Turbulent Burning Velocity Research Articles

Effect of Ionic Wind Induced by DC Electric Field on Biogas/Air Turbulent Premixed Flame Structure

ABSTRACT Effect of DC electric field on lean premixed biogas turbulent flame structure at low and medium turbulence intensity was studied experimentally using OH-PLIF. OH signal distribution, turbulent burning velocity and flame structure geometric parameters were derived. Results show that downward electric field could broaden the downstream burned gas and push the flame tip down. The flame base is closer to the burner rim in downward electric field for the modification of thermal diffusion and flow modification caused by ionic wind. The effect on flame tip height reduction is more obvious for biogas flame with a higher CO2 ratio. According to the flame structure analysis, the turbulent burning velocity of biogas is increased slightly due to the modification of flow field by ionic wind generated by downward electric field, and the increment decreases with the increase of turbulence intensity. In addition, there’s a decrease in mean flame surface density and an increase in centerline flame brush thickness. The flame surface curvature PDF distribution is nearly not affected by the application of DC electric field. For turbulent flames, the effect of electric field is non-uniform and inconstant, which induces extra flow perturbation. This perturbation is different from turbulence, and the combined effects lead to the variation of flame structure.

Flame structure, turbulent burning velocity and its unified scaling for lean syngas/air turbulent expanding flames

A systematic experimental study of lean premixed syngas/air turbulent expanding flames has been conducted under a wide range of turbulence intensities (0–3.54 m/s), initial pressures (0.5–5 bar), and hydrogen volumetric fractions up to 80% (20%, 50% and 80%). Flame structure and turbulent flame propagation dynamics were investigated. Results show that the flame becomes more refined and wrinkled with the increasing of both turbulence intensity and initial pressure, leading to a larger flame area and the associated turbulent burning velocity (ST). With hydrogen fraction increased, ST is also enhanced significantly, which is mainly due to the promotion of laminar burning velocity (SL) and diffusional-thermal instability. ST/SL is nearly kept constant with hydrogen fraction, which is a trade-off between strengthened diffusional-thermal instability and weakened turbulence stretch. A unified scaling of ST is obtained, indicating that turbulent Reynolds number (ReT) is a practical method to correlate ST when Lewis number is close to unity. Furthermore, at least in the interpretation domain, ST of spherical flames continually increases as the flame expands, which has been referred as flame acceleration phenomenon. It appears that only effective turbulence intensity itself is not able to reflect acceleration phenomenon completely. Turbulent expanding flames follow a self-similar propagation law and the quantitative ST dependence with flame expanding is ST∼R0.5 approximately.

Numerical and experimental study on turbulence statistics in a large fan-stirred combustion vessel

Particle image velocimetry (PIV) has become a popular non-intrusive tool for measuring various types of flows. However, when measuring three-dimensional flows with two-dimensional (2D) PIV, there are some uncertainties in the measured velocity field due to out-of-plane motion, which might alter turbulence statistics and distort the overall flow characteristics. In the present study, three different turbulence models are employed and compared. Mean and fluctuating fields obtained by three-dimensional computational fluid dynamics modeling are compared to experimental data. Turbulence statistics such as integral length scale, Taylor microscale, Kolmogorov scale, turbulence kinetic energy, dissipation rate, and velocity correlations are calculated at different experimental conditions (i.e., pressure, temperature, fan speed, etc.). A reasonably isotropic and homogeneous turbulence with large turbulence intensities is achieved in the central region extending to almost 45 mm radius. This radius decreases with increasing the initial pressure. The influence of the third dimension velocity component on the measured characteristics is negligible. This is a result of the axisymmetric features of the flow pattern in the current vessel. The results prove that the present vessel can be conveniently adopted for several turbulent combustion studies including mainly the determination of turbulent burning velocity for gaseous premixed flames in nearly homogeneous isotropic turbulence.Graphic abstract

Modeling the displacement speed in the flame surface density method for turbulent premixed flames at high pressures

