Slot Burner Research Articles

Parametric Study of the Effects of a Vortex Generator on the Combustion Characteristics of Liquid Petroleum Gas and Physical Air–Fuel Flow on a Slot Burner

Parametric Study of the Effects of a Vortex Generator on the Combustion Characteristics of Liquid Petroleum Gas and Physical Air–Fuel Flow on a Slot Burner

Improved combustion stability of biogas at different CO2 concentrations using inhomogeneous partially premixed stratified flames

Biogas is a renewable and clean energy source that plays an important role in the current environment of low- carbon transition. If high-content CO2 in biogas can be separated, transformed, and utilized, it not only realizes high-value utilization of biogas but also promotes carbon reduction in the biogas field. To improve the combustion stability of biogas, an inhomogeneous, partially premixed stratified (IPPS) combustion model was adopted in this study. The thermal flame structure and stability were investigated for a wide range of mixture inhomogeneities, turbulence levels, CO2 concentrations, air-to-fuel velocity ratios, and combustion energies in a concentric flow slot burner (CFSB). A fine-wire thermocouple is used to resolve the thermal flame structure. The flame size was reduced by increasing the CO2 concentration and the flames became lighter blue. The flame temperature also decreased with increase in CO2 concentration. Flame stability was reduced by increasing the CO2 concentration. However, at a certain level of mixture inhomogeneity, the concentration of CO2 in the IPPS mode did not affect the stability. Accordingly, the IPPS mode of combustion should be suitable for the combustion and stabilization of biogas. This should support the design of highly stabilized biogas turbulent flames independent of CO2 concentration. The data show that the lower stability conditions are partially due to the change in fuel combustion energy, which is characterized by the Wobbe index (WI). In addition, at a certain level of mixture inhomogeneity, the effect of the WI on flame stability becomes dominant.

DNS of a non-premixed CH4/O2 flame in a supercritical CO2 environment

3D Direct Numerical Simulation (DNS) is performed to investigate turbulent non-premixed methane oxy-combustion in a CO2-rich environment at supercritical conditions (300 bar). The study focuses on a shear-layer configuration typical of slot burners, where the central slot carries the fuel and the oxidant mixture O2/CO2 flows on both sides of the central jet. The simulation solves the compressible Navier–Stokes equations coupled with the translated volume formulation of the Peng–Robinson cubic equation of state (VTPR-EoS), utilizing a reduced 23-species kinetic scheme derived from the ARAMCO 2.0 complete mechanism. The extremely high pressure results in turbulent scales smaller than those observed at lower pressures. Both premixed and diffusion flame zones are identified, with heat primarily released in non-premixed rich regions, leading to a CO mass fraction of up to 0.55. The flow exhibits uphill (counter-gradient) diffusion for both CO2 and H2O, primarily influenced by the interaction of binary diffusion coefficients and individual species chemical potential gradients. Values in the range of 2.52−2.62 are obtained for the fractal dimension of temperature iso-surfaces. Mixture fraction probability density functions (PDFs) are compared with the standard presumed β−PDF, revealing an underestimation of mixing. Differently from flames at atmospheric pressures, the Dufour effect in the energy diffusion flux, is not negligible. Furthermore, fugacity coefficients must be considered in the calculation of chemical species kinetic rates.

A subgrid-scale model to account for thermo-diffusive effects in artificially thickened LES models for lean turbulent premixed ammonia/hydrogen flames

With the introduction of carbon-free fuels and their rapid implementation, accurate combustion models are needed to design decarbonized engines, gas turbines or burners. Recent experimental and numerical work have highlighted the high burning rate of lean hydrogen and hydrogen/ammonia flames and their deviation from flamelet assumptions, pointing out the need to revise classical combustion models for Reynolds Averaged Navier–Stokes (RANS) or Large-Eddy Simulations (LES). In this work, a novel subgrid-scale model accounting for stretch effects for lean ammonia/hydrogen (NH3/H2) flames in the Thickened-Flame LES framework (TFLES) is proposed as an extension to the standard Charlette model Charlette et al. (2002). This model, called ESE−NH3, is based on 1-D premixed strained flames and validated a priori with Direct Numerical Simulations (DNS) and used to perform LES simulation of a turbulent slot burner configuration. To assess the performance of the novel subgrid model, LES predictions are compared a posteriori to the DNS and to LES employing the standard Charlette model. The comparison shows that the LES computation with the novel subgrid-scale model accounting for stretch effects predicts flame lengths closer to the DNS. This shows that thermo-diffusive effects can be integrated simply in the TFLES framework. Moreover, results show that these effects are significant in all lean NH3/H2 flames even at low H2 contents.

