A Helium-Hydrogen Mixture for the Measurement of Mixture Fraction and Scalar Gradient in Non-premixed Reacting Flows

摘要

A novel fuel for the measurement of mixture fraction in a flame is examined. The fuel composition, 80percent helium and 20percent hydrogen on a mole basis, restores monotonicity to the Rayleigh scattering-mixture fraction relationship in flames, so only a single measurement is needed to determine the mixture fraction in the measurement plane. An analytical expression is developed to determine the relationship between measurement noise and the resulting noise in the calculated scalar dissipation. The characteristics of scalar dissipation are measured for non-reacting and several different reacting flows at jet Reynolds numbers up to 10,000 using planar Rayleigh scattering. The measurements in the non-reacting case agree well with previous measurements, although the scalar gradient shows much stronger anisotropy. The measurements made in the flames indicate that the distribution of scalar dissipation is different from that observed in liquids and non-reacting gases. The distribution of dissipation is found to differ from the log-normal distribution observed in the non-reacting case, and the cumulative distribution functions did not collapse into one curve when normalized by the mean dissipation value. The probability distribution of the scalar gradient orientation indicates that the flow is highly anisotropic at our measurement planes.