Flame-wall Interaction Research Articles

Gasoline compression ignition (GCI) combustion of pump-grade gasoline fuel under high compression ratio diesel engine

In this paper, gasoline compression ignition (GCI) combustion was investigated under a high compression ratio (CR = 22:1) system that was developed for a high-efficiency heavy-duty diesel engine platform. The experiment includes Single-Cylinder Engine (SCE) testing of two combustion systems with distinctively different piston designs: wave and stepped-lip. Validated Computational Fluid Dynamics (CFD) simulations provide insights on the in-cylinder phenomenon under GCI combustion. While early spray/flame and ignition characteristics were driven mainly by nozzle configuration in each combustion system, flame-wall and flame-flame interaction during the late-cycle combustion process were strongly affected by the piston's wall-flow-guided feature. The wave piston is unique with a recirculation flow, also known as radial mixing zone (RMZ). RMZ results from in-cylinder flow interactions of adjacent flames, which play a crucial part in enhancing late-cycle soot oxidation.Meanwhile, the stepped-lip piston splits flame jet (at the time of impingement) upward and downward to head and bowl, respectively. Overall, the stepped-lip piston showed lower heat loss due to reduced spray penetration. In contrast, wave piston demonstrated superior end-cycle mixing, reducing soot emissions. Both systems provided similar indicated thermal efficiency (ITE). Comparison between diesel and GCI shows that GCI can reach diesel-like efficiency while maintaining low soot emissions. However, there is a remaining challenge in GCI combustion with higher carbon monoxide (CO) and unburned hydrocarbon (UHC) emissions than diesel combustion.

Flame Transfer Functions and Dynamics of a Closely Confined Premixed Bluff Body Stabilized Flame With Swirl

Abstract The flame transfer function (FTF) and flame dynamics of a highly swirled, closely confined, premixed flame is studied over a wide range of equivalence ratios and bulk velocities at a fixed perturbation level at the dump plane. The operating conditions are varied to examine the ratio of flame height to velocity in scaling the FTF. The enclosure geometry is kept constant, resulting in strong flame-wall interactions for some operating conditions due to varying flame height. The resulting effect on the FTF due to changes in the “effective flame confinement” can therefore be studied. For sufficiently high equivalence ratio, and the resulting sufficiently small effective confinement, modulations of the FTF are observed due to interference of the perturbations created at the swirler and at the dump plane. The small length scales and high velocities result in modulations centered at high frequencies and spanning a wide range of frequencies compared to previous studies of similar phenomena. A critical point was reached for increasing effective confinement, where the modulations are suppressed. This is linked to a temporal shift in the heat release rate where the flame impinges on the combustion chamber walls. The shift reduced the expected level of interference, demonstrating effective confinement is important for the FTF response. Additionally, a distributed time lag (DTL) model with two time lags is successfully applied to the FTFs, providing a simple method to capture the two dominant time scales in the problem, recreate the FTF, and examine the effect of effective confinement.

CO emissions and temperature analysis from an experimental and numerical study of partially premixed methane flames impinging onto a cooking pot

The increase of thermal efficiency compatible with low carbon monoxide (CO) emissions is a challenging and permanent target in the design of any burning technology such as domestic gas cooking burners. The main goal of this work is to provide a deeper insight into the CO formation and its relationship with the flame structure when natural gas (NG) is burnt in these devices. Given their geometrical complexity, a simpler configuration is employed to carry out new experimental tests assuming similar operating conditions. Then, numerical modeling is validated and subsequently used to deeply analyze the flame-wall interaction phenomena. The simplified burner consists of an axisymmetric, partially premixed methane flame impinging perpendicularly onto the bottom wall of a water pot. The influence of burner-to-pot distance, flame thermal power, primary aeration and inside-pot water temperature on CO emissions and thermal efficiency is evaluated. A decrease in CO emissions is observed as primary aeration or wall temperature increases. Nevertheless, non-monotonic trends appear for changes in burner-to-pot distance or flame thermal power. The trends are numerically well captured selecting the detailed GRI-Mech 3.0 chemistry mechanism. The analysis of the computational results reveals that CO emissions and thermal efficiency are strongly related to the relative boundary position of the inner premixed flame cone and the wall. The growth of the distinct zones of the flame where CO chemically reacts with primary and secondary air is constrained by the presence of the pot wall. This fact drives the final CO concentration, and incidentally the thermal efficiency, leading to a factual criterion for the design of NG burners.

