Lean Premixed Flames Research Articles

Efficient emission modelling in lean premixed flames with pre-tabulated formation characteristics

It is practically important to accurately predict NOx and CO emissions in lean premixed flames for the development of fuel-efficient and low-emission combustion systems. In this study, the efficient modelling with pre-tabulated formation characteristics of NOx and CO is systemically investigated. Their formation characteristics for turbulent flames in the flamelet and thin reaction zone regimes are first quantified with one-dimensional adiabatic premixed flames through the incorporation of turbulence induced diffusion. Results show that the formation of NOx and CO can be characterized by two different stages, i.e., a rapid increase of both in flame front and the linear growth for NOx and exponential decay for CO in post-flame zone. An efficient NOx and CO modelling approach is formulated and demonstrated in a full-scale methane gas turbine combustor, in which species NOx and CO are transported and solved, with the source terms being modelled by pre-tabulated formation characteristics from one-dimensional premixed flames with detailed chemical kinetics. Realizable k-epsilon model and finite rate/eddy dissipation model in conjunction with a two-step global mechanism are employed to primarily predict the flow and flame characteristics. Results show that the predicted NOx and CO agree with experimental measurements over a wide range of equivalence ratios. The discrepancy in NOx emission can be accounted by fuel/air unmixedness. For CO, the rapid increase due to the incomplete CO oxidation resulting from its increasing characteristic oxidation time near lean blow-out (LBO) is correctly captured. It is further shown that the predicted CO emission is sensitive to combustion/kinetics model parameters and the accurate prediction of flame shape and dynamics is crucial to capture the trend of CO formation near LBO.

Effects of oxy jet in cross-flow on the combustion instability and NOx emissions in lean premixed flame

Combustion instability and NOx emission are crucial factors for modern gas turbine combustors, which seriously hampers the research and development of advanced combustors. To eliminate combustion instability and NOx emissions simultaneously, effects of the oxy (CO2/O2, N2/O2, Ar/O2, and He/O2) jet in cross-flow on combustion instability and NOx emissions are experimentally studied. In this research, the flow rate and oxygen ratio of the combustor are varied to evaluate the control effectiveness. Results denotes that all the four oxyfuel gas: CO2/O2, N2/O2, Ar/O2, and He/O2, could suppress combustion instability and NOx emissions. The CO2/O2 dilution can achieve a better damping results than the other three cases. There are peak values or lowest points of sound pressure amplitude as the parameter of oxy jet in cross-flow changes. Mode transition appears in both acoustic signal and CH* chemiluminescence of the flame. But the turning point of mode transition is different. Under the CO2/O2 cases, the NOx emission decreases from 22.3 ppm to 15.2 ppm, the damping ratio of NOx is 40.39%. The flame shape and length were changed under different jet in cross-flow dilutions. This research could promote the application of jet in cross-flow methods on combustion instability or pollutant emissions in gas turbines.

Large Eddy Simulation of Lean Mixed-Mode Combustion Assisted by Partial Fuel Stratification in a Spark-Ignition Engine

Abstract Partial fuel stratification (PFS) is a promising fuel injection strategy to improve the stability of lean combustion by applying a small amount of pilot injection right before spark timing. Mixed-mode combustion, which makes use of end-gas autoignition following conventional deflagration-based combustion, can be further utilized to speed up the overall combustion. In this study, PFS-assisted mixed-mode combustion in a lean-burn direct injection spark-ignition (DISI) engine is numerically investigated using multi-cycle large eddy simulation (LES). A previously developed hybrid G-equation/well-stirred reactor combustion model for the well-mixed operation is extended to the PFS-assisted operation. The experimental spray morphology is employed to derive spray model parameters for the pilot injection. The LES-based model is validated against experimental data and is further compared with the Reynolds-averaged Navier–Stokes (RANS)-based model. Overall, both RANS and LES predict the mean pressure and heat release rate traces well, while LES outperforms RANS in capturing the cycle-to-cycle variation (CCV) and the combustion phasing in the mass burned space. Liquid and vapor penetrations obtained from the simulations agree reasonably well with the experiment. Detailed flame structures predicted from the simulations reveal the transition from a sooting diffusion flame to a lean premixed flame, which is consistent with experimental findings. LES captures more wrinkled and stretched flames than RANS. Finally, the LES model is employed to investigate the impacts of fuel properties, including heat of vaporization (HoV) and laminar burning speed (SL). Combustion phasing is found more sensitive to SL than to HoV, with a larger fuel property sensitivity of the heat release rate from autoignition than that from deflagration. Moreover, the combustion phasing in the PFS-assisted operation is shown to be less sensitive to SL compared with the well-mixed operation.

