Gas Turbine Combustor : Design Methods BY P M V Subbarao Associate Professor Mechanical Engineering Department I I T Delhi Another Method of Generating High Enthalpy Fluid using Fuel….. All for generation of Motive Power…..

Moving Fluids Moving fluids possess flow energy, also called convected energy or transport energy. The total energy of non flowing fluid: e = u + ½V2 + gz The total energy of flowing fluid: e = u + pv + ½V2 + gz. This total energy is also called as Methalpy. The term u + pv is called as Enthalpy, h. The term u is a strong function of temperature alone. The term pv is a strong function of pressure and temperature. Therefore, high enthalpy of fluid can be achieved by high pressure and Moderate temperature or Moderate Pressure and High Temperature.

Brayton Cycle for Power Generation Natural Resource

Generation of Direct Motive Power 5 4

Brayton Cycle for Aircraft Propulsion D 1'-11" 10" 1'-2" 1 2 3 4 Compressor Turbine + Nozzle Turbofan Burner

Combustors Heat input (through Natural Resources) to the gas turbine Brayton cycle is provided by a combustor. The combustor accepts air from the compressor and delivers it at an elevated temperature to the turbine. A combustor is a direct fired air heater in which fuel is burned almost stoichiometrically. The compressor discharges at least three times the stoichiometric air. Only one third participates in combustion.

Types of Combustors

Gas Turbine Combustor

Basics of Combustor

First Law Analysis of Furnace:SSSF dm C.V. = 0 dt dE C.V. Conservation of Mass: m comp. air + m fuel - mfluegas = 0 First Laws for SG in SSSF Mode: S m in (h + ½ V2+gZ) in = S m fluegas (h + ½ V2+gZ)fluegas Temperature of flue gas will be very high. Turbine materials cannot withstand such a high temperature. More Dilution air is required. m fuel m fluegas m compressed air

Combustors have three features: Combustion products are mixed with the remaining air to arrive at suitable turbine inlet temperature. Combustors have three features: A recirculation zone A burning zone A dilution zone. Recirculation Zone: The function of Recirculation zone is to evaporate, partly burn and prepare the fuel for rapid combustion within the remainder of the burning zone. Burning Zone: Almost complete combustion occurs in burning zone. High temperature products of combustion leave the burning zone. Dilution Zone: The function of dilution zone is to mix the hot gas with the remaining excess air. The mixture leaving this zone should have a temperature and velocity distribution acceptable to turbine.

Performance of Combustor Combustor efficiency is a measure of combustion completeness.

The loss of pressure in combustor (p3 <p2) is a major problem. The total pressure loss is usually in the range of 2 – 8% of p2. The pressure loss leads to decrease in efficiency and power output. This in turn affects the size and weight of the engine. 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.

Basic Anatomy of A Combustor Casing Liner Swirler Liner holes

Flow Through Combustor

Air Distribution in A Combustor

Complete Anatomy of A Combustor

Design Constraints: Flow Velocity

Design Constraints: Flammability Characteristics Rich Mixture Saturation Line Flammable Vapour Spontaneous Ignitionn Lean Mixture SIT of Aviation fuels: 501 – 515 K Mixture Temperature

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 Unstable Stable Unstable CLP

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. Residence time tres in main burner is given by

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. 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.

Liner Holes: Liner area to casing area is around 0.6. The diameter of holes in primary zone should be no larger than 0.1 of the liner diameter. Ten rings of eight holes each will give good efficiency. Types of Combustor arrangements. Tubular Tube-annular Annular.

Types of Combustors

                                        

                                                  

                                                  

Contemporary Main Burners Engine Type TF39 Annular TF41 Cannular J79 JT9D F100 T63 Can Air Flow (kg/sec) 80.7 61.2 73.5 110 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

Using the steam or Gas to make the Power Using the steam or Gas to make the Power !   Rotating the shaft is the ultimate goal of any power plant. As you have probably noticed, from the text and pictures above, there is no shaft. Which leads to the question: "now that you have all this super energized steam or Gas, how do you get work from it ? " A boilers / Combustor is only one part of a larger operation, granted, it's a large part but most important part of the operation is it's ability to apply all this steam power.  

The Steam/Gas Turbine The more modern method of extracting mechanical energy from thermal energy is the steam turbine. Steam turbines/Gas Turbines have been the norm in various land based power plants for many years. Motive power in a steam/gas turbine is obtained by the rate of change in momentum of a high velocity jet of steam impinging on a curved blade which is free to rotate. The steam/gas is expanded in a nozzle, resulting in the emission of a high velocity jet. This jet of steam impinges on the moving vanes or blades, mounted on a shaft. Here it undergoes a change of direction of motion which gives rise to a change in momentum and therefore a force.

Principle of Turbine Through Newton’s Second Law

Classification of steam turbines Impulse turbine In impulse turbine, the drop in pressure of steam takes place only in nozzles and not in moving blades. This is obtained by making the blade passage of constant cross-sectional area. Impulse-Reaction turbine In this type, the drop in pressure takes place in fixed nozzles as well as moving blades. The pressure drop suffered by steam while passing through the moving blades causes a further generation of kinetic energy within these blades, giving rise to reaction and add to the propelling force, which is applied through the rotor to the turbine shaft. The blade passage cross-sectional area is varied (converging type).

The simple Impulse turbine It primarily consists of: a nozzle or a set of nozzles, a rotor mounted on a shaft, one set of moving blades attached to the rotor and a casing. A simple impulse turbine can be diagrammatically represented below. The uppermost portion of the diagram shows a longitudinal section through the upper half of the turbine, the middle portion shows the actual shape of the nozzle and blading, and the bottom portion shows the variation of absolute velocity and absolute pressure during the flow of steam through passage of nozzles and blades. Example: de-Laval turbine

Compounding of impulse turbine Compounding is done to reduce the rotational speed of the impulse turbine to practical limits. A rotor speed of 30,000 rpm is possible, which is pretty high for practical uses. Compounding is achieved by using more than one set of nozzles, blades, rotors, in a series, keyed to a common shaft; so that either the steam pressure or the jet velocity is absorbed by the turbine in stages. Three main types of compounded impulse turbines are: a) Pressure compounded, b) velocity compounded and c) pressure and velocity compounded impulse turbines.

Pressure compounded impulse turbine This involves splitting up of the whole pressure drop from the steam chest pressure to the condenser pressure into a series of smaller pressure drops across several stages of impulse turbine. The nozzles are fitted into a diaphragm locked in the casing. This diaphragm separates one wheel chamber from another. All rotors are mounted on the same shaft and the blades are attached on the rotor. The pressure and velocity variation are shown in the next diagram.

Impulse-Reaction turbine This utilizes the principle of impulse and reaction. It is shown diagrammatically below: There are a number of rows of moving blades attached to the rotor and an equal number of fixed blades attached to the casing. The fixed blades are set in a reversed manner compared to the moving blades, and act as nozzles. Due to the row of fixed blades at the entrance, instead of nozzles, steam is admitted for the whole circumference and hence there is an all-round or complete admission.