Effects of preferential diffusion on the burning intensity of curved flames
Using the Bunsen flame as a model curved flame, the coupled influence of preferential diffusion and flame stretch on the burning intensity of lean and rich mixtures of methane, propane, butane, ethylene, and hydrogen with air have been experimentally studied. Their results substantiate theoretical predictions and quantify previous experimental observations that, for mixtures whose effective Lewis numbers (Le) are less than unity, the flame temperature (Tf) is less than the adiabatic flame temperature (Tad). This temperature also decreases towards the flame tip, which has the largest curvature and therefore may locally extinguish. For mixtures with Le>1, Tf appear to be greater than Tad and the burning intensity increases towards the flame tip. For the Le ≈ 1 mixtures the behavior is somewhat neutral. The dominance of diffusional transport in influencing the intensity of curved flames is further demonstrated by showing that the tip opening of the highly diffusive hydrogen/air flames occurs at constant hydrogen equivalence ratios of about 1.15 to 1.20, being almost independent of the flow intensity and uniformity.
References (13)
- KaskanW.E.
- LewisB. et al.
- TsujiH. et al.
- IshizukaS. et al.
- TsujiH. et al.
- SatoJ.
Cited by (83)
Influence of slit asymmetry on blow-off and flashback in methane/hydrogen laminar premixed burners
2024, Combustion and FlameOne way to decarbonize the heat production sector is to gradually replace natural gas with hydrogen. The design of laminar premixed burners capable of operating with both methane and hydrogen is however challenging as these fuels have drastically different burning properties, which narrows the operating range. While methane flames face limitations in stabilization due to blow-off at high power, hydrogen flames tend to be susceptible to flashback at low power. This study investigates the effects of slits symmetry breaking on the blow-off of methane flames and flashback of hydrogen flames through two-dimensional direct numerical simulations of canonical asymmetrical slit configurations, revealing uneven interactions between the main openings and smaller auxiliary slits. The equations governing the reactive flow dynamics are coupled to a heat transfer solver in the solid phase to elucidate the thermal and hydrodynamic mechanisms determining the operability limits. Viscous dissipation in the small auxiliary slits together with a substantial preheating of the fresh gases by heat redistribution through the solid phase is found to govern flame stabilization in asymmetrical geometries, improving blow-off resistance of methane-air flames. Then, flashback of hydrogen-air flame is found to be driven by the competition between (i) the mass flow rate distribution between the main and auxiliary slits, (ii) preheating of the gas through the auxiliary slits and (iii) the ability of the main and auxiliary slits to quench the flame. The interplay of these phenomena gives rise to complex behaviors, wherein asymmetrical configurations could exhibit significantly enhanced resistance to flashback compared to symmetrical geometries. This conclusion, verified for different burner thicknesses and slit spacing, may be used to guide the design of fuel-flexible laminar burners.
Novelty and significance statement
1. Analyzing the effect of slits symmetry breaking on blow-off of methane flames and flashback of hydrogen flames for multi-perforated laminar burners.
2. Defining a strategy for wall heat flux redistribution to assess the asymmetrical interaction between the main and auxiliary flames through the flame-holder.
3. Demonstrating and explaining the positive impact of symmetry breaking on the blow-off resistance of methane flames.
4. Highlighting the substantial and non-monotonous impact of symmetry breaking on hydrogen flame flashback.
5. Identifying the interplay of thermal and hydrodynamic mechanisms in asymmetrical slits leading to improvement of flashback resistance for hydrogen-air flames compared to symmetrical configurations.
Predicting flashback limits in H<inf>2</inf> enriched CH<inf>4</inf>/air and C<inf>3</inf>H<inf>8</inf>/air laminar flames
2023, Combustion and FlameThe 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 /Air and /Air mixtures with hydrogen contents ranging from to and varying fuel-to-air ratios adjusted according to the iso– and the iso– hybridization strategies. The analysis reveals the existence of two different flashback regimes and a critical effective Lewis number for the fuel mixture, , 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.
Combustion of suspensions of mixed aluminum and silicon carbide in the products of hydrocarbon flames
2023, Combustion and FlameA stabilized methane/air Bunsen flame is seeded with powder fuel mixtures containing various combinations of spherical, micron-sized aluminum (Valimet H-2 with nominal d50 = 3.5 µm and H-15 with nominal d50 = 20 µm) and silicon carbide (SiC) at different mass ratios over a ∼0-400 g/m3 concentration range. It is observed that in dispersions of only H-2 aluminum, a bright white aluminum flame front forms and couples to the methane-air flame, resulting in a sustained flame speed even at concentrations beyond 400 g/m3. In contrast, dispersions of only H-15 powder decrease flame speed rapidly and cause an open-tipped methane-air flame at concentrations beyond 200 g/m3, similar to inert SiC powder. When blended together into H-2:H-15 1:1 (mass ratio) mixture dispersions, an aluminum flame front forms and couples to the methane-air flame, with a flame speed comparable to unitary H-2 aluminum in contrast to mixtures of H-2:SiC 3:1 which produce no noticeable difference in flame speed from purely inert mixtures of SiC at nearly 200 g/m3. Mixtures of H-2:SiC 3:1 demonstrate an aluminum flame coupling to the methane flame, but with an increased separation between the fronts that was not observed in previous hybrid flames studies. The flame temperatures, flame coupling, and aluminum combustion efficiency behaviors are attributed to the effective amount of slowly reacting or inert solid material in the mixed powder fuels. The behaviors are consistent with a simple hybrid flames model, developed in previous work, where the effectively reduced heat of reaction of the powder fuel is unable to support sufficient heat feedback to the methane-air flame permitting effective secondary flame front formation and flame-coupling. From this understanding, a method for determining the relative energy contribution of a lower reactivity component in a fuel mixture when it is thermally driven by a higher reactivity fuel is demonstrated using the secondary front formation and flame coupling as a benchmark for reaching a certain heat of reaction of the fuel solid mixture. It is estimated that the H-15 component of a H-2:H-15 1:2 mixture contributes to approximately 55% of the thermal energy required to achieve the same behavior as a H-2:SiC 3:1 fuel mixture.