The effect of swirl burner design configuration on combustion and emission characteristics of lean pre-vaporized premixed flames

The present paper experimentally provides a quantitative comparison between the aerodynamic flow field at non-reaction conditions (cold flow) and the combustion and emission characteristics (hot conditions) of two swirl burners having the same swirl number of 0.55 but with two different configurations. The first burner features an annular outer swirler (swirl angle of 40°) and multiple central circular jets, leading to “low swirl combustion”, whereas the second burner has a blocked central passage and an outer swirler (swirl angle of 35°) resulting in “high swirl combustion”; featuring a central recirculation zone. The cold flow study is conducted using a calibrated five-hole probe while the combustion and emission characteristics are obtained via measurements of flame temperatures and the dry volumetric analysis of CO and NOx emissions. Each burner is coaxially mounted to a vertical cylindrical combustor; inside which a lean (equivalence ratio: ϕ = 0.75), pre-vaporized (preheated to a temperature of 523 K) partially premixed mixture of Jet A-1 fuel and air are admitted. The former low swirl combustion burner gives a bulk stable and uniform reaction zone (W-shape) close to the burner exit with a very slight lifting; confirming the concept of “stable lifting” while reducing the pressure drop across the burner exit and eliminating hot spots in the reaction zone. The high swirl combustion burner exhibits a strong central recirculation zone (better flame stability) that is surrounded by an outer V-shape reaction zone having comparatively higher peak levels of temperature, CO and NOx that would lead to greater liability of higher exhaust emission.

Large-Eddy Simulation of a Lifted High-Pressure Jet-Flame with Direct Chemistry

ABSTRACT A large-eddy simulation is presented of a challenging high-pressure jet flame case that is representative of state of the art, dry low-NO x and low-CO real gas turbine combustion. A reaction scheme is developed for lifted lean premixed high pressure methane jet flames, and tested by three-dimensional large-eddy simulation of an experiment, for which very detailed data are available. Auto-ignition-delay times of different mixtures of fresh gas and products have been introduced as a novel optimization criterion for the mechanism development. The new mechanism has been developed by a genetic algorithm-based reduction and optimization, and consists of 15 species and 18 reactions. The large-eddy simulations are performed using a finite rate chemistry (FRC) approach and the dynamic thickened flame (DTF) model to investigate a lifted jet flame at high pressure in a gas turbine model combustor. In the simulations, the novel mechanism is compared to a similar mechanism that was generated without this criterion, and the well-established Lu19 mechanism. With the new mechanism, the LES predicts the flame as accurately as with Lu19, at a significantly lower cost. Further post processing with Lagrangian tracer particles confirmed that ignition events occur in the region corresponding to the liftoff height estimated in the experiment, which is corroborated by a chemical explosive mode analysis (CEMA). Overall, the newly developed mechanism with the novel optimization criterion was found to provide a better agreement with the experiments than previous mechanisms of similar cost, or a comparable agreement to a mechanism of significantly higher cost.

