Stability characteristics of a single-phase free convection loop are presented. In the experiments, water was placed inside a toroidal glass loop oriented in a vertical plane. The lower half of the loop was heated and the upper half was cooled. At low heat-transfer rates and also a t high heat-transfer rates the free convection flow was observed to be steady. For the intermediate range, however, the flow was found to be highly oscillatory. Stability predictions are also developed. The comparison between theory and experiment yields favourable agreement.Observations of unstable behaviour have been reported previously for single-phase fluids in the vicinity of the thermodynamic critical point. In these situations it has been assumed that the unusual behaviour of the fluid properties in the near-critical region necessarily constitutes the underlying cause of such instabilities. In contrast t o this view, analyses by Keller (1966) and Welander (1967) indicate that instabilities can occur for ordinary fluids as well. Results of the present study confirm this contention, since instabilities were clearly observed for water at atmospheric pressure and moderate temperatures.

A toroidal thermosyphon consisting of a fluid-filled torus located in a vertical plane was studied analytically and experimentally. Good agreement was obtained between analytical predictions and measurements for large values of the angular displacement of the heated and cooled sections. For smaller angular displacements, the analytical predictions of steady state flow rates were found to exceed the corresponding experimentally observed values. The discrepancies were attributed to a reverse flow phenomenon. Some analytically predicted flows were not physically achievable. In these situations the flow would either reach a steady condition in the opposite direction, or it would oscillate indefinitely.

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