Steam Flow in Nozzles: Velocity Coefficient at Low Steam Speeds

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Abstract
A considerable amount of controversy was raised by one aspect of the results reported by the Steam Nozzles Research Committee of the Institution over the years 1923–30. This was concerned with the variation of the velocity coefficient of a steam nozzle at low steam speeds, the Committee's results showing a tendency to higher values of coefficient at very low speeds. The work described in the present paper was undertaken to provide additional experimental evidence on this point and, in order to be comparable with that of the Committee, was carried out on apparatus employing the same general principles, but embodying certain improvements in detail. The impulse plate method was used in determining the steam velocity at the exit from the test nozzle. The apparatus was designed on a small scale for use with a small self-contained steam generating unit, and particular care was accordingly given to accuracy in all the necessary measurements. The design eliminated frictional effects in the determination of the force on the impulse plate, and tests could be carried out with wet steam as well as superheated. Two series of tests were made, one with steam superheated after expansion and the other with expansion into the supersaturated region. The exit velocity varied from about 500 to 1,200 ft. per sec., the final pressure being nominally atmospheric in all cases. The results showed that the velocity coefficient decreased continuously from the maximum to the minimum steam velocity, and that the values of the coefficient were substantially the same for the two series at corresponding speeds. This result is not in agreement with the Steam Nozzles Research Committee's work, but confirms the conclusions of other investigators in this field.

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