Analysis and simulation of startup transient in two-phase natural circulation loop

Abstract

The main objective of this research is to analyze and simulate the startup transient in two-phase natural circulation loop. Two rectangular loops have been designed and constructed to simulate a two-phase flow under two different configurations. Fast Fourier Transform (FFT) is used to analyze the oscillation of unstable temperature and differential pressure. A computer program, TEXAS, has been modified to simulate the two-phase flow in the rectangular natural circulation loop and compared with the experimental data. The same water temperature difference across the heater was measured both when the cooling system was turned on and turned off. The temperature oscillation was observed in both loops under unstable boiling condition. The temperature oscillation was due to two factors. The first factor was the presence of the horizontal tube. The second was the flow instabilities, known as geysering and flashing-induced density wave oscillation, which occurred when water flowed from small channel to large channel in the heated section. The frequency of the temperature oscillation at the condenser outlet increased when the heat flux increased. The results from the computer simulation agreed with the experimental results. However, the simulation had some limitation, and still required further modification.