Thermal Hydraulic Flow Oscillation Characteristics in Multiformed Channels under Natural Circulation and Low-Pressure Conditions

Thermal hydraulic instabilities in vertical multicombined channels under natural circulation conditions have been experimentally investigated. The multicombined channels consist of two parts; a nonheated single-channel section installed downstream and a heated parallel-channel section installed upstream. The length of each channel section could be varied to investigate the flow oscillation characteristics of the instabilities. The flow oscillation modes at the parallel-channel section could be roughly classified into three different types according to the length ratio of each channel, such as in-phase (natural circulation oscillation), out-of-phase (geysering, density wave oscillation), and intermediate-phase. The objective of this study is to experimentally clarify the transport mechanisms of those flow oscillations. The resulting geysering could be further classified into Griffith's type and Aritomi's type according to the difference in input thermal conditions. Especially in case that the length of the single section was almost equal to that of the parallel section, the intermediate-phase flow oscillation was observed. The intermediate-phase flow oscillation was basically the out-of-phase flow oscillation due to the pressure fluctuation in the parallel channels, but the range of the flow oscillation periodically increased or decreased in a certain width due to the hydrostatic head fluctuation in the single channel.