Controlled Conjugate CFD Comparison of Counter-Flow and Parallel-Flow Concentric Tube Heat Exchangers Under Identical Reynolds Conditions for Engine Cooling and Waste Heat Recovery

This study presents a controlled three-dimensional conjugate CFD comparison of counter-flow and parallel-flow concentric tube heat exchangers under identical Reynolds number conditions (Re = 1000–2000). By isolating the flow configuration as the only varying parameter, the intrinsic influence of flow arrangement on thermo-hydraulic performance is systematically evaluated. Unlike enhancement-focused studies involving geometric modification or advanced working fluids, the present study focuses exclusively on the influence of flow arrangement under identical operating conditions. The analysis focuses on heat transfer rate, outlet temperature distribution, pressure drop, thermo-hydraulic performance index, and a normalized heat transfer ratio (Ψ). The results show that the counter-flow configuration consistently enhances heat transfer by 3.17–4.29% compared to parallel-flow operation, while maintaining nearly identical pressure-drop values. This improvement is attributed to the preservation of a higher logarithmic mean temperature difference (LMTD) along the exchanger length, sustaining the thermal driving force under laminar flow conditions. In contrast, the parallel-flow configuration exhibits a rapid decay in temperature difference near the inlet region, limiting effective heat transfer. Although heat transfer increases with Reynolds number in both configurations, the thermo-hydraulic performance index decreases due to the relatively higher increase in hydraulic resistance. Comparison with classical laminar flow behavior confirmed the physical consistency and reliability of the numerical model. The findings demonstrate that counter-flow arrangement provides a measurable thermal advantage without additional hydraulic penalty. The study offers a physically consistent and practically relevant framework for the design and optimization of concentric tube heat exchangers used in engine cooling and waste heat recovery applications.