Design of Gas Turbine Combustors

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

PEMP RMD510

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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Introduction

PEMP RMD510

Location of combustor in a gas trubine engine

A typical gas turbine engine

Combustor

Rolls Royce y Turbomeca Adour Mk102 16


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Introduction

PEMP RMD510



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


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Introduction

PEMP RMD510

 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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Introduction

PEMP RMD510

 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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Parts of a Combustion Chamber

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Distribution of Air in a Combustor

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Air Flow Pattern in a Combustor

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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

PEMP RMD510

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

PEMP RMD510

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.


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Combustion System Components

PEMP RMD510

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

PEMP RMD510

Main Combustor of GE CF5-80C 16


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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


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Combustor Design Approach

PEMP RMD510

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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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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PEMP RMD510

• 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


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Design of Inlet Diffuser

PEMP RMD510

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 (<40% of the compressor exit velocity head) Short length (use of splitter vanes may be considered) No flow separation, except in dump region Uniform flow, both circumferentially and radially D Dynamic i fl flow stability t bilit att all ll operating ti conditions diti Insensitivity to changes in compressor exit flow pattern @ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Effect of Inlet Flow Conditions 

Inlet swirl



Presence of upstream struts



Radial distribution of compressor exit velocity



Reynolds number



M h number Mach b

 Turbulence

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Types of Diffusers Faired Diffuser

Fig. 3-9 / 3-10 Lefebvre

Dump Diffuser

Fig. 3-11 Lefebvre

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Vortex Controlled Diffuser Fig. i 3-13 / 3-14 Lefebvre f b

Hybrid Diffuser

Fig. 3-15 / 3-16 Lefebvre

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Types of Diffusers

Step Diffuser

Fig. 6 Mellor

Fig 8 Mellor Fig.

Multiple passage Diffuser

Controlled Diffuser 16


Fig. 7 Mellor

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R l ti Merits Relative M it off Various V i Diffuser Diff Types T Diffuser Type

Merits

Drawbacks

Aerodynamic or faired

 Low pressure loss

 Relatively long  Performance susceptible to thermal distortion and manufacturing tolerances  Performance sensitive to variations in inlet velocity profile

Dump

 Relatively short  Insensitive to variations i inlet in i l t flow fl conditions diti  High performance  Short length  Low pressure loss  High performance  Short length  Low pressure loss  Low bleed air requirement

 Pressure loss about 50% higher than faired type

Vortex controlled

Hybrid

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 Requires minimum of 4% air bleed  Design procedures not fully established  Design procedures not fully established  Bleed air pressure too low for turbine cooling


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Inlet Diffuser Design Requirements

PEMP RMD510

 Low pressure losses: In general the diffuser pressure losses

should be less than 40% of the compressor exit velocity head 

Short length: Special features, like splitter vanes, can be used to reduce length



No flow separation except in dump regions



Uniform flow, both radially and circumferentially



Dynamic flow stability at all operating conditions



Insensitivity to changes in compressor exit flow patterns or exit flow conditions

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PEMP RMD510

Design of Combustion Chamber Flow Path

 Design of combustion chamber flow path is related to the inlet diffuser

design.  These

designs must be coordinated because the combustor cowl and passage contours are very important for efficient diffuser operation.



Conversely, diffuser pressure recoveries must be known in order to Conversely select appropriate cooling and dilution hole sizes.

 The

combustor flow path should have a shortest length that meets all design requirements. Increased length adds to engine weight and requires more liner cooling flow. 

New combustor Ne b t flow fl path th designs de i aree generally e e ll based b ed on previous e i successful designs. Design improvements tend to be evolutionary.

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Combustion Chamber Flow Path – Design Variables

PEMP RMD510

 Combustor Dome height, Hd  Combustor

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dome velocity, Vd



Combustor length to height ratio, Lc/Hd



Combustor passage velocity, Vp



Fuel Injector spacing, B



Space rate, SR



Reference velocity, Vref



Reference velocity head, qref



Inlet velocity head, q3


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Air Flow Distribution

PEMP RMD510

Total T t l combustor b t airflow, i fl Wc, is i distributed di t ib t d to t different diff t combustor b t locations l ti to t achieve different design goals. The airflow distribution does not vary significantly with combustor operating conditions. The most important airflows are: W3

Compressor exit flow

Wtc

Turbine cooling airflow, which bypasses combustor

Wc

Combustor air flow (Wc = W3-W Wtc)

