An optical study of sound generation in lean premixed combustion

Abstract

This thesis presents an experimental and theoretical study of sound generation in lean, premixed, propane-air flames. Sound generation by axisymmetric, forced, laminar flames is investigated first. The sound generated by the formation and consumption of separated pockets is determined. The second part of the thesis quantifies the capability of Flame Chemiluminescence Tomography (FCT) to directly study the flame dynamics responsible for sound generation. Suitable techniques are not yet available to acquire time-resolved, 3D measurements of entire turbulent flames. FCT is therefore investigated on non-axisymmetric, forced, laminar flames.

Chemiluminescence imaging and microphone measurements are used to relate the acoustic pressure emitted by forced, laminar flames to the flame geometry at different phases of the forcing cycle. The formation and consumption of separated pockets is shown to be a significant source of sound. A model of sound generation by conical and spherical flame elements is presented, where the flame is treated as an infinitely-thin zone of heat generation. It is shown that increases in the flame displacement speed with increasing curvature is responsible for a significant increase in the amplitude of the radiated sound. The merging of the preheat and reaction zones of oppositely propagating flames that occurs in the terminal moments of both the formation and consumption of separated reactant pockets has previously been identified as a potentially significant source of combustion noise. However, in forced, laminar, lean, propane-air flames the sound produced by this merging is found to be small relative to the sound generated by negative flame stretch that occurs before opposite flames are within two thermal flame thicknesses of each other.

To assess the capabilities of FCT, a single camera is used to acquire 36 equally-spaced, coplanar views of forced, premixed, non-axisymmetric, laminar flames at 100 phases of their forcing cycle. A Multiplicative Algebraic Reconstruction Technique (MART) algorithm is then used to reconstruct the time resolved chemiluminescence field using different numbers of equally-spaced views. Algorithms for measuring the flame surface area, the mean curvature, the normal component of the flame propagation velocity and the chemiluminescent flame thickness are demonstrated and the sensitivity of these measurements to the number of views used in the reconstruction is assessed. Less than 10 views may be sufficient to measure flame surface area, flame curvature and flame surface speed. However, at least 18 views appear to be required in order to obtain useful measurements of flame thickness.