Effective Lewis Number Research Articles

Effect of hydrogen addition on the consumption speed of lean premixed laminar methane flames exposed to combined strain and heat loss

This study presents a numerical analysis of the impact of hydrogen addition on the consumption speed of premixed lean methane-air laminar flames exposed to combined strain and heat loss. Equivalence ratios of 0.9, 0.7, and 0.5 with fuel mixture composition ranging from pure methane to pure hydrogen are considered to cover a wide range of conditions in the lean region. The 1-D asymmetric counter-flow premixed laminar flame (aCFPF) with heat loss on the product side is considered as a flamelet configuration that represents an elementary unit of a turbulent flame and the consumption speed is used to characterise the effect of strain and heat loss. Due to the ambiguity in the definition of the consumption speed of multi-component mixtures, two definitions are compared. The first definition is based on a weighted combination of the consumption rate of the fuel species and the second one is based in the global heat release rate. The definition of the consumption speed based on the heat release results in lower values of the stretched flame speed and even an opposite response to strain rate for some methane-hydrogen-air mixtures compared to the definition based on the fuel consumption. Strain rate leads to an increase of the flame speed for the lean methane-hydrogen mixtures, reaching a maximum value after which the flame speed decreases with strain rate. Heat loss decreases the stretched flame speed and leads to a sooner extinction of the flamelet due to combined strain and heat loss. Hydrogen addition and equivalence ratio significantly impact the maximum consumption speed and the flame response to combined strain rate and heat loss. The effect of hydrogen on the thermo-diffusive properties of the mixture, characterised by the Zeldovich number and the effective Lewis number, are also analyzed and related to the effect on the consumption speed. Two definitions of the Lewis number of the multi-component fuel mixture are evaluated against the results from the aCFPF.

Investigation of cellular characteristics of hydrogen-ethanol flame at elevated temperatures and pressures

The blending of hydrogen into ethanol for use is attracting increasing interest and it is necessary to study its laminar combustion characteristics for better application in combustion units. The instability of hydrogen-ethanol spherically expanding flame has been investigated at initial temperatures of (370 K, 400 K, and 450 K), initial pressures of (2–4 bar), and equivalence ratios 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. The thermal-diffusion effect was found to stabilize the flame surface under all conditions, as judged jointly by the effective Lewis number and the critical Lewis number. The main contribution to flame instability therefore came from hydrodynamic effects, and this was conclusively verified by looking at the trends in the global growth rates of the various instability parameters. The effect of changes in initial pressure and initial temperature on the critical conditions for flame instability (critical radius and Peclet number) was analyzed using linear stability theory. It was found that flames were more prone to cell instability at higher pressures and temperatures. In addition, as the equivalence ratio increased, the monotonic variation of the critical Peclet number and the non-monotonic variation of the flame thickness caused a trend of decreasing and then increasing critical radius. It was noteworthy that the most unstable state for all conditions studied was at about Ф=1.2. The comparison between theoretical calculations and experimental results was generally consistent, with only some differences at Ф = 1.4. Moreover, empirical correlation formulas for the critical Peclet number (Pec) and Markstein number (Ma) were proposed.

Effect of Lewis number on the shape and structure of an ultra-lean swirling premixed flame

Effect of Lewis number on the shape and structure of an ultra-lean swirling premixed flame

Regime and morphology of polyhedral bunsen flames

Intrinsic flamefront cellular instabilities wrinkle, otherwise smooth, laminar flamefronts leading to excess surface area and, thus, burning rate. Since such instabilities increase flame propagation speed and burning rate without any external influence, such as turbulence, they are of both fundamental and practical interest. While extensively studied for freely propagating expanding or planar flames, cellular instabilities can also occur in burner-stabilized premixed flames. In this study, we present an experimental investigation of flamefront polyhedral instability occurring in premixed Bunsen flames for practically relevant lean hydrogen mixtures. Through systematic experiments, we delineate the effect of flame temperature, Lewis number, and flow rate on the stability and morphology of such flames. The results are presented as regime maps to clearly identify the conditions for which the flame manifests the instabilities and further illustrate how the transitional boundaries shift due to changes in experimental conditions. Moreover, the morphology of the flames was also investigated by identifying changes in the polyhedral structure as a function of operating conditions. Finally, we explain the observed dynamics of the polyhedral flames with the aid of analyzes of dispersion relation and residence time.

