Impact of exothermicity on steady and linearized response of a premixed ducted flame

Thermoacoustic instabilities arise in power generation devices such as gas turbines and aero-engines when acoustic modes couple with unsteady heat released due to combustion in a positive feedback loop. This work focuses on the development of a reduced order model for understanding flame dynamics in the case of flameholder-stabilized premixed combustion in a duct—a situation typical in many of these applications. Similar to earlier studies in reduced order modeling of this flow, we employ a G-equation formulation to obtain kinematical representation of the premixed flame and ignore the impact of the unsteady (vortical) fluid dynamics downstream of the flameholder. Unlike those studies, however, we retain the impact of combustion exothermicity in the form of a density jump and associated volume generation at the flame front as well as the steady portion of the baroclinic vortical effect. The reduced order model yields analytical solutions for the flame location and for linear transfer functions between imposed (acoustic) perturbation and combustion heat release. We validate these solutions against numerical simulations and other results in literature. The role of expansion (dilatation) and baroclinic aspects of exothermic effects are discussed in detail. Results show that for realistic density ratios across the flame, the flow is accelerated in the streamwise direction on account of combustion exothermicity and the effects of confinement. This not only alters the flame location but also changes the linearized dynamics of the flame and brings into question conclusions drawn from similar analyses in which exothermicity effects were neglected. This is discussed in the context of modeling and controlling thermoacoustic instabilities.     

Thermo-acoustic velocity coupling in a swirl stabilized gas turbine model combustor

Limit-cycle thermo-acoustic velocity coupling mechanisms are studied in a perfectly-premixed swirl-stabilized combustor using data from 10 kHz repetition-rate stereoscopic particle image velocimetry (S-PIV) and OH planar laser induced fluorescence (PLIF). Five cases over a range of thermal powers and equivalence ratios were investigated, each of which underwent different amplitude limit-cycle oscillations. Proper orthogonal decomposition (POD) of the velocity data showed that each case contained a dynamic helical vortex core (HVC) that rotated around the combustor and greatly affected the flame behavior. Flow and flame statistics were compiled as a function of both the phase in the thermo-acoustic cycle and a phase representing the azimuthal position of the HVC relative to the measurement plane. These data were used to determine the thermo-acoustic energy transfer field at each HVC azimuthal angle, as described by the Rayleigh integral. It was found that periodic deformations of the HVC caused large-scale flame motions, resulting in regions of positive and negative energy transfer. The deformation of the HVC was linked to a swirl number wave that propagated from the burner nozzle. While the mechanism of thermo-acoustic coupling was the same for all cases, the phase between heat release and pressure oscillations varied significantly. This phase relationship was determined by the interaction of the pressure field, swirl wave, HVC deformation, and flame response. It was shown that these can be described by the combination of a Helmholtz resonator and a convective disturbance.     

Combustion behavior and stability map of hydrogen-enriched oxy-methane premixed flames in a model gas turbine combustor

