Mixing under transcritical conditions: an a-priori study using direct numerical simulation

In this study we investigate mixing under transcritical conditions by means of direct numerical simulation. Atemporal jet configuration is used together with thermodynamic conditions matching those of experiments oncryogenic injection of nitrogen at supercritical pressures. Two cases, representing supercritical and transcriticalconditions, are simulated using a spectral element based, low-Mach numberflow solver, coupled with detailedreal-fluid equation of state and transport properties. The objective is to shed light on the transcritical mixingprocess and to concurrently investigate, in a-priori fashion, the sub-grid scale modeling of thermodynamic andtransport properties in the context of large eddy simulation. Relevant qualitative and statistical differencesbetween supercritical and transcritical jets are observed, despite the same initial Reynolds number. The use of apresumedβ-pdf for sub-grid scale mixing is investigated and is shown to significantly improve, with respect tothe largely used“no-model”, the evaluation of thermodynamic and transport properties.