Large eddy simulation of evaporating two-phase flows

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Large eddy simulation of evaporating two-phase flows

The objective of this study is to develop a CFD tool for performing reliable large eddy simulation (LES) of the compressible evaporating two-phase turbulent flow in a gas turbine combustor. The KIVA-3V code originally developed by Los Alamos National Laboratory is used as a baseline code. The KIVA-3V code has been modified to facilitate LES calculations. Both the temporal and spatial accuracies of the original KIVA-3V code have been improved to second order. A one-equation subgrid scale (SGS) turbulence model is implemented to describe the unresolved turbulent subgrid effect. To ensure that there are sufficient particle numbers to capture the dynamic droplet dispersion process, the ETAB breakup model coupled with a new hybrid droplet-particle algorithm is also implemented into the code. Furthermore, the effect of the subgrid scale (SGS) velocity on the droplet dispersion is included. The SGS velocity is computed from the subgrid turbulent kinetic energy predicted by the one-equation SGS turbulence model. A new collision model based on the concept of "particle cloud" is proposed and implemented in the code. The new model greatly reduces the grid-dependence of the original O'Rourke model in a Cartesian mesh. The gas solver of the new LES version of KIVA-3V code, which will be referred as KIVA-LES hereby) is validated against large eddy simulations of natural and forced plane impinging jets. Predictions were carried out for different inflow conditions, which include a natural plane impinging jet with a random perturbation on the inflow plane and a forced plane impinging jet with a Strouhal number of 0.36, locked both in phase and laterally in space. The first simulation was performed to quantitatively study the mean flow and turbulence statistics. The computed field variables and turbulence intensity of streamwise velocity agreed well with the experimental results. The second simulation was performed to study the vortex structures of a forced plane impinging jet. The predictions captured the typical vortex structures of this kind of flow, such as spanwise rollers, successive ribs, cross ribs and wall ribs were reproduced by the simulation, which were also previously detected by the experiment of Sakakibara et al. (103) with digital particle image velocimetry (DPIV) system, but to our -best knowledge never wholly reproduced by numerical simulations to date. Moreover, the study has also led to some new findings related to the formation and evolution of successive ribs, cross ribs and wall ribs. The new collision model is tested against analytical solutions of simplified realistic collision problems in a box volume. The grid-dependence of the model is also checked against some spray test cases. The new collision scheme is computationally more efficient than the frequently used O'Rourke's (87) scheme since it abandons a sampling procedure to compute the collision number. The new model delivers sufficient accuracy in calculating the collision numbers in cases with uniformly distributed droplets although O'Rourke's model seems to perform better for these scenarios. However, for the prediction of a real spray in Cartesian gird, the new model has delivered much improved results. The predictions of the new model do not show any grid-dependent artefacts. KIVA-LES with the Lagrangian spray models is used to predict non-evaporating and evaporating diesel fuel sprays. The computed results are compared with the experimental data by Hiroyasu and Kadota (55) and Naber and Siebers (81), as well as the predictions of the original KIVA-3V. The predictions are in good agreement with the data. The large scale vortical structures are reproduced by the LES simulations, which cause "branch-like" spray shape and influence the spray penetration depth. The predictions have also captured the differences between the dense and dilute regions of the sprays. The LES analysis of diesel sprays has also demonstrated that SGS velocity has significant influence on the predicted spray angles. Most importantly, grid-convergent results, which were difficult to obtain with the original KIVA-3V, have been obtained in the present study. Finally, the validated code is used to study evaporating two-phase spray flow in a coaxial gas turbine model combustor. The predictions were compared with some published experimental data. This is a first step towards a more comprehensive numerical analysis of practical industrial combustors where multiple inlets and more complex combustor geometry are encountered. Good agreement with the data is achieved. The predictions have captured the "ring-like" vortex just downstream the annulus and "worm-like" streamwise vortical structure further downstream. The axial droplet mass flux and Sauter mean radius (SMR) are well predicted. Overall the present study has demonstrated the capability of KIVA-LES with the newly developed collision model to provide reasonably accurate predictions of evaporating two-phase flows in coaxial gas turbine combustors

The effect of convective motion within liquid fuel on the mass burning rates of pool fires – a numerical study

