Measured heat transfer from thin wires in air

Measurements were performed on a wire made of platinum-plated tungsten with a diameter of 5µm and a length of 10mm. The aspect ratio of 2000 ensures negligible influence from end effects. The sensor wire was mounted between supporting prongs and connected to a variable power supply via a series resistor and a current shunt resistor. The voltages across the wire and the shunt resistor were measured using a Keysight data acquisition unit 34970a. The wire resistance and heat dissipation rate were calculated. The wire temperature was determined from the resistance using calibrated data.

Measurements were carried out in two wind tunnels. In the velocity range of 0.3–3 m/s, a small closed-loop wind tunnel was used, where air velocity and temperature could be independently controlled within ranges of 0.3–3 m/s and 10–60°C, respectively. Additional measurements in the 5–30 m/s range were performed in an open-circuit calibration wind tunnel at temperatures between 20 and 40 °C.

For selected values of the wind tunnel temperature and velocity, the current flowing through the wire was manually adjusted to reach a desired sensor temperature, and steady-state measurements were then taken as mean values over a one-minute interval. Air and sensor temperatures were varied in ten-degree increments. Using this procedure, a map of data was compiled across various air velocities, air temperatures, and wire temperatures.

Before conducting heat transfer measurements, the sensor wire was calibrated for temperature in a calibration chamber with circulating air at a range of 10–75°C, and resistance-temperature data were fitted to a second-order polynomial. The estimated uncertainty in measured temperatures is ±0.05 K. The electrical power is measured with an uncertainty of less than ±0.2%, and the uncertainty in air velocity is estimated at ± (0.02 m/s + 0.5%) for velocities up to 3 m/s (i.e., in the smaller precision wind tunnel) and ± (0.05 m/s + 0.5%) for higher velocities. The turbulence intensity was less than 0.7% and 0.5%, respectively.