The objective of this study is to investigate the void fraction distributions by examining the Probability Distribution Function (PDF) of local void fraction in a 3x3 rod bundle geometry. A series of experiment has been carried out with air-water two-phase flow. The experimental tests were performed under the flow conditions of Jg = 0.076-0.24 m/s and Jf = 0.14, 0.19 and 0.24m/s. Two sets of conductivity probe arrangements were used to measure the local void fractions in the regions of "rod-gap" and "sub-channel". It was observed that while the superficial gas velocity increases, the peak regions of probability distribution function may become broadened; whereas the peak values of PDF may be shifted to the left as superficial liquid velocity increases. In addition, the PDF peak values of both regions tended to stay closer and become more overlapped with increasing superficial gas and liquid velocities.

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