Effects of differential diffusion on hydrogen flame kernel development under engine conditions

The role of differential diffusion during early flame kernel development under engine conditions – part II: Effect of flame structure and geometry

The role of differential diffusion during early flame kernel development under engine conditions - part I: Analysis of the heat-release-rate response

The understanding and prediction of the early development of flame kernels are of high practical importance for the robust relight of aviation gas turbines and the control of cycle-to-cycle variations (CCV) of spark-ignition engines. CCV are known to correlate strongly with early flame kernel development and complicate the optimization of such engines in terms of safety, thermal efficiency, and engine emissions. The flame kernel initiated by a spark is initially small, in the very early combustion phase typically smaller than the size of the turbulent integral length scales. Therefore, the development of the flame kernel is dominated by local, intermittent flow fluctuations and can vary under the same nominal conditions. In this study, the effects of turbulence on the early development of premixed iso-octane and hydrogen turbulent flame kernels under realistic engine conditions are investigated through direct numerical simulations. Multiple realizations were simulated under the same nominal conditions for both fuels. Significant variations in flame kernel interactions with turbulence can be identified among different realizations. The fuel consumption rate varies by a factor of two, which is remarkable considering that only statistical differences in the local flow field are present between different realizations. Effects of different flow features of the initial flow fields on the flame kernel development were analyzed. It was found that the flow motion on the scale of the ignition radius, specifically the fluid deformation, which is characterized by the invariants of the strain rate tensor, determines the global shape of the kernel, while the variations of the kernel growth rate are mostly driven by the variations of the smallest turbulent scales. In particular, turbulence influences the flame surface area growth mainly through the tangential strain rate at the flame surface, which is shown to result from the small-scale turbulent motion. Due to differential diffusion effects, hydrogen and iso-octane exhibit significantly different flame responses to curvature, which is comprehensively studied for both fuels. The findings in this study will guide the development of combustion models that are capable to capture variations of the early flame kernels based on the local turbulence dissipation rate.

Differential diffusion effects in counter-flow premixed hydrogen-enriched methane and propane flames

Large eddy simulations of non-reacting H2/CO2 jets mixing with air are performed and the calculations are compared with the experiments reported by Smith et al. [“Laser Raman scattering measurements of differential molecular diffusion in non-reacting turbulent jets of H2/CO2 mixing with air,” Phys. Fluids 7, 1455–1466 (1995)]. The influence of differential diffusion effects for Reynolds numbers Re = 1000–8000 is analyzed and a differential diffusion parameter, ξ, is defined on the basis of normalized H2 and CO2 concentrations in order to quantify the effects of differential diffusion with increasing Reynolds number. The analysis is made not only in physical space but also with scatter plots and histograms. The simulation results reveal that differential diffusion effects are significant at downstream locations (more than 15 nozzle diameters away from the inlet) only for the lower Reynolds numbers (Re = 1000–2000). However, differential diffusion effects are present for all Reynolds numbers examined close to the inlet (closer than 10 nozzle diameters). This is not only confirmed by the mean results of the differential diffusion parameter, ξ, but also by looking at the histograms of ξ. This is an important indication that differential diffusion can be important in turbulent reacting flows if laminarization of the flow or weakening of turbulent diffusion occurs. Including differential diffusion effects in turbulent reactive flows involving mixtures with vastly different mass diffusivities can, therefore, improve the accuracy of numerical simulations. Results obtained assuming equal species mass diffusivities revealed that differential diffusion effects do not have any significant influence in the velocity field.

Spark ignition of lean and dilute fuel-air mixtures provides emission reductions of NOx. Furthermore, operation at the lean-dilute limit increases engine efficiency through reduced pumping loses and reduced heat transfer. However, ignition near the lean flammability limit becomes more stochastic and exhibits substantially decreased flame propagation rates. In this work, spark ignition and the subsequent flame kernel development and propagation are studied in a constant volume optical combustion vessel. The vessel provides full field orthogonal and line-of-site optical access via sapphire windows. Additionally, an automated process controller with a versatile gas system enables the creation of a wide range of fuel-air mixtures, including lean and dilute mixtures of hydrocarbons, oxygen, nitrogen, carbon dioxide, and other gases. Ambient conditions including in-chamber temperature and pressure levels, along with dilution conditions, can be set independently. Ignition is provided by an automotive spark plug in the chamber. Optical diagnostics including simultaneous CH* chemiluminescence and shadowgraph imaging are utilized to characterize initial kernel growth and flame development under elevated pressure conditions, from atmospheric to 17.3 bar. Chemiluminescence images are quantified to determine flame intensity and kernel radius to understand the success of initial flame kernel development and propagation. Increasing the pressure yields a slower rate of flame kernel development and propagation, with a thickening flame front, which in turn increases the effects of buoyancy and heat loss. Leaning the mixture can yield unsuccessful kernel development due to heat loss to the large electrode which may cause a failed sustaining of combustion. This knowledge on kernel development near the lean limit benefits the engine community by characterizing the importance of ambient conditions including pressure and mixture properties in sustaining flame growth and propagation.

