Laser Raman scattering measurements of differential molecular diffusion in nonreacting turbulent jets of H2/CO2 mixing with air

Abstract

In this paper we report measurements of differential diffusion in nonreacting jet flows. Pulsed laser Raman scattering spectroscopy is used to measure species concentrations in nonreacting jets of H2/CO2 into air over a range of Reynolds numbers from 1000 to 64 000. Measurements are also made in strained laminar opposed flows of H2/CO2 against air, where differential diffusion effects on relative species concentrations are observed to be independent of strain rate. In laminar opposed flows and in a laminar jet (Re=1000), measurements of average species concentrations show significant differential diffusion effects. In jet flows of higher Reynolds numbers, only instantaneous species concentrations are affected by differential diffusion; measured species concentrations are the same, on average, as the species concentrations that would have been observed if all species diffusivities were equal. Instantaneous differential diffusion effects in turbulent jets are quantified by the variance of a differential diffusion variable z, where z is the difference between the normalized mole fractions of H2 and CO2 (and normalization refers to dividing each species mole fraction by the mole fraction of that species in pure jet fluid). Measurements show that zRMS diminishes with increasing Reynolds number. Histograms of z show negative skewness in the low Reynolds number jet. At higher Reynolds numbers the histograms become more symmetric, although asymmetries do remain at higher Reynolds numbers in the data obtained near the jet edge, where there is an interface between jet fluid and coflowing air.