TECHNICAL PAPER

ADVANCES IN GAS TURBINE TECHNOLOGY

Author: Cdr AM Jaffar Sirajuddin, Trials Officer, GTTT(Mbi)

Synopsis. This paper aims at bringing out the advancements made in Gas

Turbine technology in the field of GT manufacturing over the last few decades which

have lead to increased efficiency, reduced SFC and lower emissions. Considering

the fact that Gas Turbines are the mainstay of Indian Navy, it is essential to absorb

the latest technology available in the field to enable selection of justifiable &

sustainable propulsion/power generation machinery, incorporating latest

advancement/developments, in the years to come.

1. Introduction. Gas Turbines, in today’s world, are an inseparable part of

modern life where these durable machines are being utilised in the field of power

generation to air transport and shipping. From being the most compact way of

delivering power, at the time of their invention, Gas Turbines have evolved into

engineering marvels due to use of advanced metallurgy, computational fluid

dynamics, heat transfer, additive technology etc. Some of the technologies, which

have enabled these machines become more efficient by converting 60% of heat

energy into useful work (combined cycles), are discussed in this paper as these are

considered most suitable for Naval requirements.

2. Performance Improvement. Gas Turbines can be described as simple

machines with a compressor, to compress the working medium, a combustor, to add

heat energy into the working medium and a turbine, to convert the heat energy to

work for driving the compressor and for producing useful work. Performance of the

GT depends on the efficiency of the individual components and their performance.

3. The primary parameters/ components that contribute to increasing the

efficiency of the heat cycle are the compressor pressure ratio and the turbine inlet

temperature. Efficiency and performance of a Gas Turbine depends on the pressure

ratio which can be obtained in the compressor and the maximum Turbine Inlet

Temperature (TIT) which the turbine can withstand without experiencing Creep. The

effect of these parameters is briefly described in the succeeding paragraphs.

4. Pressure Ratio. Dependence of thermal efficiency of the Gas Turbine on

the pressure ratio obtained from the compressor is shown in the figure 1 below. It is

evident from the graph, that any increase in pressure ratio will improve the

performance and the efficiency of the Gas Turbine. It may be noted from the graph

that the thermal efficiency reaches above 60% as the pressure ratio is increased to

40:1.

Figure 1 – Dependence of Thermal Efficiency on Pressure Ratio

5. Turbine Inlet Temperature. Similarly, Turbine Inlet Temperature (TIT),

also play a major role in determining the performance/efficiency of the Gas Turbine.

As TIT approaches 2000K from 1000K, the thermal efficiency peaks to 55% from

50% as shown in figure 2 below.

Figure 2 – Dependence of Thermal Efficiency on TIT

6. Traditional Methods of Improving the Thermal Efficiency of GT Cycle.

In order to improve the efficiency of gas turbine power plants, one of the

following three methods are used.

(a) Gas Turbine cycle with Regeneration. The temperature at the

exhaust of the gas turbine is higher than the temperature of the air at the exit

of the compressor. In order to utilize the heat energy of the exhaust of the

turbine, which is a waste to the atmosphere, the compressed air is heated in a

heat exchanger called regenerator. In a counter flow heat exchanger, it is

theoretically possible to heat the air discharged by the compressor reversibly

to the temperature of the exhaust leaving the turbine and to cool the turbine

exhaust gases equal to the temperature of the discharge of the compressor

air.

(b) Gas Turbine cycle with Reheating. Turbine output can be

increased by reheating of the gas during expansion in two or stages. In

between the turbine stages, an additional combustors or reheaters are added

in order to heat the gases.

(c) Gas Turbine cycle with Intercooling. As it is known that the majority

of the power generated by the turbine is utilized for running compressor, it is

important to reduce the compressor power consumption. This can be

achieved by compressing the air in two or more number of stages by providing

an intercooler in between the stages such that law of compression

approaches to isothermal compression.

Figure 3 – Braton Cycle with Intercooling, Reheat and Regeneration

7. Why improve Manufacturing Process & not the GT Cycle. However,

utilising the above methods comes with a penalty of bulkiness of the Gas Turbine

which is not a viable option onboard warship. So researchers around the world have

made efforts in improving performance of the simple cycle Gas Turbine with the

power to weight ratio advantage offered by them in mind. This paper aims at bringing

the technological advancements being made in the field of design and manufacturing

of the Gas Turbines. The paper discusses improvements in Compressor Design

using CFD, increasing TIT by using Investment Casting & development of Single

Crystal Turbine Blades, using Additive Manufacturing processes, application of

advanced Thermal Barrier Coatings, employing Ceramic Matrix Composites for

Blade manufacture & using advanced Blade Cooling Technologies.

8. Computational Fluid Dynamics in Compressor Design. Computational

Fluid Dynamics has vastly changed the design of compressors and fluid flow,

thereby increasing the compressor efficiency. 3D design modelling using advanced

software has in some cases increased pressure ratios upto 30:1. The higher

pressure ratio reduces thermal loads from the higher firing temperature of this design

while keeping exhaust temperatures optimal for combined cycle application.

