Nucleation of steam in high-pressure nozzle experiments

Abstract

The nucleation characteristics of high-pressure saturated/subcooled steam were studied in Laval nozzles. By using nozzles designed for different expansion rates and varying the inlet stagnation state in wide limits, the Wilson lines and the fog structure (droplet size and number count) were determined between 0.5 and 5 MPa pressure for expansion rates ranging between 10 000 and 200 000 s−1. The results show that Wilson point nucleation typically occurs in such fast expansions at supersaturated conditions where 3.5-5 per cent moisture would be present in the case of equilibrium flow. Higher expansion rates entail higher supersaturation, causing the formation of smaller, but more numerous, fog droplets. The reported measurements were made in the early seventies and were originally evaluated on the basis of the IFC-67 Steam Tables. The present evaluation uses the new IF-97 equations and reveals significant differences between the old and the new steam tables, leading to severe discrepancies in the value of subcooling at the Wilson point.