Axial-flowm Icroturbiwniet h Electromagnegtenicer Ator

8/2/2019 Axial-flowm Icroturbiwniet h Electromagnegtenicer Ator

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AXIAL-FLOW ICROTURBINEIT H ELECTROMAGNETICE NE RAT O R:DESIGN, FD SIMULATION, AND PROTOTYPE DEMONSTRATIONA . S. Holmes, G. Hong, K. R. Pullen, 2K .R. uffard

Dept. of Electrical & Electronic EngineeringDept. of Mechanical Engineering

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Imperial College London, Exhibition R oad, London SW7 2A2, UKABSTRACTWe have developed a MEMS power conversion device thatcombines an axial-flow turbine with an axial-fluxelectromagnetic generator. This device can generateelectrical power when placed in an air-stream, and is aimedat applications such as flow sensing and power generation forremote sensors. Recent experimental performance data arepresented for prototypes, and compared with CFD(computational fluid dynamics) simulations. The currentdevices have a volume o f approximately 0.5 cm3, generate 1mW of output power at a pressure drop of 8 mhar and a flowrate of 35 litres per minute, an d will operate at pressure dropsdown to a few mhar.1. INTRODUCTIONIn recent years there has been growing interest in alternativepower sources that might eliminate the need for chemicalbatteries in portable electronic devices. For low powerapplications (up to mW levels), there is the intriguingpossibility of scavenging energy from the surroundingsthereby eliminating the need for an installed energy source,while at higher power levels there are fuel-burning devicesthat can potentially offer longer shelf-life and/or higherpower d ensity than batteries.Energy scavenging power generators to date have beenbased mainly on resonant mass-spring-damper systems inwhich damping is effected, at least in part, by a transducerthat converts mechanical energy to electrical energy [1,2].Such devices work most effectively when excited by a low-amplitude, reciprocating motion at a frequency close toresonance, making them potentially very effective forextracting energy from vibrating machinery. Non-resonantdevices in which the spring is omitted are also underdevelopment. These have been shown to he more efficientfor larger, low frequency excitations such as might heexperienced by a portable or wearable device [ 3 ] .Work on fuel-huming MEMS power generators has beenfocused mainly on microturbines. The most highly developedwork in this area is that of MIT, where there is an extensivegas turbine program [3 ] . The MIT project aims to develop afully integrated device that combines compressor, burner,turbine and electrical generator in a single device fabricatedas a multi-wafer stack. Other groups have worked on turho-generator subsystems, for example by combining a turbineand an electrical generator on a single chip [SI.

Recently at Imperial College we have been developingan energy scavenger based on an axial-flow turhine with anintegrated axial-flux electromagnetic generator. The device,shown schematically in Fig. 1, ha s a sandwich structureconsisting of two silicon stators placed either side of apolymer rotor. Axial gas flow through the device drives therotor, while permanent magnets embedded in the rotorinduce ou tput voltage in planar coils on the stators.Silicon [7 US

Ai r flow , oil Upper/ statorRotor

\ owerstatorJSoR magnetic Permanentmaterial magnet(4

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Figure I : Axial-flow microturbine with integrated axial-fluxelectromagnetic generator: (a) cross-section of device; (b )exploded view, cut away to show rotor blade and guide vaneprofiles (generatorp arts not shown).The axial-flow turhine geometry was chosen because itallows operation at low pressure ratios, as required forextraction of power directly from an ambient gas flow.Axial-flow turbines are difficult to realize by conventionalmicrofabrication methods, because to produce the necessary

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curved profiles on the rotor blades and guide vanes i t isnecessary to fabricate structures where the sidewall angleevolves with depth in a controlled manner. W e have used acombination of conventional UV lithography and lasermicromachining to produce SU 8 polymer parts with therequired profiles. The ax ial-flux geometry was chosen for thegenerator because i t is co mpatible with planar spiral coils,which are well sui ted to m icrofabrication. This g e o m e e i swell known from larger scale machines [6], and has alsobeen used previously in mm-scale motors [7].

2. DEVICE ESIGN ND FABRICATIONPrototype devices have been fabricated by a combination ofsil icon deep reactive ion etching (DRIE), electroplating, andlaser microm achining. The device size was dictated by thedecision to use conventional ball-race bearings ( 3 mm-dia)and commercial 1 mm-dia N dBFe (neodym ium boron iron)permanent magnets. This led to a design with inner and outerradii of 2.1 mm and 3.7 mm for the electromagneticgenerator section, and 4.5 mm and 6.0 mm fo r th e turbineannulus. The rotor thickness was made equal to thepermanent magnet length of 1 mm .To form the stators, soft magnetic material waselectroplated into an annular cavity formed on the backsideof a 4"-dia silicon subs trate by D RIE. Two -layer spiral coilswere then formed on the front side by copper electroplatinginto thick resist moulds. The coils were em bedded in SUS,which also formed the m ask for through-wafer etching of theturbine fluid channel. Further details of this process arecontained in [ X I . Fig. 2 shows an SE M image of a partiallyfabricated stator, imm ediately prior to embedding o f thesecond coil layer.

