LES-CMC simulations of different auto-ignition regimes of hydrogen in a hot turbulent air Co-flow

Large-Eddy Simulation (LES) results in combination with first-order Conditional Moment Closure (CMC) are presented for a hydrogen jet, diluted with nitrogen, issued into a turbulent co-flowing hot air stream. The fuel mixes with the co-flow air, ignites and forms a lifted-like flame. Global trends in the experimental observations are in general well reproduced: the auto-ignition length decreases with increase in co-flow temperature and increases with increase in co-flow velocity. In the experiments, the co-flow temperature was varied, so that different auto-ignition regimes, including low Damkohler number situations, were obtained (no ignition, random spots, flashback and lifted flame). All regimes are recovered in the simulations. Auto-ignition is found to be the stabilizing mechanism. The impact of different detailed chemistry mechanisms on the auto-ignition predictions is discussed. With increasing air temperature, the differences between the mechanisms considered diminish. The evolution of temperature, H2O, H, HO2 and OH from inert to burning conditions is discussed in mixture fraction space

Experimental investigation on spark ignition of annular premixed combustors

The ignition behaviour of a multiple-burner annular combustion chamber consisting of 12 or 18 bluff-body premixed methane-air swirl burners was investigated experimentally. The study focusses on the mechanism of lightround, namely the burner-to-burner flame propagation. Side visualization of the spreading flame by 5 kHz OH* chemiluminescence showed that propagation from burner to burner did not follow a purely azimuthal direction, but a sawtooth pattern with downstream and sideways motion from one burner to the following, bringing flame to the downstream part of the recirculation zone of the adjacent burner before being convected upstream leading to full burner ignition. This pattern was more pronounced when the burners where fitted with swirlers. Top visualization and image processing were used to quantify the speed of lightround as a function of inter-burner spacing, bulk velocity, equivalence ratio, and swirling feature. It was found that flame spread from burner to burner following two balanced modes of propagation. These are turbulent flame propagation combined with volumetric expansion in the inter-burner region and convection within the next un-ignited burner. The results presented in this paper bring new insights into the ignition of realistic gas turbines.The authors acknowledge financial assistance from EPSRC and Rolls–Royce Group

Prediction of Global Extinction Conditions and Dynamics in Swirling Non-premixed Flames Using LES/CMC Modelling

The Large Eddy Simulation (LES)/three-dimensional Conditional Moment Closure (CMC) model with detailed chemistry is applied to predict the operating condition and dynamics of complete extinction (blow-off) in swirling non-premixed methane flames. Using model constants previously selected to provide relatively accurate predictions of the degree of local extinction in the piloted jet flames Sandia D−F, the error in the blow-off air velocity predicted by LES/3D-CMC in short, recirculating flames with strong swirl for a range of fuel flow rates is within 25 % of the experimental value, which is considered a new and promising result for combustion LES that has not been applied before for the prediction of the whole blow-off curve in complex geometries. The results also show that during the blow-off transient, the total heat release gradually decreases over a duration that agrees well with experiment. The evolution of localized extinction, reactive scalars and scalar dissipation rate is analyzed. It has been observed that a consistent symptom for flames approaching blow-off is the appearance of high-frequency and high-magnitude fluctuations of the conditionally filtered stoichiometric scalar dissipation rate, resulting in an increased fraction of local extinction over the stoichiometric mixture fraction iso-surfaces. It is also shown that the blow-off time changes with the different blow-off conditions.This work was financially supported by Engineering and Physical Sciences Research Council (EPSRC) and Rolls-Royce through a Dorothy Hodgkin Postgraduate Award. This work used the computational resources from ARCHER cluster of the UK National Supercomputing Service (http://www. archer.ac.uk) under the project of United Kingdom Consortium on Turbulent Reacting Flows (UKCTRF) and the Darwin Supercomputer of the University of Cambridge High Performance Computing Service (http:// www.hpc.cam.ac.uk/).This is the final version of the article. It first appeared from Springer via http://dx.doi.org/10.1007/s10494-015-9689-