Autoignition-induced flashback in hydrogen-enriched laminar premixed burners
2023, International Journal of Hydrogen EnergyThis paper investigates the possibility of autoignition as a flashback initiation mechanism in laminar premixed burners powered by an increasing content of hydrogen in the fuel blend. A specific experimental setup provides an optical access inside a generic burner reproducing the main features of domestic boilers. This window is used to gather high-speed intensified images during transition to flashback. Zooms into the location of flashback initiation provide clear evidence for the existence of two distinct regimes. In the first one, flashback is initiated by a flame stabilized above the burner that is able to propagate upstream through one of the burner holes. In the second regime, flashback is initiated by autoignition of the fresh gases inside the burner, as they flow along the hot metallic internal wall. These observations are corroborated over a large range of equivalence ratio and hydrogen content. They demonstrate the dependence of the initiation regime on the burner wall temperature and highlight the role of the crossover temperature of hydrogen-enriched mixtures in triggering autoignition. An autoignition Damkohler number Dai that compares a residence time of gases along the hot wall and the autoignition delay time is defined. The ratio Dai is found to be more than an order of magnitude higher for autoignition-induced cases compared to the cases initiated by a flame propagating upstream, further supporting the existence of two distinct mechanisms leading to flashback. These results carry substantial implications on the design process of H2-enriched laminar premixed burners.
Hydrogen substitution of natural-gas in premixed burners and implications for blow-off and flashback limits
2022, International Journal of Hydrogen EnergyTwo multi-perforated premixed burners, designed for natural gas, are fueled with increasing hydrogen content to assess the limits of H2 substitution and investigate potential risks associated to it. The burners feature a different design, which affects flame stabilization and heat exchange between the fresh mixture and the hot burner walls. First, results are presented by means of stability maps that were collected at constant power and over a wide range of equivalence ratio, from pure methane-air to pure hydrogen-air mixtures. The impact of hydrogen addition on blow-off and flashback limits is then analyzed. On one side, it is observed that hydrogen addition increases blow off resistance, extending the operating range towards ultra-lean conditions. On the other side, hydrogen raises the thermal load on the burner favoring flashback. It is shown that the competition between the bulk velocity at the burner outlet and the laminar burning velocity is not a reliable parameter to predict flashback occurrence, while the thermal state of the burner represents a determining factor. An analysis of the thermal transient reveals a strict correspondence between the onset of flashback for a given mixture composition and the burner surface temperature. Results highlight the challenges linked to the design of fuel-flexible systems, pointing out practical limits of H2 substitution in burners designed for operation with natural gas.
Flame Surface Density based mean reaction rate closure for Reynolds averaged Navier Stokes methodology in turbulent premixed Bunsen flames with non-unity Lewis number
2022, Combustion and FlameThe Flame Surface Density (FSD) based mean reaction rate closure proposed by Prof. K.N.C. Bray and co-workers in the context of Reynolds Averaged Navier-Stokes simulations has been a-priori analysed using a Direct Numerical Simulation (DNS) database of turbulent premixed Bunsen flames with different characteristic Lewis numbers representing the strict flamelet regime (i.e. high Damköhler number and low Karlovitz number combustion). The statistical behaviours of stretch factor, orientation factor and wrinkling length scale have been assessed for non-unity Lewis number conditions to identify their Lewis number dependencies. The assumption of presumed bimodal distribution has been found to be rendered invalid close to the nozzle exit where the unresolved wrinkling remains relatively small even when the flow parameters at the nozzle exit represent high Damköhler number and low Karlovitz number conditions. Although the PDF of reaction progress variable shows some resemblance to a bimodal distribution away from the nozzle exit, the Bray-Moss-Libby expressions which can be derived for infinitely large values of Damköhler number have been found to show considerable deviations from the Reynolds averaged reaction progress variable and reaction progress variable variance extracted from DNS data even though these cases represent the wrinkled/corrugated flamelets regime combustion based on nozzle exit conditions. This suggests that it might be necessary to solve a modelled scalar variance transport equation even in the strict flamelet regime instead of using the algebraic relation for the scalar variance. It has been found that the characteristic Lewis number has a major influence on the orientation factor, wrinkling length scale, and the stretch factor. Furthermore, these parameters are found to be (sometimes strong) functions of the axial distance from the nozzle exit. Known parameterisations for the wrinkling length scale and the stretch factor have been shown to be unable to capture the correct variation across the flame brush based on a-priori analysis of DNS data. The variability of the orientation factor and the inadequacy of existing relations to approximate other quantities such as the stretch factor, wrinkling factor and the Reynolds averaged reaction progress variable have the potential to severely limit the performance of algebraic FSD based mean reaction rate closures in turbulent premixed flames within the flamelet regime.
- *
Present Address: Department of Mechanical Engineering, University of California, Davis, California 95616.