Investigation of grid-enhanced two-phase convective heat transfer in the dispersed flow film boiling regime

A two-phase dispersed droplet flow investigation of the grid-enhanced heat transfer augmentation has been done using steam cooling with droplet injection experimental data obtained from the Penn State/NRC Rod Bundle Heat Transfer (RBHT) facility. The RBHT facility is a vertical, full length, 7 × 7-rod bundle heat transfer facility having 45 electrically heated fuel rod simulators of 9.5 mm (0.374-in.) diameter on a 12.6 mm (0.496-in.) pitch which simulates a portion of a PWR fuel assembly. The facility operates at low pressure, up to 4 bars (60 psia) and has over 500 channels of instrumentation including heater rod thermocouples, spacer grid thermocouples, closely-spaced differential pressure cells along the test section, several fluid temperature measurements within the rod bundle flow area, inlet and exit flows, absolute pressure, and the bundle power. A series of carefully controlled and well instrumented steam cooling with droplet injection experiments were performed over a range of Reynolds numbers and droplet injection flow rates. The experimental results were analyzed to obtain the axial variation of the local heat transfer coefficients along the rod bundle. At the spacer grid location, the flow was found to be substantially disrupted, with the hydrodynamic and thermal boundary layers undergoing redevelopment. Owing to this flow restructuring, the heat transfer downstream of a grid spacer was found to be augmented above the fully developed flow heat transfer as a result of flow disruption induced by the grid. Furthermore, the presence of a droplet field further enhanced the heat transfer as compared to single phase conditions. From the RBHT steam cooling with droplet injection data, it was found that a second-stage augmentation occurs under wet grid conditions at a distance of approximately 10 diameters downstream of the grid. This second-stage augmentation, which is a direct consequence of a sharp increase of the droplet interfacial area due to the breakup of liquid ligaments downstream of the grid, was not observed under dry-grid conditions nor was it observed in single-phase steam cooling tests. It was also found that the general practice of classifying a grid as wet or dry based solely on the thermocouple temperature is insufficient, as a large scatter was observed in the data.