Michael Casey

Flow Analysis in a Pump Diffuser—Part 1: LDA and PTV Measurements of the Unsteady Flow

This paper describes experimental research aimed at improving our understanding of the complex unsteady three-dimensional flow field associated with the interaction between a pump impeller and its vaned diffuser. The paper provides the results of experiments carried out using Laser Particle Tracking Velocimetry (LPTV) and Laser Doppler Anemometry (LDA), in which time-resolved details of the unsteady flow field in a vaned diffuser of a medium specific speed pump have been obtained as a function of the local position of the pump impeller blades. Detailed flow field measurements have been carried out at several measurement positions in the diffuser and at a number of operating points along the pump characteristic. The measurement results have been analyzed to elucidate some interesting flow features observed in this typical pump diffuser. These include three-dimensional flow at the impeller outlet, flow separation in the diffuser channel, unsteady recirculation of the flow from the diffuser into the impeller, the passage of vorticity in the impeller blade wakes through the diffuser, and periodic unsteadiness and turbulence in the diffuser flow channel. The relevance of these flow features to the stability of the pump characteristic is discussed.

The Efficiency of Turbocharger Compressors With Diabatic Flows

In most compressors the flow is adiabatic, but in low-speed turbochargers, the compression process has both heat transfer and work input. This paper examines different compressor efficiency definitions for such diabatic flows. Fundamental flaws in the use of the isentropic efficiency for this purpose are identified, whereas the polytropic efficiency can be used with or without heat transfer without ambiguities. The advantage of the polytropic approach for a practical application is demonstrated by analyzing the heat transfer in a turbocharger compressor. A simple model of the heat transfer allows a correction for this effect on the polytropic efficiency at low-speed to be derived. Compressor characteristics that have been corrected for this surprisingly large effect maintain a much higher efficiency down to low-speeds.

Experimental studies of the jet of a Pelton turbine

Abstract The jet discharged from the nozzle of a Pelton turbine is a key item of such hydropower systems and its precise shape and position are highly relevant to the optimum design of the turbine buckets to match the incoming flow. Experimental investigations, primarily with laser Doppler anemometry, have been used to identify the important fluid dynamic structures of the jet and its free surface. It is shown that weak secondary flows generated by the bends or bifurcations in the distributor of the Pelton turbine system are still present in the jet leaving the injectors. These cause small flow disturbances which affect the shape, orientation, and the topology of the jets and lead to a shift of the jet core from the axis of the nozzle.

Design of Industrial Axial Compressor Blade Sections for Optimal Range and Performance

A design system for the blade sections of industrial axial compressors has been developed. The method combines a parametric geometry definition method, a powerful blade-to-blade flow solver (MISES) and an optimization technique (breeder genetic algorithm) with an appropriate fitness function. Particular effort has been devoted to the design of the fitness function for this application which includes non-dimensional terms related to the required performance at design and off-design operating points. It has been found that essential aspects of the design (such as the required flow turning, or mechanical constraints) should not be part of the fitness function, but need to be treated as so-called “killer” criteria in the genetic algorithm. Finally, it has been found worthwhile to examine the effect of the weighting factors of the fitness function to identify how these affect the performance of the sections. The system has been tested on the design of a repeating stage for the middle stages of an industrial axial compressor. The resulting profiles show an increased operating range compared to an earlier design using NACA65 profiles.Copyright

A NEW STREAMLINE CURVATURE THROUGHFLOW METHOD FOR RADIAL TURBOMACHINERY

This paper describes a newly developed streamline curvature throughflow method for the analysis of ra dial or mixed flow machines. The code includes curved walls, curved leading and trailing edges, and internal blade row calculating stations. A general method of specifying the empiri cal data provides separate treatment of blockage, losses, an d deviation. Incompressible and compressible fluids are allowed, including real gases and supersonic relative flow in blade ro ws. The paper describes some new aspects of the code. In pa rticular, a relatively simple numerical model for spanwise mixi ng is derived, the calculation method for prescribed pres sure ratio in compressor bladed rows is described, and the method used to redistribute the flow across the span due to chokin g is given. Examples are given of the use and validation of the code for many types of radial turbomachinery and these show it is an excellent tool for preliminary design.

Estimation of the performance of turbocharger compressors at extremely low pressure ratios

Abstract Constant speed lines on turbocharger compressor performance maps usually have no test data at low pressure ratio and high flow. The engine, however, can force the turbocharger compressor to operate in this region and whole-engine simulation systems need to extend the test data in some way to include this region of the map. A physically based method of estimating the performance at low pressure ratios from measurements made in the rest of the map has been developed and is described in this article. The method requires no detailed geometrical information on the impeller and the stage but extracts the necessary information from the test data available; hence it can be applied to any measured compressor data. The test data over the measured range of speed and flow are used to estimate the work transfer and loss characteristics of the impeller, and the impeller throat area. The Euler equation justifies a linear extrapolation of the work input to higher outlet flow and a new technique based on the density ratio is used to extrapolate the losses. This accounts for choking at the impeller inlet in a similar way to choking at a one-dimensional duct of varying area, where losses are also a function of density ratio. The stage performance at low pressure ratios can then be obtained by recombining the extrapolated loss and work characteristics at higher flows. The method allows the measured performance map to be extrapolated to low pressure ratios on a sound physical basis and to identify physical aspects of the flow in this extreme off-design region, such as the location of increased risk of oil blow-by.

