Lokanath Mohanta

Experimental Investigation of Inverted Annular Film Boiling in a Rod Bundle during Reflood Transient

Abstract Heat transfer results for subcooled and saturated inverted annular film boiling (IAFB) obtained from a 7×7 rod bundle during transient reflood are presented in this paper. The test section consists of heater rods of 9.5-mm diameter and 12.6-mm pitch arranged in a square array. Flooding rates considered are 0.076 and 0.152 m/s, pressure varied from 138 to 414 kPa, and inlet subcooling up to 83 K. Evaluation of the data includes estimation of the local void fraction and Nusselt number during IAFB as well as in the inverted slug film boiling (ISFB) regime, which occurs when the inverted annular liquid column disintegrates. Experimental heat transfer results are compared with several film boiling models, and a new correlation for the Nusselt number is proposed for the IAFB and ISFB regimes. Predicted Nusselt numbers using the new correlation deviate from the experimental data by an average error of 15% and root-mean-square error of ∼30%.

Experimental Studies of Spacer Grid Thermal Hydraulics in the Dispersed Flow Film Boiling Regime

Abstract Spacer grids have been found to enhance downstream convective heat transfer and to strongly influence droplet size distributions through early spacer grid rewet and droplet breakup. Existing models for enhancement of heat transfer and droplet breakup, however, do not appear to accurately account for these interactions between the coolant and the spacer grid. Data from two series of rod bundle heat transfer tests, low injection rate forced reflood tests, and droplet injection tests are presented in this paper to describe the effects of the spacer grids during dispersed flow film boiling. Heat transfer downstream of the spacer grids is clearly enhanced by the presence of the droplets, while the downstream droplet size was found to depend on the condition of the spacer grid: dry or wetted. Results of this study demonstrate the need to adequately account for the separate modes of dry and wet spacer grid heat transfer enhancement in predicting the thermal-hydraulic behavior during reflood transients.