Dynamic stabilization of a hydrogen premixed flame in a narrow channel

Combustion of hydrogen can help in reducing carbon-based emissions but it also poses unique challenges related to the high flame speed and Lewis number effects of the hydrogen flame. When operated with conventional burners, a hydrogen flame can flashback at higher volumetric flow rates than a methane flame due to the difference in stabilization mechanisms of the two fuels. Due to these differences, conventional burners cannot offer similar operational ranges for hydrogen than that for hydrocarbon flames. An exploration into the unique stabilization behaviour of hydrogen flames is required which could help in envisioning non-conventional burner concepts for keeping hydrogen flames stable. Stability conditions, which describe the kinematics of premixed flames with spatially and temporally changing flow parameters, are crucial for such an exploration. Stability conditions are usually hypothesized for stable flames, where a flame upon perturbation is assumed to return to its original position. Alternatively, in the case of flashback/blow-off, it refers to a flame moving upstream of the burner or being convected out of the domain. However, it is also of interest to understand how and why a flame could move to a new location when the velocity and strain fields are varying with time and space at the original and the new location. In this paper, we investigate the flame stabilization by 1) observing the hydrogen flame’s upstream movement in a multi-slit configuration when a geometrical change is made, and 2) changing strain and velocity fields in a dynamic and periodic manner using numerical tools such that the unique behaviour of a hydrogen flame can be captured. We vary the location of high flow strain periodically in a channel by manipulating the boundary condition along a wall. It is found that a hydrogen flame follows this point in a periodic manner, also propagating against the inflow which is considerably faster than its unstretched burning velocity. Spatial and temporal stability conditions, that explain the mechanism behind the flame’s movement from its original position to a new position, are analyzed from the simulation data, advancing our knowledge on the flame movement in an unsteady setting and providing important insights into the stabilization mechanism of hydrogen flames.

Analysis of Lewis number effects on dynamic response of laminar premixed flames

The dynamic response of premixed flames with different fuel Lewis numbers is crucial to understand for the design of safe combustion chambers with wide operating range. Due to preferential diffusion effects associated with fuel Lewis number for lean mixtures, a premixed flame responds differently to flame stretch. These responses could manifest in the flame’s response to small perturbations. In order to investigate this effect, in this study, lean premixed hydrogen, methane and propane flames at the same adiabatic unstretched flame speed are studied using numerical simulations. Steady unperturbed flames show significant differences in flame stand-off distances, flame height, flame stabilization limits, burner temperature and recirculation zone length. Flames are perturbed using a step function excitation and noticeable changes in the gain and phase of the flame transfer function are found even when the flame burning velocity and the inlet flow speed are kept the same. Maximum gain of the response is found to increase with the mixture Lewis number and is shown to be directly dependent on the flame tip burning strong or weak. Phase difference at low frequencies is found to depend on the average of the time scales of the recirculation vortex which in turn depends on the flame burning stronger or weaker under the influence of positive stretch at the flame base, axial convective wave and perturbation travelling along the flame front. Time for the flame to settle to a new position after the step excitation is found to correlate well with the sum of time scales of vortex, flame tangential speed and flame tip speed. Overall, it is found that hydrogen flame responds the fastest to the perturbations and exhibits smaller increase in gain compared to methane and propane.

Analysis of nanobiofilm flow of Carreau fluid with the effect of buoyancy forces and activation energy: A numerical approach

