Turbulent Premixed Flame Propagation in Microgravity

The structure and propagation characteristics of turbulent premixed flames have been investigated for some time. For example, the turbulent flame speed has been typically estimated m terms of the laminar flame speed SL and the local turbulence intensity u. However, the exact form of the functional relation: uT/SL = f(SL,u ) has not yet been determined although various models have been proposed. A key reason for this uncertainty is the inability to experimentally resolve all scales of motion that could contribute to the wrinkling of the premixed flame and hence, to an increase the effective burn speed. In a typical high Reynolds number flow, the small scales are impossible to resolve m an experiment (e.g., using LDV). However, turbulent fluctuations at the small scales (e.g., Kolmogorov scale) could be resolved if the Reynolds number is reduced (for example, by reducing the flow velocity). A flow field that provides sustained turbulent flow in a relatively low Reynolds number is Couette flow. It has been shown m past studies that stationmy turbulent Couette flow could be achieved at Reynolds number as low as 1800. Once combustion is included, however, the large-scale turbulent stresses responsible for momentum transport can be overwhelmed by the buoyant stresses in lg. Since this phenomenon is not present in realistic high Reynolds number devices, a microgravity environment is required to suppress the buoyancy effects while still maintaining reasonable operating pressure, temperature and observable scales at all sizes.