E.S. Richardson

Effects of equivalence ratio variation on lean, stratified methane–air laminar counterflow flames

The effects of equivalence ratio variations on flame structure and propagation have been studied computationally. Equivalence ratio stratification is a key technology for advanced low emission combustors. Laminar counterflow simulations of lean methane–air combustion have been presented which show the effect of strain variations on flames stabilized in an equivalence ratio gradient, and the response of flames propagating into a mixture with a time-varying equivalence ratio. ‘Back supported’ lean flames, whose products are closer to stoichiometry than their reactants, display increased propagation velocities and reduced thickness compared with flames where the reactants are richer than the products. The radical concentrations in the vicinity of the flame are modified by the effect of an equivalence ratio gradient on the temperature profile and thermal dissociation. Analysis of steady flames stabilized in an equivalence ratio gradient demonstrates that the radical flux through the flame, and the modified radical concentrations in the reaction zone, contribute to the modified propagation speed and thickness of stratified flames. The modified concentrations of radical species in stratified flames mean that, in general, the reaction rate is not accurately parametrized by progress variable and equivalence ratio alone. A definition of stratified flame propagation based upon the displacement speed of a mixture fraction dependent progress variable was seen to be suitable for stratified combustion. The response times of the reaction, diffusion, and cross-dissipation components which contribute to this displacement speed have been used to explain flame response to stratification and unsteady fluid dynamic strain.

NUMERICAL INVESTIGATION OF FORCED IGNITION IN LAMINAR COUNTERFLOW NON-PREMIXED METHANE-AIR FLAMES

Simulations of forced ignition of non-premixed laminar counterflow flames are used to study the effect of strain rate on ignition success. A one dimensional calculation is performed, using detailed methane chemical kinetics and treating the spark as an instantaneous heat release in an inert mixing layer. Ignition success depends on the mixture composition at the spark location, resulting in lean and rich ignitability limits for a given spark that can be different from the fuels static flammability limits. The difference is attributed to the finite spark width and the diffusion of heat from the spark to the flammable mixture. Ignition is prohibited by excessive strain rates, in some cases at levels well below the extinction value. In the case of successful ignition, the high temperature reached due to the spark energy causes local auto-ignition and subsequently two reaction zones propagate away from the spark to consume the premixed reactants in the mixing layer. In the case of unsuccessful ignition, despite the auto-ignition achieved in the sparked region, the strain rate is sufficiently high for the heat and radicals to diffuse without resulting in a flame.

Terascale direct numerical simulations of turbulent combustion-fundamental understanding towards predictive models.

Advances in high-performance computational capabilities enable scientific simulations with increasingly realistic physical representations. This situation is especially true of turbulent combustion involving multiscale interactions between turbulent flow, complex chemical reaction, and scalar transport. A fundamental understanding of combustion processes is crucial to the development and optimization of next-generation combustion technologies operating with alternative fuels, at higher pressures, and under less stable operating conditions, such as highly dilute, stratified mixtures. Direct numerical simulations (DNS) of turbulent combustion resolving all flow and chemical features in canonical configurations are used to improve fundamental understanding of complex flow processes and to provide a database for the development and validation of combustion models. A description of the DNS solver and its optimization for use in massively parallel simulations is presented. Recent DNS results from a series of three combustion configurations are presented: soot formation and transport in a nonpremixed ethylene jet flame, the effect of fuel stratification in methane Bunsen flames, and extinction and reignition processes in nonpremixed ethylene jet flames.

Resolution Requirements in Stochastic Field Simulation of Turbulent Premixed Flames

The spatial resolution requirements of the Stochastic Fields probability density function approach are investigated in the context of turbulent premixed combustion simulation. The Stochastic Fields approach is an attractive way to implement a transported Probability Density Function modelling framework into Large Eddy Simulations of turbulent combustion. In premixed combustion LES, the numerical grid should resolve flame-like structures that arise from solution of the Stochastic Fields equation. Through analysis of Stochastic Fields simulations of a freely-propagating planar turbulent premixed flame, it is shown that the flame-like structures in the Stochastic Fields simulations can be orders of magnitude narrower than the LES filter length scale. The under-resolution is worst for low Karlovitz number combustion, where the thickness of the Stochastic Fields flame structures is on the order of the laminar flame thickness. The effect of resolution on LES predictions is then assessed by performing LES of a laboratory Bunsen flame and comparing the effect of refining the grid spacing and filter length scale independently. The usual practice of setting the LES filter length scale equal to grid spacing leads to severe under-resolution and numerical thickening of the flame, and to substantial error in the turbulent flame speed. The numerical resolution required for accurate solution of the Stochastic Fields equations is prohibitive for many practical applications involving high-pressure premixed combustion. This motivates development of a Thickened Stochastic Fields approach (Picciani et al. Flow Turbul. Combust. X, YYY (2018) in order to ensure the numerical accuracy of Stochastic Fields simulations.

