Differential diffusion of helium concentration and temperature in a turbulent jet

When scalars have unequal molecular diffusivities, differential diffusion can occur.  However, the consequences of differential diffusion are commonly neglected – an assumption that is typically justified by the argument that the effects of molecular properties are small at high Reynolds numbers.  Yet, this assumption has proven to be questionable, especially at moderate Reynolds numbers (e.g. Lavertu et al., J. Fluid Mech., 2008).  The present work investigates differential diffusion of two scalars (helium concentration and temperature) in a turbulent jet of air, measured by way of a thermal-anemometry-based interference probe (Hewes and Mydlarski, Meas. Sci. Technol., 2021) combined with a cold-wire thermometer.  Statistics of the instantaneous differences of the non-dimensionalized scalar concentrations are investigated.  Of particular interest are the effects of the Schmidt number(s) on the differential diffusion.  These are inferred by comparing the present results (in which the Schmidt / Prandtl are 0.2 and 0.7, respectively) with those of Lavertu et al. (J. Fluid Mech., 2008), who studied the same flow, but using liquid-phase jets in which the Schmidt numbers of the scalars were 2000 and 5000.