New measurement approaches for film thickness and wall temperature in falling film heat exchangers

Abstract

Papers presented to the 11th International Conference on Heat Transfer, Fluid Mechanics and Thermodynamics, South Africa, 20-23 July 2015.

Falling film evaporation is used in various fields, e.g. food and pulp & paper industry. Evaporation is very energy intense and relatively small efficiency improvements to the techniques can lead to large savings in absolute numbers. Falling film evaporation is affected by the wave dynamics; hence further knowledge of the behaviour of the liquid film flow can promote efficiency improvements. In this work, two new measurement approaches have been investigated. The first approach is to use a laser triangulation scanner combined with a high-speed camera where the laser scanner continuously measures the film thickness along a vertical line, resolving the flow pattern in high detail. The second approach is to measure local wall temperatures, enabling calculation of local heat transfer coefficients at any desired location. These methods has been tested and evaluated in a falling film test facility. Both approaches have proven to give valuable insights into the process and the results are in good agreement with literature data.