Turbulent mass transport and rates of reaction in a confined hydrogen-air diffusion flame

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Volumetric reaction rates and mass-transport coefficients are reported as a function of position for a ducted, two-dimensional, turbulent, hydrogen-air diffusion flame. Air and hydrogen velocities were 100 and 64 ft/sec, respectively. Basic data consisted of wall static pressures and a complete mapping of total pressure and chemical composition at eight axial locations. These data were used to compute the density, velocity, and mass fraction distributions in the flame. Mass-transport coefficients were determined by numerical solution of the time mean continuity equation for an inert species. Reaction rates were obtained by substituting the known mass-transport coefficient into the continuity equation for a reacting species and solving for the reaction rate, other terms being known. The mass-transport coefficients are at least one order of magnitude greater than the molecular diffusivities and are about one-tenth of previously reported values for confined, rod-stabilized, premixed propane-air flames with the same inlet air velocity. A fivefold increase in the mass-transport coefficient was found for the region of the flame studied. The magnitude and increase of the coefficient is due primarily to the generation of turbulence by the flame. The maximum reaction rates of oxygen are higher than those of a premixed flame with comparable inlet velocity, but the reaction zone is considerably thinner and the integrated reactant consumption is lower. A correlation exists between oxygen and hydrogen concentrations, independent of position and time. It is shown that the existence of the correlation precludes the simultaneous existence of significant amounts of hydrogen and oxygen and implies that local burning occurs at a reactant interface. It follows that turbulent mixing controls the reaction rate. An equation is developed which shows that volumetric reaction rate is directly proportional to the mass-transport coefficient.