applied here for black liquor combustion, it is also suitable and can be used for other different applications, such as the combustion of biomass, coal or heavy fuel oil and metallurgical processes.

Detailed analysis of turbulent flows in air curtains

Abstract: In order to prevent entrainment, an air curtain should provide a jet with low turbulence level, and enough momentum to counteract pressure differences across the opening. Consequently, the analysis of the discharge plenum should be taken into consideration. Hence, the main object of this paper is to study the discharge chamber geometry and the presence of blades for ?ow orientation. This analysis is carried out in order to understand their in?uence on the jet produced. Studies presented are based on detailed numerical simulations and on experimental measurements. The in?uence of the turbulence model, boundary conditions and computational domain is investigated.

Comparison of heat-transfer reduction in drag-reduced turbulent channel flows with different fluid and thermal boundary conditions

Abstract: Direct numerical simulations of drag-reduced viscoelastic turbulent channel flow with heat transfer were carried out for four kinds of rheologically different fluids, that is, with different values of Weissenberg number and viscosity ratio. Two different thermal boundary conditions were considered. We present the budget of temperature variance and the relationship between the Heat-Transfer Reduction (HTR) and the Drag Reduction (DR) for each rheologically-different fluid. A case with a low viscosity ratio was found to give rise to high DR, with relatively low HTR compared with that obtained with a high Weissenberg number, suggesting dissimilarity between the heat and momentum transports.

Large Eddy Simulation of non-reacting gas flow in a 40 MW pulverised coal combustor

Abstract: The Large Eddy Simulation (LES) of non-reacting ?ow in a full-scale single coal burner test facility is presented. The LES was run with the in-house code PsiPhi of Imperial College, using immersed boundary conditions and the Smagorinsky model. The burner quarl and upstream furnace were discretised with 45 million uniform cubic cells. The LES reveals highly unsteady ?ow and identi?es major recirculation zones crucial for coal ?ame stabilisation. LES results show a good accordance with available Reynolds-Averaged Navier?Stokes (RANS) data from FLUENT. The cost of the LES is reasonable (2 weeks CPU time, 4 nodes) given the wealth of time-resolved data it provides.

The criteria of convergence in conjugated simulation of fluid flow and heat transfer in the channel formed by tube bank fins

Abstract: The criteria of convergence of many reported numerical studies of conjugate heat transfer in tube bank fin heat exchangers have not been clearly addressed. In this paper, the criteria are addressed clearly. The results show that when the relative error of any field variable or characteristic number is less than certain value is chosen as convergence criterion, the balance between the heat transferred from and to the fins cannot achieve. This implies that in numerical method using conjugated method, the physical requirement of the studied problem must be enforced in choosing the criteria of convergence.

Numerical study on performance and fin efficiency of wavy fin-and-tube heat exchangers

Abstract: Three-dimensional numerical studies were performed for the performance of wavy fin-and-tube heat exchangers in Body-Fitted Coordinates (BFC) system. Effects of geometric parameters on air-side heat transfer and fluid flow characteristics and fin efficiency were examined. The results showed that with the increase in Reynolds number, wavy angle, fin thickness and the decrease in fin pitch and transverse tube pitch, the heat transfer performance are enhanced; however, pressure drops are also increased. So, in practical applications, the wavy angle had better be located between 10° and 20° and the fin pitch should be located between 1.2 mm and 2.0 mm. The fin efficiency and average fin surface temperature decrease with the increase of Reynolds number, wavy angle, fin pitch and transverse tube pitch. With the increase of fin thickness, the fin efficiency and average temperature on fin surface also increase.

Numerical study of droplet impact and filling in a microgroove

Abstract: The droplet impact and filling behaviour in a microgroove, which can be used to produce a very narrow microline, is studied numerically by solving the conservation equations of mass and momentum. The droplet interface is tracked by a Level-Set (LS) method, which is extended to treat the no-slip and contact angle conditions at the immersed solid surface of a microgroove. The numerical results show that the droplet filling time can be significantly reduced through the combination of hydrophilic and hydrophobic microstructures. Further, the effects of impact velocity and groove geometry on the droplet motion in a microgroove are investigated.

