Kousuke Isomura

Experimental verification of the feasibility of a 100 W class micro-scale gas turbine at an impeller diameter of 10 mm

The feasibility of a 100 W class micro-scale gas turbine with a centrifugal impeller of 10 mm diameter has been studied by experimentally verifying the four major component performance requirements found from cycle analysis. The rotor is required to rotate at 870 000 rpm to generate the compressor pressure ratio 3, and it has successfully been achieved by using hydroinertia gas bearings. A compressor efficiency higher than that required by the target cycle has been measured. After correcting the effect of the heat leakage, approximately 65% of the compressor adiabatic efficiency is estimated to be achievable. The combustor has achieved stable self-sustained combustion at a combustion efficiency higher than 99.9%. The heat conduction analysis based on measured data showed that it is possible to keep the compressor below 170 °C when the turbine inlet temperature is 1050 °C. All four requirements are proven to be achievable, and hence, the feasibility of the micro-scale gas turbine at an impeller of 10 mm diameter has successfully been proven at component level.

Development of high-speed micro-gas bearings for three-dimensional micro-turbo machines

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.

Feasibility Study of a Gas Turbine at Micro Scale

Feasibility of a small centrifugal gas turbine with its impeller diameter around a centimeter has been studied. From cycle analysis, minimum requirements for each component were set. Then, feasibility to achieve these minimum requirements were studied, component by component. The results show that the micro-scale gas turbine is feasible at impeller diameter 1cm. The largest issue will be the isolation of the heat between the compressor and the combustor to prevent the large reduction of the compressor efficiency.Copyright

Development of Microturbocharger and Microcombustor for a Three-Dimensional Gas Turbine at Microscale

A microscale gas turbine is under development at Tohoku University in Japan. Current objective of the project is to reveal the performance of the gas turbine at microscale with optimum aerodynamic shape. Therefore the engine to be tested will be fabricated by machining using a micro-5-axis end mill to realize three-dimensional modeling. The first step of the development has been split into the development of microturbocharger and microcombustor, to prevent the problem of the heat flow effect pointed out in the previous study [1]. The heat flow from the combustor to compressor will become relatively large at microscale, and this will degrade the performance of the compressor. The goal of the first step of the development is to achieve the required performance of the components to realize the gas turbine cycle, without the heat effect. Those are, 62% compressor efficiency, 870,000 rpm shaft rotating speed, and the self sustained combustion. A microscale turbocharger has been designed. The compressor impeller of diameter 10mm is expected to produce a pressure ratio of 3, and 68% compressor adiabatic efficiency. The bearings to realize the design rotational speed are hydrodynamic type gas bearing. Fabrication of the herring-bone grooves have been attempted, and successfully formed on a cylindrical surface by new etching procedure. A technique to fabricate three-dimensional turbine impellers at microscale by powder sintering of ceramics has been demonstrated. A semi-microcombustor has been fabricated and shown successful performance by burning hydrogen fuel.Copyright

Hydroinertia gas bearings for micro spinners

Externally pressurized gas bearings with large bearing clearance are successfully used in ultra-high-speed micro spinners. For example, a micro spinner 4 mm in diameter is stably operated at more than 20 krps and its whirl ratio exceeds 20. In such bearings, the inertia effect of the gas flow in bearing clearance becomes predominant and its Mach number exceeds 1. As a result, gas pressure in the bearing clearance becomes negative. These bearings are called hydroinertia gas bearings, in contrast to 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 shown. Experimental results on micro spinners and trial design of hydroinertia gas bearings for micro gas turbines are also discussed.

Effects of Reynolds Number and Tip Clearances on the Performance of a Centrifugal Compressor at Micro Scale

The performance of a centrifugal compressor at micro-scale has been assessed by a combination of experiments and numerical calculations. A micro-compressor of 10mm diameter has been fabricated and tested at the condition of 75μm tip clearance. The test results showed higher adiabatic efficiencies than the target value for the compressor to realize a 100W class gas turbine generator. However, this is due to the heat leakage, and a heat leakage model is proposed to be added in the numerical simulations. After matching the calculation results to the experimental results by modeling the heat loss through the wall, calculations of various Reynolds number and tip clearances have been conducted. The study shows that the rate of efficiency reduction due to the increase of the tip clearance is within the range of the data that Pampreen [5] used to construct his model, and the micro-compressor can achieve the target efficiency either by reducing the tip clearance to 15μm, which is practically too small to be achieved. The alternatives methods to achieve the target efficiency are to increase the blade height for about three times, or scale up the compressor. These methods require the increase of the power output of the target gas turbine engine. The study shows that the target efficiency can be better achieved by scaling up the impeller to 15mm diameter to allow increasing the tip clearance to 50μm.Copyright

Hydroinertia Gas Bearing System to Achieve 470m∕s Tip Speed of 10mm-Diameter Impellers

A microbearing tester driven by an air turbine of 10 mm diameter has been developed, and successfully used to test hydroinertia gas bearings with a shaft of 4 mm diameter. The effects of bearing gas pressure conditions and bearing length to diameter ratio (LID) on the maximum achievable rotation speed were investigated. The maximum rotation speed of 890,000 rpm, which corresponds to the DN number (the product of a shaft diameter in millimeter and a rotation speed in rpm) of 3,560,000, was achieved. At 890,000 rpm, the tip speed of the turbine reaches approximately 470 m/s. Using the bearing system developed, the turbo components of a 100 W class gas turbine and an air pump for 1 kW class fuel cells can be tested.