Effects of coflow velocity and coflow moisture contents on the formation and emissions of CO/NO in non-premixed impinging flames

The emission characteristics of non-premixed impinging flames are investigated using PIV experiments and numerical simulations. Different coflow conditions are conducted to focus on the pollutant emissions with the flame-wall interaction. Results show that flame dynamics obtained from experiments are in good agreement with that calculated from numerical simulations, and flame instability is intensified in near wall region in large coflow velocity case. In addition, the increase of coflow velocity may promote CO production, especially in low mean mixture fraction region (0.07-0.26). The distribution of CO is dispersed in large coflow velocity case, which is related with a strong convection and diffusion process. Besides, it is found that the decrease of flame temperature and the large wall quenching distance is observed with the increase of coflow moisture content, which is associated with the weakened combustion progress and the migration of reaction zone due to the decrease of O2 concentration in coflow mixture and the increase of specific heat capacity. The decrease distribution of local emission indexes of CO and NO is also shown in high coflow moisture content case, which can weaken the production of CO, thermal NO and prompt NO, and accelerate the consumption of NO. Moreover, CO reduction occurs mainly in high CO concentration region, while NO reduction is observed in the whole flow field. Results indicate that small coflow velocity and high coflow moisture content are beneficial to supress pollutant emissions, which can provide reference data for the optimization of clean combustion devices.

Monitoring combustion instabilities of stratified swirl flames by feature extractions of time-averaged flame images using deep learning method

The present article investigates the application of deep learning methods to monitor combustion instabilities based on time-averaged flame images. Experiments on BASIS (Beihang Axial Swirler Independently-Stratified) burner are carried out in a wide range of operating conditions to cover various flame shapes. We design a CNN (Convolutional Neural Networks) named BIM (BASIS Image Monitor) to extract features from flame shapes and determine the thermoacoustic states of the flames ahead of time. Experiments of 112 operating conditions are conducted, and their images are fed into BIM for training and testing purposes. BIM achieves around 99% prediction accuracy after a short training process lasting less than 20 minutes. Features extracted by BIM and visualized by Class Activation Maps (CAM) method reflect the underlying statistical links between flame images and their stabilities. Several phenomena (lift-off flame, flame-wall interaction, outer shear layer (OSL) flame and fuel mixing process) and their contributions to stabilities can be captured and quantified by CAM. After that, we test BIM with eight unknown operating conditions, and BIM can still perform well, identifying their stabilities and unstable features accurately. This paper explores the monitoring accuracy and feature extraction abilities of CNN in combustion. The BIM model shows potential to be a precursor in monitoring and active control systems of combustion instabilities.

On explosion characteristics of premixed syngas/air mixtures with different hydrogen volume fractions and ignition positions

This study analyzes the effect of both ignition position and hydrogen volume fraction on the flame propagation structure and overpressure wave. The interpretations are based on the propagation dynamics of the premixed syngas/air explosions at different ignition positions with different hydrogen volume fractions investigated at a constant equivalence ratio of 1 in a semi-open rectangular combustion chamber experimentally. The initial temperature T0 and pressure p0 were 298 K and 0.1 MPa, respectively. The experimental results are consistent with those obtained using the proposed analytical theory, indicating that the proposed theory predicts the early flame acceleration stage and the flame front motion well under some limited conditions. Flame–wall interaction and combustion instability decrease the theoretical flame front position than the experimental value and increase the wall contact time than the experimental value in some cases. At the same ignition location, both the overpressure growth rate and the maximum overpressure increase with increasing hydrogen volume fraction. When ignited at 0 mm off the closed end, the increased volume fraction of hydrogen would cause the initial location of the tulip flame to approach the opening end of the duct, but at an earlier time. The closer the ignition position to the opening end is, the more intense the overpressure and flame leading edge oscillation are, and the period of overpressure is similar to that of the flame oscillation. Finally, the ignition position and hydrogen volume fraction have obvious effects on overpressure, the structure and development of the flame front.

