Insight into the inner structure of stretched premixed ammonia-air flames

Ammonia as a fuel has sparked significant interest in the combustion community. Although, using ammonia has a lot of advantages including no carbon emissions, ammonia-air flames are characterized as thick flames with low flame speeds. It is important to understand the flame structure to know the combustion process better. Flame thickness is an important property of the flame which characterizes the reactivity of the flame. Identifying the preheat zone is necessary to determine the fresh gas surface which is used to determine flame speed. Also, understanding the behavior of the important species emitted helps to demonstrate the reaction pathway which may be implemented in chemical kinetics schemes. Further, it is interesting to know the effect of curvature on the emission of excited species which gives direct knowledge on the influence of curvature on the flame reactivity. It was seen that the change in reactivity was manifested as a change in thickness of the species. The experiments presented here were performed on a Bunsen burner at atmospheric conditions. The laminar flame speeds have been evaluated over a range of equivalence ratios by choosing the isotherm as specified by the definition of the flame speed which are slightly higher than the values obtained from the literature. Chemiluminescence from NH* and NH₂* was studied for different equivalence ratios. A 1D simulation performed in Chemkin-Pro-was used to compare the behavior of the counterpart non-excited species. This comparison helps to correlate excited and non-excited species and also to define the structure of the ammonia-air flame. Both NH* and NH₂* have been determined as heat release rate markers.     

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.     

Numerical simulation of finite disturbances interacting with laminar premixed flames

Compression waves can be generated during combustion processes and subsequently interact with flames to augment their behaviour. The study of these interactions thus far has been limited to shock and expansion waves only. In this study, the interaction of finite compression waves with a perturbed laminar flame is investigated using numerical simulations of the compressible Navier–Stokes equations with single-step chemical kinetics. The interaction is characterised using three independent parameters: the compression wavelength, the pressure ratio of the disturbance, and the perturbation amplitude of the flame interface. The results reveal a wide range of behaviours in terms of flame length and heat release rate that could occur during such an interaction. The results are compared to the classical reactive Richtmyer–Meshkov instability and the role of baroclinic torque and vorticity generation are shown to be primary drivers of the flow instability.     

Reaction zone structures and mixing characteristics of partially premixed swirling CH4/air flames in a gas turbine model combustor

The mixing, reaction progress, and flame front structures of partially premixed flames have been investigated in a gas turbine model combustor using different laser techniques comprising laser Doppler velocimetry for the characterization of the flow field, Raman scattering for simultaneous multi-species and temperature measurements, and planar laser-induced fluorescence of CH for the visualization of the reaction zones. Swirling CH₄/air flames with Re numbers between 7500 and 60,000 have been studied to identify the influence of the turbulent flow field on the thermochemical state of the flames and the structures of the CH layers. Turbulence intensities and length scales, as well as the classification of these flames in regime diagrams of turbulent combustion, are addressed. The results indicate that the flames exhibit more characteristics of a diffusion flame (with connected flame zones) than of a uniformly premixed flame.     

Influences of heat flux on extinction characteristics of steady/unsteady premixed stagnation flames

This paper investigates the extinction characteristics of premixed stagnation flames (PSFs) with controlled heat losses and flow disturbances. The low-frequency air flow pulsations that imitate the operational transients in practical combustors were specially introduced. The tunable diode-laser absorption spectroscopy (TDLAS) measurement was applied to obtain the temperature profile and wall heat flux. It is found that, for steady flame with a fixed equivalence ratio, the extinction stretch rate dramatically increases as the wall heat flux decreases. The extinction criterion is summarized as a global Karlovitz number of 0.57 by establishing a relationship between the global and local stretch rates. Numerical simulations reveal that the local extinction Karlovitz number of steady PSFs is approximately 1.0 regardless of the conditions such as heat flux and equivalence ratio. Further experiments present that the air pulsations with a repetition of ～5 Hz significantly deteriorate the flame stability. Particularly, for unsteady perturbed flames, the extinction stretch rate exhibits a nonlinear trend, yielding two regimes with discrepant sensitivities to wall heat flux. The unsteady simulation then highlights a local stretch rate overshoot in the presence of pulsation. It is caused by the time delay between the inlet velocity and flame front movement that eventually leads to poor flame stability. Moreover, in the high heat-flux regime, a smaller local stretch rate overshoot results in the weak dependence of extinction limits on heat fluxes.     

