Effective Lewis Number Research Articles

Thermodynamics of hydromagnetic boundary layer flow of Prandtl nanofluid past a heated stretching cylindrical surface with interface slip

BackgroundThe mathematical modelling of a non-Newtonian fluid in the backdrop of applied magnetic field against an electrically conducting liquid is recognized as magnetohydrodynamics (MHD), which has important implications in chemical engineering and the biological sciences. Illustrations of such magneto-fluids include molten metals, electrolytes, blood plasma, and salty water. The applications of flowing liquids over cylindrical geometries is a fascinating due to its wide range importance in engineering. The aforementioned uses of MHD encourage scientists and analysts to establish new mathematical modelling in the area of fluid dynamics. Therefore, we considered the Prandtl fluid flow with loaded nanoparticles over a cylindrical tube with slip triggered by wall shear stress. The transport equation involving thermophoresis and Brownian diffusions are modelled using the Buongiorno model. The modelling of energy and entropy are established considering the effects of frictional dissipation, non-uniform heat source, thermal stratification, and Ohmic heating. MethodsThe formulated mathematical model leading to a complex system of partial differential equations with numerous integrated parameters. Using numerical algorithm RK-45 with a tolerance limit of 10−6, the system of resultant ordinary differential equations is simulated. The solver is used repeatedly to select the parameters that best meet the intended boundary conditions. Significant findingThe results of the current study revealed that the influence of heat source and thermal stratification on fluid temperature are conflicting. Additionally, the magnetic variable and Brinkman parameter have an escalating relationship with the rate of entropy development. The Bejan number detract with the magnetic variable. For distinct estimations of slip factors, heat and mass transport behave identically. However, the effect of slip is higher as compared to no slip scenario. The effect of Lewis number against nanoparticles concentration is opposite.

Effects of unburnt reaction progress on stretch flame dynamics under elevated temperatures

Flames in practical combustors and engines are inevitably subject to the effects of upstream chemical reaction progress and stretch, induced by elevated thermodynamic conditions and flow non-uniformities, respectively. Recent shock tube experiments and simulation studies on flame propagation have shown that flame speed under engine-relevant conditions can be enhanced with non-negligible upstream chemical reaction progress, especially when low-temperature heat release is involved in the unburnt mixture. On the other hand, depending on the mixture equivalence ratio and diluents, nonequidiffusion (including the non-unity Lewis number effect and the preferential diffusion effect) can couple with the flame stretch to fundamentally affect the flame propagation, which can manifest as either facilitation or suppression. Depending on the transport property of a reacting mixture, there hence can be either inhibition or promotion from unburnt reaction progress and stretch effects on flame propagation. In the current work, through one-dimensional numerical simulations of transient planar and spherical flames of n-heptane/air under elevated thermodynamic conditions, the combined effects of upstream chemical reaction progress and stretch on flame propagation are investigated. Results show that for both lean and rich n-heptane/air mixtures, flame speed can be substantially promoted with reaction progress, while the rich mixtures can exhibit opposite stretch dependence after first-stage ignition. Different definitions of effective Lewis number are adopted to explain the change in Markstein length for spherical flame in lean and rich mixtures reformed by the low temperature chemistry. This work fills an important gap in laminar premixed flame research relevant to practical combustion systems and can provide useful insight into local turbulent flame behaviors and phi-sensitivity in engine combustion.

Effect of Turbulence and Diffusional–Thermal Instability on Hydrogen‐Rich Syngas Turbulent Premixed Flame

As a kind of fuel with low‐carbon emissions, hydrogen‐rich syngas has a large resource potential and is a promising alternative energy resource. A series of experiments in a constant volume combustion bomb are carried out to investigate the effects of turbulence and diffusional–thermal (DT) instability on a hydrogen‐rich syngas turbulent premixed flame. The flow field in the constant volume combustion bomb is calibrated, and the turbulent flow field's homogeneity and isotropy were investigated. The effects of different speeds of fan (1366–4273 rpm), hydrogen fractions (55%–95%), pressures (1.0–3.0 bar), and equivalence ratios (0.6–1.0) on the hydrogen‐rich syngas turbulent premixed flame are studied. According to the analyses, the flow field in the experimental device exhibits good homogeneous and isotropic characteristics. With the increase of turbulence intensity, equivalence ratio, hydrogen fraction, or pressure, the turbulent flame propagation speed increases gradually. As the hydrogen fraction increases or the equivalence ratio decreases, the effective Lewis number decreases, and the effect of DT instability on the flame increases. The research in this article is crucial for the clean and efficient utilization of hydrogen‐rich syngas and the design of related burners.