The article describes an experimental study and comparison of the combustion behavior and determines the stability map of turbulent premixed H₂-enriched oxy-methane flames in a model gas turbine combustor. Static stability limits, in terms of flashback and blow-out limits, are recorded over a range of hydrogen fraction (HF) at a fixed oxygen fraction (OF) of 30% and a particular inlet bulk velocity, and the results are compared with the non-enriched case (HF = 0%). The static stability limits are also recorded for different inlet bulk velocity (4.4, 5.2, and 6 m/s) and the results are compared to explore the effect of flow dynamics on operability limits of H₂-enriched flames. The stability maps are presented as a function of equivalence ratio (0.3–1.0) and HF (0%–75%) plotted on the contours of adiabatic flame temperature (AFT), power density (PD), inlet Reynolds number (Re) and reacting mixture mass flow rate ($\dot{m}$) to understand the physics behind flashback and blow-out phenomena. The results indicated that both the flashback and blow-out limits tend to move towards the leaner side with increasing HF due to the improved chemical kinetics. The stability limits are observed to follow the Reynolds number indicating its key role in controlling flame static stability limits. The results showed that H₂ enrichment is effective in the zone from HF = 20% up to HF = 50%, and O₂ enrichment is also effective in a similar zone from OF = 20% up to 50%, with wider stability boundaries for H₂ enrichment. Axial and radial temperature profiles are presented to explore the effect of HF on the progress of chemical reactions within the combustor and to serve as the basis for validation of numerical models. Flame shapes are recorded using a high-speed camera and compared for different inlet velocities to explore the effects of H₂-enrichment and equivalence ratio on flame stability. The equivalence ratio at which a transition of flame stabilization from the inner shear layer (ISL) to the outer recirculation zone (ORZ) occurs is determined for different inlet bulk velocities. The value of the transition equivalence ratio is found to decrease while increasing the inlet bulk velocity. Flame shapes near flashback limit, as well as near blow-out limit, are compared to explore the mechanisms of flame extinctions. Flame shapes are compared at fixed adiabatic flame temperature, fixed inlet velocity and fixed flow swirl to isolate their effects and investigate the effect of kinetic rates on flame stability. The results showed that the adiabatic flame temperature does not govern the flame static stability limits.     

Combustion instability of a lean premixed prevaporized gas turbine combustor studied using phase-averaged PIV

A strong, naturally-occurring “growl” combustion instability was studied for the case of a lean premixed prevaporized (LPP) combustor that shows great promise in reducing pollutant emissions. Phase-averaged particle image velocimetry (PIV) was applied for the first time to an LPP device to measure phase lags and spatial correlations. The extensive data set includes spatial and temporal correlations between six parameters: combustor pressure, plenum pressure, injection velocity, heat release rate (Rayleigh index), flame liftoff distance and flame centroid. Measured phase angles and time lags are consistent with the MIT model of Ghoniem et al., along with the concept of “equivalence-ratio oscillation” discussed by Lieuwen et al. Frequency and phase data prove that a dual-mode Helmholtz resonance is driven by an equivalence ratio oscillation. One common modeling assumption is shown to be not valid; the length of an attached flame is not what is oscillating; instead the flame base oscillates violently due to periodic liftoff and flashback and this presents modeling challenges. Growl boundaries and the effects of varying some geometric lengths were recorded.     

Thermoacoustic streaming in a tube with isothermal outer surface

Thermoacoustic oscillations at a cycle-steady state in a tube with an isothermal outer wall, and with one end closed and the other end connected to a wave generator, is analyzed based on a linearized theory. From the global mass conservation, an analytical solution has been obtained for the cross-sectional and cycle averaged axial velocity. It is shown that this averaged velocity is non-vanishing due to the mass streaming effect. By analyzing the global momentum balance, it is found that the cycle-averaged pressure depends on the momentum streaming and friction force, and a conservation relationship exists between the momentum streaming and the cycle-averaged pressure for the flow oscillating at a high frequency in a wide tube. An investigation of the global energy balance leads to an expression for thermoacoustic energy streaming. Furthermore, it is shown that the refrigeration effect is mainly caused by the non-vanishing mean velocity, and therefore the mass streaming and the energy streaming are intimately connected.     

The effect of dilution on the diffusive-thermal instability of the rich premixed hydrogen deflagration

The study focuses on the dynamics of rich premixed diluted hydrogen-oxygen flames. The problem of unsteady flame propagation is treated numerically within a 1D framework with a detailed model for the chemical kinetics and mixture averaged molecular diffusion. The effect of mixture dilution on the onset of the diffusive-thermal pulsating instabilities of freely propagating flames is investigated by studying the characteristics of the pulsating instabilities e.g. critical pressure of the onset, period and amplitude of pulsations. The dilution with helium, nitrogen and argon shows quantitatively a similar effect on the flame instability: namely, the critical pressure for the onset of oscillations is essentially reduced, while the period of oscillations increases. It is also shown that the neutral stability boundary is affected to a leading order through the reduction of the adiabatic flame temperature rather than by modification of the species molecular diffusion. The observed reduction of the critical pressure for the onset of oscillations and the sensitivity of the stability boundary of a steady flame propagation regime to the thermo-chemical system parameters open new perspectives for additional experimental validation of mechanisms of chemical kinetics.     

