Critical point pressure sensitivity

A new means of heat transfer known as the piston effect was identified in 1989. The piston effect is where the expanding thermal boundary layer acts like a piston which compresses the bulk fluid. An examination of the equations for the conservation of mass, momentum, and energy identified the significant thermophysical properties as the thermal conductivity, the volume expansivity, and the isothermal compressibility. The thermal conductivity and the volume expansivity determine the thickness of the thermal boundary layer. The isothermal compressibility of the bulk fluid determines the pressure response of the bulk fluid to a given volume change. Previous researchers used only the van der Waals equation of state at conditions within mK of the critical point. The research described herein focuses on the pressure response of a fluid near the critical point to a sudden change in the boundary temperature. The use of the van der Waals equation of state for numerical simulation of the piston effect results in underpredicting the magnitude of the pressure wave by approximately 30 percent while overpredicting the acoustic heating by approximately 15 percent compared to using all fluid properties from a real gas equation of state. When evaluating the piston effect at conditions typical of cryogenic storage systems the pressure response of the fluid was observed to be six orders of magnitude larger than had been previously reported. The extent of the acoustic heating resulted in temperature increases in the bulk fluid that were four orders of magnitude larger. The real gas equation of state was used to compare the pressure and temperature response of oxygen and hydrogen due to a thermal disturbance at the boundary. The pressure rise in hydrogen after five acoustic time periods was only 17% of the pressure rise in oxygen. The temperature increase in hydrogen was only 30% of the temperature rise in oxygen. On the diffusion time scale the pressure rise in the oxygen is an order of magnitude larger than the pressure rise in hydrogen for the same thermal penetration depth. The temperature rise in oxygen is four times greater than the temperature rise in hydrogen.