Compressor efficiency is enhanced by better stationary and rotating blade geometry

made possible by the three-dimensional computational fluid dynamic codes.

9. Better aerofoil shapes by the use of CFD design software both in rotating and

stationary blades has improved blade design helping the industry in trimming the

stages of the compressor and also reducing the number of blades in each stage

without compromising on the performance and mass flow. Notable instances in

improved compressor design include Westinghouse and its technology alliance

partners Rolls-Royce, Mitsubishi Heavy Industries and FiatAvio bumped the 501G`s

mass flow 25 percent over the 501F by increasing the mean diameter of the stages

and incorporating several advancements to individual components. The 501G`s

compressor, for example, has 17 stages, one more than the 501F, and provides an

increased pressure ratio of 19.2:1.

10. In another case, to achieve a similar pressure ratio of 16:1, Siemens was able

to eliminate two compressor stages (from 17 to 15) and trim cooling flow

requirements by using the CFD codes. With GE`s MS6001FA, designers were able

to increase inlet compressor flow by 40 percent over its previous design (MS6001B).

The blades in the first two rows of the unit`s 18-stage compressor feature a

transonic, tailored airfoil shape that is neither double circular arc nor NACA-65 in

form. The blades incorporate an advanced high-bypass ratio aircraft engine design

that minimizes shock losses at operating speeds.

Figure 4 – Flow Analysis Using CFD

Figure 5 – Analysis of Heat Transfer in Turbine Blades using CFD

11. There is a huge advantage in terms of space saving, thereby cost saving in

design of compressor with CFD. One such case is reduction of the size of suction

chamber (air initake) of the Gas Turbine. A firm reported that there was a saving of

40% in space and cost by optimising the suction chamber using the CFD.

Figure 6 – Reduction in space requirement of Suction Chamber

12. Increasing Turbine Inlet Temperature. This is the single most important

factor which is restricting improvement of the performance of the Gas Turbines which

in turn is a function of the metallurgy being used for turbine blades. TIT has

increased from 740 K during the advent of Gas Turbine to 1400-1600 K today. Some

manufacturers have also achieved 1800 K and constant efforts are made in this field.

Key developments which are enabling engineers across the world to improve the TIT

are use of thermal barrier coating, blade cooling techniques, use of single crystal

blades and very recently Additive Manufacturing.

13. Single Crystal Turbine Blades. Normally, metals are composed of

many crystals – ordered structures of atoms arranged in a regular lattice, which form

naturally as the metal cools from a molten state. These crystals are typically of the

order of tens of microns in size, positioned in many orientations. At high

temperatures and under strain, the crystals can slide against each other, and

impurities can diffuse along the boundaries between the grains. This is known as

creep, and it badly affected early turbine blades, which were forged from steel and

later nickel bars.

14. Casting single crystals, with no grain boundaries reduced creep largely due to

non-availability of grain boundaries. It is a highly complex process with internal

cooling channels already in place. The lack of these grain boundaries inhibits creep

from occurring in this way. Creep will still occur in single crystal turbine blades but

due to different mechanisms that occur at higher temperatures. The single crystal

turbine blade does not have grain boundaries along directions of axial stress which

crystalline turbine blades. Single crystal turbine blades have the mechanical

advantage of being able to operate at a much higher temperature than crystalline

turbine blades. The turbine blades are able to operate at these high temperatures

due to the single crystal structure and the composition of the nickel based

superalloy.

Figure 7 – Single Crystal Blade Grain Structure

15. Blade Cooling Technology. Cooling of turbine blades can be achieved

by air or liquid cooling. Air cooling allows the discharged air into main flow without

any problem. Quantity of air required for this purpose is 1–3% of main flow and blade

temperature can be reduced by 200–300 °C. There are many techniques of cooling

used in gas turbine blades, convection cooling, film cooling, transpiration cooling,

effusion cooling, pin fin cooling etc. which fall under the categories of internal and

external cooling. While all methods have their differences, they all work by using

cooling air (often bled from the compressor) to remove heat from the turbine blades.

Figure 8 – First Stage Guide Vane with Drilled Holes for Cooling

16. Jet Impingement Cooling. Jet impinging on the inner surfaces of the

airfoil through tiny holes in the impingement insert is a common, highly efficient

cooling technique for first-stage vanes. Impingement cooling is very effective

because the cooling air can be delivered to impinge on the hot region. Jet

impingement cooling can be used only in the leading-edge of the rotor blade, due to

structure constraints on the rotor blade under high speed rotation and high

centrifugal loads.

Figure 9 – Jet Impingement Cooling Technique

17. Additive Manufacturing. Additive manufacturing uses computer-

aided-design (CAD) software or 3D object scanners to direct hardware to deposit

material, layer upon layer, in precise geometric shapes. As its name implies, additive

manufacturing adds material to create an object. By contrast, when you create an

object by traditional means, it is often necessary to remove material through milling,

machining, carving, shaping or other means. Although the terms "3D printing" and

"rapid prototyping" are casually used to discuss additive manufacturing, each

process is actually a subset of additive manufacturing. Below figure depicts some of

the components manufactured using Additive Manufacturing Technology.

