ME 475/675 Introduction to Combustion Lecture 34.
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Transcript of ME 475/675 Introduction to Combustion Lecture 34.
![Page 1: ME 475/675 Introduction to Combustion Lecture 34.](https://reader035.fdocuments.in/reader035/viewer/2022062216/56649dda5503460f94ad00fa/html5/thumbnails/1.jpg)
ME 475/675 Introduction to
CombustionLecture 34
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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
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Midterm 2
• Scaled = 10xSqrt(Raw)• Scaled average = 82
35 45 55 65 75 85 9535
45
55
65
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85
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105
115
Midterm 1
Mid
term
2
35 40 45 50 55 60 65 70 75 80 85 90 95 100 1050
1
2
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4
Unscaled
Unscaled score
# of
stud
ents
35 40 45 50 55 60 65 70 75 80 85 90 95 100 1050
1
2
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4
Scaled
Axis Title
Axis
Title
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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)
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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
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Simplified Quenching Analysis for a slot
• To quench, we need:
• , so need • If ; and since and
�̇�′ ′ ′𝑉 𝑄𝑐𝑜𝑛𝑑𝑄𝑐𝑜𝑛𝑑
𝑑
𝛿𝐿
𝑇𝑥b=2
b>2
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Quenching will take place when
• , = 2 or larger• But
�̇�′ ′ ′𝑉 𝑄𝑐𝑜𝑛𝑑𝑄𝑐𝑜𝑛𝑑
𝑑
𝛿𝐿
𝑇𝑥b=2
b>2
𝑑𝛿
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Data, Table 8.4 page 291
•
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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,
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Flammability Limits
• Flames only propagate within certain equivalence ratio ranges• ,
• See page 291, Table 8.4 for limits
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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?
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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
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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
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•