Investigations on Slug Flow in a Horizontal Pipe Using Stereoscopic Particle Image Velocimetry and CFD Simulation With Volume of Fluid Method

Abstract

Investigations on gas-liquid flows in horizontal pipes are of immanent importance for Reactor Safety Research. In case of a breakage of the main cooling circuit of a Pressurized Water Reactor (PWR), the pressure losses of the gas-liquid flow significantly govern the loss of coolant rate. The flow regime is largely determined by liquid and gas superficial velocities and contains slug flow that causes high-pressure pulsations to the infrastructure of the main cooling circuit. Experimental and numerical investigations on adiabatic slug flow of a water-air system were carried out in a horizontal pipe of about 10 m length and 54 mm diameter at atmospheric pressure and room temperature. Stereoscopic high-speed Particle Image Velocimetry in combination with Laser Induced Fluorescence was successfully applied on round pipe geometry to determine instantaneous three-dimensional water velocity fields of slug flows. After grid independence studies, numerical simulations were run with the open-source CFD program OpenFOAM. The solver uses the VOF method (Volume of Fluid) with phase-fraction interface capturing approach based on interface compression. It provides mesh refinement at the interfacial area to improve resolution of the interface between the two phases. Furthermore, standard k-ε turbulence model was applied in an unsteady Reynolds averaged Navier Stokes (URANS) model to resolve self-induced slug formation. The aim of this work is to present the feasibility of both relatively novel possibilities of determining two-phase slug flows in pipes. Experimental and numerical results allow the comparison of the slug initiation and expansion process with respect to their axial velocities and cross-sectional void fractions.

Numerical simulation of evaporating two-phase flow: application to concentrated solar plants with direct steam generation

“…Although two-phase slug flows were extensively studied experimentally, their complexity and the computational costs made that only few studies were conducted to simulate 3D slug flows in horizontal tubes, and even fewer have compared it to experimental data [7][8][9]. It can be shown using an instability analysis based on the Kelvin-Helmholtz theory [1], that waves are formed when the velocity difference between liquid and vapor exceeds a calculated value.…”