In the past few years, many technical strategies, such as molding, condenser heat exchanger, liquefied metal filtration, fusion control and nuclear reactor coolant, that involve hydromagnetic fluxes and thermal intensification in porous media have been observed. This study investigates the Carreau nanofluid of nanobiofilm through stretching/shrinking sheet with a stagnant point flow, nanoparticles and convecting microbes. The orthogonal ([Formula: see text] impinge) coating stagnant point circulation of a medium is considered, although the sheet may be stretched/shrinked as the procedure utilized in industry. The variations in the fluid (dynamic viscosity, thermal conductivity, mass permeability) and microbes are utilized. The similarity transformation factors are used to transform the system of partial differential equations into a nonlinear system of ordinary differential equations. To find the solution of a system of equations, the Runge–Kutta method with shooting technique has been used. The flow rate, temperature and concentration, as well as the heat transfer rate, and the physical quantities have been discussed. The nanoparticle volume fraction increases with the increasing effect of activating energy as well as thermophoresis parameter, but it decreases with the enhancing effect of Lewis number (Le) and Brownian motion parameter (Nb). The graphs and tables display the illustration of the influence of different parameters.

Convective Heat Transfer in Magneto-Hydrodynamic Carreau Fluid with Temperature Dependent Viscosity and Thermal Conductivity.

This study is aimed to explore the magneto-hydrodynamic Carreau fluid flow over a stretching/shrinking surface with a convectively heated boundary. Temperature-dependent variable thermophysical properties are utilized to formulate the problem. The flow governing equations are obtained with boundary layer approximation and constitutive relation of the Carreau fluid. The shooting method is utilized to obtain graphical and numeric outcomes. Additionally, initial guesses are generated with the help of Newton's method. The effect of Weissenberg number, Magnetization, stretching ratio, Prandtl number, suction/blowing parameter, and Lewis number is obtained on velocity, temperature and species continuity profile and analyzed. Shear stress rates and Nusselt number outcomes under body forces influences are present in tabulated data and discussed. It is observed that in absence of magnetization force, B = 0 and strong mass suction 5≤S≤7.5 effect high rates of Nusselt number is obtained. It is concluded that under the influence of power law index and non-linearity parameter maximum heat transfer and reduced shear stress rates are obtained.

Logistic-Regression-Based Meta-Analysis of Factors Affecting Flame Stability in a Shock Tube

ABSTRACT This work describes a meta-analysis performed in an effort to better understand the parameters controlling flame stability in shock-tube flame experiments. The data set used in the analysis was aggregated from multiple prior studies employing the shock-tube flame speed method, as well as previously unreported results. Flames are first qualitatively classified as being smooth, distorted, or wrinkled based on end-wall images. A subset of trials capturing the stability transition from stable to unstable flames are then selected from each set of experiments. Across all data sets, the transition between stable and unstable flames is analyzed as a binary classification problem using logistic regression. The resulting classification model, cast in terms of a multivariate power-law expression for the critical temperature ( T crit ) at which flames become unstable, is found to be highly predictive of the stability transition. Consideration of the trained model coefficients allows the effects of different experimental parameters on T crit to be identified. Model parameters provide significant new insight into the selection of diluents in experiments; the use of a test gas with a high heat capacity ratio is found to have the most beneficial effect toward increasing T crit , while the effects of Lewis number and molecular weight are determined to be negligible. Ignition-location and pressure effects are less well quantified due to limited available data and are identified as key areas of further study.

Suppression of the turbulent kinetic energy and enhancement of the flame-normal Reynolds stress in premixed jet flames at small Lewis numbers

We report the effects of the Lewis number on the turbulent kinetic energy, Reynolds stresses, and scalar fluxes in premixed jet flames with strong mean shear. The direct numerical simulation (DNS) of piloted turbulent premixed jet flames is performed under the same condition except for different Lewis numbers to isolate the Lewis number effects. The turbulence intensity within the mean flame brush is found to decay more rapidly along the axial distance with a smaller Lewis number. This Lewis number dependence results from the coupling of the mean shear and the combustion heat release, where the mean shear is the primary source of turbulence production. We show that, under the strain imposed by the mean shear and turbulence fluctuation, the heat release of premixed jet flames is significantly enhanced at the Lewis number smaller than unity, which is represented by the rising flamelet consumption speed and flame surface area. As a consequence, the mean shear within the flame brush is suppressed at small Lewis numbers via the dilatation effect, and the weakened mean shear production causes the fast turbulence suppression along the axial direction. The enhanced heat release at small Lewis numbers also increases the flame-normal Reynolds stress, and leads to the counter-Boussinesq behavior of the shear Reynolds stress and the counter-gradient transport of scalar fluxes. We model the Lewis number effects on local turbulence and scalar transport using the stretched laminar flame speed and thickness, and achieve overall good estimations of the flame-normal Reynolds stress and scalar transport at small Lewis numbers.