Modelling of a Bluff-Body Stabilised Premixed Flames Close to Blow-Off

As emission legislation becomes more stringent, the modelling of turbulent lean premixed combustion is becoming an essential tool for designing efficient and environmentally friendly combustion systems. However, to predict emissions, reliable predictive models are required. Among the promising methods capable of predicting pollutant emissions with a long chemical time scale, such as nitrogen oxides (NOx), is conditional moment closure (CMC). However, the practical application of this method to turbulent premixed flames depends on the precision of the conditional scalar dissipation rate,ζ ⟨Nc|ζ⟩, model. In this study, an alternative closure for this term is implemented in the RANS-CMC method. The method is validated against the velocity, temperature, and gas composition measurements of lean premixed flames close to blow-off, within the limit of computational fluid dynamic (CFD) capability. Acceptable agreement is achieved between the predicted and measured values near the burner, with an average error of 15%. The model reproduces the flame characteristics; some discrepancies are found within the recirculation region due to significant turbulence intensity.

Lean blow-out prediction in an industrial gas turbine combustor through a LES-based CFD analysis

The use of lean premixed flames to control combustion temperature and limit nitric oxides (NOx) formation is a common practice in all the applications of land-based gas turbines irrespective of their size and power production capacity. Despite the capability of lean premixed combustion to nominally reduce NOx emissions well below the most strict international regulations, such technology still requires improvements to overcome the impact on whole engine operability due to possible onset of combustion instabilities and flame lean blow-out (LBO). Both phenomena involve highly unsteady processes resulting in challenging numerical modeling. Particularly critical is the prediction of lean blow-out mechanisms where the role of finite rate chemistry and turbulence cannot be easily simplified. The current study aims at proving the capability of a CFD methodology based on Large Eddy Simulation (LES) and on an extended version of the Turbulent Flame Closure (TFC), to describe and properly catching the dynamics of flame extinction at the leanest operating conditions in typical turbulent lean premixed flames adopted in gas turbine combustors. The methodology is first validated on a laboratory test case where detailed experimental results are available. The extended TFC model well describes the LBO process observed in an atmospheric swirl stabilized burner investigated at the University of Cambridge. The proposed approach is then applied to the investigation of a real scale combustor operated with a swirl lean burner, representing the standard device adopted by Baker-Hughes on the heavy-duty 16 MW class engine Nova LT16. An accelerated numerical procedure is used to trigger the LBO, considering both constant firing temperature and constant pilot split conditions. An excellent agreement is observed with respect to the data obtained on a full-annular test rig, confirming the validity of the developed LBO modeling strategy which can be used in design phase to improve the overall engine operability.

LES of a turbulent swirl flame using a mesh adaptive level-set method with dynamic load balancing

A turbulent lean premixed swirl flame is numerically investigated by solving the Navier–Stokes equations with a finite-volume large-eddy simulation (LES) method. A combined G-equation progress variable approach is applied to model the combustion process using a solution adaptive level-set solver. The finite-volume and the level-set solver are parallelized and coupled on a joint hierarchical Cartesian mesh, where each solver can individually use and adapt a subset of the mesh cells. The level-set solver adapts the mesh locally in a band close to the flame front location to automatically satisfy the high accuracy requirements of the level-set solution. The numerical results for the turbulent swirl flame are in excellent agreement with experimental findings. Since the flame shape varies during the computation, the number of cells associated with the flame shifts between the individual subdomains which changes the workload distribution on the parallel computing processes. To achieve a high parallel efficiency, a dynamic load balancing method is applied which determines a new partitioning of the grid using estimated cell weights based on measured computing times to redistribute the cells among all subdomains accordingly. The efficiency of the dynamic load balancing scheme to reduce load imbalances is demonstrated for a large-scale simulation of the investigated swirl flame on 170,000 compute cores. That is, the dynamic load balancing scheme reduces the computing time of this simulation by approximately 30%.

