Saturation of thermoacoustic mixture separation

Spatiotemporal evolution of molar fraction in acoustic-resonance tube filled with He-Ar mixture

This study proposes a time-evolution method to simulate acoustic gas-mixture separation. The proposed method can calculate the separation process without any arbitrary parameters except for space and time resolutions. The molar-fraction distribution within the system during the separation can be calculated, and the results show that a large molar-fraction gradient occurs in the separation tube, while a distribution also occurs in the resonance tube. Although the simulation results for the case with a high-pressure amplitude diverge from that of the experiment during the process of the separation, the simulation results for the case with a low-pressure amplitude agree well with those of the experiment.

Revisiting acoustical gas-mixture separation

This study experimentally investigated acoustically driven gas-mixture separation. Acoustic wave propagation in a narrow tube can induce gas-mixture separation. A binary mixture of helium and argon was used as the gas mixture. The pressure amplitude of the acoustic wave and initial molar fraction of the helium gas were investigated. The obtained experimental data indicated that the molar fraction initially increased with increasing pressure amplitude, whereas the saturated molar fraction did not show a clear dependence on the pressure. Although the degree of separation was smaller with purer helium, gas-mixture separation occurred under all conditions within the experimental range.

Thermoacoustic Soret separation

In a fluid mixture in a channel with an axial time-averaged temperature gradient, high-amplitude oscillating flow can greatly increase the axial flux of thermal diffusion (Soret) separation of the components of the mixture. The enhancement occurs when the oscillating lateral temperature gradient greatly exceeds the axial gradient, causing a large oscillating concentration that can be favorably time-phased with the oscillating flow. This process can occur even with a negligible pressure oscillation or with a negligible temperature response to pressure, as is the case in most liquid solutions. The thermal boundary condition imposed by realistic solids on thermoacoustic liquids is imperfect, adding mathematical complications that are absent for typical gases, for which the solid surface is temporally isothermal. Compared with gas mixtures, the high Lewis number in typical liquid solutions reduces the separation flux associated with the time-averaged temperature gradient, but it also reduces the remixing associated with the time-averaged mole-fraction gradient. For large enough channels, the second-law separation efficiency is only slightly reduced from that of steady liquid Soret separation.

High-purity thermoacoustic isotope enrichment

In a tube many wavelengths long, thermoacoustic separation of a gas mixture can produce very high purities. A flexible wall allows a spatially continuous supply of acoustic power into such a long tube. Coiling the tube and immersing it in a fluid lets a single-wavelength, circulating, traveling pressure wave in the fluid drive all the wavelengths in the tube wall and gas. Preliminary measurements confirm many aspects of the concept with neon ((20)Ne and (22)Ne) and highlight some challenges of practical implementation.

Acoustics and precondensation phenomena in gas-vapor saturated mixtures

Starting from fundamental hydrodynamics and thermodynamics equations for thermoviscous fluids, a new modeling procedure, which is suitable to describe acoustic propagation in gas mixtures, is presented. The model revises the boundary conditions which are appropriate to describe the condensation-evaporation processes taking place on a solid wall when one component of the mixture approaches saturation conditions. The general analytical solutions of these basic equations now give a unified description of acoustic propagation in an infinite, semi-infinite, or finite medium, throughout and beyond the boundary layers. The solutions account for the coupling between acoustic propagation and heat and concentration diffusion processes, including precondensation on the walls. The validity of the model and its predictive capability have been tested by a comparison with the description available in the literature of two particular systems (precondensation of propane and acoustic attenuation in a duct filled with an air-water vapor saturated mixture). The results of this comparison are discussed to clarify the relevance of the various physical phenomena that are involved in these processes. The model proposed here might be useful to develop methods for the acoustic determination of the thermodynamic and transport properties of gas mixtures as well as for practical applications involving gas and gas-vapor mixtures like thermoacoustics and acoustics in wet granular or porous media.

