Design of Low Power Radial Turbine for ORC Test Rig

Pietrobon, Luke (2013). Design of Low Power Radial Turbine for ORC Test Rig Honours Thesis, School of Engineering, The University of Queensland.

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Author Pietrobon, Luke
Thesis Title Design of Low Power Radial Turbine for ORC Test Rig
School, Centre or Institute School of Engineering
Institution The University of Queensland
Publication date 2013
Thesis type Honours Thesis
Supervisor Not Known
Total pages 47
Language eng
Subjects 0913 Mechanical Engineering
Formatted abstract
This thesis investigates the limitations of radial turbines for use in low power applications. The minimum feasible power output of a radial turbine is determined based on analytical analysis. The physical mechanisms and operational constraints that lead to this limit are established. Performance trends relating to low power radial turbines are established and discussed in order to determine areas that most prominently affect turbine efficiency and lead to the low power limit. The validity of operational constraints is scrutinized and analysis is conducted using an altered geometrical constraint more suitable to low power radial turbines.

Current literature provides evidence that reducing turbine geometry leads to large efficiency losses but fails to define a power at which these losses lead to unworkable turbine geometries. Determining this point provides a limit to the suitability of radial turbines in low power applications. Radial turbines are analysed using a previously developed preliminary turbine design methodology. This procedure incorporates a brute force analysis searching a defined range for input variables at given intervals. In this way, a large set of potentially viable combinations of system parameters are analysed and suitable turbine geometries developed.

It was determined that the low power limit based on the original model developed for turbine analysis is 14kW. The main contributing factors leading to this power limit is a constraint placed on the Mach number at the turbine rotor inlet and a limitation placed on minimum rotor blade height. The Mach limit is enforced as the sonic condition cannot be exceeded in internal viscous flows. This condition causes choked flow in the turbine and sets the limit for mass flow rate. The limitation placed on minimum blade height was determined based on typical values for radial turbines used in turbomachinery and enforced to ensure ease of manufacture.

Investigation into the validity of these limitations concluded that turbine minimum blade height could be reduced but would lead to a reduction in turbine efficiencies. Reducing this limit from 2mm to 1mm provided geometries for feasible turbines operating at 9kW and above. All analyses conducted for turbines operating below this power failed the rotor inlet Mach number feasibility check and were therefore not valid designs. It was determined that the low power limit in radial turbines is caused by choked flow.
Keyword Low Power Radial Turbine

Document type: Thesis
Collection: UQ Theses (non-RHD) - UQ staff and students only
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Created: Fri, 11 Dec 2015, 14:19:58 EST by Asma Asrar Qureshi on behalf of Scholarly Communication and Digitisation Service