Numerical Investigation on Two-Phase Water Density Wave Oscillations in a Pipe under Various Heating Conditions

This study utilizes the finite volume method with a two-fluid model to analyze water characteristics in a boiling channel when density wave oscillation occurs. A 2-D numerical model is utilized to characterize both axial and radial mass and energy exchange, as well as the mechanisms involving flow and heat transfer during density wave oscillation under uniform and non-uniform heating conditions. The numerical model was used to predict the heating power threshold values of two-phase water flowing upward in a vertical pipe under type II density wave oscillation at the working pressure and an inlet–outlet pressure difference of 125 kPa, when the average heating power increased from 44.06 kW/m2 at a speed of 0.5 kW/(m2·s). The flow and heat transfer characteristics before and after the type II density wave oscillation were determined and compared to the flow and temperature fields at various moments. Along the pipe axis, the wall heat flux was loaded under uniform, standard-sine, and inverted-sine types, which represent three typical types of wall heat flux arrangements. Flow oscillation appeared the earliest under the inverted-sine-type heat flux with low vapor quality (approximately 0.13) with a void fraction of 0.6 at the outlet.