Combustion dynamics of a low-swirl combustor

The methodology for the measurement of dynamic combustor behavior has never been clearly established, due to the complexities associated with unsteady premixed flames and the difficulties in their measurement. The global and local distribution of Rayleigh index and the flame response functions are the main parameters normally employed to quantify and describe combustion dynamics. The Rayleigh index quantifies the thermoacoustic coupling, while the flame response function is a measure of the response of the system to outside disturbances. The primary objective of this work is to investigate the combustion dynamics of a commonly used low-swirl burner and to develop tools and methods for examining the dynamics of a combustion system. To this end, the effect of acoustic forcing at various frequencies on flame heat release behavior has been investigated. The current work uses OH-PLIF imaging of the flame region to produce phase-resolved measurements of flame behavior at each frequency. The response of the flame to the imposed acoustic field over the range of 22-400 Hz is then calculated from the processed images. This provides a starting point for an extension/extrapolation to practical acoustic ranges (∼5000 Hz). It was found that the thermoacoustic coupling was mainly evident in the shear mixing zone, producing a toroidal Rayleigh index distribution pattern. The phase shift of the flame fluctuation from the imposed acoustic wave seems to be very closely coupled to the vortices generated at the flame boundary due to shear mixing (Kelvin-Helmholtz instability), thus inducing the alternating toroidal structures. The peak value of the flame response function coincides with the peak absolute value of the Rayleigh index.     

Soot volume fraction in a piloted turbulent jet non-premixed flame of natural gas

Planar laser-induced incandescence (LII) has been used to measure soot volume fraction in a well-characterised, piloted, turbulent non-premixed flame known as the “Delft Flame III”. Simulated Dutch natural gas was used as the fuel to produce a flame closely matching those in which a wide range of previous investigations, both experimental and modelling, have been performed. The LII method was calibrated using a Santoro-style burner with ethylene as the fuel. Instantaneous and time-averaged data of the axial and radial soot volume fraction distributions of the flame are presented here along with the Probability Density Functions (PDFs) and intermittency. The PDFs were found to be well-characterised by a single exponential distribution function. The distribution of soot was found to be highly intermittent, with intermittency typically exceeding 97%, which increases measurement uncertainty. The instantaneous values of volume fraction are everywhere less than the values in strained laminar flames. This is consistent with the soot being found locally in strained flame sheets that are convected and distorted by the flow.     

On the stability of a turbulent non-premixed biogas flame: Effect of low swirl strength

Biogas like other low calorific value fuels has a very narrow stable region when operating in diffusion flame mode owing to their low burning velocity in conjunction with the unburned flow high velocity. This paper presents an experimental study on the effect of the burner geometry on the stability limits of a turbulent non-premixed biogas flame. The main focus of the study is on the role of the low swirl strength of the co-airflow, and the fuel nozzle diameter. The results revealed that the swirl plays a dominant role on the flame mode (attached or lifted) as well as on its operating/stability limits. However, the results revealed that the swirl effect prevails only at relatively moderate to high co-airflow velocity. That is, the swirl does not have an apparent effect at weak co-airflow when the flame is attached. Whereas, it becomes dominant at relatively high co-airflow velocity where the attached flame lifts off and stabilizes at a distance above the burner. Correlations were proposed to describe the lifted biogas flame blowout limits.     

EINOx scaling in a non-premixed turbulent hydrogen jet with swirled coaxial air

The effect of swirl flow on pollutant emission (nitrous oxide) was studied in a non-premixed turbulent hydrogen jet with coaxial air. A swirl vane was equipped in a coaxial air feeding line and the angle of the swirl vane was varied from 30 to 90 degrees. Under a fixed global equivalence ratio of φ_G = 0.5, fuel jet air velocity and coaxial air velocity were varied in an attached flame region as u_F = 85.7–160.2 m/s and u_A = 7.4–14.4 m/s. In the present study, two mixing variables of coaxial air and swirl flow were considered: the flame residence time and global strain rate. The objective of the current study was to analyze the flame length behavior, and the characteristics of nitrous oxide emissions under a swirl flow conditions, and to suggest a new parameter for EINOx (the emission index of nitrous oxide) scaling. From the experimental results, EINOx decreased with the swirl vane angle and increased with the flame length (L). We found the scaling variables for the flame length and EINOx using the effective diameter (d_F,eff) in a far-field concept. Normalized flame length (L divided by d_F,eff) fitted well with the theoretical expectations. EINOx increased in proportion to the flame residence time (∼τ_R^1/2.8) and the global strain rate (∼S_G^1/2.8).     

