Colburn ‘j’ factor analysis for offset strip fins in compact heat exchangers

Compact heat exchangers frequently utilize offset strip fins to enhance thermal performance in constrained volumes, relying on repeated boundary layer disruption and flow mixing to achieve high convective heat transfer. This paper presents a focused numerical investigation on the j factor, aiming to derive correlations that account for variations in key geometric parameters such as lance length (l), fin height (h), fin thickness (t), and fin spacing (s). Numerical simulations using computational fluid dynamics were conducted across a broad range of Reynolds numbers spanning laminar, transitional, and turbulent regimes on ANSYS Fluent 2024. Using insights from the data, we derive continuous correlation expressions for j in terms of Reynolds number, Re and dimensionless geometric parameters. The resulting generalized Colburn j factor correlations are applicable across both laminar flow (300 ≤ Re ≤ 1500) and turbulent flow (2000 ≤ Re ≤ 6000) regimes with air being used as the fluid medium and provide a unified predictive model for Offset Strip Fin (OSF) performance in both flow regimes. These correlations offer improved predictive accuracy and serve as robust tools for optimizing compact heat exchanger performance across the practical range of commercially manufactured OSFs. The impact of pressure drop, while acknowledged as critical to overall design, is not addressed in this paper and will be explored in future work.