Heat transfer measurements in heat exchangers by liquid crystal thermography
by
Jan Stasiek
Technical University of Gdansk, Poland
Coauthors: Giovanni Tanda (University of Genova, Italy), Michele Ciofalo (University of Palermo, Italy)

Liquid crystals have been employed in heat transfer experiments for years. However, their use has mainly been qualitative until the recent development and wide-spread availability of inexpensive image digitiser has made routine digital image analysis feasible. As known, thermochromic liquid crystals modify incident white light and display colours whose wavelength is a function of temperature. They can be painted or sprayed on a surface or suspended in a fluid and used to visualise temperature fields.

Liquid crystal change in appearance is concentrated over a narrow range, typically few Kelvin degrees: this field is called "colour-play interval" and represents the temperature range within they can be used as temperature indicators.

The interpretation of colour images displayed by liquid crystals is performed by the true-colour image processing.

The image-processing system includes a video-camera to record the image, a frame grabber to digitise it and an appropriate software to convert the colour pattern into the temperature field. This last step can be performed under two distinct procedure: (i) the use of the separate colour component red, blue and green (R,G,B) and either a multiple regression method or a neural network to obtain the correlation equation between colour and temperature; (ii) the conversion of the image, pixel by pixel, from the RGB domain into the HSI (Hue, Saturation, Intensity) domain. In this way, only the Hue parameter can be retained since it is related to temperature through a relationship that can be obtained by a calibration experiment. The hue-temperature relationship depends not only on the liquid crystal compound but also on the experimental conditions, i.e. lighting, angle of view, presence of glasses between the video-camera and the LC layer. Information about saturation and intensity are not useful for the temperature reconstruction and can be abandoned, thus reducing the size of the digitised image and saving memory into the computer.

In this work examples of liquid crystals applications to the study of convective heat transfer are illustrated [1-5]. Attention is focused to the use of liquid crystal thermography to the study of compact heat exchangers performance. The complex geometry that often characterises these devices do not generally allow heat transfer investigation by traditional invasive sensors;

conversely, optical methods, and in particular the liquid crystal thermography, are useful tools to gain whole-field and real-time information about flow and thermal fields. Three different configurations have been considered: (i) a corrugated-undulated passage, (ii) a rectangular channel roughened by square ribs, and (iii) a rectangular channel finned by staggered or in-line diamond-shaped elements. All these geometry are frequently encountered in compact heat exchangers. Experiments were performed by using pre-packaged liquid crystal films consisting of a liquid crystal layer deposited onto a plastic sheet having a background colour with black paint. The film is glued onto the surface whose temperature distribution is to be detected. Under given conditions of heat transfer at the wall (for instance, uniform heat flux) and of mass flow rate, the distributions of heat-transfer coefficients have been extracted.

References [1] G.Tanda, J.Stasiek and M.W.Collins “Application of holographic interferometry and liquid crystal thermography to forced convection heat transfer from a rib-roughened channel”, Proc. of Int. Conf. on Energy and Environment ICEE ‘95, pp.

434-441, Shanghai, China, 8-10 May 1995. [2] G.Tanda, M.Ciofalo, J.Stasiek and M.W.Collins “Experimental and numerical study of forced convection heat transfer in a rib-roughened channel”, Proc. of XIII Italian Heat Transfer Conf., pp.243-254, Bologna, 22-23 June 1995.

[3] G.Tanda, J.Stasiek and M.W.Collins “An experimental study by liquid crystals of forced convection heat transfer from a flat plate with vortex generators”, Proc. of IV UK National Conference on Heat Transfer, pp.141-145, Manchester, U.K., 2 6-27 September 1995. [4] Stasiek J., Collins M.W., Ciofalo M., and Chew P. “ Investigation of Flow and Heat Transfer in Corrugated Passages. Part I: Experimental Results”, Int.J. Heat Mass Transfer, Vol.39, 1996, pp.149-164.

[5] Stasiek J., “Experimental Study of Heat Transfer and Fluid Flow Across Corrugated-Undulated Heat Exchanger Surfaces”, Int.J. Heat Mass Transfer, Vol.41, 1998, pp.899-914.

Date received: January 22, 1999