Influence of micro-explosions on the stability of laminar premixed water-in-fuel emulsion spray flames

A model is presented for a one-dimensional laminar premixed flame, propagating into a rich, off-stoichiometric, fresh homogenous mixture of water-in-fuel emulsion spray, air and inert gas. Due to its relatively large latent heat of vaporisation, the water vapour acts to cool the flame that is sustained by the prior release of fuel vapour. To simplify the inherent complexity that characterises the analytic solution of multi-phase combustion processes, the analysis is restricted to fuel-rich laminar premixed water-in-fuel flames, and assumes a single-step global chemical reaction mechanism. The main purpose is to investigate the steady-state burning velocity and burnt temperature as functions of parameters such as initial water content in the emulsified droplet and total liquid droplet loading. In particular, the influence of micro-explosion of the spray’s droplets on the flame’s characteristics is highlighted for the first time. Steady-state analytical solutions are obtained and the sensitivity of the flame temperature and the flame propagating velocity to the initial water content of the micro-exploding emulsion droplets is established. A linear stability analysis is also performed and reveals the manner in which the micro-explosions influence the neutral stability boundaries of both cellular and pulsating instabilities.     

Unsteady propagation of premixed methane/propane flames in a mesoscale disk burner of variable-gaps

Unsteady flame propagation of air-premixed methane and propane flames was investigated in a new mesoscale disk burner, of which disk-gap could be precisely varied. To begin with, the quenching disk-gaps on the flammability limits were measured. In most cases, with the slight increase of the disk-gap, cellular flame structures could be generated. The initiation of such cellular structures could be explained by the thermally induced hydrodynamic instability, and it could be enhanced if the Lewis number was sufficiently small. When the disk-gap was sufficiently larger than a critical value (approximately 1.5 times the quenching distance), the cellular flame structure changed into a smooth one in the azimuthal direction. With a further increase of the disk-gap, the flame propagation velocities approached to constant values. These values were comparable to the laminar burning velocities except for the propane-rich conditions, in which much larger propagation velocities were observed. The flame stretch effects (coupled with Le-effects) within a narrow space were suspected as the reason. The structural transition of the premixed flame could be investigated successfully through various disk-gaps, from the smallest quenching-scale to the ordinary large scales via whole mesoscales including Hele–Shaw scales.     

Effects of radiation heat loss on laminar premixed ammonia/air flames

Ammonia (NH₃) direct combustion is attracting attention for energy utilization without CO₂ emissions, but fundamental knowledge related to ammonia combustion is still insufficient. This study was designed to examine effects of radiation heat loss on laminar ammonia/air premixed flames because of their very low flame speeds. After numerical simulations for 1-D planar flames with and without radiation heat loss modeled by the optically thin model were conducted, effects of radiation heat loss on flame speeds, flame structure and emissions were investigated. Simulations were also conducted for methane/air mixtures as a reference. Effects of radiation heat loss on flame speeds were strong only near the flammability limits for methane, but were strong over widely diverse equivalence ratios for ammonia. The lower radiative flame temperature suppressed the thermal decomposition of unburned ammonia to hydrogen (H₂) at rich conditions. The equivalence ratio for a low emission window of ammonia and nitric oxide (NO) in the radiative condition shifted to a lower value than that in the adiabatic condition.     

Comparative analysis of methods for heat losses in turbulent premixed flames using physically-derived reduced-order manifolds

Heat losses have the potential to substantially modify turbulent combustion processes, especially the formation of pollutants such as nitrogen oxides. The chemistry governing these species is strongly temperature sensitive, making heat losses critical for an accurate prediction. To account for the effects of heat loss in large eddy simulation (LES) using a precomputed reduced-order manifold approach, thermochemical states must be precomputed not only for adiabatic conditions but also over a range of reduced enthalpy states. However, there are a number of methods for producing reduced enthalpy states, which invoke different implicit assumptions. In this work, a set of a priori and a posteriori LES studies have been performed for turbulent premixed flames considering heat losses within a precomputed reduced-order manifold approach to determine the sensitivity to the method by which reduced enthalpy states are generated. Two general approaches are explored for generating these reduced enthalpy states and are compared in detail to assess any effects on turbulent flame structure and emissions. In the first approach, the enthalpy is reduced at the boundary of the one-dimensional (1D) premixed flame solution, resulting in a single enthalpy deficit for a single premixed flame solution. In the second approach, a variable heat loss source term is introduced into the 1D flame solutions by mimicking a real heat loss to reduce the post-flame enthalpy. The two approaches are compared in methane–air piloted turbulent premixed planar jet flames with different diluents that maintain a constant adiabatic flame temperature but experience different radiation heat losses. Both a priori and a posteriori results, as well as a chemical pathway analysis, indicate that the manner by which the heat loss is accounted for in the manifold is of secondary importance compared to other model uncertainties such as the chemical mechanism, except in situations where heat loss is unphysically fast compared to the flame time scale. A new theoretical framework to explain this insensitivity is also proposed, and its validity is briefly assessed.     

