Application of a borescope to studies of gas–liquid flow in downward inclined pipes

The aim of the present study is to investigate stratified downward gas–liquid pipe flow with a non-intrusive measurement technique that is based on a borescope connected to a digital video camera. The borescope-based technique enables to determine the instantaneous cross-sectional distribution of both phases within the pipe. Water and air were used as working fluids. Quantitative data was extracted from sequences of recorded video images by applying a developed data processing technique for instantaneous gas–liquid interface boundaries determination. Experiments were performed for a wide range of downward pipe inclinations and gas and liquid flow rates. The instantaneous and time-average cross-sectional holdup for each set of flow parameters was calculated. Particular attention was given to the study of the interface shape that in many occasions was not flat and was characterized by the penetration of the liquid along the pipe periphery. Temporal variation of the surface elevation was also studied and various regimes characterizing interfacial waves were defined using both the recorded time series of the instantaneous depth of the water layer and the Fourier analysis of those records.