• Energy Research

The need for an accurate description of the fuel composition is emerging in recently published studies on the interaction of droplets and flames. In this context, hydrophilic fuels are of particular importance, since they are expected to interact with the water formed during combustion reactions. The impact of such interactions onto the reaction process is still not explored enough. This work aims to contribute to the understanding of such interactions by investigating the impact of accurately describing the heat and mass transfers on hydrophilic fuel droplets interacting with flames. For that, a recently proposed phase change model is employed, demonstrated to be one of the few capable of characterizing the differential diffusion of vapor into the gas phase for hydrophilic fuels interacting with reacting flows. Numerical simulations of freely propagating flames in quiescent droplet mists are conducted with a detailed chemistry description. Different scenarios focus on the impact of water addition in the gaseous or liquid phase. Results demonstrated the multi-component phase change significantly impacts the flame speed in humid air and/or with hydrous ethanol, and neglecting this effect leads to strong miscalculation in flame speed. This is shown to be a consequence of the hydrophilic property of the chosen fuel, which allows the conversion from single-component to multi-component droplets, thus modifying the flame structure.

Measurement of the burning velocity of unstretched laminar hydrogen/air premixed flames suffers from large uncertainties owing to the highly diffusive nature of hydrogen that can give rise to flame instability. This paper reports on a numerical study of the structures and stability of laminar premixed CH4/O2/CO2 flames and H2/O2/N2 flames anchored to a heat-flux burner using a high-order numerical method with detailed chemical kinetic mechanisms and detailed transport properties. The aim is to elucidate the effect of the flow and temperature inhomogeneity generated by the burner plate holes on flame structures and burning velocity. Heat transfer flux between the burner plate and the surrounding gaseous mixture is investigated under various standoff distances and burner plate temperatures. The burning velocity and the detailed flow, temperature and species distributions in flames with a zero net heat flux between the flames and the burner plate are analyzed. It is found that for the methane flames, when the standoff distance is sufficiently small, the burner can essentially suppress the intrinsic flame instability, but the plate holes can give rise to flame wrinkles of the size of the holes. At high standoff distances, the non-uniformity of the flow from the burner plate holes has a minor effect on the flame surface wrinkling; however, large-scale cellular structures can appear on the flame surface due to intrinsic flame instability. For the studied methane flames the effect of non-uniformity of the flow from the burner plate holes on the burning velocity is fairly small. For the studied hydrogen flames the burner plate could not totally suppress the intrinsic flame instability. The intrinsic flame instability can give rise to a significant increase in the flame surface area and mean burning velocity, with more than 25% increase in the burning velocity.

In this work, an experimental study investigating dielectric barrier discharge (DBD) plasma-assisted ignition in methane-air flows at near-atmospheric pressure is presented. Discharges are produced using 10 ns duration high voltage pulses at maximum 3 kHz pulse repetition rate. The goal is to check the feasibility of plasma as an ignition source for ignition-stabilized combustion in a wide range of equivalence ratio, pressure, and flow speed conditions. To that end, four important characteristics are investigated: 1) Ignition dynamics, 2) Minimum number of pulses required for ignition, 3) Plasma energy per pulse, gas temperature, and the effective reduced electric field, and 4) Plasma NOx production. In continuous plasma mode, we observe an elongated flame outside of the discharge region when a methane-air mixture flows through the discharge. The elongated flame is due to repetitively ignited kernels moving downstream with the flow. High-speed intensified imaging is used to explore plasma morphology and ignition dynamics. Plasma has a filamentary nature which is perceived as moving with the flow due to pulse-to-pulse memory effects. For fuel-air mixtures, ignition is followed by kernel expansion and splitting, flame propagation, and consecutive ignition. The time delay between consecutive ignition events is found to be a function of flow speed, because ignition is achieved only when a previously ignited kernel moves out of the plasma region, which happens faster at higher flow speeds. We performed optical emission spectroscopy in burst mode to further characterise plasma and ignition parameters. In air, plasma gas temperature and hence the reduced electric field increases because of pulse-to-pulse effects. By comparing the air plasma temperature with the methane auto-ignition temperature, we conclude that the observed ignition is low-temperature ignition. Parametric studies of NOx measurements suggest that plasma NOx emissions are higher for low flow speeds, high pulse repetition rates, and low-pressure conditions. Overall, our DBD filamentary plasma is found to be a fast, repetitive, and low-temperature ignition source that can be used for ignition-stabilized combustion at near atmospheric conditions.

