Heat Transfer and Flow Resistance Performance of Elliptical Aluminium-Finned Steel Tubes for Air-Cooled Heat Exchangers

Literature Overview

The paper by Chen Yaping and Sun Fengxiang, published in Thermal Power Generation (2003, Vol. 32, No. 3, pp. 22-23), investigates the thermal-hydraulic performance of rolled-fin elliptical aluminium-fin steel tubes used as air coolers in power generation equipment. The study employs the Wilson plot method to separately determine the heat transfer coefficients on both the tube-side and air-side, and evaluates the structural integrity of the composite finned tube assembly. This work is particularly relevant to steel pipe engineers because the base tube material, the fin-rolling process, and the interface between steel and aluminium represent critical metallurgical and manufacturing challenges that directly affect service life.

Core Technical Content and Methodology

The authors tested rolled-fin elliptical aluminium-fin steel tubes, where the base tube is carbon steel and the fins are mechanically rolled onto the outer surface from aluminium sheets. The elliptical cross-section of the steel tube was selected to increase the surface-area-to-volume ratio compared to a circular tube of equivalent hydraulic diameter, thereby enhancing both convective heat transfer and structural rigidity. The Wilson plot method, originally developed by Wilson in 1915, was used to decouple the individual heat transfer coefficients from the overall heat transfer coefficient by varying the flow velocity on one side while holding the other constant. This graphical technique eliminates the influence of fouling resistance and provides a reliable means of extracting the clean-surface performance of the finned tube.

From a steel pipe manufacturing perspective, several process variables are critical. The elliptical tube must be drawn or expanded from a circular seamless or ERW pipe through a die set that maintains wall thickness uniformity. The rolling process for attaching aluminium fins involves cold deformation of both the steel tube surface and the aluminium fin sheet, creating a mechanical interlock at the interface. The fin density, fin height, and tube outer diameter are all design parameters that must be balanced against pressure drop considerations.

Material and Process Considerations from a Metallurgical Perspective

The dissimilar metal interface between carbon steel and aluminium is a major concern for long-term durability. Galvanic corrosion in the presence of moisture and oxygen can lead to preferential attack of the aluminium fin material. The steel tube base material, typically a low-carbon grade such as Q235 or A106 Gr. B, must have adequate formability to withstand the rolling operation without cracking. The elliptical geometry introduces residual stresses during forming, particularly at the major and minor axis regions where the curvature varies significantly.

The rolling process is analogous to a cold-joining operation and must be carefully controlled. Insufficient rolling depth leads to fin detachment under thermal cycling, while excessive depth can cause micro-cracking in the steel tube surface. The residual stress state introduced during rolling also affects the fatigue life of the tube under cyclic thermal loading. From a welding standpoint, if any field repair or end-connection welding is required on these tubes, the presence of the aluminium fins near the weld zone presents a significant challenge, as aluminium oxide contamination can lead to poor weld penetration and porosity in steel welds.

Engineering Practice Implications

The findings confirm that elliptical aluminium-fin steel tubes offer a favourable combination of enhanced heat transfer, manageable flow resistance, and adequate structural strength for application in air-cooled generators and large transformers. For pipe fabrication shops, the key takeaway is that the elliptical forming process requires precision die design and controlled forming speeds to avoid thickness variation exceeding 10% of the nominal wall thickness. Post-forming, the tubes should undergo a stress-relief annealing at 550-650 degrees Celsius for 1-2 hours to reduce residual stresses before the fin-rolling operation.

In terms of quality control, the critical inspection points include: dimensional verification of the elliptical profile using coordinate measurement or laser scanning; visual and dye penetrant examination of the tube surface for cracks introduced during forming; and a pull-off test on sample fins to verify the bond strength between the steel tube and aluminium fins. The bond strength should exceed 150 N per fin to ensure reliable performance under vibration and thermal cycling conditions.

Study Insights and Reflections

This paper, while primarily a heat transfer study, provides valuable input for steel pipe engineers regarding the manufacturability limits of non-circular cross-section tubes. The elliptical geometry, though advantageous for thermal performance, introduces manufacturing complexity that must be carefully managed. The Wilson plot method, though classical, remains a practical tool for characterising heat exchanger performance when detailed numerical models are not available. The study reinforces the principle that material selection, forming process, and geometric design must be considered as an integrated system rather than in isolation. For engineers involved in specifying or manufacturing such heat exchanger tubes, attention to the steel-aluminium interface integrity and the residual stress state after forming should be paramount concerns in any quality assurance plan.