Stabilized,flat iron flames on a hot counterflow burner

Metal powder combustion has traditionally been studied to mitigate the risk of industrial accidents and to determine the contributions of metals as additives to the performance of energetic materials. Recently, there has been growing interest in exploring the potential of metal powders as recyclable, zero-carbon energy carriers as an alternative to the hydrocarbons known to contribute to climate change. The present work introduces, for the first time, a stabilized flat iron flame. The counterflow burner used in this work is comprised of an inverted ceramic nozzle which sits above, and is aligned axially with, a lower nozzle producing a laminar flow of particles suspended in an oxidizing gas. A stabilized methane flame sits inside the top nozzle and the hot combustion products impinge upon the two-phase flow from the bottom nozzle, creating a stagnation plane. Spherical iron powder, with 90% of the particles less than 2.5 µm in size, is pre-loaded into a piston and dispersed using mixtures of 30% and 40% oxygen balanced in argon. Flame speeds are measured using particle image velocimetry (PIV), while flame temperatures are determined using multicolour pyrometry. It is found that flame speeds range between 30 cm/s and 45 cm/s for both oxidizing mixtures. Despite having fuel loadings below stoichiometric concentrations, the observed particle combustion temperatures are close to the adiabatic flame temperature of the stoichiometric mixture, indicating combustion in the diffusion-controlled regime for these small particles. Finally, the independence of the flame speeds with respect to oxygen concentration suggests flame propagation in the discrete regime.     

The differential diffusion effect of the intermediate species on the stability of premixed flames propagating in microchannels

The propagation of premixed flames in adiabatic and non-catalytic planar microchannels subject to an assisted or opposed Poiseuille flow is considered. The diffusive–thermal model and the well-known two-step chain-branching kinetics are used in order to investigate the role of the differential diffusion of the intermediate species on the spatial and temporal flame stability. This numerical study successfully compares steady-state and time-dependent computations to the linear stability analysis of the problem. Results show that for fuel Lewis numbers less than unity, Le_F < 1, and at sufficiently large values of the opposed Poiseuille flow rate, symmetry-breaking bifurcation arises. It is seen that small values of the radical Lewis number, Le_Z, stabilise the flame to symmetric shape solutions, but result in earlier flashback. For very lean flames, the effect of the radical on the flame stabilisation becomes less important due to the small radical concentration typically found in the reaction zone. Cellular flame structures were also identified in this regime. For Le_F > 1, flames propagating in adiabatic channels suffer from oscillatory instabilities. The Poiseuille flow stabilises the flame and the effect of Le_Z is opposite to that found for Le_F < 1. Small values of Le_Z further destabilise the flame to oscillating or pulsating instabilities.     

Impact of the gravity field on stability of premixed flames propagating between two closely spaced parallel plates

The stability of a planar flame front propagating between two parallel adiabatic plates inclined at an arbitrary angle is investigated in the frame of narrow-channel approximation. It is demonstrated that buoyancy forces can suppress the hydrodynamic (Darrieus–Landau) and cellular (diffusive-thermal) instabilities for sufficiently large value of the gravity parameter for the case of downward-propagating flames. The stability analysis reveals that in the case of oscillatory diffusive-thermal instability, the flame front cannot be stabilized in the similar way. Finally, the stability results are compared satisfactorily with unsteady numerical simulations.     

