An experimental investigation of forced convection heat transfer in channels with rib turbulators by using the liquid crystal thermography

An optical technique for measuring local heat transfer coefficients in enhanced heat transfer channels is discussed. The technique is based on the use of thermochromic liquid crystals applied to the test surface. The two-dimensional temperature distribution can be easily visualised as a colour distribution. The colour pattern is observed by a CCD videocamera and the recorded image is digitised and converted from the RGB (Red, Green, Blue) to the HSI (Hue, Saturation, Intensity) domain. Among these new parameters, the hue can be directly related to surface temperature through a linear and repeatable relationship, determined by means of previous calibration experiments. If a uniform heat flux condition is applied to the test surface, the pattern of the heat transfer coefficient can be obtained. This optical technique was applied to the study of forced convection heat transfer characteristics in a rectangular channel with rib turbulators mounted on one of the surfaces. Continuous and broken parallel ribs, deployed normal to the main direction of air flow, were considered, for different values of the geometric parameters. The range of variation of the Reynolds number, based on the hydraulic diameter of the channel, was 8000 - 34000.