1,721,017 research outputs found
Numerical Simulation of Non-Reacting and Reacting Flows in a 5MW Commercial Gas Turbine Combustor
No full textThis study is concerned with numerical simulation of a simple swirl flame and a 5MW commercial gas turbine combustor both operating on methane/air. Validation is performed for turbulent flow and combustion models against some measurement data (http://public.ca.sandia.gov/TNE/swirlflames.html). Evaluation is performed for the standard k-e and the realizable k-e models in the nonreacting swirl flow and the EBU(eddy breakup) and the PPDF(presumed probability density function) models in the reacting flow of the 5 MW commercial combustor. Independent simulations arc carried out for the main and pilot nozzles to avoid flashback and to provide realistic inflow boundary conditions for the combustor. Important geometrical details such as air swirlers, vane passages and liner holes are taken into account. Different turbulence models result in similar flow patterns with varying sizes of the recirculation pockets in the central region and at the outside corner. The EBU and the PPDF models show similar downstream distributions of mean flow and temperature, while the EBU shows a lifted flame with a stronger effect of swirl due to limited increase of axial momentum by volume expansion near the nozzle.11Nsciescopu
Numerical study on spray and combustion characteristics of diesel and soy-based biodiesel in a CI engine
No full textThis study is to investigate spray, combustion and emission characteristics of mixtures of diesel and soy-based biodiesel in a CI engine. A surrogate mechanism is developed by combining the skeletal mechanisms of methyl butanoate and n-heptane. Thermophysical properties are validated against measured tip penetrations and Sauter mean diameters (SMDs) of nonreacting sprays. Biodiesel shows spray characteristics of larger SMD's, lower volatility and slower mixing with ambient air due to larger viscosity and surface tension. Engine simulation is performed by the CMC-ISR model to resolve coupling between chemistry and turbulence. There is no noticeable difference in ignition delays of diesel and biodiesel, while flame propagates slower with slower evaporation and mixing for biodiesel. Results are in good agreement for pressure traces, NOx and trends of variation of CO and PM in a range of operating conditions. NOx decreased for BD 60% with increasing biodiesel fraction, while PM and CO decreased due to the oxygen content in biodiesel in the test engine. Crown Copyright (C) 2013 Published by Elsevier Ltd. All rights reserved.X111314sciescopu
Validation of analytical expressions for turbulent burning velocity in stagnating and freely propagating turbulent premixed flames
No full textA general expression is derived for the turbulent burning velocity, S-T, from the continuum form of the E transport equation and shown to be valid in all turbulent premixed combustion regimes. It involves the inverse length scale, 1/L-w, for which new analytical relationships are proposed in the laminar flamelet and the distributed reaction regime. They are combined to give new predictive relationships for the ST in the two limiting regimes and extended to be applicable in the intermediate regime as well. They involve flamelet thickness, mean curvature, molecular and turbulent diffusivities at the leading edge without any tuning constants. The proposed relationships are shown to be consistent with measurements in literature at varying pressures, laminar flame speeds, turbulent intensities and mixture compositions. Convincing agreement is achieved for ST and 1/L-W, for different turbulence and laminar flame properties of stagnating compressible flames and in parametric study with respect to turbulent intensity, laminar flamelet thickness and integral length scale for freely propagating incompressible flames. There is no gravity and the Lewis number is assumed unity for simplification. (C) 2011 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X111213sciescopu
Zone conditional modeling of premixed turbulent flames at a high Damkohler number
No full textThe zone conditioning approach is applied to generic mass and momentum conservation equations in unburned and burned gas. Conditionally averaged equations include additional source terms with flame surface density to reflect interfacial transports between two zones. Zone conditional averaging does not involve countergradient diffusion, which is due to description in terms of mixture properties in Favre averaging. Asymptotic analysis shows that flame propagation characteristics are determined by gradient diffusion by turbulence and an externally imposed pressure gradient at the leading edge. Two-dimensional CFD simulations are performed for two typical premixed turbulent flames stabilized in an impinging jet. With adjusted constants to match the (c) over bar profiles, conditional axial velocities show good agreement with experimental data for the two test flames. The zone conditional model provides an intuitive set of governing equations suitable for a solution procedure of premixed combustion devices at a high Damkohler number. (C) 2004 Published by Elsevier Inc. on behalf of The Combustion Institute.X1118sciescopu
Statistically Steady Incompressible DNS to Validate a New Correlation for Turbulent Burning Velocity in Turbulent Premixed Combustion
No full textIncompressible 3-D DNS is performed in non-decaying turbulence with single step chemistry to validate a new analytical expression for turbulent burning velocity. The proposed expression is given as a sum of laminar and turbulent contributions, the latter of which is given as a product of turbulent diffusivity in unburned gas and inverse scale of wrinkling at the leading edge. The bending behavior of U (T) at higher u' was successfully reproduced by the proposed expression. It is due to decrease in the inverse scale of wrinkling at the leading edge, which is related with an asymmetric profile of FSD with increasing u'. Good agreement is achieved between the analytical expression and the turbulent burning velocities from DNS throughout the wrinkled, corrugated and thin reaction zone regimes. Results show consistent behavior with most experimental correlations in literature including those by Bradley et al. (Philos Trans R Soc Lond A 338:359-387, 1992), Peters (J Fluid Mech 384:107-132, 1999) and Lipatnikov et al. (Progr Energ Combust Sci 28:1-74, 2002).X11610sciescopu