Flow Analysis in a Pump Diffuser—Part 2: Validation and Limitations of CFD for Diffuser Flows

This paper describes an investigation into the use of CFD for highly loaded pump diffuser flows. A reliable commercial Navier-Stokes code with the standard k-e turbulence model was used for this work. Calculations of a simple planar two-dimensional diffuser demonstrate the ability of the k-e model to predict the measured effects of blockage and area ratio on the diffuser static pressure recovery at low loading levels. At high loading levels with flow separation the k-e model underestimates the blockage caused by the recirculation in the flow separation region and overestimates the pressure recovery in the diffuser. Three steady-state calculations of a highly loaded vaned diffuser of a medium specific speed pump have been carried out using different inlet boundary conditions to represent the pump outlet flow. These are compared to LDA measurement data of the flow field and demonstrate that although the Navier-Stokes code with the standard k-e turbulence model is able to predict the presence of separation in the flow, it is not yet able to accurately predict the static pressure rise of this highly loaded pump diffuser beyond the flow separation point.

The Matching of a Vaned Diffuser With a Radial Compressor Impeller and Its Effect on the Stage Performance

The matching of a vaned diffuser with a centrifugal impeller is examined with a one-dimensional (1D) analysis combined with extensive experimental data. A matching equation is derived to define the required throat area of the diffuser relative to the throat area of the impeller at different design speeds and validated by comparison with a wide range of compressor designs. The matching equation is then used to give design guidelines for the throat area of vaned diffusers operating with impellers at different tip-speed Mach numbers. An analysis of test data for a range of high pressure ratio turbocharger compressor stages is presented in which different matching between the diffuser and the impeller has been experimentally examined. The test data includes different impellers with different diffuser throat areas over a wide range of speeds. It is shown that the changes in performance with speed and diffuser throat area can be explained on the basis of the tip-speed Mach number which causes both the diffuser and impeller to choke at the same mass flow. Based on this understanding, a radial compressor map prediction method is extended to include this parameter, so that more accurate maps for matched and mismatched vaned diffusers can be predicted.

Design of industrial axial compressor blade sections for optimal range and performance

Background: The blade sections of industrial axial flow compressors require a wider range from surge to choke than typical gas turbine compressors in order to meet the high volume flow range requirements of the plant in which they operate. While in the past conventional blade profiles (NACA65 or C4 profiles) at moderate Mach number have mostly been used, recent well-documented experience in axial compressor design for gas turbines suggests that peak efficiency improvements and considerable enlargement of volume flow range can be achieved by the use of so-called prescribed velocity distribution (PVD) or controlled diffusion (CD) airfoils. Method of approach: The method combines a parametric geometry definition method, a powerful blade-to-blade flow solver and an optimization technique (breeder genetic algorithm) with an appropriate fitness function. Particular effort has been devoted to the design of the fitness function for this application which includes non-dimensional terms related to the required performance at design and off-design operating points. It has been found that essential aspects of the design (such as the required flow turning, or mechanical constraints) should not be part of the fitness function, but need to be treated as so-called killer criteria in the genetic algorithm. Finally, it has been found worthwhile to examine the effect of the weighting factors of the fitness function to identify how these affect the performance of the sections. Results: The system has been tested on the design of a repeating stage for the middle stages of an industrial axial compressor. The resulting profiles show an increased operating range compared to an earlier design using NACA65 profiles. Conclusions: A design system for the blade sections of industrial axial compressors has been developed. Three-dimensional CFD simulations and experimental measurements demonstrate the effectiveness of the new profiles with respect to the operating range.

Transonic radial compressor inlet design

Abstract The design of radial compressor inlets for transonic flow is examined. A theoretical model [1] quantifies the losses in the tip sections caused by the choke margin (incidence) and the blockage of the blades. It identifies clear design rules for the tip sections: to achieve the highest efficiency, these require minimum blockage (low blade thickness and splitter vanes) and low choke margin (close to the unique-incidence condition). Simulations of the NASA rotor 37 transonic axial compressor (with CFX-TASCflow) are used to validate the use of three-dimensional viscous computational fluid dynamics (CFD) for transonic compressor inlets and to demonstrate that the key performance features suggested by the simple model are also modelled in three-dimensional viscous flow simulations. The simple model together with CFD simulations has been used for the design of tip sections at the inlet of a transonic radial compressor. CFD simulations were used to select the position of the shock to give a low choke margin, to reduce the preshock Mach number and also to optimize the shape and position of the leading edge of the splitter vanes.