Chris Robinson

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.

A unified correction method for Reynolds number, size, and roughness effects on the performance of compressors:

An equation is derived that relates the changes in turbomachinery efficiency with Reynolds number to the changes in the friction factor of an equivalent flat plate. This equation takes into account the different Reynolds number and roughness dependencies of the individual components, and can be used for whole stages and multistage machines. The new method is sufficiently general to correct for changes in Reynolds number due to changes in fluid properties or speed, changes in machine size, or changes in the surface roughness of components for all types of turbomachinery, but is calibrated here for use on axial and radial compressors. The method uses friction factor equations for a flat plate which include fully rough behaviour above an upper critical Reynolds number, a transition region depending on roughness and a region with laminar flow below the lower critical Reynolds number. The correction equation for efficiency includes a single empirical factor. Based on a simple loss analysis and a calibration with over 30 sets of experimental test data covering a wide range of machine types, a suggestion for the variation of this factor with specific speed has been made. Additional correction equations are derived for the shift in flow and the change in pressure rise with Reynolds number and these are also calibrated against the same data.

AN OPTIMIZATION TECHNIQUE FOR RADIAL COMPRESSOR IMPELLERS

A software tool has been created to aid in automated impeller design within an integrated design system for radial flow impellers. The design tool takes the results from the 1D preliminary design process and uses these to define a parameterized blade geometry, which incorporates features that are required for low mechanical stresses and simple manufacturing. This geometry is then adjusted to minimize a global objective function using a throughflow computation. The adjustment is based on selection with a breeder genetic algorithm. The initial population includes “elite” designs from a database of earlier well-proven experience, and the final design is honed to perfection with a hill-climbing method. With the help of a suitable global objective function incorporating mechanical and aerodynamic criteria, and taking into account wide experience with the design of impellers, the tool provides a fast screening of various design possibilities to produce a geometrical input for more advanced computational fluid dynamic and mechanical analysis. This is demonstrated through the redesign of an impeller previously designed by conventional methods. Comparisons of the results of the CFD analysis of the new impeller with that of the earlier design demonstrate that the tool can rapidly produce nearly optimal designs as an excellent basis for further refinement by the more complex analysis methods.Copyright

Impeller-Diffuser Interaction in Centrifugal Compressors

This paper reports several CFD analyses of a centrifugal compressor stage with a vaned diffuser at high pressure ratio using different techniques to model the rotor-stator interaction. A conventional steady stage calculation with a mixing-plane type interface between the rotor and stator was used as a baseline. This simulation gave excellent agreement with the measured performance characteristics at design speed, demonstrating the ability of the particular steady simulation used to capture the essential features of the blockage interaction between the components.A full annulus simulation using a transient rotor-stator interaction (TRS) method was then used at the peak efficiency point to obtain a fully unsteady reference solution, and this predicted a small increase in peak efficiency. Finally, a computationally less expensive unsteady calculation using a Time Transformation (TT) method was carried out. This gave similar results to the fully transient calculation suggesting that this is an acceptable approach to estimate unsteady blade loading from the interaction.The impeller diffuser spacing was then reduced from 15 to 7% of the impeller tip radius using the more affordable TT approach. This identified an increase in efficiency of 1% and predicted unsteady pressure fluctuations in the impeller which were 116% higher with the closely spaced diffuser.Copyright

The Cordier Line for Mixed Flow Compressors

The specific speed and specific diameter of radial, mixed and axial flow compressors can be plotted in a Cordier diagram, and the best compressors then lie in a relatively narrow band, known as the Cordier or Balje line. This line exhibits a distinctive s-shape, and it is shown in this paper that this is due to the variation of the centrifugal effect on the pressure rise of the different compressor types. A new equation for the Cordier line in the mixed flow region based on the pressure rise coefficient is developed and calibrated with data from mixed flow pumps and ventilators. Together with other empirical relationships for the expected efficiency as a function of the specific speed this provides some useful new guidelines for the preliminary design of mixed flow compressors. These guidelines are then examined by carrying out a preliminary design of a high-speed mixed-flow micro-compressor that is analyzed using CFD and tested to justify the approach.Copyright

The Design of a Family of Process Compressor Stages

The design of a new family of process compressor stages is described. The paper discusses the choice of master and derived stages to cover the required flow range and provides guidelines for the design of the stage components, including the impeller, diffuser and the return channel. Details are given of the mechanical and aerodynamic design process and the computational tools used for this. The test results show that the performance objectives have been achieved. Results from testing of some of the stages are compared with CFD simulations. These show that the inclusion of real geometry features, such as the shroud and hub leakage paths and the end-wall fillets, is necessary to obtain good agreement with the measured performance.

A New Streamline Curvature Throughflow Method for Radial Turbomachinery

This paper describes a newly developed streamline curvature throughflow method for the analysis of radial 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 empirical data provides separate treatment of blockage, losses, and deviation. Incompressible and compressible fluids are allowed, including real gases and supersonic relative flow in blade rows. The paper describes some new aspects of the code. In particular, a relatively simple numerical model for spanwise mixing is derived, the calculation method for prescribed pressure ratio in compressor bladed rows is described, and the method used to redistribute the flow across the span due to choking 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.Copyright