Global hydrodynamic instability and blowoff dynamics of a bluff-body stabilized lean-premixed flame

We experimentally study the hydrodynamic instability of a lean-premixed flame stabilized behind a circular cylinder. On reducing the equivalence ratio (ϕ) at a fixed Reynolds number (ReD), we find that the flame transitions from a steady mode to a varicose mode and then to a sinuous mode. By examining time-resolved CH* chemiluminescence images and analyzing how the Strouhal number scales with ReD, we determine that the varicose mode is convectively unstable, maintained by the amplification of disturbances in the turbulent base flow, whereas the sinuous mode is globally unstable as a result of the constructive interaction between the two diametrically opposite shear layers (Bénard–von Kármán instability). We attribute the emergence of the sinuous global mode to the flame moving sufficiently far downstream with decreasing ϕ that it is out of the wavemaker region. Finally, we investigate the lean blowoff dynamics and find that local flame pinch-off, which occurs at the end of the recirculation zone, is a reliable precursor of global flame blowoff.

Combustion characteristics of n-heptane spray combustion in a low temperature reform gas/air environment

This paper presents a large eddy simulation study of n-heptane spray combustion in an n-heptane low temperature reform (LTR) gas environment in a constant volume combustion chamber, under conditions relevant to single-fuel reactivity controlled compression ignition (RCCI) combustion engines. The LTR gas is made up of partially oxidized intermediate species from rich n-heptane/air mixture in an external constant temperature reformer. It is found that a higher reform temperature results in a longer ignition delay time of the n-heptane spray and a higher liftoff length, due to the chemical effect of the LTR gas and the difference in the reaction zone structures. A significantly different spray flame structure is identified in the RCCI case from that of single-fuel spray combustion. After the onset of high temperature ignition, a double-layer flame structure is established in the RCCI case, with a diffusion flame layer and a lean premixed flame layer. The lean premixed flame affects the flow field, which significantly suppresses the mixing around the spray tip. As a result, the RCCI case exhibits a lower NOx formation but a higher soot formation than the single-fuel case.

Dilution effects of CO2, Ar, N2 and He microjets on the combustion dynamic and emission characteristics of unsteady premixed flame

To evaluate the control effectiveness of jet in cross flow on thermoacoustic instability of lean premixed flame, the dilution effects of CO2, Ar, N2 and He microjets on the dynamic and emission characteristics of premixed flame were experimentally studied in a laboratory-scale gas turbine combustor. Three factors of jet in cross flow were investigated—Injection flow rate, inner diameter of injector and injecting medium. CO2 performs better than other three gas medium in both combustion instability and NOx emissions control. Results denote that an optimal sound pressure damping ratio can be reached as 80.3% with injection flow rate of 3.5 L/min. And the damping ratio of heat release rate can be as high as 75%. NOx emissions concentration can be reduced to 15.9 ppm. The NOx emission damping ratio can be as high as 35.4%. Flame becomes dispersed and flat under microjets with high flow rate. The flame length is reduced to 54 mm when both the sound pressure amplitude and the NOx emission concentration are all at the bottom, Flame length after passive control only equals to 40% of the original. The results confirmed that suppression effectiveness of combustion instability is associated with the relative molecular masses of the jet in cross flow. However, the detailed triggering mechanism and the coupling of combustion instability with jet in cross flow still needs exploration. This study demonstrated that the passive control with microjets has the potential to be applied to combustion systems under combustion instability.

Validation of leading point concept in RANS simulations of highly turbulent lean syngas-air flames with well-pronounced diffusional-thermal effects

While significant increase in turbulent burning rate in lean premixed flames of hydrogen or hydrogen-containing fuel blends is well documented in various experiments and can be explained by highlighting local diffusional-thermal effects, capabilities of the vast majority of available models of turbulent combustion for predicting this increase have not yet been documented in numerical simulations. To fill this knowledge gap, a well-validated Turbulent Flame Closure (TFC) model of the influence of turbulence on premixed combustion, which, however, does not address the diffusional-thermal effects, is combined with the leading point concept, which highlights strongly perturbed leading flame kernels whose local structure and burning rate are significantly affected by the diffusional-thermal effects. More specifically, within the framework of the leading point concept, local consumption velocity is computed in extremely strained laminar flames by adopting detailed combustion chemistry and, subsequently, the computed velocity is used as an input parameter of the TFC model. The combined model is tested in RANS simulations of highly turbulent, lean syngas-air flames that were experimentally investigated at Georgia Tech. The tests are performed for four different values of the inlet rms turbulent velocities, different turbulence length scales, normal and elevated (up to 10 atm) pressures, various H2/CO ratios ranging from 30/70 to 90/10, and various equivalence ratios ranging from 0.40 to 0.80. All in all, the performed 33 tests indicate that the studied combination of the leading point concept and the TFC model can predict well-pronounced diffusional-thermal effects in lean highly turbulent syngas-air flames, with these results being obtained using the same value of a single constant of the combined model in all cases. In particular, the model well predicts a significant increase in the bulk turbulent consumption velocity when increasing the H2/CO ratio but retaining the same value of the laminar flame speed.