Three-Dimensional numerical modelling of the Marmore waterfalls

Abstract: This paper describes the application of a three-dimensional Computational Fluid Dynamic (CFD) model to simulate the Marmore waterfall (Italy), which is part of the complex hydraulic system Nera Velino rivers, Piediluco lake. The domain reconstruction has been performed through a non-conventional photogrammetry topographic surveying. The simulations have been performed employing a CFD model that solves the Navier-Stokes equations with the Large Eddy Simulations turbulence closure scheme and the Volume of Fluid multiphase method to handle the free-surface flow. A good agreement is achieved between the CFD model and the real waterfall flow.

Combustion and heat transfer characteristics at the combustion chamber of an oxy-fuel boiler for CO2 capturing: a numerical simulation

Abstract: A 0.5 MW class-oxy-fuel boiler was developed to capture CO2 from the exhaust gas. Flue gas recirculation (FGR) was adopted to make it possible to retrofit existing facility by changing its burner. A co-axial burner was devised to support three combustion modes, which are conventional air combustion, oxy-fuel combustion and oxy-fuel combustion with FGR. Numerical simulations were conducted for the three combustion modes to study the detailed physics inside the combustion chamber of the boiler. The temperature field obtained from the simulation reproduces the flame structures in general, while the results under-predict the spanwise mixing for the air and FGR modes. The FGR combustion yields similar heat transfer characteristics with the conventional air combustion, which implies the possibility to have a CO2-free boiler by retrofitting the burner while keeping other parts of the boiler.

Computational study of free-stream turbulence effects on film cooling using two different models

Abstract: A computational and experimental investigation of three-dimensional mean flow field resulting due to the free stream turbulence on film cooling performance has been reported in this present paper. A row of square jet with L/D ratio of 2.5 as a reference case has been employed. The free stream turbulence intensity of 8% and the blowing ratio of 0.5 are investigated to check the impact of jet velocity on the cooling performance. The objective of the present work was to carry out a detailed investigation of the mean flow field and flow structure. Numerical simulations were performed using the commercial code FLUENT 6.2.16 based on the finite volume method with two different models, namely, k-e, RSM models. The results of the preliminary computational analysis show that the flow field produced by the free stream provides a serious impact on the film cooling performance.

Investigation of the temperature and species distributions in co-flow methane?oxygen laminar diffusion flames

Abstract: Investigation of the structure of co-flow methane?oxygen diffusion flame is presented. A numerical model with a 43-steps chemical kinetics mechanism and an optically thin radiation sub-model is employed in simulations. Through simple co-flow diffusion flame experimental arrangement, visible flame extents have been measured. The methane oxygen flames have four distinct visible regions; a dark inner zone, a reddish zone, a highly luminous zone and a blue or bluish-white zone. The variation of the extents of these zones with increasing oxygen flow rate is discussed. The effect of oxygen flow rate on the species and temperature distribution is studied in detail.

Study on the heat transfer of the plate-fin heat sink under impinging jet cooling

Abstract: This paper aims at investigating the heat transfer phenomena of the plate-fin heat sinks under impinging jet cooling. The impinging jet cooling has the advantage of larger heat removal. The finite element method is used to simulate the heat transfer phenomena of the plate-fin heat sinks under impinging jet cooling. The simulation results are verified by comparing with the experimental data. The parameters discussed in this study include the fin size, the Reynolds number and the impingement distance. The results of this study show that the effect of Reynolds number dominates to the heat removal. The heat removal increases with the Reynolds number obviously. In addition, the temperature profile on the fin surface is affected by the arrangement of the fin.

Numerical investigation on fluid flow and heat transfer characteristics in a peristaltic micropump

Abstract: A two-dimensional unsteady numerical simulation using Immersed Boundary method is carried out to investigate the fluid flow and heat transfer characteristics in a channel of peristaltic micro pump. The heat transfer target wall is a portion of the immovable top wall, while the rest of the wall including the movable bottom wall with fluctuating in the form of a sinusoidally progressive wave maintains the adiabatic condition. The obtained result shows a pair of large-scale recirculation in the case of relatively larger amplitude enhances the convection heat transfer and reduces the size of reverse flow region near the target wall.

