Design of Gas Turbine Combustors

Introduction. Location of combustor in a gas trubine engine. A typical gas turbine engine. Combustor. Rolls Royce Turbom...

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PEMP RMD510

Design of Gas Turbine Combustors Session delivered by: Prof Q. Prof. Q H. H Nagpurwala

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Session Objectives

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The h discussion di i in i this hi session i will ill enable bl the h delegates to:  comprehend the constructional features of different types of combustors  understand the combustor design guidelines  undertake design of the combustors for gas turbine application

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Introduction

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Location of combustor in a gas trubine engine

A typical gas turbine engine

Combustor

Rolls Royce y Turbomeca Adour Mk102 16

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Introduction

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Heat input to the gas turbine Brayton cycle is provided by the combustor.

 The

combustor accepts air from the compressor and delivers it at an elevated temperature to the turbine.

 The

overall air/fuel ratio of a combustion chamber ((combustor)) can vary between 45:1 and 130:1. 

However, the fuel will burn efficiently at or close to the stoichiometric t i air/fuel i /f l ratio ti off 15:1 15 1 only. l 

So, the fuel is burned with only part of the air entering the combustor in the primary combustion zone. 

Combustion products are then mixed with the remaining air in the secondary and dilution zones to arrive at a suitable turbine inlet temperature. 16

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Introduction

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 Air

from the engine compressor enters the combustor at a velocity of about 150 m/s, which is far too high for sustained combustion to take place. 

Hence, the air is first decelerated to a velocity of about 25 m/s in a pre-diffuser.



However, the speed of burning kerosene at normal fuel-air ratios is only about 5-10 meters per second; hence any fuel lit even in the prediffused air stream also would be blown away. away  Therefore,

a region of low axial velocity is created in the combustor, through swirlers so that the flame will remain alight throughout the range of engine operating conditions.

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Introduction

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 The

high pressure air from the engine compressor is already heated to about 450 deg C.

 The Th

temperature off the h air i is i raised i d to about b 1300 K in i the h combustor b at constant pressure. The temperature rise in the combustor is limited by the material used in the first stage of the turbine. 

Present day aeroengines are designed for high TET of the order of 1800 K (with efficient turbine blade cooling techniques), because high TET enhances overall gas turbine cycle efficiency. efficiency  These

high TETs require combustor primary zone flame temperatures of the order of 2000 K, which, in turn, necessitate the development of newer materials and efficient cooling techniques apart from the need for low loss, efficient and complete combustion.

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Parts of a Combustion Chamber

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Distribution of Air in a Combustor

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Air Flow Pattern in a Combustor

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Types of Combustor

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C b Can C Combustor 



 

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This type of combustion chamber i so arrangedd that is th t air i from f the th compressor enters each individual chamber through the adapter. Each individual chamber is composed of two cylindrical tubes, the combustion chamber liner and the outer combustion chamber. Combustion takes place within the liner. Airflow into the combustion area is controlled by small louvers located in the inner dome, and by round holes and elongated louvers along the length of the liner. @ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Types of Combustor

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A l Combustor C b t Annular 



 

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The primary compressed air is introduced into an annular space formed by a chamber liner around the turbine assembly. The space between the outer liner wall and the combustion chamber housing permit the flow of secondary y cooling g air from the compressor. Primary air is mixed with the fuel for combustion. Secondary (cooling) air reduces the temperature of the hot gases entering the turbine to the proper level l l by b forming f i a blanket bl k t off cooll air around these hot gases. @ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Types of Combustor

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C A l C b t Can-Annular Combustor 

 



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The combustion chambers are completely surrounded by the airflow that enters the liners through various holes and louvers. This air is mixed with fuel which has been sprayed under pressure from the fuel nozzles. The fuel-air mixture is ignited g byy igniter plugs, and the flame is then carried through the crossover tubes to the remaining liners. The inner casing assembly is both a support and a heat shield; also, oil lines run through it.

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Combustion System Components

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1 1.

Diffuser: A di Diff diverging i passage, which hi h reduces d the th velocity l it off compressor exit it air flow from ~Mach 0.3 to Mach 0.05-0.1 in combustor passages with minimum pressure loss.

2.

Cowls: Structures attached to dome which guide flow from diffuser into the combustor passages with minimum pressure loss.

3.

Dome: Front end of the combustor structure which provides shelter and means of flame stabilisation (e.g. swirlers) for the primary combustion zone.

4.

Liners: Thin metal shells extending from the dome to the turbine nozzle for control of combustion and dilution air jets and cooling air film. film The liners protect the engine casing and internal shafts form the hot combustion products.

5.

Casings: Engine structural shells which carry thrust loads. Casings also comprise inner and outer passage boundaries.

6.

Fuel Injectors: Devices which provide fuel to the primary zone, usually through the dome.

7.

Igniter: Spark plug located in dome or primary zone.

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Combustion System Components

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Main Combustor of GE CF5-80C 16

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Combustor Design Requirements 



O Operability bili

P f Performance

 Ground start



Combustion efficiency

 Altitude relight



Pressure drop



Lean blow out



Exit temperature distribution



Bleed airflows

 Emissions

 Configuration

 Smoke

 Size



Carbon monoxide (CO)

 Weight



Unburned hydrocarbons





Oxides of Nitrogen (Nox)

Maintainability

 Thermal h l 

growthh

Mounting Method



Durability  Structural integrity 

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C li life Cyclic lif

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Combustor Design Approach

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Combustor design and development efforts rely very heavily on previous experience.





Design rules usually involve empirical correlation of data from previous designs. 

CFD simulations are also used in conjunction with the empirical correlations.



Ongoing g g efforts are aimed to reduce reliance on empirical p correlations and development tests. Computational models will play an increasing role in future combustor designs. 

Design D i rules l actually t ll usedd in i industry i d t tend t d to t vary from f manufacturer f t to manufacturer.

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@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Combustor Design/Test Relationship Design Phase

Test Activity

Preliminary Design • Diffuser Diff flow fl path th • Combustor flow path • Initial air flow distribution

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• Diffuser Diff water t table t bl model d l • Fuel injector drop size • Swirler/primary zone flow field characterisation • Linear heat transfer model

Detailed Design • Refine design features and air flow distribution

• Low pressure sector combustor rig • Annular diffuser model

Combustor Development • Final hole pattern and air fl distribution flow di t ib ti

• High pressure sector combustor rig • Full scale annular combustor rig

@ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Design of Inlet Diffuser

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The compressor exit Th it velocity l it from f the th modern d gas turbine t bi engines i is i typically t i ll in i the range of 150-170 m/s and the corresponding velocity head may be as high as 10% of the total pressure. The function of the diffuser is to recover a large proportion of this energy and to keep the total pressure losses low with resulting lower specific fuel consumption. For an air velocity of 170 m/s and a combustor temperature ratio of 2.5, the pressure loss l incurred i d in i combustion b i would ld be b about b 25% off the h compressor pressure rise. Hence, the air velocity must be reduced prior to combustion to about 1/5 of the compressor exit velocity.

Diffuser Design Requirements:      

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Low pressure losses (