Design and performance prediction of desiccant coated heat exchanger using ANFIS–AI tool and dynamic model

• Development of ANFIS–AI tool for design and performance optimization of DCHE. • Predicted optimal design and inlet condition of DCHE using ANFIS–AI tool. • Investigated the influence of design and inlet parameters on DCHE performance. • Assessed adsorption kinetics of DCHE using dynamic model based on Laplace transform. • Examined the interactive effect of Lewis and Stanton numbers on performance of DCHE. To improve indoor air quality, enhance the energy exchange capabilities and minimize the spread of microbial pollutants, desiccant coated heat exchanger seems a promising alternative to conventional heat exchangers such as rotary wheel and adsorber beds. Thus, precise prediction of the design and performance characteristics of desiccant coated heat exchanger is vital for improving the overall air conditioning system performance. Therefore, in the present investigation, an adaptive neuro-fuzzy inference system with artificial neural network fuzzy logic has been implemented to predict the exit parameters of desiccant coated heat exchangers. The advantage of an adaptive neuro-fuzzy inference system is its structure that combines the neural network's learning ability, excellent information representation capability of fuzzy logic, and adaptive algorithm. Silica gel is taken as the coated desiccant. The impact of inlet and design parameters on the performance characteristics of desiccant coated heat exchanger during dehumidification has been evaluated using the artificial intelligence adaptive neuro-fuzzy inference system. Also, the optimal design/inlet conditions for the given operating and design parametric range are obtained. For the obtained optimal conditions, the maximum valves of dehumidification capacity, vapor pressure difference ratio, and sensible energy load ratio are 2.01 kg/h, 0.84, and 0.135, respectively. Further, a dynamic model based on the Laplace transform is developed to assess desiccant coated heat exchangers' adsorption kinetics and thermal effects. Moreover, the interactive effect of Lewis number and Stanton number on the performance parameters has been examined judiciously. Adsorption kinetics of silica gel shows that the water adsorped per mass desiccant is about 0.1129 g/g at the end of sorption time.

Turbulent flame speed measurement of NH3/H2/air and CH4/air flames and a numerical case study of NO emission in a constant volume combustion chamber (C.V.C.C.)

Ammonia combustion is a meaningful method to retrieve stored amounts of excess variable renewable energy. The practical combustors like engines and gas turbines fired by the common hydrocarbon fuels need to be operated by the carbon-free like ammonia or ammonia/hydrogen blends shortly. Thus, in this study, the turbulent flame speed (ST) and NO emission potentials of the NH3/H2 blend (NH3/H2 = 40/60, 50/50, and 60/40 vol%) are investigated in comparison with the methane/air flames in a constant volume combustion vessel. The effects of Lewis number (Le), turbulent intensity (u′=0.78–2.34 m/s), and pressures (1, 5 bar) on normalized ST/SL are considered and validated against literature proposed correlations. Flame morphologies show that the turbulent flame wrinkling characteristics are mainly dominated by the turbulent intensity (u′/SL), turbulent length scale (lT/lF) at different pressures, and differential-diffusion term (Le). The differential-diffusion and turbulent stretch, jointly determine the ST/SL of NH3/H2/air and CH4/air mixtures. The scaling parameter ReT,flame/Le2 can describe the self-similar propagation characteristics of ST/SL of spherical flame and consider the differential-diffusion effect. The further Damköhler number, Da modification on the general turbulent flame speed correlation proposed as: ST/SL ∼ ADaB·(ReT,flame/Le2)0.5 unifies different hydrogen content cases by considering the turbulent stretch effect. When literature experimental data are also correlated together, the scaling law becomes: ST/SL ∼ Re0.5/Le ∼ (u'/SL)0.5(lT/lF)0.5Le-1 with equal weighting factors of u'/SL and lT/lF terms, this follows the Damköhler's second hypothesis at highly turbulent regimes. The impact of turbulent intensity and residence time on NO emissions in the flue gas of NH3/H2/air and CH4/air flames are independent of turbulent intensity due to the long residence time. Simulated results show that residence time has contrary effects on NO emissions of NH3/H2/air and CH4/air flames due to the contrary role of the thermal-NOx pathway. Practical utilization of NH3/H2/air in gas turbine combustors could increase the flame residence time by increasing the back-flow zone in the post-flame zone and swirl number to suppress the NOx emissions.

