Stability performance of series coupled natural circulation system with different operating procedures

Series coupled natural circulation system with the last loop being an open-air loop finds application as an infinite mission time passive decay heat removal system in nuclear power plants and solar thermal systems. Hence the stability behavior of series coupled rectangular loops with horizontal heater and horizontal cooler orientation was experimentally studied. The loop was subjected to various operating procedures viz., start up from rest, power raising from a stable steady state and power step down from an unstable state to witness that the stability threshold was not a unique value, thus confirming the existence of a hysteresis phenomenon. The stability threshold enhancement of the system with orifice plates was maximum for the lowest orifice diameter. The coupled system was chosen with an unstable first loop and the instability was found to transmit from loop#1 to the subsequent loops. However, the oscillation amplitude was highly attenuated and seemed to be nearly stable in the 2nd and 3rd loop without any flow reversals even when the first loop was unstable with chaotic bidirectional pulsing. In case of bi-directional pulsing in loop#1, the heat exchanger between the first two loops operated alternatively in co-current and counter-current mode with no change in the flow direction of loop#2. A delay in the flow initiation was found for start-up from rest, which increased from the first loop to the last loop and decreased with the increase in power input. Further, the numerical simulation with the one-dimensional model reproduced the oscillatory behavior in loop#1, with the predicted stability threshold being considerably lower compared to the experiments. The attenuation of the oscillations in the loop#2 as observed in the experiments could be reproduced. Predicted transients showed practically no oscillations in loop#3 corroborating the experimental observations. The attenuation of the oscillations in loop#2 and the absence of oscillations in loop#3 were attributed to the damping caused by the wall and also due to the higher Lt/D ratio in loop#2 and a vertical air flow channel in loop#3 respectively. The phase plots indicated a near periodic oscillation compared to the highly chaotic pattern in the experiments, presumably due to the higher power at which instability was observed in the experiments. The deviations were also due to the secondary flows being generated in the large diameter loop as the simulation was based on 1D along with the heat loss being neglected.