Experimental study of void fraction in an 8 × 8 rod bundle at low pressure and low liquid flow conditions

•Systematic two-phase flow data is collected at low pressure and low liquid velocity.•Gravitational forces are more significant at low pressure conditions.•The effect of recirculation patterns may be observed in the new data.•Uncertainty in earlier correlations may be attributed to secondary flow patterns.

An experiment has been performed to measure void fraction at stagnant to low liquid flow conditions in a rod bundle. An 8 × 8 rod bundle test facility scaled from a boiling water reactor design was utilized at atmospheric pressure with air and water as working fluids. Superficial liquid velocity ranged from 0 to 1.0 m/s and superficial gas velocity ranged from 0.03 to 10.0 m/s. Area-averaged measurements of superficial liquid velocity, superficial gas velocity, and void fraction are used to calculate distribution parameter from a kinematic constitutive equation of the drift-flux model. Results indicate a significant increase in distribution parameter when mixture volumetric flux is relatively low. This observation may be attributed to recirculating flow patterns. An investigation is conducted for existing rod bundle drift-flux correlations because they may not appropriately consider these mechanisms at low liquid flow and low pressure conditions. Results suggest that improvements may be made if the effects of recirculating flow are taken into consideration.