Excitation of thermoacoustic oscillations by small premixed flames

Experiments have been carried out in which very small lean premixed flames closely representative of those formed by modern multiport domestic gas burners have been subjected to controlled acoustic perturbation. PLIF from CH has been used to visualise the flame response and the heat-release-rate fluctuations have been evaluated directly from the flame images. It is shown that small laminar flames can amplify the effects of acoustic velocity fluctuations by mechanisms that do not involve resonant heat loss to the burner and that the fluctuations in flame-front area are not adequately characterised by a Strouhal number alone. The measured transfer function is compared with the predictions of various analytical formulations and a new model of the flame oscillation is proposed which applies specifically to situations in which the design of the burner renders the flame base immobile.     

Experimental and numerical studies on the premixed syngas swirl flames in a model combustor

Experimental and numerical investigations were performed to study the combustion characteristics of synthesis gas (syngas) under premixed swirling flame mode. Four different type of syngases, ranging from low to high H₂ content were tested and simulated. The global flame structures and post emission results were obtained from experimental work, providing the basis of validation for simulations using flamelet generated manifold (FGM) modelling approach via a commercial computational fluid dynamic software. The FGM method was shown to provide reasonable agreement with experimental result, in particular the post-exhaust emissions and global flame shapes. Subsequently, the FGM method was adopted to model the flame structure and predict the radical species in the reaction zones. Simulation result shows that H₂-enriched syngas has lower peak flame temperature with lesser NO species formed in the reaction zone.     

Development and characterization of a low-NOx partially premixed hydrogen burner using numerical simulation and flame diagnostics

The utilization of hydrogen as a fuel in free jet burners faces particular challenges due to its special combustion properties. The high laminar and turbulent flame velocities may lead to issues in flame stability and operational safety in premixed and partially premixed burners. Additionally, a high adiabatic combustion temperature favors the formation of thermal nitric oxides (NO). This study presents the development and optimization of a partially premixed hydrogen burner with low emissions of nitric oxides. The single-nozzle burner features a very short premixing duct and a simple geometric design. In a first development step, the design of the burner is optimized by numerical investigation (Star CCM+) of mixture formation, which is improved by geometric changes of the nozzle. The impact of geometric optimization and of humidification of the combustion air on NO_x emissions is then investigated experimentally. The hydrogen flame is detected with an infrared camera to evaluate the flame stability for different burner configurations. The improved mixture formation by geometric optimization avoids temperature peaks and leads to a noticeable reduction in NO_x emissions for equivalence ratios below 0.85. The experimental investigations also show that NO_x emissions decrease with increasing relative humidity of combustion air. This single-nozzle forms the basis for multi-nozzle burners, where the desired output power can flexibly be adjusted by the number of single nozzles.     

The effect of premixed stratification on the wave dynamics of a rotating detonation combustor

Typical injection schemes of rotating detonation combustors inject fuel locally into the combustion channel, creating stratified fuel-rich and fuel-lean mixing regions. In this study, premixed hydrogen and air rotating detonations are explored in a rotating detonation combustor through premixing part of the fuel into the oxidizer flow. The objective is to investigate the effect of premixing on the operation of the combustor. Three premixing schemes are examined where the detonation wave speeds are analyzed. The results show that in premixing, the fuel-lean regions became more favorable for continuous detonation propagation when premixed with the bypass fuel, resulting in higher detonation wave speeds. This phenomenon is shown to be independent of the global fuel-air equivalence ratio and the amount of fuel premixed into the oxidizer. As such, combustor performance and the operational regime could be improved with lean hydrogen premixing amounts in the main flow oxidizer.     

