2 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 r efrigerant 113 have been conducted in vertical tubes containing helica l inserts. The flow pattern observations in vertical tubes were genera lly 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 be fore any swirling flow is seen at low and moderate void fractions. At liquid flow rates somewhat below those required for annular flow, a ne w flow pattern, with a swirling outer liquid layer and central bubbly core, was observed. Boiling heal transfer rates were determined for tw o wire helix geometries. In both cases, boiling heat transfer coeffici ents tended to be below those observed in a smooth tube for similar co nditions. Predictions based on a modified boiling suppression approach yielded moderate agreement.