Abstract
In this study, the problem of lean propane-air premixed flame acceleration from closed to open end during the early stages of burning in small-size tubes with isothermal walls was considered. In particular, the effects of tube radius, slip/non-slip wall conditions, and the wall temperature on the flame propagation and shape were investigated numerically. Five stages of flame propagation are identified: 1) spherical expansion of the flame front; 2) finger shape expansion of the flame front before touching the wall; 3) flame propagation in the tube subjected to flame-wall interactions; 4) transformation of the flame shape into tulip form; 5) conversion of the tulip shape flame to finger. Our results show that the tube radius, wall condition and its temperature significantly affect flame propagation regimes even in the first instance of the flame propagation in the tubes. We find that increasing tube radius has, overall, the effect of increasing the flame propagation speed in isothermal tubes. Also, depending on tube radius and wall condition, the wall temperature can increase or decrease the flame propagation speed in the isothermal tubes. Furthermore, the results demonstrate that imposing either slip or non-slip condition on the tube’s walls impressively affects flame acceleration and its configuration in the early stages. We observe that, unlike flame propagation forms in the tubes with slip walls, the early exponential flame propagation phase in the tubes was generally followed by a linear flame propagation phase in the tubes with a non-slip wall condition. We obtain that flame propagation in tubes with slip wall conditions are more sensitive to variations in tube radius and wall temperature compared to non-slip conditions. We also see that, contrary to the exponential flame propagation phase, increasing the non-slip wall temperature reduces the flame propagation speed in the linear part of the flame propagation, while such an increase in temperature leads to oscillations in the flame propagation speed in the tubes with slip walls.
Highlights
Predicting flame shape and acceleration in confined spaces and chambers is vitally important for safety issues and designing meso- and micro-scale combustors
This deviation may be related to the effect of other parameters, such as the tube radius, non-adiabatic condition on the real tube wall, and the propane-air overall chemistry scheme which has been applied in the present study
The results showed that imposing the non-slip condition on the wall could exponentially increase the flame acceleration in an adiabatic tube, while imposing the slip wall condition could significantly suppress flame acceleration, especially after that the flame skirt touched the walls
Summary
Predicting flame shape and acceleration in confined spaces and chambers is vitally important for safety issues and designing meso- and micro-scale combustors. It could help designers to better understand and control hydrodynamic and thermodiffusive instabilities in small-scale combustors and increase their combustion efficiency. In this regard, the most typical geometry in considering safety issues and energyproduction devices corresponds to relatively long channels, with the flame propagating from a closed channel end to an open one. Shelkin related the flame acceleration to non-slip conditions at the tube walls and to flow turbulence. According to the Shelkin mechanism, non-slip channel walls make the flow non-uniform, which bends the flame, increases the burning rate, and leads to a stronger flow. The positive feedback between the flame and the flow results in flame acceleration
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