Effect of unheated rods on subchannel average distribution parameter for steam-water boiling flows in a rod bundle

A subchannel analysis code can compute two-phase flow behavior with a spatial resolution of subchannel size. The subchannel analysis code is a powerful computational tool when the geometric structures of a rod bundle exhibit highly heterogeneous characteristics. Since the computational cell size is the subchannel flow area, the mass, momentum, and energy conservation equations, as well as constitutive equations, should be formulated, considering a proper averaging of the equations. The distribution parameter characterizing the phase and mixture volumetric flux distributions is a key constitutive equation to calculate the relative velocity between gas and liquid phases, which significantly affects the void fraction. Unheated rods, such as water rods, are commonly used in BWR fuel assemblies. The presence of the unheated rods is expected to impact the phase distribution and the distribution parameter and, consequently, the void fraction. Thus, this study modeled the distribution parameters for subchannels containing unheated rods in subchannels. The developed distribution parameter models were validated using NUPEC rod bundle test data. The mean absolute relative deviations between the models and the experimental data were 0.714 %, 0.711 %, and 1.56 % for the subchannels containing one, two, and four unheated rods in subchannels, respectively. The developed distribution parameter models are expected to enhance the reliability of subchannel analysis codes. © 2025 Elsevier Ltd.