Determination of local heat transfer coefficient based on bulk mean temperature using a transient liquid crystals technique

Recent advances in thermochromic liquid crystal (TLC) thermography has improved its usefulness as a very effective technique for measurements of temperature and heat transfer. One of the approaches to determine the local heat transfer coefficient, known as the transient technique, is to monitor the temporal evolution of surface temperature in conjunction with the solution of a transient heat conduction model penetrating to the wall substrate. The local heat transfer coefficient resulted from such a transient test, by nature, has its reference temperature based on the inlet temperature of the test rig, rather than the local bulk mean temperature. The latter during a transient test varies with both time and streamwise location. The heat transfer coefficient based on the inlet temperature presents difficulty in data interpretation in designs of heat exchangers, particularly for flow channels with large length-to-diameter ratios. This study evaluates four different approaches and their theoretical background for determining the local bulk mean temperature and the sensible local heat transfer coefficient. Using a test model of a turbine blade cooling passage with delta-wing shaped vortex generators mounted on one of the passage walls, the magnitudes of the sensible heat transfer coefficient resulted from various approaches vary as much as 40%. Validated with the experimental data, two of the four methods yield superb data accuracy. Nevertheless, one of them stands out as the best choice, as it requires much less processing time and implementation effort. (C) 1998 Elsevier Science Inc. All rights reserved.