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Text | A MICRO TURBINE DEVICE WITH ENHANCED MICRO AIR BEARINGS | 001
Stresa, Italy, 26-28 April 2006
A MICRO TURBINE DEVICE WITH ENHANCED MICRO AIR BEARINGS
1 Singapore Institute of Manufacturing Technology, Singapore 638075 2 Data Storage Institute, 5 Engineering Drive 1, Singapore 117608
3 MEMS & Packaging Lab, AIST, Namiki 1-2, Tsukuba, 305-8564 Japan
As part of progress in developing a micro gas turbine engine, this paper presents the design, fabrication and testing of a silicon-based micro turbine device, which is driven by compressed air. To improve its rotational speed and stability, the turbine device has enhanced journal air bearing and thrust air bearings. The thrust air bearings are utilized for supporting the rotor from both its top- and bottom- sides. The top thrust air bearing employs pump-in type spiral grooves, and the bottom bearing uses pump-out type grooves. The journal air bearing is a plain journal with a critical short journal length (L) and a small radial clearance (C). Aspect ratio (L:C) above 20:1 is realized using optimized DRIE process. The micro turbine device has been fabricated and tested. The turbine device can sustain stable operations, and a rotation speed of 15,000 rpm has been reported during test.
Micro gas turbine engine [1-2] is one of the promising solutions to provide high-density power source for microsystems. We are developing a silicon-based micro gas turbine engine , which consists of a combustor, a radial inflow turbine and a centrifugal compressor. A micro combustor composed of seven layers of silicon structures has been developed, which can sustain a stable combustion with an exit temperature as high as 1600 K . The micro turbine is another critical component in micro gas turbine engine, which will generate power output and drive the compressor.
The critical concerns for a turbine device are friction and wear during operation. Micro air bearing is preferred in microsystems due to its miniaturized dimension and capability to achieve high speed and low wear operations. Some researchers have employed micro air bearings into rotational microsystems [4-6].
A conventional rotor has a dimension of a few millimeters along its axial direction. However, the rotor in a silicon-based micro turbine has an axial dimension (L) of less than one millimeter and a diameter (D) of several millimeters. The commonly accepted assumptions and design rules for conventional air bearings need to be revised due to the consequences of scaling down as well as the current limitations in micro fabrication techniques. Piekos  has investigated the performance of a short journal bearing in micro turbine system. A categorization of non-dimensional variables was suggested including loading and fluid.
This paper reports on design, fabrication and testing of a silicon-based micro turbine device. The turbine device consists of dynamic journal bearing and thrust air bearings, which are designed based on numerical simulation and fabricated through DRIE process. The turbine is driven by compressed air instead of exhausted gas from a micro combustor during test. Experimental investigations demonstrate that stable operations can be sustained, and a rotation speed of 15,000 rpm has been achieved.
2. DESIGN OF MICRO TURBINE STRUCTURE
The turbine device to be developed consists of three layers of silicon dies and two acrylic plates for tubing and clamping. Fig. 1 shows the schematic design of the turbine device and Fig. 2 illustrates the exploded view with the acrylic plates omitted. There are three flow paths, which are for rotor driving, static journal air bearing and thrust air bearing, respectively. The 3rd wafer provides a platen for bottom-side dynamic thrust air bearing and the flow paths for driving air as well as for static air bearings. The blades profile for both the rotor and stator are fabricated from the 2nd wafer. The 1st wafer provides the air outlet and a platen for topside dynamic thrust air bearing.
Silicon wafers of 0.4 mm thick are used for 1st and 3rd layers of wafers. A wafer of 0.8 mm thick is used for the 2nd wafer, from which both the rotor and stator are
©TIMA Editions/DTIP 2006 -page-
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