Multi-dimensional void fraction measurement of transient boiling two-phase flow in a heated rod bundle

Abstract

A subchannel void sensor (SCVS) was developed to measure the cross-sectional distribution of void fraction in a 5×5 heated rod bundle with o.d. 10 mm and heated length 2000 mm, and applied to a boiling two-phase flow experiment under the atmospheric pressure condition assuming at an accident or in a spent fuel pool in a boiling water reactor (BWR). The SCVS comprises 6-wire by 6-wire and 5-rod by 5-rod electrodes. The wire electrodes of 0.2 mm in diameter are arranged in lattice patterns between the rod bundle, while the electric conductance value in a region near one wire and another corresponds to local void fraction in the central-subchannel region. The local void fractions at 32 points (= 6×6-4) can be obtained as a cross-sectional distribution. The local void fractions near the rod surface at 100 points (= 4×25) can be also estimated by the conductance value in a region between one wire and one rod. The devised sensors are installed at five height levels along the axis to acquire two-phase flow behavior. A pair of SCVS is mounted at each level and placed 30 mm apart to estimate the one-dimensional phasic velocity distribution based on the cross-correlation analysis of both layers. The temporal resolution of void fraction measurement is 1600 frames (cross-sections) per second. The axial and radial power profile of the heated rod bundle are uniform, and eight pairs of sheath thermocouples are embedded on the heated rod to monitor its surface temperature distribution. The boiling two-phase flow experiment, which simulated a boil-off process, was conducted with the devised SCVS and experimental data was acquired under various inlet flow velocity, rod bundle power and inlet subcooling conditions. The experimental results were presented by the axial and cross-sectional distributions of void fraction, phasic velocity and bubble-chord length.

Measurement of subchannel void fraction in 5 × 5 rod bundles using an impedance void meter

“…In its verification, the area-averaged superficial gas velocity estimated by the measured results coincided well with that measured with the gas flow meter (within an error of 20%). Moreover, the SCVS was applied in heated 5 × 5 rod bundles further by Arai et al [19,20]; these could measure 32 central subchannel regions and 100 rod surface regions in the same way. The time resolution of the 132 points void fraction measurement was up to 5000 frames (cross-sections) per second.…”

“…Moreover, many types of impedance void meter have been developed and applied in research on two-phase flow, such as the double ring type [24] in pipes, face-to-face plates [21], and quadrant arcs [25] in rod bundles. It should be noted that, in some way, the SCVS [5,19,20] could also be considered as one type of electrical impedance void meter.…”

“…The gamma ray attenuation technique [6][7][8], neutron radiography [9][10][11], and fast neutron scattering method [12] are too expensive compared with other methods, and they are dangerous if the ray irradiates the human body. Secondly, the response time is emphasized for the rapid change of phase distribution in two-phase flow, which indicates that only electrical methods (SCVS [5,19,20], multi-sensor probe [21], and electrical impedance void meter [21]) can capture the dynamic features of the void fraction in rod bundles. Thirdly, regarding accuracy, almost all methods can reach a reasonable accuracy when the signals are dealt with or calibrated properly as discussed in their papers.…”

“…Moreover, the multi-sensor probe [21] and quick closing valve method [13] cost lots of time for the probe moving or repeating experiments. Lastly, some methods (quick closing valve method [13], SCVS [5,19,20], and multi-sensor probe [21]) disturb the two-phase flow field, which may result in wrong results at downstream measuring points; these methods can be difficult to apply to obtain the real-time void fraction in industrial equipment. In conclusion, the electrical impedance void meter would be the best way to measure the void fraction in rod bundles.…”