Prediction of 3-D nano-mesh flows by a micro-flow LBM and its evaluation against MD simulations

Abstract: In this paper, a Lattice Boltzmann Method (LBM) is evaluated for three-dimensional (3-D) continuum to non-continuum transitional flow regimes, and its performance for flows in a nano-porous medium is discussed. The equations used in the LBM are modified Lattice Boltzmann Equations (LBEs) introducing the diffuse-scattering wall boundary condition and a Knudsen number dependent relaxation-time along with the regularisation procedure. The evaluation is carried out by comparing the results with those of the Molecular Dynamics (MD) simulation. The results of the flows at Knudsen numbers Kn = 0.1 and 0.2 show good agreement in both simulations and confirm the reliability of the LBM.

Forced convection analysis of coupled heat and mass transfer in a channel filled with a reactive porous medium

Abstract: Investigation of the fluids purification process, either by filtration or by undesirable substances destruction, is conducted. A convective flow in a reactive porous medium with an axisymmetrical geometry is considered. The momentum equation for the forced convection through a porous substrate is modelled by using the Darcy-Brinkman-Forchheimer model, while the temperature and the concentration fields are obtained subsequently from the energy and diffusion equations. The effects of the main governing parameters such as solid-to-fluid conductivity ratio, Reynolds number, Biot number, as well as the modified Frank-Kamenetskii number (FK)m are studied. The comparison with previously published work shows an excellent agreement.

Numerical simulation for turbulence-driven secondary motion over a streamwise external corner

Abstract: Numerical simulation, based on a Reynolds Stress Model (RSM), of turbulent flow over a streamwise external corner is presented. This work is an extension of an earlier experimental study (Moinuddin et al., 2004), which identified a pair of counter-rotating vortices placed around the corner of a 6 m long model. Experimental data measured at an early station is used as the inlet condition for the numerical simulation. Mean flow and turbulence statistics from numerical simulation are compared with the experimental data at a downstream station and they are found to be in excellent qualitative agreement. For the scaled mean flow data, quantitative agreement is also very good. Investigation reveals that vorticity production by secondary shear stress is dominant in generating secondary flow over an external corner, which is opposite for the case of an internal corner as found by Xu and Pollard (2001).

Dust collection simulation inside a wire-pipe type electrostatic precipitator system

Abstract: The collection mechanism inside the Electrostatic Precipitator System (EPS) is determined by the electric field, the fluid flow and the particle motion. In this study, the collection phenomena inside a wire-pipe type EPS were simulated by coupling the Electrohydrodynamics (EHD) simulation with the particle dynamics. The presented numerical method was verified by comparing to the analytical solution for an electric field and to the available numerical results for a dust collection of wire-plate type EPS. From the present EHD simulation, the collection mechanism of the wire-pipe type EPS was investigated for various operating conditions.

Numerical simulation of unsteady 3D air-water turbulent flow in a water cannon

Abstract: A numerical study for the 3D unsteady structure of air-water flow of a water cannon has been carried out. As the Reynolds number of fluid is high, coupling with Large Eddy Simulation (LES), the Volume of Fluid (VOF) and Continuum Surface tension Force (CSF) method, the high-speed turbulent flow has been computed. The influences of electrovalve's opening and cannon's inclination on internal dynamics of two-phase in barrel and atomisation of jet have been investigated in detail. The driving forces of air, jet core length, spray angle, and the velocity field are obtained from the computational results.

Open-source interface to Bird's DSMC code for complex interaction

Abstract: A plug-in-based interface to the 2D Direct Simulation Monte Carlo (DSMC) programme DS2 written by G.A. Bird is proposed. The interface enables the user to read and modify certain variables and data structures inside the simulation at runtime. With this tool, the user can apply the DSMC method to custom problems and rely on a mature DSMC code. Some of the most interesting data structures are the surface properties like adsorption coefficients, reaction probabilities and temperature and the properties of the inlet streams. All those quantities can now be altered at runtime within the DS2 programme with the proposed interface. The various features of the plug-in and the coupling between them are demonstrated using four exemplary geometries.