“Heavy” species Ludwig–Soret transport effects in air-breathing combustion

摘要: Abstract In the combustion community there seems to be a persistent view that Ludwig–Soret (L-S) (thermally driven, non-Fickian) species diffusion will only important for low molecular weight species, such as H and H2 in H2/air flames. present paper we mobilize extend some relevant research which, among other things, proves this particular viewpoint untrustworthy, replacing it with more robust readily used criteria tailored high (Sc > 1) species. As an immediate corollary of results presented/discussed here, many previous “L-S-free” theoretical studies (large or small) “Lewis number effects combustion” must re-examined extended since “extreme” nonunity Lewis significant L-S transport frequently “go together.” Often, lack direct experimental information about thermal factor αT, even complexity underlying kinetic theory expressions are “justify” neglecting transport. To overcome suggest exploit two remarkably simple but rational approximate methods estimate αT “heavy” via already familiar Schmidt νA/DBA, ideal gas, temperature limit. We also provide, corollary, correlations/estimates anticipating simultaneous consequences Fick diffusional transport, including appreciable modifications recovery mainstream chemical energy (Rosner [1] ) surface-catalyzed combustion. Using instructive examples nonpremixed combustion, demonstrate importance heavy fuel vapor (even normal paraffins CnH2n+2 smaller than eicosane (n = 20; Table 1 ), well flame intermediates (including soot precursors, polycyclic aromatic hydrocarbons (PAHs) up coronene (C24H12; 2 ). conclude is not bona fide particles appreciably influenced by “thermophoresis” laminar flames—even (Table 2) dimensionless “thermophoretic diffusivity”: αTDBA/vg, 0.47, far from particle-limit value 0.5385. Put another way, applications effect, while known flames involving H(g) H2(g), does “disappear” heavier vapors, “suddenly” reappear bona-fide nanoparticles (e.g., “soot”)!