Two Mechanisms of Vorticity Generation in Combusting Flow Fields

Citation

Hendricks, Gavin Julian (1986) Two Mechanisms of Vorticity Generation in Combusting Flow Fields. Dissertation (Ph.D.), California Institute of Technology. doi:10.7907/kj79-eq14. https://resolver.caltech.edu/CaltechETD:etd-03192008-104812

Abstract

In Part 1 of the thesis the behavior of a diffusion flame in an unbounded flow with an imposed pressure gradient is analysed. The problem is formulated using the compressible boundary layer equations, and the assumption of infinite reaction kinetics is employed. The equations are transformed to the equivalent incompressible equations by the application of the Howarth transformation. Solutions to these equations are obtained for a functional form of the pressure gradient which admits similarity solutions. Two stoichiometric fuel—air ratios are considered, φ = 1 which yields a symmetric flow field about the flame, and φ = 0.058 which corresponds to the combustion of methane in air and yields an unsymmetric flow field.

For favorable pressure gradients the fluid in the vicinity of the flame is accelerated more than the fluid in the free stream. The acceleration of the fluid as it is convected downstream causes an augmentation in the fuel mass consumption rate, the mechanism of which is similar to that of a strained diffusion flame in an unbounded counterflow. For adverse pressure gradients a reverse flow develops in the vicinity of the flame which severely affects the mass consumption rate of fuel. For a flame with unit stoichiometry, recirculation zones develop on either side of the flame which eventually lead to extinction. For the stoichiometric ratio corresponding to the combustion of methane in air, the recirculation zones are situated on the fuel side of the flame and no tendency toward extinction is shown.

In Part 2 a numerical study is done to investigate the formation of large vortex structures observed in the combustion chambers of air breathing engines under certain conditions. It has been proposed that these vortex structures are formed by a surging flow over the flameholding device which exists when longitudinal modes of the combustion system are excited. In the present study the surging flow is generated by passing a weak shock wave over a rearward facing step. The fluid entering the chamber is of high density (representing the cold fuel—air mixture) whereas the fluid in the chamber has a low density (the combustion products). The vortex formed by the surging flow induces a downward velocity in the high density fluid toward the lower wall. It is found that larger surge velocities result in the formation of stronger vortices (which induce higher downward velocities), whereas an increase in the mean velocity causes no significant change in the flow field. The time taken for the high density fluid to reach the lower wall is therefore decreased as the surge velocity is increased. By considering these results, a possible model for the sustenance of the vortex shedding mechanism is proposed.

Thesis Files