Nusair Hasan

Fast Heating Induced Thermoacoustic Waves in Supercritical Fluids: Experimental and Numerical Studies

Thermoacoustic waves in near-critical supercritical carbon dioxide are investigated experimentally on acoustic time scales using a fast electrical heating system along with high speed pressure measurements. Supercritical carbon dioxide (near the critical or the pseudocritical states) in an enclosure is subjected to fast boundary heating with a thin nickel foil and an R-C circuit. The combination of very high thermal compressibilities and vanishingly small thermal diffusivities of the near-critical fluid affect the thermal energy propagation, leading to the formation of acoustic waves as carriers of thermal energy (the so called piston effect). The experimental results show that under the same temperature perturbation at the boundary, the strength of the acoustic field is enhanced as the initial state of the supercritical fluid approaches criticality. The heating rate, at which the boundary temperature is raised, is a key factor in the generation of these acoustic waves. The effect of different rates of boundary heating on the acoustic wave formation mechanism near the critical point is studied. The thermoacoustic wave generation and propagation in near-critical supercritical fluid is also investigated numerically and compared with the experimental measurements. The numerical predictions show a good agreement with the experimental data.

Experimental and numerical investigations of resonant acoustic waves in near-critical carbon dioxide

Flow and transport induced by resonant acoustic waves in a near-critical fluid filled cylindrical enclosure is investigated both experimentally and numerically. Supercritical carbon dioxide (near the critical or the pseudo-critical states) in a confined resonator is subjected to acoustic field created by an electro-mechanical acoustic transducer and the induced pressure waves are measured by a fast response pressure field microphone. The frequency of the acoustic transducer is chosen such that the lowest acoustic mode propagates along the enclosure. For numerical simulations, a real-fluid computational fluid dynamics model representing the thermo-physical and transport properties of the supercritical fluid is considered. The simulated acoustic field in the resonator is compared with measurements. The formation of acoustic streaming structures in the highly compressible medium is revealed by time-averaging the numerical solutions over a given period. Due to diverging thermo-physical properties of supercritical fluid near the critical point, large scale oscillations are generated even for small sound field intensity. The strength of the acoustic wave field is found to be in direct relation with the thermodynamic state of the fluid. The effects of near-critical property variations and the operating pressure on the formation process of the streaming structures are also investigated. Irregular streaming patterns with significantly higher streaming velocities are observed for near-pseudo-critical states at operating pressures close to the critical pressure. However, these structures quickly re-orient to the typical Rayleigh streaming patterns with the increase operating pressure.

Acoustically Augmented Flow and Transport in Supercritical Fluids

Acoustically augmented flow and transport in supercritical fluid (CO2) generated by standing wave in a cylindrical enclosure is simulated. The oscillatory flow field in the enclosure is created by the vibration of one of the end walls of the enclosure. A novel application of the acoustically augmented flow in membrane contactors used in supercritical fluid extraction process is demonstrated numerically. The predicted results from the present study can be utilized to enhance the transport mechanisms in these fluids. The geometric parameters and the frequency of the vibrating wall are chosen such that the lowest acoustic mode propagates along the enclosure. A real-fluid model for representing the thermo-physical and transport properties of the supercritical fluid is considered. The fully compressible form of the Navier–Stokes equations is used to model the flow fields and an implicit time-marching scheme is used to solve the equations. The formation of the acoustic field in the enclosure is computed and fully described and the acoustic boundary layer development is predicted. The interaction of the wave field with viscous effects and the formation of streaming structures are revealed by time-averaging the solutions over a given period. Due to diverging thermo-physical properties of supercritical fluid near the critical point, large scale oscillations are generated even for small sound field intensity. The effects of near-critical property variations and system pressure on the formation process of the streaming structures are also investigated.Copyright

Enhancing Supercritical Fluid Extraction Using Acoustic Excitations

A computational fluid dynamics model of supercritical fluid extraction of a solute (caffeine) from a fixed bed of porous solid matrix (coffee beans) using a supercritical solvent (carbon dioxide) is developed. The mathematical model is developed considering diffusion-controlled transport in the particle and film mass transfer resistance around the particle. Accurate representations of the transport properties of supercritical carbon dioxide are considered. The conservation equations are numerically solved using an implicit finite volume method. Supercritical fluid extraction of a solute from a solid matrix has a slow dynamics even when solute free solvent is re-circulated and therefore improvements in the mass transfer process are required. The effect of acoustically excited flows on supercritical fluid extraction in a fixed bed extractor is investigated. Harmonically oscillating inlet wall boundary condition is used to model a piezoelectric transducer. The use of acoustic excitation represents a potential efficient way of enhancing mass transfer processes. Application of acoustic excitations at the fundamental frequency of the extractor (f = 996 Hz) increased the overall yield by about 15%. The effects produced by compressions and decompressions, as well as by radiation pressure and streaming contribute to the enhancements.Copyright

Numerical Studies of Thermoacoustic Convection in Near-Critical Fluids

Thermoacoustic convection in carbon dioxide near its critical point is investigated numerically. A real-fluid model has been developed taking into account all the relevant fluid property variations near the critical point, including the bulk viscosity. The thermo-physical properties of the near-critical supercritical fluids are given as functions of both pressure and temperature due to their strong divergence near the critical point. As a layer of supercritical fluid is heated rapidly, the combination of very high thermal compressibility and vanishing thermal diffusivity near the critical points of fluids affect thermal energy propagation, leading to the formation of acoustic waves as carriers of thermal energy. The rapidity of the boundary heating is a key factor in the generation of these acoustic waves. We also study the effect of different rates of boundary heating for the temperature equilibration mechanism near the critical point. As the critically diverging bulk viscosity plays a significant role on the transport processes near the critical point, effect of bulk viscosity on the flow field and heat transport induced by thermoacoustic waves and buoyancy in supercritical fluids is also investigated numerically. The predicted results from the present study can be utilized to understand the thermal transport mechanism in near-critical fluids.Copyright