Numerical study of the lean premixed PRECCINSTA burner with hydrogen enrichment

Hydrogen combustion is one of the most promising solution to achieve a global decarbonization in power production and transports. Pure hydrogen combustion is far from becoming a standard but, during the energy transition, hydrogen co-firing can be a feasible and economically attractive shortterm measure. The use of hydrogen blending gives rise to several issues related to flashback, NOx emissions and thermo-acoustic instabilities. To improve the understanding of the effect of hydrogen enrichment, herein a numerical analysis of lean premixed hydrogen enriched flames is performed by means of 3D unsteady CFD simulations. The numerical model has been assessed against experimental results for both cold and reacting flows in terms of velocity profile (average) and flame shape (mean OH* radical fields). The burner under investigation is the swirl stabilized PRECCINSTA studied at the Deutsches Zentrum für Luft-und Raumfahrt (DLR). The DLR’s researchers have shown the effect of hydrogen addition on the flame topology and combustion instabilities at various operating conditions in terms of thermal power, equivalence ratio and H2 volume fraction. Simulations are in good accordance with experimental data both in terms of velocity and temperature profiles. The numerical model provides a qualitative estimation of the flame shape.

Erratum to "impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame"

Erratum to "impact of burner plenum acoustics on the sound emission of a turbulent lean premixed open flame"

The shape and fluctuation of hydrogen-propane-butane-air lean premixed flames formed on a flat flame burner

The shape and fluctuation of hydrogen-propane-butane-air lean premixed flames formed on a flat flame burner

The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under the adiabatic and non-adiabatic conditions

The effects of steam addition on the unstable behavior of hydrogen-air lean premixed flames under the adiabatic and non-adiabatic conditions were investigated using numerical calculations. We adopted the detailed chemical reaction mechanism for hydrogen-oxygen combustion, modeled with seventeen reversible reactions of eight reactive species and a diluent. Two-dimensional unsteady reactive flow was treated, based on the compressible Navier-Stokes equations. The burning velocity of a planar flame decreased as the steam addition and heat loss increased. A sufficiently small disturbance was superimposed on a planar flame to study the characteristics of intrinsic instability. We obtained the relation between the growth rate and wave number, i.e. the dispersion relation, and the linearly most unstable wavelength, i.e. the critical wavelength. As the steam addition increased, the unstable range became narrower, and the critical wavelength became longer. Taking account of heat loss, we obtained smaller growth rates and narrower unstable range. The superimposed disturbance developed owing to intrinsic instability, and then the cellular shape of flame fronts appeared. In cellular flames, compared with planar flames, high (low) concentration of active chemical species was found in downstream of convex (concave) fronts. This was caused by high (low) temperature at convex (concave) fronts due to the diffusive-thermal effects. The concentration of active chemical species became lower with increasing the steam addition and heat loss, which was because of the reduction of flame temperature. Moreover, the burning velocity of a cellular flame increased monotonically with an increase in the scale of premixed flames. The burning velocity of a cellular flame normalized by that of a planar flame increased as the steam addition and heat loss increased. The numerical results denoted that the steam addition and heat loss had a great influence on the unstable behavior of hydrogen-air lean premixed flames.

