An experimental study of the performance of an air-to-air, multi-loop, two-phase thermosiphon heat-exchanger system, utilizing R-11 as the working fluid, has been carried out to investigate the effect of its previous thermal history for hot air to cold air temperature differences up to 50 Celsius degrees. Tests were run on a four loop system with equal face velocities of 1.3, 2.2 and 3.1 m/s at each coil and on a single loop system with a face velocity of 2.2 m/s.

An hysterisis loop was found to exist on each plot of heat-exchanger effectiveness vs . overall temperature difference for temperature differences below 45 Celsius degrees. Boiling was not initiated until a difference of 13 degrees was reached. However, once initiated, boiling was not completely stopped until the temperature difference dropped below 4 degrees. The shape of the two bounding curves which form these hysteresis loops could be approximated very well by exponential curves.

Units: SI