Acoustic fields in binary gas mixtures: mutual diffusion effects throughout and beyond the boundary layers

The acoustic behavior in thermo-viscous gas mixtures, both in proximity of walls and far from them (outside the boundary layers), involves deviations from the adiabatic and laminar movements in pure gases, which result from the influence of several diffusive fields, namely, shear, entropic, and concentration variation fields (their energy being provided by the acoustic field itself). Owing to the boundary conditions, that are slip condition, isothermal condition and concentration flux vanishing on the walls, a strong coupling between these fields occurs inside the boundary layers while their effects appear to be simple additive processes in the bulk of the medium. Although recent literature on this subject leads to interesting results, opening the way to several new issues [R. Raspet et al., J. Acoust. Soc. Am. 105, 65-73 (1999); R. Raspet et al., J. Acoust. Soc. Am. 112, 1414-1422 (2002); G. W. Swift and P. S. Spoor, J. Acoust. Soc. Am. 106, 1794-1800 (1999); D. A. Geller and G. W. Swift, J. Acoust. Soc. Am. 111, 1675-1684 (2002)], the results available still have limitations because they do not provide complete solutions for the propagative and diffusive fields throughout and beyond the boundary layers. The present work aims at providing these solutions in the whole domains considered. The results allow interpreting analytically the behavior of the fields above mentioned in closed cavities and ducts, and particularly in spherical cavities which are best suited to develop metrological applications.

Thermoacoustic mixture separation with an axial temperature gradient

The theory of thermoacoustic mixture separation is extended to include the effect of a nonzero axial temperature gradient. The analysis yields a new term in the second-order mole flux that is proportional to the temperature gradient and to the square of the volumetric velocity and is independent of the phasing of the wave. Because of this new term, thermoacoustic separation stops at a critical temperature gradient and changes direction above that gradient. For a traveling wave, this gradient is somewhat higher than that predicted by a simple four-step model. An experiment tests the theory for temperature gradients from 0 to 416 K/m in 50-50 He-Ar mixtures.

Simulation of ultrasonic-driven gas separations

The separation of components in a gas mixture is important for a wide range of applications. One method for achieving this separation is by passing a traveling acoustic wave through the gas mixture, which creates a flux of the lighter components away from the transducer. A series of simulations was performed to assess the effectiveness of this method for separating a binary mixture of argon and helium using the lattice kinetics method. The energy transport equation was modified to account for adiabatic expansion and compression. The species transport equation was modified to include a barodiffusion term. Simulations were performed on two different scales; detailed acoustic wave simulations to determine the net component flux as a function of local concentration, pressure, etc. and device scale simulations to predict the gas composition as a function of time inside a gas separation cylinder. The method is first validated using data from literature and then applied to mixtures of argon and helium. Results are presented and discussed.

Theory of inert gas-condensing vapor thermoacoustics: propagation equation

The theory of acoustic propagation in an inert gas-condensing vapor mixture contained in a cylindrical pore with wet walls and an imposed temperature gradient is developed. It is shown that the vapor diffusion effects in the mixture are analogous to the heat diffusion effects in the thermoacoustics of inert gases, and that these effects occur in parallel with the heat diffusion effects in the wet system. The vapor diffusion effects can be expressed in terms of the thermoviscous function F(lambda) used in the theory of sound propagation of constant cross-section tubes. As such, these results can be extended to any shape parallel-walled tube. The propagation equations predict that the temperature gradient required for onset of sound amplification in a wet-walled prime mover is much lower than the corresponding temperature gradient for an inert gas prime mover. The results of a measurement of the onset temperature of a simple demonstration prime mover in air with a dry stack and with a stack wetted with water provide a qualitative verification of the theory.

Thermodynamic efficiency of thermoacoustic mixture separation

The acoustic power loss in the thermoacoustic mixture-separation process is derived, including the contributions due to a nonzero gradient in concentration. The significance of the gradient-dependent term is discussed. The limiting thermodynamic efficiency of the separation is calculated. Under reasonable circumstances, the efficiency approaches 10(-2) nHnL(delta m/m(avg))2, where nH and nL are the mole fractions of the two components of the mixture, and delta m/m(avg) is the fractional difference between the molar masses of the two components. This efficiency is of the same order of magnitude as that of some other, more conventional separation methods.