Pressure drops for steam and water flow in heated tubes

Measurements of pressure drop for a steam and water flow inside several heated tubes were obtained at two different laboratories (Chalk River Laboratories and École Polytechnique). The test sections used to carry out the experiments were constructed with Inconel-600 tubes, and were cooled internally with an upward flow of water. A reduction in frictional pressure gradient with an increase in heat flux was observed for single-phase flows at the same cross-sectional-average conditions. After boiling was initiated, the two-phase friction multiplier increased with increasing heat flux in the bubbly-flow region, but decreased in the annular-flow region. As the heated surface approached dryout, the two-phase friction multiplier exhibited a maximum value and decreased with increasing thermodynamic quality, until dryout occurred. The decreasing trend was attributed to a reduction in the liquid-film thickness and the termination of liquid entrainment. Following the occurrence of critical heat flux, a significant drop in frictional pressure gradient was noted, caused by the change in the near-wall sublayer to a low-viscosity vapor blanket.