Transient thermal response of a hot-wire anemometer

The ability of a thermal anemometry system to accurately measure unsteady fluid velocity depends on the electrical control system as well as the thermal properties of the sensor. The present work is a numerical study of the thermal transient response of a hot-wire. A conventional constant temperature anemometer with an ideal feedback amplifier as well as a pulse width modulated system were used to model the electrical current supplied to the sensor to maintain a nominally constant sensor resistance. The agreement between these two electrical models confirmed that the response characteristics are only due to thermal effects. The thermal response was tested by providing a known input function for the cooling velocity, and comparing this with the output of the model. The first test used a step input function. It was found that the thermal transient effects along the length of the sensor caused the system to initially under predict the actual velocity increase; this was followed by an exponential increase to the steady state velocity. Secondly, the model was tested with sinusoidal inputs over a wide frequency range. The ratio: indicated-velocity/input-velocity, as a function of the input frequency was used to characterize the 'thermal frequency response'.