ME 525: Combustion Lecture 16: Laminar Premixed Flame

1. ME 525: CombustionLecture 16: Laminar Premixed Flame Detailed Chemistry: H2-Air and CH4-Air flames • COSILAB calculates detailed flame structure for one • dimensional laminar premixed flames and you will have • opportunities to study these flames in upcoming HW 8. • Structure of rich premixed flames is much more complicated • than that of lean premixed flames. • Many applications such as land based low NOx gas turbines • are operating with lean premixed flames.

2. Structure of Premixed Flames • Flame consists of three zones: Preheat zone, Reaction zone, Recombination zone • Reaction zone can be further divided into a zone in which intermediates like HO2 and H2O2 and radicals like H atom and OH are formed. The fist three of these are consumed rapidly within the flame but OH exists for larger distance (and time). • Flame structure is determined by advection flow of unburned gases towards and diffusion of heat and radical species away from the reaction zone. • Diffusion dominated by the H atom. • Considerable HO2 formed by H + O2 + M → HO2 + M in preheat zone.

3. Structure of Premixed Hydrogen Flames • HO2 then forms H2O2 which is advected towards high T reaction zone and then breaks down to form OH. • OH plays very important role in final oxidation • Recombination zone: rate of temperature rise is rapid because of exothermic radical recombination reactions like H + OH + M → H2O + M.

4. Structure of Stoichiometric H2/O2 Premixed Flame Speed ~ 230 cm/s H2 HO2 H2O O2 H2O2 H T, K OH

5. Structure of Lean (Ф = 0.6) H2/O2 Premixed Flame Speed ~ 104 cm/s T, K HO2 H2 H2O O2 H2O2 H OH

6. Structure of Premixed Ф = 1.0 Methane/Air Flames Speed ~ 39 cm/s T, K O2 CH4 CO2 H2O CO H2 OH H HO2 H2O2

7. Structure of Premixed Ф = 0.6 Methane/Air Flames Speed ~ 13 cm/s O2 T, K CH4 H2O CO2 CO H2 OH H HO2 H2O2

8. Structure of Premixed Hydrocarbon Flames • In the first ½ mm methane preheats and diffuses towards its reaction zone but does not react. The first active species formed do not involve carbon at all. • Hydrogen diffuses very far into the reactant side and with the plenty of O2 available there, forms HO2 then forms H2O2. • The high diffusivity and high reactivity of hydrogen makes the first part of a methane flame look like it was a hydrogen flame. • Methane decomposes by reactions with the H radical & others? • OH plays very important role in H-atom abstraction from HCs in the “pyrolysis” zone and then in oxidation of CO and H2. • Next Homework will allow you to learn more about lean premixed flames • Heat release curve peaks in latter part of the reaction zone because of the reactions that convert CO to CO2, including the CO + OH → CO2 + H reaction. Read COSILAB Manual. COSILAB use is for Purdue educational purpose. Do not transfer COSILAB information!

9. Analysis of Premixed HC Flames with detailed chemistry • Assumptions • Steady state • 1-D, constant area flame • No radiation heat transfer • No shaft work or viscous dissipation • Potential energy changes are negligible • Kinetic energy changes are negligible • Diffusion of heat and mass follows Fourier and Fick’slaws, no thermal diffusion is now an option*. • Diffusion coefficient is the same for each species • Le = 1 Read COSILAB Manual. COSILAB use is for Purdue educational purpose. Do not transfer COSILAB information!

10. Analysis of Premixed Hydrocarbon Flames • Conservation of Energy: Useful for Flame Calculations • Multiply Conservation of Species by and add over all • Use definitions above, recognize items like: to obtain the equation used in PREMIX flame calculations. Review COSILAB Manual please.