Heat transfer enhancement with tube inserts: How can we define the best benefit?

This study recommends the use of two criteria FG-1a and FG-1b for evaluating the benefits that could be obtained when different heat transfer enhancement techniques are implemented in the heat exchanger design, instead of the most commonly used criterion PEC, based on the constraint of fixed pumping power. When the thermal performances of two heat transfer channels are compared, they must be put under equal conditions, such as fixed heat transfer area, mass-flow, and initial temperature. It is also important whether an external thermal resistance of the channel is available or not. The first case is typical for experiments in shell-and-tubes heat exchangers where the objective is an increase in the heat flow, whereas the second case is encountered in experiments with electrical heating of the tube wall, where the objective is a decrease in the driving temperature difference with fixed heat flow. An additional constraint in both cases is the augmentation entropy generation number, Nsa ? 1. Through manifold examples, using different twisted tape inserts, we demonstrate how these two criteria have to be implemented for assessing the thermal benefits. The use of the criterion PEC is connected with many erroneous results and misunderstood conclusions that have been revealed in this study.