Second Law Analysis of Combustion Processes in Turbulent Flows

Abstract

Combustion remains the dominant source of usable-energy destruction in most industrial and transportation
systems, yet standard first-law combustion analysis reports only heat release and species conversion, leaving the
irreversibility of the combustion event itself unquantified. We performed a second-law (entropy-generation) analysis
of premixed methane-air combustion in turbulent flows using Reynolds-averaged Navier-Stokes simulations
coupled to a reduced 19-species chemical mechanism, across 42 operating conditions spanning turbulence
intensities from 0.5 to 6.0 m/s and equivalence ratios from 0.7 to 1.3, resolving entropy generation from four distinct
sources: chemical reaction, heat conduction, species diffusion, and viscous dissipation. Volumetric
entropy-generation rate peaked at stoichiometric conditions (phi = 1.0), reaching 18.4 kW/(m3 K) at a turbulence
intensity of 3.5 m/s, a 46% increase over the laminar-flame baseline of 12.6 kW/(m3 K) measured under otherwise
identical conditions. Chemical-reaction irreversibility dominated the entropy budget at every condition tested,
contributing 61-68% of total entropy generation, with heat conduction across the flame front contributing a further
21-27% and species diffusion and viscous dissipation together accounting for the remaining 8-14%. Increasing
turbulence intensity from 0.5 to 6.0 m/s at stoichiometric conditions raised total entropy generation by 58% while
shortening the flame brush thickness by 34%, indicating that turbulence promotes irreversibility primarily by
intensifying local temperature and species gradients rather than by altering the reaction pathway itself. Second-law
(exergetic) combustion efficiency fell from 0.79 under near-laminar conditions to 0.68 at the highest turbulence
intensity tested, a 14% relative decline (95% CI: 11-17%, n = 42 simulated conditions, bootstrap resampling).
Off-stoichiometric operation on the lean side (phi = 0.8) reduced peak entropy-generation rate by 23% relative to
stoichiometric combustion at matched turbulence intensity, at the cost of a 9% reduction in net heat release. These
results quantify a direct, previously unresolved trade-off between turbulence-enhanced burning rate and second-law
efficiency in premixed combustion, and provide entropy-generation maps that can guide burner design toward
operating envelopes that limit exergy destruction without unacceptably compromising heat-release rate.