Numerical investigation on characteristics of onset of flow instability (OFI) and flow and heat transfer in petal-shape fuel assembly

Onset of Flow Instability (OFI) marks the threshold at which a two-phase flow system transitions from stable to unstable operation. Particularly in narrow flow channels, the premature occurrence of this point will significantly elevate risks of nuclear reactors. Apply the models of Eulerian two-phase flow and the RPI wall boiling, the subcooled flow boiling in the petal-shaped fuel assembly is simulated. This study reveals the mechanism of key operating parameters affect the OFI point, and explains the flow field patterns and heat transfer characteristics near the OFI cases. The result show that when the heating power increases, the critical mass flow rate at the OFI increases, while the effect of inlet subcooling is opposite: the higher the subcooling, the lower the critical mass flow rate. Among all parameters, operating pressure has the most significant influence. An increase in operating pressure reduces the critical mass flow rate at the OFI, but raises the critical void fraction required for instability. Before the OFI, bubbles mainly adhere near the heated wall. After the OFI, the vortex flow rapidly detaches bubbles from the wall and transports them toward the channel center, while bubble accumulation appears in the vortex core region. After the OFI, large-scale vortex structures are generated in the channel, especially in the rear half. These vortices are driven by strong pressure differences and significantly enhance transverse mixing between subchannels, increasing the local cross flow velocity by 200%-300%. The intense mixing effectively promotes heat transfer from the wall to the channel center, resulting in a more uniform temperature distribution across the cross section in the rear half of the channel after OFI, and the wall temperature becomes lower than that under higher flow rate conditions, while the overall convective heat transfer coefficient is improved.