We propose a model of the local displacement speed for the large-eddy simulation (LES) of turbulent premixed combustion with the flame surface density (FSD) method. This model accounts for flame stretch and curvature effects in order to improve the prediction of LES–FSD for turbulent premixed flames with the non-unity Lewis number at high pressures. The proposed model is validated by a priori and a posteriori tests, with the direct numerical simulation (DNS) and LES–FSD of statistically planar turbulent premixed flames of lean hydrogen at 1 and 10 atm and a range of turbulence intensities. The a priori test confirms that the proposed model accurately estimates the negative correlation of the displacement speed and the flame curvature. In the a posteriori test, the comparison of DNS and LES–FSD results demonstrates that the proposed model improves the prediction on the turbulent burning velocity and the turbulent flame thickness, compared with those from the existing displacement–speed model. The stretch effect in the present model leads to the rise of the turbulent burning velocity and the suppression of the “bending” phenomenon in LES–FSD, consistent with the DNS result.

Effect of DME addition on turbulent flame structure in lean premixed CH4/DME/air mixtures

An experimental study was conducted on DME addition to lean premixed CH4/air Bunsen flame in order to investigate differential diffusion and turbulence effects on flame structure. Three DME volume fractions of 0, 20% and 50% were adopted in the presence of carefully controlled laminar burning velocity at a near-constant level to highlight the differential diffusion effects. By extracting the flame front from the OH-PLIF images, the flame front curvature, turbulent burning velocity and mean flame surface density were obtained and discussed. Results show that DME addition and turbulence will eventually make the curvature distribution more symmetrical. In addition with the turbulence intensity increasing, the DME addition seems to have minor effect on curvature. The turbulent burning velocity is decreased with DME addition, which is in agreement with earlier research results for Le > 1 mixtures. In this study, we prove that the mean flame surface density at different height can qualitatively represent the local consumption speed (ST,LC). DME addition only changes the ST,LC distribution structure of the flame, while turbulence not only changes the structure, but also increases the maximum ST,LC value. It is also found that the decrease of turbulent burning velocity (ST,GC), caused by DME addition at low turbulence intensity, is mainly manifested by an increase of the flame height. Moreover at thin reaction zone, the interaction between turbulence and differential diffusion effect promotes the change of ST,LC distribution. Therefore, the effect of differential diffusion on ST,GC is enhanced by turbulence as the diffusion layer is broadened.

Measurements of explosion parameters for diethyl ether/air mixtures at pre-ignition quasi-isotropic turbulence

This paper presents a detailed investigation of the explosion characteristics (i.e., maximum explosion pressure, maximum rate of the pressure rise) and combustion properties (i.e., laminar burning velocity, turbulent burning velocity) of diethyl ether (DEE)/air mixtures under different pre-ignition quasi-isotropic turbulence velocity. The concentration of DEE/air mixtures is the equivalence ratio ϕ= ~1.1. The experiment is performed by systematically measuring the pressure evolutions in a 20-L explosion spherical vessel. The present measurement shows that the peak explosion pressure (Pmax) increased nonlinearly with increasing pre-ignition turbulence velocity. The peak pressures reach the maximum values of 0.93 MPa at pre-ignition turbulence velocity 6.2 m/s, and the lowest are 0.57 MPa at pre-ignition turbulence velocity 1.0 m/s. The experimental result of the laminar burning velocity (uL) is ~ 0.55 m/s, and the largest data of turbulent burning velocity (ut) are 1.8 m/s at pre-ignition turbulence velocity at 6.2 m/s. In this work, the model gives the expression on the laminar burning velocity (uL) and turbulent burning velocity (ut) versus the pre-ignition turbulence velocity Urmsin accordance with Williams's theoretical model.

