TTA – Thermal Transient Anemometer

1. 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.

2. 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.

3. Automotive Applications Underhood cooling circuit HVAC ducts – flow rate distributions and thermal energy loss upstream of the register

4. Underhood Cooling Obvious objectives: transfer the thermal energy from the liquid media to the passing wind. Obvious statement of success:

5. 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 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.

6. Control electronics Component Elements of the TTA • 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.)

7. 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

8. 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.

9. 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

10. Morris and Foss (2003) –Transient…HWA For h ≈ constant, T(t) for heat transfer dominated by the forced convection term is exponential since (4) (5) Rn = Rn(T0)[1+(TnT0)] (6) (7) (8)

11. Calibration Process Fit R(t) data to: Utilize calibration data to determine spatially averaged velocity for the cell as: (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.)

12. Typical Calibration Data

13. 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.

14. Calibration ‘Hot Box’ Schematic

15. Data for the determination of  Symbols show R(T) for different cells. ( = [slope]-1)

16. 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).

17. 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. 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.

18. Ford Haus Heater – Temperature Sensors Solid State Temp. Sensors. Pitot probe with adjacent Therms. Couple for velocity measure-ments. 5 lengths of 0.005” (0.127 mm) tungsten wire

19. Thermal Profile at level of sensor wire 5.66 m/s 1.49 m/s ← Location of Sensor Wire →

20. Velocity Profile at level of sensor wire

21. 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 Evaluate

22. 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.

23. 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.

24. 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.)