Spark ignition of turbulent premixed and non-premixed opposed jet flames

The probabilistic nature of ignition of premixed and non-premixecl turbulent opposed-jet flames has been examined and the flame structures following ignition have been visualized directly and with OH-PLIF. It has been found that high bulk velocities decrease the ignition probability in all locations and for all flames. Ignition is sometimes possible even in locations where there is negligible probability of finding flammable mixture and is sometimes impossible in locations with high probability of flammable fluid. The edge flame propagation speed is also estimated.

Investigation of extinction in unsteady flames in turbulent combustion by 2D-LIF of OH radials and flamelet analysis

An experimental and theoretical investigation of premixed turbulent combustion in an engine simulator is presented. The distribution of hydroxyl radicals formed in the combustion of propane/air mixtures was visualized by 2D-LIF and used to monitor the progress of the combustion process. For stoichiometric mixtures, images showed a continuous wrinkled flame front, while in lean (λ=1.5) mixtures, local flame extinction was observed as discontinuities in the reaction zone. A bright active reaction zone was still observed in flame inlets and closed concave structures. The effects of self-absorption and of collisional quenching on the fluorescence signal are considered and appear to have only a minor net influence on the shape and width of the flame front. The images are evaluated and interpreted in terms of the Lewis number effect and the laminar flamelet model. Analysis was performed by determining the contour lines of the images (specifically, the ratios of average maximum to equilibrium OH concentration) and comparing with corresponding ratios from unstrained flame simulations. The results show that although the degree of turbulence is not high enough for straining effects to be important, flamelet curvature does play a significant role in the combustion of lean mixtures; this is manifested by a mean effective flame velocity that is less than the laminar burning velocity. © 1991 Combustion Institute.

Analysis of in-cylinder hydrocarbons in a multi-cylinder gasoline HCCI engine using gas chromatography

Gasoline Homogeneous Charge Compression Ignition (HCCI) combustion has been studied widely in the past decade. However, in HCCI engines using negative valve overlap (NVO), there is still uncertainty as to whether the effect of pilot injection during NVO on the start of combustion is primarily due to heat release of the pilot fuel during NVO or whether it is due to pilot fuel reformation. This paper presents data taken on a 4-cylinder gasoline direct injection, spark ignition/HCCI engine with a dual cam system, capable of recompressing residual gas. Engine in-cylinder samples are extracted at various points during the engine cycle through a high-speed sampling system and directly analysed with a gas chromatograph and flame ionisation detector. Engine parameter sweeps are performed for different pilot injection timings and quantities at a medium load point. Results show that for lean engine running conditions, earlier pilot injection timing leads to partial oxidation of the injected pilot fuel during NVO, while the fraction of light hydrocarbons remains constant for all parameter variations investigated. The same applies for a variation in pilot fuel amount. Thus there is evidence that in lean conditions, pilot injection-related NVO effects are dominated by heat release rather than fuel reformation. © 2009 SAE International.

Fluid mechanics of pulse pressure-gain combustors

The fluid dynamic operation of a valveless pulse combustor has been studied experimentally and numerically. Through phase-locked chemiluminescence and pressure measurements it is shown that mechanical energy is created periodically in the flame surface, with an efficiency of 1.6%. This mechanical energy leaves the pulse combustor through unsteady jets at the aerovalve inlet and the tailpipe exit stations. Two thermodynamically distinct flows are identified: a flow that is transported from inlet to exit and participates in combustion along the way, and a flow that is ingested and then ejected from the combustor without undergoing combustion. It is the latter of these two flows which has the greatest quantity of net work done on it. Copyright © 2008 by the American Institute of Aeronautics and Astronautics, Inc.

Model-based thermoacoustic oscillation control via ℋ ∞ loop-shaping and integral quadratic constraints

Lean premixed prevaporized (LPP) technology has been widely used in the new generation of gas turbines in which reduced emissions are a priority. However, such combustion systems are susceptible to the damage of self-excited oscillations. Feedback control provide a way of preventing such dynamic stabilities. A flame dynamics assumption is proposed for a recently developed unsteady heat release model, the robust design technique, ℋ ∞ loop-shaping, is applied for the controller design and the performance of the controller is confirmed by simulations of the closed-loop system. The Integral Quadratic Constraints(IQC) method is employed to prove the stability of the closed-loop system. ©2010 IEEE.

