ACOUSTIC SATURATION OF THE CRITICAL SPEEDING-UP

It has recently been found that the heat transfer in the vicinity of t he liquid-vapor critical point is governed by a fourth heat transfer p rocess called Piston Effect. This effect has been described as the str ong expansion of a thin initial thermal boundary layer (due to the div erging compressibility of near-critical fluids), driving in the rest o f the fluid a set of compression waves which adiabatically increase th e temperature, on a shorter and shorter time-scale as the critical poi nt is approached (Critical Speeding Up). In the present work, matched asymptotic descriptions techniques applied to the Navier-Stokes equati ons show that very near the critical point, a new regime of heat trans fer appears where thermal equilibration is governed only by acoustic p henomena. In this new regime, temperature relaxation takes place withi n a few acoustic typical times only, showing that the Piston-Effect me chanism can be as fast as the acoustic propagation, but no faster. Thi s phenomenon is called the Acoustic Saturation of the Piston Effect.