Time-Accurate Conjugate Analysis of Transient Measurements of Heat Transfer Coefficients

Abstract

Time-accurate and steady conjugate computational fluid dynamics analyses were performed to examine a class of transient methods, known as the one-equation and the two-equation methods, for measuring the heat transfer coefficient via two test problems: a channel with pin fins and a duct with ribs. For the test problems studied, the one-equation method that used the one-dimensional approximate solution to calculate the heat transfer coefficient was found to yield heat transfer coefficients within 5% error relative to the actual heat transfer coefficient at the time when the surface temperature reached 37.6°C, which is a commonly used reference temperature. The one-equation method that used the zero-dimensional solution to calculate the heat transfer coefficient on the pin-fin surface was found to produce relative errors up to 160%. In general, the two-equation method produced much larger relative errors than the one-equation method. More important, it was found that the heat transfer coefficient could vary with time during the transient and that variation could be negligible at some locations but 30% or higher elsewhere. When the heat transfer coefficient at a location varied appreciably in time, it begged the following question: Which of the measured heat transfer coefficients is the correct one? Thus, it is important to measure and assess the heat transfer coefficient variation in time during the transient, and not just the heat transfer coefficient measured at some reference temperature.