Film thickness measurements in falling annular films
Liquid films falling under the influence of gravity are widely encountered in a variety of industrial two-phase flow applications (distillation columns, nuclear reactor cores, etc.). In addition, the falling annular film represents a fundamental limiting case of the annular flow regime of two-phase gas-liquid flows. The literature on annular falling films is dominated by studies concerning the average film thickness. Information on more detailed characteristics of the film thickness variations and information on the velocity profile within the film and wall shear stress are much less common. The statistical description of the film thickness is complicated by the fact that practically all flows of interest occur in the turbulent regime. Due to the complex and unsteady nature of the turbulent annular falling film, no complete theories or models have yet been developed on the subject. Experimental studies are needed to gain insight into the basic mechanisms that govern this complex flow.The primary purpose of this thesis research was to characterise the film thickness of falling annular films at high and very high Reynolds numbers using non-intrusive imaging techniques. Another objective was to develop ray-tracing techniques to reduce optical distortion and obtain high-quality experimental data. Instantaneous film thickness measurements of falling annular films were extracted at five different Reynolds numbers in the range Re = 1000 ~ 6000 for the fully developed turbulent regime using an automated optical measurement technique. From visual observation of the images obtained it was found that waves were not axisymmetric, i.e., there was substantial azimuthal variation in film thickness. The turbulent waves appeared to be similar in appearance to very large breaking ocean waves driven by strong winds. The random nature of these falling annular films was subjected to statistical analysis.Statistical characteristics of film thickness were studied at Reynolds numbers in the range Re = 1000 ~ 6000. A correlation for dimensionless mean film thickness was obtained in the turbulent flow regime. The dimensionless mean film thickness obtained here was found to be in reasonable agreement with the other established experimental and theoretical studies. It was shown that the Reynolds number influences the statistical characteristics of film thickness such as standard deviation and coefficient of variation. The additional data obtained here shows that the standard deviation continues to increase in proportion to the mean film thickness in the turbulent regime. In other words, in the lower turbulent zones the films are thin and less wavy, whereas in the higher turbulent zones the films are thicker and extremely wavy in nature.The probability density distributions were also obtained. It was found that the measured probability density distributions were asymmetric. They all had a maximum peak and were skewed to the right hand side with a long tail that stretched to over six times the peak value. The maximum peak could be considered to represent the modal value of the film thickness or the substrate film thickness. The increase in skewness and the decrease in the height of the peak with liquid Reynolds number could be attributed to the presence of large disturbance waves which ride on the substrate film. This enhances the waviness of the film.A common problem in imaging flows in cylindrical tubes is the optical distortion caused by the wall curvature. To minimize this problem the cylindrical tube was surrounded by an optical correction box with flat walls filled with water. In addition, an advanced ray tracing model was employed to reduce optical distortion effects in the cylindrical tube. This technique increased the accuracy of the imaging technique and enabled quantitative measurements of film thickness to be made.
DegreeMaster of Science (M.Sc.)
SupervisorBugg, James D.
CommitteeTorvi, David A.; Sumner, David
Copyright DateOctober 2006