Two-Phase Flow Patterns in Microchannel Vaporization of CO2 at Near-Critical Pressure

Carbon dioxide (CO ₂ /R-744) is receiving renewed interest as a refrigerant, in many cases for systems with microchannel heat exchangers that have high pressure capability, efficient heat transfer, and compact design. A good understanding of two-phase flow of evaporating CO ₂ in microchannels is needed to analyze and predict heat transfer. A special test rig was built in order to observe two-phase flow patterns using a horizontal glass tube with ID 0.98 mm. Flow visualization experiments were conducted for temperatures 20°C and 0°C and for mass flux ranging from 100 to 580 kg m^?2 s^?1 . The observations showed a dominance of intermittent (slug) flow at low x and wavy annular flow with entrainment of droplets at higher x. The aggravated dryout problem reported from heat transfer experiments at high mass flux could be explained by increased entrainment. The flow pattern observations did not fit generalized maps or transition lines showed in the literature.