Premixed vs diffusion flames — Hinglish notes

5.3.5 · Chemistry › Combustion Chemistry (Propulsion Bridge)

YEH distinction exist kyun karti hai

Propulsion ke liye importance: ek engine designer jaanbujhkar regime choose karta hai. Premixed fast, controllable heat release deta hai lekin flashback aur knock ka risk hota hai. Diffusion flashback se safe hai (unmixed fuel mein wapas burn nahi ho sakta) lekin mixing-limited aur sooty hota hai. Rockets aur gas-turbine afterburners isi trade-off par jeete hain.

Controlling idea: rate-limiting step

Yeh single line sab kuch bata deti hai (speed, structure, soot, stabilization kahan hoti hai).

Laminar premixed flame speed derive karna (scratch se)

Hume chahiye $S_{L}$ , woh speed jis par flame fresh mixture ko khaati hai. KYA chahiye: ek balance reaction se generate hone wali heat aur agli layer ko preheat karne ke liye conduct hone wali heat ke beech.

Step 1 — Physics setup karo. Ek 1-D flame socho, fresh gas $S_{L}$ speed par aa rahi hai. Flame ka ek preheat zone hai thickness $δ$ wala. Uske andar, conduction ko gas ko $T_{u}$ (unburned) se $T_{b}$ (burned) tak raise karna hoga. Yeh step kyun? Flame tabhi aage badh sakti hai jab woh fresh gas ko ignition tak preheat kare — conduction woh messenger hai.

Step 2 — Do characteristic times match hone chahiye.

Zone ke across diffusion (thermal) time: $t_{d i f f} \sim \frac{δ ^{2}}{α}$ , jahan $α = k / (ρ c_{p})$ thermal diffusivity hai.
Chemical reaction time: $t_{c h e m} \sim \frac{1}{ω ˙ / ρ}$ (reaction rate per mass ka inverse).

Yeh step kyun? Flame tabhi steady hoti hai jab "heat pahunche" aur "fuel jale" same timescale par. Agar chemistry slower hoti, heat usse aage nikal jaati aur quench ho jaati; agar faster hoti, zone thin ho jaata.

Step 3 — Speed aur thickness relate karo. Ek wave jo thickness $δ$ ko time $t_{d i f f}$ mein cross kare, woh move karti hai $S_{L} \sim \frac{δ}{t _{d i f f}} = \frac{δ}{δ ^{2} / α} = \frac{α}{δ} .$ Saath hi zone mein gas ka residence time hai $t_{r es} = δ / S_{L}$ , aur combustion chahta hai $t_{r es} \sim t_{c h e m}$ , toh $δ \sim S_{L} t_{c h e m}$ .

Step 4 — $δ$ eliminate karo. $δ \sim α / S_{L}$ ko $δ \sim S_{L} t_{c h e m}$ mein substitute karo: $\frac{α}{S _{L}} \sim S_{L} t_{c h e m} \Rightarrow S_{L}^{2} \sim \frac{α}{t _{c h e m}} .$

Diffusion flame: woh kahan baith ti hai?

Ek reaction $F + s O_{2} \to$ products ke liye (s = mass of O₂ per mass of fuel), oxidizer stream mein oxidizer mass fraction $Y_{O, 2}$ aur fuel stream mein fuel mass fraction $Y_{F, 1}$ ke saath: $Z_{s t} = \frac{Y _{O, 2} / s}{Y _{F, 1} + Y _{O, 2} / s} .$ Yeh step kyun? Flame par, fuel aur oxygen exactly usi ratio mein pahunchte hain jis ratio mein wo ek doosre ko consume karte hain, isliye local fuel/oxidizer balance space mein ek single surface define karta hai.

Diffusion flame ki koi propagation speed nahi hoti — yeh kisi cheez mein travel nahi karti. Iski "speed" replace ho jaati hai is baat se ki mixing reactants ko kitni fast deliver karti hai (scalar dissipation rate $χ$ ). Mixing bahut zyada push karo ( $χ$ bahut bada) aur flame extinguish (blow out) ho jaati hai kyunki reactants heat release se faster dilute ho jaate hain.

