CFD modelling of air flow through a finned coil heat exchanger to improve heat transfer and pressure drop predictions

Abstract

In the current study, a method of lessening computational expense and model design effort is investigated for finned coil heat exchangers (FCHXs) by using STAR-CCM+ as simulation tool. The simulation model prediction accuracy, in terms of the air side thermal-hydraulic characteristics of a staggered tube, true-to-industry (TTI) sized FCHX model, is compared to a repeatable, representative segment (RS) model of the same FCHX across a wide air flow continuum ranging between laminar to fully turbulent. The level of confidence of these models is validated based on a comparison with previous experimental data from a renowned source using the Colburn j-factor and Fanning friction factor (f-factor) as reference and illustrate a reasonable agreement. The RS model type is found to be a suitable approach, limiting computational expense compared to the TTI model, which showed a minor improvement of the heat transfer and pressure drop predictions by only 1.18% and 1.83%, respectively. In order to reduce simulation model design effort in the next phase of the study, the model prediction results of a plain fin RS model are compared to a wavy fin RS model. Wavy fin FCHXs are commonly found in industry and create a few extra design challenges for simulation purposes when compared to a plain fin FCHX. The results of a plain fin RS model is found to yield large inaccuracies compared to the wavy fin RS model and beckons the need to parametrically test the effects of geometrically modifying a plain fin RS model in order to increase model prediction accuracy. Detailed analysis of the effect on the heat transfer and pressure drop performance is done by evaluating related parameters such as the fin pitch, longitudinal tube pitch and transverse tube pitch. The increase in fin pitch is found to cause an increase in heat transfer performance (in terms of the Nusselt number) due to a substantial hydraulic diameter increase, although a decrease in the heat transfer coefficient and pressure drop is seen. A decrease in the longitudinal and transverse tube pitches causes an increase in heat transfer and pressure drop performance, whereby the effect of the transverse tube pitch is found to yield the closest results comparison in relation to the wavy fin RS model’s results. The average prediction accuracy for the entire flow range was found for the heat transfer to be predicted with an error deviation of 3.22% and pressure drop of 4.44%, which was acceptably accurate. Although the variation in transverse tube pitch proved to be acceptable for this study, more research has to be done in future to confirm this finding using a wavy fin model incorporating a variation of waviness heights (and waviness angles) and a different set of geometrical parameters before a final conclusion can be made.