Conditional Moment Closure Modelling for Dual-Fuel Combustion Engines with Pilot-Assisted Compression Ignition

Dual-fuel combustion is an attractive approach for utilizing alternative fuels such as natural gas in compression-ignition internal combustion engines. In this approach, pilot injection of a more reactive fuel provides a source of ignition for the premixed natural gas/air. The overall performance combines the high efficiency of a compression-ignition engine with the relatively low emissions associated with natural gas. However the combustion phenomena occurring in dual-fuel engines present a challenge for existing turbulent combustion models because, following ignition, flame propagates through a partially-reacted and inhomogeneous mixture of the two fuels. The objective of this study is to test a new modelling approach that combines the ability of the Conditional Moment Closure (CMC) approach to describe autoignition of fuel sprays with the ability of the G-equation approach to describe the subsequent flame propagation. The effects of partially-ignited fuel on the flame propagation speed is taken into account by a new laminar flame speed model. This methodology can be used for the full range of fuel substitution from perfectly-premixed through to pure diesel operation. The hybrid modelling approach is used to simulate n-heptane pilot jet-ignited combustion of a premixed methane air charge in a rapid compression-expansion machine apparatus. The results show that the hybrid model adequately captures ignition and transition to premixed flame propagation, and the sensitivity of the predictions to the flame speed modelling and ignition criteria is explored.

Micromixing effects in air pollution modelling

Predicting the dispersion of reacting pollutants close to their source is a topic of importance in Air Quality Modelling. The conventional method of neglecting species concentration fluctuations is not valid for such small-scale problems. Various methods that incorporate segregation are reviewed here and their use for typical atmospheric dispersion problems is illustrated through numerical simulations of a simplified problem. By comparison with experimental data, it is found that micromixing can affect the evolution of the mean reaction rate and that the models presented here are more accurate than if segregation were not included. Further work should focus on the interfacing of these models with practical Air Quality calculations.

A Thickened Stochastic Fields Approach for Turbulent Combustion Simulation

The Stochastic Fields approach is an effective way to implement transported Probability Density Function modelling into Large Eddy Simulation of turbulent combustion. In premixed turbulent combustion however, thin flame-like structures arise in the solution of the Stochastic Fields equations that require grid spacing much finer than the filter scale used for the Large Eddy Simulation. The conventional approach of using grid spacing equal to the filter scale yields substantial numerical error, whereas using grid spacing much finer than the filter length scale is computationally-unaffordable for most industrially-relevant combustion systems. A Thickened Stochastic Fields approach is developed in this study in order to provide physically-accurate and numerically-converged solutions of the Stochastic Fields equations with reduced compute time. The Thickened Stochastic Fields formulation bridges between the conventional Stochastic Fields and conventional Thickened-Flame approaches depending on the numerical grid spacing utilised. One-dimensional Stochastic Fields simulations of freely-propagating turbulent premixed flames are used in order to obtain criteria for the thickening factor required, as a function of relevant physical and numerical parameters, and to obtain a model for an efficiency function that accounts for the loss of resolved flame surface area caused by applying the thickening transformation to the Stochastic Fields equations. The Thickened Stochastic Fields formulation is tested by performing LES of a laboratory premixed Bunsen flame. The results demonstrate that the Thickened Stochastic Fields method produces accurate predictions even when using a grid spacing equal to the filter scale. The present development therefore facilitates the accurate application of the Stochastic Fields approach to industrially-relevant combustion systems.

Exploitation of phase change materials for temperature control during the fast filling of hydrogen cylinders

This paper explores the use of phase change materials for the fast filling of hydrogen cylinders in order to limit the rise in the gas temperature by enhancing heat transfer from the gas. It is necessary to limit the temperature rise because the structural performance of the cylinder materials can be degraded at higher temperatures. Initially, two computational approaches for modelling the fast filling of hydrogen cylinders are presented and validated; the first is an axisymmetric computational fluid dynamics simulation and the second is a single-zone approach with one-dimensional conjugate heat transfer through the cylinder walls. The effect of incorporating paraffin wax-based phase change material within the cylinder structure has been investigated using the single-zone model. The predictions show that use of pure paraffin wax does not help to reduce the gas temperature due to its low thermal conductivity, however materials with improved thermal conductivity, for example mixtures of paraffin wax and graphite, can facilitate reduced fill times. The impact of phase change material is assessed in the case of a production hydrogen-powered passenger car. Without use of phase change material it is not possible to reduce the fill time below three minutes unless the gas supply is pre-cooled. While the fill time can be reduced by precooling the gas supply, the phase change material reduces the degree of pre-cooling required for a given fill time by 10-20 K, and reduces the minimum power consumption of the cooler by as much as 0.5% of the fuel’s calorific value.

LES Loss Prediction in an Axial Compressor Cascade at Off-Design Incidences With Free Stream Disturbances

It is well known that an axial compressor cascade will exhibit variation in loss coefficient, described as a loss bucket, when run over a sweep of incidences, and that higher levels of free stream turbulence are likely to suppress separation bubbles and cause earlier transition (see e.g. [23]). However, it remains difficult to achieve accurate quantitative prediction of these changes using numerical simulation, particularly at off-design conditions, without the added computational expense of using eddy-resolving techniques. The aim of the present study is to investigate profile losses in an axial compressor under such conditions using wall-resolved Large Eddy Simulation (LES) and RANS. The work extends on previous work by Leggett et al.[11] with the intention of furthering our understanding of loss prediction tools and improving our quantification of the physical processes involved in loss generation. The results show that while RANS predicts losses with good accuracy the breakdown of these losses are attributed to different processes, meaning that optimisation of a compressor cascade profile, based solely on RANS, may be hard to achieve.