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Figure 2: SE M photograph o partially fabricated stator,showing electroplatedplanar coils. Scale bar is 200p.The rotors were preformed by 2-layer SUS photoli thography,and then laser machined to define the curved turbine bladeprofiles (see Fig. 3 ) . A variable aperture mask, undercomputer numerical control , was used during lasermachining to achieve the required blade profiles [ 9 ] . Theguide vanes were formed in SUS stator inserts using a similarapproach. The rotor blades and guide vanes were designed onpaper using standard turbomachinery practice, assuming a

nominal shaft speed of 30,000 rpm and a pressure ratio of1.05. The same cross-sectional shape was used both for therotor blades the inlet stator guide vanes, with straight guidevanes being used o n the ou tlet stator.

Figure 3: SEMphotagraph of SU 8 polymer rotor with lasermicromachined 3 0 blades, viewedporn inlet side. Scale baris 1 mm.Following the laser machining process, permanent magnetsand a precision machined steel shaft were manually insertedinto the cavities in the rotor, and secured with SUS whichwas app lied with a needle and cured by UV flood exposureand heating. The entire device was then assembled on ametal jig w ith integra l pins to align the tw o stators. An SUSspacer was used to define the gap between the stators. Fig. 4shows a completed device viewed from the o utlet side, withthe guide va ne insert removed to reveal the rotor,

Figure 4:dimensions. including m etal casing are 18x18x9 mm.Photograph of un assembled protofype. Outer

3. CFD SIMULATIONSCFD numerical simulations of the turbine stage have beencam ed out using CFX-TA SCflow software. The geometrymodelled is shown in Fig. 5 , and corresponds to the actualblade profiles realised. These differ from the paper design inthat no attempt was made to introduce rounding on theleading edges of the blades. Also, the outlet stator wasomitted as it was not includ ed in th e prototype devices.

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A relatively coarse mesh of 30,000 elements was used tomodel overall stage performance, refined near the bladesurfaces to improve the m odelling of flow separation. A k-wShear Stress Transport (SST) turbulence model was used asthis is recommended for the accurate prediction of flowseparation near smooth walls for low-Reynolds numbercomputations. Walls were modelled as smooth and tipclearances were not included as the priority was to produceperformance maps in order to understand the hasic operatingcharacteristics of the turbine.

1 --0.20Stator (SI)

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Figuremm).5: Geometry used for CFD model (all dimensions in

Figure 6 shows a relative Mach number plot at midspan,assuming a rotation speed of 70,000 rpm and a pressure dropacross the stage of 50 mbar. Even at this rotation speed themaximum relative Mach number is fairly low (less than 0.3),indicating that the turbine should be far from choked whenoperated at or below the design speed.Figure 7 is a streamline plot, which reveals regions offlow separation around the rotor and stator blades. Theseregions correspond to regions of high loss.Near the fronts ofthe blades, this can be attributed to the sq uare leading edges.Rounding of these edges would improve performance, butwould make the fabrication process more complex byrequiring additional laser machining steps.By varying the CFD model boundary conditions (inletpressure and rotational speed) a set of theoreticalperformance curves was produced for the turbine stage overa wide ran ge of operating conditions. These indicated that theoptimum (i.e. maximum efficiency) operating point for themachine is at a somew hat higher pressure ratio of around 1. ,and at a higher rotation speed of around 140 krpm. Thetheoretical power output of the device turbine under theseconditions was found to be in excess of IO W. This resultillustrates the high theoretical power densities that can beachieved, although such high power operation would not bepossible in our devices because of materials limitations.

4. EXPEFUMENTALESTSFunctional tests of prototype devices have been performedwith compressed nitrogen. The turbine was inserted into a

test jig comprising a long tube with a bore matched to theouter diameter of the turbine annulus. Inlet and outlet coneswere attached to central portions of the stators on either sideof the device to reduce pressure losses associated with abruptchanges in the channel cross-section. The pressure dropacross the device was measured by a manometer, connectedvia small (0.5 mm dia) holes in the tube immediatelyupstream and downstream of the device. The flow rate w asmeasured by a precision gas flow meter (Cole Palmer, 16series, range 0-250 SLPM). Th e temperatures at the entranceand exit of the device were also measured, usingthermocouples.

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Gi0.30

0.250.20

0.150.10

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eFigure 6:Relative Mach num ber at mid-span fo r rotationspeed of 70 b p m an dpressu re drop o f 50 mbar.

