Non-premixed Mode Research Articles

Numerical investigation on the influence of swirl number to high-temperature zone evolution and outlet temperature distribution in multi-stage combustor

The outlet temperature distribution plays a crucial role in determining the lifespan and stability of downstream thermal components. It poses a significant challenge in the design and regulation of high-temperature combustors. This study aims to investigate the influence of swirl number on the outlet temperature distribution in multi-stage combustor. The impact of varying fourth-stage swirl numbers on flow structure, hot streaks, and temperature distribution is compared and analyzed. The results indicate that increasing the swirl number significantly improves the air–fuel mixing within the combustion zone, leading to the expansion of high-temperature regions. Additionally, the high-temperature region migrates upstream along the primary zone. With an increase in the swirl number, the relative area of hot spots within the combustor also increases. Remarkably, the swirler configuration with a swirl number of 1.5 exhibits the most minimal exit temperature gradient and the highest level of temperature distribution uniformity, quantified by an outlet temperature distribution factor (OTDF) of 0.26. Furthermore, changes in the swirl number affect the distribution of combustion modes within the combustor. Increasing the swirl number promotes the premixed combustion mode at the pilot stage exit and enhances temperature uniformity at the swirler exit. Thorough comprehension of the temperature distribution in the primary zone enables control of the outlet temperature distribution from its origin. Uniformity amplitude (UA) of HCO and primary temperature distribution factor (PTDF) are defined to describe the temperature distribution in primary zone. At a swirl number of 1.5, the UA and PTDF also shows the lowest values of 0.2754 and 0.3019, respectively. Overall, when improving the distribution of HCO under both premixed and non-premixed modes, a more uniform temperature distribution across the primary zone can be achieved, effectively optimize the exit temperature distribution.

Role of secondary hydrogen injection on flame stabilization of ammonia/air swirling flames

Ammonia is widely recognized as one of the most advanced hydrogen carriers and can be utilized as a fuel in gas turbines. Premixing hydrogen into the ammonia carbon free fuel system can substantially enhance stable limits, but it significantly promotes cross-reactions, leading to increased NO production. Recent findings related to other fuels suggest that the alternative approach for mitigating combustion instabilities involves introducing minimal pure hydrogen at the chamber inlet in a non-premixed mode. This may introduce a novel approach to stabilize ammonia/air swirl flames by extending the stability through a minimal hydrogen via secondary injection, with minimal impact on increasing nitrogen oxide emissions. In the present study, the flame stabilization mechanism and nitrogen oxide emission behaviors of swirl ammonia/air flames by a secondary hydrogen injection were compared with the premixed ammonia/ hydrogen/air flames. OH-/NO-PLIF and PIV techniques were applied to reveal the lean blow-off characteristics and the reacting flow features. The NOx emissions were measured by the FTIR gas analyzer. The large eddy simulation method with a developed dynamic thickened flame model was employed to further reveal the experimental findings. Experimental results show that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. The secondary hydrogen injection exhibits the stronger enhancement ability for the lean/rich blow-off limits, primarily due to the local diffusion hydrogen flame at the flame root providing more active radicals and higher temperature gases. The NO emission shows an increase with the addition of premixed hydrogen, while in the secondary hydrogen injection, NO emission deteriorates due to higher temperatures, with the NO emission increasing by less than 10% compared with the ammonia flame. In the large eddy simulation analyses, the physical effects of the secondary hydrogen injection enhance the flame stability by increasing the resistance of the flame root to extinction. The heat release rate and the mass fractions of NH and H near the flame root are significantly increased by 2% secondary hydrogen injection. The enhancement of the ammonia decomposition process is stronger with secondary hydrogen injection than with fully premixed hydrogen addition. For 2% secondary hydrogen injection case, the local hydrogen injection to form a non-premixed combustion mode can provide much higher capability to increase the local heat release rate compared to the fully premixed mode.Novelty and significance statement: Introducing small amount of hydrogen to ammonia flame can effectively improving the flame stabilization in a swirl combustor. In this study, it is found that the flame stabilization limits are largely depended on the way in which hydrogen is introduced. Comparing the way of premixing hydrogen in the unburned mixture, a larger effect on blow-off limits extension was found when introducing a separate non-premixed hydrogen at the chamber inlet, which is mentioned as the secondary hydrogen injection. The lean blow-off limits can be extended from about 0.67 to 0.59 when only introducing 2% secondary hydrogen injection by heat value. The NO emission shows in the secondary hydrogen injection, NO emission deteriorates by less than 10% compared with the ammonia flame. A thickened flame model for non-premixed combustion was established and verified adequate with velocity field and flame structure. The mechanisms of enhancing flame stabilization by hydrogen introduction including physical and chemical effects for premixing and injection cases were revealed. Furthermore, the difference in the mechanisms of the two cases was emphasized.

