Shinichi Togo

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.

Turbo test rig with hydroinertia air bearings for a palmtop gas turbine

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.

Characteristics of the magnetic gear using a bulk high-Tc superconductor

A magnetic gear which can transmit force without contact has operational benefits of requiring neither vibration, noise, nor lubrication. Still, its transmission force is considerably smaller than a mechanical gear. Strong magnets are needed to improve magnetic gear transmission force. For this purpose, a magnet formed by the pinning effect in a bulk superconductor might have magnetic flux density several times stronger than rare-earth permanent magnets. We propose a noncontact magnetic gear using a bulk high-Tc superconductor. In this paper, a three-dimensional electromagnetic field analysis program has been developed to analyze characteristics of the proposed gear. Two superconducting magnetic gears with different magnet configuration are manufactured, and their characteristics are measured. The analysis and the experiment explore the possibility of a magnetic gear using the pinning magnet formed in the superconductor.

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

Design and Workout of ACROSS Transmitters Using Air Bearings

Abstract The generation and extraction of heat due to inherent friction at the ball bearings to support the large force is an important technical problem in current seismic ACROSS transmitters of the rotary type for stable maintenance and routine operation for continuous active monitoring over years. This paper proposes the use of gas bearings to improve the relevant situation, since they are expected to be useful in reduction of heat generation and energy loss to eliminate the peripheries for heat extraction. Since no experience has been reported so far to sustain the eccentric rotor by gas bearings, we made some test experiments of eccentric rotors supported by air bearings, and we confirmed that they are ideal for supporting the centrifugal force generated by rotation of an eccentric rotor for the ACROSS transmitters of the next generation, if it is designed appropriately to meet our utility purposes.

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.

Study on characteristics of porous ceramic gas bearings.

Porous gas bearings show high load capacity and bearing stiffness. Conventional metal sintered porous gas bearings are the surface choked type and there are some difficulties in controlling gas permeability. Recently, porous ceramics with low and homogeneous permeability can be easily obtained and machined with high accuracy without any surface chok-ing. In this paper, analysis of porous ceramic gas bearings are presented and steady state characteristics are calculated for the cases of externally pressurized, self acting, and hybrid type operation. Pressure distribution in porous ceramics is also calculated when the surface is slightly choked to improve pneumatic hammer instability.