DYNAMICS AND EXTINCTION OF NON-ADIABATIC PARTICLE-LADEN PREMIXED FLAMES

Propagation and extinction of gaseous premixed flames seeded with inert particles were analytically and numerically investigated for the one-dimensional, planar H 2 /O 2 /N 2 and CH 4 /air flames, with particular interest on the joint effects of gas-particle heat conduction, gas-particle radiation heat loss, and gas-particle velocity lag. Analytical expressions were derived for the non-adiabatic particle-laden flame speed and the state of extinction allowing for the first two effects. These results reduce to the classical theories for the non-adiabatic particle-free flame and the adiabatic particle-laden flame in the limits of vanishing particle concentration and vanishing radiative heat loss, respectively. It is shown that small particles behave like a diluent gas and induce a flammability limit based on the particle concentration. For moderate particle size, there are multiple flame regimes and extinction points due to the combined effects of the gas-particle conduction heat transfer and radiative heat loss. For large particle size, the fast flame extinction limit is extended, while the slow flame regime is narrowed. Numerical calculations allowing for all effects show that the particle velocity lag greatly affects the flame speed and the state of extinction. Comparison of the numerical and analytical results with the literature experimental data shows good agreement for small particle size. It is emphasized that the existence of multiple solutions is an intrinsic feature of particleladen flames. Introduction In many practical combustion processes, particles are present either intrinsically, as in coal burning and soot particles in flames, or through intentional addition, as in water mist and metal salt particles in fire extinguishment. The presence of these particles in flames enriches the combustion phenomena and complicates the physical process through (1) gasparticle velocity lag, (2) gas-particle heat conduction, (3) gas-particle heat loss to environment, (4) particle-particle radiative heat transfer, and (5) particle-gas radiative transfer. For systems in which the particle loading is not high such that reabsorption through particle-particle and particle-gas radiative transfer is small, the dominant heterogeneous effects are those of processes 1 through 3. For the three processes identified above, the effects of gas-particle heat conduction were first analytically investigated by Joulin [1] and Mitani [2]. The results show that while small particles act as a gaseous diluent such that the flame speed monotonically decreases with increasing particle concentration, for particle sizes exceeding a critical value the flame speed responds non-monotonically to the particle loading, in the form of an inverse-S curve, such that between the two monotonically decreasing flame speed regimes there is a regime of moderate particle loading within which the flame speed is triple valued. Experimentally, the effects of gas-particle conduction were investigated by Mitani and Niioka [3]. The results show that the flame spee