Experimental investigation of parallel-walled and divergent-walled open thermosyphons

Abstract Natural convection experiments are performed for plane-walled, slot-like open thermosyphons having either parallel principal walls or divergent principal walls. The fluid in the thermosyphon and its surroundings is water ( Pr ≅ 5). When the two principal walls are heated and maintained at the same uniform temperature which exceeds that of the surroundings, unstable operation is encountered. The instability is believed due to inherent obstructions to the transport of cool fluid from the surroundings to the thermosyphon, such transport being needed to make up for the hot fluid discharged from the thermosyphon to the surroundings. Steady-state Nusselt number results are obtained for the one-sided-heated thermosyphon. For the parallel-walled case, a universal Nusselt number correlation is achieved as the 0.25 power of the product of the inverse aspect ratio and the Rayleigh number. This correlation yields heat transfer coefficients that are independent of the interwall spacing. Furthermore, aside from a 7% difference in a multiplicative constant, the correlation is identical to that for natural convection in a one-sided-heated vertical channel open at both the top and bottom. Divergence of the principal walls of the one-sidedheated thermosyphon has only a small effect on the Nusselt number, with departures of no more than 5% from those for the parallel-walled case.