Formation, prediction and analysis of stationary and stable ball-like flames at ultra-lean and normal-gravity conditions

Abstract

In this research work, we report on the numerical predictions and analysis of stable, stationary and closed burner-stabilized reacting fronts under terrestrial-gravity conditions for ultra-lean hydrogen-methaneair premixed mixtures with a 40% hydrogen (H 2 ) and 60% methane (CH 4 ) fuel composition, specified on a molar basis. The transition from a cap-like to ball-like flame shape with decreasing inlet equivalence ratio is predicted in agreement with experimental observations. The predicted flames are compared to both flames that were studied in experiments and numerical solutions of perfectly-spherical flame balls in the absence of gravity and convection. The comparison includes flame size, lean limits, and when pertinent, standoff distances, all for two different reaction mechanisms. The absolute molar consumption rates of both H 2 and CH 4 for the limit flame attain maximum values that are significantly larger than those of the corresponding gravity-free flame ball. The fuel supply mechanism of the normal-gravity limit flame is similar to the fuel supply of flame balls in that it is driven by diffusion even away from the flame front. Heat conduction to the tube wall of the burner and convective heat loss are the dominant forms of heat loss. Furthermore, simulations with inclusion of multicomponent transport and Soret and Dufour effects show that the flame size increases for both flame balls and the burner-stabilized flames. For the latter, a slight modification in the stabilization position is found owing to the intensification of the consumption rates of both H 2 and CH 4 when these effects are accounted for. In summary, the present work considers a new configuration that allows the study of stable and stationary ball-like flames at ultra-lean and nearlimit conditions, and advances the understanding of such flames via detailed numerical computations.

Large-scale flame structures in ultra-lean hydrogen-air mixtures

“…Recirculation zone consisting of two large vortices is formed in the wake of the flame directly after its upward moving starts [42] (see streamlines presented in figure 3a). Fresh mixture is attracted to the trailing edge of the flame kernel by the vortical flows and deficient reactant diffusion that determines non-zero reaction rate on the flame trailing edge [24]. Flame stretch due to vortical flows leads to the characteristic cap-shaped flame structure formation (see Fig.…”

Effect of pressure on the lean limit flames of H2-CH4-air mixture in tubes

“…An experimental setup similar to the one used in [15] is employed in this study. In order to extend this setup to elevated pressures, it is redesigned to fit in the high pressure cell which was used in [17].…”

“…Subsequently, stationary ball-like flames in H 2 -CH 4 -air mixtures near the lean limit with well defined boundary conditions were stabilized in a downward flow [14] at normal pressure. A detailed comparison between experimental ball-like and numerical ball-like flames at normal pressure has been conducted by Hernández-Pérez et al [15], analyzing the effects of transport, chemistry and heat loss on the formation and stabilization of the ball-like flames.…”

“…The aim of this work is to experimentally and numerically investigate lean limit flames with low Lewis number at elevated pressures and normal gravity. In order to keep the consistency with the previous study at normal pressure [15], a 40% H 2 and 60% CH 4 fuel composition (specified on a molar basis) is selected. Lean limit flames at elevated pressures are experimentally and numerically examined as a function of the inlet equivalence ratio, φ.…”

CFD Modeling of the Combustion Characteristics and Stability in Catalytic Micro-Channels

“…The Reynolds number is a criterion of whether fluid flow is absolutely steady or on the average steady with small unsteady fluctuations (Kim et al, 1987). The ability of the system to operate under fuel-lean conditions can expected to be beneficial, since it could reduce unwanted combustion products such as coke, nitric oxide, and carbon monoxide (Hernández-Pérez et al, 2015). It could also reduce the operating temperature, which in turn could increase the lifetime of the system (Okuno et al, 2017).…”