LES study of diesel flame/wall interaction and mixing mechanisms at different wall distances

In this paper, the flame-wall interaction of reacting diesel spray under engine like conditions is investigated using large eddy simulations. The aim of this study is to understand the influence of the distance between the wall and the spray nozzle on the air entrainment rate, which is a key variable in formation/oxidation process of soot. Three experimental cases are investigated, a free jet case and two wall impingement cases with a distance from nozzle to wall of 30 mm and 50 mm, which are considered as characteristic wall impingement distances for light- and heavy-duty bores in diesel engines, respectively. The optical soot measurements imply a positive influence of wall on the rate of soot oxidation. Numerical simulations are employed to elucidate importance of different mechanisms for the air entrainment, i.e., air entrainment prior to flame lift-off position, enhanced mixing due to the wall impingement and enhanced mixing by the entrainment wave. The results show that oxidation process after the end of injection is driven by a different mixing mechanism depending on the distance to the wall. The 30 mm case resulted in a “mixing boost”, where the dominant mixing mechanism is the wall impingement vortex mixing, which gives rise to the fastest soot decay among the cases. The mixing in the 50 mm case is governed by a late wall impingement vortex mixing, giving rise to a low, but a constant air entrainment rate, i.e., a “mixing plateau”. The free jet case resulted in mixing governed by the entrainment wave mechanism. Both wall impingement cases have faster soot oxidation rate compared with the free jet case, but due to a different underlying mixing process. LES is shown to be able to replicate the line-of-sight measurements of natural OH* chemiluminescence and distribution of soot region from the optical soot diagnostics.

Reduced modeling of Flame-Wall-Interactions of premixed isooctane-air systems including detailed transport and surface reactions

In this work, the Reaction-Diffusion Manifold (REDIM) method, a method for model reduction, is applied to a premixed isooctane-air system with Flame-Wall-Interactions (FWI). In order to provide a highly accurate reduced kinetic model, a detailed model for the diffusive processes is applied and complex boundary conditions that account for heterogeneous wall reactions are implemented.The REDIM is constructed and validated by comparing results of detailed and reduced kinetics in the system state space. The results of the reduced computations are compared with those of the detailed computations. It is shown that the reduced kinetics reproduce the results of the FWI very accurately. In particular, the difference between detailed kinetics with and without wall reactions is larger than the difference between detailed and reduced kinetics with heterogeneous wall reactions, which demonstrates the quality of the model reduction.

Effects of the cold wall boundary on the flame structure and flame speed in premixed turbulent combustion

The near-wall flame structure is analyzed based on the direct numerical simulation (DNS) data of a statistically stationary counterflow-like flame-wall interaction (FWI) configuration. According to the relative orientation between the flame normal and the wall normal, a flame element can be either head-on or entrained. Within the quenching zone of FWI, the flame elements are more likely to be entrained, while the head-on flame elements are more likely to appear beyond the influence zone. The flame in the quenching zone thickens considerably which has implications on the variation of the displacement speed of different progress variable isosurfaces. To characterize the dynamics of the local flame elements, a local mean density-weighted displacement speed across the flame front is introduced and investigated. Moreover, the scalar dissipation rates conditional on non-dimensional temperature and progress variable are found to be considerably different in the quenching zone because of significant differences in the distributions of progress variable and non-dimensional temperature in the near-wall region.

