Validations of Thermohydraulic Models for Geysering in a Natural Circulation Loop Using an Impedance Needle Probe

Abstract

This paper describes the validations of thermohydraulic models that predict the geysering instability observed at the riser section of a short-tube natural circulation evaporator (STNCE) at low heat flux and low pressure. The validations have been made by measuring the void fraction in the steam-water system at the riser section of a STNCE with the help of an impedance void-needle probe. The calibration of the probe shows that in the range of low void fractions the probe indicates lower values while in the range of high void fractions it indicates higher values compared to those measured by the manometer. Measured data of void fraction have been analyzed by thermohydraulic models. Both measured data and simulated data of temporal variation of void fractions during geysering have been observed to be periodic pulses of high amplitudes. Effects of riser height and loop resistance on the geysering frequency have been demonstrated in detail. The paper also discusses both the measurement uncertainties and the model uncertainties.

Subcooled boiling oscillations in natural circulation boiling loop at low pressure

“…However, a cycle of the observed geysering oscillations consists of three stages, as also reported by Yadigaroglu and Paruya et al The stages of the cycle LMNL (symbolically denotes a cycle for geysering) are shown in the phase portraits of p Hi ‐ T Ho presented in Figures a, b. The points L, M, and N form a triangular shape.…”

“…The diagonal line NL is refilling stage (gradual increase of static head due to bubble condensation in the riser causing low flow or reversed flow). Unlike the geysering instability of a periodic nature observed by Yadigaroglu and Paruya et al, the present experiments support that the geysering periods are not purely periodic. It is also noted in Figures a, b that the chaotic geysering‐oscillations become stronger as Q increases.…”

“…Particularly, the understandings of the instabilities in NCBL at boiling incipience in subcooled boiling are not complete. Here, we extend the studies of Paruya et al rescaling the NCBL with a longer riser and a longer heated section (HS). Our experimental setup also differs from the existing ones with respect to the scaling ratio of NCBL.…”

Nonlinear Analysis of Chaotic Time Series in a Natural Circulation Boiling Loop

“…because of low driving heads. Experimental studies at the conditions of low pressures and low heat fluxes report five major instabilities that occur in NCBL (see the works of Wissler et al., Ledinegg, Chexal and Bergles, Fukuda and Kobori, Aritomi et al, Wu et al, Kyung and Lee, Jiang et al, Zboray et al, Hsieh et al, Jain et al, and Paruya et al). The instabilities include flow excursion or flow reversal, geysering instability, natural circulation oscillation, flashing-induced instability, and Type I (low steam quality) density-wave oscillation (DWO).…”