A Comparison Between Conventional and MILD Combustion Processes of Turbulent Stratified Mixtures

ABSTRACT The relative enhancements of volume-integrated burning rate and flame surface area under turbulence and heat release rate contributions due to different combustion and flame propagation modes have been compared between conventional and Moderate or Intense Low Oxygen Dilution (MILD) combustion of stratified mixtures (for a global mean equivalence ratio of 0.8) for the first time using three-dimensional Direct Numerical Simulation (DNS) data. The burning rate enhancement under turbulence is found to be comparable to the enhancement of flame surface area for the conventional stratified flames considered here, whereas the burning rate enhancement is found to be considerably greater than flame area augmentation in the MILD cases analyzed here. It has been found that almost all the heat release rate arises due to the lean-premixed mode of combustion for the conventional stratified flame cases considered here. In contrast, the lean premixed mode of combustion accounts for about 50% of the total heat release rate for the MILD cases where rich-premixed and non-premixed modes of combustion contribute significantly to the overall heat release rate. Under the conditions analyzed here, more than 75% of heat release rate arises from the propagation-dominated regions for both MILD combustion and stratified flame cases, but the trend is stronger in the conventional stratified mixture combustion. More than 80% of heat release rate in the globally lean conventional stratified flame cases considered here originates from back-supported flame propagation, whereas both front and back-supported flame propagation modes have comparable contributions to heat release in the MILD combustion cases. The non-premixed mode of combustion has been found to play a more important role in the MILD combustion cases than in the case of conventional stratified flames. This implies that the closures, which work reasonably well for MILD combustion of homogeneous mixtures, may not be sufficient for modeling of MILD combustion of stratified/inhomogeneous mixtures.

The effect of methane addition on reacting hydrogen jets in crossflow

The combustion characteristics in a jet in crossflow configuration are numerically investigated using Large Eddy Simulation (LES) with subgrid scale modelling for partially premixed combustion. The time-averaged statistics are compared with the experimental measurements. Both the temperature and velocity comparisons show good agreement with measurements. The distribution of reaction regions of nitrogen-diluted hydrogen fuel jet is then examined in both physical and mixture fraction space. The results demonstrate that the fuel consumption in potential core, near-field and far-field regions occurs at various mixture fraction ranges. In the potential core region, reactions predominantly occur between the lean and stoichiometric mixture fraction range. This range broadens to include fuel-rich conditions in the near-field region and narrows again to fuel-lean combustion in the far-field region. With these changes in the mixture fraction range, the fractional contribution from different combustion modes also changes. A closer study of these changes reveals that the fractional contribution from the non-premixed mode is highest at the end of the potential core region. The dependency of these combustion modes on the fuel composition is studied by increasing the volume fraction of methane in the jet. This resulted in altered chemical kinetics and thereby increased the fractional contribution of the non-premixed mode, particularly in the potential core region.

Enhancing ammonia combustion performance using hydrogen peroxide-enriched air: A computational fluid dynamics analysis

