ABSTRACT
An experimental investigation into low pressure subcooled flow boiling in a vertical annular test section was undertaken. A traversing single-beam gamma densitometer was used for measuring the axial distribution of the area-averaged void fraction and a high speed video system was used to visualize the subcooled flow boiling phenomena at various axial locations along the test section. The high speed photography results confirmed the fact that bubble detachment was not the cause of the NVG phenomenon. A new NVG model was developed based on the balance between vapour generation and condensation rates at the NVG point. The model is consistent with the Saha and Zuber’s model (1974) for thermally controlled net vapour generation. The predictions of the proposed model are in good agreement with available low pressure NVG data for subcooled flow boiling in vertical conduits.