Development of an algebraic wall heat transfer model for LES in IC engines using DNS data

This paper presents a novel algebraic wall-modeled large-eddy simulation (WMLES) approach for wall heat transfer (WHT) in internal combustion engines (ICE) using recent high-fidelity simulation data from two real ICE and a flame-wall interaction (FWI) setup. The model formulation is based on intuitive arguments rather than simplified forms of near-wall governing equations. Input information from the two wall-normal wall-adjacent nodes is used, facilitating the interpretation of the local state of the thermal boundary layer (TBL). With filtered direct numerical simulation (DNS) data (i.e. a ‘perfect’ LES), model performance is evaluated locally at different near-wall resolutions. The proposed model is compared to a widely used wall function (WF) approach and to a data-driven model.

Conjugate heat transfer analysis of methane/air premixed flame – Wall interaction: A study on effect of wall material

Conjugate heat transfer analyses of methane/air premixed flames propagating toward walls are performed without employing any wall model in terms of one- and two-dimensional numerical simulations, and the flame-wall interaction is investigated in detail. To describe the combustion reaction of methane, a two-step global reaction model is used. In one-dimensional numerical simulations, the effects of wall material, i.e., the insulation and Al alloy, and ambient pressure on the wall heat flux are investigated. In two-dimensional numerical simulations, on the other hand, the effect of turbulence on the wall heat flux is investigated. The results of the one-dimensional numerical simulations show that the wall heat flux on the insulation wall is higher than that on the Al alloy wall at the moment the flame reaches the wall. Thereafter, the heat flux on both walls rapidly reduces, however heat flux on the insulation wall reduces more than that on the Al alloy wall, because the gas temperature gradient on the insulation wall becomes smaller. This reduction effect of the wall heat flux due to the insulation is more efficient at higher pressure conditions, and reaches 1.8 % at 4 MPa. On the other hand, the two-dimensional numerical simulations show that the propagation speed in the vicinity of the wall after the flame first reaches the wall is slower in the turbulent case than that in the laminar case. This is due to the fact that the advection of the flame front with high temperature toward the unburnt side is suppressed by the turbulent eddies.

Effects of Fuel Lewis Number on the Near-wall Dynamics for Statistically Planar Turbulent Premixed Flames Impinging on Inert Cold Walls

ABSTRACT The flame-wall interaction in a quasi-steady configuration, where a statistically planar premixed flame is pushed by the inflow of unburned reactants and stabilizes at a distance away from the wall, has been analyzed for different fuel Lewis numbers and inlet turbulence intensities. The extent of flame-wall interaction has been found to increase with increasing inlet turbulence intensity and increasing fuel Lewis number due to the greater extent of flame wrinkling and smaller flame stabilization distance from the wall, respectively. The increasing trend of turbulent flame speed with decreasing fuel Lewis number and increasing inlet turbulence intensity act to increase the wall heat flux magnitude. However, the quenching distance remains comparable to the quenching distance predicted by one-dimensional conventional head-on quenching simulations. It has been found that the wall Stanton number and the skin friction coefficient are of the same order of magnitude, but the Reynolds-Colburn analogy does not remain strictly valid in the case of flame-wall interaction in this configuration. For a given value of bulk mean inlet velocity to laminar burning velocity ratio, the flames with smaller stabilize further away from the wall due to their higher turbulent flame speed and thus the flame-wall interaction events are less frequent for small values of . The drops in temperature and reaction rate magnitude lead to reductions in the values of dilatation rate, normal strain rate, and flame displacement speed in the flame quenching zone; with the increased likelihood of finding negative values for these quantities in the near-wall region. The decrease in displacement speed with decreasing wall-normal distance leads to predominantly positive normal strain rates induced by flame propagation in the quenching zone. These behaviors act to reduce the magnitude of the reaction progress variable gradient in the quenching zone, which has implications on the behaviors of the Flame Surface Density and scalar dissipation rate in the near-wall region.