The effect of H2O2 addition to the air on ammonia combustion was investigated in the current research using a computational fluid dynamics approach. The numerical simulation was conducted in a 10-kW laboratory-scale furnace. The oxidizer and fuel were injected into the furnace in a non-premixed mode. Furthermore, a kinetic study was carried out to analyze the sensitivity of NO production during combustion and to determine reaction pathways under various conditions. The findings indicate that the addition of H2O2 to the mixture increases flame temperature and NO levels, while decreasing N2O levels. Moreover, the study demonstrates that, with a maximum concentration of only 10 ppm, the amount of NO2 is very low under various percentages of H2O2 and different operating conditions. Furthermore, it is concluded that during ammonia/air combustion, lowering the oxidizer inlet temperature and increasing wall heat extraction may cause the combustion to become unstable. Under pure air oxidizer conditions, where Tin = 973 K and Twall = 1273 K, the combustion is stable. However, instability occurs when Twall falls to 1173 K. In this instance, adding merely 5 % H2O2 to the oxidizer is sufficient to provide self-sustaining and stable burning. More H2O2 must be introduced into the furnace to maintain stable combustion as Tin and Twall continue to decline. Interestingly, while adding H2O2 raises NO levels, decreasing the inlet and wall temperatures at higher H2O2 concentrations can help regulate NO emissions. These findings clearly indicate that introducing H2O2 into the fuel mixture could be a promising strategy for reducing the inlet temperature and enhancing heat extraction, which will both reduce energy consumption and increase system efficiency.

Experimental and numerical study on flow/flame interactions and pollutant emissions of premixed methane-air flames with enhanced lean blowout hydrogen injection

This study investigates experimentally and numerically the stability, combustion, and emissions characteristics of premixed swirl-stabilized methane/air flames with enhanced lean blowout (ELBO) injection of hydrogen in the non-premixed mode for higher turndown ratio gas turbines. The study analyzed the effects of hydrogen fraction (HF: volumetric percentage of H2 in the total fuel) ranging from 0 to 25% and equivalence ratio (φ) ranging from 0.5 to 0.893 on flow/flame interactions and flame stability, structure, and emissions. The introduction of non-premixed H2 yields two conflicting outcomes, with enhancements in reaction kinetics and flame speed yet counteracted by increased hydrogen jet momentum dispersing the flame. The combustor lean blowout (LBO) limit remains relatively unaffected (φoverall≈ 0.5) by H2 injection. Flame temperature is primarily influenced by φ and minorly by HF. Increasing HF resulted in localized temperature rises around H2 jets without an overall increase in flame temperature, offering a promising strategy for reduction of NOx emissions. Nevertheless, using H2 as a pilot fuel leads to an increase in intermediate species like OH, H, and O, contributing to flame stabilization. However, this practice also results in higher CO and unburned hydrocarbons emissions due to the inherent characteristics and heightened reactivity of H2.

Enhancement of Radiative Power Using a Divergent Splitter Plate Design in H2–Air Non-Premixed Micro-Combustor

Abstract Micro-combustion based power generation devices can be considered as future alternatives to batteries in miniature electronic devices. Micro-combustors operating in non-premixed mode are free from flashback but face the challenge of properly mixing fuel and air within a small volume. In this work, the effect of a divergent fuel–air splitter design on the mixing performance and combustion characteristics of H2–air fueled diffusion micro-combustor is studied. The laminar reacting flow is simulated using the finite volume method and a detailed hydrogen kinetic mechanism. Three divergent splitter designs are compared with the commonly used rectangular splitter to study the effect on radiation power, an essential parameter for thermophotovoltaic power generation. The best-performing divergent and base rectangular splitter designs are investigated in detail. The study shows that the micro-combustor with divergent splitter design reduces mixing distance (Lmix) by 5–23% depending on inlet velocity and channel height. With the divergent splitter, the peak value of the heat release rate also increases slightly, implying enhanced combustion. The divergent splitter increases the high-temperature surface area of the outer wall as compared to the rectangular splitter. This leads to the micro-combustor with divergent splitter producing significantly higher radiation power (>10%) than the rectangular splitter for larger channel heights and higher inlet velocities.

