Question bank — Premixed vs diffusion flames

False — it does not travel into anything; there is no wave. Its pace is set by how fast mixing delivers reactants, not by a propagation speed.

False — the gas flows out at $S_L$ while the flame tries to travel in at $S_L$; the two cancel, so it only looks still (see Flashback and blow-off limits).

False — yellow is soot glowing (incandescence), not high temperature; the blue premixed flame is actually hotter and cleaner.

False — the chemistry is usually faster than in premixed flames; they are slow because they wait on mixing (transport-limited).

True — with mixing already done, the premixed flame speed is set by kinetics + heat conduction; the diffusion flame's chemistry is fast, so transport is the bottleneck.

False — the inner cone is premixed (entrained air), the outer mantle is a diffusion flame burning leftover fuel in room air. One device, both regimes.

True — $S_L\sim \sqrt{\alpha \cdot (1/t_{chem})}$ (derived in the box above), so it rises with both thermal diffusivity $\alpha$ and reaction rate.

False — a diffusion flame cannot burn back into unmixed fuel, since there is no premixed gas to propagate through; that is precisely why designers pick it for safety.

True — since $S_L\sim \sqrt{\alpha /t_{chem}}$ and rate $\propto 1/t_{chem}$, $S_L$ grows as the square root of the rate, not linearly.

False — small $Z_{st}$ means the flame sits deep on the air side; only ~5.5 % of local mass is fuel-derived, which is why the fuel-rich interior soots (see Soot formation and the rich premixed zone).

Wrong — push mixing too hard (scalar dissipation $\chi$ too high) and reactants get diluted faster than heat is released, so it blows out despite fast chemistry.

Wrong — thickness and speed trade inversely, $\delta \sim \alpha /S_L$ (see the formula box: a wave crossing a thin zone in a fixed diffusion time is faster), so a thinner zone accompanies a faster flame.

Wrong — blow-off is caused by gas speed exceeding $S_L$; you lower the velocity (or raise $S_L$) to keep the flame anchored (see Flashback and blow-off limits).

Wrong — it depends on the local equivalence ratio at the reaction zone; diffusion flames burn near stoichiometric locally and can reach comparable peak $T$, but bulk fuel-rich or lean loading shifts it (see Adiabatic flame temperature).

Wrong — Damköhler $=t_{mix}/t_{chem}$; a high value means chemistry is fast relative to mixing, so mixing is the slow step (see Damköhler number).

Wrong — Diesel sprays fuel into already-hot compressed air and burns mostly as a diffusion flame as it mixes, while SI (gasoline) is premixed (see Diesel vs SI engine combustion).

Wrong — it is thin because conduction and reaction balance there; conduction is exactly what preheats the next layer and lets it advance.

Because the premixed flame is a self-propagating wave eating into fresh mixture, while a diffusion flame just sits where reactants meet — nothing is being "propagated into."

Conduction is the messenger that carries heat from the hot reacted gas back into the cold fresh layer to bring it to ignition; that preheating is what lets the wave advance.

If heat arrived faster than fuel burned the zone would thin and quench; if slower, heat would outrun reaction — steadiness requires both on the same timescale.

Its interior is fuel-rich (the reaction shell sits far on the air side), so fuel pyrolyzes without enough local oxygen, forming soot that then glows yellow.

Diffusion flames die from dilution — reactants thinned faster than heat releases; premixed flames instead lose or win the race between gas velocity and $S_L$ at the anchor point.

Because chemistry is fast, the reaction collapses onto the surface where the fuel/oxidizer ratio is exactly stoichiometric — the $Z=Z_{st}$ isosurface (see Mixture fraction Z and conserved scalars).

Premixed gives fast, controllable heat release but risks flashback and knock; diffusion is flashback-safe but mixing-limited and sooty — the design lives on that balance.

$t_{chem}\to \infty$, so $S_L\sim \sqrt{\alpha /t_{chem}}\to 0$ — the flame cannot propagate and the mixture is below its flammability limit.

The outflow and the flame's inward travel cancel perfectly, giving a stationary, marginally stabilized flame — the ideal anchored condition between flashback and blow-off.

No — with fuel alone there is nothing to react; "premixed" requires fuel and oxidizer blended together, otherwise there is no self-sustaining wave.

Yes — a Bunsen with the air hole partly open, or a lifted turbulent flame, shows a premixed inner region and a diffusion outer region simultaneously.

With almost no mixing, reactants barely reach the surface, so heat release is feeble and the flame is weak and stretched — this is starvation (too few reactants delivered), the opposite failure from the high-$\chi$ case, where the flame dies by dilution (reactants over-stirred and cooled below reaction temperature).

With more oxygen available per unit mass, $Z_{st}$ rises, moving the flame surface toward the fuel side and raising local temperature — hotter, thinner, and closer to the fuel exit.