Toward calibration-free wall shear stress measurement using a dual hot-film sensor and Kelvin bridges

Abstract

Combined heat transfer and wall shear stress measurements on the inner wall of a fully developed turbulent air flow in a round pipe were presented in this paper. The heat transfer was measured using a flush-mounted dual hot-film sensor composed of a non-electric conductive membrane sandwiched in between two thin nickel films. Each of the two films was installed in a branch of a separate Kelvin bridge, and operated at a same temperature; the bottom film served as an active heat insulator so that the Joule heat from the upper film transferred only to the air. Both theoretical analysis and measurements indicated Nū · Pe-1/3 ≈ 0.91 for 300 ≤ Pe ≤ 2500, where Nū was the averaged Nusselt number and Pe was the Péclet number. A calibration-free technique for wall shear stress measurement based only on measuring the Joule heat flux was found to be feasible, providing the Péclet number was in the range of 300-2500 and the film temperature was sufficiently large.