Scalar dissipation and mean reaction rates in premixed turbulent combustion

A general equation for a variance parameter, appearing as a crucial quantity in a simple algebraic expression for the mean chemical rate, is derived. This derivation is based on a flamelet approach to model a turbulent premixed flame, for high but finite values of the Damköhler number. Application of this equation to the case of a planar turbulent flame normal to the oncoming flow of reactants gives good agreement with DNS data corresponding to three different values of the Damköhler number and two values of the heat release parameter. © 2011.

Modeling evaporation effects in conditional moment closure for spray autoignition

Simulations of an n-heptane spray autoigniting under conditions relevant to a diesel engine are performed using two-dimensional, first-order conditional moment closure (CMC) with full treatment of spray terms in the mixture fraction variance and CMC equations. The conditional evaporation term in the CMC equations is closed assuming interphase exchange to occur at the droplet saturation mixture fraction values only. Modeling of the unclosed terms in themixture fraction variance equation is done accordingly. Comparison with experimental data for a range of ambient oxygen concentrations shows that the ignition delay is overpredicted. The trend of increasing ignition delay with decreasing oxygen concentration, however, is correctly captured. Good agreement is found between the computed and measured flame lift-off height for all conditions investigated. Analysis of source terms in the CMC temperature equation reveals that a convective-reactive balance sets in at the flame base, with spatial diffusion terms being important, but not as important as in lifted jet flames in cold air. Inclusion of droplet terms in the governing equations is found to affect the mixture fraction variance field in the region where evaporation is the strongest, and to slightly increase the ignition delay time due to the cooling associated with the evaporation. Both flame propagation and stabilization mechanisms, however, remain unaffected. © 2011 Taylor & Francis.

Effects of turbulent reynolds number on the displacement speed statistics in the thin reaction zones regime of turbulent premixed combustion

The effects of turbulent Reynolds number on the statistical behaviour of the displacement speed have been studied using three-dimensional Direct Numerical Simulation of statistically planar turbulent premixed flames. The probability of finding negative values of the displacement speed is found to increase with increasing turbulent Reynolds number when the Damkhler number is held constant. It has been shown that the statistical behaviour of the Surface Density Function, and its strain rate and curvature dependence, plays a key role in determining the response of the different components of displacement speed. Increasing the turbulent Reynolds number is shown to reduce the strength of the correlations between tangential strain rate and dilatation rate with curvature, although the qualitative nature of the correlations remains unaffected. The dependence of displacement speed on strain rate and curvature is found to weaken with increasing turbulent Reynolds number when either Damkhler or Karlovitz number is held constant, but the qualitative nature of the correlation remains unaltered. The implications of turbulent Reynolds number effects in the context of Flame Surface Density (FSD) modelling have also been addressed, with emphasis on the influence of displacement speed on the curvature and propagation terms in the FSD balance equation. © 2011 Nilanjan Chakraborty et al.

Statistics and modelling of turbulent kinetic energy transport in different regimes of premixed combustion

