A novel interferometric method for simultaneous measurement of film thickness and film interface temperature for a horizontal tube falling film evaporator for MED systems

Horizontal tube falling film evaporation spans a lot of applications, especially in multi-effect desalination application. Hence, the improvement of its gain output ratio is of prime importance, and for that, the studies on the characteristics associated with the flow, mainly falling film thickness and falling film interface temperature, are inevitable. As the falling film flow is a delicate fluid structure, non-intrusive measurement techniques have to be preferred for the experiment. In this work, we introduce a novel interferometric method to calculate film interface temperature and a shadowgraph approach to estimate film thickness simultaneously. A Mach-Zehnder interferometer with infinite fringe setting was used to capture the thermal fluctuations around the horizontal tube falling film evaporator. Interferometric method implemented in this work also serves as an image visualization technique and does the purpose of qualitative as well as quantitative analysis. An in house designed experimental setup was fabricated for producing a thin and uniform falling film with a fully wetted flow over the horizontal copper tube. A heater controller setup was incorporated into it for accurate temperature control. This method holds the advantage of scanning falling film thickness over circumferential angle for the range 5 degrees < theta < 175 degrees, which is an improvement to state of the art. Furthermore, dynamic film thickness and instantaneous film interface temperature studies were also incorporated to prove the efficacy of the technique and to test the robustness of the algorithms implemented. CFD analysis was also carried out for the entire process by means of VOF scheme and was found to be in good agreement with the experimental results, which proves our technique to be a good one. 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. (c) 2021 Elsevier Ltd. All rights reserved.

Automated summary

Horizontal tube falling film evaporation is widely used in multi-effect desalination applications. This study introduces a novel interferometric method to calculate film interface temperature and a shadowgraph approach to estimate film thickness. The experimental results show that this method has good reproducibility and measurement capability.