To improve numerical simulation of liquid pool fires and remove the need for experimentally measured or empirically calculated mass burning rates as boundary conditions, a fully coupled three-dimensional (3-D) numerical formulation, which directly solves convective motion in the fuel region by incorporating inhomogeneous heat feedback, is formulated. The fire dynamics is modelled using the large eddy simulation (LES) approach. Incompressible laminar flow formation is applied to the liquid fuel region, assuming constant thermo-physical properties except for the density which follows the Boussinesq approximation. The numerical formulation of the two phases is solved using a fully coupled conjugate heat transfer approach at the pool surface. The coupled model is validated against published measurements for a thin-layer heptane pool fire and a deep methanol pool fire. The convective motion within the liquid phase is found to have important effects on the pool fire mass burning rate and its neglection would result in a fast rise and over-prediction of the mass burning rate

Computational analysis of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures

The present study aims to gain insight of the mechanisms and characteristics for pulsating and uniform flame spread over liquid fuel at subflash temperatures. A specific goal is to use the validated three-dimensional (3-D) numerical model to reveal fine details of the gas and liquid phase flows as well as the resulting flame characteristics, which are challenging to obtain experimentally. To facilitate the study, 3-D formulations have been developed to explicitly solve the transport equations in both phases. A compressible solver was formulated for flame propagation in the gas phase using a one-step chemical reaction expression and mixture-averaged diffusion coefficients for the gaseous species. An incompressible solver with temperature dependent thermo-physical properties was employed to describe the convective motions and heat transfer in the liquid fuel region. Validation has been conducted for both uniform and pulsating spreads over a narrow 1-propanol tray with varying fuel depths through comparing the predicted flame front evolution with published measurements. Further qualitative comparison has also been conducted for some predicted fine features of the gas and liquid phase flows and flame spreading characteristics with published experimental observations. For both the uniform and pulsating spread, the detailed flame structure including the main diffusion flame and a small stratified premixed flame at the front have been captured. Wherever relevant, the detailed predictions were also used to shed light on some discrepancies in previously reported features in different laboratory studies and numerical simulations. Finally, the detailed 3-D predictions were used to illustrate fine features of the subsurface convective flow and its relative position to the flame front, the relative magnitudes of the subsurface flow velocity and that of the spread rate as well as the role of the thermocapillary-driven subsurface flow in the flame spread mechanism

Turbulent combustion modeling using a flamelet generated manifold approach - a validation study in OpenFOAM

An OpenFOAM based turbulence combustion solver with flamelet generated manifolds (FGMs) is presented in this paper. A series of flamelets, representative for turbulent flames, are calculated first by a one-dimensional (1D) detailed chemistry solver with the consideration of both transport and stretch/curvature contributions. The flame structure is then parameterized as a function of multiple reaction control variables. A manifold, which collects the 1D flame properties, is built from the 1D flame solutions. The control variables of the mixture fraction and the progress variable are solved from the corresponding transport equations. During the calculation, the scalar variables, e.g., temperature and species concentration, are retrieved from the manifolds by interpolation. A transport equation for NO is solved to improve its prediction accuracy. To verify the ability to deal with the enthalpy loss effect, the temperature retrieved directly from the manifolds is compared with the temperature solved from a transport equation of absolute enthalpy. The resulting FGM-computational fluid dynamics (CFD) coupled code has three significant features, i.e., accurate NO prediction, the ability to treat the heat loss effect and the adoption at the turbulence level, and high quality prediction within practical industrial configurations. The proposed method is validated against the Sandia flame D, and good agreement with the experimental data is obtained

Numerical simulation of carbon dioxide dispersion from vertical vent release

Numerical simulation of carbon dioxide (CO2) dispersion from two vertical vent release tests, i.e. test 7 and test 11, recently commissioned by National Grid is studied. For Test 7, the vent pipe is 3 m high with 1 inch diameter while Test 11 is 3 m high and 2 inch in diameter. The flow is subjected to a transient wind condition where both the wind direction and magnitude are changing with time. The far field computations started from a source condition derived from curve fitting the near-field simulations conducted by Leeds University. For test 7 the output at one hundred and sixty eight-diameter (168-D) distance from the pipe exit was used whereas in test 11 the output at 255 vent exit diameters (255-D) downstream from the vent exit is used, at this point all the solid particles are vaporised and the mixture is in pure-gas phase. The CO2 release continues at a steady condition. For test 7 the maximum release velocity is about 80 m/s at 168-D distance and for case 11 the maximum release velocity is roughly 45 m/s at the vent centre line at 255-D distance from the vent exit

Dispersion of carbon dioxide from vertical vent and horizontal releases - a numerical study

