Modeling of dispersed flow film boiling with two flow, five field Eulerian-Eulerian approach and effects of spacer grids on heat transfer

In the case of a postulated loss of coolant accident (LOCA) in a nuclear reactor, an accurate prediction of clad temperature is needed to determine the safety margins. The large break LOCA analyses can be divided in to three time periods. These periods are blowdown, refill and reflood. During the blowdown and reflood phases of the LOCA, when the local void fraction is greater than 80% and the wall is at a temperature above minimum film boiling temperature (Tmin), heat is transferred from the fuel rod to a continuous vapor flow with dispersed droplets. The high void fraction mixture of droplets and vapor provide cooling to prevent the clad temperature from exceeding the safety limit. The heat transfer process for high void fraction mixture is called dispersed flow film boiling (DFFB). This thesis has been modeled DFFB in the reflood phase of a LOCA in a pressurized water reactor (PWR) rod bundle.;In this study, the modifications and modification requirements for the COBRA-TF code to obtain a five field Eulerian - Eulerian modeling for two-phase DFFB is described. COBRA-TF is a best estimate code developed for the rod bundle analysis and has four fields, namely, vapor, entrained drop and continuous liquid film. COBRA-TF has a detailed reflood package which takes effect of spacer grids on heat transfer into account. This study has a detailed description of code's solution scheme and the models used for dispersed flow film boiling.;The dispersed flow film boiling heat transfer model of the COBRA-TF code has been modified by adding a small droplet field to the code as the fifth field. The effect of smaller, thermally more active droplets on heat, mass and momentum transfer during DFFB has been modeled.;Since the large drop break up due to spacer grids is one of the reasons for small droplet generation, the spacer grid models of the COBRA-TF have been revised and modified. In addition to small droplet generation, the spacer grid rewet is an important aspect of heat transfer during DFFB. Since wet spacer grids provide a large interfacial area for the heat transfer between the superheated vapor and the liquid deposited on the spacer grid, the grid rewet has been modeled and effect of wet grid on heat transfer has been investigated. (Abstract shortened by UMI.)...