Direct numerical simulation of a transitional, cryogenic cavitating mixing layer of liquefied natural gas behind a flat splitter plate using a homogenous equilibrium mixture cavitation model

Abstract

A major challenge in the design of liquefied natural gas (LNG) pumps is concerned with the accurate prediction of flow behaviour at off-design conditions where destructive phase change from liquid to vapor - named cavitation - at cryogenic conditions may emerge undesirably and downgrade the machine performance. As a first step of our research towards addressing this challenge, the present study employs a homogenous equilibrium mixture (HEM) –based cryogenic cavitation solver to run direct numerical simulation (DNS) of a fundamental case study of transitional cavitating mixing layer of LNG behind a 3D planar flat plate, which resembles actual conditions of flow behind the vanes of industrial LNG sub-merged pumps. Assessment of the preliminary findings shows that the mixing layer is mainly driven by different modes of the well-known Kelvin-Helmholtz (K–H) instabilities, which develop coherent vortex structures that tend to shed off periodically across the mixing layer. The cavity structures are formed via transient interactions of the shedding vortices, and in the meantime of their pairing processes. The shedding of cavities, which mostly appear as “horse-shoe” and “croissant”-shaped vapor structures, maintains by the unsteady sheet cavity formed on the splitter upper wall and the latent heat exchange mechanisms, beside the dominant K–H instabilities. The presence of three-dimensionality is found to enhance instability of the mixing layer by increasing the frequency of phase change successions.

Keywords

• Cryogenic
• Cavitation
• Mixing layer
• LNG
• Direct numerical simulation