Liquid film thickness measurements on a plate based on brightness curve analysis with acute PLIF method

Abstract

Normal planar laser induced fluorescence (N-PLIF) method has been widely applied in liquid film imaging and thickness measurements in circular tubes with large curvatures, but meets great challenges in liquid film thickness measurements in circular tubes with small curvatures or at plates. An alternative method is to place both the incident laser sheet and the camera below the bottom interface of a plate, forming the acute PLIF (A-PLIF) method with an acute laser optical path and camera alignment. However, the liquid film thickness measurements based on direct imaging with the A-PLIF method are still challenged by the decrease of image contrast and indistinct interfaces. This work further explores the A-PLIF method with a novel image processing approach through both static liquid film and moving liquid film measurements. In static liquid film measurements, brightness curves along the liquid film thickness direction are extracted to compare with numerical simulation results based on ray-tracing approach, in which the laser intensity attenuation, laser sheet thickness, fluorescence spectrum and top interface reflection are considered. The comparison indicates that out-of-focus imaging is the dominant factor for the expanding of measured brightness curves. Three distinct regions, i.e. the increasing region, slowly decreasing region and rapidly decreasing region, are identified on the brightness curves and the main fluorescence source of each region is analyzed theoretically. It is concluded that the total width of the two decreasing regions shows strong correlation to the liquid film thickness, which facilitates the evaluation of liquid film thickness through peak position determination of the brightness curve and linear fitting of the rapidly decreasing region. This method is further tested in the moving liquid film thickness measurement in a cross-flow tunnel at various cross-sections, which shows that it can be applied to the liquid film imaging and thickness measurements on small curvature or flat surfaces.