Various experimental studies of laboratory flames revealed trends that advocated the sig-nificance of differential diffusion. In particular, flames involving hydrogen fuels had shown that differential diffusion could explain observations of higher temperatures and radical production (such as OH and NO). This section reviews experimental studies in literature and presents their major findings in relation to differential diffusion.
Drake et al. [36] observed evidence of differential diffusion effects on H2, H2O, and N2 concentrations in fuel rich regions of a mildly turbulent flame of Reynolds number 1500 and 2200 H2 in air jet flame in laser-Raman spectroscopy measurements. Correlation plots of concentration versus temperature for H2 and N2 departed from the equal diffusiv-ity, adiabatic, and equilibrium calculations. Temperature measurements were found to be higher than the equilbrium solution. In the fuel rich regions, N2 and H2O concentrations measurements were found to be higher than predicted and H2 concentrations were lower.
Predicted departures were not consistent with the measured departures. Drake et al. [36]
considered the effects of differential diffusion as the explanation for their observations of super-equilibrium temperatures. Drake et al. proposed several possible assumptions in the equilbrium calculations that might have been the source of the deviations: neglect of ra-diative heat loss, finite rate chemistry, buoyancy, and differential diffusion. Calculations of maximum radiative heat loss showed that the resulting changes in temperature and concen-trations were much smaller than the deviations observed. The calculated time scales that brought the theoretical and measurements into agreement by considering non-equilibrium effects were not compatible with the shorter time scales in hydrogen combustion. Buoyancy effects were also not able to account for the magnitude of deviations observed. Drake et al.
concluded that the most plausible explanation for the observations were due to differential diffusion effects of H2 from the rich regions. Drake et al. [37] extended the investigation by including data for Re = 8500 flame and deviations were still observed close to the nozzle (less than 50 nozzle diameters), though suggested that finite-rate chemistry may also play a role.
A different set of experiments with H2-air non-premixed flame were conducted by Bar-low and Carter [38], with flames of Re = 10, 000 and different levels of helium dilution in the fuel. In addition to Raman scattering measurements for major species concentrations (H2, O2, H2O, and N2), Rayleigh scattering techniques were used to also obtain concentrations for OH and NO. Even at this Reynolds number, differential diffusion effects were observed close to the nozzle. Super-equilibrium concentrations were observed for OH, which were found to be close to calculations for a strained laminar flame which considers differential diffusion. Interestingly, the helium dilution of the flame, which minimized the effects of radiation, resulted in trends (super-equilibrium temperature and OH concentrations) that were qualitatively the same as the undiluted case, and thereby strengthened the conjec-ture of the significance of differential diffusion. Temperaconjec-ture and O concentration in turn affected the production rate of NO [39].
Super-equilibrium temperatures and increased NO production rates near the nozzle were also observed by Meier et al. in the Raman/LIF measurements of H2-air non-premixed flame with various levels of dilution by N2, at different Reynolds numbers (6200 and 8800) [40], as well as with different nozzle diameters (at Re = 10, 000) and with lift-off conditions (separation of flame from the nozzle) [41]. Like the helium diluted flames studied by Barlow and Carter, the nitrogen dilution reduced radiation heat losses, which reduced uncertainty caused by these losses. To further investigate the influence of differential diffusion, Meier et al. examined the correlation of the NO measurements to two different mixture fraction definitions, one based on the elemental mass fraction of H and another on the elemental O
mass fraction. The elemental mixture fractions were defined by the expression ξα = Yα− Yα,o
Yα,f − Yα,o, (3.4)
where the subscript α indicates the element, and f and o indicates fuel and oxidizer streams, respectively. It was found that the peak NO shifted to the rich side of the stoichiometric mixture fraction for the H element mixture fraction and to the lean side for the O element mixture fraction. The defined mixture fraction did not conserve the stoichiometric value under the presence of differential diffusion, since the elemental mixture fractions themselves diffused differently. In other words, the stoichiometric value of mixture fraction shifted to the rich side of the hydrogen element mixture fraction due to a higher rate of diffusion of hydrogen element, and by conservation, the opposite effect occurred for the oxygen element mixture fraction. Meier et al. found that differential diffusion effects became more pronounced as the exit velocity was decreased, while a constant Reynolds number was kept by increasing the nozzle diameter. As well, a slightly stronger influence of differential was found when the flame was lifted.
Differential diffusion effects were also observed in CO2 diluted hydrogen flames studied by Masri et al. [42] and Smith et al. [43]. At the high temperature of the flame, CO2 was not completely inert, and reacted to produce CO. Masri et al. investigated high Reynolds numbers (15000 to 30000) flames with blowoff, and found that although differential diffusion effects on major species were not too important, the effects on minor radicals like H, which were important in controlling extinction, were significant. Smith et al. explored a wide range of reacting jets of Reynolds numbers 1,000 to 30,000, and found differential diffusion effects to be present throughout the range of Reynolds numbers by examining the elemental mixture fractions of H and C defined by Eq. 3.4. Significant differential diffusion effects occurred on the fuel rich side of the flame, and caused a greater net flux of hydrogen toward reaction zone from diffusion between hydrogen and H2O compared to the diffusion between CO2 and H2O. Smith et al. also noted, by comparing with nonreacting measurements, that the presence of chemical reactions accentuated the differential diffusion of H2 by the existence of a sink for H2 (and source for H2O) which caused steeper concentrations gradients. The same trends were observed by Bergmann [44] in a methane (CH4)/H2/N2 flame, who also found deviations in the elemental mixture fractions of H and C from equal diffusivity results at five nozzle diameters from the jet exit.