The Effect of Methane–Ammonia and Methane–Hydrogen Blends on Ignition and Light-Around in an Annular Combustor

Abstract The use of hydrogen and ammonia in gas turbines, either alone or blended with natural gas, poses various technical challenges for combustion systems, including ignition. Depending on the fuel composition, the laminar flame speed and the ratio of unburned to burned gas density (dilatation ratio) of hydrogen and ammonia flames can be well outside the range seen in natural gas flames. Previous studies in annular combustion chambers have provided evidence of the importance of these properties in determining the ignition dynamics including light-around times. So far, these studies have mostly considered hydrocarbon fuels, have been limited to only a few runs, and have not yet systematically investigated variations in the dilatation ratio and the flame speed but rather have considered them as a lumped parameter. To investigate these effects in more detail, experiments characterizing the light-around times were carried out on an atmospheric annular combustor in which the dilatation ratio and the laminar flame speed was independently varied. This was achieved by varying the equivalence ratio and employing a variety of different hydrocarbon fuels (ethylene, propane, and methane) and fuel blends of methane–ammonia and methane–hydrogen. Light-around times were evaluated from global chemiluminescence measurements obtained using an azimuthal array of photomultipliers placed round the combustor chamber as well as high speed imaging. To improve statistical certainty, more than 3000 ignition and light-around times were measured with 30 repetitions obtained for each operating condition. To provide some insight into the light-around dynamics in specific cases, 900 of the 3000 sets included high-speed OH* chemiluminescence images. Light-around times for premixed pure hydrocarbon flames showed a similar dependence on the laminar flame speed as reported in previous studies. For the range of ammonia fuel blends investigated, an increase in laminar flame speed leads to a predictable increase in the flame propagation speed, as in the case of hydrocarbon fuel. Furthermore, collapse of this dependence for all blends could be achieved when corrected for an effective Lewis number, noting that all Lewis numbers for these blends were above unity. However, for hydrogen fuel blends, a decrease in dilatation ratio was found to decrease the light-around time counter to existing experimental results on the ignition of hydrocarbon fuels for which we currently do not have an explanation.

Propagation and extinction of premixed H2[sbnd]O2[sbnd]N2 edge-flames

The effects of equivalence ratio (ϕ), global strain rate (σ) and mixture strength on the shapes and propagation rates (Uedge) of premixed edge-flames in H2O2N2 mixtures were investigated experimentally using a counterflow slot-jet apparatus. Results are presented for both single (premixed gas against cold inert gas) and twin (premixed gas against premixed gas) configurations. Both advancing (Uedge > 0) and retreating edge-flames (Uedge < 0) were characterized. Flame images show that for lean mixtures where H2 is the deficient species and thus the effective Lewis number (Leeff) is less than unity, the leading edge-flame has a stronger burning intensity than the trailing strained premixed flame. For mixtures with low ϕ (thus low Leeff) at near-extinction conditions corresponding to sufficiently high or low σ, transitions from continuous to broken structures were observed due to diffusive-thermal instabilities. Such transitions enable flames to survive under conditions where extinction would have occurred without transition. The combined effects of ϕ, adiabatic flame temperature and σ were found to correlate well in terms of the effect of a non-dimensional strain rate (ε) on a scaled propagation rate Ũ.

