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 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

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 ? 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

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

Thermochemistry of Combustion

Modeling of Actual Combustion