Question 1

Why does the adiabatic flame temperature peak slightly above φ = 1.0?

Accepted Answer

The peak adiabatic flame temperature occurs near φ ≈ 1.05–1.10 rather than exactly at φ = 1.0 due to dissociation reactions. At stoichiometric conditions (φ = 1.0) some CO₂ and H₂O dissociate endothermically into CO, H₂, and O₂ at high temperatures, absorbing heat. A slightly fuel-rich mixture suppresses this dissociation, yielding a marginally higher temperature. For methane/air, the difference is roughly 30–50 K between φ = 1.0 (~2230 K) and φ = 1.05 (~2270 K).

Question 2

What is the difference between AFR, λ (lambda), and φ (equivalence ratio)?

Accepted Answer

These three quantities all describe the fuel–air mixture: AFR (Air-Fuel Ratio) is the mass ratio of air to fuel. λ (excess air ratio or lambda) = AFR_actual / AFR_stoich. φ (equivalence ratio) = AFR_stoich / AFR_actual = 1/λ. At stoichiometric: AFR = AFR_stoich, λ = 1, φ = 1. Lean mixture: λ > 1, φ < 1. Rich mixture: λ < 1, φ > 1. For methane, AFR_stoich ≈ 17.2 (mass basis).

Question 3

How does hydrogen fuel compare to methane in combustion performance?

Accepted Answer

Hydrogen (H₂) key differences vs. methane: ① Stoichiometric AFR ≈ 34.3 vs. 17.2 (H₂ needs twice the air per kg fuel). ② LHV ≈ 120 MJ/kg vs. 50 MJ/kg — much higher gravimetric energy density. ③ Laminar flame speed ~2 m/s vs. 0.4 m/s — 5× faster, raising flashback risk. ④ Zero CO₂ emissions — only water produced. ⑤ High adiabatic flame temperature (~2480 K) promotes NOx formation. Hydrogen is attractive as a zero-carbon fuel but requires burner redesign for flame stability and NOx control.

Question 4

How can this calculator be used in CFD combustion simulations?

Accepted Answer

Typical CFD workflow applications: ① Setting inlet boundary conditions (mixture fraction, temperature) for ANSYS Fluent, OpenFOAM, or Star-CCM+ combustion simulations. ② Selecting target equivalence ratio for gas turbine or engine design studies. ③ Preliminary CO/NOx regulatory compliance screening before running full detailed chemistry (Chemkin, FlameMaster). ④ Validating simple combustion models — adiabatic flame temperature should match the value from detailed mechanisms within ~100 K for lean mixtures. Note: actual flame temperatures are typically 10–20% lower than adiabatic due to heat losses.

Combustion Calculator · Air-Fuel Ratio · Adiabatic Flame Temperature

Theory