Studies on laminar boundary layer diffusion flames in a confined flow.

Abstract

When a gaseous fuel is uniformly injected through a porous flat plate into a laminar boundary layer of an oxidant stream, a two-dimensional diffusion flame can be established which exhibits some special features. Despite its simplicity and practical relevance, the studies on such flames are surprisingly very few. The study reported in this thesis makes use of both experimental and theoretical approaches to unravel some special features of the diffusion flames in a laminar boundary layer over a porous flat plate in a confined flow. Rather extensive literature review of diffusion flames in related areas is included in various chapters of the thesis to project the present work in a proper perspective. The experiments have been conducted in a horizontal subsonic combustion tunnel. The test section is a rectangular combustion chamber with a porous flat plate forming one of the horizontal walls. The experimental set-up has provisions to vary parameters like oxygen concentration in the free stream, fuel concentration in the injectant, temperature of the free stream oxidant, nature of confinement along the combustion chamber length, and the direction of injection of fuel with reference to the gravitational vector. Experimental data on the location of the diffusion flame, temperature distribution, and velocity distribution across the boundary layer have been obtained for various conditions using direct photography, thermocouple traverse, and the particle track technique respectively. There appear to be three distinct regimes of such a flow, namely:

a stable combustion region where a steady diffusion flame prevails, an unstable combustion region where an oscillatory flame exists in the boundary layer which upon transition into the third regime gets blown out.

The boundaries of these three regimes have been obtained and plotted for various experimental conditions. The influence of the confined nature of the flow has been studied by varying the flow area along the length of the combustion chamber without affecting either the conditions at the porous plate or the two-dimensionality of the flow. The measured flame temperature just before extinction has been used to infer the causes for extinction by various modes. In the unstable region, at low injection velocities, the two-dimensional flame moves forward and backward all along the plate. This low-frequency oscillatory flame has been examined by direct motion picture photography. One important feature in the stable combustion regime is the appearance of velocity overshoot near the flame zone in the presence of a favourable pressure gradient. It is suggested that the confined nature plays a significant role in inducing the favourable pressure gradient and the consequent velocity overshoot. Experiments with combustion chambers of varying degrees of confinement support this view. The role of buoyancy has also been examined by two series of comparative experiments with fuel being injected upwards and downwards. While the buoyancy-induced pressure gradient is favourable for upward injection, it is adverse for downward injection. In the numerical study of the problem, the governing conservation equations have been formulated in the x–w coordinates where x is the streamwise distance and w is the non-dimensional stream function. To highlight the influence of the confined nature alone, the effects of gravity have been excluded. The governing differential equations with the appropriate boundary conditions have been solved on a digital computer by marching integration technique based on GENMIX—a computer program developed by Patankar and Spalding for solving two-dimensional parabolic boundary layer flows. The predicted diffusion flame characteristics compare well with experimental observations. Some discrepancies between theory and experiments can easily be traced to the simplifying assumptions employed in the analysis. The analysis demonstrates the suitability of the method in predicting the favourable pressure gradient due to the confined nature of the flow and the consequent velocity overshoot near the flame. The model has reproduced qualitatively the effects of various parameters on flame structure as observed in experiments. Although the analysis pertains to steady-state combustion only, it does give an indication of some of the limiting conditions for extinction. The thesis concludes with a summary of the important observations as a result of the present study along with some suggestions and scope for further research.