This paper presents the results of an experimental investigation conducted for high heat flux subcooled boiling heat transfer and pressure drop in a tubular channel under both smooth- and swirl-flow of high velocity water. High heat flux flow boiling is of interest to Fusion reactor first wall cooling. Test conditions covered a mass flux range from 5 to 10 Mg/m2 s, inlet temperatures from 100 to 175°C and system pressures from 2.0 to 5.0 MPa. The maximum heat flux tested was 12 MW/m2. The test section diameter used in this study was 5.30 mm (I.D.) with an axial heated length of 356 mm. To ensure accurate results, a significant number of heat balance tests were performed with a minimum and maximum heat balance error of 1.5%. Swirl-flow tests were performed using twisted tape inserts with thickness 0.8 mm with twist ratios between 2 and 4. To measure heat transfer performance, 15 miniature thermocouples were used to measure the tube outside wall temperature at various axial and circumferential positions. Differential pressure transducers were used to measure the axial pressure drop at several locations along the test section under single- and two-phase conditions.