Lagrangian conditional statistics of turbulent n-heptane spray combustion in different injection conditions
No full text3D DNS is performed for n-heptane sprays that go through ignition and combustion in different injection conditions. Conditional statistics are obtained for multiple Lagrangian groups of sequentially evaporating fuel in the group and the collective combustion regime. Ignition occurs close to but leaner than the most reactive mixture fraction for fuel of the longest residence time and low scalar dissipation rates. Combustion propagates with strong interaction among neighboring groups after ignition of the preceding flame group in the given conditions. Multiple flame groups are required for accurate description of combustion of a spray over long injection duration or multiply injected sprays. Reasonable agreement is shown between DNS and model predictions of conditional evaporation and scalar dissipation rates in the range of meaningful probabilities. Budgets are checked for all component terms in the balance equation of conditional sensible enthalpy with evaporation source terms. (C) 2012 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X1145sciescopu
Assessment of the finite-volume method and the discrete ordinate method for radiative heat transfer in a three-dimensional rectangular enclosure
No full textThe finite-volume method (FVM) and the discrete ordinate method (DOM) are implemented to assess their capability to predict radiative heat transfer in a three-dimensional enclosure. A varying optical thickness and a nonuniform temperature profile are assumed to reproduce a typical furnace. Results show that the FVM performs better than the DOM in optically thin media, while they show comparable accuracy in optically thick media. The lower-order FVM and DOM may lead to erroneous results due to the my effect in optically thin media with a highly nonuniform temperature profile. The 8 X 12 FVM and the S-8 DOM have shown good accuracy in all the test cases in this article.X1127sciescopu
DNS analysis of propagation speed and conditional statistics of turbulent premixed flame in a planar impinging jet
No full textDNS is performed for turbulent premixed flames stabilized in planar impinging jets. Test cases are defined to allow independent evaluation of the effects of turbulent intensity and mean strain rate. Conventional NSCBCs are extended to allow multidirectional flow and compatibility of different types of boundaries. The turbulent displacement speeds are well predicted by the asymptotic relationship in terms of turbulent diffusivity and 1/L-w at the leading edge. Higher speeds of stagnating flames are due to higher 1/L-w's of the stretched flame brushes than those of freely propagating ones. Gradient diffusion may occur with a negative slip velocity throughout the flame region for a high turbulent intensity and a high mean strain rate. Turbulent intensities are anisotropic with the axial component increasing by production in unburned gas, while there is not much production of turbulence in burned gas near the wall. (C) 2010 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X1157sciescopu
Conditional statistics of nonreacting and reacting sprays in turbulent flows by direct numerical simulation
No full textConditional statistics concerning evaporation and combustion of a spray are investigated in homogeneous, isotropic, and decaying two-dimensional (2D) turbulence. Randomly distributed, polydisperse droplets of n-heptane go through single-step combustion chemistry. Attention is focused on parametric effects of initial Sauter mean radius (SMR), turbulence level and droplet velocity in both reacting and non-reacting cases. A simple linear model for the conditional evaporation rate is proposed and validated against DNS data. A conventional P-probability density function (pdf) is shown to be valid with no peak occurring on the fuel side. The amplitude mapping closure (AMC) model works well for the conditional scalar dissipation rate with evaporating and reacting sprays. Parametric study shows that initial SMR and droplet velocity are major factors affecting conditional flame structures, whereas the effect of reaction is not significant except during autoignition. (C) 2006 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X1129sciescopu
Assessment of closure schemes in second-order conditional moment closure against DNS with extinction and ignition
No full textThe performance of second-order conditional moment closure (CMC) depends on models to evaluate conditional variances and covariances of temperature and species mass fractions. In this paper the closure schemes based on the steady laminar flamelet model (SLFM) are validated against direct numerical simulation (DNS) involving extinction and ignition. Scaling is performed to reproduce proper absolute magnitudes, irrespective of the origin of mismatch between local flamelet structures and scalar dissipation rates. DNS based on the pseudospectral method is carried out to study hydrogen-air combustion with a detailed kinetic mechanism, in homogeneous, isotropic, and decaying turbulent media. Lewis numbers are set equal to unity to avoid complication of differential diffusion. The SLFM-based closures for correlations among fluctuations of reaction rate, scalar dissipation rate, and species mass fractions show good comparison with DNS. The variance parameter in lognormal PDF and the constants in the dissipation term have been estimated from DNS results. Comparison is made for the resulting conditional profiles from DNS, first-order CMC, and second-order CMC with correction to the most critical reaction step according to sensitivity analysis. Overall good agreement ensures validity of the SLFM-based closures for modeling conditional variances and covariances in second-order CMC. (c) 2005 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X1110sciescopu
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