Experimental investigation was conducted with CO2 at high pressure, ranging 5.0 to 6.5 MPa, in horizontal mini-channels of 0.51, 1.0 and 2.0 mm in diameters. In smaller bores tube, e.g. 0.51 and 1.0 mm, the phase stratification is not serious and the flow pattern becomes quasi-axi-symmetric, while in 2.0 mm or larger tubes phase stratification becomes significant as in conventional sized tubes. This phase stratification, together with the intermittent flow behavior, causes the heat transfer deterioration at the upper wall. Existing criteria as a whole were insufficient in predicting flow patterns. In order to predict complicated two-phase flow behavior, including inherent void fraction fluctuation and flow patterns, the discrete bubble model based on a pattern dynamics approach was developed, so as to be applicable to the horizontal mini-channels. The simulated void fraction fluctuations successfully provided not only the flow pattern maps in terms of statistical properties, but also quantitative estimation of the heat transfer deterioration at the upper wall of the tube of 2.0 mm in diameter. Existing correlations were evaluated through the comparison with the present experimental data, and a nucleate boiling mode was found a dominant mode over the boiling heat transfer at high pressure in mini-channels.

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