Yuriy Shoshin

EXPERIMENTAL AND COMPUTATIONAL STUDY OF LEAN LIMIT METHANE-AIR FLAME PROPAGATING UPWARD IN A 24 MM DIAMETER TUBE

Lean limit methane-air flame propagating upward in a 24 mm diameter tube was studied experimentally and by numerical simulations. Gas velocity field was measured using Particle Image Velocimetry (PIV) method. The experimental measurements are compared with the results of numerical simulations and with previous similar measurements performed for a standard flammability tube. The experimentally determined lean flammability limit in 24 mm diameter tube was 4.9±0.03% CH4 by volume, against determined earlier flammability limit 5.1–5.2% CH4 in a standard tube. Maximum stretch rate value was found to be at flame leading point and was higher in a 24 mm diameter tube than in a standard flammability tube. The observed extension of the lean flammability limit is attributed to the strengthening effect of positive flame stretch on lean methane-air flames, characterized by Lewis number, Le < 1. Numerical simulations of limit methane/air flame in a 24 mm diameter tube demonstrated presence of negative flame speed at the flame leading edge.

Effect of Soret diffusion on lean hydrogen/air flames at normal and elevated pressure and temperature

The influence of Soret diffusion on lean premixed flames propagating in hydrogen/air mixtures is numerically investigated with a detailed chemical and transport models at normal and elevated pressure and temperature. The Soret diffusion influence on the one-dimensional (1D) flame mass burning rate and two-dimensional (2D) flame propagating characteristics is analysed, revealing a strong dependency on flame stretch rate, pressure and temperature. For 1D flames, at normal pressure and temperature, with an increase of Karlovitz number from 0 to 0.4, the mass burning rate is first reduced and then enhanced by Soret diffusion of H2 while it is reduced by Soret diffusion of H. The influence of Soret diffusion of H2 is enhanced by pressure and reduced by temperature. On the contrary, the influence of Soret diffusion of H is reduced by pressure and enhanced by temperature. For 2D flames, at normal pressure and temperature, during the early phase of flame evolution, flames with Soret diffusion display more curved flame cells. Pressure enhances this effect, while temperature reduces it. The influence of Soret diffusion of H2 on the global consumption speed is enhanced at elevated pressure. The influence of Soret diffusion of H on the global consumption speed is enhanced at elevated temperature. The flame evolution is more affected by Soret diffusion in the early phase of propagation than in the long run due to the local enrichment of H2 caused by flame curvature effects. The present study provides new insights into the Soret diffusion effect on the characteristics of lean hydrogen/air flames at conditions that are relevant to practical applications, e.g. gas engines and turbines.

Numerical and experimental studies of torus-like flame around the vortex filament in a premixed reactant flow

ABSTRACT In this work we present numerical studies and experimental observations of premixed torus-like flames formed around the filament of a steady vortex in a flow of premixed reactants at lean conditions. The numerical results were obtained within the diffusive-thermal model while the experimental observations were carried out for pure methane-air and 50% hydrogen +50% methane-air mixtures. The parallels between such flames and the flame ball are observed owing to delivering of reactants to the curved flame front and removing of combustion products solely by diffusion.

On the correlation of inverted flame blow-off limits with the boundary velocity gradient at the flame holder surface

Conductive heat losses from the base of a lean methane–air inverted flame stabilized behind the trailing edge of a thin rod have been experimentally evaluated. The results favor the view that the heat losses to the flame holder play a crucial role in the inverted flame stabilization and blow-off. Simple estimations have been performed, which indicate that the well-established correlation between the mixture composition and the boundary velocity gradient at the flame holder, usually considered as a proof of the flame stretch theory of blow-off, can be explained without involving the flame stretch concept. The suggested explanation of this correlation is based on the assumption that the heat loss to the flame holder is the main factor that determines the inverted flame blow-off behavior and on the similarity between the mechanisms of energy and momentum diffusion in gases (Pr≈ 1).