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

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 (Nu) over bar . Pe (-1/3) approximate to 0.91 for 300 <= Pe <= 2500, where (Nu) over bar was the averaged Nusselt number and Pe was the Peclet 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 Peclet number was in the range of 300-2500 and the film temperature was sufficiently large.