Effects of pressure and heat loss on the unstable motion of cellular-flame fronts caused by intrinsic instability in hydrogen-air lean premixed flames

Effects of pressure and heat loss on the unstable motion of cellular-flame fronts caused by intrinsic instability in hydrogen-air lean premixed flames

Flame-resolved transient simulation with swirler-induced turbulence applied to lean blowoff premixed flame experiment

This article presents a flame-resolved transient simulation of the Cavaliere et al premixed flame experiment [1] to investigate the mechanisms that lead towards lean blowoff/blowout (LBO). The computational domain includes the swirler to capture the unsteady turbulent 3D velocity field generated. The computational grid design intent is to minimize the use of the subgrid scale models in order to resolve most of the turbulence scales in both the fresh and the burnt gases as well as the flame front thickness throughout the computational domain, resulting in a structured mesh with a grid count of 236 million cells. A transient sequence corresponding to a step change of equivalence ratio Φ from 0.7 to 0.55 is modeled. The combustion chemistry mechanism consists of a single step overall reaction. The computational results are compared to experimental data and an overall good agreement is observed, except for the latest times of the LBO sequence. The validated computational results are analyzed with dynamic mode decomposition (DMD) technique, flow field visualizations, signals time-traces and space-time diagrams of a posteriori reconstructed OH fields to investigate the underlying mechanism leading to lean blowoff for this flame. In addition, an evaluation of known state of the art LBO mechanisms reported in literature is carried out. It includes the precessing vortex core, flame sheet holes, inner recirculation zone (IRZ) dynamics, and heat losses. The analysis shows that a key phenomenon leading to lean blowout for the present configuration is associated with the convective motion of cooler combustion products into the IRZ as the equivalence ratio is decreased.

Thermoacoustic instability analysis of a laminar lean premixed flame under autoignitive conditions

To address engineering challenges encountered in developing advanced gas turbine combustors, theoretical analysis and numerical simulation were performed to investigate the thermoacoustic instability of a laminar lean premixed flame under autoignitive conditions. The analysis was carried out within the scope of weak autoignition tendencies and employed a three-step mechanism, which included the chain-initiation reaction. The numerical simulation was performed as a validation using a skeletal mechanism for diesel fuel under the typical operating conditions of gas turbines. The results showed that the three-step mechanism could qualitatively describe the high-temperature chemistry involved in autoignition and flame propagation. The analysis indicated that the flame propagation speed was accelerated by the accumulation of pre-flame reactions. Under certain conditions, the flame speed showed a linear correlation with the pre-flame length. The results were consistent with the existing literature. In addition, the equilibrium position of the flame oscillation, which was excited by velocity fluctuation, could return to the initial position, and pre-flame reactions could alter the phase delay. These results suggested a new mechanism for thermoacoustic instability because the overall heat release rate fluctuated with the flame front. The flame might amplify, damp, high-pass filter, or low-pass filter the acoustic energy based on the phase difference between the velocity and pressure oscillations.

Experimental study of compact swirl flames with lean premixed CH4/H2/air mixtures at stable and near blow-off conditions

An experimental study on the compact lean premixed flames stabilized on a bluff-body and swirl burner is performed. The flames are featured with a strengthened Inner Recirculation Zone (IRZ). The objective is to identify the critical causes of blow-off near the lean limit. Meanwhile, the effects of hydrogen addition on the flame stabilization are also examined. Premixed CH4/H2/air mixtures are adopted with varied hydrogen fractions up to 80%. Flames far from and close to the blow-off conditions are both obtained for comparison. Simultaneous PIV/OH-PLIF measurements are performed, and flow-flame dynamics are analyzed. Results show that the flow straining near the flame root is dominant to induce the root extinction and initiate the blow-off. The strain is observed to mainly result from the shear deformation. The weakened root burning intensity is demonstrated with the CH* chemiluminescence. With hydrogen addition, the flame root is strengthened due to the enhanced resistance to the strain. The increased burning velocity and temperature with hydrogen are also beneficial for the flame stabilization. The inner shear layer vortices are anticipated to promote the blow-off by convecting cold unburned mixture into the IRZ.