Study of non-premixed turbulent flame of hydrogen/air downstream Co-Current injector

For three decades, hydrogen has been identified as a versatile potential fuel concurrent to the conventional fuel such as gasoline. In order to fully implement it and to develop the combustion based power devices that may supply much higher energy density, it is very essential to understand the mechanism of Hydrogen/Air combustion. In this work, Computational Fluid Dynamics (CFD) numerical simulations have been performed to study the combustion of non-premixed turbulent hydrogen-air mixture with different equivalence ratios and different mass flow rates and its effect on different species formation, peak temperature and NO_x formation. The performance of the combustor is evaluated by using FLUENT software under adiabatic wall condition. Generalized finite rate chemistry model was used to analyze the hydrogen-air combustion system. The combustion is modeled using multi-step reaction mechanism with 14 species, until complete conversion of fuel to H₂O. Through such a systematic analysis, a proper controlled operation condition for the combustor is suggested which may be used as a guideline for combustor design. Results reported in this work illustrate that the CFD simulation can be one of the most powerful, beneficial and economical tool for combustor design and for optimization and performance analysis. They are more sensitive to the model of the transport properties while the reasonable results can be achieved even with the use of global reaction mechanism and a simple turbulence model as k- ε, which are not excessively time and memory consuming. From an environmental point view, this study shows that the radical production (OH and NO) is very small although maximum temperature reached exceeded 2000 (K). The mass fraction of NO is much lower if we increase the air inlet velocity, which makes the cold reaction mixture do not promote the NO formation by dissociation.     

Simultaneous stereo particle image velocimetry and double-pulsed planar laser-induced fluorescence of turbulent premixed flames

A new technique was developed and applied to the study of flame structure and flame-vortex interaction in turbulent premixed flames. Turbulent premixed flames were probed using simultaneous stereo particle image velocimetry (PIV) and a double-pulsed acetone planar laser-induced fluorescence system (PLIF). Two double-pulsed Nd:YAG lasers operating at 532 and 266 nm were used for the PIV and acetone PLIF measurements, respectively. The stereo PIV images were acquired using two double-frame CCD cameras, and two ICCD cameras were used to capture the PLIF signal. The diagnostic system was applied to study turbulent methane-air stoichiometric premixed flames at relatively high Reynolds numbers. Flame merging and the creation of pockets of both products and reactants were detected, and a very strong interaction between the flame front and the vortex structures was suggested in the simultaneous PIV/PLIF images. Double-pulsed PLIF data obtained for different time delays allowed statistical study of flame development. Three-dimensional turbulent fluxes of mean progress variable were obtained. It was shown that the fluxes obey the gradient diffusion hypothesis. The proposed diagnostic increases flexibility and range of measurements available for premixed flames.     

Large eddy simulation of soot formation in a turbulent non-premixed jet flame

A recently developed subgrid model for soot dynamics [H. El-Asrag, T. Lu, C.K. Law, S. Menon, Combust. Flame 150 (2007) 108-126] is used to study the soot formation in a non-premixed turbulent flame. The model allows coupling between reaction, diffusion and soot (including soot diffusion and thermophoretic forces) processes in the subgrid domain without requiring ad hoc filtering or model parameter adjustments. The combined model includes the entire process, from the initial phase, when the soot nucleus diameter is much smaller than the mean free path, to the final phase, after coagulation and aggregation, where it can be considered in the continuum regime. A relatively detailed but reduced kinetics for ethylene-air is used to simulate an experimentally studied non-premixed ethylene/air jet diffusion flame. Acetylene is used as a soot precursor species. The soot volume fraction order of magnitude, the location of its maxima, and the soot particle size distribution are all captured reasonably. Along the centerline, an initial region dominated by nucleation and surface growth is established followed by an oxidation region. The diffusion effect is found to be most important in the nucleation regime, while the thermophoretic forces become more influential downstream of the potential core in the oxidation zone. The particle size distribution shows a log-normal distribution in the nucleation region, and a more Gaussian like distribution further downstream. Limitations of the current approach and possible solution strategies are also discussed.     

