Frequency domain method was employed to evaluate DWO in HTR-10 in the study of Niu et al. to investigate DWO in single and two parallel helically coiled tubes. More recently, experimental and theoretical studies were performed by Papini et al. carried out experiments to study DWO in helically coiled tube steam generators. Most helically coiled tube correlations are formed based on straight pipe correlations by introducing a correction factor to adapt to the new geometry. Previous and current researches on flow instabilities mainly focus on straight tubes, whereas the research on helically coiled tubes has received much less attention. Numerous theoretical and experimental works are collected in several excellent literature reviews. Two-phase flow instabilities including DWO have been studied since the 1960s many researchers contributed to understand the instability mechanisms, namely, Yadigaroglu and Bergles, Ishii, and Fukuda and Kobori. It is well known that DWO is induced in boiling system by the interaction between the single-phase flow and two-phase flow pressure drops, the inlet mass flow rate, and the void fraction distribution. DWO is the most representative instability encountered in boiling systems. Hence, it is imperative to avoid flow instabilities and determine the safe operating regions of steam generators through rational design and accurate definition of the threshold values of system parameters. In steam generators, extensive attention is attached to flow instabilities, as they can lead to mechanical vibrations, thermal fatigue, problems of system control, and heat transfer surface burnout issues.
The most prominent characteristic of flow in helically coiled tubes is the secondary flow induced by centrifugal force due to the curvature of the pipe, thus resulting in significantly larger heat transfer and friction factors with respect to straight pipes. They are widely adopted in different plants, such as HTGR, SMR, and IRIS. Helically coiled tube steam generators are widely used in nuclear industry for their compactness, easy manufacture, and enhanced heat transfer efficiency. IntroductionĪ steam generator is a type of heat exchanger specifically designed to transfer heat from a coolant into water, producing steam that can be utilized for power generation. The comparison results agreed well which showed that the correction is effective. A correction factor was introduced to reduce the error of RELAP5 when modeling helical geometry. The theoretical analyses of the in-house program were compared to the simulation results of RELAP5.
RELAP5/MOD3.2 code was also used to simulate DWO in H-OTSG. According to the analyses, the impact law of the geometrical parameters to the system stability was obtained. Using Nyquist stability criterion of the single variable, the stability cases were studied with an in-house computer program.
Based on the steady-state calculation, the mathematical model of single-channel system was established, and the transfer function was derived. This paper presents research on density wave oscillations (DWO) in a typical countercurrent H-OTSG. The flow instability of the secondary side of the H-OTSG is particularly serious, because the working condition is in the range of low and medium pressure. Helically coiled tube Once-Through Steam Generator (H-OTSG) is one of the key equipment types for small modular reactors.