RECOMPRESSION PHENOMENA IN STEAM NOZZLES: PART I

Abstract

The purpose of the investigation was to find when and how recompression occurs in the flow of steam through a nozzle, its causes and effects, how the results can be predicted, and what losses are caused by these phenomena. Four types of recompression were observed: equilibrium, latent, vena contracta, and shock recompression.It is shown that steam in flowing through a nozzle of varying cross-section responds to the changes of area within certain limits and recompression phenomena may be expected. The Venturi effect has been observed in single nozzles, and in both parts of a composite nozzle.The behaviour of steam in equilibrium recompression in convergent-divergent nozzles can be predicted by means of an equation from which a valuers obtained for the pressure pr at which overexpansion ends and an increase in pressure begins; the pressure pr depends on the inlet pressure to the nozzle and on the difference in pressure at inlet and outlet.The recompression due to the contraction of the section can be controlled and avoided by suitable provision in the design. The compression following a change of state of a flowing fluid can be controlled by adjusting the rate of expansion of the fluid in specified pressure ranges. Novel evidence of latent recompression is found in a break or notch in the pressure expansion curve plotted from search tube observations.The experiments were carried out at inlet pressures within the range at which turbine-condition curves may cross the saturation line between the superheat and wet regions on the Mollier diagram, and the effects of recompression to be expected under varying load conditions can be interpreted for particular nozzle designs.Comparative studies of the effect of the nozzle shape on recompression phenomena and the losses resulting therefrom have indicated that the conditions for favourable efficiency require a larger inlet radius at the throat and a length shorter than the lengths commonly found in practice.