Unsteadiness in Condensing Flow: Dynamics of Internal Flows with Phase Transition and Application to Turbomachinery

Abstract

Steady flows of condensable fluids may become unsteady if one component of the fluid starts to condense. In high-speed expansion flows, typical for large-scale steam turbines, the subcooled vapour state collapses after nucleation, typically in flow regimes close to Mach number 1. After the formation of steady shocks, instantaneous thermal choking initiates selfexcited high-frequency oscillations which is the focus of this article. The driving mechanism is the interaction of compressibility and energy supply in flows close to maximum mass flux density and is therefore not controlled by the viscosity of the fluid. Additional viscosity-driven excitation mechanisms exist and superpose the primary diabatic instability, especially in axial cascades. Typical are shedded shear layers from blade trailing edges and the periodic interference of wakes separating from the stator with the rotor blades. This article presents a review of the authors and various co-workers' research, supplemented by important references to complete the subject under consideration. This article starts with an introduction in the most simple flow model of given heat addition in constant area flow and ends with mixed homogeneous/heterogeneous condensation in a transonic axial cascade stage with a high-resolution sliding interface for preservation of submicron condensate convected from the stator into the rotor. Numerical simulations are compared with experiments of flows with and without carrier gas.

Instability in two-phase flows of steam

“…Some unsteady conditions are experienced in nucleating flows of steam in convergent-divergent nozzles, when the zone of rapid condensation occurs at a short distance downstream of the throat and the pressure rise associated with heat release is of a magnitude to propagate upstream (e.g. Barschdorff 1971;Yousif et al 1972;Saltanov 1976;Skillings & Jackson 1987;White & Young 1993;Adam & Schnerr 1997;Schnerr 2005), but these lead to steady oscillations. The unstable conditions considered in the present paper differ from these in character and form and occur over a much wider range of conditions.…”

Results of the International Wet Steam Modeling Project

“…The pressure rise becomes steeper as a consequence of heat release taking place closer to sonic conditions and eventually a true, aerodynamic shockwave becomes embedded within the condensation zone. Further reduction in inlet temperature results in self-excited oscillations, detailed studies of which have been undertaken by Schnerr 7 (especially for moist air flows) and others. Although these oscillatory regimes are of considerable scientific interest and may well be of practical relevance to turbine flows, they are perhaps less important for initial validation studies and have not been included in the present project.…”

Effects of turbulence and inlet moisture on two-phase spontaneously condensing flows in transonic nozzles

“…Traditionally, two-phase transonic flows in Laval nozzles and turbine blade cascades are modeled in an approximation that ignores the effects of viscosity, thermal conductivity, and turbulence (e.g. [1][2][3][4][5][6]). The disregard of molecular viscosity and conductivity is caused by high flow velocities, at which the molecular mechanisms of momentum and heat transfer are indeed not of primary importance as compared to turbulent transfer mechanisms.…”

“…Predicted (1-3) and measured(4)(5)(6) variations in pressure along the nozzle by Deich and Philippov[19]. (1)and(4)-T 0 = 352 K, X 0 = 0; (2) and (5) -T 0 = 352 K, X 0 = 0.004; (3) and (6) -T 0 = 370 K, X 0 = 0.…”