Comparison of disturbance wave parameters with flow orientation in vertical annular gas-liquid flows in a small pipe

The interfacial wave structure of the liquid film in both upward and downward annular gas-liquid flows in an 11.7 mm pipe were investigated using the Brightness Based Laser Induced Fluorescence technique (BBLIF). Film thickness measurements were carried out with high spatial and temporal resolution between 330 and 430 mm from the inlet, where the properties of disturbance waves are almost stabilised. Using a tracking algorithm to detect disturbance waves, a full characterisation in terms of their velocity, frequency, longitudinal size and spacing was carried out. Direct comparison between both flow orientations while testing the same flow conditions shows that although the flow orientation does not affect the velocity of disturbance waves, the fraction of film surface occupied by the disturbance waves is smaller in upwards flow. Thus, more liquid travels in the base film in upwards flow, which is consistent with the base film thickness measurements. These observations, together with qualitatively different behaviour of ripple wave velocity in upwards and downwards flows, studied using 2D Fourier analysis, indicate that the role of gravity is much more important on the base film than on disturbance waves. This supposedly occurs due to a local decrease in the interfacial shear stress on the base film surface because of the resistance of the disturbance waves to the gas stream in upward flow.