Feasibility of a 100W class micro-scale gas turbine with centrifugal impeller of diameter 10mm has been studied experimentally. The rotor that is required to rotate at 870,000 rpm to generate the compressor pressure ratio 3 has successfully been achieved the stable operation by using hydroinertia gas bearings. The compressor efficiency higher than that required by the target cycle has been measured. The combustor has achieved stable self-sustained combustion at the combustion efficiency higher than 99.9%. The heat conduction analysis based on measured data showed that it is possible to keep the compressor under the compressor exit temperature 170degC when the turbine inlet temperature is 1050degC. Based on these results, the feasibility of the micro-scale gas turbine at impeller diameter 10mm has successfully been proven at component level.

A micro-high-speed bearing test rig has been designed and fabricated to develop air bearings for a micro-machine gas turbine. The micro-machine gas turbine requires bearings of diameter 4 mm to operate stably at 870 000 rpm. Based on the preliminary analysis, it has been found that a large bearing gap and very high rotor balance are required to achieve low viscous loss at high speed and to prevent the rotor from hitting the bearing at critical speed. Therefore, a hydro-inertia gas bearing has been selected with half-split bearing sleeves. A hydro-inertia bearing is a type of static air bearing which has large bearing clearance to generate supersonic flow in the bearing gap. Currently, a rotor speed as high as 770 000 rpm has been achieved in the test.

Externally pressurized gas bearings with large bearing clearance are successfully used in ultra high speed micro spinners. For example, a micro spinner 4mm in diameter is stably operated more than 20krps and its whirl ratio exceeds 20. In such bearings, inertia effect of the gas flow in bearing clearance becomes predominant and its Mach number exceeds 1. As the results, gas pressure in the bearing clearance becomes negative. These bearings are called as hydroinertia gas bearings contrasted with the conventional hydrostatic gas bearings. Static characteristics of hydroinertia gas bearings are analyzed by considering the viscous effect of gas as wall friction, and the optimum design of hydroinertia gas bearings is showed. Experimental results on micro spinners, trial design of hydroinertia gas bearings for micro gas turbines and its experimental results are also discussed.

Micromachine gas turbine with centrifugal impellers of 10 mm diameter fabricated by 5-axis micro-milling is under development at Tohoku University, in conjunction with Ishikawajima-Harima Heavy Industries Co., Ltd. (IHI), Tohoku-Gakuin University, and Sankyo Seiki Mfg. Co., Ltd. The development is currently at the stage of proving the feasibility of the gas turbine cycle by component tests. Micro-combustors have been developed for both hydrogen and methane fuel. Over 99.9% of the combustion efficiency has been realized in both combustors and the baseline configuration of the combustor for the gas turbine is set. A compressor of 10 mm diameter has been developed as a micromachined turbocharger. The performance test of the micromachined turbocharger has been started, and ran up to 566 000 rpm, which is approximately 65% of the design speed. Compressor performance has been successfully measured along a constant speed line at 55% of the design speed.

This paper describes a turbo test rig to test the compressor of a palmtop gas turbine generator at low temperature (<100 • C). Impellers are 10 mm in diameter and have three-dimensional blades machined using a five-axis NC milling machine. Hydroinertia bearings are employed in both radial and axial directions. The performance of the compressor was measured at 50% (435 000 rpm) and 60% (530 000 rpm) of the rated rotational speed (870 000 rpm) by driving a turbine using compressed air at room temperature. The measured pressure ratio is lower than the predicted value. This could be mainly because impeller tip clearance was larger than the designed value. The measured adiabatic efficiency is unrealistically high due to heat dissipation from compressed air. During acceleration toward the rated rotational speed, a shaft crashed to the bearing at 566 000 rpm due to whirl. At that time, the whirl ratio was 8.