This study addresses the scalar structure of turbulent oxy-fuel syngas non-premixed jet flame in the context of direct numerical simulation (DNS) and detailed chemistry. The main objective is to identify the influence of the Reynolds number on oxy-fuel flame structure, and to clarify the differences of scalar structures between oxy-fuel syngas and syngas-air flames at a similar higher Reynolds number. Two oxy-syngas flames at Reynolds numbers of 3000 and 6000 and one syngas-air flame at Reynolds number of 6000 were simulated. These studies show that the existence of CO2 in the oxidant has a profound effect on scalar structure of oxy-fuel syngas flame compared to syngas-air flame.

In this research work, we report on the numerical predictions and analysis of stable, stationary and closed burner-stabilized reacting fronts under terrestrial-gravity conditions for ultra-lean hydrogen–methane–air premixed mixtures with a 40% hydrogen (H2) and 60% methane (CH4) fuel composition, specified on a molar basis. The transition from a cap-like to ball-like flame shape with decreasing inlet equivalence ratio is predicted in agreement with experimental observations. The predicted flames are compared to both flames that were studied in experiments and numerical solutions of perfectly-spherical flame balls in the absence of gravity and convection. The comparison includes flame size, lean limits, and when pertinent, standoff distances, all for two different reaction mechanisms. The absolute molar consumption rates of both H2 and CH4 for the limit flame attain maximum values that are significantly larger than those of the corresponding gravity-free flame ball. The fuel supply mechanism of the normal-gravity limit flame is similar to the fuel supply of flame balls in that it is driven by diffusion even away from the flame front. Heat conduction to the tube wall of the burner and convective heat loss are the dominant forms of heat loss. Furthermore, simulations with inclusion of multicomponent transport and Soret and Dufour effects show that the flame size increases for both flame balls and the burner-stabilized flames. For the latter, a slight modification in the stabilization position is found owing to the intensification of the consumption rates of both H2 and CH4 when these effects are accounted for. In summary, the present work considers a new configuration that allows the study of stable and stationary ball-like flames at ultra-lean and near-limit conditions, and advances the understanding of such flames via detailed numerical computations.

The use of biomass-derived ethanol in spark-ignitionengines is an interesting option to decarbonize transport and increase energy security. An engine cycle code valid for this fuel, could help to explore its full potential. Crucial building blocks to model the combustion in ethanolengines are the laminarburningvelocity and flamethickness of the ethanol–air–residuals mixture at instantaneous cylinder pressure and temperature. This information is often implemented in engine codes using correlations. A literature survey showed that the few available flamethicknesscorrelations have not yet been validated for ethanol. Also, none of the existing ethanollaminarburningvelocitycorrelations covers the entire temperature, pressure and mixture composition range as encountered in spark-ignitionengines. Moreover, most of these correlations are based on measurements that are compromised by the effects of flame stretch and the occurrence of flame instabilities. For this reason, we started working on new correlations based on flame simulations using a one-dimensional chemical kinetics code. In this paper the published experimental data for the laminarburningvelocity of ethanol are reviewed. Next, the performance of several reaction mechanisms for the oxidation kinetics of ethanol–airmixtures is compared. The best performing mechanisms are used to calculate the laminarburningvelocity and flamethickness of these mixtures in a wide range of temperatures, pressures and compositions. Finally, based on these calculations, correlations for the laminarburningvelocity and flamethickness covering the entire operating range of ethanol-fueled spark-ignitionengines, are presented. These correlations can now be implemented in an engine code.

The manner in which an ultra-lean hydrogen flame stabilizes and blows off is crucial for the understanding and design of safe and efficient combustion devices. In this study, we use experiments and numerical simulations for pure H2-air flames stabilized behind a cylindrical bluff body to reveal the underlying physics that make such flames stable and eventually blow-off. Results from CFD simulations are used to investigate the role of stretch and preferential diffusion after a qualitative validation with experiments. It is found that the flame displacement speed of flames stabilized beyond the lean flammability limit of a flat stretchless flame (ϕ=0.3) can be scaled with a relevant tubular flame displacement speed. This result is crucial as no scaling reference is available for such flames. We also confirm our previous hypothesis regarding lean limit blow-off for flames with a neck formation that such flames are quenched due to excessive local stretching. After extinction at the flame neck, flames with closed flame fronts are found to be stabilized inside a recirculation zone.