A Thermochromic Liquid Crystals Image Analysis technique to investigate temperature polarization in spacer-filled channels for Membrane Distillation

The analysis of flow fields and temperature distributions is of paramount importance in the development and optimization of new spacer-filled channel geometries for Membrane Distillation modules. The literature reports only few studies on the experimental characterization of such channels and, to the authors' knowledge, none of them present local information concerning the temperature distribution on the membrane surface. In the present work, a non-intrusive experimental technique named TLC-IA-TP is presented: it is based on the use of Thermochromic Liquid Crystals (TLCs) and digital Image Analysis (IA) and it is applied here for the first time to the analysis of Temperature Polarization (TP) in spacer-filled channels typically adopted in thermally-driven membrane separation processes. In particular, this technique allows the local distribution of convective heat transfer coefficients to be determined, thus providing (i) useful indications on strengths and weaknesses of some spacer arrangements and (ii) valuable benchmark data for Computational Fluid Dynamics (CFD) studies. For the purpose of the present work, the technique's fundamentals are presented, along with a comprehensive assessment of its accuracy. Results of some preliminary measurements on commercial spacers are also reported.