This paper describes some of the key experimental results and features obtained from a study on upward flow boiling of water in a vertical tube at sub-atmospheric pressures. The experiments were conducted at 250, 500 and 1000 mbar (abs) exit pressures. From the experimental results, several interesting features and effects were observed, namely, heat transfer coefficient maxima at around zero thermodynamic quality were observed for high inlet liquid subcoolings at low sub-atmospheric pressures, which were attributed to local thermal non-equilibrium instability (Jeglic and Grace, 1965). Such effects were also observed in boiling of pure hydrocarbons in vertical tubes (Kandlbinder, 1997; Urso et al., 2002) and were explained and explored quantitatively with a thermal non-equilibrium slug flow model (Barbosa and Hewitt, 2005) that associates vigorous bubble growth at sub-atmospheric pressures to the formation of large Taylor bubbles separated by subcooled liquid slugs.
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