Design Framework for Turbo Combustor

P M V Subbarao

Professor

Mechanical Engineering Department

Design for performance, safety and Reliability…..

Simple Burner

Fuel

Air

Burning Velocity

Flow velocity

• Burning Velocity > Flow Velocity : Flash Back Limit

• Burning Velocity < flow Velocity : Blow Off Limit

• Burning Velocity = Flow Velocity : Stable Flame.

Stability & Flammability Limits

Air Flow rate

Fue

l Flo

w r

ate

Rich Mixture

Lean Mixture

Blow off Flash Back Stable Flame

Classification of Combustors

• Basis for this classification:• A burner handles finite amount of fuel.• Arrangement of multiple burners.• There are currently three basic types of Burner

Arrangements• The multiple-chamber or can type. • The annular or basket type. • The can-annular type.

Types of Combustors

Types of Combustors

Three Dimensional View of Can Combustor

Geometrical details of Can Type Combustor

Flow Through Can type Combustor

Velocity Distribution in A CAN

~c=750 m/s~ M=0.3

Contemporary Main BurnersEngine Type

TF39

Annular

TF41

Cannular

J79

Cannular

JT9D

Annular

F100

Annular

T63

Can

Air Flow

(kg/sec)

80.7 61.2 73.5 110 61.2 1.5

Fuel Flow

(K/hr)

5830 4520 3790 7300 4800 107

Length (m)

0.53 0.42 0.48 0.45 0.47 0.24

Diameter

(m)

0.85 0.61 0.81 0.965 0.635 0.14

P (kPa) 2630 2160 1370 2180 2520 634

Tcomb oC 1346 1182 927 1319 1407 749

Performance Requirements

High combustion efficiency. This is necessary for long range. • Stable operation. • Combustion must be free from blowout at airflows

ranging from idle to maximum power and at pressures representing the aircraft's entire altitude range.

• Low pressure loss: It is desirable to have as much pressure as possible available in the exhaust nozzle to accelerate the gases rearward High pressure losses will reduce thrust and in-crease specific fuel consumption.

Performance Requirements contd…..• Uniform temperature distribution : The average temperature of gases entering the

turbine should be as close as possible to the temperature limit of the burner material to obtain maximum engine performance.

• High local temperatures or hot spots in the gas stream will reduce the allowable average turbine inlet temperature to protect the turbine.

• This will result in a decrease in total gas energy and a corresponding decrease in engine performance.

Operational Requirements• Easy starting. Low pressures and high velocities

in the burner make starting difficult. • Small size. A large burner requires a large

engine housing with a corresponding increase in the airplane's frontal area and aerodynamic drag.

• This will result in a decrease in maximum flight speed Excessive burner size also results in high engine weight, lower fuel capacity and payload, and shorter range.

• Burners release 500 to 1000 times the heat of a domestic oil burner.

• Without this high heat release the aircraft gas turbine could not have been made practical.

Operational Requirements Contd…• Low carbon formation : Carbon deposits can block

critical air passages and disrupt airflow along the liner walls, causing high metal temperatures and low burner life.

• All of the burner requirements must be satisfied over a wide range of operating conditions.

• Airflows may vary as much as 50:1, • fuel flows as much as 30:1, and • fuel-air ratios as much as 5:1.• Burner pressures may cover a ratio of 100:1, while

burner inlet temperatures may vary by more than 450ºC.

• Low-smoke burner. Smoke not only annoys people on the ground, it may also allow easy tracking of high-flying military aircraft.

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

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

Combustion Design Considerations

• 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.

Combustion Design Considerations• Ratios for casing ranges from two – to – four.• Wobbe Number: Wobbe number is an

indicator of the characteristics and stability of the combustion process.

• 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.

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

LL S

The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.

u

SlDa L

L

i

L

T

Regimes of Turbulent Flame

Da

Re

1

108

10-4 108

Weak Turbulence

Reaction Sheets

Distributed Reactions

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

LHVmηhmhm fuelcombustorin0,airiex,0,igas,

LHVmηTcmTcm fuelcombustorin0,airp,airiex,0,ip,igas,

Modeling of Combustion• CXHYSZ + 4.76 (X+Y/4+Z) AIR + Moisture in Air + Moisture in

fuel → P CO2 +Q H2O +R SO2 + T N2 + U O2 + V CO• Exhaust gases: P CO2 +QH2O+R SO2 + T N2 + U O2 + V CO

kmols.• Excess air coefficient : .• Volume fraction = mole fraction.• Volume fraction of CO2 : x1 = P * 100 /(P+R + T + U + V) • Volume fraction of CO : x2= VCO * 100 /(P +R + T + U + V) • Volume fraction of SO2 : x3= R * 100 /(P +R + T + U + V) • Volume fraction of O2 : x4= U * 100 /(P +R + T + U + V)• Volume fraction of N2 : x5= T * 100 /(P +R + T + U + V)• These are dry gas volume fractions.• Emission measurement devices indicate only Dry gas volume

fractions.

Emission Standards

• 15% oxygen is recommended in exhaust.

• NOx upto 150 ppm.

• SO2 upto 150 ppm.

• CO upto 500 ppm.• HC upto 75 ppm.• Volume fractions of above are neglected for the

calculation of specific heat flue gas.

2222

2222222

ONOHCO

ONp,NOHp,OHCOp,COp,fluegas UMTMQMPM

cUMcTMcQMcPMc

kJ/kgK1000

T0.39

1000

T1.27

1000

T1.670.45c

32

COp, 2

kJ/kgK1000

T0.20

1000

T0.586

1000

T0.1071.79c

32

steamp,

kJ/kgK1000

T0.42

1000

T0.96

1000

T0.481.11c

32

Np, 2

kJ/kgK1000

T0.33

1000

T0.54

1000

T0.00010.88c

32

Op, 2

Specific Heat of flue gas :

LHVmηTcmTcm fuelcombustorin0,airp,airex0,p,fluegasigas,

•For a given mass flow rate of fuel and air, the temperature of the•exhaust can be calculated using above formula.•If mass flow rates of fuel and air are known.

•Guess approximate value of specific heat of flue gas.•Calculate T3.•Calculate cp,flue gase.•Re calculate T3.

•Repeat till the value of T3 is converged.

Total Pressure Loss in Turbo Combustor

• The loss of pressure in combustor (p0,ex <p0,in) is a major problem.

• The total pressure loss is usually in the range of 2 – 8% of p0,in.

• The pressure loss leads to decrease in efficiency and power output.

• This in turn affects the size and weight of the engine. • There are several methods of quantifying the total pressure

loss in a combustor,Relative to the total inlet pressure :

in0,

ex0,in0,combustor0, p

ppΔp

Relative to the inlet Dynamic pressure :indyn,

ex0,in0,combustor0, p

ppΔp

Relative to a reference dynamic pressure:ref

ex0,in0,combustor0, p

ppΔp

Combustion Terms• Reference Velocity: The theoretical velocity for

flow of combustor inlet air through an area equal to the minimum cross section of the combustor casing. (20 – 40 m/s).

• Profile Factor: The ratio between the maximum exit temperature and the average exit temperature.

Air Distribution in A Combustor

Velocity Distribution in A CAN

Inlet Exit