Partially premixed flamelet modeling in a hydrogen-fueled supersonic combustor

In scramjet combustors, the combustion process is usually partially premixed, that is, both the non-premixed and the premixed regimes should be taken into account. Based on the multi-regime flamelet (MRF) model proposed for low Mach number flows, a modified MRF model that applies to supersonic flow conditions has been developed. Taken a hydrogen-fueled model combustor as test case, the good agreement between the calculation and experiments was obtained. The distribution of weighting coefficient, which is defined based on the concept of combustion regime index, shows that the flow field in the supersonic combustor is partially-premixed. The premixed regime distributes in the backflow region, the shear layer and the boundary layer. Comparisons between the results of steady laminar flamelet (SLF) model and the modified MRF model show that the latter one gives a more precise prediction of temperature profiles, indicating the modified MRF model has better versatility and accuracy.     

Turbulent consumption speed via local dilatation rate measurements in a premixed bunsen jet

The mean local reaction rate related to the average expansion across the front and computed from the mean velocity divergence is evaluated in this work. Measurements are carried out in a air/methane premixed jet flame by combined PIV/LIF acquisitions. The procedure serves the purpose of obtaining values of a turbulent flame speed, namely the local turbulent consumption speed S_LC, as a function of the position along the bunsen flame. With the further position that the flamelet assumption provides a proportionality between turbulent burning speed normalized with the laminar unstretched one and the turbulent to average flame surface ratio, the proportionality constant, i.e., the stretching factor becomes available. The results achieved so far show the existence of a wide region along which the bunsen flame front has a constant stretching factor which apparently depends only on the ratio between turbulent fluctuations and laminar flame speed and on the jet Reynolds number.     

Experimental investigation of the stability and emission characteristics of premixed formic acid-methane-air flames in a swirl combustor

Formic acid (FA) is a potential hydrogen energy carrier and low-carbon fuel by reversing the decomposition products, CO₂ and H_2, back to restore FA without additional carbon release. However, FA-air mixtures feature high ignition energy and low flame speed; hence stabilizing FA-air flames in combustion devices is challenging. This study experimentally investigates the flame stability and emission of swirl flames fueled with pre-vaporized formic acid-methane blends over a wide range of formic acid fuel fractions. Results show that by using a swirl combustor, the premixed formic acid-methane-air flames could be stabilized over a wide range of FA fuel fractions, Reynolds numbers, and swirl numbers. The addition of formic acid increases the equivalence ratios at which the flashback and lean blowout occur. When Reynolds number increases, the equivalence ratio at the flashback limit increases, but that decreases at the lean blowout limit. Increasing the swirl number has a non-monotonic effect on stability limits variation because increasing the swirl number changes the axial velocity on the centerline of the burner throat non-monotonically. In addition, emission characteristics were investigated using a gas analyzer. The CO and NO concentrations were below 20 ppm for all tested conditions, which is comparable to that seen with traditional hydrocarbon fuels, which is in favor of future practical applications with formic acid.     

The impact of equivalence ratio oscillations on combustion dynamics in a backward-facing step combustor

The combustion dynamics of propane-air flames are investigated in an atmospheric pressure, atmospheric inlet temperature, lean, premixed backward-facing step combustor. We modify the location of the fuel injector to examine the impact of equivalence ratio oscillations arriving at the flame on the combustion dynamics. Simultaneous pressure, velocity, heat-release rate and equivalence ratio measurements and high-speed video from the experiments are used to identify and characterize several distinct operating modes. When the fuel is injected far upstream from the step, the equivalence ratio arriving at the flame is steady and the combustion dynamics are controlled only by flame-vortex interactions. In this case, different dynamic regimes are observed depending on the operating parameters. When the fuel is injected close to the step, the equivalence ratio arriving at the flame exhibits oscillations. In the presence of equivalence ratio oscillations, the measured sound pressure level is significant across the entire range of lean mean equivalence ratios even if the equivalence ratio oscillations arriving at the flame are out-of-phase with the pressure oscillations. The combustion dynamics are governed primarily by the flame-vortex interactions, while the equivalence ratio oscillations have secondary effects. The equivalence ratio oscillations could generate variations in the combustion dynamics in each cycle under some operating conditions, destabilize the flame at the entire range of the lean equivalence ratios, and increase the value of the mean equivalence ratio at the lean blowout limit.     