The effect of hydrogen addition on biogas non-premixed jet flame stability in a co-flowing air stream

An investigation of the stability limits of biogas jet non-premixed (diffusion) flames in a co-flowing air stream was conducted. The stability limits were determined experimentally for two different methane–carbon dioxide mixtures that represent the typical biogas composition. Moreover, the effect of jet nozzle diameter was also investigated. It was found that with the presence of a significant amount of CO₂ in the fuel, the stability limits were very low and the flames can only be stabilized over a very small range of co-flowing air velocities. As expected, an increase in carbon dioxide concentration resulted in the narrowing of the region for stable flames. However, it was shown that the flame stability of such mixtures can be enhanced very significantly over a much wider range of co-flowing air velocities by introducing a small amount of hydrogen into the fuel. Results obtained in the current experimental setup indicate that an increase in the stability limits by approximately four-fold when 10% (by vol.) of hydrogen is added under the same operating conditions. The effect of the addition of hydrogen on the enhancement of biogas stability is most significant with a 10% initial addition. The degree of enhancement diminishes with further increases in hydrogen addition from 10% to 30%.     

Effects of Damköhler number on flame extinction and reignition in turbulent non-premixed flames using DNS

Results from a parametric study of flame extinction and reignition with varying Damköhler number using direct numerical simulation are presented. Three planar, non-premixed ethylene jet flames were simulated at a constant Reynolds number of 5120. The fuel and oxidizer stream compositions were varied to adjust the steady laminar extinction scalar dissipation rate, while maintaining constant flow and geometric conditions. Peak flame extinction varies from approximately 40% to nearly global blowout as the Damköhler number decreases. The degree of extinction significantly affects the development of the jets and the degree of mixing of fuel, oxidizer, and combustion products prior to reignition. The global characteristics of the flames are presented along with an analysis of the modes of reignition. It is found that the initially non-premixed flame undergoing nearly global extinction reignites through premixed flame propagation in a highly stratified mixture. A progress variable is defined and a budget of key terms in its transport equation is presented.     

Stability characteristics of non-premixed turbulent jet flames of hydrogen and syngas blends with coaxial air

The stability characteristics of attached hydrogen (H₂) and syngas (H₂/CO) turbulent jet flames with coaxial air were studied experimentally. The flame stability was investigated by varying the fuel and air stream velocities. Effects of the coaxial nozzle diameter, fuel nozzle lip thickness and syngas fuel composition are addressed in detail. The detachment stability limit of the syngas single jet flame was found to decrease with increasing amount of carbon monoxide in the fuel. For jet flames with coaxial air, the critical coaxial air velocity leading to flame detachment first increases with increasing fuel jet velocity and subsequently decreases. This non-monotonic trend appears for all syngas composition herein investigated (50/50 → 100/0% H₂/CO). OH^∗ chemiluminescence imaging was performed to qualitatively identify the mechanisms responsible for the flame detachment. For all fuel compositions, local extinction close to the burner rim is observed at lower fuel velocities (ascending stability limit), while local flame extinction downstream of the burner rim is observed at higher fuel velocities (descending stability limit). Extrema of the non-monotonic trends appear to be identical when the nozzle fuel velocity is normalized by the critical fuel velocity obtained for the single jet cases.     

Acoustic excitation effect on NOx reduction and flame stability in a lifted non-premixed turbulent hydrogen jet with coaxial air

The effects of acoustic excitation on the reduction in nitric oxidant (NO_x) emission were experimentally investigated in non-premixed lifted hydrogen jet flames with coaxial air. The purpose of the present work was to analyze the acoustic forcing effect on the flow field, the reaction zone, and NO_x emission, and to study the mechanisms of NO_x reduction and flame stabilization. To analyze of the flow field, a PIV method was used that incorporated two Nd-YAG lasers and a CCD camera. The reaction zone was visualized by taking OH* chemiluminescence images with a 307.1 ± 5 nm narrow band pass filter and an ICCD camera. A flow condition was carefully selected at u_F = 150, 200, 250 m/s and u_A = 12, 16, 20 m/s, which was sustainable for acoustic excitation in a lifted flame region. The frequency was swept from 150 to 1000 Hz in 5 Hz steps. From the measurements of the flow field, the reaction zone, and NO_x emission, we concluded that NO_x emission was reduced and minimized at the resonance frequency. The vortex that was generated by acoustic forcing promoted air entrainment and enhanced the fuel-air mixing rate. This premixing effect resulted in a lower flame temperature, and thus lower NO_x emissions. In addition, the liftoff height periodically fluctuated due to the stretch effect as the vortex interacted with the flame base.     

