Fundamental studies of combustion of droplet and vapour mixtures

There are few experimental data of a fundamental nature that clearly demonstrate the similarities and differences in burning rates between single phase and two phase combustion, either in laminar or turbulent conditions. Such data are essential towards a better understanding of the spray combustion phenomena as well as a whole system. In the present study, experimental investigations of combustion of droplet and vapour air mixtures under quiescent and turbulence conditions have been conducted in a fan stirred combustion vessel. Aerosols were generated by expansion of gaseous pre-mixture to produce a homogeneously distributed suspension of fuel droplets. Spherically expanding flames following central ignition were employed to quantify the flame structure and propagation rate. The effect of droplets on flame propagation was investigated by comparing the burning rate of gaseous mixtures at initial pressure and temperature close to those of aerosol mixtures. In quiescent conditions, aerosols of two different fuels, isooctane and ethanol, were investigated at near atmospheric conditions. The effect of fuel droplets, up to 31 J.1m diameter, on laminar flame propagation was examined at a wide range of equivalence ratios. In the early stages of flame development, inertia of fuel droplets leads to local enrichment in equivalence ratio which increases the initial burning rate of lean aerosols but decreases that of rich ones. For the later stages of flame propagation, the presence of liquid droplets causes earlier onset of instabilities and cellularity than for gaseous flames, particularly at rich conditions. This leads to an enhanced burning rate and is probably due to heat loss from the flame and local disturbances due to droplet evaporation and subsequent diffusion processes. In turbulent studies, the effect of isooctane droplets up to 14 J.1m in diameter on flame propagation was examined at various values of root mean square turbulence velocities between zero and 4.0 mls. It is suggested that during early flame development, the turbulence was found to induce droplet motion before flame initiation which dominated over those resulting from the flame, negating the effect of droplet inertia. In the later stages, the presence of droplets in a low turbulent flame resulted in a significant burning rate enhancement. However, this enhancement became progressively less important as turbulent wrinkling became dominant. Between low and high turbulence, there was a transition regime between instability dominated and turbulence dominated regimes. As a consequence, the burning rate enhancement due to droplets under this transition range was rather complex.