Computational investigation of the dynamic response of a supercritical natural circulation loop to aperiodic and periodic excitations

• Dynamic analysis of a rectangular natural circulation loop with supercritical water.
• Step, ramp, exponential and sinusoidal excitation imposed over steady-state values.
• Exponential variation most stable during both power upsurge and down-surge.
• Longer power transition period preferable for both ramp and exponential profiles.
• Identification of natural frequency of loop under periodic excitation using FFT.

Dynamic response of a supercritical natural circulation loop under transient variation in heater power has hardly received any attention till date. Therefore a computational model of an open rectangular loop working on supercritical water is developed. 1D conservation equations are solved numerically, along with property relations. Steady-state results are validated with literature and grid- and time-sensitivity tests are performed to ensure accuracy of transient solution. Stability threshold is estimated following nonlinear analysis. Step, ramp, exponential and sinusoidal excitations are imposed on the system. Step rise in heater power is found to introduce instability into the system and has the most destabilizing influence. System takes long time to regain steady-state, if the final power is within stability boundary. Ramp and exponential profiles are found to provide favourable response during both power upsurge and down-surge, with the exponential transition being slightly more preferable from stability point of view. Longer period of transition allows the system to suppress unstable fluctuations a better way. Application of sinusoidal transition results in distorted periodic response, attempting to follow the imposed signal only after sufficient time since application of periodic signal. Fast Fourier transform of resultant discrete data series exhibits two distinct peaks, with the larger one corresponding to the imposed sinusoid and the other one signifying the natural frequency of the system.