dr. r. nagarajan professor dept of chemical engineering iit madras
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Dr. R. Nagarajan
Professor
Dept of Chemical Engineering
IIT Madras
Advanced Transport PhenomenaModule 7 Lecture 32
1
Similitude Analysis: Flame Flashback,
Blowoff & Height
Flashback of a Flame in a Duct:
Depends on existence of region near duct where local
streamwise velocity < prevailing laminar flame speed, Su
No flame can propagate closer to wall than “quenching
distance” q, given by:
mixture thermal diffusivity
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
q u uconst. / S
2
Flashback of a Flame in a Duct:
Critical condition for flashback is of “gradient” form, i.e.,
U/d :Su/q, or:
Multiplying both sides by d2/u leads to correlation law
of Peclet form:
Basis for accurate flashback predictions in similar
systems
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
2
u
u ufb
S dUconst.
2u
fb u
SUconst.
d
3
Flashback of a Flame in a Duct:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
Correlation of flashback limits for premixed combustible gases in tubes (after Putnam and Jensen (1949))
4
“Blow-Off” from Premixed Gas Flame “Holders”:
Oblique premixed gas flames can be stabilized in ducts even
at feed-flow velocities >> Su
Anchor is well-mixed zone of recirculating reaction products
e.g., found immediately downstream of bluff objects (rods,
disks, gutters), in steps of ducts
Sharp upper limit to feed-flow velocity above which blow-out
or extinction occurs Ubo
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
5
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
Schematic of an oblique flame “ anchored” to a gutter-type (2 dimensional) flame-holder (stabilizer) in a uniform stream of premixed combustible gas (Su<U<Ubo)
6
“Blow-Off” from Premixed Gas Flame “Holders”:
Su measure of reaction kinetics
Similitude to GT combustor efficiency example yields:
where
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
1boRe fct Dam, Arr, , ,Pr,Sc,geometry
bobo
u
U LRe
v
7
“Blow-Off” from Premixed Gas Flame “Holders”:
Based on SA of Su data:
Solving for Peclet number at blow-off:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
22
boO F
u u
L / UDam . fct Arr, , ,v ,v
/ S
3bo u
O Fu u
U L S Lfct , Arr, , ,v ,v ,Pr,Sc,geometry
8
“Blow-Off” from Premixed Gas Flame “Holders”:
Experimentally:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
4bo u
u u
U L S Lfct ,geometry
9
“Blow-Off” from Premixed Gas Flame “Holders”:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
Test of proposed correlation of the
dimensionless "blow-off” velocity,
for a flame
stabilized by a bluff body of transverse
dimension L in a uniform, premixed gas
stream ( adapted from Spalding (1955))
bo u u uU L / vs.S L /
10
“Blow-Off” from Premixed Gas Flame “Holders”:
Alternative approach: recirculation zone likened to
WSR
3D stabilizer of transverse dimension L exhibits
recirculation zone with effective volume given by:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
35WSR eff
V L . fct Re,Pr,Sc,geometry
11
“Blow-Off” from Premixed Gas Flame “Holders”:
Fuel-flow rate into recirc/ reaction zone can be written
as:
Blow-out occurs when corresponding volumetric fuel
consumption rate is near
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
26F u F ,um U . L . fct Re,Pr,Sc,geometry
'''F F ,max WSRbo eff
m r . V
'''F ,maxr
12
“Blow-Off” from Premixed Gas Flame “Holders”:
Rearranging:
Additional insight: blow—off velocity scales linearly with
transverse dimension of flame stabilizer, at sufficiently
high Re
Experimentally verified
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
2
7 2bo uO F
u u
U L S L. fct Arr, , ,v ,v ,Re,Pr,Sc,geometry
13
Laminar Diffusion Flame Height:
Buoyancy and fuel-jet momentum contribute to height,
Lf, of fuel-jet diffusion flame
Simple model: relevant groupings of variables
Beyond realm of ordinary dimensional analysis
Treat hot “flame sheet” region as cause of natural
convective inflow of ambient oxidizer
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
14
Laminar Diffusion Flame Height:
For any fuel/ oxidizer pair, when buoyancy dominates:
If fuel-jet momentum dominates, at constant Re:
Rj = ½ dj
Fr Froude number:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
1 32 /f
j
L. Re Fr const
R
1 3 1 32 / /f
j
L. Re Fr const' . Fr
R
2j jFr U / gR
15
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
Correlation of laminar jet diffusion flame lengths ( adapted from Altenkirch et al. (1977))16
Fuel Droplet Combustion at High Pressures:
Burning time, tcomb, depends on pressure
In rockets, aircraft gas turbines, etc., pressures > 20 atm may be
reached
Based on diffusion-limited burning-rate theory & available
experimental data at p ≥ 1 atm:
K burning rate constant
Dependent on fuel type & environmental conditions
Independent of droplet diameter
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
20comb p ,t d / K
17
Fuel Droplet Combustion at High Pressures:
Each fuel also has a thermodynamic critical pressure, pc, to
which prevailing pressure, p, may be compared:
Hence, following correlation may be obtained:
Corresponding-states analysis for high-pressure droplet
combustion is reasonably successful
Allows estimation of burning times where data
are not available
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
1
1c c
K p, fuel type,... pvs
K p ,same fuel ,... p same fuel
18
Fuel Droplet Combustion at High Pressures:
PARTIAL MODELING OF CHEMICALLY REACTING SYSTEMS
Approximate correlation of fuel-droplet burning rate constants at elevated pressures ( based on data of Kadota and Hiroyasu (1981))
19
Configurational Analysis (Becker, 1976):Establishing conditions of similarity by forming a
sufficient set of eigen-ratiosLeads to similitude criteria in the form of dimensionless
ratios of: Inventories Source strengths Fluxes Lengths Time, etc.
