DNS analysis of small-scale turbulence-scalar interactions in evaporating two-phase flows

Scalar dissipation rate (SDR) is a key quantity in turbulent flow modeling since it measures the scalar mixing intensity. It is well known that turbulence-scalar interaction (TSI) processes play an essential role in turbulent scalar mixing and drive to a large extent the SDR evolution. These processes are characterized by the tensor inner product between the scalar gradient vector and the strain-rate tensor. Direct numerical simulations are conducted to analyze the physics of this interaction in vaporizing turbulent two-phase flows. The well known alignment of the scalar gradient with the most compressive principal direction of the strain-rate tensor - resulting in production of the scalar gradient by turbulence - is recovered in statistics collected sufficiently far from the liquid-gas interface. By contrast, the action of the turbulence-scalar interaction is progressively attenuated as we approach this interface, where the scalar gradient tends to have a direction intermediate between the extensive and the compressive directions. This result questions the validity of passive-scalar turbulence concepts and closures that are commonly used for to tackle the modeling of scalar behavior in vaporizing two-phase flows featuring (or not) subsequent chemical reactions.