The Cool Flames Experiment

A space-based experiment is currently under development to study diffusion-controlled, gas-phase, low temperature oxidation reactions, cool flames and auto-ignition in an unstirred, static reactor. At Earth's gravity (1g), natural convection due to self-heating during course of slow reaction dominates diffusive transport and produces spatio-temporal variations in thermal and thus species concentration profiles via Arrhenius temperature dependence of reaction rates. Natural convection is important in all terrestrial cool flame and auto-ignition studies, except for select low pressure, highly dilute (small temperature excess) studies in small vessels (i.e., small Rayleigh number). On Earth, natural convection occurs when Rayleigh number (Ra) exceeds a critical value of approximately 600. Typical values of Ra, associated with cool flames and auto-ignitions, range from 104-105 (or larger), a regime where both natural convection and conduction heat transport are important. When natural convection occurs, it alters temperature, hydrodynamic, and species concentration fields, thus generating a multi-dimensional field that is extremely difficult, if not impossible, to be modeled analytically. point has been emphasized recently by Kagan and co-workers who have shown that explosion limits can shift depending on characteristic length scale associated with natural convection. Moreover, natural convection in unstirred reactors is never strong to generate a spatially uniform temperature distribution throughout reacting gas. Thus, an unstirred, nonisothermal reaction on Earth does not reduce to that generated in a mechanically, well-stirred system. Interestingly, however, thermal ignition theories and thermokinetic models neglect natural convection and assume a heat transfer correlation of form: q=h(S/V)(T(bar) - Tw) where q is heat loss per unit volume, h is heat transfer coefficient, S/V is surface to volume ratio, and (T(bar) - Tw ) is spatially averaged temperature excess. Newtonian form has been validated in spatially-uniform, well-stirred reactors, provided effective heat transfer coefficient associated with unsteady process is properly evaluated. Unfortunately, it is not a valid assumption for spatially-nonuniform temperature distributions induced by natural convection in unstirred reactors. This is why analysis of such a system is so difficult. Historically, complexities associated with natural convection were perhaps recognized as early as 1938 when thermal ignition theory was first developed. In 1955 text Diffusion and Heat Exchange in Chemical Kinetics, Frank-Kamenetskii recognized that the purely conductive theory can be applied at sufficiently low pressure and small dimensions of vessel when influence of natural convection can be disregarded. was reiterated by Tyler in 1966 and further emphasized by Barnard and Harwood in 1974. Specifically, they state: It is generally assumed that heat losses are purely conductive. While this may be valid for certain low pressure slow combustion regimes, it is unlikely to be true for cool flame and ignition regimes. While this statement is true for terrestrial experiments, purely conductive heat transport assumption is valid at microgravity (mu-g). Specifically, buoyant complexities are suppressed at mu-g and reaction-diffusion structure associated with low temperature oxidation reactions, cool flames and auto-ignitions can be studied. Without natural convection, system is simpler, does not require determination of effective heat transfer coefficient, and is a testbed for analytic and numerical models that assume pure diffusive transport. In addition, mu-g experiments will provide baseline data that will improve our understanding of effects of natural convection on Earth.