Effective Lewis number and burning speed for flames propagating in small-scale spatio-temporal periodic flows

Propagation of premixed flames having thick reaction zones in rapidly-varying, small-scale, zero-mean, spatio-temporal periodic flows is considered. Techniques of large activation energy asymptotics and homogenization theory are used to determine the effective Lewis number Leeff and the effective burning speed ratio ST/SL, which are influenced by the flow through flow-enhanced diffusion. The resultant effective diffusivity matrix is, in general, neither a scalar nor a diagonal matrix and therefore induces anisotropic effects on the propagation of multi-dimensional flames. As the flow Peclet number Pe becomes large, the flow-enhanced fuel diffusion coefficient and the thermal diffusivity behave respectively like (PeLe)σ and Peσ, where Le is the Lewis number and σ≤2 is a constant which depends on the flow and the direction of flame propagation. The maximal value σ=2 is achieved for steady, unidirectional, spatially periodic shear flows, while for steady two-dimensional square vortices, we have σ=1/2. In general, the constant σ is determined by solving a linear partial differential equation. The scaling laws for the diffusion coefficients lead to corresponding scaling laws for the effective Lewis number and the effective burning speed ratio of the form Leeff≃Le1−σ and ST/SL∼(Pe/Le)σ/2. Effects of thermal expansion and volumetric heat loss on the flame are also briefly discussed. In particular, it is shown that the quenching limit is enlarged by a factor 1/Leσ for Le<1 and diminished by the same factor for Le>1, due to the flow-enhanced diffusion. The potential implications of the results to better understand turbulent combustion are discussed. A special emphasis is placed on the dependence of the flame on Le in the presence of high-intensity, small-scale flows. In particular, it is shown that this dependence is intimately linked to the flow through Taylor-dispersion like enhanced diffusion, rather than through the traditional molecular diffusion coupled with curvature effects. The flow-dependent effective Lewis number identified may also provide an explanation to the peculiar experimental observation that turbulence appears to facilitate ignition in Le>1 mixtures and to inhibit it in Le<1 mixtures. Novelty and significance statementAn original study, combining asymptotic analysis and homogenization theory, is applied to describe flame propagation in small-scale, spatio-temporal periodic flow fields. Scaling laws are derived for the effective burning speed and the effective Lewis number for high-intensity small-scale flows, which are useful to better understand the behaviour of turbulent premixed flames in the distributed reaction zone regime. The formula for the flow-dependent effective Lewis number identified herein may explain the peculiar experimental observation that turbulence appears to facilitate ignition in Le>1 mixtures and to inhibit it in Le<1 mixtures. The high-intensity small-scale flows are shown to increase the quenching limit due to volumetric heat losses in Le<1 mixtures and decrease it in Le>1 mixtures

On the flow-facilitated ignition in a mixture with low Lewis number

The imposed laminar flow has been shown to facilitate ignition under certain conditions in our previous work (Chen et al. 2023). Such flow-facilitated ignition (FFI) was observed only for mixtures with sufficiently large effective Lewis numbers, i.e., Le>>1. It is not clear whether FFI can also occur in a mixture with Le<1 and Le∼1. This study will investigate the occurrence of FFI in a mixture with low Lewis number. We conduct multi-dimensional simulations of forced ignition by a pair of electrodes in both static and flowing hydrogen/air mixtures. A simplified ignition model is used in simulations. The laminar flow direction is perpendicular to the pair of electrodes. We consider different values for equivalence ratio (ϕ=0.4 and 1.0), pressure (P = 0.3–1 atm), electrode gap distance (dgap=0.64–3.88 mm) and flow velocity (Uin=0–10 m/s). The simulation results show that FFI can occur in a fuel-lean (ϕ=0.4) H2/air mixture with Le≈0.43 at sub-atmospheric pressure. This indicates that large Lewis number is not a necessary condition for FFI occurring in simulations considering a simplified ignition model. Here, the favorable conditions for FFI are large critical ignition radius at sub-atmospheric pressure, sufficiently small electrode gap distance and moderate flow velocity. Moreover, the reduction in heat loss to the electrodes by the imposed flow is found to be crucial for FFI. To further understand the boundary conditions of FFI, an ignition mode regime diagram in terms of flow velocity and normalized critical ignition radius (i.e. the ratio of critical ignition radius and electrode gap distance) is proposed. The present study provides insights on the effects of laminar flow on forced ignition of premixed flames.

