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
Obvious objectives: transfer the thermal energy
from the liquid media to the passing wind.
Obvious statement of success:
.


Q   A (C pT )V nˆdA  A
(C pT )V nˆdA
exit
inlet
(II )
(I )
Underhood Cooling (cont.)
Obvious 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
 Cp Tamb )  I
(m
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.
Component Elements of the TTA
• Control
electronics
• A frame with 8
• A frame with 16
cells to fit a heat
cells to fit the
exchanger
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 22 (6161cm2) wind tunnel.
A 20-cell frame:
Frame Perimeter
Tungsten
Wires
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+(TambT0)]
2) Introduce heating current (I) such that:
I2RsensorTsensor≲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
qconv ]n  hnDs x[Tn  Tamb ].
(4)
hd
 Nu  0.42Pr ]0f.26  0.57Pr ]0f.33 Re ]0f.45
kf
(5)
Rn = Rn(T0)[1+(TnT0)]
(6)
N
Rs   Rn  Ts
(7)
n 1
For h ≈ constant, T(t) for heat transfer dominated by the
forced convection term is exponential since
d
Ts  (1 /  )[Ts  Tamb ]
dt
(8)
Calibration Process
Fit R(t) data to:
R ( t )  R (0)
 exp( t / )
R max  R (0)
Utilize calibration data to determine spatially
averaged velocity for the cell as:
1
 A  BV n

(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
Wire Resistance vs. Time
R(t )  R(0)
R* 
Rmax  R(0)
1
0.8
Exponential decay
region evaluated for
curve fit
R*
0.6
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
v1 = 1.58 m/s
v2 = 3.08 m/s
v3 = 9.21 m/s
0.4
0.2
0
0
0.05
0.1
0.15
0.2
0.25
0.3
0.35
0.4
0.45
0.5
time (s)
Evaluated Region
Calibration Results
0
25
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
-0.25
20
-0.5
-1
1
 (s )
ln(R*)
15
v1 = 1.58 m/s
v2 = 3.08 m/s
v3 = 9.21 m/s
-0.75
-1
10
a = 3.663 s-1 b = 5.768 sn-1/mn n = 0.4981 = 0.009
5
-1.25
0
0
0.01
0.02
0.03
0.04
0.05
0.06
time (s)
0.07
0.08
0.09
0.1
0.11
0
1
2
3
4
5
Velocity (m/s)
6
7
8
9
10
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
9.00
60.00
8.50
50.00
8.00
T-T0 (Kelvin)
Resistance (Ohms)
Data for the determination of 
7.50
7.00
6.50
6.00
280.00
40.00
30.00
20.00
10.00
300.00
320.00
340.00
Temperature (K)
Symbols show R(T) for
different cells.
360.00
0.00
0.00
T-T0
Linear (T-T0)
0.05
0.10
0.15
[R(T)/R(T0)]-1
( = [slope]-1)
0.20
0.25
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 Heater
Ford Haus: A sub-atmospheric flow facility that allows the operator and test chamber to be on the upwind side
of the external prime mover.
View from entry door into
the 2.6m x 1.83m “Haus”.
Insulation is visible through
the clear plastic side wall of
the plenum chamber.
Ford Haus Heater –
Temperature Sensors
Pitot probe
with
adjacent
Therms.
Couple for
velocity
measurements.
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
10
120
5.66 m/s
9
100
1.49 m/s
Temp. ( C )
80
7
6
60
5
← Location of Sensor Wire →
40
4
3
20
2
0
0
50
100
150
200
Position (mm from left)
250
300
350
1
400
Velocity at 4" position with Pitot (m/s)
8
Velocity Profile at level of sensor wire
Velocity Profile Across range of interest @ ambient temp
4.5
4.4
4.3
Hotwire Volts
4.2
4.1
4.0
3.9
3.8
3.7
3.6
3.5
100
150
200
250
distance (mm) from reference
300
350
Basic Data from 3 Calibrations
Tamb’ = 22°C
Tamb’’ = 72°C
Tamb’’’ = 103°C
V = 1.5 – 6.5 m/s
V = 1.5 – 6.0 m/s
V = 1.5 – 6.0 m/s
Evaluate
1

 A  B(Re)
0.45
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.
10.00
1/Tau = 1.4478 * RE + 0.6652
9.00
0.45
1/Tau = 1.59 * RE
+ 0.6117
8.00
1/Tau
0.45
1/Tau = 1.0067 * RE
+ 1.8013
72 C
7.00
103 C
6.00
0.45
1/Tau = 1.227 * RE
+ 1.2308
Linear
(22 C)
5.00
4.00
2.00
22 C
2.50
3.00
3.50
4.00
4.50
RE0.45
5.00
5.50
6.00
6.50
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.
10.00
9.00
1/Tau
8.00
22 C
7.00
()
6.00
5.00
4.00
2.00
0.45
1/Tau = A + B * RE
A = .0062 Delta T
B = 1.26 @ 22 C
B = 1.42 @ 72 C
B = 1.59 @103 C
2.50
3.00
3.50
4.00
4.50
RE0.45
5.00
5.50
6.00
72 C
103 C
6.50
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.)