An experimental study on the instability of RP-3 aviation kerosene/air premixed flame

In the present work, experiments were carried out in a constant volume chamber to study the unstable flame propagation processes of RP-3/air premixed flame at initial pressures of 0.1, 0.2 and 0.3 MPa, initial temperatures of 420, 450 and 480 K and over a wide equivalence ratio range from 1.0 to 1.7. Results show that the RP-3/air premixed flame tends to be unstable with increased equivalence ratio and initial pressure. The measured transition acceleration onset and calculated critical flame radius Rcr agree well with each other. The effects of diffusional- thermal, hydrodynamic instability and stretch rate in spherically expanding flames on α show that α of RP-3/air premixed flame strongly depends on effective Lewis number Leeff and Markstein number Ma. With the increase of equivalence ratio, acceleration exponent α tends to increase and the flame radius corresponding to the onset of similarity acceleration decreases.

Experimental and numerical study of the laminar burning velocity of syngas in oxyfuel conditions

ABSTRACT This study evaluates the laminar burning velocity of oxyfuel syngas mixtures. Experimental measurements were performed in standard conditions (298 K and 1 atm), varying the oxyfuel syngas composition (H2/CO/CO2/O2) and equivalence ratio for lean mixtures. The experimental setup uses the heat flux method, validated with CH4/air and H2/CO/air measurements. The results were compared with six chemical kinetic mechanisms developed for syngas, aiming at evaluating their predictive capabilities. The mixtures presented a tendency to the appearance of cellular instability in the flame front that was prevented to a certain extent. The CO2 effects on the flames were explored through numerical analysis, isolating the thermodynamic properties, transport properties, thermal radiation, and chemical effects. The CO2 or N2 effect as bath gas in the oxidizer is also evaluated in terms of the laminar burning velocity and mixture’s effective Lewis number. Limitations due to cellular instability were studied, analyzing the influence of fuel dilution and composition on the onset of cellular instability at the flames. The results showed a good prediction of the experimental data by the evaluated kinetic mechanisms. The expected reduction in the laminar burning velocity with CO2 dilution is primarily due to the effect of carbon dioxide’s thermodynamic properties, followed by its transport properties (when compared to N2 dilution), chemical effects, and heat losses by radiation. Cellular instability is attenuated with high dilution levels due to the reduction of hydrodynamic instability. At the same time, the increase of the H2 concentration in the mixture leads to more unstable flames due to both hydrodynamic and thermal-diffusive instability. A laminar burning velocity threshold above which the measurements were not possible due to the effects of cellular instabilities was identified and analyzed in detail.

Lewis number effects on laminar and turbulent expanding flames of NH3/H2/air mixtures at elevated pressures

This study clarifies the effects of Lewis number (Le) on laminar and turbulent expanding flames of NH3/H2/air mixtures. The laminar burning velocity (SL) and turbulent burning velocity (ST) were measured using a medium-scale, fan-stirred combustion chamber with ammonia/hydrogen molar ratio (NH3/H2) of 50/50 and 80/20 under the maximum pressures of 5 atm. The lean laminar flame with NH3/H2 = 50/50 is significantly accelerated by the diffusional–thermal instability, which dominated the trend of ST,c=0.1 with the equivalence ratio (ϕ). The lean normalized turbulent burning velocity (ST/SL) increases with the decrease of hydrogen content due to the weakening effects of SL. However, the ST/SL reaches peak with hydrogen volumetric content less than 20% due to effects made by diffusional–thermal instability than SL did. The turbulent flame of NH3/H2/air mixtures is characterized by self-similar acceleration propagation, and propagation with Le < 1 is faster. A modified correlation considering the effects of Le was proposed, as (d<r>/dt)/σSL = 0.118(ReT,flameLe−2)0.57, which was able to predict not only the self-similar propagation of NH3/H2/air but also the previous syngas/air flames. The Kobayashi correlations modified by three kinds of Le power exponents were used to clarify the effects of Le by comparing their fitting parameters and predictive powers on experimental data and literature data. Similar pre-factors, power exponents and the goodness of fit (R2) were obtained with Le ranging from 0.58 to 1.62, which suggested that the determination of Le power exponent had no significant effect on the prediction accuracy of the ST/SL trend with data of Le near unity. This might be attributed to the fact that the variation ranges of the dimensionless number that characterizes the experimental conditions is much larger than that of the Le.

