Flow regime maps were obtained for an adiabatic air-water two phase flow through a flow channel with 8×8 rod bundle, which simulated a typical rod bundle in a BWR. Impedance void meters were used to measure the area averaged void fraction at various axial locations in the flow channel The Cumulative Probability Distribution Functions (CPDF) of the signals from the impedance meters were fed to a self organizing neural network to identify the flow regimes. The flow regimes were identified at seven axial locations in the channel in order to understand the development of the flow regimes in axial direction. The experimental flow regime transition boundaries agreed well with the theoretical ones obtained using Mishima and Ishii (1984) model. In addition, the two impedance void meters located across a spacer grid, which were used to study the change in the flow regime across the spacer grid.

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