For determination of the heat transfer coefficient from a transient experiment, the test section is heated to a constant temperature followed by cooling under forced flow conditions. The mainstream flow is started by turning on the digitally controlled blower. Fluid velocity is recorded at the inlet of the test section by the pitot static tube. The measurement indicates that the transient period for establishing the flow is around 3-5 seconds, reckoned from the start of the blower. The sequence of images of the LCT sheet is recorded after the full flow rate is reached. The rate of cooling of the surface of the heated test section is recorded as a set of color images. Images obtained in a transient experiment, with one solid rib mounted on the test surface is shown in Figure 6.6.

Figure 6.6: An image sequence showing changes in color during the cooling of a plate downstream of a solid rib (arrow indicates the flow direction); figures (a) to (d) are arranged in an increasing time sequence.

Here, red indicates the coldest region followed by green, yellow and blue respectively for higher temperatures. Initially the plate is heated up to the clearing point temperature and correspondingly the LC sheet appears to be uniformly blue. Once the flow is established, the surface cools under forced convection conditions. A gradual change in temperature occurs with time. Subsequent images (Figures 6.6 (b-d)) show the color-temperature distribution at later times. The effect of a reattaching boundary layer in to cool preferentially the surface downstream of the rib. Equivalently, it is a region of high heat transfer (coefficient). Streaks of yellow and red indicate zones of high heat transfer as clearly revealed in Figure 6.6(d) within the reattachment region. Comparatively higher temperatures upstream of the rib points towards low heat transfer where the flow is stagnant within the recirculation zone. Overall, the images show that LCT can be used as a visualization tool for qualitative information. These data (images) are subsequently digitized frame by frame. The history of color distribution at each pixel yields the temperature history of the test surface.