The effect of mixture inhomogeneity and turbulence on the flame front curvature and flame surface density of turbulent planar flames of natural gas

The present study is an extension of our earlier mixing field investigation in partially premixed flames. In the current work, the link between the mixing field and flame structure, and hence flame curvature and surface density are experimentally studied. The flame structure is captured using a high-speed OH-PLIF technique, whereas the crosswise mixing fields were previously presented in our earlier study. A concentric flow slot burner is employed for producing turbulent planar partially premixed natural gas flames at different levels of mixture inhomogeneity. The effects of the level mixture inhomogeneity, the equivalence ratio, the air to fuel stream velocity ratio, and Reynolds number “Re” on the flame characteristics are investigated. The link between the previously reported data on the mixing field and the flame structure are examined. The correlations between the mixing field and the flame structure were clearly observed in the current study. That is, as the mixture becomes highly inhomogeneous, in rich input jet conditions cases, altering Reynolds number and the mixing length led to a pronounced change in both flame structure and flame curvature while altering L/D influences the flame structure. On the other hand, the effects of the mixing length and Reynolds number on the flame structure and curvature in a more homogeneous mixture were not significant. The data showed that the flame surface density was inversely proportional to the level of the mixture inhomogeneity and directly proportional to the Reynolds number. The significant effect of the jet equivalence ratio on the level of mixture inhomogeneity leads to a significant change in the flame structure and flame curvature. In addition, if the mixture inhomogeneity is less pronounced the variation s of the mixing length and Reynolds number lead to minimal effects on the flame structure and flame curvature.

The mixing field and flame structure near the reaction zone of turbulent planar flames at different levels of mixture inhomogeneity

Turbulent flames with compositionally inhomogeneous mixtures are commonly used in many combustion systems. In this work, turbulent planar jet flames issued from a concentric flow slot burner, CFSB, were used to study the impact of mixture inhomogeneity near the flame sheet. The CFSB burner can control the mixing inhomogeneity by changing the mixing length “L” between the concentric fuel and air slot nozzles. At various levels of mixture inhomogeneity, the mixing field, presented by mixture fraction, Z, distribution, and the flame structure was investigated via simultaneous Rayleigh and OH-PLIF imaging technique. Our previous study investigated the mixing field immediately downstream of the burner exit in non-reacting conditions. The PDFs of Z showed that the mixing field covered a wide range of mixture fractions, where the high flame stabilization occurred when a large portion of the PDF(Z) was located within the fuel flammability limits. This work showed that further downstream, the highly stabilized flames were also obtained when the range of fluctuations in the mixture fraction was close to the stoichiometric mixture fraction. Moreover, plotting the mixing field using the scatter plot within the mixing regime diagram for various flame conditions showed that the mixing field downstream of the burner exit consistently follows the mixing diagram classification. Moreover, a close inspection of the flame structure showed that the flame sheet varies from thick, and corrugated at a low and high levels of mixing to thin flame sheet but less corrugated and that at a particular mixing normalized length L/D = 7. The flame corrugation data showed that the flame stability occurs at a minimum corrugation factor.

Direct numerical simulations of methane, ammonia-hydrogen and hydrogen turbulent premixed flames

Three turbulent premixed flames with the same unstretched laminar flame speeds and thicknesses are analyzed and compared using 3D Direct Numerical Simulation (DNS) in a slot burner configuration at atmospheric conditions: a CH4/air flame at ϕ=1, an NH3−H2/air (46% vol. H2) at ϕ=1 and an H2/air flame at ϕ=0.45.While both stoichiometric methane and ammonia-hydrogen flames behave similarly, the lean hydrogen flame brush is twice as short with less flame surface, and exhibits significant alteration of its local flame structure: for the H2 flame, the thickness of flamelets decreases significantly while burning rates increase drastically. This is observed for flame elements convexly curved with respect to fresh reactants (positive curvature) because of preferential diffusion, where thermo-diffusive instabilities generate long-tail structures that continuously head toward the fresh gases, and also for near-flat flame elements because of local strain effects. Opposite behaviors are observed for flame elements concavely curved (negative curvature) where fuel depletion caused by unfocusing of hydrogen and strain effects are unable to increase the consumption, even leading to near-extinction of the flame. The turbulent methane and ammonia-hydrogen flames studied in this work do not exhibit the instabilities seen in the pure hydrogen flame. However, local flame-flame interactions are observed in the cusp regions of all three flames, which significantly increase the displacement speed as well as flame surface destruction.A 1D-3D comparison indicates that strain effects in regions of low curvature and preferential diffusion effects in regions of positive curvature can be modeled using counterflow flamelets to account for stretch effects on the turbulent hydrogen flame propagation in addition to turbulent wrinkling.