Experimental study on the intrinsic instabilities of spherically expanding CH4/H2/CO2/O2 flames

To study the instabilities of spherically expanding CH4/H2/CO2/O2 flames with an equivalence ratio of 0.80, isochoric combustion experiments were carried out in a 36 L constant volume bomb at an initial temperature (298±2 K) and atmospheric pressure (100 kPa). The crack length Lcrack, average cell area Scell, critical flame radius Rc and critical Peclet number Pec of CH4/H2/CO2/O2 flames were evaluated. The results show that the evolution of Lcrack at low hydrogen fractions is slightly affected by oxygen enrichment but significantly affected at high hydrogen fractions. Thermodiffusive instability dominates the evolution of Scell at low hydrogen fractions, but hydrodynamic instability dominates at high hydrogen fractions. At lower oxygen fractions, hydrogen addition results in more significant enhancement of the thermodiffusive and hydrodynamic instabilities. As the oxygen fraction increases, the maximum size of the thermodiffusive cells decreases, and the thermodiffusive instability weakens due to the increasing effective Lewis number, but the flame thickness decreases. This indicates that the weak thermodiffusive instability is amplified by the small flame thickness. With increasing oxygen fraction, the unstable region of the peninsula by the linear stability theory narrows in the Pe–n plane but broadens in the R–n plane. Therefore, the unstable peninsula in the R–n plane is consistent with the trend of crack growth observed in the experiments. Additionally, the experimental critical flame radius Rc,1 is closer to Rc calculated by the linear stability theory than Rc,2, where Rc,1 and Rc,2 represent the cross-cracking of the thermodiffusive cells and the onset of the hydrodynamic cells, respectively. Pecl exhibits a linear dependence on the Markstein number Ma. Moreover, a power law correlation of the experimental Rc with respect to oxygen enrichment and hydrogen addition is proposed.

Experimental and numerical study on laminar burning velocity and premixed combustion characteristics of NH3/C3H8/air mixtures

Propane (C3H8) is the major component of liquefied petroleum gas, the combustion of C3H8 and ammonia (NH3) should be concerned. However, there is a scarcity of experimental data on NH3/C3H8/air mixtures to validate the kinetic models, especially at elevated pressures. This work aims to provide some new laminar burning velocity data and investigate the premixed combustion characteristics of NH3/C3H8/air mixtures in detail. The measurement was performed at various equivalence ratios (ϕ = 0.6–1.4), ammonia ratios (α = 20 %, 50 %, 80 %), and initial pressures (P0 = 0.1–0.5 MPa). Besides, five kinetic models were compared against the experimental data, the optimal performing model was used for numerical simulation. The results show that when the mole fraction of C3H8 in fuels is 50 %, the peak laminar burning velocity is about 28.5 cm/s. And the peak value decreases to 19.0 cm/s when the initial pressure rises to 0.5 MPa. The burned gas Markstein length decreases with the increase of equivalence ratio mainly due to the remarkable effect of the effective Lewis number, and it also decreases with the increase of initial pressure because of the decreasing flame thickness. C3H8 promotes the combustion intensity of NH3, which is reflected in the enhancement of flame temperature and radical pool, but C3H8 has no obvious effect on the mixture heating value of the NH3/C3H8/air mixtures. NH3 substituting can effectively reduce CO and CO2 emissions, and the reduction effect is more significant under high NH3 mole fraction conditions.

