TTA – Thermal Transient Anemometer

Anemos: Greek for wind

Anemometer: to measure the wind

Thermal Transient: A heated sensor will lose energy to the passing wind. The higher the speed, the faster the loss and the shorter the “time constant ()” of the temperature decrease.

Ergo: utilize as the anemometer’s output.

TTA – Thermal Transient Anemometer

Developed for underhood cooling circuit diagnostic evaluations: – velocity and temperature distributions averaged over

segments of the in-line heat exchanger

Developed under the sponsorship of

DiamlerChrysler Challenge Fund (originally with Mr. Clem Mesa, continued with Mr. Michael Zabat)

Patent Pending – MSU

Commercialized by DFTI – Digital Flow Technologies, Inc.

Automotive Applications

Underhood cooling circuit

HVAC ducts – flow rate distributions and thermal energy loss upstream of the register

Underhood Cooling

ObviousObvious objectives: transfer the thermal energy from the liquid media to the passing wind.

ObviousObvious statement of success:

)()(

ˆ)(ˆ)(.

III

dAnVTpCinletAdAnVTpC

exitAQ

Underhood Cooling (cont.)

ObviousObvious Problem: it is not feasible to construct a measurement scheme to obtain the infinite number of data points to evaluate the exit integral – assuming Tinlet=Tamb such that

TTA Strategy: obtain approximations to the spatial integral for area segments whose sum is the complete area of interest.

Diagnostic strategy: make the segments small enough that problem areas (e.g., downwind from crash members) are apparent.

I)TCm( ambp

Component Elements of the TTA

• Control electronics

• A frame with 8 cells to fit a heat exchanger

• A frame with 16 cells to fit the subject heat exchanger

(The control electronics schematic is provided in the TTA portion of www.dift-us.com.

See the MST article.)

Custom Frames

A representative frame, mounted for calibration in the TSFL 22 (6161cm2) wind tunnel.

A 20-cell frame:Tungsten Wires

Frame Perimeter

Pitot Probe

Sensor Wires

Typical Tungsten wire diameter = 5-8mil (0.127 to 0.203 mm)

Sensor wires are robust a la wind loads, dust, etc. impact.

• Hairs and grit will change the heat transfer coefficients but these can be cleaned off.

• Plastic deformation will nullify the basic calibration.

Three Stage Functioning of the Control Electronics (per cell)

1) Obtain Tamb from R(Tamb)=R(T0) [1+(TambT0)]

2) Introduce heating current (I) such that:

I2RsensorTsensor≲Tmax • where Tmax≲oxidation temperature

3) Cease heating current • utilize measurement current (ca 10ma) to record R(t) during

the temperature “decay” to Tamb

Morris and Foss (2003) – Transient…HWA

For h ≈ constant, T(t) for heat transfer dominated by the forced convection term is exponential since

].[] ambnsnnconv TTxDhq

45.033.026.0 ]]57.0]42.0 fefrfruf

RPPNk

hd

sn

N

ns TRR

1

Rn = Rn(T0)[1+(TnT0)]

])[/1( ambss TTTdt

d

(4)

(5)

(6)

(7)

(8)

Calibration Process

Fit R(t) data to:

Utilize calibration data to determine spatially averaged velocity for the cell as:

)/texp()0(RR

)0(R)t(R

max

nBVA1

(Note, this form of the TTA transfer function is motivated by that for a constant temperature anemometer. It is supported by the observed agreement with experimental data.)

Typical Calibration Data

)0(

)0()(*

max RR

RtRR

Wire Resistance vs. Time

0

0.2

0.4

0.6

0.8

1

0 0.05 0.1 0.15 0.2 0.25 0.3 0.35 0.4 0.45 0.5

time (s)

R*

v1 = 1.58 m/s

v2 = 3.08 m/s

v3 = 9.21 m/s

Exponential decay region evaluated for curve fit

Evaluated Region

-1.25

-1

-0.75

-0.5

-0.25

0

0 0.01 0.02 0.03 0.04 0.05 0.06 0.07 0.08 0.09 0.1 0.11

time (s)

ln(R

*)

v1 = 1.58 m/s

v2 = 3.08 m/s

v3 = 9.21 m/s

Note: ln(R*) = -t/

V1: = 93.4 ms, = 576 s, = 6.17e-3

V2: = 72.3 ms, = 312 s, = 4.32e-3

V3: = 47.6 ms, = 225 s, = 4.73e-3

Calibration Results

0

5

10

15

20

25

0 1 2 3 4 5 6 7 8 9 10

Velocity (m/s)

1 (

s-1)

a = 3.663 s-1 b = 5.768 sn-1/mn n = 0.4981 = 0.009

  tau (ms) σ (μs) σ/tau

V1=1.58 m/s 93.4 576 6.17E-03

V2=3.08 m/s 72.3 312 4.23E-03

V3=9.21 m/s 47.6 225 4.73E-03

Determination of the Temperature Coefficient of Resistance ()

Can be influenced by the alloys in the Tungsten, by the annealing processes, by witchcraft.

