Comparison of Local and Averaged Air-Side Heat Transfer Coefficients on Fin-and-Tube Heat Exchangers Obtained With Experimental and Numerical Methods

Abstract

Numerical methods are often used to obtain two-dimensional air-side heat transfer coefficients (HTCs) on heat exchanger (HX) fin surfaces. The model's accuracy is usually verified through averaged HTCs by comparing with published experimental results. However, substantial disagreement is not uncommon and can hardly be explained by averaged HTCs. This study focuses on comparing experimental, local air-side HTCs to numerical ansys fluent results. A mass transfer experimental method was employed to obtain HTC distributions on the fin-and-tube HX fin surfaces. Therefore, disagreements between the experimental and numerical results can be explained in detail. There are several significant findings: inaccurate predictions of local HTCs are observed even though the averaged HTCs from the numerical method may agree with the averaged experimental results under some conditions. The models fail to capture horseshoe vortices, underestimate the HTCs in the wake region of the tubes, and overpredict row-by-row HTC degradation. Moreover, the accuracy of the numerical model decreases when the complexity of geometry increases. For the flat plate, numerically obtained HTCs agree with the experimental results within 10%. However, the error is more than 30% for the eight-row HX. Nonetheless, the model's accuracy becomes worse at higher airflow velocities. Oversized fin-and-tube HXs with multiple tube rows are often selected as a result of using the underpredicted averaged air-side HTCs from the numerical computational fluid dynamics (CFD) simulations. Thence, the authors have proposed a corrective method to improve the accuracy of the numerical model.