Effect of unsteady relative thermal and concentration boundary layer thickness on mixed convection in a partially heated contaminated enclosure

Double diffusive flow under mixed convection has become a significant area of research. This study aims to evaluate the Lewis number effect in such a double-diffusive problem in a roof-based air-ventilated system. The Lewis number 0.01 ≤ Le ≤ 5 was chosen for the study, considering other parameters as constant to observe the impact of different heat and mass transfer properties varying the dimensionless time 0.1 ≤ τ ≤ 1. The Galerkin residual technique was chosen to generate the governing equations. The outcomes have been plotted in streamlines, isotherms, isoconcentration, heat and mass transfer, average fluid temperature and mass concentration. The major findings elucidate that heat and mass transfer rate augments when Lewis number goes higher and vice versa. Moreover, when the thermal diffusivity is higher than the mass diffusivity, the average fluid temperature and mass concentration become lower. Overall, this study will be a guide on double-diffusive system design, such as roof-based air ventilation systems.

Experimental investigations of laminar and turbulent burning velocities of thermally cracked hydrocarbon fuels

A comprehensive understanding of the flame propagation characteristics of thermally cracked fuels is crucial for developing advanced scramjets using regenerative cooling techniques. In the present study, both the laminar and turbulent burning velocities of thermally cracked hydrocarbon fuels at typical pyrolysis temperatures were systematically measured using a newly developed, fan-stirred, constant-volume bomb. The results show that variations in fuel composition have no significant influence on the laminar burning velocities of pyrolysis gases and liquids. In addition, the promotional effects of pyrolysis gas on flame propagation of thermally cracked fuel depend on the gas-production rate and equivalence ratio. The normalized turbulent burning velocities of typical thermally cracked fuels can be well scaled using a power law with regard to the turbulent Reynolds number (ReT, f). The fitting exponents gradually increase with increasing equivalence ratio, indicating a stronger promotional effect by turbulence for fuel-rich flames. For premixed turbulent flames in the wrinkled/corrugated flamelet regimes, turbulence and preferential diffusion simultaneously affect the flame propagation process. The wrinkling effects of turbulence are weakened by thermal-diffusional stability when the combustible mixture has an effective Lewis number (Leeff) greater than one. By contrast, the wrinkling process is promoted by thermal-diffusional instability, and the flame propagation is simultaneously enhanced by turbulence and thermal-diffusional instability. A unified scaling law for normalized turbulent burning velocities of thermally cracked fuels is proposed based on ReT,f Leeff–0.6, which accounts for the impacts of preferential diffusion. The effective Lewis number corrected scaling law can be applied to thermally cracked fuels at different pyrolysis temperatures, with a maximum relative error not exceeding 15%.

Effects of differential diffusion on hydrogen flame kernel development under engine conditions

The early flame kernel development in spark ignition engines is crucial for engine performance. For non-unity-Lewis-number mixtures, it can be significantly influenced by differential diffusion due to the large curvature of the small kernel. Differential diffusion can lead to thermodiffusive instabilities for lean hydrogen/air flames, which are enhanced for high pressures but diminish for increasing temperatures. In this study, direct numerical simulations of lean hydrogen flame kernels under engine conditions have been performed to investigate how differential diffusion affects their growth with an effective Lewis number far smaller than unity and if thermodiffusive instabilities appear under realistic engine conditions with elevated in-cylinder pressure and high unburned temperature. It is found that turbulence triggers the instabilities for flame kernel sizes far smaller than the critical radius of the onset of cellular instabilities for laminar flames. The strong thermodiffusive instabilities significantly facilitate the flame kernel growth. The normalized fuel consumption rate is increased by a factor of up to four, due to an enhanced propagation speed. This is remarkable as it was found in earlier studies that for laminar flames the effects of instabilities become much weaker under these conditions. Thermodiffusive instabilities also lead to large variations of the local fuel/air equivalence ratio resulting in temperatures up to 500 K above the adiabatic temperature, which impacts NOx formation. In addition, thermodiffusive instabilities alter the mechanisms of flame surface area formation. The production and destruction of the surface area by flame propagation are significantly increased. A transition phase can be identified during the formation of the negative curvature regions from the initial spherical kernel.