Wa

Fuel atomising air flow admitted through the fuel injector

Ws

Swirler airflow

Wp

Primary air jets which interact with swirler flow in the primary zone

Wdil

Dilution air jets downstream of the primary zone to provide the dome cooling airflow

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Wdc

Dome cooling airflow

Wlc

Liner cooling airflow

Wd

Combustor dome flow (Wd = Wa + Ws + Wdc) @ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Preliminary Design Procedure

PEMP RMD510

Fig. 24 Mellor

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Primary Zone Design

PEMP RMD510

• Flame Holding Concepts

• Swirler Design Approaches

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Dilution Zone Design

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• Dilution Flow Distribution • Mixing Mi i U Uniformity if it • Profile Trim

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Combustor Cooling

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• Louvered Liners • Cooling Slots • Thermal Th l Barrier B i Coatings C ti • Augmented Backside Convection • Segmented Wall Construction • Quasi Transpiration Cooling

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Combustor Cooling

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Combustion Chamber Performance

PEMP RMD510

A combustion chamber must be capable of allowing fuel to burn efficiently over a wide range of operating conditions without incurring a large pressure loss. loss In addition, addition if flame extinction occurs, then it must be possible to relight. In performing these functions, the flame tube and burner atomizer components must be mechanically reliable Because the gas turbine engine operates on a constant pressure cycle, l any loss l off pressure during d i the th process off combustion b ti must be kept to a minimum. In providing adequate turbulence and mixing, a total pressure loss varying from about 5 to 10 per cent of the air pressure at entry to the chamber is incurred.

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Combustion Intensity

PEMP RMD510

The heat released by a combustion chamber or any other heat generating unit is dependent on the volume of the combustion area. Thus to obtain the required high power output, Thus, output a comparatively small and compact gas turbine combustion chamber must release heat at exceptionally high rates. For example, a Rolls-Royce Spey engine will consume in its ten flame tubes 3402 kg of fuel per hour. The fuel has a calorific value off approximately i t l 8888 kJ/s kJ/ kkg (18 (18,550 550 British B iti h Thermal Th l Units U it per lb), lb) therefore each flame tube releases nearly 4076 kJ/s (232,000 British Thermal Units per minute). Expressed in another way, this is an expenditure of potential heat at a rate equivalent to approximately 40.8 MW for the whole engine.

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Combustion Efficiency

PEMP RMD510

The combustion efficiency of most gas turbine engines at sealevel take-off conditions is 100 per cent, which reduces to 98 per cent at altitude cruise conditions. The values vary as shown in the figure, because of the reducing air pressure, temperature and fuel/air ratio.

Combustion C b ti efficiency ffi i andd air-fuel ratio 16


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Combustion Stability

PEMP RMD510

Combustion stability means smooth burning and the ability of the flame to remain alight over a wide operating range. range For any particular type of combustion chamber there is both a rich and a weak limit to the air/fuel ratio, ratio beyond which the flame is extinguished. An extinction is most likely to occur in flight during a glide or dive with the engine idling, when there is a high hi h airflow i fl andd only l a small ll fuel f l flow, fl i.e., i a very weakk mixture strength.

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Combustion Stability (… contd.)

PEMP RMD510

Combustion stability limits

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Combustion Stability (… contd.)

PEMP RMD510

The range of air/fuel ratio between the rich and weak limits is reduced with an increase of air velocity, and if the air mass flow is increased beyond a certain value, flame extinction occurs. A typical stability loop is illustrated in the figure. The operating range defined by the stability loop must obviously cover the required air/fuel ratios and mass flow of the combustion chamber. The ignition g pprocess has weak and rich limits similar to those shown for stability. The ignition loop, however, lies within the stability loop since it is more difficult to establish combustion under ‘cold’ cold conditions than to maintain normal burning. burning

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Carbon Formation

PEMP RMD510

High pressure ratio engines tend to produce exhaust smoke at take-off k ff conditions. di i This Thi indicates i di that h carbon b particles i l are being b i formed in over-rich regions of the primary zone in conditions of low turbulence, at high g temperature p and ppressure. However, smoke represents an almost negligible loss in combustion efficiency of less than 0.3 per cent.

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Combustor Materials

PEMP RMD510

The containing walls and internal parts of the combustion chamber must be capable of resisting the very high gas temperatures in the primary zone. In practice, this is achieved by using the best heat resisting materials available and by cooling the inner wall of the flame tube as an insulation from the flame. The combustion chamber must also withstand corrosion due to the products of combustion, creep failure due to temperature gradients, and fatigue due to vibrational stresses.

