Transient thermal response of a hot-wire anemometer

Abstract

The ability of a thermal anemometry system to accurately measure unsteady fluidvelocity depends on the electrical control system as well as the thermal propertiesof the sensor. The present work is a numerical study of the thermal transientresponse of a hot-wire. A conventional constant temperature anemometer with anideal feedback amplifier as well as a pulse width modulated system wereused to model the electrical current supplied to the sensor to maintain anominally constant sensor resistance. The agreement between these twoelectrical models confirmed that the response characteristics are onlydue to thermal effects. The thermal response was tested by providing aknown input function for the cooling velocity, and comparing this with theoutput of the model. The first test used a step input function. It wasfound that the thermal transient effects along the length of the sensorcaused 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 frequencyrange. The ratio: indicated-velocity/input-velocity, as a function of theinput frequency was used to characterize the ‘thermal frequency response’.