Investigations on two-phase flow resistances and its model modifications in a packed bed

During severe core melting accidents of light water reactors with failures of all cooling systems, the molten core fuel (corium) would meet and interact with residual coolant water (FCI), then break up and fragment into a porous debris bed. Therefore, the debris bed coolability depending mainly on the flow friction laws, will be in great significance to nuclear reactor safety. Motivated by reducing the uncertainties in debris coolability assessment, this paper reports an experimental study on two-phase flow resistance in porous packed beds, and a modified model is proposed to predict the pressure drops of two-phase flow through packed beds with coarse particles. The tests are performed on the DEBECO-LT facility which is constructed to investigate the friction laws of adiabatic single and two-phase flow in a particulate bed. The spherical particles of 1.5 mm, 2 mm, 3 mm, 4 mm, 6 mm and 8 mm in diameter are packed separately in the cylindrical test section with the inner diameter of 120 mm and the height of 600 mm. The quality of experimentation and instrumentation can be ensured by the good agreement between the measured pressure drops of single phase flow tests and the predictions of Ergun equation. Then air-water co-current two-phase flow tests are conducted to investigate the flow resistance characteristics, providing the basic data for verifying, analyzing and modifying the existing models. The results show that: 1) when two-phase fluids flow upward through the porous beds packed with the smaller particles (e.g. 1.5 mm and 2 mm), the flow resistance increases gradually with the fluid flowrate, and the predictions of Reed model are more comparable with the measured pressure drops. While for the beds with larger size parties (e.g. 4 mm, 6 mm, and 8 mm), the flow resistances of two phase flow show a down-up tendency and hardly be predicted well by previous models; 2) it is believed that the interfacial drag plays non-ignorable role to the flow resistances for the beds packed with larger size particles, and should be considered specifically in the analysis models; 3) A modified model is proposed to predict the two-phase flow resistance in the packed bed with coarse particles. Compared with the previous models, the predictions of modified model show a favorable agreement with the experimental data under different conditions.