Interview—Combustion
Interviewer: Prof. Dryer
Prof. Dryer’s interview is very helpful because you will learn a lot through that.
Please read Prof. Glasssman’s textbook before you take it (and Ken is very helpful).
Other references are:
Prof. Dryer’s notes for course MAE427
Prof. Dryer’s notes for course MAE531, Combustion, 2001
Read through Prof. Law’s notes on combustion (to be a book) several times. Most of Prof.
Law’s questions during exam are based on his notes. I think Prof. Law’s notes are very
very very helpful in learning combustion.
First Interview: Combustion and Physics of Gases
Prof. Dryer (Dec. 1st, 2004)
At first, you should know how to solve problems on the ‘Bible’ (old questions).
1, N2H4+NO2=2H2O+1.5N2
(1), why are the products H2O and N2? (smallest chemical potential)
(2), rich shift of adiabatic flame temperature. (fuel rich: N2H4=NH3+H2, which is
exothermic)
(3), why is diffusion flame temperature less than stoichiometric adiabatic flame
temperature? (leakage, radicals..) (Due to finite reaction rates--Prof.. Glassman’s book)
(4), how to determine adiabatic flame temperature? (heat of formation)
(5), how to get heat of formation experimentally? (spectrum vibration frequency)
(6), estimate ignition energy ( ~(Tignition-T0)*(flame thickness)**3 )
2, NO mechanism
(1) thermal NO mechanism (Prof.. Glassman’s book)
O+N2=NO+N, N+O2=NO+O, N+OH=NO+H
(2) prompt NO mechanism (Prof.. Glassman’s book)
CH+N2=HCN+N, C2+N2=2CN, CN+H2=HCN+N, CN+H2O=HCN+OH, overshoot of
O and OH radiacals
(3) NO2 mechanism (at low temperature)
N2+O=N2O, N2O+O=2NO, N2O=N2+0.5O2
(4) fuel-bound nitrogen NO mechanism (Prof.. Glassman’s book)
Note: at low temperature, (3) dominates; at high temperature (3) is replaced by (1)
3, extinguish flame temperature
(1) CnHm is almost the same (about 1200-1400K); (2) H2 (about 1100K) ???
4, chain-branching
(1) critical chain branching ratio (Prof. Classman’s book)
(2) addition of CH4 makes H2-O2 less explosive because:
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compare H+O2=OH+O (chain branching) with
CH4+M=CH3+H+M
CH3+O2=CH2O+OH or CH3O+H is very slow (slow branching)
CH3+C2H6=C2H5+CH4
C2H5+M=C2H4+H+M
C2H5+O2=C2H4+HO2
(3) O+H2=OH+H competes with
H+O2+M=HO2+M, HO2+H=H2O2, H2O2+M=OH+OH+M
So H+O2+M=HO2+M is chain termination or chain carrying, never chain branching
Suggestion on physics of gases:
(1), spectrum; (2), radiation; (3), molecules; (4)…
Second Interview: Combustion and Physics of Gases
Prof. Dryer (April. 11th, 2005)
1, NO formation (four mechanism). How about SOx ? Soot formation in diffusion flame
and premixed flame (different…)?
2, explain H2-O2 explosion limits? Branching factor?
3, Spalding’s flame holding theory (resistance time vs. ignition delay)
4, radiation (N2—no absorption, CO2---yes,how?)
5, characteristic temperature for transition, rotation and vibration.
6, flame speed affected by pressure, thermal conductivity
7, quenching diameter? Proportional to flame thickness
8, heat capacity of N2H4? (3 transition, 3 rotation, 12 vibration)
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