Experimental Study of Flow and Heat Transfer Characteristics in Helically-coiled Tube and Evaluation of Empirical Formulas

The flow and heat transfer characteristics of helically-coiled tubes are crucial for the design of spiral tube steam generators. In this paper, the flow and heat transfer characteristics of a vertical helically-coiled tube with an inner diameter of 8.8 mm and a helical diameter of 568 mm were experimentally investigated in a wide pressure range: 0.2-14.1 MPa. The mass flow rate is 49-1 902 kg/(m2·s). The heat flux of the experimental section is 14.5-580 kW/m2. In this experiment, the flow rate of the main loop is adjusted by the valve opening, the system pressure is adjusted by the high-pressure nitrogen cylinder, the metering pump and the pressure relief valve, the inlet fluid parameters of the experimental section is adjusted by the input power of the preheater and the direct-current (DC) voltage loaded in the preheating section, and the heating heat flux is adjusted by the DC voltage loaded in the experimental section. Finally, the friction coefficients of single-phase and two-phase, as well as heat transfer coefficients of single-phase, subcooled boiling, saturated boiling and dry out under different working conditions were obtained. Comparison and analysis of the experimental results with the empirical relational formulas of recent years revealed that the empirical formulas of Akagawa’s, Hart’s, and Ito’s predicted the single-phase friction coefficients with high accuracy within ±5%. The secondary flow increases the critical Reynolds number, which is about 10 000 in this experiment. In the range of straight tube laminar flow (Re＜2 300), the influence of secondary flow is greater. When the Reynolds number increases, the energy dissipation of secondary flow to the turbulent region is much smaller than that of laminar flow region. The current empirical formula has at least 10%-20% deviation in predicting the two-phase friction coefficient and the heat transfer coefficient in different regions. The relative average deviation between the friction coefficient of the two phases and the calculation formulas of Chen, Guo, Ferraris and M-N is about ±20%, and the prediction accuracy difference between the empirical formulas of spiral tube and straight tube is not obvious. The heat transfer coefficient of single-phase water section has the smallest relative average deviation from Guo et al. empirical formula, which is 18.2%. The relative average deviation between the heat transfer coefficient of the subcooled boiling zone and Hardik’s empirical formula is the smallest, which is −21.1%. The heat transfer coefficient of saturated boiling region has the smallest relative average deviation from the modified Chen’s formula, which is 7.5%. The relative average deviation of heat transfer coefficient of dry zone from Gao’s empirical formula is the least, which is 17.9%. The results of the analysis can provide a reference for the design of helically-coiled tube steam generators.