Acceleration of turbulent combustion DNS via principal component transport

We investigate the implementation of principal component (PC) transport to accelerate the direct numerical simulation (DNS) of turbulent combustion flows. The acceleration is achieved using the transport of PCs and the tabulation of the closure terms in the PC-transport equations using machine learning. Further acceleration is achieved by a treatment for bottlenecks associated with the acoustic time steps for low Mach number flows. The approach is implemented in 2D and 3D on a laboratory scale lean premixed methane-air flame stabilized on a slot burner. DNS based on the transport of thermochemical scalars (species and energy) is also carried out, first to develop a 2D DNS database for PC-transport equations’ closure terms and, second, to validate the approach against species DNS in 2D and 3D, a principal goal of the present effort. The results show that surrogate PC DNS can reproduce instantaneous profiles as well as statistics associated with turbulence, flame topology properties and measures of flame-turbulence interactions. The study also demonstrates that parametric simulations with surrogate PC DNS can be implemented at a fraction of the cost of a full 3D DNS with species and energy transport.

Co-flow jet diffusion flames in a multi-slot burner: Flow field and emissions

Motivated by the application of internally air-assisted flares and recent anomalous data that black carbon (BC) emissions changes are not monotonic with the amount of air added inside the fuel stream, a burner was designed to study the flow, emissions, and stability aspects of the existence of both normal and inverse jet diffusion flames in close proximity. Since the radius of curvature of a burner affects all aspects of combustion, such as dynamics, stability, flame structure, and emissions, a slot burner configuration, inspired by the Wolfhard-Parker burner, was adopted. This multi-slot burner consists of five parallel rectangular slots (i.e., the central slot for the inner air, sandwiched between two fuel slots, and surrounded by two outer air slots). When operated with laminar fuel and air flows, this burner produced flame sheets at each fuel-air mixing layer with open optical access to all the flows. Flow fields were characterized using two-dimensional two-component particle image velocimetry, while simultaneous single-lens reflex photography was used to establish the overall height and the location of the flames. Emission measurements specifically targeted BC, a precursor of soot, and oxides of nitrogen (NOx). The experimental test conditions involved constant flows of outer air and propane, and variable inner air flow. At one extreme, i.e., zero inner air flow, two flame bases were produced at the interfaces between the fuel and outer air streams. These flames merged to produce a normal diffusion flame. With the addition of inner air, two more flame bases appeared (referred to as the inner flame) and formed an inverse diffusion flame. With increasing inner air flow, the inner flame transitioned from a closed-tip flame, to an open-tip flame, and eventually to a lifted flame. With increasing inner air flow, the NOx emissions remained constant while the BC emissions, which were 3 g/kg-fuel with no inner air, rose by an order of magnitude as it became an open-tip flame. Only when the inner flame finally lifted off due to the inner air flow, did the BC emissions collapse to near zero. Phenomenological models associated with the importance of partial premixing were proposed to explain this collapse, thereby generalizing this finding to other combustion systems when attempting to reduce BC emissions through secondary internal air addition.

Development of laminar burning velocity measurement system in premixed flames with hydrogen-content syngas or strong oxidizer conditions in a slot burner

This study presents the experimental determination of laminar burning velocity for critical premixed flames with multiple fuels or strong oxidizers. Experiments were conducted with a slot burner under methane/air, methane/nitrous oxide, and syngas (CH4/CO/H2)/air premixed flames conditions with varying equivalence ratios from 0.8 to 1.4. With the flame surface area determined from the Schlieren measurement system and stretch effect corrected by Markstein length, unstretched laminar burning velocity can be garnered according to the conservation of mass. First, the experimental results of methane/air premixed flame velocities were validated by comparing with one-dimensional unstretched burning velocities through numerical simulations with GRI 3.0 mechanism. The experimental results in the equivalence ratio ranging from 0.85 to 1.2 demonstrated errors less than 3.8%. Then, the validated burning velocity measuring techniques were implemented under methane/nitrous oxide and syngas/air premixed flame conditions. Finally, appropriate chemical mechanisms, such as USM or UGM, can be validated for the numerical simulation of critical premixed flames via this proposed laminar burning velocity measuring technique.

