The Loss of Coolant Accident (LOCA) is a design basis accident for light water reactors that usually determines the limits on core power. During a LOCA, film boiling is the dominant mode of heat transfer prior to the quenching of the fuel rods. The study of film boiling is important because this mode of heat transfer determines if the core can be safely cooled. One important film boiling regime is the so-called Inverted Annular Film Boiling (IAFB) regime which is characterized by a liquid core downstream of the quench front enveloped by a vapor film separating it from the fuel rod. Much research have been conducted for IAFB, but these studies have been limited to steady state experiments in single tubes.In the present work, subcooled and saturated IAFB are investigated using high temperature reflood data from the experiments carried out in the Rod Bundle Heat Transfer (RBHT) test facility. Parametric effects of system parameters including the pressure, inlet subcooling, and flooding rate on the heat transfer are investigated. The heat transfer behavior during transition to Inverted Slug Film Boiling (ISFB) regime is studied and is found to be different than that reported in previous studies. The effects of spacer grids on heat transfer in the IAFB and ISFB regimes are also presented. Currently design basis accidents are evaluated with codes in which heat transfer and wall drag must be calculated with local flow parameters. The existing models for heat transfer are applicable up to a void fraction of 0.6, i.e. in the IAFB regime and there is no heat transfer correlation for ISFB. A new semi-empirical heat transfer model is developed covering the IAFB and ISFB regimes which is valid for a void fraction up to 90% using the local flow variables. The mean absolute percentage error in predicting the RBHT data is 11% and root mean square error is 15%. This new semi-empirical model is found to compare well with the reflood data of FLECHT-SEASET experiments as well as data from single tube experiments. The root mean square error in predicting the FLECHT-SEASET data is 20% whereas for single tube data it is 12%.In previous studies, the transition criterion from the IAFB to the ISFB regime is purely empirical. In this work, a theoretical stability analysis of a liquid jet co-flowing with its vapor in a tube is carried out to seek a better understanding of the underlying physics of the regime transition. The effect of heat and mass transfer at the interface is included in the stability analysis. Also, the effect of viscous force is included in the stability analysis, by employing the viscous potential flow method. The wavelength that is responsible for breakup of the liquid core in IAFB is predicted in the present analysis and is compared with the adiabatic experiments of IAFB from the literature. The effects of various controlling parameters including the relative Weber number, vapor Reynolds number, velocity ratio, density ratio and viscosity ratio of vapor and liquid are studied to understand the physics of transition. Finally a physics-based heat transfer model is proposed for heat transfer in the ISFB regime using the wavelength obtained from the stability analysis.

Post-dryout heat transfer is an important phenomenon in the analysis of nuclear fuel rods during a hypothetical loss of coolant accident (LOCA). Heat transfer in transition boiling increases as intermittent wetting of the heated surface takes place. In film boiling, a vapor film covers the heated wall and prevents direct contact heat transfer from the liquid to the rod. The boundary between these two heat transfer regimes occurs at Tmin, the minimum film boiling temperature, sometimes called the Leidenfrost point. Understanding the sensitivity of Tmin to pressure, subcooling, peak power, surface conditions and flow rates is critical because Tmin determines when the vapor film breaks, and the heat transfer significantly improves. In this work, the minimum film boiling temperature has been determined for different ranges of pressure, subcooling, flow rate and peak power for various flooding rate tests. Pressure, liquid subcooling and flow rate were varied from 0.14 MPa to 0.42 MPa (20 to 60 psia), 5 K to 83 K (9 to 150F), 0.076 m/s to 0.2032 m/s (3 to 8 in/s), and 1.31 kW/m to 2.3 kW/m (0.4 to 0.7 kW/ft) respectively with the peak power varying from 1.31 kW/m to 2.3 kW/m (0.4 to 0.7 kW/ft) and the initial peak rod temperature varying from 1033 K to 1144 K (1400 to 1600 F). The experiments were performed using an electrically heated 77, 3.66 m (12 ft) Inconel rod bundle array with constant and variable flooding rate capabilities at the NRC-PSU Rod Bundle Heat Transfer (RBHT) facility. The latter was also equipped with seven mixing vane spacer grids with which to study the effects on Tmin. Data reduction was performed by using an inverse heat conduction code to calculate the surface temperature and heat flux. The results indicate that the system pressure, inlet subcooling, flooding rate and spacer grids have appreciable impact on the minimum film boiling temperature. The experimental results presented in this work can be used to determine the transition boiling regime in the limit of TCHF

Inverted annular flow can be visualized as a liquid jet-like core surrounded by a vapor annulus. While many analytical and experimental studies of heat transfer in this regime have been performed, there is very little understanding of the basic hydrodynamics of the post-CHF flow field. However, a recent experimental study was done that was able to successfully investigate the effects of various steady-state inlet flow parameters on the post-CHF hydrodynamics of the film boiling of a single phase liquid jet. This study was carried out by means of a visual photographic analysis of an idealized single phase core inverted annular flow initial geometry (single phase liquid jet core surrounded by a coaxial annulus of gas). In order to extend this study, a subsequent flow visualization of an idealized two-phase core inverted annular flow geometry (two-phase central jet core, surrounded by a coaxial annulus of gas) was carried out. The objective of this second experimental study was to investigate the effect of steady-state inlet, pre-CHF two-phase jet core parameters on the hydrodynamics of the post-CHF flow field. In actual film boiling situations, two-phase flows with net positive qualities at the CHF point are encountered. Thus, the focus of the present experimental study was on the inverted bubbly, slug, and annular flow fields in the post dryout film boiling region. Observed post dryout hydrodynamic behavior is reported. A correlation for the axial extent of the transition flow pattern between inverted annular and dispersed droplet flow (the agitated regime) is developed. It is shown to depend strongly on inlet jet core parameters and jet void fraction at the dryout point. 45 refs., 9 figs., 4 tabs.