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Variable-viscosity mixing in the very near field of a round jet

One of the persistent challenges in turbulent mixing is understanding of the phenomenology associated with heterogeneous mixtures of gases. The turbulence in single, homogeneous fluid has drawn much attention. However, heterogeneous mixing of gases is prevalent in real flows, whether nonreactive or reactive. This paper is dedicated to advancing our understanding of the turbulent mixing phenomenology in a round jet flow of propane issuing in an oxidizer. The ratio of the propane/oxidizer kinematic viscosities is 1/5.5, whereas their densities are nearly equal. Therefore, the main physical property that differentiates the two fluids is their viscosity. The focus of this paper is on the role played by the viscosity gradients on the turbulent flow, over the first several diameters. The addressed questions concern a comparison between the present flow, which is a variable-viscosity flow (hereafter, VVF), and an air jet flow issuing in the air (constant-viscosity flow, hereafter CVF). The comparison is made at the same initial conditions, i.e. the same jet momentum per surface unit, M0 = ρ0U20 = 130 and 360 kg m−1 s−2 (U0 is the injection velocity and ρ0 is the fluid density). For doing so, an experiment has been designed and both the velocity and the scalar fields have been measured by using a technique based on hot-wire anemometry and Rayleigh light scattering, as well as laser Doppler velocimetry which provides two velocity components. The comparison criteria between the VVF and CVF are the instantaneous aspect of the turbulent velocity field, the axial mean velocity () and the rms (root mean squares) of the two measured velocity fluctuations. The one-point kinetic energy budget, which involves the mean energy dissipation rate, is developed and the expression of the mean energy dissipation rate is revisited. The latter involves additional terms reflecting the velocity–viscosity gradient correlation. This contribution is positive, thus enhancing the real mean energy dissipation rate in VVF. We show that, in comparison with the CVF, the VVF evolves much faster toward a self-similar regime, the decay of the axial mean velocity is enhanced and the mean energy dissipation rate is more important. We discuss, finally, a possible mechanism for the accelerated turbulence decay observed in VVF. Firstly, an external viscous fluid is brought inwards of the jet core by the jet frontier instabilities (nozzle-wake instabilities, Kelvin–Helmholtz instabilities and instabilities associated with the variable-viscosity interface). Secondly, these viscous-fluid blobs lead to enhanced local velocity gradients, to greater local dissipation and to increased production of lateral velocity fluctuations behind these irregular obstacles. While this scenario is speculative, it is likely to explain the observed statistics of the velocity field.