Synergistic interactions of thermodiffusive instabilities and turbulence in lean hydrogen flames

Interactions of thermodiffusive instabilities and turbulence have been investigated by large-scale Direct Numerical Simulations (DNS) in this work. Two DNS of turbulent premixed lean hydrogen/air flames have been performed in a slot burner configuration at the same jet Reynolds number of Re=11,000 and Karlovitz number of Ka≈15 using a detailed chemical mechanism. Realistic transport models are employed in one case, which features the characteristic patterns of thermodiffusively unstable flames, such as strong variations of the heat release and super-adiabatic temperatures. In the other case, the diffusivities of all species are set equal to the thermal diffusivity (unity Lewis numbers assumption) and thermodiffusive instabilities are therefore suppressed.The local burning of the turbulent flame without thermodiffusive instabilities is similar to an unstretched laminar flame and the turbulent flame speed increases only due to the increase of flame surface area in agreement with previous studies for flames at similar conditions. In contrast, the thermodiffusively unstable flame features a strong enhancement of the turbulent flame speed, which is not only caused by flame wrinkling, but is greatly increased due to significant variations of the local reaction rates. These are caused by variations of the local equivalence ratio due to the differential diffusion of hydrogen. A comparison with a thermodiffusively unstable laminar flame at the same conditions reveals that the variations of the local equivalence ratio and local reaction rates are significantly enhanced in the turbulent flame due to higher fluctuations of curvature and an enhanced average strain rate induced by turbulence. Thus, turbulence and thermodiffusive instabilities show synergistic effects, which are reflected in a significantly higher fuel consumption rate per flame surface area. The flame surface area generation, which is governed by the tangential strain and the flame propagation in curved flame segments, is also different in the two cases. Most noteworthy, the tangential strain rate is shown to be determined by the smallest turbulent structures in both turbulent flames and to be unaffected by the thermodiffusive instability mechanism. However, thermodiffusive instabilities lead to a production of flame surface area in convexly curved flame segments, featuring the formation of tongue-like structures that penetrate into the unburned gas, which do not exist in the turbulent flame with unity Lewis numbers. This is linked to an enhancement of the flame displacement speed with curvature in the thermodiffusively unstable flame, while in the absence of instabilities, a reduction of the flame displacement speed with increasing curvature is observed, leading to a destruction of flame surface area.These findings suggest that thermodiffusive instabilities are sustained in turbulent flows and even show synergistic interactions with turbulence, which needs to be accounted for in turbulent combustion models.

One zirconia-based ceramic coating strategy of combustion stabilization for fuel-rich flames in a small-scale burner

From the perspective of the thermal/chemical performance management of a slot burner, three kinds of blended coatings with the low thermal conductivity and certain chemical activities, including 30wt.%Y2O3/ZrO2, 30wt.%MgO/ZrO2 and 30wt.%Al2O3/ZrO2, were prepared on stainless steel walls plated by atmospheric plasma spraying. The effects of coating material, wall temperature, and equivalence ratio on fuel-rich methane flame characteristics were investigated experimentally. The OH planar laser-induced fluorescence (OH-PLIF) technique was used to obtain OH intensity and flame anchoring location. The physical and chemical properties of different coatings before and after spraying were analyzed by a series of material and surface characterization techniques. Results show that compared with pure ZrO2 coating, the quenching distances of three ZrO2-based blended coatings with Y2O3, MgO, and Al2O3 decrease to some extent, resulting in the minimum quenching distance of Al2O3/ZrO2 coating, and the maximum one for Y2O3/ZrO2 coating. Meanwhile, the blended coating walls result in the higher flame anchoring location than that of pure ZrO2 in most cases, in which the coating of Y2O3/ZrO2 displays the highest location. The blended coatings have higher thermal conductivity, which is beneficial to balance part of the heat loss through the axial heat recirculation of wall structures, and thus expanding stability limits at low temperatures. In addition, partial lattice solid solutions are formed in mixture systems after the ultra-high-temperature thermal spraying, leading to more surface lattice oxygen and stronger chemical promotion. In summary, reasonable allocation of ZrO2-based ceramic components as coating wall is an effective alternative strategy to control flame stabilization in small-scale combustion devices.

