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 (CT), 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 multilayer 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 (PI) 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 Red = 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, to investigate drag reduction mechanisms, and to enhance environmental perception in flow field.