Hot Wire Anemometry

1. Measure the width of the slit, W, and record this value in table 1.
2. Verify that the data acquisition system follows the schematic in Figure 2.
3. Connect the positive port of the pressure transducer (see Figure 2 for reference) to the plenum pressure tap ().
4. Leave the negative port of the pressure transducer open to the atmosphere. Hence, the reading of this transducer will be directly; as required by equation (7).
5. Start the program for hot-wire calibration. Set the sample rate at 100 Hz for a total of 1000 samples (i.e. 10s of data).
6. Make sure that channel 0 in the data acquisition system corresponds to the voltage of the hot-wire anemometer.
7. In the field corresponding to channel 0, select the value of the constant to 0.45.
8. Set the hot-wire anemometer at the position of the vena contracta (on the centerline, at x = 1.5 W).
9. Make sure that channel 1 in the data acquisition system corresponds to the signal of the pressure transducer.
10. Enter the values of local air density (typically 1.2 kg/m³ for average local conditions) and conversion constant from volts to pressure (76.75 Pa/V) in the fields corresponding to the pressure transducer. Record these values in table 1. With this, the data acquisition system will report the data directly in velocity in m/s according to equation (7).
11. Cover the stack completely to establish the condition for maximum velocity at the jet.
12. Turn on the flow facility.
13. Acquire a dataset.
14. Change the stack plate for one with a lower restriction (larger diameter)
15. Acquire a dataset.
16. Repeat steps 1.15 and 1.16 for a total of at least four times. Make sure that the last repetition is carried out with the stack fully unrestricted (lowest jet velocity).
17. The data acquisition program will carry out the least-squares calculation and report the calibration constants automatically. Record these values on table 1.

Table 1. Basic parameters for experimental study.

Parameter	Value
Slit width (W)	19.05 (mm)
Air density (r)	1.2 (kg/m3)
Transducer calibration constant (m_p)	76.75 (Pa/V)
Calibration constant A	5.40369 (V2)
Calibration constant B	2.30234 (V2 s0.65m-0.65)

Figure 3. Hot-wire anemometer circuitry. (A): Wheatstone bridge circuit to ensure constant temperature in the hot-wire. (B): detail of the structure of a hot-wire. Please click here to view a larger version of this figure.

Figure 4. Flow control in the flow system. The stack on top of the plenum serves the purpose of diverting flow from the jet slit allowing to control the jet's exit velocity. Please click here to view a larger version of this figure.