Detailed investigation of the mixing field and stability of natural gas and propane in highly turbulent planar flames

In most practical combustion devices, the actual combustion process occurs within different mixture inhomogeneity levels. Investigating the mixture fraction field upstream of the reaction zones of these flames is an essential step toward understanding their structure, stability, and emission formation. In this study, the mixture fraction fields were measured for turbulent non-reacting inhomogeneous mixtures immediately downstream from the slot burner exit, using Rayleigh scattering imaging. The slot burner had two concentric slots. The inner air slot can be recessed at distances upstream from the exit of the outer fuel slot, allowing various degrees of mixture inhomogeneity. Mixture fraction field statistics and the two-dimensional gradient were utilized to characterize the impact of the air-to-fuel velocity ratio, global equivalence ratio, fuel composition, Reynolds number, and the premixing length on the mixture mixing field, and thus flame stability. These impacts were evaluated by tracking the normalized mean mixture fraction and mixture fraction fluctuation transition across the regime diagram for partially premixed flames. The results showed that the air-to-fuel velocity ratio was the critical parameter affecting the mixture fraction field for the short premixing length. Stability results showed that the level of mixture inhomogeneity mainly influenced the flame stability. High flame stability is achieved if a large portion of the inhomogeneous mixture fraction is within the fuel flammability limits.

Generalization Capability of Convolutional Neural Networks for Progress Variable Variance and Reaction Rate Subgrid-Scale Modeling

Deep learning has recently emerged as a successful approach to produce accurate subgrid-scale (SGS) models for Large Eddy Simulations (LES) in combustion. However, the ability of these models to generalize to configurations far from their training distribution is still mainly unexplored, thus impeding their application to practical configurations. In this work, a convolutional neural network (CNN) model for the progress-variable SGS variance field is trained on a canonical premixed turbulent flame and evaluated a priori on a significantly more complex slot burner jet flame. Despite the extensive differences between the two configurations, the CNN generalizes well and outperforms existing algebraic models. Conditions for this successful generalization are discussed, including the effect of the filter size and flame–turbulence interaction parameters. The CNN is then integrated into an analytical reaction rate closure relying on a single-step chemical source term formulation and a presumed beta PDF (probability density function) approach. The proposed closure is able to accurately recover filtered reaction rate values on both training and generalization flames.

Mapping the stability of free-jet biogas flames under partially premixed combustion

Enhancement of biogas combustion characteristics will increase the possibility of the direct utilization of such an eco-friendly fuel in practical combustors. We report a full study of biogas combustion under partially premixed mode to investigate the stability of biogas free-jet flames using a concentric flow slot burner was used to evaluate the biogas combustion characteristics. Five mixtures of biogas ranging from 0%CO2 up to 40%CO2 were investigated to study the effect of CO2 proportion on the stability of biogas flames. A new well-defined stability procedure was followed by studying the four major observed phenomena in biogas flames: Stable Flame, Partially-lifted Flame, Fully-lifted Flame, and Extinction. Each phenomenon was clearly defined in addition to describing the operating conditions in terms of Reynolds Number (Re) and Equivalence ratio (ϕ). The effect of the premixing ratio (LD) on the combustion stability of biogas flames was also studied for five premixing ratios LD3, LD5, LD7, LD10, and LD16 to optimize the flame stability. The temperature of stable biogas flames was also measured at different CO2 percentages. The results showed that LD10 is the optimum premixing ratio to generate a stable biogas flame at all tested CO2 proportions. Partially premixed combustion in slot burner manages to sustain a stable biogas flame up to 30%CO2. However, increasing the CO2 to 40% eventually led to a weak and unstable flame regardless of the premixing ratio.

Structure and stability characteristics of turbulent planar flames with inhomogeneous jet in a concentric flow slot burner

Turbulent flames with compositionally inhomogeneous mixtures are common in many combustion systems. Turbulent jet flames with a circular nozzle burner were used earlier to study the impact of inhomogeneous mixtures, and these studies showed that the nozzle radius affects the flame stability. Accordingly, planar turbulent flames with inhomogeneous turbulent jet are created in a concentric flow slot burner (CFSB) to avoid this effect in the present study. The stability characteristics, the mixing field structure, and the flame front structure were measured, and the correlations between stability and the mixing field structure were investigated. The mixture fraction field was measured in non-reacting jets at the nozzle exit using highly resolved Rayleigh scattering technique, and the flame front was measured in some selected turbulent flames using high-speed Planar Laser-Induced Fluorescence (PLIF) of OH technique. The data show strong correlations between flame stability and the range of mixture fraction fluctuations. The flames are highly stabilized within a mixing field environment with the range of fluctuation in mixture fraction close to the range of the flammability limits. The mixing field structure is also illustrated and discussed using a mixing regime diagram and showed that the scatter of the data of the different cases is consistent with the classified mixing regimes. Lean flames are stabilized in the current slot burner. The flame front structure topology varies consistently from thin, small curvature at the low level of turbulence and higher equivalence ratio to more wrinkled, larger curvature, but a thicker structure at a higher level of turbulence and lower equivalence ratio.

