The Role of Impeller Outflow Conditions on the Performance of Vaned Diffusers
Highly loaded impellers, typically used in turbocharger and gas turbine applications, exhaust an unsteady, transonic flow that is nonuniform across the span and pitch and swirling at angles approaching tangential. With the exception of the flow angle, conflicting data exist regarding whether these attributes have substantial influence on the performance of the downstream diffuser. This paper quantifies the relative importance of the flow angle, Mach number, nonuniformity, and unsteadiness on diffuser performance, through diffuser experiments in a compressor stage and in a rotating swirling flow test rig. This is combined with steady and unsteady Reynolds-averaged Navier–Stokes (RANS) computations. The test article is a pressure ratio 5 turbocharger compressor with an airfoil vaned diffuser. The swirling flow rig is able to generate rotor outflow conditions representative of the compressor except for the periodic pitchwise unsteadiness and fits a 0.86 scale diffuser and volute. In both rigs, the time-mean impeller outflow is mapped across a diffuser pitch using miniaturized traversing probes developed for the purpose. Across approximately two-thirds of the stage operating range, diffuser performance is well correlated to the average impeller outflow angle when the metric used is effectiveness, which describes the pressure recovery obtained relative to the maximum possible given the average inflow angle and Mach number and the vane exit metal angle. Utilizing effectiveness captures density changes through the diffuser at higher Mach numbers; a 10% increase in pressure recovery is observed as the inlet Mach number is increased from 0.5 to 1. Further, effectiveness is shown to be largely independent of the time-averaged spanwise and unsteady pitchwise nonuniformity from the rotor; this independence is reflective of the strong mixing processes that occur in the diffuser inlet region. The observed exception is for operating points with high time-averaged vane incidence. Here, it is hypothesized that temporary excursions into high-loss flow regimes cause a nonlinear increase in loss as large unsteady angle variations pass by from the rotor. Given that straight-channel diffuser design charts typically used in preliminary radial vaned diffuser design capture neither streamtube area changes from impeller exit to the diffuser throat nor vane incidence effects, their utility is limited. An alternative approach, utilizing effectiveness and vane leading edge incidence, is proposed.
Flow instability evolution in high pressure ratio centrifugal compressor with vaned diffuser
Abstract Compressor stability is an essential issue for developing single-stage high pressure ratio centrifugal compressors. In this paper, a comprehensive stability map of a centrifugal compressor stage with the peak pressure ratio of 6.2 has been illustrated. Fast response pressure transducers are mounted inside the casing or the back-plate wall to measure internal transient behaviors. From Fourier analysis of pressure signals, various instabilities across time and length scales are identified. At low speeds, the impeller inlet rotating instability develops in scale and evolves into the inducer stall, which eventually induces the mild surge and deep surge of the compression system. At middle speeds, the compressor successively experiences stable state, mild surge, rotating instability, and deep surge. The mild surge region coincides with the dip region of the compressor S-shape pressure rise curve, and deep surge occurs when the compressor pressure rise raises to the second peak. At high speeds, mild surge and deep surge abruptly occur without preceding stall or rotating instability. To explain the complex surge behavior, the mechanical analogy between the compression system and the mass-spring-damper system is applied. Both mild surge and deep surge transients are found to belong to the dynamic instability of the mass-spring-damper system, and the exact form of the surge state will be determined by eigenvalues of the system governing equation.
Characterization of Unsteady Impeller-Blade Loading in a Centrifugal Compressor With a Discrete-Passage Diffuser
Unsteady numerical simulations in a centrifugal compressor stage consisting of an impeller followed by a discrete passage diffuser are employed to characterize unsteady impeller blade loading. For a stage typical of modern design, peak dynamic loads can be as high as 3.5 times greater that the peak steady-state loads. Unsteady load fluctuations of 0.2 times the tip dynamic pressure are observed in the forward 30% splitter chord, indicating a significant upstream propagation of the unsteady pressure disturbance. There can thus be an upstream manifestation of a downstream stimulus. The upstream manifestation is the considerable unsteady loading in the splitter blade leading edge while the downstream stimulus is the unsteady interaction between the rotating impeller and the stationary downstream diffuser. Increased relative frame diffusion, characterized by the DeHaller number, increases the sensitivity to downstream disturbances and leads to decreased attenuation of the upstream disturbance. Increased relative Mach number decreases the relative frame disturbance wavelength and thus improves the attenuation of the upstream disturbance. The length scale of spatial variation in unsteady loading is set by the relative Mach number, diffuser passing time, and the acoustic speed.Copyright © 2007 by ASME