ME 475/675 Introduction to

CombustionLecture 34

Announcements• Integrated BS/MS Degree• http://www.unr.edu/engineering/academics/accelerated

• Term Project• Add 2% to HW• http://wolfweb.unr.edu/homepage/greiner/teaching/MECH.475.675.Combustion/TermProjectAssignment.pdf

Midterm 2

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Flame Quenching, Mixture Flammability, Ignition •What does it take to ignite a mixture? •What does it take to extinguish a flame?• “Williams Criteria” (rule of thumb)• Ignition will occur if enough energy is added to a slab of thickness (laminar

flame thickness) to raise it to the adiabatic flame temperature, Tad. • A flame will be sustained if its rate of chemical heat release insides a slab is

roughly equal to heat loss by conduction out of the slab• Example extinguishment methods

• Pass a flame through a narrow tube or slot so it losses too much heat to the surfaces• Dilute using water (or thermal?)• Interrupt chemical kinetics (halogens)• Blow reaction away (loses fuel or heat)

Cold Wall Quenching

•Quenching distance d• Smallest dimension d that allows flame to pass• Experimentally determined by shutting off flow of a premixed

stabilized flame• dtube = (1.2 to 1.5) dtube

d d

Simplified Quenching Analysis for a slot

• To quench, we need:

• , so need • If ; and since and

�̇�′ ′ ′𝑉 𝑄𝑐𝑜𝑛𝑑𝑄𝑐𝑜𝑛𝑑

𝑑

𝛿𝐿

𝑇𝑥b=2

b>2

Quenching will take place when

• , = 2 or larger• But

�̇�′ ′ ′𝑉 𝑄𝑐𝑜𝑛𝑑𝑄𝑐𝑜𝑛𝑑

𝑑

𝛿𝐿

𝑇𝑥b=2

b>2

𝑑𝛿

Data, Table 8.4 page 291

•

Example 8.4, page 290Turn in next time for EC

• Consider the design of a laminar-flow, adiabatic, flat-flame burner consisting of a square arrangement of thin-walled tubes as illustrated in the sketch below. Fuel-air mixture flows through both the tubes and the interstices between the tubes. It is desired to operate the burner with a stoichiometric methane-air mixture exiting the burner tubes at 300 K and 5 atm.• Determine the mixture mass flowrate per unit cross-sectional area at the design

condition. • Estimate the maximum tube diameter allowed so that flashback will be

prevented.

• Methane (CH4)/air,

Flammability Limits

• Flames only propagate within certain equivalence ratio ranges• ,

• See page 291, Table 8.4 for limits

Example 8.5, p 294 (turn in next time for EC)• A full propane cylinder from a camp stove leaks its contents of 1.02 lbm (0.464

kg) into a 12’x14’x8’ (4.66 m x 4.27 m x 2.44) room at 20C and 1 atm. After a long time, the fuel gas and room are well mixed. • Is the mixture in the room flammable?

Ignition

• The minimum electrical spark energy capable of igniting a flammable mixture.• It is dependent on the temperature, pressure and equivalence ratio of the mixture

• What is the critical (minimum) radius of a spark that will propagate

• ; ; ; ;

( 𝑑𝑇𝑑𝑥 )𝑅𝐶𝑟𝑖𝑡

Energy to bring critical volume to Tb

•

• • ; •

• not normally considered reliable

• Agrees with measurements at low pressure • Need lots of energy at low pressure• Hard to restart jet engines at high pressures• decreases as Tu increases• Table 8.5 page 298 Different fuels

•