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.

The effect of pressure on the hydrodynamic stability limit of premixed flames

The effect of pressure on the hydrodynamic stability limits of lean methane–air premixed flames is investigated with Direct Numerical Simulation based on multi-step chemistry and using a simplified one-step chemistry formulation. The dependency on pressure p of the cut-off length scale λc, that separates stable from unstable wavelengths of the initial perturbation, is computed for a number of different conditions. An increase of pressure causes a significant decrease of the cut-off length, as observed already in previous simulations and experiments. However, this decrease cannot be ascribed only to the decreased flame thickness due to elevated pressures, but the cut-off is reduced significantly even if normalized by either the thermal flame thickness ℓT or the diffusive flame thickness ℓD. For the conditions analyzed, the cut-off can be well approximated by the power-law λc∝p−0.8, while the thermal and diffusive flame thicknesses, in accordance with previous experiments, are proportional and scale as ℓT∝ℓD∝p−0.3. Therefore, the non-dimensional cut-off scales as λc/ℓT∝λc/ℓD∝p−0.5. This behavior is linked to the increase of the Zeldovich number with pressure, caused by higher inner layer temperatures at higher pressures, which is a result of increased importance of chain termination reactions. The same behavior is observed also in a one-step chemistry approach if the Zeldovich number, appearing explicitly in the one-step model equations, is varied with pressure according to the results from multi-step chemistry. The analysis is extended to the non-linear phase of the instability, when typical strong cusps are observed on the flame surface, simulating a two-dimensional slot burner for different pressures; it is confirmed that the same pressure effects are observed also in more complex settings and in the non-linear regime.

Structure and propagation of n-heptane/air premixed flame in low temperature ignition regime

This paper presents a large eddy simulation of n-heptane/air turbulent premixed combustion in a reactor assisted turbulent slot (RATS) burner under different preheating conditions. N-heptane/air mixture at an equivalence ratio of 0.6, pressure of 1 atm and temperature of 600, 650 and 700 K is considered to investigate the effect of low temperature chemistry on turbulent burning velocities and flame regimes, including chemically frozen (CF) regime where the fuel/air mixture inside the burner is chemically frozen, low temperature ignition (LTI) regime where the fuel/air mixture inside the burner undergoes LTI reactions, and transition regime from CF to LTI. The results show that the flame in the LTI regime exhibits the highest turbulent burning velocity. Differential diffusion is found to play an important role in the LTI regime whereas it is less important in the CF regime. To investigate the effect of LTI reactions on the flame, a series of two-dimensional laminar flames are simulated, in which the effect of turbulence on the flames is eliminated. The results show that in the LTI regime, the laminar burning velocity is drastically enhanced and the heat release zone is broadened. Budget term analysis shows that the enhanced rate of production and diffusion towards the preheat zone of the flames and the smaller gradient of reactant mass fraction are the main reasons behind the increased laminar burning velocity in the LTI regime.

Numerical simulations of soot-radiation from buoyant turbulent wall-bounded diffusion flames

Buoyant, turbulent diffusion flames anchored on a slot burner in the presence of an isothermal, low-temperature (333 K) wall have been numerically simulated. A laminar smoke point based subgrid (flamelet) radiation model has been applied toward estimation of overall radiant fractions and radiant power distributions of the ethylene fires. The model uses a turbulent micro-scale strain rate and an enthalpy defect that influence the flamelet soot radiation. In the presence of the isothermal, low-temperature wall, the radiant fractions of the fires are observed to decrease along with a reduction of their peak radiant power output. Predicted flame heights, radiant fractions and radiant power distributions are found to reproduce observed experimental trends and exhibit quantitative agreement with measurements. Comparison of radiant power distributions are also made for fires adjacent to high-temperature (1000 K) isothermal and adiabatic walls and peak radiant power output for both scenarios are found to be higher compared to the isothermal, low-temperature wall-bounded fire.