Effects of Mean Inflow Velocity and Droplet Diameter on the Propagation of Turbulent V-Shaped Flames in Droplet-Laden Mixtures

Three-dimensional carrier phase Direct Numerical Simulations of V-shaped n-heptane spray flames have been performed for different initially mono-sized droplet diameters to investigate the influence of mean flow velocity on the burning rate and flame structure at different axial locations from the flame holder. The fuel is supplied as liquid droplets through the inlet and an overall (i.e., liquid + gaseous) equivalence ratio of unity is retained in the unburned gas. Additionally, turbulent premixed stoichiometric V-shaped n-heptane flames under the same turbulent flow conditions have been simulated to distinguish the differences in combustion behaviour of the pure gaseous phase premixed combustion in comparison to the corresponding behaviour in the presence of liquid n-heptane droplets. It has been found that reacting gaseous mixture burns predominantly under fuel-lean mode and the availability of having fuel-lean mixture increases with increasing mean flow velocity. The extent of flame wrinkling for droplet cases has been found to be greater than the corresponding gaseous premixed flames due to flame-droplet-interaction, which is manifested by dimples on the flame surface, and this trend strengthens with increasing droplet diameter. As the residence time of the droplets within the flame decreases with increasing mean inflow velocity, the droplets can survive for larger axial distances before the completion of their evaporation for the cases with higher mean inflow velocity and this leads to greater extents of flame-droplet interaction and droplet-induced flame wrinkling. Mean inflow velocity, droplet diameter and the axial distance affect the flame brush thickness. The flame brush thickens with increasing droplet diameter for the cases with higher mean inflow velocity due to the predominance of fuel-lean gaseous mixture within the flame. However, an opposite behaviour has been observed for the cases with lower mean inflow velocity where the smaller extent of flame wrinkling due to smaller values of integral length scale to flame thickness ratio arising from higher likelihood of fuel-lean combustion for larger droplets dominates over the thickening of the flame front. It has been found that the major part of the heat release arises due to premixed mode of combustion for all cases but the contribution of non-premixed mode of combustion to the total heat release has been found to increase with increasing mean inflow velocity and droplet diameter. The increase in the mean inflow velocity yields an increase in the mean values of consumption and density-weighted displacement speed for the droplet cases but leads to a decrease in turbulent burning velocity. By contrast, an increase in droplet diameter gives rise to decreases in turbulent burning velocity, and the mean values of consumption and density-weighted displacement speeds. Detailed physical explanations have been provided to explain the observed mean inflow velocity and droplet diameter dependences of the flame propagation behaviour.

The relation between flame surface area and turbulent burning velocity in statistically planar turbulent stratified flames

The relation between turbulent burning velocity and flame surface area for turbulent stratified flames has been analyzed using a direct numerical simulation database. The simulations have been carried out for different initial turbulence intensities and initial integral length scales of equivalence ratio fluctuation for a given root-mean-square value in a globally stoichiometric mixture. Additionally, statistically similar turbulent premixed flames have been considered for comparison. The turbulent burning velocity and flame surface area for stratified flames are found to be significantly smaller than in premixed flames for slow mixing cases (large scalar integral length scale and low turbulence intensity), although this trend weakens in fast mixing cases (low scalar integral length scale and high turbulence intensity). In slow mixing cases, the increase in the burning rate occurs at a smaller proportion than the increase in the flame surface area. The contributions of the components arising from tangential diffusion of displacement speed and cross-scalar dissipation rate to the turbulent burning velocity are found to be negligible in comparison to that arising from the combined reaction and normal diffusion component for all cases. The increased probability of finding fuel-lean and fuel-rich mixture affects the contribution of turbulent burning velocity arising from the combined reaction and normal diffusion component of displacement speed for slow mixing cases. Front-supported flame elements dominate over back-supported flame elements for large scalar length scale cases, but this behavior reverses for small scalar length scale cases. These findings suggest that increases in the burning rate and flame surface area do not take place in the same proportions for turbulent stratified combustion.