Formation,growth, and transport of soot in a three-dimensional turbulent non-premixed jet flame

The formation, growth, and transport of soot is investigated via large scale numerical simulation in a three-dimensional turbulent non-premixed n-heptane/air jet flame at a jet Reynolds number of 15,000. For the first time, a detailed chemical mechanism, which includes the soot precursor naphthalene and a high-order method of moments are employed in a three-dimensional simulation of a turbulent sooting flame. The results are used to discuss the interaction of turbulence, chemistry, and the formation of soot. Compared to temperature and other species controlled by oxidation chemistry, naphthalene is found to be affected more significantly by the scalar dissipation rate. While the mixture fraction and temperature fields show fairly smooth spatial and temporal variations, the sensitivity of naphthalene to turbulent mixing causes large inhomogeneities in the precursor fields, which in turn generate even stronger intermittency in the soot fields. A strong correlation is apparent between soot number density and the concentration of naphthalene. On the contrary, while soot mass fraction is usually large where naphthalene is present, pockets of fluid with large soot mass are also frequent in regions with very low naphthalene mass fraction values. From the analysis of Lagrangian statistics, it is shown that soot nucleates and grows mainly in a layer close to the flame and spreads on the rich side of the flame due to the fluctuating mixing field, resulting in more than half of the total soot mass being located at mixture fractions larger than 0.6. Only a small fraction of soot is transported towards the flame and is completely oxidized in the vicinity of the stoichiometric surface. These results show the leading order effects of turbulent mixing in controlling the dynamics of soot in turbulent flames. Finally, given the difficulties in obtaining quantitative data in experiments of turbulent sooting flames, this simulation provides valuable data to guide the development of models for Large Eddy Simulation and Reynolds Average Navier Stokes approaches.     

Investigations on the self-excited oscillations in a kerosene spray flame

A laboratory scale gas turbine type burner at atmospheric pressure and with air preheat was operated with aviation kerosene Jet-A1 injected from a pressure atomiser. Self-excited oscillations were observed and analysed to understand better the relationship between the spray and thermo-acoustic oscillations. The fluctuations of CH^∗ chemiluminescence measured simultaneously with the pressure were used to determine the flame transfer function. The Mie scattering technique was used to record spray fluctuations in reacting conditions with a high speed camera. Integrating the Mie intensity over the imaged region gave a temporal signal acquired simultaneously with pressure fluctuations and the transfer function between the light scattered from the spray and the velocity fluctuations in the plenum was evaluated. Phase Doppler anemometry was used for axial velocity and drop size measurements at different positions downstream the injection plane and for various operating conditions. Pressure spectra showed peaks at a frequency that changed with air mass flow rate. The peak for low air mass flow rate operation was at 220 Hz and was associated with a resonance of the supply plenum. At the same global equivalence ratio but at high air mass flow rates, the pressure spectrum peak was at 323 Hz, a combustion chamber resonant frequency. At low air flow rates, the spray fluctuation motion was pronounced and followed the frequency of the pressure oscillation. At high air flow rates, more effective evaporation resulted in a complete disappearance of droplets at an axial distance of about 1/3 burner diameters from the injection plane, leading to a different flame transfer function and frequency of the self-excited oscillation. The results highlight the sensitivity of the self-excited oscillation to the degree of mixing achieved before the main recirculation zone.     

Fire dynamics simulation of a turbulent buoyant flame using a mixture-fraction-based combustion model

Fire dynamics simulations of a 7.1-cm buoyant turbulent diffusion flame were performed using a mixture-fraction-based combustion model. In our previous work, good agreement between the measured and the calculated fire flow field was achieved with carefully selected domain and grid sizes using a Lagrangian thermal-element combustion model. The Lagrangian thermal-element model exhibits qualitative as well as quantitative differences in the measured and calculated temperature profiles in the flame zone. The number of Lagrangian thermal elements must be carefully selected and the model is not designed to provide insights into the species distributions in the fire. To address these issues, a mixture-fraction-based combustion model was used in the present work. The domain and grid size dependence using this model are documented. Comparisons between the measured and the calculated velocities, mixture fractions and temperatures show that the mixture-fraction-based combustion model captures the qualitative and quantitative fire behavior very well.     

Analysis of entropy generation in a hydrogen-enriched turbulent non-premixed flame

The analysis of local entropy generation and exergy loss was performed in a turbulent non-premixed H₂-enriched CH₄–air bluff-body flame. Detailed chemical kinetic, transport properties, and turbulence-chemistry interaction were taken into account in using laminar flamelet model for the simulation of combustion process via an in-house, finite volume code. The analysis was based on local entropy generation calculation. Results showed that thermal conduction made the most contribution to entropy generation followed by chemical reaction and mass diffusion, while the contribution of viscous dissipation was negligible. Entropy generation resulting from thermal conduction occurs in a large volume of the domain, while entropy generation resulting from chemical reaction and mass diffusion occurs only near the bluff surface. The effect of H₂ addition to fuel and air preheating on the entropy generation rate was investigated. It was observed that entropy generation and exergy loss were decreased by H₂ addition, mainly due to a decrease in the chemical reaction component of entropy generation, while entropy generation resulting from thermal conduction slightly increased and entropy generation resulting from mass diffusion remained almost constant. Entropy generation resulting from heat conduction by preheating combustion air decreased, while entropy generation resulting from chemical reaction and mass diffusion remained almost constant. The decrease of thermal conduction contribution in entropy generation is so significant that, by preheating air up to 750 K in the case of pure CH₄, chemical reaction becomes the main source of irreversibility. These investigations show that H₂ addition and preheating the combustion air both lead to the improvement of the second law efficiency, although the second law efficiency is more sensitive to flame structure and air temperature.     

