Determination of temperature maps using a method based on thermochromic liquid cristals (TLC)

Thermal treatments have been of interest to the food processing industry. Their successful implementation requires a thorough knowledge of temperature histories at every point of the treated product. An original non-intrusive technique has been developed to visualize and to measure temperature distributions inside a cavity (container, tube, plate...) containing either a newtonian or a non-newtonian fluid. The method consists of using Thermochromic Liquid Crystals (TLC's) as a non-intrusive temperature sensor. TLC's, type TCC1075 from Merck Clevenot, are mixed in a parallelepipedic tank with a fluid at a concentration of 0.01 % by weight. Thermochromic reaction of the liquid crystals is then observed through a white colored wide light beam generated by a complete light source system (a source equipment and a set of cylindrical lenses). Liquid crystals microcapsules reflect the incident light in a temperature range between 35 and 50°C with a color variation from red to blue as the temperature to be measured rises due to heating. To digitalize and to treat the images, an acquisition system has then been developed and applied to obtain temperature maps. An RGB (Red, Green, Blue) video sequence is recorded using a high performance video camera, and then digitized. The images are recorded regularly beginning with the color change phenomenon. A computer program written in MATLAB® environment, converts the images from the RGB space to the HSI one (Hue, Saturation, Intensity). In the present study, we are interested only in the hue value which conveniently characterizes the color tint. A correlation linking the hue values to temperatures has been previously defined. The thermal map of a particular image can thus be established by assigning every given temperature to a hue value. In the future, the use of this method will potentially be of interest in the fields of fluid mechanics and heat transfer, not only in small scale laboratory works but also directly in pilot scale. / Les traitements thermiques sont très intéressants dans les procédés agro-industriels. Leur succès exige une connaissance approfondie de la cartographie des températures du produit traité. Une technique originale non invasive a été développée pour visualiser et pour mesurer les distributions de température à l'intérieur d'une cavité (un container, tube, assiette) contenant soit un fluide newtonien soit un fluide non-newtonien. La méthode consiste à utiliser des cristaux liquides thermochromiques (TLC) comme capteur de température. Les TLC de type TCC1075 de Merck Clevenot sont mélangés dans un tank parallélépipédique avec un fluide à une concentration de 0.01%. La réaction thermochromique des cristaux liquides est alors observée à partir d'un rayon de lumière blanche généré par une source (une source et un jeu de lentilles cylindriques). Les micro-capsules contenant des cristaux liquides réfléchissent la lumière incidente à une température entre 35 et 50°C avec une variation de couleur du rouge au bleu lorsque la température à mesurer augmente pendant le chauffage.