A new technique for dynamic heat transfer measurements and flow visualization using liquid crystal thermography

A new technique for dynamic heat transfer coefficient measurements and flow visualization is described. This technique uses a surface with a low thermal mass (a thin tissue) embedded with thermochromic liquid crystals (TLCs) and heated uniformly with infrared radiation. For air heat transfer coefficient measurements, the frequency response is estimated at 0.3-0.5 Hz. Depending on the local heat transfer coefficient and the magnitude of its fluctuations, it is estimated that surface temperature fluctuations can be detected to 100 Hz. These surface temperature fluctuations are driven by changes in local heat transfer coefficients, caused by dynamic flow behavior such as vortex shedding, and are captured by video recordings of the hue of the liquid crystals. The video images provide time-dependent heat transfer coefficient distribution and time-dependent surface flow visualization. Two applications are used to illustrate this technique: flow on a surface downstream of a protruding cylinder in cross-flow with vortex shedding, and flow downstream of a shallow cylindrical surface dimple. Images of the time-dependent surface temperature distribution downstream of the protruding cylinder are presented. They show the fluctuations in the surface heat transfer coefficient due to vortex shedding. The temperature distributions downstream of the cylindrical dimple were found to be relatively steady within the frequency limits of this technique. Heat transfer coefficient contours are presented for this case.