Interferometric analysis of flow around a horizontal tube falling film evaporator for MED systems

Abstract In this work, an optical shadow method (non-intrusive) was used to evaluate the falling film thickness around the circumference of a horizontal tube. A Mach-Zehnder Interferometer was used to visualize the isotherm formation. Most of the well established non-intrusive techniques for measuring film thickness around the cylinder fails to measure the film thickness near to entry region ( θ 30 ∘ ) and towards the exit region ( θ > 140 ∘ ) because of the limitation and complexity of the apparatus. Relavance of this work lies on the simplicity of the measurement and the range of circumferential angles that can be investigated ( 10 ∘ ≤ θ ≤ 170 ∘ ). This technique involves image visualization, thus, serves as a tool for analyzing the film both quantitatively and qualitatively. A lucid algorithm was developed for image processing along with a set of intensity profile tracking procedures making the whole process of film thickness measurement more obvious. The measured film thickness is showing good agreement with the commonly used empirical formula, and the effectiveness of using those empirical formulas for the small diameter tubes was analyzed. Further, this study was extended to determine the effects of flow rate and feed inlet temperature on film thickness. Film thickness variation around the circumference of the cylinder is showing an increase in trend with increasing Reynolds number, but a change in trend with the varying feed inlet temperature is found to be marginal. This study is further extended for different positions of the tube in the order of impingement of film or feed inlet as top, middle and bottom tubes and the film thickness variation was again found to be marginal. Dynamic characterization at θ = 10 ∘ was studied and the maximum film thickness variation was found to be 11.8%. A standard error of mean analysis was performed on every data set which validates the reproducibility and the dynamic measurement capability of the technique used.