Investigation of dryout oscillation dependency on surface wettability in printed circuit steam generator

Abstract

A Printed Circuit Steam Generator (PCSG) is one of the promising steam generator types that is manufactured with diffusion bonding process, so it has advantages such as high surface-to-volume ratio, better affordability, and superior structural integrity under high pressure and temperature conditions. Nonetheless, PCSG is a once-through steam generator, thus dryout and two-phase instability occur in the flow path. Due to these phenomena, periodical alternating stress induced by dryout thermal fluctuation can finally lead to the thermal fatigue failure. To overcome the limitation of dryout in once-through steam generator channels, a mitigation strategy by controlling wettability of the channel surface is investigated in this thesis. To experimentally analyze the dryout phenomena, a test section with semi-circular bare surface channel with 4mm diameter and another test section with identical geometry but hydrophobic surface were manufactured, and experiments with two test sections were conducted. As a result, frequencies of dryout thermal fluctuation on both the bare and the hydrophobic surfaces were measured as 0.1 to 1Hz but the maximum temperature amplitude was smaller for the hydrophobic surface by 20℃. Furthermore, to assess thermal fatigue failure under real nuclear reactor operating condition, a dryout frequency prediction model and a dryout amplitude prediction model are newly suggested based on the newly obtained experimental data in this thesis and the previous studies. To analyze thermal fatigue failure under more challenging conditions than a light water reactor condition, a PCSG for PGSFR operating condition was newly designed and evaluated. From the design results of PCSG and the newly proposed dryout thermal fluctuation prediction model, the frequency was estimated to be 51Hz under the PGSFR operating condition. With the predicted variables, PCSGs under PGSFR operating condition with bare surface and hydrophobic surface are modeled with finite element method, and it was observed that the PCSG with hydrophobic surface has smaller alternating stress induced by dryout thermal fluctuation under the identical thermal-hydraulic condition.