Numerical investigation of the combustion regime in pilot-ignited high-pressure direct-injection low reactivity fuel combustion with the injection system of eccentric configuration

The pilot-ignited high-pressure direct-injection (HPDI) combustion with the eccentric configuration is a practical approach to achieve high thermal efficiency and clean combustion. However, the combustion process and combustion regime still remain unclear. In this study, the effects of two key parameters “offset” (distance between concentric and eccentric injectors) and “dwell” (interval between concentric and eccentric injection timing) on the combustion process, combustion regime, and the corresponding performance are numerically investigated systematically. Results show that the interaction between the flame kernels of pilot fuel and the plumes of main fuel determines the combustion mode of each main fuel plume, and thereby different combustion regimes in the pilot-ignited HPDI engine can be generated by controlling the offset and dwell. Specifically, under the concentric configuration or the eccentric configuration with very small offset and dwell, all the main fuel plumes are in the non-premixed mode, exhibiting a diffusion combustion (DC) regime. Under the eccentric configuration with medium offset and dwell, some of the main fuel plumes are in the non-premixed and the others in the partially premixed mode, exhibiting a special combustion regime, which is named the multi-mode co-existing combustion (MMCC) in this study. Under the eccentric configuration with large offset and dwell, all the main fuel plumes are in the partially premixed mode, exhibiting a partially premixed combustion (PPC) regime. Among the three combustion regimes, the diffusion combustion regime exhibits the lowest ISFC and unburned HC emission but high NOx and soot emissions. The ISFC of the partially premixed combustion regime is almost double that of the other two regimes which is far from being acceptable. The MMCC regime shows a better comprehensive performance: achieving much lower ringing intensity and soot emission with almost the same ISFC compared to the diffusion combustion regime. In summary, the pilot-ignited HPDI combustion with the eccentric configuration can potentially achieve high fuel economy with low emissions, under the premise of the ratio of non-premixed and partially premixed combustion modes in the MMCC regime is well controlled.

Numerical investigations of supersonic combustion characteristics of thermally cracked fuels at typical pyrolysis temperatures

Insight into supersonic combustion characteristics of thermally cracked fuels in a scramjet combustor is crucial for the development of next-generation supersonic flight vehicles. In the present study, four typical thermally cracked fuels at pyrolysis temperatures of 450, 550, 600, and 650 ℃ (denoted as ckori, ck550, ck600, and ck650) were selected. Improved delayed detached eddy simulation (IDDES) modeling with a compact skeletal mechanism and a transported multi-regime flamelet model was carried out to simulate the supersonic combustion of the target fuels in a full-scale scramjet combustor. Supersonic flow, mixing, and combustion characteristics of these fuels were quantitatively compared. Discrepancies in the combustion process are mainly at the cavity region, where the mixing efficiencies and combustion intensities of the first three fuels increase monotonically with the increasing pyrolysis temperature, while that of ck650 locates between ck550 and ck600. The increase in pyrolysis gas content will enhance the mixing process, while the increase in injection pressure will suppress the heat/mass transport between fuel and incoming air. The mixing process dominates the distributions of intermediate radicals and main products. The first three fuels tend to combust under fuel-rich condition, while ck650 tends to combust under the stoichiometric condition, which explains the distributions of OH, CO, and CO2 concentrations. The premixed and non-premixed combustion modes coexist in the model scramjet combustor, and the non-premixed combustion mode dominates the combustion process of thermally cracked fuel.

Numerical study on the ignition and combustion of a CH4 jet flame in diluted and undiluted coflows

This study numerically investigates the ignition and evolution of CH4 jet flames in a hot coflow (JHC) using large eddy simulation (LES). Three coflow oxygen fractions are carefully selected to compare moderate or intense low-oxygen dilution (MILD, YO2,C = 6 % and 9 %) and high temperature combustion (HTC, YO2,C = 23.3 %). Systematic investigations are performed on flame ignition, lift-off, reaction progress, and stabilization mechanisms. The results show that for the MILD cases, both the ignition process and final stable flame are controlled by turbulent mixing and chemistry in a state far from chemical equilibrium. In contrast, the reactions are much stronger for HTC, and the final flame is controlled by chemistry through equilibrium reactions. Moreover, for all three cases, the reactions always progress more rapidly at the lean side than at the rich side; thus, autoignition dominates the former, whereas flame propagation is more important for the latter. In addition, the premixing of fuel and oxidant prior to main reactions is crucial for realizing MILD combustion; therefore, the flame is lifted and stabilized by the autoignition of partially premixed reactants in the fuel-lean region. In contrast, the final HTC flame attaches to the burner exit and is stabilized by both the lean-side autoignition and rich-side flame propagation. For all three cases, the lean-side and stoichiometric flames adopt the nonpremixed mode, whereas the rich-side reactions are in the premixed mode, except for the flame base.