Inhibition of turbulent methane/air premixed bunsen flames by dimethyl methylphosphonate

There is little knowledge about the inhibition efficiency of dimethyl methylphosphonate (DMMP) against turbulent flames, in which vortices of different sizes corrugate and stretch the flame front. This study is to investigate the performance and the mechanism of DMMP in inhibiting turbulent methane/air premixed Bunsen flames. Several important parameters are chosen to characterize the combustion of the flames, including flame brush height, flame brush thickness and turbulent burning velocity. It is found that the flame brush expands both horizontally and vertically with the increase of turbulent intensity. For all the flames, the centerline mean flame brush thickness increases linearly with flame height. Turbulent burning velocity decreases monotonically with the increasing of DMMP addition. However, the normalized turbulent burning velocity increases with the normalized turbulent intensity. Compared with the performance in laminar premixed flames, the inhibition efficiency depends on the turbulent intensity. For low-intensity turbulent flames, the inhibition efficiency is higher than that in laminar premixed flames, but the relative inhibition efficiency decreases with DMMP addition. This can be attributed to the relative size of the vortices and the flame front thickness: when the Kolmogorov length scale is less than the preheat zone thickness but greater than the reaction zone thickness, vortices reduce the burning velocity; when the Kolmogorov length scale is less than the reaction zone thickness, vortices facilitate the combustion. The trend is opposite in high-intensity turbulent flames, which is mainly attributed to the generating of the flame pockets in the flame tips.

Experimental investigation on ammonia combustion behavior in a spark-ignition engine by means of laminar and turbulent expanding flames

Ammonia combustion appears as a meaningful way to retrieve stored amounts of excess variable renewable energy, and the spark-ignition (SI) engine has been proposed as a practical conversion system. The present work aims at elucidating the combustion characteristics of ammonia blends in engine-relevant turbulent conditions. To that end, laminar and turbulent flame experiments were conducted in a constant-volume vessel at engine-relevant conditions of 445 K and 0.54 MPa to assess the combustion behavior of ammonia/hydrogen/air, ammonia/methane/air and methane/hydrogen/air mixtures observed in an all-metal single-cylinder SI engine. Results show that the respective accelerating or decelerating effects of hydrogen or methane enrichment observed in the SI engine could not be sufficiently explained by the measured laminar burning velocities of the mixtures. Since the latter are very low, the studied combustion regimes are at the boundary between the thin and broken reaction zones regimes, and thus strongly influenced by flame-turbulence interactions. The quantification of the flame response to turbulence shows much higher effects for ammonia blends, than for methane-based fuels. The aforementioned opposite effects of ammonia enrichment with hydrogen or methane are observed on the turbulent burning velocity during the turbulent flame experiments and correlated to the thermochemical properties of the reactants and the flame sensitivity to stretch. The latter may explain an unexpected bending effect on the turbulent-to-laminar velocity ratio when increasing the hydrogen fraction in the ammonia/hydrogen blend. Nevertheless, a very good correlation of the turbulent velocity was found with the Karlovitz and Damköhler numbers, that suggests that ammonia combustion in SI engines may be described following the usual turbulent combustion models. This encourages further investigations on ammonia combustion for the optimization of practical systems, by means of dedicated experiments and numerical simulations.

Burning Velocity of Turbulent Methane/Air Premixed Flames in Subatmospheric Environments.

The aim of our work was to study turbulent premixed flames in subatmospheric conditions. For this purpose, turbulent premixed flames of lean methane/air mixtures were stabilized in a nozzle-type Bunsen burner and analyzed using Schlieren visualization and image processing to calculate turbulent burning velocities by the mean-angle method. Moreover, hot-wire anemometer measurements were performed to characterize the turbulent aspects of the flow. The environmental conditions were 0.85 atm, 0.98 atm, and 295 ± 2 K. The turbulence–flame interaction was analyzed based on the geometric parameters combined with laminar flame properties (which were experimentally and numerically determined), integral length scale, and Kolmogorov length scale. Our results show that the effects of subatmospheric pressure on turbulent burning velocity are significant. The ratio between turbulent and laminar burning velocities increases with turbulence intensity, but this effect tends to decrease as the atmospheric pressure is reduced. We propose a general empirical correlation as a function between ST/SL and u′/SL based on the experimental results obtained in this study and the equivalence ratio and pressure we established.

