Abstract: This computational study investigates the nonlinear dynamics of unstable convection in a 2D thermal convection loop (i.e., thermosyphon) with heat flux boundary conditions. The lower half of the thermosyphon is subjected to a positive heat flux into the system while the upper half is cooled by an equal-but-opposite heat flux out of the system. Water is employed as the working fluid with fully temperature dependent thermophysical properties and the system of governing equations is solved using a finite volume method. Numerical simulations are performed for varying levels of heat flux and varying strengths of gravity to yield Rayleigh numbers ranging from 1.5 × 10^2 to 2.8 × 10^7. Simulation results demonstrate that multiple regimes are possible and include: (1) conduction, (2) damped, stable convection that asymptotes to steady-state, (3) unstable, Lorenz-like chaotic convection with flow reversals, and (4) high Rayleigh, aperiodic stable convection without flow reversals. Delineation of the various flow regimes, as characterized by the temporal evolution of bulk mass flow rate, is obtained in terms of heat flux, gravity, and the Rayleigh number. Temporal frequencies of the oscillatory behavior and residence time in a circulatory direction are explored and described for the various thermal and gravitational forcing (Rayleigh number) applied to the system.
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