Numerical simulations of far-field carbon dioxide dispersion were conducted for a vertical vent release and a horizontal release from a shock tube. These scenarios had also been studied experimentally at field scale commissioned by National Grid. This work and the experiments both form part of the National Grid dense phase CO2 pipeLine TRANSportation (COOLTRANS) research programme. All tests involved releases of dense phase CO2 into an atmospheric flow. The dispersing plumes were subjected to transient wind conditions where both the direction and magnitude of the wind fluctuated with time. As part of the COOLTRANS research programme, the far-field dispersion simulations started from source terms derived from the near-field simulations conducted by the University of Leeds and outflow simulations conducted by University College London. The numerical model used for the far-field simulations is based on OPENFOAM, which is an object-oriented open source computational fluid dynamics toolbox. A dedicated solver CO2FOAM has been developed within the framework of OPENFOAM for simulating dispersion from dense phase CO2 releases. This has included the implementation of the homogeneous equilibrium method for fully compressible two-phase flow, treatment of the transient atmospheric boundary conditions and the time-varying inlet boundary conditions. The experimental measurements were supplied to the authors after the predictions were completed and submitted to National Grid. Hence, the validation reported here is indeed “blind.” While further fine tuning of the model and validation is still underway, the relatively good agreement between the predictions and measurements in the present study has demonstrated the potential of CO2FOAM as an effective predictive tool for far-field CO2 dispersion in the context of pipeline transportation for carbon capture and storage

Extension of the eddy dissipation concept and smoke point soot model to the LES frame for fire simulations

The eddy dissipation concept (EDC) is extended to the large eddy simulation (LES) framework following the same logic of the turbulent energy cascade as originally proposed by Magnussen but taking into account the distinctive roles of the sub-grid scale turbulence. A series of structure levels are assumed to exist under the filter width “Δ” in the turbulent energy cascade which spans from the Kolmogorov to the integral scale. The total kinetic energy and its dissipation rate are expressed using the sub-grid scale (SGS) quantities. Assuming infinitely fast chemistry, the filtered reaction rate in the EDC is controlled by the turbulent mixing rate between the fine structures at Kolmogorov scales and the surrounding fluids. In order to extend the laminar smoke point soot model (SPSM) to LES, the partially stirred reactor (PaSR) concept is used to relate the filtered soot formation rate to the soot chemical time scale, which is assumed to be proportional to the laminar smoke point height (SPH) of the fuel. The turbulent mixing time scale for soot is computed as a geometric mean of the Kolmogorov and integral time scale. A new soot oxidation model is also developed by imitating the gas phase combustion within EDC. The newly extended EDC and SPSM are implemented in the open source FireFOAM solver and tested with two medium scale heptane and toluene pool fires with promising results

An efficient approach to achieve flame acceleration and transition to detonation

This paper presents a novel method to accelerate flame propagation and transition to detonation in a coiled channel. The objective is to bring to light the basic understanding of the phenomenon and to show its potential in the fields of highly efficient combustion or propulsion. It was found that the flame evolution in the coiled channel is significantly different from that in a straight channel. In the flame acceleration stage, the flame propagation velocity increases exponentially in the coiled channel while it increases linearly in the straight channel, primarily due to the existence of a strong velocity gradient in the transverse direction in the coiled channel. Deflagration to detonation transition (DDT) was only observed in the coiled channel under current settings, being triggered by a series of local explosions at the boundary layer. In general, the coiled channel can greatly accelerate the flame and shorten the distance of the DDT compared with the straight channel

Large eddy simulation of a medium-scale methanol pool fire using the extended eddy dissipation concept

The eddy dissipation concept (EDC) is extended to the large eddy simulation (LES) framework following the same logic of the turbulent energy cascade as originally proposed by Magnussen but taking into account the distinctive roles of the sub-grid scale turbulence. A series of structure levels are assumed to exist under the filter width “Δ” in the turbulent energy cascade which spans from the Kolmogorov to the integral scale. The total kinetic energy and its dissipation rate are expressed using the sub-grid scale (SGS) quantities. Assuming infinitely fast chemistry, the filtered reaction rate in the EDC is controlled by the turbulent mixing rate between the fine structures at Kolmogorov scales and the surrounding fluids. The newly extended EDC was implemented in the open source FireFOAM solver, and large eddy simulation of a 30.5 cm diameter methanol pool fire was performed using this solver. Reasonable agreement is achieved by comparing the predicted heat release rate, radiative fraction, velocity and its fluctuation, temperature and its fluctuation, turbulent heat flux, SGS and total dissipation rate, SGS and total kinetic energy, time scales, and length scales with the corresponding experimental data