Spontaneous ignition of isolated n-heptane droplets at low, intermediate, and high ambient temperatures from a mixture-fraction perspective

Detailed numerical simulations of isolated n-heptane droplets autoignition have been conducted at pressures of 5 and 10 atm for several values of the initial ambient gas temperature. The ignition modes considered included low-, intermediate-, and high-temperature ignition. The analysis was conducted from a mixture-fraction perspective. For sufficiently low values of the ambient gas temperature, two-stage ignition was observed. Under these conditions, low-temperature reactions played an important role in the transition of the system to a fully burning state. As the initial value of the ambient gas temperature increased, the influence of the low-temperature reactions on the ignition process decreased and eventually became marginal for temperatures above 900 K. Comparisons against homogeneous reactor calculations showed that the ignition location in mixture fraction space could be reasonably predicted for the high-temperature case, whereas discrepancies occurred for the intermediate- and low-temperature ones due to the shift in the maximum reactivity of the system caused by the cool flame appearance. The dependence of the ignition process on the initial droplet diameter was also studied for different values of the initial ambient gas temperature. It was found that, for all cases investigated, a value of the droplet diameter existed for which ignition took the least time to occur. For smaller droplets, the ignition transient was longer and eventually a burning flame did no longer appear when the droplet was initially too small. For the low-temperature case, the minimum ignition delay time was determined by the competition between the quicker ignition of the cool flame and the longer second induction time resulting from the decrease in the droplet size; for the high-temperature case, it was the results of phenomena occurring early during the droplet lifetime

Forced ignition of turbulent spray flames

This paper reviews the current state of knowledge on the initiation of a flame in a spray through the action of a spark or through local deposition of heat, and the subsequent flame development, in uniform and non-uniform dispersions of droplets and in the presence of turbulent flow. These processes are of importance in various applications such as gas turbine ignition (relight) and safety related to flammable liquid mists. The review focuses on the initial kernel development, the evolution of a spherical or edge flame, and the ignition of the spray flame when viewed at the whole combustor scale. The factors that determine success or failure of the ignition process at the various phases of the overall burner ignition are discussed through experiments and Direct Numerical Simulations, while modelling efforts are also assessed. The fuel volatility, droplet size, overall fuel-to-air ratio, and the degree of pre-evaporation are the important factors that distinguish spray ignition from gaseous flame ignition, and the extra fluctuations introduced by the random droplet locations, and how this may affect modelling and flame evolution, are highlighted. The flame propagation mechanism in laminar and turbulent sprays is one of the key aspects determining overall ignition success. Suggestions for future research are discussed.European Commission, Engineering and Physical Sciences Research Council (Grant ID: EP/J021644/1), Rolls-Royce Grou

Proper orthogonal decomposition analysis of a turbulent swirling self-excited premixed flame

Thermoacoustic oscillations constitute a serious threat to the integrity of combustion systems. The goal of the present work is to determine the effect of the equivalence ratio (φ), inlet flow velocity (U), and burner geometry on the characteristics of the self-excited oscillations and to reveal the dominant mechanisms. It also focuses on the data post-processing aiming at extracting information about the dynamics that are not captured through classical ensemble-averaging, and hence the Proper Orthogonal Decomposition technique is used. Experiments were conducted with a fully-premixed air/methane flame stabilized on a conical bluff body. Self-excited acoustic instabilities were induced by extending the length of the combustion chamber downstream of the bluff body. The flame was visualised using OH* chemiluminescence and OH PLIF at 5 kHz. Proper Orthogonal Decomposition (POD) and Fast Fourier Transform analysis were conducted on the imaging data. A strong effect of the chamber length was found, which primarily drove the generation of acoustic oscillation and flame-vortex interaction. Significant differences in the flame roll-up were found when either the burner geometry or the equivalence ratio was altered. Changes were detected in the frequency of oscillations, which showed a general trend to increase with φ and U and decrease with the length of the duct. Analysis of the POD modes allowed an estimate of the convection speed of the flame structures associated with the dominant frequency and it was found that this convection speed was about 1.5 U for most conditions studied

