Joost J. Brasz

Analysis of Jet/Wake Mixing in a Vaneless Diffuser

A time-dependent, three-dimensional Navier-Stokes algorithm has been developed to study the unsteady flowfield interaction in the impeller and vaneless diffuser of a centrifugal compressor. This study describes the flow phenomena taking place in the entry region of a vaneless diffuser receiving the distorted impeller discharge flow. In the diffuser entry region, the initial dynamic variations in the velocity and flow angle (caused by the so-called jet/wake flow profile of the fluid leaving the impeller passages) mix out quickly. It was also found that the steady-state distortion in the hub-to-shroud plane, with higher through-flow velocities at the hub than at the shroud, persisted over a large area of the diffuser. Fairly large fluctuations in static pressure also persist well into the region where the velocity and flow angles have damped out. The reason for the relatively fast mixing of the dynamic velocity fluctuations is that these fluctuations are transmitted by convection with the local velocity of the fluid particles. Conservation of mass and momentum cause two flows (jet and wake) with different radial and tangential velocities and angles entering a radial vaneless diffuser to mix out rapidly. However, dynamic fluctuations in static pressure propagate with the speed of sound and can persist at larger diffuser radius ratios without violating the continuity and momentum equations.Copyright

Centrifugal Compressor Impeller Aerodynamics: An Experimental Investigation

The ability to acquire blade loadings (surface pressure distributions) and surface flow visualization on an unshrouded centrifugal compressor impeller is demonstrated. Circumferential and streamwise static pressure distributions acquired on the stationary shroud are also presented. Data was acquired in a new facility designed for centrifugal compressor aerodynamic research. Blade loadings calculated with a blade–to–blade potential flow analysis are compared with the measured results. Surface flow visualization reveals some complex aspects of the flow on the surface of the impeller blading and hub. In a companion paper, Dorney and Davis (1990), a state–of–the–art, three–dimensional, time–accurate, Navier Stokes prediction of the flow through the impeller is presented.Copyright

The Effect of Impeller Inlet Annular Turning Vanes on Multistage Centrifugal Compressor Performance

The flow path of multistage centrifugal compressors is characterized by two 180-degree turns per stage: the inlet turning bend connecting the radial inflow return channel with the radial outflow impeller and the cross-over bend connecting the radial outflow diffuser with the return channel. Due to higher flow velocity and larger width to turning radius ratio, the turning losses are substantially higher in the inlet bend than in the return channel bend. Performance measurements were taken using different annular through-flow turning vane arrangements designed to reduce the inlet turning losses and increase the overall efficiency of the multistage centrifugal compressor. The experiments have shown consistent efficiency gains with corresponding capacity increases by adding multiple annular turning vanes in the inlet bend. The performance improvement potential of the vanes depends strongly on the positioning of these vanes in the flow passage. Based on these results, an empirical turning loss model was developed with the capability to predict the performance improvement achievable with correctly positioned single or multiple turning vanes in the impeller inlet bend area.Copyright