Boundaries among bubbly and slug flow regimes in air-water two-phase flows in vertical pipe of poor wettability

摘要

Much effort has been devoted to-understand the characteristics of gas-liquid two-phase flows in vertical and horizontal pipes used in a variety of engineering fields such as mechanical, chemical and atomic energy engineering (Wallis, 1965; Akagawa, 1980; Hetsroni, 1982; Ueda, 1989; Jpn. Soc. Mech. Eng.,1989). Most researchers have paid their attention to pipes of good wettability, i.e., pipes wetted with liquid. Information on the flow patterns, velocities of liquid and gas, pressure losses, heat transfer in the pipes of good wettability have been extensively accumulated. On the other hand, investigations on the effects of wettability on the characteristics are very limited (Barajas and Panton, 1993), although pipes of poor wettability have been widely used in materials engineering (Sugita, 1998) and atomic energy engineering. This lack of research is partly because the liquids i.e., molten metals used in these engineering fields are not transparent, and partly because the melting temperatures of the molten metals are usually very high, so that measurements of the aforementioned characteristics are dangerous. Accordingly, adequate sensors for measuring the characteristics under such severe conditions are absent at the present stage. Model experiments, therefore, are expected to be carried out, but it is not easy to control and 'keep the wettability of a pipe for a long time even in the model experiments. Surface treatment is ""commonly applied to change the wettability of the pipe wall. The advancing contact angle. which is used to quantitatively represent the wettability (Iida and Guthrie, 1988), rapidly decreases due to contamination and in most cases, the pipe becomes wetted with liquid in a relatively short time. Accordingly, long-range experiments are difficult except in very rare cases.