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 Fuel Flow rate 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

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 Burners Engine Type TF39 Annular TF41 Cannular J79 Cannular JT9D Annular F100 Annular T63 Can Air Flow (kg/sec) Fuel Flow (K/hr) Length (m) Diameter (m) P (kPa) T comb o C

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 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 n ~ 1.8

Combustion Stability Characteristics CLP Stable 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. 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.

Residence time t res 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 t reaction

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 ? S l : 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 l i is a measure of the size of the large energy containing sturctures of the flow. The Kolmogrov scale l k defines the smallest structure of the flow where small-scale kinetic energy is dissipated via molecular viscosity. Important dimensionless parameters: Turbulent Reynolds Number: Eddy turnover time:

Characteristic Chemical Reaction Time: The ratio of the characteristic eddy time to the laminar burning time is called the Damkohler Number Da.

Regimes of Turbulent Flame Da Re Weak Turbulence Reaction Sheets Distributed Reactions

Thermochemistry of Combustion

Modeling of Actual Combustion

Modeling of Combustion C X H Y S Z +  4.76 (X+Y/4+Z) AIR + Moisture in Air + Moisture in fuel → P CO 2 +Q H 2 O +R SO 2 + T N 2 + U O 2 + V CO Exhaust gases: P CO 2 +QH2O+R SO 2 + T N 2 + U O 2 + V CO kmols. Excess air coefficient : . Volume fraction = mole fraction. Volume fraction of CO 2 : x 1 = P * 100 /(P+R + T + U + V) Volume fraction of CO : x 2 = VCO * 100 /(P +R + T + U + V) Volume fraction of SO 2 : x 3 = R * 100 /(P +R + T + U + V) Volume fraction of O 2 : x 4 = U * 100 /(P +R + T + U + V) Volume fraction of N 2 : x 5 = 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. NO x upto 150 ppm. SO 2 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.

Specific Heat of flue gas :

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 T 3. Calculate c p,flue gase. Re calculate T 3. Repeat till the value of T 3 is converged.

Total Pressure Loss in Turbo Combustor The loss of pressure in combustor (p 0,ex <p 0,in ) is a major problem. The total pressure loss is usually in the range of 2 – 8% of p 0,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 : Relative to the inlet Dynamic pressure : Relative to a reference dynamic pressure:

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