Flow Instability and Bifurcation in Gas-Loaded Reflux Thermosyphons

In vertical gas-loaded two-phase reflux thermosyphons, the temperature and concentration gradients between the active and shut-off regions can create double-diffusive mixed convection flows. When the noncondensable gas molecular weight is greater than that of the vapor, different steady and time-dependent gas recirculation patterns develop, similar to those observed during Rayleigh-Be´nard convection in vertical slots. Experimental results presented here show that bifurcations from axisymmetric to three-dimensional, then asymmetric, and finally multiple distinct gas-flow patterns occur as the Rayleigh number increases. Ultimately, a sudden transition to a large-amplitude oscillatory gas flow takes place. Detailed concentration maps and power-density spectra from point wet-bulb temperature measurements clearly chart these transitions. These results impact the design of many condensing systems where noncondensables are present.