Predicting flashback limits in [formula omitted] enriched [formula omitted]/air and [formula omitted]/air laminar flames

The influence of hydrogen blending on the flashback limit of dihedral premixed laminar flames stabilized on a slit burner is investigated via two-dimensional direct numerical simulations with conjugate heat transfer and detailed chemistry. Flashback limits are determined for CH4/H2/Air and C3H8/H2/Air mixtures with hydrogen contents ranging from 0% to 100% and varying fuel-to-air ratios adjusted according to the iso–Tad and the iso–δT hybridization strategies. The analysis reveals the existence of two different flashback regimes and a critical effective Lewis number for the fuel mixture, Lec=0.5, controlling the switch from one regime to the other. Simulations are also used to explore the dynamics of flames during flashback in these two regimes. They show that for mixtures above the critical effective Lewis number, flashback is symmetric whereas for mixtures below the critical value, an asymmetric flame propagation is observed through the burner slit. These results highlight the impact of preferential diffusion on the stabilization mechanism of hydrogen fuel blends.

Experimental and numerical investigation of combustion characteristics of carbon-free NH3/H2 blends in N2O

A detailed experimental and numerical investigation of the combustion characteristics of carbon-free NH3/H2 blends in N2O was performed in the present work. The experiments with various hydrogen fractions (α = 0–1.0) and equivalence ratios (φ = 0.6–1.4) were performed at ambient pressure (1 atm) and temperature (283 K) in a standard 3.375-L cubic combustion vessel. The explosion behaviors (maximum explosion pressure Pmax and maximum pressure rise rate (dp/dt)max) were obtained from the pressure-time curves. Further, the combustion characteristics (e.g. laminar burning velocity (SL), thermal expansion ratio (σ), flame thickness (δ), effective Lewis number (Leeff) and Markstein length (Lb)) were calculated numerically by means of chemical kinetic analysis using Zhang's mechanism (Zhang et al., Combust. Flame, 2017, 182: 122–141). Besides, sensitivity analyses of the explosion pressure and laminar burning velocity were performed to identify the contribution of dominant elementary reactions. The results show that, for combustion supported by N2O, the flame instability and the potential of deflagration to detonation transition (DDT) should be considered for the evaluation of Pmax and (dp/dt)max. Moreover, the hydrodynamic instability of premixed H2/NH3/N2O flame reaches its peak near the stoichiometric ratio and increases monotonously with the hydrogen fraction. The diffusion-thermal instability is more significant in the leaner mixture, and it is not invariably enhanced with the increase of hydrogen fraction but is relevant to the composition of the mixture. In addition, the sensitivity analysis of explosion pressure demonstrates that R80 (N2O + H2N2+H2O) and R70 (N2O+(M) = N2+O (+M)) are the most dominant reactions contributing to the pressure rise. The sensitivity analysis of laminar burning velocity shows that R70 (N2O+(M) = N2+O (+M)), R71 (N2O + HN2+OH) and R80 (N2O + H2N2+H2O) are the most dominant reactions enhancing laminar burning velocity.

Study on the laminar combustion characteristics and kinetic of IC8H18/NH3 premixed flames

To promote the transition to a carbon neutral society, it is necessary to conduct a comprehensive research on the utilization of ammonia contained in gasoline engine. In the present study, the laminar burning velocity (SL), Markstein length (Lb) and flame instability of isooctane/ammonia (IC8H18/NH3)-air mixtures were analyzed at the initial temperatures (T) of 400 K and 470 K, initial pressure (P) of 1–4 atm, ammonia mole fraction (XNH3) of 0%-70% and an equivalence ratio (ϕ) ranging from 0.8 to 1.3. Also, the kinetic mechanism of IC8H18/NH3 as proposed by Cai et al. [Combust. Flame, 162, 2015] was further optimized and compared with the mechanism of CRECK and Cai. The results show that the difference between the Cai and present mechanisms is largely attributable to the variation in sensitivity of NH3 oxidation-related reaction to SL. In addition, it is widely known that SL is dependent on the initial temperature and pressure. In order to improve the applicability of IC8H18/NH3 fuel mixtures, their temperature (α) and pressure (β) coefficients were determined given varying ϕ and XNH3. Meanwhile, a mathematical correlation for SL was also conducted. The analysis of Lb was also conducted to obtain the results showing that Lb decreases with ϕ increasing, except for NH3-air flame. Moreover, as XNH3 increases, Lb varies between lean and rich flames. For lean-flames, NH3 addition leads to a significant decrease in Lb, while for rich-flames, NH3 addition causes a marginal increase in Lb. All these are attributed to the variation in Zel’dovich number (Ze), effective Lewis number (Leeff) and flame thickness (δ). Furthermore, the instability of flame suggests that the addition of ammonia enhances the stability of the flame due to the reduced molecular diffusion effect. Moreover, an increase in XNH3 contributes only to a slight decrease of critical Peclect number (Pecr), and a sharp rise in the critical flame radius (Rcr) results from the increase of flame thickness (δ).

