Visualization of unsteady gas/vapor expansion flows

Abstract

High speed expansion flows of pure vapors or gas/vapor mixtures are important to many technical applications, e.g. to steam turbines, jet engines, and for safety control of pressurized power plants. The sudden cooling of the fluid flow leads to condensation and nonequilibrium two-phase flow with instabilities and periodic shock formation at mean frequencies of about 1 kHz. Modelling and control of this dynamical problem is not only important with respect to erosion, it also may cause flutter excitation and serious demolition of technical facilities. In numerical simulations, the time dependent 2-D Euler equations coupled to four equations describing the process of homogeneous nucleation and droplet growth are solved by a MUSCL-type finite volume method. The results are compared with experiments carried out in an atmospheric supersonic wind tunnel. By application of this numerical method to internal flows (nozzles) we found different modes of instabilities including bifurcations. At the stability limit a sharp frequency minimum was found for symmetric oscillations in slender nozzles. It separates oscillation modes where the oncoming subsonic flow remains unchanged from the oscillatory state where a shock monotonically moves upstream into the oncoming flow. For different nozzles we detected a new unsymmetric oscillation mode with a complex system of upstream moving oblique shocks. Here the frequency curve shows the typical structure of a bifurcation problem, which is definitely not controlled by viscous effects but by instabilities of the interaction of flow and phase transition process.