Hence, a hot-box has been constructed to determine for each cell of a completed frame.

Calibration ‘Hot Box’ Schematic

Data for the determination of

6.00

6.50

7.00

7.50

8.00

8.50

9.00

280.00 300.00 320.00 340.00 360.00

Temperature (K)

Res

ista

nce

(O

hm

s)

0.00

10.00

20.00

30.00

40.00

50.00

60.00

0.00 0.05 0.10 0.15 0.20 0.25[R(T)/R(T0)]-1

T-T

0 (K

elv

in)

T-T0

Linear (T-T0)

Symbols show R(T) for different cells.

( = [slope]-1)

Compensation for =(V) if Tamb(test)≠Tamb(cal)

Conflicting information from basic heat transfer sources.

Fabrication and use of a test facility to directly evaluate the effect of Tamb(test)≠Tamb(cal).

Ford Haus HeaterFord Haus: A sub-atmospheric flow facility that allows the operator and test chamber to be on the upwind

side of the external prime mover.

Insulation is visible through the clear plastic side wall of

the plenum chamber.

View from entry door into the 2.6m x 1.83m “Haus”.

Ford Haus Heater – Temperature Sensors

Pitot probe with

adjacent Therms.

Couple for velocity

measure-ments.

Solid State Temp.

Sensors.

5 lengths of 0.005”

(0.127 mm)

tungsten wire

Thermal Profile at level of sensor wire

Left-Right Thermal Profile

0

20

40

60

80

100

120

0 50 100 150 200 250 300 350 400

Position (mm from left)

Tem

p. (

C )

1

2

3

4

5

6

7

8

9

10

Vel

oci

ty a

t 4"

po

siti

on

wit

h P

ito

t (m

/s)

← Location of Sensor Wire →

5.66 m/s

1.49 m/s

Velocity Profile at level of sensor wire

Velocity Profile Across range of interest @ ambient temp

3.5

3.6

3.7

3.8

3.9

4.0

4.1

4.2

4.3

4.4

4.5

100 150 200 250 300 350

distance (mm) from reference

Ho

twir

e V

olt

s

Basic Data from 3 Calibrations

Tamb’ = 22°C V = 1.5 – 6.5 m/s

Tamb’’ = 72°C V = 1.5 – 6.0 m/s

Tamb’’’ = 103°C V = 1.5 – 6.0 m/s

Evaluate45.0(Re)

1BA

Calibration Data

It is inferred that the jump discontinuity for the 22°C=Tamb values represents the transition to vortex shedding. Future measurements for 72°C and 103°C=Tamb cases will test the hypothesis with higher velocity values.

1/Tau = 1.4478 * RE + 0.6652

1/Tau = 1.227 * RE0.45 + 1.2308

1/Tau = 1.59 * RE0.45 + 0.6117

1/Tau = 1.0067 * RE0.45

+ 1.8013

4.00

5.00

6.00

7.00

8.00

9.00

10.00

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

RE0.45

1/T

au

22 C

72 C

103 C

Linear(22 C)

Compensation for Elevated Tamb

The “A” terms, which represent free convection and heat loss by conduction, have been divided by their respective (Thot-Tamb) values. The averae of the three ratios was 0.0062. The three “low Re” data sets were brought to a common ordinate-intercept as A=0.0062 ΔT(°C). The three calibrations could then be made to agree by scaling the B’ terms with respect to ΔT.

1/Tau = A + B() * RE0.45

A = .0062 Delta TB = 1.26 @ 22 CB = 1.42 @ 72 CB = 1.59 @103 C

4.00

5.00

6.00

7.00

8.00

9.00

10.00

2.00 2.50 3.00 3.50 4.00 4.50 5.00 5.50 6.00 6.50

RE0.45

1/T

au

22 C

72 C

103 C

Conclusions

1.) Step change in heat transfer, not previously seen with the TTA, is present with larger diameter sensor wires.

2.) Installed (in a frame) evaluation of is required for accurate Tambient measurements.

3.) Elevated (cf velocity calibration) temperature effects for a test condition must be addressed. (This presentation is advanced cf the associated SAE paper (#05VTMS-103). Further work is in progress.)