Experimental study of flow boiling heat transfer and dryout characteristics at low mass flux in helically-coiled tubes

• We experimentally research including a very low mass flux of 88.4 kg/(m2 s) to consider loss of coolants accidents in nuclear power plants.
• We present suggestions to estimate flow boiling heat transfer coefficients and dryout quality in helically coiled tubes.
• The modified dryout correlations are suggested to cover the extended mass flux and pressure range.

The goals of this study were to understand flow boiling phenomena in helically-coiled tubes and to predict their flow boiling heat transfer coefficients (HTC) and dryout qualities. Flow heat transfer and dryout were experimentally investigated in coiled tubes of 12-mm inside diameter and 17-mm outside diameter. The helical diameters Dh of the coiled tubes used were 606 mm and 977 mm for the HTC experiments and 606 mm, 977 mm, and 1290 mm for the dryout experiments. In flow boiling experiments mass flux G was 88.4–530.5 kg/(m2 s), system pressure P was 1–6 MPa, and heat flux q″ to the tube wall was 30.0–1145.3 kW/m2. For dryout experiments, q″ was 131.3–1764.9 kW/m2. Flow boiling heat transfer in the coiled tubes was significantly affected by nucleate boiling and convective boiling, and not by secondary flow due to the helical coiling. HTC in the coiled tubes were accurately predicted (standard deviation 29%) by Steiner and Taborek HTC correlation for straight vertical tubes. Dryout qualities were significantly influenced by the coiled tubes. The effects of Dh, G, and P were greatly affected by secondary flow caused by the coiled tubes. Dryout in the coiled tubes can be only predicted using dryout correlations for them. The dryout map proposed by Berthoud and Jayanti was modified to consider the effects of redeposition and gravity; this modified correlation accurately predicted dryout qualities (standard deviation 10.1%) within the range of experimental conditions. These results will help guide design of cooling systems that use helical coils.