Worked examples

Worked example 1 — In flames ko classify karo

System	Premixed ya Diffusion?	Kyun
Candle	Diffusion	wax vapor (fuel) surrounding air se sirf flame surface par milta hai
Bunsen, air hole khula	Premixed	air tube exit se pehle entrain hoti hai
Diesel engine	Diffusion (mostly)	fuel already-hot compressed air mein spray hota hai, mix hote hue jalta hai
Gasoline SI engine	Premixed	fuel+air intake mein mix hote hain, phir spark-ignited
Welding torch inner cone	Premixed; outer plume Diffusion	ek hi flame mein dono present hain

Yeh kyun matter karta hai: Bunsen ka inner blue cone IS ek premixed flame hai; faint outer mantle ek diffusion flame hai jo room air mein leftover fuel jalaati hai. Ek device, dono regimes.

Common mistakes (steel-manned)

Active recall

Recall Test yourself (answers chhupa lo)

Woh kaunsa single sawaal hai jo premixed ko diffusion flames se alag karta hai?
Kis regime mein $S_{L}$ meaningful hai, aur doosre mein $S_{L}$ kyun nahi hota?
$S_{L} \sim α / t_{c h e m}$ ko do timescale steps mein derive karo.
Diffusion flame ko physically kya limit karta hai, aur woh extinguish kyon hoti hai?
Candle flame yellow kyun hoti hai lekin stove flame blue kyun hoti hai?

Recall Feynman: ek 12-saal ke bache ko explain karo

Socho matches aur dominoes ki ek row. Premixed flame mein gunpowder pehle se hi har jagah air ke saath mix hai, toh jab tum ek jagah light karo toh aag khud hi line mein chalti hai — woh running speed flame speed hai. Diffusion flame mein, fuel aur air alag rakhe hote hain, jaise matches ko ek box mein aur striking strip ko doosre mein rakhna; aag sirf exactly wahan hoti hai jahan wo touch karte hain. Candle doosri type hai: pigala wax "fuel" upar float karta hai aur air se sirf bright edge par milta hai, toh flame wahaan hang karti rehti hai jab tak fuel aata rahe.

Connections

Laminar flame speed (S_L) — woh quantity jo humne derive ki; $ϕ$ , $T_{u}$ , pressure par depend karti hai.
Mixture fraction Z and conserved scalars — diffusion flames ka math backbone.
Flashback and blow-off limits — $S_{L}$ vs gas velocity ka direct consequence.
Soot formation and the rich premixed zone — diffusion flames kyun glow karti hain.
Damköhler number — $D a = t_{f l o w} / t_{c h e m}$ "mixing- vs reaction-limited" ko formalize karta hai.
Adiabatic flame temperature — speed derivation mein use hone wala $T_{b}$ set karta hai.
Diesel vs SI engine combustion — is poore note ka propulsion payoff.

Woh kaunsa single sawaal hai jo premixed ko diffusion flames se alag karta hai?

Kya fuel aur oxidizer flame tak pahunchne se PEHLE mix hue (premixed) ya unhe diffusion se flame PAR mix karna padta hai (diffusion).

Laminar premixed flame ki speed kya limit karti hai?

Chemical kinetics combined with fresh gas mein heat conduction — yeh reaction-limited hoti hai.

Diffusion flame ko kya limit karta hai?

Molecular/turbulent mixing (transport-limited); chemistry comparatively fast hoti hai.

Laminar flame speed aur thickness ki scaling do.

S_{L} \sim α / t_{c h e m}

aur

δ \sim α / S_{L}

Diffusion flame ki koi propagation speed kyun nahi hoti?

Yeh unburned mixture mein travel nahi karti; yeh stoichiometric mixture-fraction surface par baith ti hai jahan reactants mixing se pahunchte hain.

Fast-chemistry diffusion flame space mein kahan hoti hai?

Us surface par jahan mixture fraction

Z = Z_{s t}

(stoichiometric) hota hai.

Air mein methane ka stoichiometric mixture fraction (approx)?

Lagbhag

Z_{s t} \approx 0.055

Premixed burner mein flashback kya cause karta hai?

Gas exit velocity

S_{L}

se neeche girti hai, toh flame tube ke andar wapas travel karti hai.

Diffusion flame extinguish kyun hoti hai?

Excessive scalar dissipation (bahut fast mixing/strain) reactants ko us se faster dilute karta hai jis se heat release hoti hai.

Candle yellow kyun hoti hai lekin stove blue kyun hota hai?

Candle ek fuel-rich diffusion flame hai jo glowing soot produce karti hai (yellow); premixed stove flame cleanly hotter jalti hai (blue).

Flame-speed formula mein

α

kya hai?

Thermal diffusivity

α = k / (ρ c_{p})

Premixed-blue ya diffusion-yellow mein se typically kaunsi hotter aur cleaner hoti hai?

Premixed blue flame.