Quantitative measurement of thermal responses of a near-critical fluid chamber subjected to bottom heating

A broader application of supercritical CO₂ in the industry has invoked the need for the understanding of the coupling mechanism between the non-linear, non-monotonous behaving thermophysical properties with the fundamental heat transfer process in the region close to the critical point or the pseudo-critical line. In this research, a near-critical convective flow when subjected to the heat from the bottom (Rayleigh–Bénard model) is studied to investigate its instability and correlation behavior between the flow pattern and heat transfer characteristic. An improved phase-shifting interferometer is applied to capture transient density and temperature field data, facilitating the investigation of local fluid behavior. The measurement is conducted under several specific working conditions close to the critical point and also along the pseudo-critical line (initial conditions T_i = 32.00 ∼ 36.00 ℃, p_i = 7.50 ∼ 8.30 MPa), and under different heat fluxes q = 164, 657, and 1477 W/m². The main findings include: (1) In general, the convection and the plume flow are observed with the temperature changes of 10 ∼ 60 mK and Nusselt number up to 8 × 10⁴ in the length scale of 7 mm; (2) The faster trigger of convective flow under the same heating conditions in the gas-like cases is mainly due to the small dynamic viscosity, while the enlarged thermal expansion coefficient β_p near the pseudo-critical line or the critical point is responsible for the instability of the thermal boundary layer; (3) The inhomogeneous distribution of Nusselt number indicates the existence of small plume flow (on the length scale about 1 mm with variation in Nusselt number of 2 × 10³), which shows an inhomogeneous transfer heat transfer intensity in a small scale. This phenomenon is more important when the critical point or the pseudo-critical line is approached, which also indicates the importance of the local effect of convective heat transfer in the design and operation of the supercritical fluids system.