Flame Surface Density and Artificially Thickened Flame Combustion Models Applied to a Turbulent Partially-Premixed Flame

The Sydney Flame provides an excellent framework for systematic analysis of premixed, partially premixed and non-premixed methane/air combustion. It represents (depending on the axial position from the fuel inlet) non-premixed, partially premixed and fully premixed combustion. We simulate the test case FJ200-5GP-Lr75-57, which represents the partially premixed mode. Large-eddy simulations (LES) are conducted using a flame surface density combustion model (FSD) using one-step chemistry and an artificially thickened flame model (ATF) with a global two-step and an analytically reduced multi-step chemistry reaction mechanism. Unity Lewis numbers are assumed (Le=1\\documentclass[12pt]{minimal} \\usepackage{amsmath} \\usepackage{wasysym} \\usepackage{amsfonts} \\usepackage{amssymb} \\usepackage{amsbsy} \\usepackage{mathrsfs} \\usepackage{upgreek} \\setlength{\\oddsidemargin}{-69pt} \\begin{document}$$Le=1$$\\end{document}) at first. The FSD and ATF models are compared using the same computational mesh, numerical scheme and boundary conditions. Both models will be analysed regarding their accuracy and computational efficiency in the regime of partially premixed combustion. A comparison of the results points out the strengths and weaknesses of the FSD models. The FSD model with inclusion of flame stretch effects yields good agreement with mean experimental temperature, CO2 and H2O mass fraction distributions in contrast to the ATF model using the global two-step mechanism, which overestimates downstream temperature, CO2 and H2O mass fractions. The FSD model performs well in all regions, which are dominated first by premixed, then partially premixed and finally non-premixed combustion along the flame. The ATF multi-step chemistry shows good results only in the premixed mode region, while mean temperature, CO2 and mass fractions are overestimated in the non-premixed mode regions at higher mixture fraction values. Including differential diffusion into the transport equations improved the ATF model results in comparison with experiments. A mesh study revealed, that the ATF model performs only after mesh refinement. In particular results are improved in the non-premixed combustion region. In contrast, the FSD model is less resolution sensitive and performs very well for both meshes. An evaluation of the Wasserstein metric provides a quantitative assessment of different ATF model setups on simulation accuracy. Finally, computational times for all simulation setups are compared.

A chemical kinetic analysis: Influence of post-flame chemistry, combustion pressure, premixing degree (fully premixed to non-premixed), and secondary air supply on NOx emissions from NH3/CH4-air combustion

Numerous studies assessed the viability of ammonia blends for the wide-scale application of ammonia in carbon-free energy systems. NOx emissions from ammonia combustion are the primary concern constraining the adoption of NH3 as a fuel. The present study explores the effect of post-flame chemistry, combustion pressure, reactant-mixing (premixing degree), and secondary air supply on NOx emissions from NH3/CH4 swirl flame. Furthermore, an extensive chemical kinetic analysis is carried out using the rate of production and sensitivity analyses to study NOx formation and destruction with the above-discussed techniques. The present study adopts the NH3/CH4 reaction model developed by Okafor et al. and a PSR-PFR chemical reactor network for predicting the NOx formation chemistry in NH3/CH4-air combustion. The analyses are conducted for the values of ammonia heat fraction in the fuel mixture (ENH3) = 0.1–0.3 and equivalence ratio (ϕ) = 0.6–1.4. The present work suggests that providing a post-flame zone is advantageous in reducing both NOx and NH3 emissions, especially for rich NH3/CH4 flames. Increasing the combustion pressure to achieve low NOx without significant ammonia slip appears to be a viable method for NH3/CH4 flames with ϕ < 1.2. Adopting a non-premixed injection mode is a feasible low NOx combustion technique for 0.9 < ϕ < 1.2, especially for higher ammonia fractions in the fuel mixture. An excess air supply of 10% reduces the NOx emissions for fuel-lean conditions (ϕ < 0.9) with a negligible rise in unburnt NH3 emissions under atmospheric conditions.