Figure 10 – Components manufacturing using Additive Manufacturing

Technique

18. Objects are digitally defined by computer-aided-design (CAD) software that is

used to create .stl files that essentially "slice" the object into ultra-thin layers. This

information guides the path of a nozzle or print head as it precisely deposits material

upon the preceding layer. Or, a laser or electron beam selectively melts or partially

melts in a bed of powdered material. As materials cool or are cured, they fuse

together to form a three-dimensional object.

Figure 11 – Turbine Blade Manufactured using AM Technique

19. By incorporating organic structures into designs, designers can eliminate

substantial weight while maintaining the part’s strength and integrity. An existing

bracket was redesigned for additive manufacturing, maintaining strength of the

original while reducing the weight by 84%.

Figure 12 – Hot section components of GT using AM Technique

20. Thermal Barrier Coating. These 100 µm to 2 mm coatings serve to insulate

components from large and prolonged heat loads by utilizing thermally insulating

materials which can sustain an appreciable temperature difference between the

load-bearing alloys and the coating surface. Thermal barrier coatings typically

consist of four layers: the metal substrate, metallic bond coat, thermally-grown oxide

(TGO), and ceramic topcoat.

Figure 12 – Thermal Barrier Coating

21. The use of TBCs (100 to 500 µm in thickness), along with internal cooling of

the underlying superalloy component, provide major reductions in the surface

temperature (100° to 300°C) of the superalloy. This has enabled modern gas-turbine

engines to operate at gas temperatures well above the melting temperature of the

superalloy (∼1300°C), thereby improving engine efficiency and performance.

Figure 13 – TBC of a Turbine Blade

22. Ceramic Matrix Composites. Ceramic matrix composites (CMCs) are a

subgroup of composite materials as well as a subgroup of ceramics. They consist of

ceramic fibres embedded in a ceramic matrix. CMCs are coming on strong for all gas

turbines and this will change the way the hot parts of gas are made. The use of

CMCs in gas turbines permit higher turbine inlet temperatures, which improves

turbine efficiency. Because of the complex shape of stator vanes and turbine blades,

the development was first focused on the combustion chamber. In the US, a

combustor made of SiC/SiC with a special SiC fiber of enhanced high-temperature

stability was successfully tested for 15,000 hours.

Figure 14 – CMC Turbine Blades

23. Efforts are being made to manufacture parts by powdering Titanium Aluminide

(TiAl) and then through Additive Manufacturing technique. Parts could thus be made

that could not be made any other way. Advanced new fuel nozzles for GE’s new fan

jets are now being made this way by computer programs. The CMC blading being

much lighter (one third of the conventional blade) in weight makes it possible for the

gas turbine disks and bearings to be reduced in size and weight. The fir tree

connection of the blades to the disks can be simplified and less costly to

manufacture.

24. Conclusion. Efficiency of GTs being used in the IN is in the range of 35-38%

for M/s Zorya make GTs and is 38% for M/s GE make GTs. With advanced

technologies, it is not far off before the efficiency of the aero /industrial Gas Turbines

crossing the 40% mark. Therefore, it is imperative to induct GTs in future with some

of the technologies in the future. In addition, these technology will enable building

Power Generation equipment with much higher capacity which occupies less space.

25. References.

Advances in Gas Turbine Technology, Edited by Ernesto Bernini and published by

INTECH WEB.ORG

Additive Manufacturing of Aerospace Propulsion Components by Dr. Ajay Misra,

Dr. Joe Grady and Robert Carter in NASA Glenn Research Center, Cleveland, OH.

Analysis of the Temperature Distribution in GT Blade Cooled by Compressed Air by

Hussain H. Al-Kayiem and Amir H. Ghanizadeh.

CMCs will revolutionize aero and land-based gas turbines by TMI Staff &

contributors.

Ceramic matrix composite from Wikipedia, the free encyclopedia. Gas turbine technology is swiftly evolving as manufacturers introduce aero-derived

advances in the pursuit of more power by Steven E. Kuehn, Senior Editor.

Jewel in the crown: Rolls-Royce’s single-crystal turbine blade casting foundry by Stuart Nathan.

Recent Studies in Blade Cooling by Je-Chin Han for Taylor & Francis Group.

Space Age Ceramics are Aviation’s New Cup of Tea by Tomas Kellner.

Overview of a Gas Turbine and the different methods to improve Thermal Efficiency

by Gubbale Sesha Saikrishna, Mallavolu Sai Nithish and Nekkanti Raviteja.

The Cooling of Turbine Blades by Zhou Qin-sheng and Wang Feng.

The Development of Single Crystal Superalloy Turbine Blades by M Gell, D N Duhl and A F Giamei. Thermal barrier coating from Wikipedia, the free encyclopedia. Thermal Barrier Coatings for Gas-Turbine Engine Applications by Nitin P. Padture,

Maurice Gell and Eric H. Jordan.