Experimental Study of Post-Dryout Heat Transfer in a Tight-Lattice 3-Rod Bundle

Fuel with a tight lattice structure in the reactor core is an important design direction for high-performance water reactors. Due to the dispersed flow characteristic, research on post-dryout heat transfer is limited. However, a better understanding of post-dryout heat transfer characteristics under accident conditions is significantly important for fuel design and safety analysis. This study experimentally investigates the characteristics of post-dryout dispersed flow heat transfer in a 3-rod tight-lattice bundle with a pitch-to-diameter ratio of 1.2. The working conditions are as follows: system pressure ranging from 6 to 10 MPa, mass flux between 65 to 200 kg/(m2s), and heat flux varying from 75 to 200 kW/m2. Circumferentially non-uniform heat transfer is obviously observed. The wall temperature is higher in the narrow gaps between rods, while lower in the vicinity of the subchannel center. The specific mechanisms of the above phenomena are analyzed. Parametric effects on post-dryout heat transfer are discussed and illustrated. Using the experimental data, commonly utilized correlations for transition boiling and film boiling are evaluated. In order to improve the prediction accuracy, new heat transfer correlations for transition boiling and film boiling in the tight-lattice under low mass flux and low heat flux are developed based on the experimental data and mechanistic analysis.