Modelling of spray flames with Doubly Conditional Moment Closure

Simulations of a pilot-stabilised flame in a uniformly dispersed ethanol spray are performed using a Doubly Conditional Moment Closure (DCMC) model. The DCMC equation for spray combustion is derived, using the mixture fraction and the reaction progress variable as conditioning variables, including droplet evaporation and differential diffusion terms. A set of closure sub-models is suggested to allow for a first, preliminary application of the DCMC model to the test case presented here. In particular, the DCMC model is used to provide complete closure for the Favre-averaged spray terms in the mean and variance equations of the conditioning variables and the present test case is used to assess the importance of each term. Comparison with experimental data shows a promising overall agreement, while differences are related to modelling choices.M.P. Sitte gratefully acknowledges funding from the Gates Cambridge Trust. The simulations were performed using the ARCHER UK National Supercomputing Service (http://www.archer.ac.uk) with computational time given by the UK Consortium on Turbulent Reacting Flows

Large Eddy Simulation of a spray jet flame using Doubly Conditional Moment Closure

A spray jet flame is modelled using Large Eddy Simulation (LES) with Doubly Conditional Moment Closure (DCMC). Since turbulent spray flames may include multiple combustion modes, the DCMC model uses both mixture fraction and reaction progress variable as conditioning variables. Conditional spray terms were included in the DCMC model to consider the coupling between evaporation and the flame structure. In the case of spatial homogeneity and in the limit of negligible mixture fraction scalar dissipation rate (SDR), the DCMC equation is shown to reproduce the flame structure of freely propagating laminar flames. For the spray jet flame, a good agreement between the simulation results and the experiments is achieved, in terms of the spray statistics, as well as the instantaneous and mean flame shape. The simulation shows important differences in the flame structure between the turbulent inner and the quasilaminar outer flame branch. The doubly-conditional parametrisation appears to be advantageous for resolving small scale effects related to droplet evaporation. Analysis of the DCMC equation suggests that the behaviour of the flame at its anchoring point is strongly influenced by non-premixed burning modes. The solution appears to be weakly affected by terms of convective transport in the DCMC equation, but significant spatial variations and temporal fluctuations of the conditional reaction rate, around 10 % of the time-based mean, persist.EPSRC, Project number: EP/R029369/

Stabilisation of swirling dual-fuel flames

© 2018 Elsevier Inc. C 7 H 16 -CH 4 -air flames stabilised in a bluff body swirl burner have been examined with flame photographs, OH ∗ chemiluminescence, and simultaneous 5 kHz OH-PLIF and Mie scattering with a focus on local an global extinction characteristics. The aim of this study is to investigate flame structure when more than one fuel is present and provide both insight and data for dual-fuel modellers. Flame imaging shows that the presence of an additional fuel affects the stabilisation characteristics of one fuel, whether it be liquid or premixed gaseous. With the addition of more CH 4 in the oxidiser channel, dual-fuel flames with C 7 H 16 spray became more premixed in appearance, evidenced by flame photographs, mean OH ∗ chemiluminescence images, and instantaneous and mean OH-PLIF images. Addition of CH 4 to such systems also forces the flame to stabilise on the outside of the swirled channel, similar to premixed CH 4 -air flames far from blow-off. However, the flame branch in the region of the shear layer directly above the bluff body edge moves further from the base of the burner with the addition of CH 4 , suggesting that typical spray flame behaviour is lost even with a small addition of CH 4 to the system. This observation is supported by global extinction curves, which show that C 7 H 16 -CH 4 -air flames appear to behave more similarly to premixed flames than spray flames, but remain of fundamental interest due to their unique stabilisation behaviour and relative insensitivity to bulk velocity changes compared to spray-only flames at similar equivalence ratios

