FAST ADIABATIC HEATING AND TEMPERATURE RELAXATION IN NEAR-CRITICAL FLUIDS UNDER ZERO-GRAVITY

Heat transport in supercritical CO2 is studied under microgravity cond itions. A large temperature and density rho region around the critical point is explored (CO2 cells were filled at critical density rho=rho( c) and off-critical densities rho=rho(c) +/- 0.18 rho(c)). Local heati ng is obtained by using a small thermistor located in the bulk fluid. Through interferometric observations, a new mechanism of thermalizatio n has been evidenced. Thermal expansion of a warm diffusing boundary l ayer around the heating thermistor is responsible for rapid adiabatic heating of the bulk fluid through the emission of pressure waves at th e border. The scaled thickness of the thermal boundary layer follows a power law. When the heat flow stops, the bulk adiabatic heating insta ntaneously vanishes and the temperature relaxation inside the thermal boundary layer follows locally a diffusive process.