Wall-to-vapor convective heat transfer is the dominant heat transfer mechanism in the reactor core during the blowdown phase and the early stage of reflooding phase in a postulated large-break loss-of-coolant-accident (LB LOCA). Experiments on single-phase convective heat transfer to vapor were conducted in partially blocked rod bundles of 90% flow blockage ratio. The experimental results showed increase in local heat transfer just downstream of the blockage caused by flow separation phenomenon. The single-phase heat transfer enhancement models implemented in the COBRA-TF code underpredicted the local heat transfer in the downstream region of the flow blockage. Hence, a new correlation was proposed to improve the prediction capability of the COBRA-TF models by more accurately describing the flow separation effect. The new correlation predicted well various sets of experimental data within 20% discrepancy.

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