Simulation of hydrogen auto-ignition in a turbulent co-flow of heated air with LES and CMC approach

Large-Eddy Simulations (LES) with the first order Conditional Moment Closure (CMC) approach of a nitrogen-diluted hydrogen jet, igniting in a turbulent co-flowing hot air stream, are discussed. A detailed mechanism (nine species, 19 reactions) is used to represent the chemistry. Our study covers the following aspects: CFD mesh resolution; CMC mesh resolution; inlet boundary conditions and conditional scalar dissipation rate modelling. The Amplitude Mapping Closure for the conditional scalar dissipation rate produces acceptable results. We also compare different options to calculate conditional quantities in CMC resolution. The trends in the experimental observations are in general well reproduced. The auto-ignition length decreases with an increase in co-flow temperature and increases with increase in co-flow velocity. The phenomena are not purely chemically controlled: the turbulence and mixing play also affect the location of auto-ignition. In order to explore the effect of turbulence, two options were applied: random noise and turbulence generator based on digital filter. It was found that stronger turbulence promotes ignition

Numerical simulations of hydrogen auto-ignition in a turbulent co-flow of heated air

Our research objective is the performance of Large-Eddy Simulation (LES) with the first order Conditional Moment Closure (CMC) of the test case experimentally studied by Markides and Mastorakos [1]. The experiment concerns auto-ignition of hydrogen, diluted with nitrogen, in a co-flow of heated air. A 19 step, nine species detailed mechanism is used for the reaction. Simulations reveal that the injected hydrogen mixes with co-flowing air and a diffusion flame is established. The configuration is sensitive to inlet boundary conditions, as all major turbulence effects are expected to be dominated by the inflow conditions. Preliminary LES results are presented. Stand-alone chemistry calculations are also presented to illustrate sensitivity on chemistry mechanisms

LOW-ORDER MODELING OF IGNITION IN ANNULAR COMBUSTORS

The SPINTHIR model, which is a Lagrangian stochastic low-order model for ignition validated and applied to several premixed and non-premixed cases, is modified in this paper to improve the numerical prediction of the flame light-round process in premixed annular combustors. This work proposes to take into account Flame Generated Turbulent Intensity (FGTI) and to impose the tubulent flame speed to the flame particles using expressions from the literature to address the current limitations in SPINTHIR. For this, using RANS CFD results as an input, the model was applied to simulate the ignition transient in a premixed, swirled bluff body stabilized annular combustor to characterize the light-round time, both in stable conditions and close to the stability limits. Several cases were analyzed, where flame speed and fuel are varied and light-round times are compared to experimental results. The proposed modifications increased the precision of the light-round time predictions, suggesting that FGTI may be an essential phenomenon to be modeled. The SPINTHIR model coupled with the Bray turbulent flame speed expression resulted in an average error of $15\%$, a maximum error of $26\%$ and minimum error of $1\%$ for the explored range of parameters. This is an attractive feature considering the low computational cost of these simulations, which take on average 75\,\si{min} per simulation in a single core of a local workstation.RC has been supported by funding from the European Union’s Horizon 2020 Research and Innovation Programme under the Marie Skłodowska-Curie Grant Agreement No. 765998, project ANNULIGHT. LCCM has been supported by funding from the Clean Sky 2 Joint Undertaking (JU) under project PROTEUS, Grant Agreement No 785349. The JU receives support from the European Union’s Horizon 2020 research and innovation programme and the Clean Sky 2 JU members other than the Union

LES/CMC Simulations of Swirl-Stabilised Ethanol Spray Flames Approaching Blow-Off.

