Some recent analytical results for chugging phenomena

Depending mainly on the temperature of the pool and the steam mass flux into it, there are different modes of condensation phenomena. Especially in a later blow-down period with practically pure steam flow the socalled chugging mode occurs. This condensation mode is characterized by high pressure peaks in the pool due to collapsing steam bubbles in the vicinity of the vent pipe outlets, where bubble simply means the steam volume surrounded by the pool fluid. According to experiments in this field chugging obviously is a stochastic process although governed by a quasi-periodic motion of the pool fluid. The reason for this triggering is the change of kinetic and potential energy of the pool fluid. In trying to attack the chugging phenomenon analytically one is faced with the problem to find a hypothesis about the physical effect that initializes chugging. Starting with the deterministic model KSWING-IV the improved model KSWING-V was developed. In this model the hypothesis for bubble collapse is based on the combined action of three instability effects: Turbulent motion of the steam-pool-interface, which is increasing with the growing bubble, this may be referred to as micro-instability. The possibility of spontaneous condensation, the probability of which increases with higher turbulence, this is a thermodynamic instability. The existence of an instable state in the sense of TAYLOR-PLESSET, which will magnify the perturbation caused by spontaneous condensation, and therefore it may be regarded as macro-instability. The chugging model KSWING-V was mainly tested against the GKSS-M1-test and showed good agreement. (orig./HP)