Flame structure analysis of turbulent premixed/stratified flames with H2 addition considering differential diffusion and stretch effects

Assessment of a methodology to include differential diffusion in numerical simulations of a turbulent flame

<div class="htmlview paragraph">The early flame kernel development period has a strong influence on the ultimate performance and emission characteristics of spark ignition engines. The fibre-optic instrumented spark plug, <span class="small-caps">FOSP</span>, is a tool used to characterise the early flame kernel development period without the need to modify production engines. Simultaneous in-cylinder pressure, fibre-optic spark plug and secondary ignition system voltage measurements have been made in the <span class="small-caps">GM</span> 2.8 L and the high swirl 3.1 L production engines modified to run on a single cylinder. The secondary ignition system voltages indicate that restrikes are occurring and that spark anemometry is a promising tool to extract information about the flow near the spark plug at the time of ignition. Further development of the technique is, however, required. In regard to <span class="small-caps">FOSP</span> data processing, it was found that i) the flame growth rate should be measured relative to the ignition point instead of the flame kernel centroid to account for the presence of restrikes, and ii) the <span class="small-caps">FOSP</span> measures a mass-weighted convection velocity. Normalisation of the measured convection velocity by the mean piston speed and unburned gas density at the time of ignition accounts for a large part of the convection velocity variations with engine speed and load; the so-normalised velocity was found to decay during the compression stroke. The initial flame growth rate was found to be strongly dependent on the convection velocity, and to increase during the compression stroke. Cyclic variations in the flame growth rate were found to correlate well with <span class="small-caps">LPP</span> and the 0-1% and 0-50% mass burn durations.</div>

<div class="htmlview paragraph">Many studies were done about the link between the flame kernel behavior and the global combustion stability. It was shown that aerodynamics and mixture preparation are predominant for flame propagation. Multidimensional optical diagnostic techniques have become an important tool to study combustion inside engines. One-point measurements of flow field have been done in order to look at the impact on global combustion analysis. But the effect of the instantaneous flow field at the vicinity of the spark plug and at the spark timing on the flame kernel was not well explored. It is the objective of this paper: to quantify as well as possible, the effects of the instantaneous local velocity field at the vicinity of the spark plug and just prior spark, on the flame kernel propagation. For that, in a classical transparent 4-valve Spark Ignition engine, fuelled with methane to provide homogeneous mixture, we firstly characterized mean flow fields, by Particle Imaging Velocimetry, for different conditions. Secondly, we get instantaneous flow field prior ignition and quasi-simultaneously, recorded chemiluminescence images. It was found that the flame kernel development is strongly related with the local aerodynamics flow-field.</div>

Experimental study on the intrinsic instabilities of spherically expanding CH4/H2/CO2/O2 flames

<div class="htmlview paragraph">The fiber optic spark plug was used in conjunction with a piezoelectric pressure transducer to collect combustion diagnostic data on four production engines designed to generate quiescent, swirl, and tumble charge motions. Spark advance was varied under low speed, low load conditions to investigate changes in flame kernel behavior and in-cylinder charge motion as functions of crank angle and spark advance. Two flame kernel models were filled to the data and a critical comparison of the models was conducted. Flame kernel behavior was represented by three values: convection velocity, growth rate, and convection direction. Convection velocity was highest in the swirl chambers. It also varied considerably among cylinders in the same engine. Growth rate correlated well with 0-2% burn but showed negligible correlations with later burn or IMEP. Convection direction proved useful in determining flow direction near the plug. Furthermore, changing flow characteristics, such as tumble breakup and turbulence generation, were evident from the fiber optic data as functions of crank angle degree, implying that the fiber optic spark plug may have increased diversity as a diagnostic tool.</div>

Effects of differential diffusion on turbulent burning velocity and flame surface area in NH3/H2/N2-air premixed flames

<div class="section abstract"><div class="htmlview paragraph">Advanced ignition systems with enhanced discharge current have been extensively investigated in research, since they are highly regarded as having the potential to overcome challenges that arise when spark-ignition engines are running under lean or EGR diluted conditions. Local flow field is also of particular importance to improve the ignitability of the air-fuel mixture in SI engines as the spark plasma channel can be stretched by the flow across the spark gap, leading to longer plasma length, thus more thermal spark energy distributed to the air-fuel mixture in the vicinity of the spark plug. Research results have shown that a constantly high discharge current is effective to maintain a stable spark plasma channel with less restrikes and longer plasma holding period. However, with the further increase in discharge current, plasma channel becomes thicker, and the stretched plasma length becomes shorter under a constant flow speed, which may suppress the advantages of the enhanced discharge current.</div><div class="htmlview paragraph">In this work, the interaction between discharge current level and plasma length under flow conditions is investigated. Whether a thick but shorter plasma channel or a thin but stretched farther plasma channel is more effective for the flame kernel formation is discussed to provide an insight into the influence of the plasma length on flame initiation. An optical combustion chamber platform along with a cross-flow generation system was used to study the plasma channel behavior; an in-house developed spark energy management module was employed to boost the discharge current level up to 3 A; in order to decouple the effect of discharge duration on flame initiation, the total discharge process was controlled within 0.8 ms.</div></div>

Direct numerical simulations (DNS) have been done for premixed laminar and turbulent NH3/H2/air flames propagating towards a cold wall. Two turbulent channel flows (with well-characterized turbulent boundary layers) at Reτ = 280 and 313 have been simulated for the turbulent flame cases. Near-wall low temperature chemistry has been considered. Effect of differential diffusion on the flame head-on quenching characteristics (quenching distance and maximum wall heat flux) has been checked in detail. Finally, two underlying mechanisms determining differential diffusion effects have been identified: (a) preferential diffusion of H radical increases the reaction rates of the near-wall low temperature (radical recombination) reactions, resulting in larger quenching distance and wall heat flux; (b) differential diffusion of fuels results in smaller flame heat release, thus, larger quenching distance and smaller wall heat flux. The effect of H radical preferential diffusion is suppressed in rich H2/air turbulent flames and strengthened in lean H2/air turbulent flames, due to the differential diffusion focusing/defocusing effects. The effect of differential diffusion of fuels is suppressed in turbulent flames, due to the enhanced mixing process, especially at low Damköhler numbers typical for NH3/H2/air flames interacting with highly turbulent flows.