The statistical behaviour of turbulent kinetic energy transport in turbulent premixed flames is analysed using data from three-dimensional Direct Numerical Simulation (DNS) of freely propagating turbulent premixed flames under decaying turbulence. For flames within the corrugated flamelets regime, it is observed that turbulent kinetic energy is generated within the flame brush. By contrast, for flames within the thin reaction zones regime it has been found that the turbulent kinetic energy decays monotonically through the flame brush. Similar trends are observed also for the dissipation rate of turbulent kinetic energy. Within the corrugated flamelets regime, it is demonstrated that the effects of the mean pressure gradient and pressure dilatation within the flame are sufficient to overcome the effects of viscous dissipation and are responsible for the observed augmentation of turbulent kinetic energy in the flame brush. In the thin reaction zones regime, the effects of the mean pressure gradient and pressure dilatation terms are relatively much weaker than those of viscous dissipation, resulting in a monotonic decay of turbulent kinetic energy across the flame brush. The modelling of the various unclosed terms of the turbulent kinetic energy transport equation has been analysed in detail. The predictions of existing models are compared with corresponding quantities extracted from DNS data. Based on this a-priori DNS assessment, either appropriate models are identified or new models are proposed where necessary. It is shown that the turbulent flux of turbulent kinetic energy exhibits counter-gradient (gradient) transport wherever the turbulent scalar flux is counter-gradient (gradient) in nature. A new model has been proposed for the turbulent flux of turbulent kinetic energy, and is found to capture the qualitative and quantitative behaviour obtained from DNS data for both the corrugated flamelets and thin reaction zones regimes without the need to adjust any of the model constants. © 2010 Springer Science+Business Media B.V.

Conditional Moment Closure for Turbulent Premixed Flames

The conditional moment closure (CMC) method has been successfully applied to various non-premixed combustion systems in the past, but its application to premixed flames is not fully tested and validated. The main difficulty is associated with the modeling of conditional scalar dissipation rate of the conditioning scalar, the progress variable. A simple algebraic model for the conditional dissipation rate is validated using DNS results of a V-flame. This model along with the standard k- turbulence modeling is used in computations of stoichiometric pilot stabilized Bunsen flames using the RANS-CMC method. A first-order closure is used for the conditional mean reaction rate. The computed non reacting and reacting scalars are in reasonable agreement with the experimental measurements and are consistent with earlier computations using flamelets and transported PDF methods. Sensitivity to chemical kinetic mechanism is also assessed. The results suggest that the CMC may be applied across the regimes of premixed combustion.

On the stabilisation of turbulent lifted flames

This study explores the stabilisation mechanisms of turbulent lifted flames by examining the scalar dissipation rate (SDR) of both passive and reactive scalars and their cross dissipation (CDR) in the stabilisation region. DNS results of a laboratory scale hydrogen turbulent lifted flame has been used for this analysis. Various definitions of the flame leading edge (FLE) has been compared and differences are illustrated. Time and spatial averaged statistic of SDR and CDR were examined. It was found that the averaged SDR for the mixture fraction at FLE was well below the reference quenching value for stoichiometric mixture. The averaged SDR for the progress variable is in the same order of the unstrained premixed laminar flame value. It was observed that the averaged CDR changed from negative to weakly positive at FLE. The change in sign was explained by a change in the relative alignment of the gradients of mixture fraction and progress variable. It was thus evident that the CDR was a good marker for stabilisation region and an important quantity in stabilisation mechanism.

Application of scalar dissipation rate modelling to industrial burners in partially premixed regimes

The objective of this paper is to test various available turbulent burning velocity models on an experimental version of Siemens small scale combustor using the commercial CFD code. Failure of burning velocity model with different expressions for turbulent burning velocity is observed with an unphysical flame flashback into the swirler. Eddy Dissipation Model/Finite Rate Chemistry is found to over-predict mean temperature and species concentrations. Solving for reaction progress equation with its variance using scalar dissipation rate modelling produced reasonably good agreement with the available experimental data. Two different turbulence models Shear Stress Transport (SST) and Scale Adaptive Simulation (SAS) SST are tested and results from transient SST simulations are observed to be predicting well. SAS-SST is found to under-predict with temperature and species distribution.

Simulation of spherical turbulent premixed flames

Outwardly-propagating spherical turbulent premixed flames are studied using Reynolds-Averaged Navier-Stokes methodology. The reaction rate closure is based on recently developed strained flamelets, where the flamelets are parametrised using the scalar dissipation rate. It is shown that the leading edge speed for the spherical flame is higher than for the planar case, for a given turbulence and thermo-chemical conditions. In addition, it is shown that including the mean curvature effects in the reaction rate closure do not influence the spherical flame speeds when compared with a model that excludes these effects.