Dynamics of upstream flame propagation in a hydrogen-enriched premixed flame

An unconfined strongly swirled flow is investigated to study the effect of hydrogen addition on upstream flame propagation in a methane-air premixed flame using Large Eddy Simulation (LES) with a Thickened Flame (TF) model. A laboratory-scale swirled premixed combustor operated under atmospheric conditions for which experimental data for validation is available has been chosen for the numerical study. In the LES-TF approach, the flame front is resolved on the computational grid through artificial thickening and the individual species transport equations are directly solved with the reaction rates specified using Arrhenius chemistry. Good agreement is found when comparing predictions with the published experimental data including the predicted RMS fluctuations. Also, the results show that the initiation of upstream flame propagation is associated with balanced maintained between hydrodynamics and reaction. This process is associated with the upstream propagation of the center recirculation bubble, which pushes the flame front in the upstream mixing tube. Once the upstream movement of the flame front is initiated, the hydrogen-enriched mixture exhibits more unstable behavior; while in contrast, the CH₄ flame shows stable behavior.     

Direct numerical simulation and analysis of a hydrogen/air swirling premixed flame in a micro combustor

A three dimensional spatially developing hydrogen/air premixed flame in a micro combustor with a moderate Reynolds number and a high swirl number is studied using direct numerical simulation. The inflow mixture is composed of hydrogen and air at an equivalent ratio of 1.0 in the jet core region, and pure air elsewhere. The maximum axial velocity at the inlet is 100 m/s. A fourth-order explicit Runge–Kutta method for time integration and an eighth-order central differencing scheme for spatial discretization are used to solve the full Navier–Stokes (N–S) equation system. A 9 species 19-step reduced mechanism for hydrogen/air combustion is adopted. Vortex and turbulence characteristics are examined. Two instabilities, namely Kalvin–Helmholtz instability and centrifugal instability, are responsible for the transition from laminar flow to turbulence. A cone-like vortex breakdown is observed both in the isothermal swirling flow and in the swirling flame. One dimensional premixed laminar flame is studied, the structure of which is compared with that of the multi-dimensional one. Probability density functions of the curvature and tangential strain rate are presented. It is shown that the flame curvature has a near zero mean, and the flame aligns preferentially with extensive strain. Finally, the turbulent premixed flame regime diagram is used to characterize the flame. It is found that most of the flame elements lie in the laminar flame regime and the thin reaction zones regime.     

Experimental study of the effects of hydrogen addition on the thermoacoustic instability in a variable-length combustor

The current study examined the self-excited thermoacoustic instability of hydrogen/methane premixed flames using a variable-length combustor (300–1100 mm). The global dynamic pressure, heat release rate oscillation, together with the flame dynamics were studied. Results showed that both the hydrogen concentration and the chamber length were critical in determining the acoustic oscillation mode and instability trend. Low-frequency primary acoustic modes (<200 Hz) were mainly excited when the hydrogen concentration was low, whereas primary acoustic modes with relatively higher frequencies (~400 Hz) tended to occur in cases with a high hydrogen proportion (>40%). For primary acoustic modes lower than 200 Hz, the primary oscillation frequency tended to increase linearly with a rising hydrogen proportion. Heat release oscillation and flame dynamics analyses demonstrated that for the flame with large-scale shape deformation, the initial addition of hydrogen would intensify the heat release oscillation. Nevertheless, a further increase in the hydrogen level tended to inhibit the heat release oscillation by weakening the flame shape deformation. Eventually, a sufficient high-level of hydrogen addition would weaken the primary acoustic modes that have similar frequencies.     

