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.
Advances and Breakthroughs in Diffusers for Compressors and Pumps: A Short History of Diffusers for Centrifugal Machines
Abstract The diffuser plays an indispensable role in setting the useable flow range and efficiency for centrifugal machinery. Key aspects of the history and design of such diffusers are overviewed followed by a look at current, novel ideas for better systems. Improved machinery can be expected in the coming years due to decades of development work in this field.
The Design Space Boundaries for High Flow Capacity Centrifugal Compressors
A methodology has been derived allowing a fast preliminary assessment of the design of centrifugal compressors specified for high specific swallowing capacity. The method is based on one-dimensional (1D) design point values using classical turbomachinery analysis to determine the inlet geometry for the maximum mass flow function. The key results are then expressed in a series of diagrams which draw out the nature of the conflicting boundary conditions of the design. In particular it is shown how the inlet casing relative Mach number causes the design flow coefficient to decrease with the total pressure ratio and determines the inlet eye diameter. Physically-based boundaries of operation are added to the diagrams giving guidelines for the proper choice of specification values to the designer. In addition, links are given to some well-known impeller efficiency correlations, so that a preliminary estimate of the performance can be made. Comparisons are made with a range of compressor data which supports the approach. The derived methodology allows any given specifications to be checked rapidly for feasibility and development risk or can be used to define a challenging specification for the design of a new product.Copyright © 2012 by ASME