Thermoacoustic convection and transport in supercritical fluids under normal and micro-gravity conditions

Zhiheng Lei

doi:10.17918/etd-3188

The generation, propagation and dissipation of thermally induced and mechanically driven acoustic waves in supercritical nitrogen and carbon dioxide were studied. Supercritical fluids are widely used in various industrial and laboratory processes as substitutes for organic solvents. These fluids also have high compressibilities, high thermal conductivities, low viscosities, and low thermal diffusivities. As the thermal diffusivity tends to zero near the critical point, acoustic waves are believed to be the primary reason for fast thermal equilibration in supercritical fluids. The compressible form of Navier-Stokes equations for Newtonian fluids was considered to model the supercritical fluids. A high-order explicit numerical scheme (FCT: flux-corrected transport, along with an accurate wall density boundary condition, was applied to accurately track the acoustic waves. The property variations (pressure, internal energy, viscosity, and thermal conductivity) of the supercritical fluids (carbon dioxide and nitrogen) as functions of temperature and density were obtained from the NIST Standard Reference Database 12 instead of simplified equations for real gases. Both one-and two-dimensional computational fluid dynamics models were developed for predicting the temporal evolution of pressure, density, temperature and flow field in supercritical fluids due to thermal and mechanical excitations. The flow fields and heat transport induced by thermally induced acoustic waves and buoyancy in supercritical fluids were investigated numerically under normal and reduced gravity conditions. The flow fields induced by mechanically driven acoustic waves in an enclosure driven by a vibrating wall were also numerically studied. The model developed was also used to investigate the interaction of thermally induced acoustic waves and mass transfer in supercritical fluids (naphthalene dissolution in supercritical carbon dioxide). The generation and propagation of thermally induced acoustic waves due to rapid heating of a solid wall in a confined supercritical fluid layer were also experimentally investigated. A high-pressure experimental facility was constructed to characterize the generation and decay of acoustic waves in supercritical nitrogen and supercritical carbon dioxide in an enclosure due to rapid heating of an end wall. The underlying physics of the 'piston effect' (fast thermal equilibration of supercritical fluids with high compressibilities) were studied and explained in detail. To the author's knowledge, this is the first attempt where detailed simulations of thermally induced and mechanically driven acoustic waves in supercritical fluids have been conducted with accurate equations of state and property functions. Better understanding of these problems will help the tailoring and optimizing the operating conditions for industrial and laboratory processes for supercritical fluids.

Thermoacoustic convection and transport in supercritical fluids under normal and micro-gravity conditions

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Thermoacoustic convection and transport in supercritical fluids under normal and micro-gravity conditions

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