Boiling-like phenomena in free-convection heat transfer at supercritical pressures

Four differently composed nickel-alloy wires and platinum wires were used as heaters to CO₂ in a wide range of pressures and temperatures. Visual observations exhibited three flow regimes: laminar flow, oscillatory flow and turbulent film flow. Boiling-like phenomena may only be attributed to the oscillatory and film regime with respect to the appearance of bubble-like structures and high heat transfer coefficients. The occurrence of these regimes depends on the thermal diffusivity of the wire material. Only laminar flow is observed for platinum wires with a high thermal diffusivity; all three regimes are observed with nichrome wires with low values of thermal diffusivity. The high heat transfer coefficients obtained with boiling-like action distinctly deviate from a correlation valid for platinum wire experiments.