A one-dimensional model is derived for natural convection in coupled loops. The physical model can be reduced to two coupled sets of ordinary differential equations of the Lorenz type. Experiments are performed to validate the model. In general, the phenomena observed in the experiments#x2014;mnamely, heat conduction, cocurrent and countercurrent steady flow, and irregular time-dependent flow, as well as the related stability bounds#x2014;are predicted reasonably well by the mathematical model. The experiments, moreover, confirm the existence of a subcritical parameter range, which is predicted by the model to alternatively exhibit steady stable as well as time-dependent behavior.
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