Investigation of a low emission peripheral vortex reverse flow (PVRF) combustor fuelled by LPG and ethylene

Peripheral Vortex Reverse Flow (PVRF) combustor is characterized by a strong peripheral recirculation zone resulting in stable combustion and low pollutant emissions. In this study, a PVRF combustor is investigated in the premixed and non-premixed modes with LPG and ethylene fuels at a maximum heat load of 6.25 kW and thermal intensity of 25 MW/m3-atm, relevant to gas turbine combustors. The fuel is injected co-axially, with respect to the air jet, using two different fuel injection diameters of 1 mm, and 2 mm. For both the fuels, the fuel injection velocity is higher as compared to the air injection velocity for fuel injection diameter of 1 mm, while it is lower for fuel injection diameter of 2 mm. CH* chemiluminescence images are acquired, and CO and NOx emissions are measured. Reynolds Averaged Navier Stokes (RANS) simulations are carried out at an equivalence ratio of 0.8 using the Eddy Dissipation Concept (EDC) model with a H2/CO/C1–C4 mechanism comprising of 50 species and 373 reactions. Delayed ignition and downstream shifting of the reaction zone facilitating higher product gas entrainment prior to reactions is observed in the non-premixed mode as compared to the premixed mode. NOx emissions were found to be lower in the non-premixed mode as compared to the premixed mode for both the fuels. This is possibly due to suppression of NO formation in the reaction zone via. prompt route which is also supported by lower CH* chemiluminescence intensity in the non-premixed mode. With LPG, the case with larger fuel injection diameter showed further delayed mixing as compared to the case with smaller fuel injection diameter, and resulted in even lower NOx emissions. For all the cases, CO emissions were of similar levels in the non-premixed and premixed modes suggesting that lower NOx emissions in the non-premixed mode is not at the expense of increase in CO emissions. Overall, the investigation shows that lower NOx emissions can be achieved in the non-premixed mode as compared to the premixed mode if more product gas entrainment is facilitated prior to the reactions by delayed mixing between fuel and oxidizer.

Combustion and emission characteristics of NH3/CH4/air in a model swirl combustor: Comparison between premixed and non-premixed modes

Ammonia (NH3) is a promising hydrogen carrier and can be used as carbon-free alternative fuel. However, NH3 combustion faces critical challenges, such as narrower flammability range and high NOx emission, which limits its applications. To extend the knowledge on NH3 combustion characteristics, intensive previous studies have been carried out, but most of them were concerned on premixed swirl flames. This work aims to experimentally provide a thorough comparison between premixed and non-premixed NH3/CH4/air swirl flames in terms of their flame structure, chemiluminescence spectrum, and pollutant emission characteristics at various NH3 volumetric blending ratios (XNH3) and global equivalence ratios (φ). The results show that, at a fixed φ of 0.85, the premixed combustion features a closely attached flame profile at any XNH3 of interest, while the non-premixed combustion exbibits a lifted flame appearance for all XNH3 conditions, hence a higher flammability limit of XNH3 is attained by the premixed combustion mode. At a fixed XNH3 of 50%, the non-premixed combustion tends to move the whole flammability range towards the rich direction slightly. Considering the monotonic relationship between chemiluminescence intensity of different excited radicals and operating parameters, OH∗/CH∗ intensity ratio is suggested to be used for sensing the change of XNH3 in both premixed and non-premixed combustion modes, while NH∗/CH∗ intensity ratio is recommended for sensing the change of φ. Comparing to premixed combustion, the non-premixed combustion produces a lower NO emission in the whole ranges of considered XNH3 and φ. Moreover, the NO emission profiles suggest the necessity of employing rich-lean technology for achieving acceptably low NO emission for both premixed and non-premixed combustion modes, while the latter shows a possibility to be operated in relatively leaner condition in the first stage.

