In this paper the study focuses on a visual investigation on the gas-water two-phase flow in a vertical circular narrow channel with 2 mm inner diameter under atmospheric pressure. Experiments were carried out with different working conditions, including different gases as gas-phase working fluids such as nitrogen, air, carbon dioxide and argon, and the gas flow rate, Q, varied between 0 ml/s (single liquid phase flow) to 9.0 ml/s, and the liquid mass flux, G, varied between 581.3 kg/m2s to 3201.8 kg/m2s. The influence of liquid mass flux, gas flow rate as well as Eo number and Mo number (using these two non-dimensional parameters to specify the effect of gas-phase properties) on the fluctuation of pressure drop and mass flux were investigated in this study. It is found that the pressure drop increases along with increasing liquid-phase flow rate with identical other working conditions, and the corresponding flow patterns are slug flow even though the liquid-phase flow rates are different. However, the pressure drop decreases at first and then increases along with gas-phase flow rate, with constant liquid flow rate (liquid mass flux), and the corresponding flow patterns include slug flow, slug-annular flow and annular flow. Based on the experimental result, it is also found that the smaller Eo number and Mo number of the gas-phase working fluid, the smaller the fluctuations of the pressure drop and mass flux would be due to the gas-phase working fluid is different.

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