Flow Boiling Heat Transfer, Pressure Drop, and Flow Pattern for $CO2$ in a $3.5mm$ Horizontal Smooth Tube

$CO2$ flow boiling heat transfer coefficients and pressure drop in a $3.5mm$ horizontal smooth tube are presented. Also, flow patterns were visualized and studied at adiabatic conditions in a $3mm$ glass tube located immediately after a heat transfer section. Heat was applied by a secondary fluid through two brass half cylinders to the test section tubes. This research was performed at evaporation temperatures of $−15°C$ and $−30°C$⁠, mass fluxes of $200kg∕m2s$ and $400kg∕m2s$⁠, and heat flux from $5kW∕m2$ to $15kW∕m2$ for vapor qualities ranging from 0.1 to 0.8. The $CO2$ heat transfer coefficients indicated the nucleate boiling dominant heat transfer characteristics such as the strong dependence on heat fluxes at a mass flux of $200kg∕m2s$⁠. However, enhanced convective boiling contribution was observed at $400kg∕m2s$⁠. Surface conditions for two different tubes were investigated with a profilometer, atomic force microscope, and scanning electron microscope images, and their possible effects on heat transfer are discussed. Pressure drop, measured at adiabatic conditions, increased with the increase of mass flux and quality, and with the decrease of evaporation temperature. The measured heat transfer coefficients and pressure drop were compared with general correlations. Some of these correlations showed relatively good agreements with measured values. Visualized flow patterns were compared with two flow pattern maps and the comparison showed that the flow pattern maps need improvement in the transition regions from intermittent to annular flow.