OH-PLIF investigation of Y2O3-ZrO2 coating improving flame stability in a narrow channel

The chemical catalytic performance of functional ceramics materials of rare earth is of great technological and scientific significance to actively regulate the flame-wall interaction in high-temperature combustion. The premixed methane-air flames in a slot burner with different wall materials were characterized by OH-PLIF technique to gain an insight into the effects of heterogeneouschemistry at gas–solid interfaces on the flame characteristics. The 304STS wall coated with ZrO2 and Y2O3-ZrO2 respectively through atmospheric plasma spray were investigated at the wall temperatures of 473 K and 773 K in this paper. The flame stability was quantitatively analyzed through the dynamic flame morphology of OH-PLIF images. The absolute maximum OH intensities in the flame cores were extracted to investigate the effects of different gap distances, wall temperatures and surface materials. The characterization of chemical interactions between flame and wall was also carried out based on the spatial distribution of OH radicals near the wall. Lastly, the heterogeneous chemistry between the gas-phase radicals in flame with the electrophilic oxygen species on the surface of Y2O3-ZrO2 coating was discussed to reveal the dominant pathways of the reduction and re-oxidation of surface lattice oxygen. Results show that a marked improvement in combustion stability was observed with using Y2O3-ZrO2 coated walls compared to the uncoated 304STS walls under the same conditions. The maximum OH intensity in flame would increase with the decrease of channel gap at higher temperature due to the unique chemical promoting effect of Y2O3-ZrO2 coating. Overall, this work indicates that coating with rare earth composite metal oxides such as Y2O3-ZrO2 can potentially serve as an effective strategy for the optimization of combustion chambers.

Comparative analysis of Reaction-Diffusion Manifold based reduced models for Head-On- and Side-Wall-Quenching flames

In this study, multi-dimensional molecular transport phenomena during Flame-Wall-Interactions (FWI) and their effects on model reduction strategies are investigated. In order to access the problem, the standard configurations of a two-dimensional Side-Wall Quenching (SWQ) flame and a one-dimensional Head-On Quenching (HOQ) flame are used and compared. In the case of the SWQ configuration it is shown that the gradients of the species scatter significantly both in the physical space and in the state space. Moreover, the gradient vector of the specific enthalpy describing energy losses towards the wall is not aligned with the gradient vectors of the species, which can be considered as a typical case while a flame in application might approach to the wall at any arbitrary transversal direction. This observation motivates to take the gradients’ scattering and multi-dimensional transport phenomena into account during model reduction to describe reliably the quenching process.The Reaction-Diffusion Manifold (REDIM) method is applied in this work. The method allows to take into account multi-dimensional transport in a very generic way. In order to generate the REDIM, gradient estimates are approximated by using a Singular-Value Decomposition (SVD) of SWQ detailed gradients fields. Two-dimensional REDIMs for both cases are constructed and compared to each other. Different transport (diffusion) models are implemented to compare quantitatively the manifolds with HOQ and SOQ gradients estimates. The comparison shows that the differences between reduced models with varying transport models is significantly larger than the differences for varying configurations (multidimensional gradient estimates). This justifies the use of a relatively simple REDIM for more complicated geometries and configurations. This simplifies the treatment and model reduction procedure significantly for such complicated transient phenomena.

Simultaneous 1D hybrid fs/ps rotational CARS, phosphor thermometry, and CH* imaging to study transient near-wall heat transfer processes

Near-wall transient heat transfer and flame–wall interaction (FWI) are topics of great importance in the development of downsized internal combustion (IC) engines and gas turbine technology. In this work we perform measurements using 1D hybrid fs/ps rotational CARS (HRCARS), thermographic phosphors (TGP) and CH* imaging in an optically-accessible chamber designed to study transient near-wall heat transfer processes relevant to IC engine operation. HRCARS provides single-shot gas-phase temperatures (40 µm spatial resolution and up to 3 mm wall-normal distances), while thermographic phosphors measures wall temperature and CH* measures the flame front position. These simultaneous measurements are used to resolve thermal boundary layer (TBL) development and associated gaseous heat loss for three important processes of gas–wall interactions: (1) an unburned-gas polytropic compression process, (2) FWI, and (3) post-flame and gas expansion processes. During a mild polytropic compression process, measurements emphasize that even a relatively small wall heat flux (≤5 kW/m2) yields an appreciable temperature stratification through a developing TBL. During FWI, thermal gradients induced by the flame are resolved within the TBL. Gases closest to the wall (y<0.2 mm) continue to experience thermal loading from polytropic compression until the flame is within ∼1.4 mm from the wall. Immediately afterwards, the wall first senses the flame as the wall temperature begins to increase. During FWI, gas temperatures up to 1150 K impinge on the wall, producing peak wall heat fluxes (620 kW/m2) and the wall temperature increases (ΔTwall=14 K). Gaseous heat loss in the post-flame gas occurs rapidly at the wall, yielding a TBL of colder gases extending from the wall as wall heat flux slowly decreases. HRCARS further captures the rapid cooling of gases in the TBL and core-gas during the mild expansion and exhaust process.

