Centrifugal Compressor Research
Research into centrifugal compressors (also known as radial compressors) has been a key part of the research into high-speed technology at LUT for the past two decades. In addition to published research papers, a number of licentiate theses and doctoral dissertations have been completed in connection with the research.
A device that pressurizes air is generally named a compressor once the pressure achieved exceeds 20% above normal atmospheric pressure.
Compressor types can be categorised according to the means of compression as statically compressive (such as reciprocating compressors) or kinetically compressive (such as axial and radial compressors), and as reverse flow compressors.
The final pressure required from a compressor varies considerably according to its end use. For example, in pneumatic transfer of 0.5 to 3 bar of additional pressure it may be necessary to perform certain tests of over 1,000 bar pressure.
Commercial centrifugal compressors were equipped with a gearbox in the past, because the speeds in typical applications are up to several tens of thousands of revolutions per minute. In high-speed technology applications where an electric motor rotates at the required speed of the compressor, the gearbox can be removed and the electrical motor can be made smaller.
Due to developments in variable frequency drives and oil-free bearings, high-speed compressors have entered the market in earnest to save energy and reduce environmental impact.
Compressor-related research can be divided into the following main areas:
- Radial compressor design
- Compressor performance measurements
- Compressor internal flow analysis
- Compressor control and system simulation
A cutaway model of the first commercial radial compressor designed at LUT.
4. Inlet guide vanes
7. Upper gas bearing
The Fluid Dynamics Laboratory has two decades of experience in the design of centrifugal compressors and compressor performance measurements.
In the design of compressors, LUT uses self-developed software, which is based on close cooperation with compressor manufacturers. For example, the impellers supplied for compressors manufactured by High Speed Tech Oy were based on LUT designs.
1. Initially the compressor's dimensions are selected and calculated based on one-dimensional calculations. The software programme CCFlow has been developed for this purpose, based on LUT's research and experience.
2. Next, the 3D geometry for the compressor impeller and flow ducts are created. This is completed using LUT-developed software (Turbo) and other programmes.
3. The geometry is then transferred for computation to a fluid dynamic programme (CFD) and strength analysis programme. The software used to calculate the flow dynamics includes the CFD software FinFlo developed by the Helsinki University of Technology.
Finally, a data file containing the geometry is produced for numerically controlled the machining and casting of a prototype model.
4. The success of the prototype design is always verified by measuring the compressor's performance in detail in the LUT test station.
In this field, four PhD students have graduated (Reunanen, Turunen-Saaresti, Tang and Jaatinen) and one licentiate thesis has been completed (Backman).
Modelling and Control
Compressor control basis
The suction chamber pressure, temperature and humidity of a compressor have a significant impact on the identification of the field values. In order to have optimal control of a centrifugal compressor, the compressor must be modelled accurately and its properties mapped accurately to the field values.
The Fluid Dynamics Laboratory has developed software for calculating and analysing the field values under different operating conditions. This creates the basis for understanding the working of the compressor and the development of control algorithms.
A high-speed compressor always requires speed control at the very least. The Fluid Dynamics Laboratory has developed control algorithms suitable for compressors manufactured by High Speed Tech Oy.
Guide vane control
The compressor's stall-free operating range can be extended by using adjustable guide or diffusion vanes.
The Fluid Dynamics Laboratory has designed an inlet guide vane control device and studied compressor use and stall limits using the equipment.
Research revealed that combined vane and rotational speed control provided high levels of efficiency and very good control.
Parallel- and/or series-connected compressor system simulation
The Fluid Dynamics Laboratory has a further field value analysis software programme with the flexibility to model multiple compressors in series or in parallel.
The programme may be utilised with any compressor if the field values representing its performance are known or measurable.
This programme has been one of the building blocks in the project carried out in cooperation with the company Sarlin Hydor, in which a simulation model was made for compressed air systems for a number of types of compressors, pipelines, containers and various transportation devices. Using the simulations, the optimal means of using different compressors in the compressed air systems were studied.
As a result of the research work conducted in cooperation with LUT and the Helsinki University of Technology, Cardo Production Finland (formerly HST) has since 1996 manufactured commercial, high-speed technology-based compressors at its plant in Lappeenranta.
The company belongs to the Swedish ABS Pump Group, which employs about 30 people in the region. Each year the company manufactures approximately one hundred high-speed compressors, which are mostly for export.