Temperature and heat flux

Abstract

Thermochromic thermochromic liquid crystal (TLC)TLC (thermochromic liquid crystal) liquid crystals (TLCs) can be applied for thermographic measurements of heat transfer and temperature in fluid mechanics, delivering important quantitative full-field data for comparison with and validation of numerical simulations. Thin coatings of TLCs at surfaces are utilized to obtain detailed heat transfer data for steady or transient processes. Application of TLC tracers allows instantaneous measurement of the temperature and velocity fields for two-dimensional cross sections of flows. These methods are based on computerized true-color analysis of digital images for temperature measurements and modified particle particle image velocimetry (PIV)PIV (particle image velocimetry) image velocimetry, which is used to obtain the flow field velocity. In this Chapter, the advantages and limitations of liquid-crystal thermography thermography are discussed, followed by several examples of thermal flow field measurements. The use of infrared thermography for non-intrusive measurement of spatially resolved surface heat transfer characteristics is described for five different measurement environments, including situations where large gradients of surface temperature are present. In the first of these, measurements are made on the surface of a therapeutic biomedical patch, where the quantity of interest is the time-varying spatially resolved surface temperature. For the other situations, the measured temperature distributions are used to deduce quantities such as the surface Nusselt numbers on the surface of a swirl chamber, the effectiveness of surface adiabatic film cooling downstream of individual shaped film cooling holes, the surface heat flux reduction ratio downstream of two rows of film cooling holes placed on a model of the leading edge of an airfoil, and thermal boundary condition information for numerical predictions of the heat transfer characteristics on the surface of a passage with an array of rib turbulators. In all of these situations, in situ calibration procedures are employed in which the camera, imaging, and data-acquisition systems are all calibrated together in place within the experimental facility as the infrared measurements are obtained. This requires separate, simultaneous, and independent measurements of surface temperatures, and produces spatially resolved results from infrared images with high levels of accuracy and resolution.