Near Wall Dynamics of Premixed Flames

Highly-resolved numerical simulations employing detailed reaction kinetics and molecular transport have been applied to flame-wall interaction (FWI) of laminar premixed flames. A multiple plane-jet flame (2D) has been considered, which is operated with premixed methane/air mixtures at atmospheric conditions and with different equivalence ratios. Free flame (FF) and side-wall quenching (SWQ) conditions have been accomplished by defining one lateral boundary as either a symmetry plane for FF or a cold wall with fixed temperature for SWQ. An equidistant grid with a resolution of 20 µm is used to resolve the FWI zone. The GRI-3.0 mechanism is used for computing chemical reaction rates. The flame is tangentially compressed when approaching the cold wall, and elongated in the FF case, causing an inversion of the sign of the tangential strain rate Kas and a considerable decrease of the total stretch rate Katot for the SWQ flame. The flame consumption speed SL decreases with decreasing normal stretch due to curvature Kac while approaching the cold wall, but it increases with decreasing Kac for the FF case, leading to an inversion of the Markstein number Matot based on Katot from positive in FF to negative in the SWQ case. The results reveal a strong correlation of flame dynamics during transitions from FWI to freely propagating flames, which may bring a new perspective for modeling FWI phenomena by means of flame dynamics. To do this, the quenching effect of the wall may be reproduced by an inversion of the Markstein number from positive to negative in the FWI zone and applying the general linear Markstein correlation, leading to a decrease of the flame consumption speed. In addition, the quenching distance evaluated from SL has been found to be almost equal to the unstretched laminar flame thickness, which compares quantitatively well with measured data from literature.

Reduction of RANS/LES combustion sub-models for quasi-dimensional spark ignition engine simulations and evaluation of the modelling assumptions with DNS

Despite the significant improvement of computational resources, large eddy simulations (LES) are too expensive to be applied to wide ranges of operating conditions and multiple engine architectures. Efficient models employed in quasi/zero-dimensional approaches may offer an attractive alternative for engine calibration or optimisation. Despite their fair predictive capability, validity is typically limited to around their calibration region and is subject to uncertainties that stem from heuristic simplifications. The validity of the individual sub-models is rarely assessed using detailed three-dimensional (3-D) simulations over a wide range of combustion regimes. The objectives of this work are the following: (i) to present a formal reduction of widely applied 3-D LES combustion sub-models found in the literature with particular emphasis on the early flame development and flame-wall interaction, (ii) to assess the assumptions/approximations introduced in quasi-dimensional (Q-D) modelling using Direct Numerical Simulation (DNS), and (iii) to refine the Q-D models used currently, using formally-derived sub-models.It is found that many of the refinements introduced noticeably improve the accuracy of the Q-D model. The functional form of the models obtained through formal reduction of various LES combustion sub-models presents similarities and agrees well with the heuristic functions that can be found in existing Q-D models. Ultimately, this study enhances the transferability of insights from fundamental investigations to real engine applications, which can be useful for future Q-D model development and validation.

