Two phase flow patterns and boiling heat transfer in tubes containing helical wire inserts .1. Flow patterns and boiling heat transfer coefficients

A coordinated series of heat transfer and flow-pattern studies using refrigerant 113 have been conducted in vertical tubes containing helical inserts. The flow pattern observations in vertical tubes were generally similar to earlier observations in horizontal lines. As previously noted, the dispersed flow pattern is replaced by annular flow with a swirling outer liquid layer. A minimum liquid flow rate is required before any swirling flow is seen at low and moderate void fractions. At liquid flow rates somewhat below those required for annular flow, a new flow pattern, with a swirling outer liquid layer and central bubbly core, was observed. Boiling heal transfer rates were determined for two wire helix geometries. In both cases, boiling heat transfer coefficients tended to be below those observed in a smooth tube for similar conditions. Predictions based on a modified boiling suppression approach yielded moderate agreement.