The pressure recovery and void fraction change of air-oil two-phase flow across a sudden expansion has been investigated experimentally over a range of flow conditions. The pressure upstream and downstream of a half-inch to one-inch sudden expansion was measured using a series of pressure taps, and capacitance sensors were used to measure the void fraction along the test section. The void fraction increases as the flow approaches the sudden expansion section, with a sudden increase immediately downstream of the expansion followed by a gradual relaxation to the fully developed value further downstream. The normalized pressure recovery coefficient using the dynamic head based on the homogeneous density and two-phase velocity is found to collapse when plotted as a function of the mass quality. The experimental pressure recovery data are compared with predictions from existing models, and are found to be in good agreement with the Delhaye model with the void fraction relation of Wallis.

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