Phenomenological model of chain-branching premixed flames

In this work, we introduce a global kinetic model that includes fuel, oxygen, products and two radical species involved in the reversible chain-branching, chain-propagation and chain-termination reactions. The model naturally extends the Zeldovich–Liñán and Zeldovich–Barenblatt–Dold models and can be used to describe both premixed and diffusion flames. Here it is applied to the problem of the deflagration wave propagation in the hydrocarbon fuel/air mixture with arbitrary equivalence ratio under the simplifying thermal-diffusive approximation. The conservation equations are solved numerically in order to obtain the velocity and structure of the combustion wave. It is demonstrated that the peak values of the adiabatic flame temperature and deflagration velocity are shifted towards the rich mixture composition if the reverse reactions of product decomposition are taken into account. The dependence of the flame speed and temperature on parameters of the system is analysed. The prospects of further investigation are discussed.     

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.     

Dynamics and stability of premixed hydrogen-air flames in square microchannels with wall temperature gradients

The dynamics and stability of premixed hydrogen-air flames in square microchannels with heated walls were investigated through three-dimensional direct numerical simulations. The inlet velocity and equivalence ratio were 1.5 m/s and 0.5. The effect of the wall temperature gradient characteristics on the flame dynamics and stability was examined varying the width and location of the wall temperature gradient for a channel height of 1.5 mm. Five distinct flame modes were observed at different wall temperature profiles: flame with repetitive extinction-ignition (FREI), pulsating flame, laterally oscillating flame, spinning flame, and steady flame modes. Furthermore, transitions between these flame modes were observed for specific inflow and boundary conditions. The effect of the channel height on the flame stability was investigated by varying the channel height from 1.0 mm to 1.677 mm for a fixed wall temperature gradient. As the channel height was increased, four of the flame modes, namely, FREI, laterally oscillating flame, spinning flame, and steady flame modes appeared sequentially. To determine whether this sequential appearance was associated with the variation of the wall heat loss, the maximum wall temperature was changed by small amounts. For a lower wall temperature, the laterally oscillating flame mode transitioned to the FREI mode, and for a higher wall temperature, unstable flame modes such as the FREI and laterally oscillating flame modes disappeared, resulting in stable flame.     

Impact of pressure fluctuations on the dynamics of laminar premixed flames

Thermo-acoustic instabilities are problematic in the design of continuous-combustion propulsion systems such as gas turbine engines, rocket motors, jet engine afterburners, and ramjets. Conceptually, the coupling between acoustics and flame dynamics can be divided into two categories: flame area fluctuations and changes in the local flame speed. The latter can be caused by the thermodynamic fluctuations that accompany an acoustic wave. This coupling is the focus of the present work. In this paper, we are concerned with the dynamics of laminar premixed flames involving large hydrocarbon species. Through high-fidelity numerical simulations, we investigate the flame response for a wide range of fuels and acoustic frequencies. The combustion of hydrogen and methane is considered for verification purposes and as baseline cases for comparison with two large hydrocarbon fuels, n-heptane and n-dodecane. We extract the phase and gain of the unsteady heat release response, which are directly related to the Rayleigh criterion and thus the stability of the system. For all fuels, we observe a local peak in the heat release gain. At high frequencies, we find that the fluctuations of the different species mass fractions decrease with the inverse of the acoustic frequency, leading to chemistry being “frozen” in the high-frequency limit. This allows us to predict the flame behavior directly from the steady-state solution.     

PDF calculations of piloted premixed jet flames

A series of piloted premixed jet flames with strong finite-rate chemistry effects is studied using the joint velocity-turbulence frequency-composition PDF method. The numerical accuracy of the calculations is demonstrated, and the calculations are compared to experimental data. It is found that all calculations show good agreement with the measurements of mean and rms mixture fraction fields, while the reaction progress is overpredicted to varying degrees depending on the jet velocity. In the calculations of the flame with the lowest jet velocity, the species and temperature show reasonable agreement with the measurements, with the exception of a small region near the centerline where products and temperature are overpredicted and fuel and oxidizer are underpredicted. In the calculations of the flame with the highest jet velocity, however, the overprediction of products and temperature and underprediction of fuel and oxidizer is far more severe. An extensive set of sensitivity studies on inlet boundary conditions, turbulence model constants, mixing models and constants, radiation treatment, and chemical mechanisms is conducted to show that any parameter variation offers little improvement from the base case. To shed light on these discrepancies, diagnostic calculations are performed in which the chemical reactions are artificially slowed. These diagnostic calculations serve to validate the experimental data and to quantify the amount by which the base case calculations overpredict reaction progress. Improved calculations of this flame are achieved only through artificially slowing down the chemical reaction by a factor of about 10. The mixing model behavior in this combustion regime is identified as a likely cause for the observed discrepancy in reaction progress.     

