Miniature Thermoacoustic Engines. Ultrasonic Ranges

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Primrose Janel Dickerson
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1 Miniature Thermoacoustic Engines in the High Audio and Low Ultrasonic Ranges Myra Flitcroft and O.G. Symko University of Utah This work was supported by the Army Space and Missile Defense Command

2 Outline Introduction / motivation Theory thermoacoustics Experimental ldetails Single engine performance Theory synchronization Synchronization observations Conclusions

3 Introduction Thermoacoustic engines convert input heat to Thermoacoustic engines convert input heat to sound

4 Threshold behavior As the temperature gradient builds up, parcels of air diffuse along stack from the hot to cold side When parcels reach the cold side, they compress quickly, generating many frequencies Onefrequencyis reinforced through positive feedback by the resonator As the gradient increases, there will be enough pops to sustain oscillation a standing wave is set up

5 Thermocoustic effects Glass blowing Taconisoscillations ill Thermoacoustic engines either prime movers or refrigerators Luminosity oscillations in variable stars

6 Thermoacoustic applications Green energy: Clean refrigeration Energy harvesting

7 Thermoacoustic Cycle

8 Motivation Thermoacoustic engines have conventionally operated at frequencies of Hz. However, theory predicts that higher frequency engines have a higher power density. They can be assembled into multiple engine arrays for higher power output. Arrays of miniature engines can be used for energy conversion at small scales.

9 Motivation Advantages of higher frequency engines Smaller engines smaller volumes higher h power density Lower masses short response time Piezoelectric power density increases with frequency Many units can be combined to form an array MEMS

10 Goals Reduce thermoacoustic engines length scales from mid audio to ultrasonic range, by first building 10 khz range devices and then further reducing to 20 khz range engines Observe in phase synchronization between a pair of ultrasonic engines

11 Quarter wave resonator x p P 1 A sin cos t x, t u x t I p, ~ 1 1 PA x u1 cos sin a m t

12 Engine Components

13 Cavity provides positive feedback piezoelectric transducer can be attached thermal anchor allows for pressurization or the use of different gases

14 Linear Thermoacoustics Thermal penetration depth Critical ii temperature gradient Acoustic power density

15 Assembly challenges

16 Experimental Details Heat was injected using heater wires. Temperature measurements were made with thermocouples read by a USB DAQ module. Pressure measurements were made with a piezoelectric i transducer read by a PCI DAQ board. All measurements were recorded using NI Signal Express and analyzed with NI DIAdem.

17 Experimental details

18 Single Engines 10 khz range 86mm 8.6 long, with 1 and 2 mm diameters 20+ khz range 34mm 3.4 long with 1 mm diameter

19 AUDIO 20Hz 2kHz 20kHz ULTRASONIC 20kHz 48kHz

20 10 khz (2mm) engine run

21 10 khz engine run

22 10 khz engine run

23 10 khz engine run

24 10 khz JTFA

25 Onset

26 Ultrasonic engine run

27 Ultrasonic engine run

28 Ultrasonic engine run

29 Engine characteristics Engine Frequency (khz) Onset delta T (ºC) Max. Pressure Amplitude (Pa) Onset time (s) 10 khz 1mm khz 2mm ultrasonic, 1mm

30 Self sustained oscillators have aean energy e gysource, but are aenot driven oscillator s vibration affects the source s vibration the phase of oscillations is random intrinsically non linear other examples are lasers, electronicoscillators oscillators, many clocks, and biological systems. An important characteristic of self sustained oscillators is that they can be synchronized.

31 Synchronization Courtesy of Arkady and Pikovsky, 2007, scholarpedia.org

32 Synchronization In thesynchronization region, theoscillators become phase locked, generally either: In phase Or anti phase Courtesy of Arkady and Pikovsky, 2007, scholarpedia.org

33 Synchronization set up

34 10 khz synchronization

35 10 khz synchronization JTFA

36 10 khz amplitude comparison

37 21 khz synchronization

38 21 khz synchronization JTFA

39 Conclusions Designed, assembled, and studied engines in 10 khz and low ultrasonic ranges. Engine performance scaled as expected with frequency. In phase synchronization i was observed at both frequency ranges. Opened the field to MEMS technology for fabrication of the devices in large arrays.

40 Thank you!

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