Conditionally averaged balance equations for modeling premixed turbulent combustion in flamelet regime

Balance equations for mass, velocity, and Reynolds stresses conditioned either to an unburned or to a burned mixture are derived from standard mass conservation, Navier-Stokes, and combustion progress variable balance equations by assuming that the probability of finding intermediate states of a reacting mixture is much less than unity. The derived equations contain three unclosed terms controlled by flamelet structure and flamelet statistics, whereas other unclosed terms are not straightforwardly affected by heat release and have counterparts in models of nonreacting flows. Equations of this type offer an opportunity to facilitate modeling the effects of heat release on turbulence. The modeling of these effects is further simplified if flamelet structure perturbations by turbulent eddies are neglected.     

Three-dimensional temperature gradients in premixed turbulent flamelets via crossed-plane Rayleigh imaging

A new technique for obtaining instantaneous, high-resolution, three-dimensional thermal structure data from turbulent flames, crossed-plane Rayleigh imaging is described and then demonstrated. Quantitative Rayleigh imaging measurements are made simultaneously in two orthogonal, intersecting laser-sheet illumination planes. At points along the line of intersection of the two laser sheets, instantaneous, three-dimensional temperature gradient data are measured. The technique has higher resolution than parallel plane measurement techniques, which have limited resolution in the direction orthogonal to the parallel planes. The technique is used to measure temperature gradient data for a lean, premixed, methane-air turbulent V-flame with an equivalence ratio of Φ=0.7, and normalized turbulence intensity (, where u^′ is the turbulence intensity and is the unstretched laminar flame speed. Measurements are also presented for a laminar V-flame and a laminar Bunsen flame for comparison. Finally, an unstretched laminar flame calculation is made. Quantitative estimates of the experimental uncertainty are presented. The primary source of uncertainty in the data is due to shot noise. Measured temperature gradient data for laminar flames differ from that of the unstretched laminar flame calculation, especially in the oxidation layer. Turbulent flame temperature gradient data indicate that the turbulent V-flame thermal structure is not significantly perturbed from the measured laminar V-flame structure. For the flame studied, the flamelet approximation is valid if the flamelet used is based on the measured laminar flame structure of the V-flame. Isothermal surface orientation data are presented and are close to parallel for most realizations. Isothermal surface density is calculated from the distribution of isothermal surface orientations and from conditional averages of the magnitude of the temperature gradient. Isothermal surface density does not vary for different isothermal surfaces.     

Hydrogen substitution of natural-gas in premixed burners and implications for blow-off and flashback limits

Two multi-perforated premixed burners, designed for natural gas, are fueled with increasing hydrogen content to assess the limits of H₂ substitution and investigate potential risks associated to it. The burners feature a different design, which affects flame stabilization and heat exchange between the fresh mixture and the hot burner walls. First, results are presented by means of stability maps that were collected at constant power and over a wide range of equivalence ratio, from pure methane-air to pure hydrogen-air mixtures. The impact of hydrogen addition on blow-off and flashback limits is then analyzed. On one side, it is observed that hydrogen addition increases blow off resistance, extending the operating range towards ultra-lean conditions. On the other side, hydrogen raises the thermal load on the burner favoring flashback. It is shown that the competition between the bulk velocity at the burner outlet and the laminar burning velocity is not a reliable parameter to predict flashback occurrence, while the thermal state of the burner represents a determining factor. An analysis of the thermal transient reveals a strict correspondence between the onset of flashback for a given mixture composition and the burner surface temperature. Results highlight the challenges linked to the design of fuel-flexible systems, pointing out practical limits of H₂ substitution in burners designed for operation with natural gas.