Figure 1: Air turbine, diameter 10 mm.
To gather initial experience, a simple turbine driven by compressed air was built first, instead of a complete gas turbine.
The rotor has a diameter of 10 mm and is supported by two ball bearings. Figure 1 shows a subassembly of nozzle disc, rotor, and bearings.
The turbine is tested for a supply pressure of 0.8 bar.
To avoid breakdown of the bearings, the speed is limited to 130,000 rpm, far below the (theoretical) optimal speed of 210,000 rpm.
Up to 50 W mechanical power is generated with an efficiency up to 24 %.
From the results, it was clear that power and efficiency would increase further with speed, beyond the speed limits of the ball bearings.
So, a major lesson drawn from this test set-up is that speed is the limiting factor for the performance and efficiency of micro turbomachinery.
The obtained efficiency may be sufficient to operate an air powered turbine, but lies far from the efficiency required for gas turbine aero-components.