Vogel et al. (2002) carried out a comparative study of various heater-foil configurations for LC based experiments. The authors introduced a new transient heater-foil method for film cooling. Thus, the film cooling effectiveness and heat transfer augmentation could be simultaneously obtained. Baughn et al. (1998) introduced the periodic transient method for heat transfer measurements. Here, the free stream is periodically heated while the changes in the local surface temperature of a model are measured. The local heat transfer coefficient is related to the frequency of periodic changes in temperature, the ratio of the change in surface temperature and that of the free stream and the model thermal properties. The primary advantage of this technique is that it approximates quite well a uniform thermal boundary condition which is often not the case for the step transient. On the other hand, it requires a complex heating arrangement for periodically heating the free stream.

In the studies mentioned above, an initially isothermal test surface is exposed to the thermal transient. The color of the test surface coated with the LC sheet starts to change with time. Each pixel will reach the prescribed temperature (color) depending upon the local heat transfer coefficient. The local heat transfer coefficient is calculated, by suitably using the solution of the classical one dimensional transient heat conduction equation. In principle, a few snap shots of the color images are adequate to reconstruct the heat transfer distribution over the surface. However, in practice, the entire cooling duration of the surface is employed so that heat transfer values are retrieved in a statistical sense by the method of least squares. In this approach, the entire calibration range of the liquid crystal sheet is required. The advantage gained by this route is insensitivity to scatter in isolated measurements and hence, a certain robustness in the prediction of the heat transfer rates.