The combustion of fuel and oxidizer dissolved in a high-density fluid such as supercritical water is often characterized by highly temperature-sensitive mass diffusivities. Such behaviour, which is Arrhenius in nature, reflects the transition from a liquid- to gas-like character of the fluid as the temperature varies from ambient conditions to elevated values characteristic of flames. For the case of supercritical diffusion flames in counterflow, this property results in a flame structure comprised of three identifiable regions. In particular, there is an outer preheat zone on both the fuel and oxidizer side of the flame in which species transport is by convection alone; a relatively thin diffusion zone in the vicinity of the stagnation plane in which a convective-diffusive balance is maintained for both species and temperature; and finally, a very thin reaction zone embedded in the thicker diffusion zone in which a reactive-diffusive balance is achieved. An analysis of all possible burning regimes then leads to a characteristic S-response for the stagnation temperature plotted as a function of Damk#xF6;hler number, where the ignition and extinction limits depend on the various parameters in the problem.
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