Impact of bioconvection on Darcy‐Forchheimer flow of MHD Carreau fluid with Arrhenius activation energy

AbstractThis research examines the bioconvection impact on mixed convective Carreau fluid flowing toward permeable stretched surface with joule heating and Darcy‐Forchheimer effect. The evaluation has been attained in the presence of convective conditions, Joule heating, thermal radiative, mixed convective and motile microorganisms, which have not been dreamed before. Flow in permeable medium is characterized by utilizing Darcy‐Forchheimer model. Appropriate similarity factors are utilized to obtain the ordinary differential equations (ODEs). The derived mathematical formulation is now numerically solved with the help of R‐K 4th order scheme based on shooting approach. The results of the motile microorganism number, velocity, Sherwood number, temperature, motile density number, drag coefficient, concentration, and local Nusselt number are revealed graphically. The effect of bioconvection Lewis number on the motile microorganisms and motile density profile are quite inverse. Further, higher values of Schmidt number decrease the concentration field.

Stability of diffusion flames under shear flow: Taylor dispersion and the formation of flame streets

Diffusion flame streets, observed in non-premixed micro-combustion devices, align parallel to a shear flow. They are observed to occur in mixtures with high Lewis number (Le) fuels, provided that the flow Reynolds number, or the Peclet number Pe, exceeds a critical value. The underlying mechanisms behind these observations have not yet been fully understood. In the present paper, we identify the coupling between diffusive-thermal instabilities and Taylor dispersion as a mechanism which is able to explain the experimental observations above. The explanation is largely based on the fact that Taylor dispersion enhances all diffusion processes in the flow direction, leading effectively to anisotropic diffusion with an effective (flow-dependent) Lewis number in the flow direction which is proportional to 1/Le for Pe≫1. Validation of the identified mechanism is demonstrated within a simple model by investigating the stability of a planar diffusion flame established parallel to a plane Poiseuille flow in a narrow channel. A linear stability analysis, leading to an eigenvalue problem solved numerically, shows that cellular (or finite wavelength) instabilities emerge for high Lewis number fuels when the Peclet number exceeds a critical value. Furthermore, for Peclet numbers below this critical value, longwave instabilities with or without time oscillations are obtained. Stability regime diagrams are presented for illustrative cases in a Le−Pe plane where various instability domains are identified. Finally, the linear analysis is supported and complemented by time dependent numerical simulations, describing the evolution of unstable diffusion flames. The simulations demonstrate the existence of stable cellular structures and show that the longwave instabilities are conducive to flame extinction.

Hydrogen addition effect on cellularization and intrinsic instability of ethyl acetate spherical expanding flame

The cellularization and intrinsic instability of ethyl acetate (EA) spherical expanding flame with different proportions of hydrogen (4%, 8%, 12%) were studied through experiments and theoretical analysis. The high-speed schlieren imaging technique combined with the image processing method was used in the constant volume combustion chamber to quantify the development of flame surface cracks and saturation time, crack length, number of cellular units, and average cellular area. The results show that the evolution of flame cracks can be mainly classified into two periods. In the first period, several initial cracks are generated on the flame surface. In the second period, cracks rapidly expand while forming a large number of small cellular units, and the surface areas also increase rapidly. The increase in the proportion of hydrogen led to an increase in minor cellular cracks and a more moderate structural evolution of flame cells during the second period. Afterwards, the involved thermodynamic parameters, including the thermal expansion ratio, flame thickness, effective Lewis number, critical Peclet number, and perturbation logarithmic growth rate of unstable flames, were quantitatively analyzed. The results show that an increase in initial pressure significantly reduces the flame thickness, leading to more unstable flame. In contrast, the proportion of hydrogen has less impact on the involved thermodynamic parameters, while the equivalence ratio is the primary factor influencing instability parameters. Additionally, the study examined the flame's self-acceleration characteristics and found that increasing the proportion of hydrogen reduces the tendency for ethyl acetate self-acceleration.