LES/CMC Modelling of a Gas Turbine Model Combustor with Quick Fuel Mixing

The first-order, single-conditioned sub-grid Conditional Moment Closure (CMC) model for Large Eddy Simulation (LES) is applied to simulate a globally lean swirling methane flame in a gas turbine model combustor that has been studied experimentally. The time-averaged velocity, mixture fraction, temperature, major species and OH mass fractions, and the heat release rate are predicted well for most locations. A transient lift-off and flashback of the flame root due to localized extinction near the burner exit is observed that is qualitatively consistent with the experimental measurements. The time-averaged temperature is over-predicted very close to the fuel injection point, while it is accurately reproduced downstream. Comparisons of instantaneous conditionally-filtered temperature in mixture fraction space shows that the LES/CMC reproduces the large scatter of the experimental data points in temperature‒mixture fraction plane that span the full range unburnt to fully burnt, but to a smaller extent, suggesting a minor under-prediction of local extinction or the inaccuracy of the present first-order, coarse-grid CMC formulation to capture locally premixed flame propagation behaviour. Periodic variation of the heat release rate is observed with a frequency close to that of the measured Precessing Vortex Core (PVC). In general, the current LES/CMC model captures most features of this high-mixing-rate nominally non-premixed swirl flame in a gas turbine model combustor in agreement with experimental measurements

Autoignition of n-decane Droplets in the Low-, Intermediate-, and High-temperature Regimes from a Mixture Fraction Viewpoint

Detailed numerical simulations of isolated n-decane droplets autoignition are presented for different values of the ambient pressure and temperature. The ignition modes considered included single-stage ignition, twostage ignition and cool-flame ignition. The analysis was conducted from a mixture fraction perspective. Two characteristic chemical time scales were identified for two-stage ignition: one for cool-flame ignition, and another for hot-flame ignition. The appearance and subsequent spatial propagation of a cool flame at lean compositions was found to play an important role in the ignition process, since it created the conditions for activating the hightemperature reactions pathway in regions with locally rich composition. Single-stage ignition was characterized by a single chemical time scale, corresponding to hot-flame ignition. Low-temperature reactions were negligible for this case, and spatial diffusion of heat and chemical species mainly affected the duration of the ignition transient, but not the location in mixture fraction space at which ignition first occurs. Finally, ignition of several cool flames of decreasing strength was observed in the cool-flame ignition case, which eventually lead to a plateau in the maximum gas-phase temperature. The first cool flame ignited in a region where the fuel / air mixture was locally lean, whereas ignition of the remaining cool flames occurred at rich mixture compositions.This is the author accepted manuscript. The final version is available from Springer via http://dx.doi.org/10.1007/s10494-016-9710-

Spark ignition of a turbulent shear-less fuel-air mixing layer

A planar methane-air mixing layer with equal velocity in the two streams has been used to examine the ignition probability and the non-premixed edge flame speed following spark ignition. The mixing layer has approximately homogeneous turbulent intensity and lengthscale. Mean local mixture fraction has also been measured for the whole flow field. The ignition and subsequent flame propagation were visualized with a high-speed camera and the flame's edges in the upstream, downstream and cross-stream directions have been identified. The average rate of flame evolution in these directions allowed an estimation of the average absolute flame speed. Ignition probability contour of the mixing layer takes a V-shape, which matches the shape of the lean and rich flammability limits with a little discrepancy in the rich side. By subtracting the uniform mean velocity resulted in estimates of the mean relative edge flame speed. This quantity was approximately 2.5SL, where SL is the laminar burning velocity of stoichiometric methane-air premixed flames. The results are consistent with DNS of turbulent edge flames.This work has been funded by the European Commission through project "TIMECOP-AE" (AST5-CT-2006-030828). Thanks to Mr. I.A. Bahena Ledezma for assistance with the experimental techniques. S.F. Ahmed wishes to thank Qatar University for the support.Scopu