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Combustion Chamber Requirements • • • • • • • •

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PEMP RMD510

Complete combustion Low total pressure loss Stability of combustion process Proper temperature distribution at exit with no “hot spots” Short length and small cross section Freedom from flameout Relight ability Operation over a wide range of mass flow rates, pressures p and temperatures


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Basic Sizing of Combustors

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Annular Combustion System

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Annular Combustion System (… contd.)

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Basic Formulae for Combustor Design

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Design Charts for Combustor Sizing

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Combustor Calculations

PEMP RMD510

Conduct basic sizing for a conventional, as opposed to DLE (Dry Low Emission), industrial engine combustor. The configuration is a single pipe combustor which has the following requirements at ISO base load: M flow Mass fl rate t = 7 kg/s k / T4

= 1400 K

P3

= 900 kPa (8.88 atm)

WF

= 0.146 kg/s

T3

= 610 K

LHV = 43124 kJ/kgg –kerosene 16


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Combustor Volume •

PEMP RMD510

As per the design guidelines set combustor loading to 1 kg/s atm1.8 m3 (based upon total mass flow and combustor volume) l ) for f 99.9 99 9 % efficiency. ffi i Hence from F5.7.2: LOADING = W/(VOL*P311.8*10(0.00145*(T31-400))) 1 =7/(VOL*8.881.8 *10(0.00145 *(610-400))) VOL = 0.068 m3

•

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Loadingg at idle should also be checked as pper the gguidelines provided, however for an industrial engine which does not have altitude operation or altitude relight then usually setting this level at ISO base load is sufficient. sufficient @ M.S. Ramaiah School of Advanced Studies, Bengaluru

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Combustion Intensity

PEMP RMD510

From 5.7.3 INTENSITY = Wf * ETA3 * LHV / (P31*VOL) INTENSITY = 0.146 * 0.999 * 43124/(8.88 * 0.068) INTENSITY = 10.41 MW/atm.m3 •

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This is considerably less than the design guideline maximum level of 60 MW/atm m3.


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Primary Zone and CAN Area •

PEMP RMD510

From the guidelines design the primary zone for an equivalence ratio of 1.02, hence: FAR = 1.02 * 0.067 FAR = 0.0683 Wprimary = 0.146 / 0.0683 Wpprimary a y = 2.14 kg/s

•

Set primary zone exit Mach number = 0.02, the lower end of the design guidelines. Hence from Chart 3.8, Q=1.3609 and taking primary i zone exit i temperature to be b 2300 K: 1.3609 = 2.14 * 23000.5 /(Acan * 900) Acan = 0.084 m2

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Combustor Radii •

PEMP RMD510

Can radius is derived from area: 0 084 = PI * Rcan 0.084 R 2 Rcan = 0.164 m

•

Set 0.1 Mach number for outer annuli, as per design guidelines, hence Q = 6.9414 kgK/s kPa m2 and: 0 5 /(Aouter 6 9414 = 7*610 6.9414 *6100.5 /(A * 900)

Aouter = 0.028 m2 0.028 = PI * ( Router2 – 0.1642 ) Router = 0.189 m

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Combustor Length •

From volume and area: L =0.068/0.084 L =0.81 m

•

PEMP RMD510

Now check residence time using V = M * SQRT ( * R * T) Time = L / V V = 0.02 * (1.333 * 287.05 * 2300)0.5 V = 18 18.76 76 m/s /

This is significantly longer than the minimum value of 3 ms given in the guidelines. H However, only l primary i zone Mach number has been used. While it is acceptable it shows that there is some scope to reduce combustor area, length and volume.

Time = 0.81/18.76 Time = 43 ms 16


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Secondary and Tertiary Air Flows •

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PEMP RMD510

From the design guidelines the secondary zone should be set up for an equivalence ratio of 0.6, 0 6 hence: FAR

= 0.6 * 0.067

FAR

= 0.0402

Wsecondary

= 0.146/0.0402

Wsecondary

= 3.45 kg/s

Wtetertiary tay

= 7 - 2.14 - 3.45

Wtertiary

= 1.41 kg/s


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Final Combustor Layout

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Summary

PEMP RMD510

• Design features of gas turbine combustors are introduced. introduced • Basics of combustion process and combustor performance pparameters like combustion intensity, y, efficiency, y, stability y are discussed. • Design point performance and basic sizing calculations of a combustor b t are shown h with ith the th help h l off an example. l

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Design of Gas Turbine Combustors

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