Strain Rate Effects on Head-on Quenching of Laminar Premixed Methane-air flames

Head-on quenching is a canonical configuration for flame-wall interaction. In the present study, the transient process of a laminar premixed flame impinging on a wall is investigated for different strain rates, while previous studies with detailed chemistry and transport focused only on unstrained conditions. Increasing strain rate leads to a reduction in the normalized quenching distance, and an increase in the normalized wall heat flux, both are considered as global flame quantities. Looking more into the local microstructure of the quenching process, CO formation and oxidation near the wall are shifted to higher temperatures under higher strain rates. Further, the local flame structure and the thermochemical state are affected by differential diffusion driven by differences in species’ gradients and diffusivities. Quenching leads to increased species’ gradients and consequently differential diffusion is amplified near the wall compared to propagating flames. However, this effect is suppressed for increasing strain rates, which is explained in more detail by a source term analysis of the transport equation for the differential diffusion parameter ZHC. Results for the global quantities and the local flame structure show that the impact of the strain rate weakens for higher wall temperatures. Finally, the analyses of the thermo-chemical quantities in the composition space shows that H2 can be a good parameter to characterize the strain rate both for propagating and quenching flamelet.

Formations and emissions of CO/NO2/NOx in the laminar premixed biogas-hydrogen flame undergoing the flame-wall interaction: Effects of the variable CO2 proportion

Experimental and numerical studies were conducted to investigate the effects of CO2 proportion on the formation processes and overall emissions of CO and NOx of the laminar premixed biogas-hydrogen flames undergoing the flame-wall interaction (FWI). The overall EICO, EINOx, EINO2 and NO2/NOx ratio were obtained. The near-wall combustion processes of the wall-impinging flame at different separated distance (H) were simulated numerically using a computational model, and four major routes (thermal route, prompt route, NNH intermediate route and N2O intermediate route) of the NO production were calculated separately. The results show that the EICO decreases at small H but increases at large H with the increased CO2 proportion, which is dominated by the cooling plate and dilution/chemical effects of CO2, respectively. This indicates that the overall EICO of biogas fuel can be reduced effectively with the enhanced CO2 proportion when the FWI effects are intensive. The overall EINOx is decreased obviously with the increased CO2 proportion owing to the declined temperature and reduced active radicals. With the increased CO2, the contributions of thermal route and N2O route are decreased considerably while the importance of prompt route and NNH route are increased evidently. The strong FWI effects can increase the importance of prompt route while decrease the contribution of thermal route effectively. The EINO2 of biogas-H2 flame is decreased with the increased CO2 proportion. The dilution effects of CO2 can improve the NO2 conversion efficiently but this NO2 conversion process can be suppressed effectively by the intensive FWI effects.

Computational analysis of premixed methane-air flame interacting with a solid wall or a hydrogen porous wall

The process of flame-wall interaction for premixed methane-air flames is investigated by direct numerical simulation. The flames propagate towards an isothermal, chemically inert surface, consisting of either a solid impermeable wall (IW) material or a hydrogen-permeable wall (PW) material. With the PW, hydrogen seeps into the domain and participate as a secondary, non-premixed fuel. The skeletal methane-air chemical reaction kinetics mechanisms of Smooke and Giovangigli and DRM22 are used with the S3D code to study the major reactions controlling the flame-wall interactions (FWI). Initially, results of said mechanisms are compared to the complete GRI 3.0 scheme. The configurations are investigated for two temperatures, 600 K and 750 K, of the wall and the unburnt gas, and for initial equivalence ratios of 0.5, 1.0 and 1.5. Results for IW are similar to previous FWI studies. The flame quenches at the wall, with maximum heat release and wall heat flux occurring close to the quenching instance. For the PW cases, the flame quenches before reaching the wall. This is explained by the mutual effects of convective heat transfer away from the wall and flame due to permeation, a high concentration of hydrogen and high local fuel-to-oxidizer ratio, reduced temperature and reduced reaction heat release. The quenching definition and flame position are based on OH radicals concentration. The observed maximum wall heat flux is much lower than for IW, and occurs some time after quenching. A discussion about the quenching process indicates that a definition based on maximum wall heat flux is inappropriate.