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The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced. [ABSTRACT FROM AUTHOR]

In this study, characteristics of the miniature vortex combustion power system were investigated with methane, propane, n-butane and i-butane / air mixtures. The system output and CO emission were examined under various equivalence ratio conditions. The experimental results for thermal input of 600W showed that the output of methane attained 20.0W (11.89V × 1.69A) for stoichiometric condition, which is evidently higher than other fuels, 18.1W for propane, 18.6W for n-butane and 18.0W with i-butane. Furthermore, the difference of the output power among those fuels became larger with decreasing the equibalance ratio, that is, the output power of propane, n-butane and i-butane drastically decreased in fuel lean condition, Φ < 0.9. As the exhaust gas temperature showed monotonical increase with the decrease in Φ, the exhaust energy was found to be increased with the reduction of Φ. However, it was found that further energy was lost by incomplete combustion in fuel lean conditions of propane, n-butane and i-butane. For those fuels, the flame become unstable and large amount of CO was emitted for Φ < 0.9 while the output of 16.0W was maintained for methane even at Φ = 0.8. According to the flame appearance obtained by quartz combustor, it was clearly observed that the flame luminosity around the flame base was weakened for propane and butane in fuel lean conditions. As such weakening was not observed for methane, it is considered that the Lewis number effects may contribute to the weakening of propane and butane flame base, and thus, drastic decrease of the thermal input in fuel lean conditions of propane and butane.

The understanding of the boundary layer flame flashback (BLF) has considerably improved in recent decades, driven by the increasing focus on clean energy and the need to address the operational issues associated with flashback. This study investigates the influence of the Lewis number (Le) on symmetric flame shapes under the critical conditions for a laminar boundary layer flashback in cylindrical tubes. It has been found that the transformation of the flame shape from a mushroom to a tulip happens in a tube of a given radius, as the thermal expansion coefficient and Le are modified. A smaller Lewis number results in a local increase in the burning rate at the flame tip, with the flame being able to propagate closer to the wall, which significantly increases the flashback propensity, in line with previous findings. In cases with a Lewis number smaller than unity, a higher thermal expansion results in a flame propagation happening closer to the wall, thus facing a weaker oncoming flow and, consequently, becoming more prone to flashback. For Le > 1, the effect of the increase in the thermal expansion coefficient on the flashback tendency is much less pronounced.

Numerical simulations of temporally-evolving droplet-laden plane jets are performed in conditions relevant to rocket main engines fed with liquid oxygen (LOx) and gaseous methane (CH 4). The computations are performed using a direct numerical simulation (DNS) solver with the liquid phase represented within the discrete particle simulation (DPS) framework. Considering the multiplicity of the physical phenomena that are involved in such conditions – e.g., atomization, dispersion, evaporation – a progressive and phenomenological methodology is retained to proceed with a complexity-increasing set of computations. Thus, the development of a purely gaseous jet is first studied with the corresponding set of data providing a reference or baseline condition. Then, other conditions are considered to analyze the influence of (i) liquid droplet evaporation and (ii) molecular mixing processes as described by two distinct multicomponent transport models. The analysis of the obtained results shows that both evaporation and molecular transport representation play a crucial role in the plane jet development and may drastically alter its characteristics before ignition and subsequent combustion stabilization may take place. Finally, the obtained results also unambiguously put into evidence the influence of the Lewis number onto the vaporization rate. [Display omitted] • Direct Numerical Simulation (DNS) of droplet-laden plane jets are conducted. • The influence of liquid droplets on the jet development is studied. • Evaporation and molecular transport play a crucial role. • The Lewis number value influences the vaporization rate. [ABSTRACT FROM AUTHOR]

Implementation of the adiabatic steady PPDF flamelet model involves a lot of variations including different scalar dissipation rate calculation methods and different mass diffusion models of the opposed jet flame. Four different look-up tables have been generated with the combinations of two different scalar dissipation rate calculation methods and two different mass diffusion models of the opposed jet flame. Simulation of a turbulent non-premixed H2 jet flame is used to discriminate the accuracy of different implementation methods by comparison with experimental data. It is observed that the turbulent flamelets are very close to their equilibrium states and the simulation result is not sensitive to the choice of dissipation rate calculation method. However, the choice of mass diffusion model has significant influence on the simulation result and excluding the Lewis number effect should be enforced for the opposed jet flame simulation.

