Double-pipe heat exchangers (DPHEs) are vital in industrial applications, and improving their performance is crucial for sustainability. This study uses three-dimensional Computational Fluid Dynamics (CFD) simulations to investigate the impact of passive flow modifications, specifically geometrically spaced and perforated ring inserts, on heat transfer and pressure drop in a DPHE. The research involved developing a 3D numerical model whose accuracy was ensured through a comprehensive mesh independence study and rigorous validation against established empirical correlations and experimental data. Subsequent simulations explored the influence of geometric spacing ( G -factor) and the number of perforations per ring. Results demonstrated that for unperforated rings P = 0 , uniform spacing maximised heat transfer, reaching a Nusselt number of 177.4 at a Reynolds number of 12,000. In contrast, strongly biased configurations exhibited superior overall performance by balancing thermal enhancement with hydraulic losses. These biased cases achieved a Performance Evaluation Criterion (PEC) of 1.065, equivalent to a 6.5% improvement compared with the uniform arrangement. The introduction of perforations significantly altered performance; a four-hole configuration with G = 1 . 00 consistently achieved the highest heat transfer and overall performance, with the Nusselt number rising to 195.8 and the PEC reaching 1.176, indicating an optimal balance between fluid mixing and flow resistance. By comparison, increasing the number of perforations further to eight reduced the pressure drop from 171 . 9 Pa for solid rings to 132 . 0 Pa , but had a less pronounced positive impact on heat transfer performance. For this configuration, the Nusselt number remained close to that of the unperforated case. Analysis of both turbulent kinetic energy (TKE) and velocity vector fields provided critical insights into the underlying mechanisms, illustrating how ring geometry and perforations disrupt boundary layers and generate beneficial turbulence. Furthermore, regression-based multivariate correlations for both the Nusselt number and friction factor were formulated as functions of Reynolds number, G -factor, and porosity. Validation against the full set of 75 CFD simulation cases demonstrated high accuracy, with 96% of the correlation-predicted Nusselt numbers deviating by less than ± 5 % from the corresponding CFD results, and the equivalent friction factor values deviating by less than ± 10 % . • Non-uniform ring spacing significantly affects DPHE thermo-hydraulic behaviour. • Four-hole perforated rings provide the best heat-transfer and pressure balance. • Strongly biased ring spacing yields higher overall PEC than weakly biased layouts. • CFD reveals mixing patterns driven by combined spacing and perforation effects. • Correlations predict Nusselt number and friction factor for 75 DPHE cases.
Laue et al. (Fri,) studied this question.