The heat transfer characteristics of CO2 and CO2-oil mixture in tubes including convective flow
boiling, gas cooling, and condensation are investigated. Two-phase flow patterns are thoroughly
investigated based on physical phenomena, which show the early flow transition to intermittent or annular
flow especially for small diameter tube. The physical phenomena for nucleate boiling of CO2 follow the
same trends with other organic fluids under the same reduced pressure. The gas cooling heat transfer is
critically dependent on the turbulent diffusivity related with buoyancy force due to the large density
difference. Under the oil presence conditions, the interaction of oil rich layer and bubble formation is the
physical mechanism for the CO2-oil mixture convective boiling. Besides, the gas cooling phenomena with
oil should be investigated based on the flow patterns formed by CO2 and oil, and the oil rich layer, whose
thickness are depends on the solubility of CO2 to oil explains the physical mechanisms of heat transfer.
The thermodynamic properties of CO2-oil were estimated by the general model based on EOS, and they
are utilized to estimate the properties for oil rich layer and oil droplet vapor core. Through these predicted
properties, the convective boiling and gas cooling heat transfer coefficients and pressure drop
theoretically estimated. Condensation of CO2 is not so different from the existing one, so the heat transfer
coefficients and pressure drop are well estimated by the existing one developed for other fluids.
