Transient and steady behavior of an open, symmetrically-heated, free convection loop

Abstract Experiments and analyses are reported for an open, free convection loop. The loop is U-shaped with the lower segment heated; the vertical legs are adiabatic and are connected to an isothermal reservoir. The loop is filled with water or a water-saturated porous medium. Experimental results include the starting transients and the friction factors and heat transfer rates at steady state for flow Reynolds numbers from 4 to 1000. Oscillations are observed with the onset of boiling. Single-phase stability analyses confirm the unstable rest states and the stable steady-states observed in the experiments, and reveal a conditional instability of the steady states. Numerical simulations of the starting transients are obtained and are compared with experiment. Results are applicable to geothermal, solar, and industrial open-loop thermosyphons.