Constant Temperature Hot-film Sensor for the Measurement of Near-wall Turbulence and Flow Direction

Wall shear stress and flow direction provide a basis for analyzing the boundary layer conditions, investigating drag reduction mechanisms, and enhancing environmental perception. This work presents novel single-loop and dual-loop hot-film sensors driven by the constant temperature, which are capable of simultaneously measuring wall shear stress and flow direction. Based on the heat transfer and fluid dynamics theory, a mathematical model is developed to analyze the flow directions. The sensors feature multi-layer structures, where the numerous leads are concealed and embedded in the insulation layer to enhance their robustness and integration. Utilizing the microelectromechanical system technology, sensor prototypes with single-loop and double-loop are fabricated. In particular, a new process method for accomplishing junction holes in the polyimide insulation layer is proposed. The sidewall-to-bottom angle of junction holes fabricated through wet etching is ~29.4°. After metal lays are deposited in the junction holes, the upper and bottom surfaces of the insulation layer are able to conduct electricity. Moreover, a testing system consisting of a microchannel and a turbulence generator is established to carry out the experimental verification. Then, the hot-film sensors are tested in the microchannel with a maximum Reynolds number of Re _d = 8600. Low-frequency turbulence as well as natural transition signals are detected by the hot-film sensors successfully. In the range of wall shear stress from 0 to 14.2 Pa, the accuracies of the dual-loop and single-loop hot-film sensors in perceiving flow directions are better than ± 3° and ± 6.5°, respectively. This work assists to analyze the boundary layer states, investigate drag reduction mechanisms, and enhance environmental perception in flow field.