Hyperacceleration effects on turbulent combustion in premixed step-stabilized flames

Experimental results are presented from an investigation of the effects of large transverse accelerations on flame propagation and blowout limits in premixed step-stabilized flames. The accelerations, which exceed ±10,000 g in the present study, induce large body forces on the high-density reactants and low-density products. These body forces can substantially alter the flame propagation mechanisms and dramatically increase the flame blowout limits. Sustained centripetal accelerations a_c ≡ U²/R are created by flowing a premixed propane–air reactant stream with equivalence ratios 0.7  Φ  1.9 at various speeds U through a semicircular channel with radius R. A backward-facing step of height h on the radially outer (a_c > 0) or inner (a_c < 0) wall stabilizes the flame. For a_c > 0 the acceleration acts to force high-density reactants into the recirculation zone and low-density products into the reactant stream, while a_c < 0 forces hot products into the recirculation zone and impedes cold reactants from entering this zone. An otherwise identical straight channel provides corresponding baseline (a_c = 0) results for comparison. The flow speed U, equivalence ratio Φ, and step height h are systematically varied for a_c = 0, a_c > 0, and a_c < 0. Shadowgraph and chemiluminescence imaging show that as a_c→ +∞ the propagation of the flame across the channel becomes independent of the flame burning velocity and instead is primarily due to large-scale “centrifugal pumping” driven by the induced body forces. For a_c → −∞ the body forces effectively segregate reactants and products to produce a nearly flat flame. In both cases, for large |a_c| values the resulting blowout limits can be substantially higher than those at a_c = 0.     

Influence of wall heat loss on the emission characteristics of premixed ammonia-air swirling flames interacting with the combustor wall

The influence of wall heat loss on the emission characteristics of ammonia-air swirling flames has been investigated employing Planar Laser-Induced Fluorescence imaging of OH radicals and Fourier Transform Infrared spectrometry of the exhaust gases in combustors with insulated and uninsulated walls over a range of equivalence ratios, ?, and pressures up to 0.5 MPa. Strong influence of wall heat loss on the flames led to quenching of the flame front near the combustor wall at 0.1 MPa, resulting in large unburned NH₃ emissions, and inhibited the stabilization of flames in the outer recirculating zone (ORZ). A decrease in heat loss effects with an increase in pressure promoted extension of the fuel-rich stabilization limit owing to increased recirculation of H₂ from NH₃ decomposition in the ORZ. The influence of wall heat loss resulted in emission trends that contradict already reported trends in literature. NO emissions were found to be substantially low while unburned NH₃ and N₂O emissions were high at fuel-lean conditions during single-stage combustion, with values such as 55 ppmv of NO, 580 ppmv of N₂O and 4457 ppmv of NH₃ at ? = 0.8. In addition, the response of the flame to wall heat loss as pressure increased was more important than the effects of pressure on fuel-NO emission, thereby leading to an increase in NO emission with pressure. It was found that a reduction in wall heat loss or a sufficiently long fluid residence time in the primary combustion zone is necessary for efficient control of NH₃ and N₂O emissions in two-stage rich-lean ammonia combustors, the latter being more effective for N₂O in addition to NO control. This study demonstrates that the influence of wall heat loss should not be ignored in emissions measurements in NH₃-air combustion, and also advances the understanding of previous studies on ammonia micro gas turbines.     

Application of a pulsed laser for soot measurements in premixed flames

A pulsed Nd: YAG laser has been used to perform scattering/extinction measurements in flat, premixed methane/oxygen flames to determine soot particle sizes, number concentrations and soot volume fractions. A discussion is given on accuracies in these determinations with respect to experimental and theoretical parameters. Absorptions were measured through a referencing method which yielded a single-pulse relative standard deviation in the normalized signal of 0.7%. The incident laser fluence was kept below 0.1 J/cm² in the focusing point in the flame to avoid soot vaporization effects. Interference in the measurements from polyaromatic hydrocarbons (PAH) is also discussed.