Effects of Variable Transport and Diffusive-Thermal Instabilities on Diffusion Flames

The objective of this work is to examine the effects of temperature-dependent transport and thermal diffusion and of diffusive-thermal instabilities on the structure and characteristics of non-premixed flames (diffusion flames). The configuration adopted is the planar unstrained flame with a bulk flow directed toward the reaction zone from either the fuel or the oxidizer sides. Included in this discussion is the no bulk flow case, where the reactants reach the reaction zone purely by diffusion. The model also allows for non-unity and distinct Lewis numbers, for the fuel and oxidizer. Results show that the variations of the thermal conductivity and the diffusion coefficients with temperature affect the flame standoff distance and flame temperature and consequently the profiles of temperature and concentrations. These results were shown to be in accord with experimental data. Extinction criteria for diffusion flames are typically expressed in terms of a critical Damkohler number Dc below which the flame temperature is too low for the reaction to be maintained. This Dc is significantly smaller for the temperature dependent case when compared with previous analyses with constant properties. Thermal diffusion, also known as the Soret effect, affects the flame standoff distance by shifting it towards the fuel/oxidizer and affects the flame temperature by making it smaller/larger for heavy/light fuels respectively. Predicted extinctions Dc are minimally affected by Soret effects except for very heavy fuels. Diffusive-thermal instabilities are examined for constant transport properties and no-Soret conditions. In the absence of a flow case the only mode of instability is planar oscillations. The effect a convection flow causes a general increase in the frequency of the oscillations. Now there is also a possibility of formation of cellular flames. Comparison is then made to experimental data to understand the effect the parameters have on cell size and on the frequency of oscillations. We examine the effect gravity has on the diffusive thermal instabilities. The addition of gravity in no bulk flow leads to cellular instabilities where unconditionally stable flames persist otherwise and has a relatively minor effect on the diffusive-thermal instabilities in the presence of a convective bulk flow.