The present work aims at the detailed understanding of the local processes in premixed combustion of hydrogen, methane and propane flames at unsteady conditions. The methodology consists of the analysis of simulations of two-dimensional flame-vortex interactions as well as statistical data obtained from threedimensional Direct Numerical Simulations (DNS) of the flame front interacting with a set of vortexes. Special attention is given to the relationship between the Lewis number (Le) of the fuel and the flame front stretch in terms of both curvature and strain rate. A large single vortex bends the flame front thus creating both positive and negative curvatures, which in turn enhance the heat release rate in some locations of the flame front and decrease it in others. The resulting effect is called “polarisation effect”. The occurrence and the strength of the polarisation effect of curvature are tightly bound up with the Lewis number of the fuel. The polarisation effect is quantified by the ratio of maximum to minimum heat release rates along the flame front, which defines the Polarisation Effect Number (PEN). The more the Lewis number of a fuel deviates from unity, the stronger the polarisation effect is. Strong polarisation effects lead finally to local flame extinction. This is demonstrated for hydrogen flames with Le = 0.29 (lean) and Le = 2.2 (rich) as well as for artificially designed cases with Le = 0.1 and Le = 10.0. Therefore, flame extinction can occur for both thermodiffusively stable and unstable flames. It is shown that choosing an appropriate mixture of real fuels with different Lewis numbers, the homogeneity of the heat release rate along the flame front could be considerably enhanced. This relatively uniform heat release rate is not sensitive to curvature, which consequently decreases the occurrence of local extinction.

The structure of a counterflow-type cylindrical diffusion flame was studied experimentally to determine the effect of stretch and curvature. The cylindrical flame used in this study had a concave curvature with respect to the fuel stream. The fuels used were propane and methane, diluted with nitrogen, argon or helium. Diluent gases were used to change the Lewis number, Lef, of the fuel. Our results indicated that (1) the fuel was preheated to a high temperature by the concentric inward heat flow from the flame because the flame shape is concave curvature with respect to the fuel flow; (2) at constant flame stretch, flame temperature increased with a decrease in flame radius when Lef > 1, but decreased when Lef < 1; and (3) at constant flame radius, flame temperature decreased with increased flame stretch rate when Lef > 1, but increased when Lef < 1.

This study investigated the turbulent flame speed (S T) of NH 3 /CH 4 /H 2 /air mixtures subjected to differential-diffusion effect characterized by sub-unity Lewis number (Le) at elevated pressures (1 and 5 atm), temperature (373 K), lean equivalence ratios (ϕ = 0.6–0.91) with and without 10% volumetric H 2 O dilution. The experiments were conducted in a fan-stirred constant volume combustion vessel, with a homogeneous turbulence intensity of u ′ = 0 - 2.34 m/s. Schlieren imaging was used to derive the instantaneous turbulent flame speed. Morphology analysis shows that at atmospheric-pressure conditions, the laminar diffusional-thermal unstable flame surface is subjected to cellular wrinkling, this instability is increased at elevated-pressure conditions. Whereas, external turbulence will aggravate the wrinkling but the increase in turbulence intensity suppressed differential-diffusion effects on the flame structure. Flame speed development at sub-unity Le cases show differential diffusion effect and propagates faster than unity Le cases even though they have similar laminar flame speed. Then, the turbulent spherical flame speeds are scaled with a power-law fitting of flame Reynolds number, Re T,flame. The normalized turbulent propagation speed (S T / S L) is much higher for the sub-unity Le flames but it decreases as hydrogen mole fraction (X H2) increases in the fuel mixture. This was explained by the synergistic effect of differential diffusion and turbulence. Water dilution and elevated pressure via increase of Karlovitz number (Ka) and turbulent Re T,flow affect the S T / S L increasing. At sufficiently low Damköhler numbers, Da , a ratio of normalized turbulent flame speed S T / S L is mainly controlled by Re T and scales as a half power law, in line with the classical hypothesis by Damköhler. Additional turbulent stretch modification with Ka or Da can correlate with different hydrogen content conditions, this is because the independent pair of scaling parameters from a set of (u ′/ S L , l T / l f , Da, Ka, Re T) is considered. The scaling parameter ((u ′/ S L) α (l T / l f) β (Le) γ) is insensitive to Lewis number, because of the dominance of relative turbulent intensity and turbulent length scale. [ABSTRACT FROM AUTHOR]

