Dryout study of a helical coil once-through steam generator integrated in a thermal storage prototype

Abstract

In this work, it is studied several dryout parameters (the dryout onset, first and total dryout, its associated azimuthal position and local steam qualities) in a triple helical coil once-through steam generator under quasi-steady working conditions (inlet pressure, PIN∈ [37.9, 39.2] bar, inlet subcooling, ΔTsub≈ 0 °C and total mass flow rate, ṁws≈ 0.082 kg·s−1) which exchanges heat with a molten salts counterflow under quasi-steady pressure and temperature conditions (atmospheric pressure and inlet temperature, TIN∈ [472, 477] °C) but transient mass flow rate, ṁms, depending on the helical coil diameter, D. For this purpose, a CFD model (Computational Fluid Dynamics model) has been used which has been evaluated with the experimental data obtained during a discharge test of the 300 kWth thermal storage prototype with integrated steam generator of the Casaccia Research Center (ENEA, Italy), previous benchmarking of two phase flow model. Results show, within the working conditions range and considering the particular steam generator characteristics, the two-phase flow behavior is uniform. This information has not been reported yet on available literature. In addition, numerical local steam qualities are compared with those calculated by using forced convective boiling correlations suitable for this type of steam generator geometry, concluding that: 270° Ruffel’s and Santini’s correlations are the most appropriate during first dryout, and to assume a local steam quality value of 0.97 during total dryout the most suitable approximation. This work can be useful for those who pretend to scale-up such a steam generator for industrial applications requiring process heat and/or power generation.