Numerical investigation of the microdynamics of the initiation of motion and startup transients in a supercritical natural circulation loop

Initiation of motion in a supercritical natural circulation loop is explored over a wide range of heater power, in an endeavor to correlate the inherent microdynamics with the regimes of heat transfer. It is impossible to predict the first direction of bulk movement, as well as the time required to attain that from quiescent condition, in a symmetric orientation, necessitating comprehensive computation. Development of recirculation vortices at both ends of the heating section, owing to the discontinuity in thermal boundary condition there, is earmarked as the first reason of instigating motion. Temperature of supercritical fluid outside the vortex fronts is found to increase uniformly till the merger of both the vortices, and it is accredited to the appearance of piston effect. Realization of such adiabatic heating in closed loops is a novel identification. Rayleigh -Taylor -type instability appears interior to the heater, resulting in chaotic patterns in the fluid segment entrapped between the two propagating fronts, forcing the system to deviate from symmetry, and accordingly enforcing the first direction of motion. Adiabatic heating is more prominent at lower powers, while the greater rate of temperature rise at higher powers allows an early dominance of buoyancy, with the thermodynamic state of the system quickly veering away from the critical point. Every loop undergoes a pair of flow reversals following the first motion to adapt to the high level of initial buoyancy and large circulation rate. While the system promptly achieves a steady-state at lower powers, it may continue to exhibit chaotic oscillations with repetitive flow reversals at higher powers, without attainment of any steady-state.