Skin Friction Measurements in a Gas-Liquid Pipe Flow Via Optical Interferometry

An instrument for the measurement of wall shear stress in two-phase flows is described. The device, termed a Laser Interferometer Skin Friction (LISF) meter, determines the wall shear by optically measuring the time rate of thinning of a thin oil film placed on the wall of the flow channel. The LISF meter has proven to be a valuable tool for measurement of wall shear in single-phase gaseous flows but, to date, had not been applied to liquid or gas-liquid flows. This paper describes modifications to the LISF meter developed to facilitate its use in two-phase flows. The instrument’s configuration, governing theory, and data reduction procedure are described. Additionally, results of validation experiments for a single-phase water flow are presented, which demonstrate the instrument’s ability to accurately measure wall shear. Measurements are also presented for two-phase, water-air flows in a duct of square cross section at various superficial gas and liquid velocities within the bubbly flow regime. Results of the measurements confirm previous observations that the addition of a very small amount of the gaseous phase increases the wall shear significantly over that in a single-phase water flow. The two-phase wall shear saturates to a maximum and then declines again as the superficial gas velocity is increased. The peak two-phase wall shear increases as the liquid superficial velocity is decreased. These trends are qualitatively in agreement with previous measurements obtained in pipes using an electrochemical technique.