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 fin surfaces. The model's accuracy is usually verified through averaged HTCs by comparing to 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 heat exchanger (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 8-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 CFD simulations. Thence, the authors have proposed a corrective method to improve the accuracy of the numerical model.

Integration of winglet type vortex generators in finned-tube heat exchangers for a sizeable capacity augmentation

“…A recent study reports that, despite variations in local values, the average values of the performance indices derived from numerical simulations of finned-tube heat exchanger closely match the experimental values. 18 Zdanski et al 19 carried out an experimental study on an inline tube bank with delta winglets on the upstream, and reported a correlation for the average Nusselt number. Here, it is interesting to know that the winglets can be integrated in a finned-tube bank in two different ways, 2 either with its leading points spread apart or its trailing points spread apart, where former is referred to as “toe-out” arrangement (also called common-flow up arrangement) and the latter is referred to as “toe-in” arrangement (also called common-flow down arrangement).…”

Nusselt Numbers and Friction Factor Determination of Air-Side Finned Heat Exchangers – Individual Correlations of Each Row

“…These studies usually describe the local heat exchange and often compare it to the average HTC. Che and Elbel [14] showed the experimental and numerical determination of heat transfer coefficients (HTCs) using the absorption-based mass transfer method. This study showed that the HTC value in the first row of FHE compared to the second row of FHE can be 30% higher.…”

Development of FEM Calculation Methods to Analyse Subcooled Boiling Heat Transfer in Minichannels Based on Experimental Results

“…There are well-known commercial computational fluid dynamics (CFD) programs. The most popular are the following: ANSYS CFX/Fluent [7,8] and COMSOL Multiphysics [9]. In [7] the authors have considered four-row finned-tube heat exchangers.…”

“…The main result was the proposition of new correlations for the air-side Nusselt number for the four-row finned tube heat exchanger. ANSYS Fluent software has been used in [8]. The study focuses on comparing experimental heat transfer coefficients on the local air side with numerical results obtained with the help of the ANSYS Fluent software.…”