Direct Numerical Simulation of Partial Fuel Stratification Assisted Lean Premixed Combustion for Assessment of Hybrid G-Equation/Well-Stirred Reactor Model

Abstract Partial fuel stratification (PFS) is a promising fuel injection strategy to stabilize lean premixed combustion in spark-ignition (SI) engines. PFS creates a locally stratified mixture by injecting a fraction of the fuel, just before spark timing, into the engine cylinder containing homogeneous lean fuel/air mixture. This locally stratified mixture, when ignited, results in complex flame structure and propagation modes similar to partially premixed flames and allows for faster and more stable flame propagation than a homogeneous lean mixture. This study focuses on understanding the detailed flame structures associated with PFS-assisted lean premixed combustion. First, a two-dimensional direct numerical simulation (DNS) is performed using detailed fuel chemistry, experimental pressure trace, and realistic initial conditions mapped from a prior engine large-eddy simulation (LES), replicating practical lean SI operating conditions. DNS results suggest that the conventional triple flame structure is prevalent during the initial stage of flame kernel growth. Both premixed and nonpremixed combustion modes are present with the premixed mode contributing dominantly to the total heat release. Detailed analysis further reveals the effects of flame stretch and fuel pyrolysis on flame displacement speed. Based on the DNS findings, the accuracy of a hybrid G-equation/well-stirred reactor (WSR) combustion model is assessed for the PFS-assisted lean operation in the LES context. It is found that the G-equation model qualitatively captures the premixed branches of the triple flame, while the WSR model predicts the nonpremixed branch of the triple flame. Finally, potential needs for improvements to the hybrid G-equation/WSR modeling approach are discussed.

Numerical simulations of supercritical CH4/O2 flame propagation in inhomogeneous mixtures following ignition

The propagation of flames in non-homogeneous medium is studied for the particular case of the combustion of methane with oxygen at high pressure, in supercritical regime. A reduced chemistry containing 17 species and 44 elementary reactions is used in two configurations: 1/ a pocket of methane in a steady environment of oxygen at 10 MPa, and 2/ a double transcritical injection in a splitter plate configuration at 5.4 MPa. High fidelity simulations are carried out with the real gas version of the SiTCom-B numerical code. The addition of a hot spot to the reactive mixtures resulted in the formation of triple flames propagating along the stoichiometric isoline whatever the configuration chosen: with or without turbulence, cryogenic temperatures, random distribution of methane or not. In the case of a high density ratio between the two fluids (CH4 and O2), the flame naturally chooses to move towards that of lower density. In the case of the combustion of one pocket of methane, the addition of a homogeneous isotropic turbulence promotes combustion by increasing the length of the flame as well as the lean premixed combustion mode compared to the rich or non-premixed combustion. A similar behavior is observed when the pocket of CH4 is replaced by a set of random pockets. In the case of the splitter plate, tribachial flames propagating along the stoichiometric line are observed again. Combustion develops in finger-like structures mostly with a non-premixed mode of combustion. However, the lean and rich premixed combustion remains and the flame spreads towards the injector. The tip of the flame then attempts to pull up the flow into a reduced space stuck between dense oxygen and a rapid flow of methane.

Effect of Strong Azimuthal Swirl on Ignition and Light-Around in an Annular Combustor

Abstract In this paper, we investigate the effect of strong azimuthal swirl on ignition dynamics in a laboratory-scale annular combustor. Bulk azimuthal swirl was produced by a novel angled injector configuration, producing swirling jet flames oriented downwards toward the combustor backplane and in the azimuthal direction, replicating a simplified version of the SAFRAN spinning combustor concept. To provide more realistic flow conditions, the design included rich-quench-lean (RQL) staging via a circumferential distribution of dilution ports and an effusion cooled combustor backplane. High-speed imaging and an azimuthal array of photomultipliers to measure OH* chemiluminescence were used to characterize the ignition dynamics for different injector velocities and global equivalence ratios. The mass flows through the injectors, dilution ports, and effusion cooled backplane were independently metered so that the injector equivalence ratio and global equivalence ratio could be separately controlled. The light-around times were found to have no clear correlation with the injector velocity since the rich injector equivalence ratio meant the flame burned in a nonpremixed mode even though the global equivalence ratio was lean due to the RQL staging. However, it was found that lower injector velocities extended the lean ignition limit based on the global equivalence ratio. The ignition sequence during light-around (order in which the injectors are ignited) was found to be highly repeatable, igniting each consecutive injector in the anticlockwise direction (the direction of bulk swirl). In rare cases, the ignition sequence was observed to branch in both directions. Finally, in an effort to extend the lean ignition limit, the effect of azimuthal staging was investigated. Two configurations were tested. In the first configuration, the injectors on one half of the annulus were operated at a fixed equivalence ratio whereas the other half of the annulus was operated at a different equivalence ratio. In the second configuration, every second injector had the same equivalence ratio. Both configurations extended the lean extinction limit but the first configuration was the most effective.

