The thesis addresses the influence of compressible effects on the stability of deflagration waves. Due to the quasi-isobaric nature of slow flames, compressible effects in laminar flames are usually neglected. Nevertheless, turbulent deflagrations may propagate at substantially higher speeds, suggesting that compressible effects may play a role in their dynamics. In the present thesis, the stability of diffusion-dominated high-speed deflagrations is addressed. The deflagration is studied in the thickened regime, hence addressing the long wavelength limit of these deflagrations. The deflagrations are modelled by the compressible reactive Navier-Stokes equations with a single-step Arrhenius reaction model. The 2D stability of the steady traveling-wave solution is studied by direct simulation. It is found that, as the flame compressibility becomes significant, not only does the growth rates of the cellular profile of the deflagration waves increase, but the traditional correlation of the burning velocity and the flame surface area become far less strong. Significant compression regions form in front of the nonlinear flames. These compression regions have been found to increase the growth rates by increasing the temperature of the unburned gas in front of the flames, as well as convecting the flame forward. The results show that the flame propagation velocity in references to the unburned gas was significantly faster than the burning velocity. The vorticity was given consideration, as the compressibility of flame increase one can expect the baroclinic source to be of greater significance. The vorticity was show to, in effect, increase as compressibility increases while unexpectedly having a stabilizing direction of rotation on the cellular structure of the flames.

Acoustic instabilities in a combustion chamber arise due to the coupling of acoustic pressure with in-phase heat-release, and are characterized by large amplitude oscillations of one or more natural modes of combustor. Even though an array of studies, both theoretical and experimental, has been conducted by a number of authors in this field to extract the flame response, most of these are based on kinematic flame models. In this dissertation, an experimental study of a subsonic flame's intrinsic response to acoustic pressure perturbations is performed for the case of a tube closed at one end and the other end opened to the atmospheric conditions. Pressure fluctuations inside the tube are measured for hot and cold side flows, and their varying trend is explained. The frequencies obtained from Fourier transform analysis exhibit a strong dependence with the distance between the stabilized flame position and open end of the tube. For different values of flame position (xf ), the values of growth constant 's' are calculated from the pressure versus time data readings procured from acoustic pressure transducer and dominant frequencies are analyzed from windowed FFT of the same. The expression for obtaining response function from the measured pressure fluctuations has been derived from the 1-D linearized conservation equations. The undamped response function plot is obtained by adding the decay rates at different frequencies inside the tube to the corresponding growth rates. Finally, the effect of blockage of pre-mixed flow on the growth rates inside the tube and consequently, the flame response values, is studied by repeating the experiment with different types of flame holders. A large number of theoretical flame-response models have been developed in modern literature, and some of these models are compared with the experimentally obtained response. Suggestions are also cited in this study so as to account for the observed deviations in trends. This includes a revisit of the intrinsic flame model by incorporating the effect of flame-area perturbations, with the aid of analyzed steady flame images.