On molecular unmixedness between the ejected original and entrained ambient fluids in a turbulent jet

Abstract

The present study investigates the molecular unmixedness between the injected and entrained fluids in a turbulent jet as well as its dependence on the initial conditions. This unmixedness can be quantified as the parameter of molecular segregation between the ejecting “fuel” fluid (A) and the entrained “oxidizer” fluid (B), defined by α≡cAcB¯/C¯AC¯B (overbar denotes time-averaging). For the first time, an expression of the parameter has been derived for the two-fluid mixing in a heated turbulent nonreactive jet. That is, α=−θ2¯/Θo−Θ¯Θ¯−Θa, where Θo and Θa denote the ejected “warm” and entrained “cold” fluid temperatures, whereas Θ and θ are the instantaneous and fluctuating temperatures of the local fluid mixture. This expression of α is well validated by comparing the measured natural-gas flames from a smooth-contraction nozzle with those from a long-pipe nozzle. Moreover, the jet-nozzle configuration is found to show a strong effect on α. Likewise, the jet density ratio (Rρ) is a highly influential factor: e.g., an increase in Rρ reduces α substantially. In contrast, the effect of the jet-exit Reynolds number is less significant. In the present paper, we try to explain these observations.