Passive control techniques for heat exchangers: Permeability-driven modulation of vortex shedding and heat transfer enhancement

This study examines the impact of porous wrapping on flow and thermal dynamics of two tandem tubes with varying permeability levels, serving as a fundamental model for heat exchanger analysis. Simulations were conducted at different Reynolds numbers (Re) and Darcy numbers (Da) across various gap ratios (t/d, gap between tubes relative to diameter) with a fixed porous layer thickness. Darcy-Brinkman-Forchheimer model was employed to assess the porous ring zone. FFT analysis revealed a dominant frequency associated with the Strouhal number (St), indicating a single vortex street for t/d < 6, with harmonic frequencies emerging at larger gaps. Vorticity structural parameter (Г) analysis further emphasized the role of porosity in modifying vorticity distribution and flow coherence, particularly in the gap region. High permeability allowed fluid penetration into the porous layer, altering wake structures and enhancing convection-driven heat transfer. In contrast, low permeability traps the flow within the porous layer, resulting in conduction-dominated heat transfer. The study also identifies the drag inversion range, a quintessential process of highlighting the destabilizing effect of the downstream tube. Comparative analyses between porous-wrapped and bare tubes revealed substantial differences in drag (35–45 % drag reduction) and Nusselt number (17–19 % increase in heat transfer), particularly at mid-range gap ratios.