Detailed measurements of heat transfer coefficients in a rectangular duct using hue-based calibrated liquid crystal

In the present study, the optical method for measuring detailed heat transfer and temperature distributions using liquid crystal is described. The liquid crystal is calibrated using a hue-based color decomposition system. Three runs were conducted and a correction algorithm is adopted to eliminate the effect of different optical paths. A hue-temperature calibration curve is obtained to designate temperature. A part of the calibration curve is adopted to demonstrate specific temperatures due to its good performance. Based on the calibration of the liquid crystal, the temperature distributions on the inner wall of a rectangular duct were investigated. Two broader walls are electrically heated. On the surface of the heating element, black paint and liquid crystal are sprayed by an air brush to map the temperature distribution. A steady state method is used to obtain the local heat transfer coefficient. The duct width to height ratio is 18. The Reynolds number range is from 1,000 to 6,000. Average Nusselt numbers are calculated and compared with the literature data. Influences on temperature distribution on the wide wall by the wall heat conduction are also investigated.