Visual identification of two-phase flow patterns in adiabatic air-water flow in porous media using a refractive index matching technique

Understanding two-phase flow patterns in porous media is essential for estimating interfacial friction, and consequently, the coolability limit of debris bed - dry-out heat flux - in the event of a severe accident in nuclear power plants. The lack of clear visual data on two-phase flow in porous media led to the development of multiple maps to identify flow patterns based on void fraction and particle size. Hence, in this paper, to visualize two- phase flow patterns, adiabatic air/water experiments were conducted with a refractive-index matching tech-nique and high-speed photography. With single-point injection experiments, multiple distinguishable flow pat-terns, like, small distorted bubbles, Y-shaped bubbles, multi-dimensional slugs, and laterally interacting channels, were observed with an increase in air flow rate. Subsequently, the top flooding approach has been adopted for simultaneous estimation of void fraction and visualization of flow patterns. At a high void fraction (>0.75), laterally interacting multidimensional slugs were observed throughout the bed, while at lower void fraction (0.36-0.75), various flow patterns co-existed in different channels of the porous bed with 10 mm hydrogel particles. This investigation highlights the presence of complicated flow patterns at different void fractions, underlining the necessity of considering these identified flow patterns to reduce uncertainty in models estimating coolability