Design & Thermo Chemistry of Turbo Combustor P M V Subbarao Professor Mechanical Engineering Department Design for performance, safety and Reliability…..
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Design & Thermo Chemistry of Turbo Combustor
P M V Subbarao
Professor
Mechanical Engineering Department
Design for performance, safety and Reliability…..
Variables Affecting the Performance
• The effect of operating variables on burner performance is--
• Pressure. • Inlet air temperature. • Fuel-air ratio. • Flow velocity.
Generalized Flammability Map
Design Constraints: Flow Velocity
Region of Stable Burning
Design Constraints: Flammability Characteristics
Mixture Temperature
a
f
m
m
Saturation Line
Flammable Vapour Spontaneous Ignition
Lean Mixture
Rich Mixture
SIT of Aviation fuels: 501 – 515 K
Flammable mist
Flash Point
Combustion Stability
• The ability of the combustion process to sustain itself in a continuous manner is called Combustion Stability.
• Stable and efficient combustion can be upset by too lean or too rich mixture.
• This situation causes blowout of the combustion process.
• The effect of mass flow rate, combustion volume and pressure on the stability of the combustion process are combined into the Combustor Loading Parameter (CLP), defined as
VolumeCombustion p
mCLP
n
mixture
• n ~ 1.8
Combustion Stability Characteristics
CLP
a
f
m
m
Stable
Unstable
Unstable
Combustion Design Details
• Cross Sectional Area: The combustor cross section is determined by a reference velocity appropriate for the particular turbine.
• Another basis for selecting a combustor cross section comes from thermal loading for unit cross section.
• Length: Combustor length must be sufficient to provide for flame stabilization.
• The typical value of the length – to – diameter ratio for liner ranges from three to six.
• Ratios for casing ranges from two – to – four.
Combustion Design Considerations• Pressure Drop: The minimum pressure drop is
upto 4%.• Volumetric Heat Release Rate: • The heat-release rate is proportional to
combustion pressure. • Actual space required for combustion varies
with pressure to the 1.8 power.
Length of Combustor : Mixture Burn Time
How to proved the time required to burn all the mixture ?
l
combcomb S
Lt
Sl : Laminar Flame velocity
It is impossible to build an air craft engine which runs more than few m/s with laminar flames
Laminar Vs Turbulent Flames
Scales of Turbulence
Turbulent Flames
• Turbulent flames are essential for operation of high speed engines.
• Turbulent flames are characterized by rms velocity flucuation, the turbulence intensity, and the length scales of turbulent flow ahead of flame.
• The integral length scale li is a measure of the size of the large energy containing sturctures of the flow.
• The Kolmogrov scale lk defines the smallest structure of the flow where small-scale kinetic energy is dissipated via molecular viscosity.
• Important dimensionless parameters:
i
T
luRe
u
liT
Turbulent Reynolds Number:
Eddy turnover time:
Characteristic Chemical Reaction Time:L
combustorL S
L
The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.
u
S
L
lDa L
combustion
i
L
T
Regimes of Turbulent Flame
Da
Re
1
108
10-4 108
Weak Turbulence
Reaction Sheets
Distributed Reactions
Length Scaling
• An estimate of the size of main burner is required during the engines preliminary design.
• The cross sectional area can be easily determined using velocity constraints.
• The length calculations require scaling laws.
• The length of a main burner is primarily based on the distance required for combustion to come to near completion.
RT
En expTfp Rate Reaction
RT
Emn
reaction expTp t • There are no universal rules for pressure and
temperature exponents.
• Typical values of n : 1<n<2.
• Typical values of m: 1.5 <m <2.5.
mixturerefaveres
m
ρAL
V
L
V
Lt
3
1.51
T
pL
• Residence time tres in main burner is given by
• The aircraft turbo combustor is designed for a Residence time scale in• Primary combustion zone or• Flame holder zone or • Mixing zone which ever is long when compared to treaction
Thermochemistry of Combustion
Modeling of Actual Combustion
LHVm
hmhmm
Δh
Δhη
fuel
in0,airex0,fuelair
ideal0,
actual0,combustor
LHVmηhmhmm fuelcombustorin0,airex0,fuelair
LHVmηhmhm fuelcombustorin0,airex0,gas
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