Effect of hydrogen addition on the laminar burning velocity and the flame stability of n-dodecane reacting with air at elevated pressures

The effects of hydrogen addition on the premixed laminar burning characteristics of n-dodecane reacting in air were experimentally investigated using the freely expanding spherical flame method. The initial operating conditions were temperature = 425 K, pressure = 1 ˗ 4 bar, equivalence ratio = 0.8–1.4, and mole fraction of H2 in n-dodecane-H2 mixture = 0–40%. Nonlinear stretch extrapolation schemes were implemented to minimize the uncertainty in the estimation of unstretched flame speed as the binary fuel mixtures of n-dodecane and H2 had non-unity Lewis numbers. All experimental operating conditions were simulated in CHEMKIN using a planar flame model with three different reaction mechanisms. The statistical uncertainty estimated for the LBV was below ±6.93%. The LBV increased by three/two times at off–stoichiometric/stoichiometric mixtures as the mole fraction of hydrogen (XH2) increased from 0 to 40% in n-dodecane. The rich mixtures of n-dodecane-air (ϕ = 1.4) were unstable to thermo-diffusive effects due to the lower mass diffusivity of n-dodecane and Le < 1, in contrast, H2 blending to the n-dodecane by keeping the other parameters being same transformed it from an unstable to a stable one. The effective Lewis number and Markstein length decreased with an increasing XH2 in n-dodecane, indicating that the mixtures were stable to thermo-diffusive effects, but with further increase in XH2, there may be a transition in flame stability. H2 addition resulted in an earlier onset of hydrodynamic instability due to a reduction in the flame thickness. In addition, sensitivity analysis was performed to identify the key reactions responsible for the enhanced reactivity associated with H2 addition. The reaction pathway and emission analysis showed a significant reduction in CO2 and CO emissions. Finally, an increase of initial pressure decreased the LBV for all investigated mixtures.

Experimental study on curvature effects and preferential diffusion&amp;nbsp;for perturbed laminar premixed ammonia-air flames

Ammonia-air flames are known for low reactivity and have been posing as a huge hindrance in employing the chemical as a sustainable fuel of tomorrow. Curvature is a parameter that could influence the flame structure and so the position of the maximum heat release rate. Flame-acoustic interactions on a Bunsen burner are performed to study the local flame response to highly perturbed flows. NH2* chemiluminescence is used to study the reactivity of these flames. Non-perturbed flames are used as a reference to understand the inherent behaviour of Bunsen ammonia flames. A case study has been chosen for an equivalence ratio ranging between 1.0 and 1.4 at atmospheric conditions to study perturbed flames. The objective is to study the effect of curvature induced by the perturbations on the reactivity of the flame. It was seen that this given case study was quite complex as the flame response was to multiple factors like the effect of Lewis number, convective-diffusion velocities, decomposition of ammonia into hydrogen, thereby, promoting preferential diffusion of hydrogen in both large-scale and locally for certain cases apart from the generated acoustic perturbation which itself dictates the flow regime of the fresh gases, etc. Since the Damköhler number was around 1, the perturbation time scales and the reactivity time scales were comparable and so none of the effects could be ignored. It was concluded that for richer flames where Le&gt;1, the negative curvature promoted the production of hydrogen leading to local enhancement in reactivity. A change in the local thickness due to the induced curvature was seen for all conditions.

