Entropy Generation Minimization in Convective Heat Exchanger Design

Povzetek

Convective heat exchangers must balance two competing sources of irreversibility, heat transfer across a finite
temperature difference and viscous friction in the flowing fluid, and entropy generation minimization (EGM) provides
a formal second-law framework for locating the operating point that minimizes their sum. We applied EGM analysis
to a plate-fin convective heat exchanger operating with air at inlet temperature 320 K and a wall heat flux of 8.5
kW/m^2, evaluating entropy generation number Ns across 42 Reynolds numbers between 800 and 24,000 for five
fin-spacing geometries (2.0-6.0 mm). Total entropy generation number, non-dimensionalized against a reference
entropy generation rate, followed the expected U-shaped curve for every geometry tested, with heat-transfer
irreversibility dominating at low Reynolds number and friction irreversibility dominating at high Reynolds number.
The minimum-entropy-generation Reynolds number shifted systematically with fin spacing, from Re = 3,400 at 2.0
mm spacing to Re = 9,100 at 6.0 mm spacing, and the corresponding minimum Ns fell from 0.462 to 0.318 across
the same range, a 31% reduction, indicating that wider fin spacing lowered achievable irreversibility at its own
optimum despite reducing heat-transfer area. At the global optimum, 4.5 mm fin spacing operating at Re = 6,800,
Ns reached its lowest value of 0.291, with the Bejan number (fraction of total entropy generation attributable to heat
transfer) equal to 0.54, indicating a near-even split between the two irreversibility mechanisms at this operating
point, compared to Bejan numbers of 0.81 and 0.22 at the low- and high-Reynolds extremes of the sweep,
respectively. A parametric sensitivity analysis varying wall heat flux by ±25% around the 8.5 kW/m^2 baseline
shifted the optimal Reynolds number by less than 6% but changed minimum Ns by up to 18%, showing that heat
flux strongly affects the achievable entropy-generation floor without substantially relocating the optimal operating
point. These results provide a quantitative, geometry-resolved basis for selecting fin spacing and flow rate in
convective heat exchanger design to minimize thermodynamic irreversibility, complementing purely
heat-transfer-rate-based design criteria with an explicit second-law optimum