A priori DNS study of applicability of flamelet concept to predicting mean concentrations of species in turbulent premixed flames at various Karlovitz numbers

Complex-chemistry direct numerical simulation (DNS) data obtained earlier from lean hydrogen-air flames associated with corrugated flame (case A), thin reaction zone (case B), and broken reaction zone (case C) regimes of turbulent burning are analysed to directly assess capabilities of the flamelet approach to predict mean concentrations of species in a premixed turbulent flame. The approach consists in averaging dependencies of mole fractions, reaction rates, temperature, and density on a single combustion progress variable c, which are all obtained from the unperturbed laminar flame. For this purpose, four alternative definitions of c are probed and two probability density functions (PDFs) are adopted, i.e. either an actual PDF extracted directly from the DNS data or a presumed β-function PDF obtained using the DNS data on the first two moments of the c(x, t)-field. Results show that the mean density and mean mole fractions of H2, O2, and H2O are well predicted using both PDFs for each c, although the predictive capabilities are little worse in case C. In cases A and B, the use of the actual PDF and the fuel-based c also offers an opportunity to well predict mean mole fractions of O and H, whereas the mean mole fraction of OH is slightly underestimated. In the highly turbulent case C, the same approach performs worse, but still appears to be acceptable for evaluating the mean radical concentrations. The use of the β-function PDFs or another combustion progress variable yields substantially worse results for these radicals. When compared to the mean mole fractions, the mean rate of product creation, i.e. the source term in the transport equation for the mean combustion progress variable, is worse predicted even for a quantity (species concentration or temperature) adopted to define c and using the actual PDF. Consequently, turbulent burning velocity is not predicted either.

An extended flamelet-based presumed probability density function for predicting mean concentrations of various species in premixed turbulent flames

Direct Numerical Simulation (DNS) data obtained by Dave and Chaudhuri (2020) from a lean, complex-chemistry, hydrogen-air flame associated with the thin-reaction-zone regime of premixed turbulent burning are analyzed to perform a priori assessment of predictive capabilities of the flamelet approach for evaluating mean species concentrations. For this purpose, dependencies of mole fractions and rates of production of various species on a combustion progress variable c, obtained from the laminar flame, are averaged adopting either the actual Probability Density Function (PDF) P(c) extracted from the DNS data or a common presumed β-function PDF. On the one hand, the results quantitatively validate the flamelet approach for the mean mole fractions of all species, including radicals, but only if the actual PDF P(c) is adopted. The use of the β-function PDF yields substantially worse results for the radicals’ concentrations. These findings put modeling the PDF P(c) on the forefront of the research agenda. On the other hand, the mean rate of product creation and turbulent burning velocity are poorly predicted even adopting the actual PDF. These results imply that, in order to evaluate the mean species concentrations, the flamelet approach could be coupled with another model that predicts the mean rate and turbulent burning velocity better. Accordingly, the flamelet approach could be implemented as post-processing of numerical data yielded by that model. Based on the aforementioned findings and implications, a new approach to building a presumed PDF is developed. The key features of the approach consist in (i) adopting a re-normalized flamelet PDF for intermediate values of c and (ii) directly using the mean rate of product creation to calibrate the presumed PDF. Capabilities of the newly developed PDF for predicting mean species concentrations are quantitively validated for all species, including radicals.