Double compression-expansion engine (DCEE) fueled with hydrogen: Preliminary computational assessment

Hydrogen (H2) is currently a highly attractive fuel for internal combustion engines (ICEs) owing to the prospects of potentially near-zero emissions. However, the production emissions and cost of H2 fuel necessitate substantial improvements in ICE thermal efficiency. This work aims to investigate a potential implementation of H2 combustion in a highly efficient double compression-expansion engine (DCEE). DICI nonpremixed H2 combustion mode is used for its superior characteristics, as concluded in previous studies. The analysis is performed using a 1D GT-Power software package, where different variants of the DICI H2 and diesel combustion cycles, obtained experimentally and numerically (3D CFD) are imposed in the combustion cylinder of the DCEE. The results show that the low jet momentum, free jet mixing dominated variants of the DICI H2 combustion concept are preferred, owing to the lower heat transfer losses and relaxed requirements on the fuel injection system. Insulation of the expander and removal of the intercooling improve the engine efficiency by 1.3 and 0.5%-points, respectively, but the latter leads to elevated temperatures in the high-pressure tank, which makes the selection of its materials harder but allows the use of cheaper oxidation catalysts. The results also show that the DCEE performance is insensitive to combustion cylinder temperatures, making it potentially suitable for other high-octane fuels, such as methane, methanol, ammonia, etc. Finally, a brake thermal efficiency of 56% is achieved with H2 combustion, around 1%-point higher than with diesel. Further efficiency improvements are also possible with a fully optimized H2 combustion system.

Lean blow-off investigation in a linear multi-burner combustor operated in premixed and non-premixed modes

The flame behaviour and the lean blow-off (LBO) mechanism of a multi-burner linear arrangement of five identical burners each fitted with a central bluff body and a swirler up- stream, operated in either premixed or non-premixed mode, was studied experimentally. The flame stability was assessed over a wide range of velocities. High-speed (5 kHz) OH∗ chemiluminescence and low-speed (10 Hz) CH2O-PLIF were employed. As the velocity in- creased, the premixed flame exhibited a shape change with significant CH2O build up in the region between adjacent burners, suggesting the presence of localised quenching where the flames interact. Close to LBO, the flame showed a pattern of lift-off and reattachment over individual bluff bodies, with movement of OH∗ and CH2O pockets across the burners. The non-premixed flame was shorter and showed less visible interaction between adjacent burners. When the fuel velocity was reduced, the flame showed intermittent blow-off/reignition prior to the complete extinction. The flame shapes and stability regions were compiled in a regime diagram and compared to a full annular combustor that used the same individual burners. Finally, the LBO conditions were correlated through a Damkh¨oler number (Da) based on laminar flame speed and compared to the annular combustor and a scaled-up single burner. In the premixed flame, the linear multi-burner LBO conditions agreed with those in the annular geometry when expressed in terms of Da, but the single burner did not. For the non-premixed flame, the correlation proved less successful. The results reported here improve the understanding of the LBO process in practical systems and can assist the development of turbulent flame models.

Numerical investigation of fuel flexibility in a small-scale flameless combustor

Numerical simulation of a laboratory flameless combustor was performed to investigate the flexibility to burn alternative fuels to natural gas. The studied fuels are biogas, syngas and a mixture of ammonia and methane. The inlet temperatures of air and fuel, the equivalence ratio and the geometrical characteristics of the combustor were maintained constant. The results show that flameless combustion is observed in the biogas and in the NH3/CH4 mixture, while the syngas burns according to the conventional non-premixed combustion mode. According to the predictions, the biogas emits 1.1 ppm of NOx and 229 ppm of CO, syngas produces 7.8 ppm of NOx and 35 ppm of CO and the NH3/CH4 mixture emits about 3900 ppm of NOx and 608 ppm of CO. The high NOx and CO emissions in the NH3/CH4 mixture show that the combustor needs to be optimized to burn a nitrogen-containing fuel.