Large Eddy Simulations (LES) with the Conditional Moment Closure (CMC) combustion model of swirling ethanol spray flames have been performed in conditions close to blow-off for which a wide database of experimental measurements is available for both flame and spray characterization. The solution of CMC equations exploits a three-dimensional unstructured code with a first order closure for chemical source terms. It is shown that LES/CMC is able to properly capture the flame structure at different conditions and agrees reasonably well with the measurements both in terms of mean flame shape and dynamic behaviour of the flame evaluated in terms of local extinctions and statistics of the lift-off height. Experimental measurements of the overall (liquid plus gaseous) mixture fraction, performed using the Laser-Induced Breakdown Spectroscopy technique, are also included allowing further assessment and validation of the numerical method. The sensitivity of the simulation results to the various boundary conditions is discussed.Rolls-Royce Group, Engineering and Physical Sciences Research Council (Grant ID: EP/J021644/1

MILD Combustion Limit Phenomena

This work contains an analysis of the existence of critical phenomena in MILD combustion systems through an exploration of classical results from high-energy asymptotics theory for extinction conditions of non-premixed flames and well-stirred reactors. Through the derivation of an expression linking burning rate to Damköhler number, the criteria for a folded S-Shaped Curve, representative of a combustion system with sudden extinction and ignition behavior, was derived. This theory is discussed in detail, with particular focus on the limitations of the global chemistry it presents. The conditions reported by various previously-published numerical and experimental investigations are then discussed in the context of this theory. Of these investigations, those with the highest level of preheat and dilution had monotonic rather than folded S-Shaped Curves, indicating a lack of sudden extinction phenomena. It suggests that MILD combustion systems are those which lack sudden ignition and extinction behavior, therefore exhibiting a smooth, stretched S-Shaped Curve rather than a folded one with inflection points. The results suggest that the delineation between folded versus monotonic S-Shaped Curves may provide a useful alternative definition of MILD combustion

Data-Driven Incompletely Stirred Reactor Network Modeling of an Aero-Engine Model Combustor

Accurate predictions of soot emissions from combustion systems are required to implement the design of low-emission aero-engine combustors that can mitigate the effects of particulate matter on human health and the environment. The use of detailed models of soot formation can be unfeasible in terms of computational costs for optimisation procedures involving a large number of numerical simulations of different combustor configurations. A reduced-order formulation for turbulence-chemistry interactions and kinetic post-processing of Computational Fluid Dynamics (CFD) simulations, i.e., the Incompletely Stirred Reactors Network (ISRN) method, has recently provided promising qualitative predictions of soot emissions while allowing the use of complex chemistry at minimal computational costs. However, loss of accuracy and uncertainty in the predictions of relevant quantities, e.g., temperature and pollutant emissions, should be accounted for when reduced-order models like the ISRN method are employed. Hence, the integration of the ISRN method with data-driven approaches included in the framework of Uncertainty Quantification (UQ) has been pursued and is presented in this work. The grid parameters of the ISRN were calibrated via a UQ approach so that the predictions of temperature within an aero-engine model combustor match those obtained by a detailed CFD simulation with accuracy higher than 90\%. The UQ approach results in the determination of a feasible set of grid parameters and information about the correlation between them. Then, the proposed methodology has been applied on soot emissions in the aero-engine combustor to obtain bounded predictions from the ISRN method that are of the same order of magnitude as the corresponding ones provided by the high-order Conditional Moment Closure (CMC) combustion model.European Union’s Horizon 2020 project CoEC, grant agreement No 95218

Direct Numerical Simulations of Dual-Fuel Non-Premixed Autoignition

Autoignition of turbulent methane/air mixing layers, in which n-heptane droplets have been added, was investigated by DNS. This configuration is relevant to dual-fuel, pilot-ignited natural gas engines under direct injection conditions. Two passive scalars were introduced in order to describe the dual fuel combustion. It was shown that the pre-ignition phase is dominated by n-heptane oxidation while methane oxidation is less intense. During the pre-ignition phase the methane/air mixing layer is distorted due to turbulence creating regions around the n-heptane droplets allowing the transport of intermediate species to the methane reaction zone. According to the passive scalars introduced, it was shown that ignition occurs at mixtures rich in n-heptane vapour. Subsequently, consumption of both n-heptane and methane is rapidly increased and promoted by the high temperatures achieved. The competition of the two fuels makes autoignition retarded relative to the pure n-heptane case, but accelerated relative to the pure methane case.This is the author accepted manuscript. The final version is available from Taylor & Francis via http://dx.doi.org/10.1080/00102202.2016.113939