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  • 8/6/2019 Acoustics 08 Kees de Blok

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    8 juli 2008 1

    AcousticsAcoustics--08 Paris08 Paris

    Low operating temperature integral systemsA novel hybrid configuration TA engine

    Kees de BlokAster Thermoakoestische Systemen

    General system aspects

    Reduction of average regenerator impedanceNovel hybrid configuration

    Experimental verification

    Conclusions

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    General system aspectsGeneral system aspects

    Fixed power driven TA engines T3 >> T2 gain proportional with load

    Acoustic power constant

    No or little hot hex losses

    Fixed temperature driven TA engines

    (T3 - T2) to be minimized

    fixed gain (per stage)

    acoustic power inversely proportional to load

    hex design crucial

    Hx-h

    Reg

    Hx-c

    heat source

    AcousticLoop power

    acoustic

    load

    T0

    T1

    T2

    T3

    Pertain to nearly all practical applications

    - Useful in TA experiments

    - Little practical use

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    General system aspectsGeneral system aspects

    Loop, transferred and available power

    walllamLoss ApP ..2

    0_

    )Re(.2

    . 210

    L

    LoadZ

    pAP =

    D0

    TA ZLp1

    (or Helmholtz type)

    Pheat Pac_out Pac_load

    p0

    Minimize ratio acoustic loss / transfered power

    Two way strategy

    1. Minimize acoustic losses

    - Avoid high local amplitude (no standing waves)- Shape and wall finish (in turbulent regime only)

    - Lower limit by thermal and viscous boundary layer losses !

    2. Maximize transferred or loop power

    - Decrease average regenerator and load impedance

    = 1.__

    C

    Hloopacoutac

    T

    TPP

    )Re(.2

    .2

    0

    _

    reg

    reg

    loopacZ

    pAP =

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    General system aspectsGeneral system aspects

    Impact of average regenerator impedance on available acoustic power

    Assumptions:

    ideal 1 stage engine

    0

    R 0

    2 1

    Impedance setting f (Rreg)

    loss calculated for resonator small signal regime (laminar)

    300 400 500 600 700 800 9000

    0.2

    0.4

    0.6

    0.8

    1

    TH

    [K]

    1-loss/P

    out

    TC

    = 300 K

    Zreg

    = 5. .c

    Zreg = 10..cZ

    reg= 20..c

    - Onset and small signal behavior depends on configuration and geometry only

    - Acoustic loop power times gain less than acoustic loss for Zreg= 20..c and TH < 410 K

    Average regenerator impedance should be set to a minimum value for low and medium

    operating temperatures

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    10

    20

    30

    30

    210

    60

    240

    90

    270

    120

    300

    150

    330

    180 0Z/Z0

    2.0 Z0

    1.5 Z0

    1.0 Z0

    0.5 Z0

    Reduction of average regenerator impedanceReduction of average regenerator impedance

    For default torus or bypass geometry

    Zreg commonly set to > 15..c

    Inertancetorus

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    Hybrid configurationHybrid configuration

    Back to basics !(re-arranging acoustic circuitry and

    apply all tricks from the past)

    Traveling wave loop (0) Initial used by Ceperly for timing

    Now used to avoid high localamplitudes

    Torus or bypass

    Compliance (3)

    Inertance (2)

    Velocity reduction

    Additional bypass (1)

    Areg > A0

    a

    b

    30

    TC

    21

    TH

    S12

    S11

    2-port

    a b

    feedback

    membrane

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    0 50 100 150 2000

    0.2

    0.4

    0.6

    0.8

    1

    freq [Hz]

    |s|

    s11

    measured

    s12

    calculated

    s11

    calculated

    Hybrid configurationHybrid configuration

    Feedback loop

    Travelling wave if

    S11 0 (no reflection)

    Oscillation if

    net forward acoustic power gain 1

    phase delay of 2-port plus feed back loopequals 2. ( = )

    Average regenerator impedance absolute value relatively low

    phase nearly independent of regeneratorflow resistance

    1

    2

    3

    4

    5

    30

    210

    60

    240

    90

    270

    120

    300

    150

    330

    180 0

    Zreg

    /.c

    1.0 .c

    0.5 .c

    0.2 .c

    0.1 .c

    0

    0

    .

    A

    cZ

    =

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    Hybrid configurationHybrid configuration

    Comparison with the classic torus or bypass configuration

    Regenerator impedance can be set to arbitrary (lower) value

    Absolute value depends on geometry and regenerator flow resistance Phase (timing) depends primarily on geometry

    Regenerator mass can be minimized

    Lower onset temperature

    Steeper slope Pac/T

    Becomes a function of acoustic power (related to heat capacity)

    Efficiency improves for lower regenerator flow resistance (as should be the casefor thermo-dynamic systems in general)

    Reduced impedance allows for multiple regenerator units

    Extended soft spot High acoustic power at given amplitude (near traveling waves)

    System more compact

    Low acoustic loss / power ratio (no extreme local amplitdes)

    Streaming suppression (e.g. membrane) on convenient location

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    Experimental verificationExperimental verification

    Measurement setup 2 stage engine

    Water circuits High temperature

    Gas fired water heater

    Max 160 C (10 bar) Low temperature

    Car radiator to air

    Flow 1.5 l.min-1

    20-30 C

    Acoustic power measurements Pressure gradient method

    Temperature measurements Water in out (T0,T3)

    Regenerator high low (T2, T1)

    #1

    Pac2

    Pac1

    TC

    Water Thigh(70-160 C)

    #2

    TH

    Water Tlow(20-30 C)

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    Experimental verificationExperimental verification

    Typical pressure amplitude distribution at oscillation (Measured Values in kPa)

    D0 = 67 mm

    Areg =100 x 120 mm

    Lreg = 1.58 mm

    Por = 0.74

    Lhex = 0.56 mm

    Air @ 98 kPa

    Frequency = 132 Hz Twater_in =148 C

    Twater_uit = 28C

    3.54 (42.4W)3.41

    2.77

    4.363.753.17

    1.87

    (32.3W)

    5.22

    - Near traveling wave in feedback loop (SWR < 1.2)

    - No extreme amplitude maxima or minima

    - High acoustic power while only 3.5% drive ratio in feed back loop (resonator)

    - Reduced regenerator impedance (Zreg 3 .c )

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    Experimental verificationExperimental verification

    Onset temperature samefor air and argon

    Text - Tint proportional

    with power GHEX 7 W.K

    -1 (air)

    Hex temperature droprelated to gas heat

    conductivity

    Slope Pac / T related toviscosity

    Slope Pac / T increaseswith power better heat exchange (Re, Nu)

    Higher efficiency

    0

    5

    10

    15

    20

    25

    30

    35

    40

    45

    0 50 100 150

    Th-Tc. [K]

    Pac1[W

    ] air int

    air ext

    argon int

    argon ext

    TOnset

    TAvailable

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    ConclusionsConclusions

    Classic standing wave resonator combined with a high regenerator impedance(set by torus or bypass) impede low and medium temperature applications

    To overcome these limitations an example of a novel hybrid configuration isproposed

    Experimental results agree well with theory behind and indicate a significantimprovement in onset temperature and power density

    With air at atmospheric pressure

    applied onset temperature difference 63 K

    acoustic power 42 W at 3.5% drive ratio

    For efficient low temperature operation at 160 C onset temperature and hextemperature drop still to high

    Hybrid configurations allows for further optimizing TA engines in the low andmedium operation temperature regime