Stability and thermal characteristics of rod-stabilized premixed flame jets

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Stability and thermal characteristics of rod-stabilized premixed flame jets


Author: Drkos, Miroslav
Title: Stability and thermal characteristics of rod-stabilized premixed flame jets
Degree: Ph.D.
Year: 2009
Subject: Hong Kong Polytechnic University -- Dissertations.
Flame monitoring systems.
Combustion engineering.
Department: Dept. of Mechanical Engineering
Pages: 1 v. (various pagings) : ill. ; 30 cm.
Language: English
Abstract: A comprehensive study of rod-stabilized butane / air flames has been carried out to provide more information on flammability limits, thermal characteristics of free-jet flames and heat transfer of impinging flames. An experimental setup, consisting of round flame jet with axially mounted solid rod had been designed and there were two non-dimensional parameters proposed to represent the variation of geometrical configurations of the nozzle: rod protrusion BR and rod-to-tube ratio BT. Primarily, regions of flame stability had been obtained experimentally for low Reynolds numbers (<3600). Equivalence ratio ranged between 0.4 - 3.0, variable rod protrusion BR (0 - 2) and rod-to-tube ratio BT (0.51 to 0.8) have been applied. Inverted, partially-inverted and regular (Bunsen) flames were fundamental flame types identified across the stable regions. Inter-regions were characterized by the existence of more than one stable flame type, primarily depended on the axial and radial distances of the ignition source from the burner port. Size of the unstable region extended with increasing BR and the effect of BT showed a similar trend. Smaller rod diameters extended the flashback region. Blow-off limits were found to be less dependent on the geometrical parameters. It has been also shown that the rod-stabilized method enabled the premixed flame to operate at very fuel-lean mixtures. Structure and temperature profiles of free-jet inverted and partially-inverted flames have been further studied separately. Inverted flames at equivalence ratio equals to 0.9 and partially-inverted flames equivalent to 1.4 were chosen representatives from respective regions. Temperature profiles, obtained experimentally at Re = 2000, demonstrate very uniform distribution over the central area with increasing height. Partially-inverted flames, characterized by occurrence of inner and outer reaction zone, further widen the area with uniform temperature distribution, in comparison with inverted flames with single reaction zone. An experimental investigation was conducted to deal with the inverted flame impinging vertically onto a flat water-cooled plate. Local and area-averaged distributions of heat flux and Nu for various Re and geometrical configurations were obtained together with flame temperature profiles for better understanding of impinging process. It has been proved that for nozzle-to-plate ratio equal to 3 and higher, heat flux distribution is nearly uniform over a large area around the stagnation and impingement region. This is predominately due to the convex shape of inverted flame that creates central core of burned gas with high temperatures. For very small H (1.0 to 2.5), the maximum local heat flux increased. Decreasing of Re lowered these maximums and shifted them slightly away from the stagnation point. Increasing of BT showed opposite trend. Impact of BR was negligible. Similarly, identical experiments were carried out to understand the heat transfer characteristics of impinging partially-inverted flames. Local and area-averaged distributions of heat flux and Nu for various geometrical configurations and Re and temperature profiles have been obtained. The presence of outer reaction zone is more significant in further nozzle-to-plate distances as it further elongates plateaus with high heat flux and Nu. Increasing of Re shifted the peaks slightly inwards for higher H values, while for small H the trend was opposite. Thermal efficiency is generally lower in partially-inverted flames than in inverted flames and is in the range of 36 - 63 % for inverted flames and 30-51 % for partially-inverted flames. Comparison of both inverted and partially-inverted flames with regular Bunsen flame and Bunsen flame with induced swirl, confirmed the major advantage of rod-stabilized flames, i.e. very uniform heat transfer characteristics over large area of the impinging plate. Semi-analytical approach has been used to predict convective heat transfer without TCHR from impinging inverted flames onto a target plate using extensive experimental data. Based on boundary layer theory, empirical correlations had been derived for local and area-averaged Nu as a function of Reynolds number, nozzle-to-plate ratio and non-dimensional geometrical parameters of the jet.

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