Thermoacoustic effects in supercritical fluids near the critical point: Resonance, piston effect, and acoustic emission and reflection

TL;DR

This paper develops a comprehensive theory of thermoacoustic phenomena in supercritical fluids near the critical point, considering boundary heat conduction and viscosity effects, and explores resonance, boundary reflection, and acoustic responses.

Contribution

It introduces a new coefficient $Z(requency)$ to characterize sound reflection at boundaries and analyzes acoustic eigenmodes and responses near the critical point.

Findings

Resonance effects are significantly enhanced near the critical point.

Thermal diffusion layers play a crucial role due to critical divergence.

The theory accurately predicts sound emission and reflection behaviors.

Abstract

We present a general theory of thermoacoustic phenomena in supercritical fluids near the critical point in a one-dimensional cell. We take into account the effects of the heat conduction in the boundary walls and the bulk viscosity near the critical point. We introduce a coefficient $Z(\omega)$ characterizing reflection of sound with frequency $\omega$ at the boundary. As applications, we examine the acoustic eigenmodes in the cell, the response to time-dependent perturbations, sound emission and reflection at the boundary. Resonance and rapid adiabatic changes are noteworthy. In these processes, the role of the thermal diffusion layers is enhanced near the critical point because of the strong critical divergence of the thermal expansion.