Analysis of Thermodynamically Induced Compressibility in Supercritical Mixing Layers
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Casotto, Cosimo
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Abstract
Research into supercritical fluids is ongoing and is motivated, among other things, by
the existence of supercritical fluids within rocket engine injectors and growing concerns
over climate change and the increased demand for energy, which have spurred technological advances in the field of supercritical CO2 gas turbines.
This research builds upon recent work reported in the literature on thermally inhomogeneous supercritical CO2 mixing layers, in which the effects of thermodynamic compressibility on supercritical mixing were investigated [1]. By comparing the findings in Ref. [1]
to newly generated data sets, this research attempts to quantitatively and qualitatively identify key differences in the turbulent structures and mixing dynamics between supercritical
and ideal gas conditions, as well as between single and multi-component mixtures. Using
an identical setup as the work reported by Purushotham in Ref. [1], Large Eddy Simulation
(LES) was performed on a thermally inhomogeneous CO2 mixing layer under ideal gas
settings and on a thermally homogeneous CH4/O2 mixing layer under supercritical fluid
conditions. Datasets were produced at distinct grid resolutions, ranging from a relatively
coarse grid to a Wall Resolved LES (WRLES) resolution, utilizing the massively parallel
computational fluid dynamics solver RAPTOR.
Consideration of the effects of compressibility and associated phenomena governing the
temporal response of pressure and temperature highlights the influence of nonlinear thermodynamic terms at states near supercriticality. In the study, time derivatives of pressure
and temperature are expressed in terms of spatial operators related to mass, momentum,
and total energy conservation in two separate equations. The terms are modulated by a
prefactor Γ = c2/(ρcp) and contain quantities such as ∂ρ/∂p and ∂ρ/∂T that strongly vary
in the vicinity of the critical point. Analysis shows that the sensitivity of the time derivatives of pressure and temperature to these modulating parameters is orders of magnitude
greater under near-critical conditions compared to the ideal gas scenario.
The structure of the turbulence within the mixing layer is investigated by analyzing
the shape of the Reynolds stress ellipsoids formed within the flow. Comparisons between
anisotropy invariance maps reveal more anisotropic turbulence at supercritical conditions.
This observation is complemented by the presence of strong density stratification both in
the thermally inhomogeneous, and multi-component flows, which inhibit velocity fluctua
tions in directions normal to the local gradient.
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Date
2025-12
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Text
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Thesis (Masters Degree)