Figure 7: Streamline plot fur same conditions as in Fig. 6.Figure 8 shows the measured variation of pressure dropwith flow rate up to around 60 LPM flow. Note that thethreshold pressure head for activation of the device is verylow at around 3 mbar. This is a key feature for directextraction of power from an ambient flow. Figure 8 alsoshows C FD predictions for the same operating range. These

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data points were obtained by entering measured values forpressure drop and rotation rate, and extracting the expectedflow rate. Given the approximate nature of the CFD model,the agreement between the experimental and theoreticalcurves is very good, lending credibility to the CFDcalculations.

o ! I0 10 20 30 40 50 60 70

Flow rate (LPM)Figure 8:variations ofpressure drop with nitrogenflo w rate.Figure 9 shows the measured variation of generator outputpower (per stator, into a matched load of 40 Q) with rotationspeed for a typical device. At a flow rate of 35 LPM, and arotation speed of 30,000 rpm, each stator can deliver a pow erof 1.1 mW, which would be sufficient for many remotesensing applications. Note, however, that the generatedpower is only a small fraction (around I%) of the expectedoutput power of the turbine as predicted b y CF D. We believethat most of this power is being dissipated in windage andbearing losses

Comparison between measured and predicte d

1000

10 0

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0.1

0.01

-Generator (expt)-Turbine (CFD)

0 10 20 30 4 0 50 60 70Row rate (LPM)

Figure 9: Comparison between measured electromagneticgenerator output pow er and C FD-p redicted turbine outputpow er, both as function o fflow rate.5. DISCUSSIONND CONCLUSIONSThe prototype devices described in this paper are alreadycapable of generating potentially useful levels of power when

subject to small pressure ratios compatible with energyscavenging from am bient flows. However, comparison of theCFD and experimental results suggests that it should hepossible either to (a) achieve similar output power levelswith a smaller turbine andor lower pressure head if lossescan he reduced, or @) generate higher output power levelswith the same turb ine if the power output capabilities o f thegenerator can be enhanced. The latter could he achieved byincreasing one or more of the following: the number of coillayers on each stator, the fill factor of the coils(copperldielectric ratio) or the number and/or stren gth of thepermanent magn ets. Placing the generator outside the turbinewould also help to balance the maximum outputs of the twodevices. W e will explore these possibilities in future work .

ACKNOWLEDGEMENTSThis work was supported by the UK Engineering andPhysical Sciences Research Council, under Grant No.G R N I 8 89 5 - Microengineered Axial-Flow Pumps andTurbines. Laser microm achining facilities were kindlyprovided by Exitech Limited, Oxford, UK.

REFERENCES[ l ] C.B. Williams, C. Sheanvood, M.A. Harradine, P.H. Mellor,T.S. Birch, R.B. Yates, Development of an electromagneticmicro-generator, IEE Proc. Circuits, Devices and Syst., vol.148(6),pp. 337-342,2001,[2] S. Meninger, J.C. Mu-Miranda, R. Amirtharajah, A.P.Chandrakasan, J.H. Lang, Vibration-to-electc energyconversion, IEEE Trans. VLSI Syst., vol. 9(1), pp. 64-76,2001.[3 ] P.D. Mitcheson, P. Miao, B.H. Stark, A S . H olmes, E.M.Yeatman, T.C. Green, Analysis and optimisation of MEMSelectrostatic on-chip power supply for self-powering of slow-

moving sensors, Proc. Eurosensors XVII, Guimaraes,Portugal, Sept 21-24.2003, pp. 492-495.[4] A.H. Epstein, Millimeter-Scale, MEMS Gas TurbineEngines, Proceedings of ASME Turbo Expo 2003, Powerfor Land, Sea and Air, Atlanta, June l6-19,2003.T.G. Wiegele, Micro-turbo-generator design and fabrication:a preliminary study, Proc. IECE 96, vol. 4, 1996, pp. 2308-2313.[6] A. Cavagnino, M. Lazzari, F. Profurno, A. Tenconi, Acomparison between the axial flu and the radial fluxstmchxes for PM synchronous motors, IEEE Trans. IndustryAppl., vol. 38(6), pp. 1517 -1524,2002.P.A. Gilles,J. Delamare, 0. Cugat, J.L. Schanen, Design of apermanent magnet planar synchronous micromotor, IndustryApplications Conference, Oct. 8-12. 2000, IEEE Conference

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Record, vol. 1, pp. 223 -227.G. Hone. A S . Holmes. M.E. Heaton. K.R. Pullen. Desim.81_ -. ~.fabrication and characterization of axial-flow &bine forflow sensing, Proc. Transducer03, Boston, June 8-12, 2003,pp. 702-705.A.S. Holmes, M.E. Heaton, G. Hong, K.R. Pullen and P.T.Rumsby, Laser Profiling of 3-1) M icroturbine Blades, Proc.LPM 2003, Munich, June 21-24,2003,

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Axial-flowm Icroturbiwniet h Electromagnegtenicer Ator