Vortex controlled heat transfer in eight row plate fin and tube exchangers: CFD derived air side Nusselt correlations

Abstract

This paper investigates and characterizes the row dependent thermal performance of an 8 row finned tube heat exchanger (HEX) with an offset tube arrangement and continuous flat fins. Computational fluid dynamics (CFD) simulations reveal distinct hydraulic thermal behavior across the tube rows, with the first, second, and eighth rows exhibiting significantly higher heat transfer efficiency than the average for the entire HEX under specific flow conditions. At air velocities below 3 ms-1, the first rows exhibit increased efficiency, while the eighth row exhibits comparatively better efficiency at velocities exceeding 6 ms-1. In the fitted range (2.5 to 10 ms-1), orders 1, 2, and 8 outperform orders 3 to 7 by approximately 15 to 25% in Nusselt number, indicating that orders 1, 2, and 8 operate under distinct flow and thermal conditions. The study also determined individual Nusselt number correlations for each order through systematic simulations over the velocity range from 2.5 to 10 ms-1. The analysis further linked these changes to specific flow mechanisms, such as boundary layer development, wake interference, and vortex shedding, which govern the distinct thermal behavior of the first, second, and last orders. These findings can enable dual optimization: lower capital costs by minimizing the number of orders for the target thermal performance, and lower operating costs by reducing pressure drop (reducing fan power).