Effects of residence time on the NOx emissions of premixed ammonia-methane-air swirling flames at elevated pressure

In this study, a bespoke single-stage swirl burner was used to experimentally investigate the effects of residence time on emissions from premixed ammonia-methane-air flames. The residence time was altered in two ways: by modifying the combustion chamber's length or by modifying the swirl number. Exhaust emissions of O₂, CO₂, CO, NO, NO₂, and N₂O were measured at an absolute pressure of 2 bar for equivalence ratios between 0.50 and 0.95 and ammonia fractions in the fuel blend between 0 and 100%. Spatial distributions of NO and OH radicals were also imaged using PLIF inside the combustion chamber at different heights above the nozzle. Data shows that increasing residence time can further advance chemical reactions, as evidenced by a reduction in O₂ concentration in the exhaust. Increasing the swirl number reduces emissions of NO, NO₂, and N₂O more efficiently than tripling the chamber's length. However, a decrease in the combustion efficiency may be responsible for a fraction of this NOx reduction when the swirl number is increased for some equivalence ratios. NO emissions are not modified when the chamber's length is increased, which is consistent with the fact that the NO-LIF signal does not decay when the distance from the nozzle increases. Therefore, NO formation is somehow restricted to within the main reaction zone of the swirling flame, that is, the zone whose height does not exceed 60 mm for this burner. Conversely, tripling the chamber's length reduces the concentrations of NO₂ and N₂O. This reduction is not reflected in a measurable increase in NO concentration because NO is present in much larger quantities than NO₂ and N₂O in flames examined here. Consistent with the fact that OH promotes NO formation via fuel-NOx pathways, a positive correlation is found between NO- and OH-LIF intensities.     

Structural response of different Lewis number premixed flames interacting with a toroidal vortex

Simultaneous measurements of temperature, CH* and OH* chemiluminescent species are carried out to explore the impact of stretch rate and curvature on the structure of premixed flames. The configuration of an initially flat premixed flame interacting with a toroidal vortex is selected for the present study and reasons for this choice are discussed. Lewis number effects are assessed by comparing methane and propane flames. It is emphasized that the flame structure experiences very strong variations. In particular, the flame is shrunk (broadened) in the initial (final) period of the interaction with the vortex where strain rate (curvature) contribution of the stretch rate is predominant. By further analysing independently the thickness of the preheat and reaction zones, it is shown that for propane flames, not only the former but also the latter is significantly altered in zones where the flame curvature is negative. Changes in the reaction zone properties are further emphasized using CH* and OH* radicals. It is demonstrated that higher thermal diffusivity plays a significant role around curved regions, in which the enhanced diffusion of heat leads to a strong increase of CH* compared to OH* intensity. As an overall conclusion, this study suggests that it would be interesting to reassess the internal flame structure at lower and moderate Karlovitz numbers since changes might appear for a moderate vortex intensity with typical size much larger than the flame thickness.     

Characteristics of self-excited spinning azimuthal modes in an annular combustor with turbulent premixed bluff-body flames

In this paper we investigate self-excited azimuthal modes in an annular combustor with turbulent premixed bluff-body stabilised flames. Previous studies have shown that both swirl and equivalence ratio influence modal dynamics, i.e. the time-varying nature of the modes. However, self-excited azimuthal modes have not yet been investigated in turbulent flames without bulk swirl, which do not generate any preferential flow in either azimuthal direction, and may therefore lead to different behaviour. Joint probability density functions of the instability amplitudes at various flowrates and equivalence ratios showed a strong bi-modal response favouring both ACW and CW spinning states not previously observed. Operating conditions leading to a bi-modal response provide a unique opportunity to investigate whether the structure of the global fluctuating heat release rate of self-excited spinning modes in both directions exhibit similar dynamics and structure. This was investigated using high-speed OH* chemiluminescence images of the annular combustor and a new rotational averaging method was applied which decomposes the spinning components of the global fluctuating heat release rate. The new rotational averaging, which differs from standard phase-averaging, produces spatial averages in a frame of reference moving with the spinning wave. The results show that the structure of the fluctuating heat release rate for spinning modes is highly asymmetric as characterised by large, crescent shaped regions of high OH* intensity, located on the far side of each flame, relative to the direction of the azimuthally propagating pressure wave. In comparison with interacting swirling flames, these results indicate that the previously observed radial asymmetry of OH* fluctuations may be introduced through advection by local swirl.     

Stability characteristics and flowfields of turbulent non-premixed swirling flames

A simple, yet representative, burner geometry is used for the investigation of highly swirling turbulent unconfined, non-premixed, flames of natural gas. The burner configuration comprises a ceramic faced bluff-body with a central fuel jet. The bluff-body is surrounded by an annulus that delivers a swirling primary flow of air. The entire burner assembly is housed in a wind tunnel providing a secondary co-flowing stream of air. This hybrid bluff-body/swirl burner configuration stabilizes complex turbulent flames not unlike those found in practical combustors, yet is amenable to modelling because of its well-defined boundary conditions. Full stability characteristics including blow-off limits and comprehensive maps of flame shapes are presented for swirling flames of three different fuel mixtures: compressed natural gas (CNG), CNG–air (1:2 by volume) and CNG–H₂ (1:1 by volume).

