Numerical investigation of systematic measurement errors in conductance-type wire-mesh sensors for rod-bundle flows

Uesawa, Shinichiro  ; Yoshida, Hiroyuki  

In void-fraction measurements using conductance-type wire-mesh sensors (WMSs) in rod-bundle flows, the measurement accuracy is governed by the non-uniform electric current density formed near the electrodes. This non-uniformity causes the instantaneous WMS signal to be strongly affected by the bubble position and shape, thereby introducing systematic errors specific to rod-bundle geometries. However, these errors have not been sufficiently evaluated. In this study, three-dimensional electrostatic simulations were performed to clarify the mechanisms and magnitudes of these errors. The current density distributions and WMS signal responses were analyzed for the subchannel, inter-subchannel, and corner-subchannel regions while varying the bubble position, bubble shape, and transmitter-receiver layer distance. The results demonstrated that the spatial characteristics of the current-density distribution differ among the three channel types. Consequently, the correlation between the WMS signal and void fraction was not unique, and the channel-dependent variations in the correlation were confirmed to arise from differences in bubble position, bubble shape, and layer distance. By consolidating these correlations, representative void-fraction conversion formulas incorporating quantitative systematic errors were developed. The findings of this study provide a quantitative basis for improving the reliability of WMS measurements and for evaluating experimental uncertainties in the validation of two-phase flow CFD simulation codes.