An experimental study of gas-liquid flow in a vertical bundle of rods in a hexagonal arrangement with spacer grids was performed. The test section was a bundle consisted of 37 rods. Experiments were performed in system water – air at room temperature. New information about characteristics of two-phase flow near a spacer grid is presented. A comparison of distribution of local heat transfer from the central rod surface and distribution of local void fraction around this rod for two ways of gas phase addition was performed. A non uniform void profile around the central rod of the bundle was obtained for both cases. Just downstream the spacer grid an increase of void fraction was obtained Gas addition to the flow results in heat transfer enhancement. This is especially clear for the case of gas supply by a single nozzle. In this case high void fraction and high heat transfer coefficient were observed in one of the subchannels. The data obtained allow to estimate the transfer of gas bubbles in rod bundles and heat transfer augmentation by the addition of gas phase into the coolant flow.

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