Large Eddy Simulations of forced ignition of a non-premixed bluff-body methane flame with Conditional Moment Closure

Large Eddy Simulations (LES) of forced ignition of a bluff-body stabilised non-premixed methane flame using the Conditional Moment Closure (CMC) turbulent combustion model have been performed. The aim is to investigate the feasibility of the use of CMC/LES for ignition problems and to examine which, if any, of the characteristics already observed in related experiments could be predicted. A three-dimensional formulation of the CMC equation was used with simple and detailed chemical mechanisms and sparks with different parameters (location, size) were used. It was found that the correct pattern of flame expansion and overall flame appearance were predicted with reasonable accuracy with both mechanisms, but the detailed mechanism resulted in expansion rates closer to the experiment. Moreover, the distribution of OH was predicted qualitatively accurately, with patches of high and low concentration in the recirculation zone during the ignition transient, consistent with experimental data. The location of the spark relative to the recirculation zone was found to determine the pattern of the flame propagation and the total time for the flame stabilisation. The size was also an important parameter, since it was found that the flame extinguishes when the spark is very small, in agreement with expectations from experiment. The stabilisation mechanism of the flame was dominated by the convection and sub-grid scale diffusion of hot combustion products from the recirculation zone to the cold gases that enter the burner, as revealed by analysis of the CMC equation.     

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.     

Influence of CH4/air injection location on non-premixed flame blow-off limits in a novel micro-combustor

Stable flame is still one of the challenging issues in the micro-combustors. Besides, the premixed flame may flash back under the large equivalence ratio, and the non-premixed combustion is an effective way to avoid the flame flash-back issue. In this work, a novel non-premixed CH₄/air micro-combustor with the flame holder, the backward-facing steps and the separated preheating channels is developed to avoid the flame flash-back issue and extend the flame blow-off limits. This paper investigates the influence of two different CH₄/air injection locations on the flame blow-off limits numerically and profoundly reveals the mechanisms in this micro-combustor. Results show that this novel configuration can considerably anchor the flame without the flame flash-back issue and significantly extend flammability owning to the flow & heat recirculations. Compared with the “Air_(out)/CH_4(in)” injection location, the “Air_(in)/CH_4(out)” injection location exhibits a comparatively larger size of recirculation, causing a more vital anchorage ability. In addition, after the preheating in the preheating channels, the CH₄/air mixture of the “Air_(in)/CH_4(out)” injection location presents a higher temperature level than that of the “Air_(out)/CH_4(in)” injection location. Consequently, compared to the “Air_(out)/CH_4(in)” injection location, the larger blow-off limits are attained for the “Air_(in)/CH_4(out)” injection location. These results provide new insights into the stable non-premixed flame and give valuable guidance for designing the similar non-premixed micro-combustor.     

Effect of syngas fuel compositions on the occurrence of instability of laminar diffusion flame

The paper presents a numerical investigation of the critical roles played by the chemical compositions of syngas on laminar diffusion flame instabilities. Three different flame phenomena – stable, flickering and tip-cutting – are formulated by varying the syngas fuel rate from 0.2 to 1.4 SLPM. Following the satisfactory validation of numerical results with Darabkhani et al. [1], the study explored the consequence of each species (H₂, CO, CH₄, CO₂, N₂) in the syngas composition. It is found that low H₂:CO has a higher level of instability, which however does not rise any further when the ratio is less than 1. Interestingly, CO encourages the heat generation with less fluctuation while H₂ plays another significant role in the increase of flame temperature and its fluctuation. Diluting CH₄ into syngas further increases the instability level as well as the fluctuation of heat generation significantly. However, an opposite effect is found from the same action with either CO₂ or N₂. Finally, considering the heat generation and flame stability, the highest performance is obtained from 25%H₂+75%CO (81 W), followed by EQ+20%CO₂, and EQ+20%N₂ (78 W).