Can start analysis from macroscopic or microscopic CVs
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
20
But, interpretation not unique:
All may be regarded as ratios of same type, e.g.,
characteristic times
Relevant even to SS problems
e.g., length ratio in forced convection system fluid
transit-time ratio
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
21
e.g., momentum-flux ratio (Re) ratio of times
required for momentum diffusion & convection through
common area, L2
e.g., Pr ratio of times governing diffusive decay of
nonuniformities of energy (L2/) and momentum (L2/)
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
22
Characteristic Times:
e.g., fluid-phase, homogeneous-reaction Damkohler
number ratio of characteristic flow time to
characteristic chemical reaction time
e.g., surface (heterogeneous) Damkohler number
ratio of characteristic reactant diffusion time across
boundary layer to characteristic consumption time on
surface
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
2m diffw m
chem,we m
e w
/ D tkC
D t
k
23
Characteristic Times:
e.g., Stokes number (governing dynamical non-equilibrium
in two-phase flows) ratio of particle stopping time to
fluid transit time (L/U)
Attractive way of dealing with complex physicochemical
problems, e.g.:
Gas-turbine spray combustor performance
Coal-particle devolatilization
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
24
Further simplifications possible through:
rational parameter groupings
Dropping parameters via sensitivity analysis
Make maximum use of available insight & data–
analytical & experimental
ALTERNATIVE INTERPRETATIONS OF DIMENSIONLES GROUPS: “EIGEN-RATIOS
25
Integrates results of:
Scale-model testing
Full-scale testing
Mathematical modeling
Allows judicious blend of all three
Based on fundamental conservation principles & constitutive
laws
Maximum insight with minimum effort
BENEFITS OF SIMILITUDE ANALYSIS
26
Yields set-up rules for designing scale-model experiments
amenable to quantitative use
Reflects relative importance of competing transport
phenomena
Leads to smaller set of relevant parameters compared to
“dimensional analysis”
Sensitivity-analysis can enable “approximate” or “partial”
similarity analysis
BENEFITS OF SIMILITUDE ANALYSIS
27
OUTLINE OF PROCEDURE FOR SA
Write necessary & sufficient equations to determine QOI in most appropriate coordinate system PDEs bc’s ic’s Constitutive equations
Introduce nondimensional variables Use appropriate reference lengths, times, temperature
differences, etc. Normalize variables to range from 0 to 1
Make suitable (defensible) approximations Drop negligible terms
28
Express dimensionless QOI in terms of dimensionless
variables & parammeters
Inspect result for implied parametric dependence
This constitutes “similitude relation” sought
Stronger than conventional dimensional analysis
Fewer extraneous criteria
OUTLINE OF PROCEDURE FOR SA
29
Apparently dissimilar physico-chemical problems lead to
identical dimensionless equations
Establishing useful analogies (e.g., between heat &
mass transfer)
Dimensionless parameters will represent ratios between
characteristic times in governing equations
Problem may simplify considerably when such ratios
0 or ∞
SIMILITUDE ANALYSIS: REMARKS
30
Approximate similitudes possible for complicated
problems
Some conditions may be “escapable”
Resulting simplified equations may have invariance
properties
Allow further reduction in number of governing
dimensionless parameters
Allow extraction of functional dependencies, simplify
design of experiments
SIMILITUDE ANALYSIS: REMARKS
31
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