It is found that with increased fuel flow, flame blow-off mode may change with swirl number, S_g. At low swirl, the flame remains stable at the base but blows off in the neck region further downstream. At higher swirl numbers, the flames peel off completely from the burner's base. Swirling CNG–air flames are distinct in that they only undergo base blow-off. In the low range of swirl number, increasing S_g causes limited improvement in the blow-off limits of the flames investigated and (for a few cases) can even lead to some deterioration over a small intermediate range of S_g. It is only above a certain threshold of swirl that significant improvements in blow-off limits appear. Six flames are selected for further detailed flowfield and composition measurements and these differ in the combination of swirl number, primary axial velocity through the annulus, U_s, and bulk fuel jet velocity, U_j. Only velocity field measurements are presented in this paper. A number of flow features are resolved in these flames, which resemble those already associated with non-reacting swirling flows of equivalent swirl obtained with the present burner configuration. Additionally, asymmetric flowfields inherent to some flames are revealed where the fluidic centreline of the flow (defined in the two-dimensional (U–W velocity pair) velocity field by the ?ω? = 0 tangential velocity contour), meanders strongly on either side of the geometric centreline downstream by about one bluff-body diameter. Flow structures revealed by the velocity data are correlated to flame shapes to yield a better understanding of how the velocity field influences the flames physical characteristics.     

Statistical characteristics of pressure oscillations in a premixed combustor

This paper describes an investigation of the statistical characteristics of self-excited and noise-driven pressure oscillations in a premixed combustor. This work was motivated by observations that certain characteristics of these oscillations appear random and cannot be entirely characterized within a deterministic framework (e.g., spontaneous, noise-induced transitions of the combustor from stable to unstable operation or cycle-to-cycle variations in the oscillating pressure). In an effort to elucidate these stochastic elements, we performed an analysis of cycle-to-cycle variations in combustor pressure whose results are described in this paper. Data obtained from our combustor shows that the probability density function of the amplitude of these oscillations transitions from a Rayleigh to a Gaussian-type distribution as the combustor moves from stable to unstable operation. These data also show that the instability phase is nearly uniformly distributed; i.e., there is no phase value with maximum probability of occurrence. We also describe a theoretical analysis of the statistical features of a non-linear combustor model that is forced by random noise. Solutions of this model are presented and shown to be in agreement with measured data. The good agreement between the predictions and measured data suggest that the analysis presented in this paper provides a useful framework for interpreting many other apparently random features of combustor stability characteristics; for example, cyclic variability, “fuzziness” in stability boundaries, or noise-induced transitions.     

The effects of swirling partially premixed flame on scaled kinetic energy transport in a gas turbine-like combustor

The multi-scale interaction between combustion and turbulence is of great importance in modifying the small-scale flame structure and kinetic energy, especially in swirling flames under practical conditions. In the present study, direct numerical simulation of swirling partially premixed flame is conducted within a model combustor under gas turbine conditions. The reactive flow is compared to the corresponding non-reactive one to investigate the influence of combustion on the scaled kinetic energy transport. Kinetic energy spectra demonstrate that the turbulent kinetic energy is reduced in the dissipative subrange while enhanced in the energetic one by the flame. The critical scale is located in the inertial subrange and close to the estimated turbulent flame thickness. Filtering analyses show that the resolved-scale kinetic energy is augmented by the increased large-scale pressure-gradient work in the reactive flow, while the subgrid-scale kinetic energy is attenuated by the enhanced small-scale viscous dissipation. The backscatter prevails in the heat release regions when the filter size is larger than the laminar flame thickness, and this effect decreases with the swirling flow developing downstream. The interaction between the kinetic energy flux and the local dilatation as well as the subgrid-scale pressure-gradient work is also investigated to achieve a comprehensive understanding about the effects of combustion on the backscatter.     

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.     

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.     

Influence of radiation losses on the stability of premixed flames on a porous-plug burner

Analysis of the planar premixed flames on a porous plug was performed numerically for finite activation energy within the diffusive-thermal model. The paper is focused on the influence of radiation heat loses on the flame standoff distance and its linear stability. We show that the presence of volumetric heat losses limits the range of the mass flow range as well as it can promote the flame instabilities of different kinds, both oscillatory and cellular. The oscillatory instability, which for freely propagating flames can be usually observed for the Lewis number larger than one, in the porous-plug case occurs also for flames with unity and lower than unity Lewis number. For flames with Le < 1 both cellular and oscillatory instabilities can be observed simultaneously in a certain range of the mass flow rate.     

Experimental and numerical determination of heat release in counterflow premixed laminar flames

This paper presents an experimental and numerical study of heat release in atmospheric laminar counterflow premixed flames. The measurements are based on simultaneous planar laser-induced fluorescence (PLIF) of OH and HCHO. These measurements are compared to numerical results obtained using detailed chemistry and multicomponent transport properties. A low Mach number formulation along the stagnation streamline is employed to describe the reactive flow. The conservation equations are completed with CHEMKIN and EGLIB packages. They are solved using finite differences, Newton iterations, and an adaptive gridding technique. The comparison is done along the burner axis for both, maximum heat release location and heat release profile width. It is shown that the product of OH and HCHO concentrations yields a result closely related to the heat release. These comparisons lead to the conclusion that the experimental method used seems to be a good tool for the determination of heat release in flames.