A direct numerical simulation analysis of turbulent V-shaped flames propagating into droplet-laden mixtures

Three-dimensional Direct Numerical Simulations (DNS) data has been used to analyse the gaseous phase combustion behaviour in a V-shaped flame configuration where fuel is supplied in the form of droplets such that an overall (i.e. liquid+gaseous) equivalence ratio of unity is maintained in the unburned gas. The analysis has been carried out for different initially mono-sized droplet diameters. A gaseous premixed V-shaped flame with the same flow conditions has been utilised to compare the flame propagation in V-shaped spray flames with a corresponding gaseous premixed flame case. It has been found that combustion in the gaseous phase mainly takes place under fuel-lean mode and the probability of finding fuel-lean burning increases with increasing droplet diameter. However, the mean equivalence ratio of the predominantly fuel-lean mixture increases with increasing axial distance from the flame holder. The presence of droplets has been found to give rise to dimples on the reaction progress variable isosurfaces, and the effects of droplet–induced flame wrinkling are reflected by the widening of the curvature probability density functions for large droplets. The heat release has been found to arise mainly from the premixed mode of combustion. Detailed analysis of reaction progress variable transport has been used to explain the mean variations of consumption speed and density-weighted displacement speed with the axial distance from the flame holder. The mean values of consumption speed, density-weighted displacement speed and turbulent burning velocity have been found to decrease with increasing droplet diameter. The flame speed statistics have been utilised to explain that the enhancement of the burning rate cannot be equated to the enhancement of the flame surface area in turbulent droplet-laden V-shaped flames.

Modeling pressure effects on the turbulent burning velocity for lean hydrogen/air premixed combustion

We investigate and model pressure effects on the turbulent burning velocity over a range of pressures and turbulence intensities with the direct numerical simulation (DNS) of statistically planar turbulent premixed flames for lean hydrogen/air mixture. DNS results indicate that the stretch factor has an impact on the turbulent burning velocity and flame surface area at elevated pressures. In particular, the enhanced stretch factor at high pressures increases the ratio of turbulent and laminar burning velocities, diminishing the “bending” effect. Based on a good consistency between turbulent and laminar burning velocities with respect to flame stretch, a lookup table formed by laminar flame data is employed to model the stretch factor in turbulent flames at various pressures. A predictive model for the turbulent burning velocity is then developed by combining sub models of the stretch factor and flame surface area. The overall good agreement between model predictions and DNS results demonstrates that the proposed model is able to quantitatively predict the turbulent burning velocity over a range of pressures and turbulence intensities in homogeneous isotropic turbulence.

Development of a turbulent burning velocity model based on flame stretch concept for SI engines

According to the US Energy Information Administration, fossil fuels will remain the main source of energy for transportation over the next decades and thus the combustion of these fuels remains an important concern.This research studied the flame propagation under engine in-cylinder conditions and developed a correlation for turbulent burning velocity based on the global flame stretch concept. To study the impact of engine operation on flame stretch, two speeds, two loads, and three fuel-air mixtures were investigated. The flame front was determined by processing images of the flame natural luminosity.A turbulent burning velocity model was developed using dimensional analysis. The model showed that the turbulent burning velocity decreased due to flame stretching. Higher engine speeds increased the turbulent burning velocity by increasing the turbulent intensity, yet a tradeoff between the flame stretch and the turbulent burning velocity due to higher engine speed was observed. In cases where the flame distortion was very high, the flame stretch may cancel out any benefits of a large enflamed area.Incorporating the flame stretch into the burning velocity model and coupling the developed model with GT-Power simulation software revealed that the stretch may result in a 35% reduction in turbulent burning velocity.

A predictive combustion model for one-dimensional gasoline engine simulation

A new predictive combustion model for a one-dimensional computational fluid dynamics tool in the multibody dynamics processes of gasoline engines was developed and validated. The model consists of (1) a turbulent burning velocity model featuring a flame radius–based transitional function, steady burning velocity that considers local quenching using the Karlovitz number and laminarization by turbulent Reynolds number, as well as turbulent flame thickness and its quenching model near the liner wall, and (2) a knock model featuring auto-ignition by the Livengood–Wu integration and ignition delay time obtained using a full-kinetic model. The proposed model and previous models were verified under a wide range of operating conditions using engines with widely different specifications. Good agreement was only obtained for combustion characteristics by the proposed model without requiring individual calibration of model constants. The model was also evaluated for utilization after prototyping. Improved accuracy, especially of ignition timing, was obtained after further calibration using a small amount of engine data. It was confirmed that the proposed model is highly accurate at the early stage of the engine development process, and is also applicable for engine calibration models that require higher accuracy.