Investigation of cellularization characteristics of hydrogen-methane-ethanol expanding spherical flame at elevated pressures

The incorporation of ethanol into hydrocarbon fuels for use is attracting increasing interest and it is necessary to investigate its inherent flame instability for better application in combustion units. The instability of hydrogen-methane-ethanol spherically expanding flame has been investigated at an initial temperature of 400 K, initial pressures of (2–4 bar), ethanol fraction of (20%, 50%, 80%), and equivalence ratios (Ф) of (0.7–1.4) using the constant volume combustion chamber (CVCC). High-speed schlieren technology was used to record flame propagation images. The effects of hydrodynamic and thermal-diffusion effect on the inherent instability of the flame were investigated. As the ethanol ratio increased, the hydrodynamic effect was enhanced. The thermal-diffusion effect was discovered to stabilize the flame surface under all conditions, as judged jointly by the effective Lewis number and the critical Lewis number. The critical conditions (critical radius and Peclet number) at the onset of unstability were evaluated, and it was found that the flames were more prone to flame instability at higher pressures. The critical Peclet number increased with the increase in the equivalence ratio when the ethanol ratio was 20%, and showed the opposite trend when the ethanol ratio was 50% and 80%. In addition, as the ethanol ratio increases, the stability of the lean mixtures flame increases, while the rich mixtures flame suffers from early onset of instability. The theoretical and experimental results were consistent, with some differences at Ф = 1.4. An empirical correlation formula for the critical Peclet number (Pec) and Markstein number (Mb) was further proposed (Pec= 18.03Mb+214.78). Finally, the Karlovitz number was used to study the instability behavior of the flame. The critical Karlovitz number (Kac) decreased with increasing Mb and the tendency of the flame to suffer from instability diminished, and the following correlation was obtained Kac=0.05635×e−0.13852Mb. Furthermore, the flame was more unstable in rich mixtures, this was consistent with the conclusion of instability derived from the critical radius.

Self-similar propagation and flame acceleration of hydrogen-rich syngas turbulent expanding flames

In order to investigate the flame self-similar propagation and flame acceleration characteristics of hydrogen-rich syngas turbulent expanding flames, the turbulent combustion experimental studies of H2/CO/air mixtures with XH2 = 55%-95%, φ = 0.6–1.0, P = 1 bar-3 bar, u’=0.809–2.533 were conducted in a fan-stirred turbulent combustion chamber with an inner diameter of 380 mm. Results show that the normalized flame propagation speed ST/SL and turbulent flame Reynolds number ReT,flame could be represented by power-law correlations. With the increase of hydrogen fraction from 55% to 95%, the power exponent decreases from 0.413 to 0.301. The increase of hydrogen fraction makes the flame acceleration weaker because that the enhancing of laminar flame speed is more obvious than the differential-diffusion effect by decreasing Le. With the decrease of equivalence ratio from 1.0 to 0.6, the power exponent increases from 0.304 to 0.411 in this study. The relative high power exponent under low equivalence ratio is due to the coupling between differential-diffusion and the flame stretch on the local wrinkled flame structures. In addition, the influence of turbulence and flame intrinsic instabilities on flame acceleration propagation process of hydrogen-enriched syngas were studied. The acceleration exponent was extracted to quantitatively investigate the flame self-acceleration propagation in the transition stage, and the relationships between acceleration exponent and turbulence intensity (u’), Kovasznay number (Kz), turbulence integral length scale (LT), flame thickness (δ), thermal expansion ratio (ρu/ρb), effective Lewis number (Leeff) were analyzed

Influence of hydrogen and methane addition in laminar ammonia premixed flame on burning velocity, Lewis number and Markstein length

