Heat transfer improvement of tubular heat exchangers with twisted nail wire inserts

Meeting the challenge of designing compact, energy-efficient, and sustainable heat exchangers requires inno vative yet practical passive heat transfer improvement techniques. This study introduces a new passive technique using innovative twisted nail wire inserts (TNWIs) in double-tube heat exchangers (DTHXs). Experiments are conducted under turbulent flow conditions (Reynolds number of 5000–21,250) with constant inlet temperatures of hot- and cold-water streams. Three distinct TNWI configurations − aligned, cross, and alternating nail arrangements − were tested at fixed nail spacing (P) to evaluate thermal-hydraulic performance and entropy generation, using a plain-tube DTHX as the baseline. Among these, the alternating configuration yields the best performance, achieving a 1.78-times higher Nusselt number (Nu), a maximum thermal performance factor (TPF) of 1.75, and the lowest entropy generation ratio (EGR), while requiring minimal material and maintaining low friction losses. Further evaluation with varying nail density ratios (L/P = 60, 30, and 20; L is the insert length) reveals that increased nail density enhances heat transfer (up to 2.29 times Nu enhancement) and reduces EGR (down to 2.15). New empirical correlations are developed for Nu, friction factor, and TPF, with prediction errors within ±10%. Compared to conventional inserts, TNWIs offer a simpler and energy-efficient alternative for advanced thermal system applications.