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

Heat transport in supercritical CO2 is studied under microgravity conditions. 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 heating is obtained by using a small thermistor located in the bulk fluid. Through interferometric observations, a new mechanism of thermalization has been evidenced. Thermal expansion of a warm diffusing boundary layer around the heating thermistor is responsible for rapid adiabatic heating of the bulk fluid through the emission of pressure waves at the border. The scaled thickness of the thermal boundary layer follows a power law. When the heat flow stops, the bulk adiabatic heating instantaneously vanishes and the temperature relaxation inside the thermal boundary layer follows locally a diffusive process.