Role of instability on the limits of laterally strained detonation waves

The present work examines the role of instability and diffusive phenomena in controlling the limits of detonations subject to lateral strain. Experiments were conducted in mixtures with varying levels of cellular instability, i.e., stoichiometric methane–oxygen (CH4/2O2), ethylene–oxygen (C2H4/3O2), and ethane–oxygen (C2H6/3.5O2). These detonations were propagated in channels with exponentially enlarging cross-sections, following the recent works of Radulescu and Borzou (2018) and Xiao and Radulescu (2020). Steady detonation waves were obtained at the macro-scale, with the near-limit reaction zone structures characterized by significant unreacted gas pockets. The turbulent flame burning velocity of these pockets was evaluated to be 30 m/s to 70 m/s, which is larger than the theoretical laminar value by a factor of 2 to 7 and smaller than the CJ deflagration speed by a factor of 2 to 3. For all the mixtures tested, the characteristic D−K relationships, relating the detonation mean propagation speed with lateral flow divergence, were obtained directly from experiments and as well from the generalized ZND model with lateral strain rates using detailed chemical kinetics. The results showed that the degree of departure between experiments and the theoretical predictions increases significantly with the detonation instability level. As compared to the laminar ZND wave, the more unstable detonations are much more detonable than the more stable detonations, with substantially larger limiting divergence rates and maximum velocity deficits. Such enhanced detonability with detonation instability can be manifested in the significantly enhanced global rates of energy release with the notably suppressed thermal character of ignition for the more unstable detonations. This globally enhanced burning mechanism is found to be realized by the intensified auto-ignition assisted by the turbulent diffusive burning of the unreacted gas pockets, substantially shortening the characteristic reaction zone lengths. Finally, the relatively good universality of the D/DCJ−Keffλ relations implicitly suggests that cell size is also a function of the detonation wave stability, since it is controlled by the reaction zone thickness of unstable detonations.

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.

The reactivity of hydrogen enriched turbulent flames

The use of hydrogen enriched fuel blends, e.g. syngas, offers great potential in the decarbonisation of gas turbine technologies by substitution and expansion of the lean operating limit. Studies assessing explosion risks or laminar flame properties of such fuels are common. However, there is a lack of experimental data that quantifies the impact of hydrogen addition on turbulent flame parameters including burning velocities and scalar fluxes. Such properties are here determined for aerodynamically stabilised flames in a back-to-burnt opposed jet configuration featuring fractal grid generated multi-scale turbulence (Ret=314±19) using binary H2/CH4 and H2/CO fuel blends. The binary H2/CH4 fuel blend is varied from α=XH2/(XH2+XF)= 0.0, 0.2 and 0.4–1.0, in steps on 0.1, and the binary H2/CO fuel blend from α=0.3−1.0 also in steps of 0.1. The equivalence ratio is adjusted between the mixture specific lower limit of local flame extinction and the upper limit of flashback. The flames are characterised using PIV measurements combined with a flame front detection algorithm. The study quantifies the impact of hydrogen enrichment on (i) turbulent burning velocity (ST), (ii) turbulent transport and (iii) the rate of strain acting on flame fronts. Scaling relations (iv) that correlate ST with laminar flame properties are evaluated and (v) flow field data that permits validation of computational models is provided. It is shown that CH4 results in a stronger inhibiting effect on the reaction chemistry of H2 compared to CO, that turbulent transport and burning velocities are strongly correlated with the rate of compressive strain and that scaling relationships can provide reasonable agreement with experiments.