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