We recently initiated exploration of a spray-equation-based theoretical model of liquid-fueled ”primary” combustors (Rosner, Arias-Zugasti and Labowsky(ChE Science, in press,2008)). Such “continuous” combustors are widely used for purposes of chemical propulsion (aircraft- or marine- gas turbines and chemical rockets) as well as in ground-based chemical synthesis applications (first step in the industrial synthesis of the commodity chemicals H2SO4, H3PO4, etc.). In the present paper we expand on this model by exploring the interacting roles of diffusion-controlled droplet evaporation (fuel vapor availability), turbulent micro-mixing, and homogeneous chemical kinetics in determining combustor operability limits and, within these limits, attainable combustion intensities---ie., volumetric rates of chemical energy release (eg. GW/m^3)). For present purposes, it is fruitful to adopt the viewpoint of ”interacting populations” of fuel droplets, turbulent eddies, and strained “gaseous diffusion flamelets”. We find that under typical aircraft gas turbine primary combustor conditions the volume fraction of active chemical reaction is only ca. O(10^-3), implying that most of the (micro-UNmixed) combustor volume is occupied by molecularly unmixed ”eddies” of partially pre-heated air, fuel-vapors, or higher temperature product gas. With this “multi-phase, flamelet” picture in mind, global “flame-out” (eg., lean blow-out; LBO) is not only due to the loss of chemical energy release associated with incomplete droplet evaporation in the available residence time, but also the localized extinction of an excessive fraction of vapor phase diffusion flamelets ---- a conclusion consistent with the frequent observation that CO and UHC emissions increase dramatically near such BO-boundaries. Our present extended numerical estimates of spray combustor performance are based on injected kerosene-like liquid hydrocarbon fuels (nominal stoichiometry:(CH1.9)n ) vaporizing/burning in compressed air at pressures up to ca. 24 atm. Experimentally available laminar premixed flame speed data and/or diffusion flamelet extinction strain rates are invoked as practical “surrogates” for the complex homogeneous chemical kinetics. Alternatively, a semi-global 2-step/5 species fuel oxidation kinetic scheme (originally developed to reproduce measured laminar hydrocarbon pre-mixed vapor/air flame behavior) is used to develop simple estimates of the maximum possible chemically-controlled combustion ”intensity” and corresponding efficiency. We are presently generalizing this approach to also predict emission indices for the principal pollutants: UHCs, CO, soot, NO and (for sulfur-containing fuels) SO3. Even in its present form our simple mathematical model provides a rational basis for making self-consistent, semi-quantitative and geometry-insensitive spray combustor performance ”maps”, including the effects of parameters sensitive to fuel injector performance. As a potentially valuable byproduct, the present model also provides an economical “test-bed” for examining various conjectures and/or proposed simplifications for the modeling and/or preliminary design of high-pressure spray combustors.

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