Surface Flow Visualization Using Encapsulated Thermal Tufts for Steady Flow and a Dynamic Heat Transfer Measurement Technique for Unsteady Flows

The photographs above show the flow direction on a surface using thermal tufts produced with embedded capsules. The capsules are short copper cylinders (3 mm diameter) containing a phase change material (water). The wall is made of a poor conductor air brushed with microencapsulated thermochromic liquid crystals. For these pictures a color play temperature of 12 deg C with a 6 deg C band was chosen. The model is cooled until the water freezes. When exposed to flow the surface approaches ambient temperature while the embedded capsules thaw to the phase change temperature and then remain isothermal. The thermal tuft appears on the surface downstream of the capsules pointing in the direction of the surface flow. If it is desired to initially heat the model, other phase change materials can be used above the ambient temperature reversing the process. The thermal tufts produced by this new surface flow visualization method are similar to those produced by the "laser thermal tuft". The sequence of photographs above show the time-dependent heat transfer coefficient distribution on a wall downstream of a protruding cylinder with vortex shedding, providing dynamic heat transfer measurements and dynamic surface flow visualization. The experimental technique uses a low thermal mass substrate (thin tissue paper) for the surface which is embedded with microencapsulated thermochromic liquid crystals and heated uniformly with infrared radiation. The hue is temperature-dependent and thus each hue represents a specific surface temperature and corresponding heat transfer coefficient (h). The frequency response is dependent on the value of h, its fluctuation, the color play temperatures, and the thermal mass of the substrate. For air, it is estimated that fluctuations in the range of 10-100 Hz can be observed. Observation of the flow fluctuations in these photographs is limited by the video frame rate of 30 fps.