City Research Online

Abstract

Flame structure analyses have been carried out on the stabilising region of two laminar diffusion flames of methane; one close to the blow-off limit and the second a stable flame for reference. The flames were formed on a slot-type burner to provide an effectively two-dimensional flame system. The experimental study involved the determination of sets of interrelated profiles - aerodynamic, thermal and compositional - using the techniques of stroboscopic particle-tracking, fine-wire thermocouple thermometry and micro-probe sampling followed by gas chromatographic analysis. The experimental data were combined with the conservation equations of species and heat energy to yield fluxes and rates of creation of material species and heat energy.

Results have shown that, for diffusion flames, the limiting rate of supply of oxidant into the stabilising region is governed, not by the mass flow, as in pre-mixed flames, by predominantly by diffusive flow across the stream tubes. Contrary to the recent suggestions of Robson and Wilson, and Bellis, the flame structure analyses carried out here strongly suggest that the magnitude of velocity at the flame base is incidental to the phenomenon of flame lift, extinction being limited by chemical reaction rate and mass diffusion rather than by the bulk flow. A picture emerges of a maximum fuel consumption rate of the same order as that in a laminar stoichiometric premixed flame. Analysis of the thermal structure shows that the major proportion of the heat liberated in the reaction zone arises, in mathematical terms, from changes in the conduction flux profile which, in turn, depend primarily on changes in the temperature profile. In order to fully exploit this result an attempt is made to develop a sufficiently rapid and accurate method for measuring flame temperature.

It is shown that the technique of laser interferometry is capable of providing the resolution and precision compatible with the more conventional probe methods that are normally associated with flame structure studies. An extended spline fit technique has been successfully adapted for the quantitative interpretation of conical flame interferograms. Finally, the results offer encouragement that a method similar to the one described can provide a complete picture of the thermal structure (temperature, fluxes of heat energy and rates of heat release) of laminar diffusion flames.

Publication Type:	Thesis (Doctoral)
Subjects:	T Technology > TA Engineering (General). Civil engineering (General) T Technology > TJ Mechanical engineering and machinery
Departments:	School of Science & Technology > Department of Engineering School of Science & Technology > School of Science & Technology Doctoral Theses Doctoral Theses

Export

Downloads