In this paper

theoretical analysis and visual experimental research are reported on the flow-state characteristics during the heat transfer of CO2 flow boiling in a horizontal tubule. A CO2 theoretical flow-state prediction model under low evaporation temperature is updated based on visual experimental results. Experimental conditions are as follows: heat flux (7.5–30 kW/m2)

mass flow rate (50–600 kg/(m2?s))

saturation temperature (?40 to 0 °C)

vapor quality (0–1)

and inner diameter (1.5 mm). Theoretical analysis indicates that the mass flow rate determines the flow pattern experienced during the boiling process. The form of annular-dryout and the dryout-mist flow areas in the theoretical flow pattern diagram mainly depends on the heat flux of boiling

while the flow-state transition depends on the saturated temperature. Visual research shows that the flow pattern prediction of CO2 during boiling in the horizontal tubule was satisfactory based on the theoretical flow pattern chart

as well as the trend under different conditions. However

there was a large deviation between the dry region and the mist flow region under experiment conditions. Thus

an updated critical heat flux prediction model was proposed based on experimental data by adding dimensionless factors of the liquid-gas viscosity ratio. Based on the updated model and experimental data

the transition curves of the annular flow-dry region

dry region-mist flow

and intermittent flow/slug flow-annular flow in the primary theoretical flow pattern chart were updated by introducing the boiling number Bo

considering the effect of mass flow rate and heat flux.

89.4% of the flow pattern data obtained from the visualization study is consistent with the updated model.