Investigations on Supersaturation in Nozzle Flow

This thesis is concerned, with attempts to examine the problem of the supersaturated condition in the expansion of steam through simple nozzles. In particular two of the recognised difficulties of the subject are explored. One is the peculiar form of the flow curve originally established by Mellanby and Kerr. The other is the failure of direct temperature measurements in the jet. Part I. of the thesis endeavours to present the somewhat conflicting facts and opinions of the various authorities on the subject and reviews briefly the experimental evidence and the ideas that have been expounded. It emphasises the importance given, In all hypotheses, to the super saturation ratio and the wide and uncertain variations that would appear to have been shown in this value. Part II is wholly devoted to the experimental work which is in two sections, dealing respectively with flow and temperature measurements. In both two types of nozzle were used - convergent and divergent. In the flow and pressure tests the main object was to check the Mellanby and Kerr type of results over a wider range and with variable surface ratio as an additional factor. The temperature tests were based on a new method which gave very striking results. In Part III, the main features of the tests are discussed. The flow curves are analysed by the different methods employed by Mellanby and Kerr and by Powell. It is shown that neither method is satisfactory and that a better agreement is obtained on the assumption of sudden reversion. The results of the temperature investigations are then advanced as proof of sudden reversion and an explanation is given of why, before reversion, the temperature readings are high. The reason for failure in the measurement of undercooled temperature is thus made clear. The supersaturation ratios and velocities at the point of reversion are examined but it is finally concluded that the sudden change to the equilibrium state is really caused by the shock wavs in high speed flow