Thermohydraulic instability of boiling natural circulation loop with a chimney. (Part I. Linear stability analysis using homogeneous two-phase flow model and experiment on thermohydraulic instability induced by flashing)

Instability of a boiling natural circulation loop with a chimney at low pressure and low heater power was investigated by linear stability analysis and experiment. A homogeneous and thermodynamic equilibrium model for two-phase flow was used. The effect of flashing induced by pressure drop in the heated channels and the chimmey was considered. The effects of coupling between two boiling channels were investigated. It was found that in-phase-mode instability was apt to occur when channel inlet subcooling was large and boiling began in the chimney. In-phase-mode instability easily occurred when channel length was shortened and chimney length was increased. Out-of-phase-mode instability was apt to occur when chimney length was decreased and boiling began in the channel.