On the transition from thermoacoustic convection to diffusion in a near-critical fluid

Acoustic waves can be generated in response to thermal disturbances near the critical point, due to the diverging compressibility. We study the thermomechanical effect in a slab of supercritical nitrogen subjected to various forms of boundary heating by numerically solving the governing hydrodynamic equations. The results show that, dependent on the rapidity of the heating, inherently different fluid-dynamical wave behaviors occur on the acoustic timescale with respect to acoustic emission, propagation, and reflection patterns. Specifically, the sudden ramp of the boundary temperature is capable of triggering a strong thermoacoustic pulse in the fluid, whose reflection at the isothermal boundary introduces complex features. In contrast, linear compressive waves dominate under the gradual heating. On a longer timescale, both types of fast processes lapse into slow thermal diffusion coupled by pronounced density inhomogeneities, via different routes nonetheless.