1,159 research outputs found
Effects of Lewis number, density ratio and gravity on burning velocity and conditional statistics in stagnating turbulent premixed flames
No full textDNS is performed to analyse the effects of Lewis number (Le), density ratio and gravity in stagnating turbulent premixed flames. The results show good agreement with those of Lee and Huh (Combustion and Flame, Vol. 159, 2012, pp. 1576-1591) with respect to the turbulent burning velocity, S-T, in terms of turbulent diffusivity, flamelet thickness, mean curvature and displacement speed at the leading edge. In all four stagnating flames studied, a mean tangential strain rate resulting in a mean flamelet thickness smaller than the unstretched laminar flame thickness leads to an increase in S-T. A flame cusp of positive curvature involves a superadiabatic burned gas temperature due to diffusive-thermal instability for an Le less than unity. Wrinkling tends to be suppressed at a larger density ratio, not enhanced by hydrodynamic instability, in the stagnating flow configuration. Turbulence is produced, resulting in highly anisotropic turbulence with heavier unburned gas accelerating through a flame brush by Rayleigh-Taylor instability. Results are also provided on brush thickness, flame surface density and conditional velocities in burned and unburned gas and on flame surfaces to represent the internal brush structures for all four test flames.X1122sciescopu
EFFECTS OF THE KARLOVITZ NUMBER ON THE EVOLUTION OF THE FLAME SURFACE DENSITY IN TURBULENT PREMIXED FLAMES
No full textDirect numerical simulation (DNS) is conducted to investigate the effects of the Karlovitz number (Ka) on displacement speed and consequent evolution of flame surface density (FSD). Parametric study is performed for the Ka between 2 and 10 in the thin reaction zone regime with independent variation of laminar flame speed and turbulent intensity. Previous study showed the effect of the turbulent intensity without noticeable influence of the Ka lower than 2.4 [I. Han, K.Y. Huh, Combust. Flame 152 (2008) 194-205]. A higher Ka involves a lower displacement speed on the positive curvature side primarily due to the influence on the normal diffusion component. It leads to a negative curvature term to act as a sink for FSD throughout a flame brush. The maximum FSD increases with increasing turbulent intensity, while a higher Ka leads to an asymmetric profile of FSD due to suppressed production at the leading edge. A higher Ka decreases total flame area and turbulent burning velocity as well, while a limiting behavior is shown for low Da cases. (C) 2009 The Combustion Institute. Published by Elsevier Inc. All rights reserved.X1131sciescopu
Conditional moment closure modeling of turbulent nonpremixed combustion in diluted hot coflow
No full textThe conditional moment closure (CMC) model is applied to predict flame structures and NO formation in the moderate and intense low oxygen dilution combustion mode. The effects of oxygen concentration in a hot diluted oxidant stream are investigated in the experimental condition of Dally et al. [Proc. Combust. Inst. 29 (2002) 1147–1154]. The GRI 2.11 Mech is used for description of chemical reaction including NOx chemistry. The conditional scalar dissipation rate, which describes the effect of turbulent mixing on finite chemistry, is calculated by integrating the transport equation for probability density function (PDF). A new PDF is proposed to describe three stream mixing in terms of a single mixture fraction. The conditional mean predictions of temperature, and CO, OH, and NO mass fractions are in good agreement with measurements. The unconditional Favre mean predictions of CO and NO mass fractions are also in reasonable agreement. Upstream underprediction of OH and NO in the low oxygen concentration case may be attributed to uncertainty in low temperature reaction mechanism and mixing prediction. Differential diffusion effects are shown to be nonnegligible in the present flames. The CMC model is an attractive choice for simulation of MILD combustion in which conditional fluctuations of reactive scalars are small enough for first-order closure of conditional mean reaction rates to remain valid.Seung Hyun Kim, Kang Y. Huh, and Bassam Dallyhttp://www.elsevier.com/wps/find/journaldescription.cws_home/703522/description#descriptio
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
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