The effect of reduced pressures (i.e., p = 1.0 atm, 0.8 atm and 0.5 atm) on the combustion efficiency of lean H 2 /air flames in a micro-combustor was investigated in this work. We found that the combustion efficiency increases as the pressure is reduced from 1.0 atm to 0.8 atm, and then decreases when the pressure is further reduced from 0.8 atm to 0.5 atm. Numerical analyses reveal that when the pressure is reduced to 0.8 atm, the laminar burning velocity, the heat release rate and the effective Lewis number are increased, while the stretch rate is decreased. The combined effects of these aspects retard the occurrence of “flame tip opening” and lead to a higher combustion efficiency. When the pressure is further reduced to 0.5 atm, the fuel supply and heat release amount are significantly reduced, resulting in much lower temperature level and weaker reaction intensity. Meanwhile, the heat-loss ratio is almost doubled. Therefore, the combustion is completely extinguished in the downstream channel and a large amount of fuel leaks out without burning. As a consequence, the combustion efficiency decreases drastically. [ABSTRACT FROM AUTHOR]

Abstract: Rotating counterflow twin premixed flame (RCTF) of hydrogen air was numerically simulated with detailed chemistry to explore the possibility of ultra-lean combustion. As a result, it was found that ultra-lean RCTF of equivalence ratio Φ =0.052, which is far leaner than the generally-recognized flammability limit Φ =0.10, is realized. It was also found that under ultra-lean conditions the flame temperature of RCTF largely exceeds the adiabatic flame temperature; e.g., at Φ =0.06 the former is 1171K, while the latter is 503K. This increase of burned gas temperature is attributed to the so-called low Lewis number effect within the flammability limit, but under an ultra-lean condition some other mechanism to increase temperature is dominant. The “pseudo local equivalence ratio” of burned gas of RCTF differs largely from that of the unburned gas due to the extraordinarily high concentration of H2O. This suggests the possibility that the local condition at the reaction zone is much richer than the unburned gas, which brings about the large temperature increase. [Copyright &y& Elsevier]

Abstract: Oscillatory propagation phenomena of ethylene premixed gas induced by absorption of externally supplied CO2 laser was investigated. Premixed gas was filled in a combustion tube of 50mm in diameter at 0.1MPa and CO2 laser beam of different diameter was irradiated along the center axis of the tube. The experimental results showed that the flame front curvature was controlled by the irradiated beam diameter and had strong effect on the oscillatory propagation behavior. When the beam diameter is narrow, the flame oscillation phenomena clearly appeared and the oscillation frequency became larger with increased flame front curvature. On the other hand, oscillation phenomena could not be observed when beam diameter was lager than a certain value. According to the time-resolved observation, it was found that the flame front curvature fluctuated with time when oscillatory propagation appeared and the instant of the maximum curvature presence corresponded to the instant of flame propagation deceleration or pause. The observed results could support the mechanism of the oscillation propagation proposed in the previous study, which included the relation of the flame front curvature and thermal-diffusive instability. [Copyright &y& Elsevier]

Abstract: This paper presents the influence of the Lewis number on laminar mixed convective heat and mass transfer in a horizontal tube with uniform heat flux and uniform concentration at the fluid–solid interface. The results of this numerical study show that, for these boundary conditions, the effect of the Lewis number on the Sherwood number is most important near the tube inlet. In the case of the Nusselt number and the wall shear stress, this effect is limited to the intermediate region between the entrance and the fully developed regions. [Copyright &y& Elsevier]