The use of Ammonia (NH3) and blends with either Methane (CH4) or Hydrogen (H2) obtained by in-situ NH3 cracking, seem to be promising solutions to partially or fully decarbonise our energy systems. To strengthen understanding of fundamental combustion characteristics of these NH3 blends, the outwardly propagating spherical flame configuration was employed to determine the flame speeds and Markstein lengths. The air/fuel mixtures were varied across a large range of compositions and equivalence ratios. In general addition of CH4 or H2 results in a linear and exponential increase in measured laminar burning velocity, respectively. Of the appraised mechanisms, Stagni and Okafor kinetics mechanisms yielded best agreement with NH3/H2 and NH3/CH4 flame speed measurements. With respect to measured Markstein length, for a fixed equivalence ratio, addition of CH4 to NH3 resulted in a linear reduction in stretch sensitivity for the tested conditions. For lean NH3/H2 flames, an initial decrease in Markstein length is observed up to 30–40% H2 addition, at which point any further addition of H2 results in an increase in Markstein Length, with a non-linear behaviour accentuated as conditions get leaner. Above stoichiometry similar stretch behaviour is observed to that of NH3/CH4. Different theoretical relationships between the Markstein length and Lewis Number were explored alongside effective Lewis Number formulations. For lean NH3/H2 mixtures, a diffusional based Lewis Number formulation yielded a favourable correlation, whilst a heat release model resulted in better agreement at richer conditions. For NH3/CH4 mixtures, a volumetric based Lewis Number formulation displayed best agreement for all evaluated equivalence ratios. For NH3/H2, changes in measured Markstein Length were demonstrated to potentially be the result of competing hydrodynamic and thermo-diffusive instabilities, with the influence of the thermo-diffusional instabilities reducing as the equivalence ratio increases. On the other hand, the addition of CH4 to NH3 results in the propensity of moderating hydrodynamic instabilities, resulting in a stabilising influence on the flame, reflected by increasing positive Markstein number values. Finally, a systematic analysis of the flame speed enhancements effects (kinetic, thermal, diffusive) of CH4 and H2 addition to NH3 was undertaken. Augmented flame propagation of NH3/CH4 and NH3/H2 was demonstrated to be principally an Arrhenius effect, predominantly through the reduction of the associated activation energy.

Experimental and numerical study on the critical initiation radii of 1,3-Butadiene/Oxygen/Helium flame under elevated pressures

A numerical and experimental study investigated the first and second critical initiation radii of 1,3-butadiene/oxygen/helium flame initiation. A minimum critical flame radius was measured and compared with a calculated first critical initiation radius since the first radius is hard to obtain experimentally. Experiments were conducted in a constant-pressure combustion chamber under pressure up to 1.5 MPa at equivalence ratios of 0.8–1.5 and a temperature of 298 K. The effects of equivalence ratio and pressure on the critical initiation radii and critical stretch rates were studied. Both critical initiation radii first decrease with increasing equivalence ratio and then increase. In contrast, with higher pressure, both critical initiation radii reduce. The critical stretch rates, which correspond to the stretch rates at the critical initiation radii, vary in the opposite direction with changing pressure and equivalence ratio. A nonlinear model and a detailed model were used for a quick estimation of critical initiation radii and theoretical analysis, respectively. The calculated results obtained using the two models demonstrated strong agreement with the experimental data. Additionally, the minimum radius and critical radius were highly linearly related to the flame thickness. The detailed model was used to analyze the effects of density ratio, Zel’dovich number, and effective Lewis number on critical initiation radii. A sensitivity analysis revealed that the Zel’dovich number dominates the trends of critical initiation radii. The results also indicated that the critical initiation radii become more sensitive to the Lewis number at low values of the Lewis number. Comparisons between the theoretical results of a detailed model and a nonlinear model were conducted. The nonlinear model was capable of accurately predicting the second critical radius well but only matched the first critical radius at smaller Markstein numbers. An empirical remedy equation of the first critical radii at large Markstein number was also presented.

Revisiting effective Lewis number of combustible mixtures

Lewis number (Le) is of importance in the field of premixed combustion. In this study, the effective Lewis numbers of combustible mixtures containing a single fuel (CH4/air, C3H8/air, H2/air, C2H4/O2/N2), two fuels (CH4/C2H4/air, H2/NH3/air, H2/CH4/N2O) and two oxidizers (CH4/N2O/O2) over a wide range of equivalence ratios were evaluated via various methodologies. The Markstein length was obtained from previous studies, or extracted experimentally from the outwardly propagating flames. Firstly, the mass diffusivity and Lewis number of a reactant in mixture were evaluated using different models. Secondly, the effective Lewis number (Leeff) was calculated based on various theories, which was compared with the experimental ones. The results indicate that depending on the components, the heat-release, volumetric and diffusion models can be optimal for the depiction of Leeff. For the studied mixtures, the measured Le based on λ(x)=x (Bechtold & Matalon, Combust. Flame, 2001, 127(1–2): 1906–1913) was found to be more universal, where λ is the non-dimensional thermal conductivity. Additionally, the experimental Lewis numbers derived from different models can be very close, and the key parameters were compared and analyzed.