An experimental study on flow boiling heat transfer and dryout characteristics in helically coiled tubes

Abstract

Flow boiling and dryout experiments were conducted in helically coiled tubes of 12 mm I.D. and 17 mm O.D. with helical diameters of 606, 977 and 1290 mm. The range of conditions investigated were the following: Mass flux 88.4-530.5 kg/m2s

System pressure 1-6 MPa

Heat flux 30.0-1145.3 kW/m2 for flow boiling experiments

Heat flux 131.3-1764.9 kW/m2 for dryout experiments.The influence of heat flux on heat transfer coefficients was investigated in order to understand the trend of the nucleate boiling heat transfer mechanism. The heat transfer coefficients were increased with the increase in heat flux. In addition, the influence of mass flux on heat transfer coefficients was investigated in order to consider convective boiling heat transfer mechanism. The heat transfer coefficients were little enhanced by the increase in mass flux. Thus, it was found that nucleate boiling was the dominant heat transfer mechanism in most experimental ranges. Klausner’s equation was used for quantitative analysis of nucleate boiling and convective boiling. The influence of system pressure and helical diameters on the heat transfer coefficients was also investigated. The effects of system parameters, such as pressure, mass flux and helical diameter, on dryout qualities were investigated. It was found that these parameters play important roles in the dryout qualities. Measured heat transfer coefficients were compared with previous correlations. Because the Steiner correlation accurately predicted the trend of nucleate boiling heat transfer and convective boiling heat transfer, the experimental data were well predicted. The correlation also predicted the superior heat transfer mechanism in the local region of flow boiling. For dryout qualities, Berthoud's correlation well estimated the dryout qualities in the redeposition zone but misestimated them in the gravity-affected zone. Thus, the correlation should be improved.