Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)23

Key Technologies for
1700℃ Class Ultra High Temperature Gas Turbine

KOICHI ISHIZAKA*1

KEIJIRO SAITOH*2

EISAKU ITO*3

MASANORI YURI*4

JUNICHIRO MASADA*5

Gas-turbine combined-cycle power generation is expected to lead to a long-term extensionof power-generation markets by providing the most clean and economic thermal power plants
which coexist with renewable energy. We are developing technologies for 1700°C-class gas
turbines as part of a national project to further improve the performance, and some of the latest
developed technologies have been applied to the development of the world's first 1600°C-class
J-series gas turbine and next-generation high-efficiency gas turbines.1 In this article, thetechnology development status, together with examples of technologies developed toward theapplication of next-generation 1700°C-class gas turbines, is described.

|1. Introduction
Innovative technology development is important for the reduction of greenhouse effect gas
emissions. We have conducted the development of key technologies required for thecommercialization of 1700°C-class gas turbines as a national project with an eye toward thecommercialization of the high-efficiency thermal power generation technology using
ultrahigh-temperature gas turbines as an advanced use of natural gas. The road map of technology
development for the realization of ultrahigh-temperature gas turbines is shown in Figure 1.

Figure 1
*1
*2
*3
*4
*5

Technology development roadmap for ultrahigh-temperature gas turbines

Chief Staff Manager, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.
Manager, Combustion Research Department, Research & Innovation Center, Mitsubishi Heavy Industries, Ltd.
Director, Business Intelligence & Innovation Department, Marketing & Innovation Headquarters, Mitsubishi Heavy Industries, Ltd.
Deputy Director, Large Frame Gas Turbine Engineering Department, Gas Turbine Technology & Products Integration Division,
Mitsubishi Hitachi Power Systems, Ltd.
Senior General Manager, Gas Turbine Technology & Products Integration Division, Mitsubishi Hitachi Power Systems, Ltd.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)24

So far, the technology development has been made in Phase I (from 2004 to 2007) for thedevelopment of the six key technologies (barrier coating, cooling, combustor, turbine, compressor,
heat-resistant material) for 1700°C-class gas turbines and in Phase II and III (from 2008 to 2015)
for the commercialization of the technologies.2 The achievements obtained in this project havebeen reflected in the development of the 1600°C-class J-series gas turbines and the next-generation
high-efficiency gas turbines. Currently, in Phase IV, as shown in Figure 2, with the increased 13
key technologies, the technology development has been continued toward the development,
manufacturing and test operation of an actual unit. In this article, the development status of theeight key technologies and the achievements obtained so far are introduced (Figure 2).

Figure 2 Development of key technologies for ultrahigh-temperature gas turbines|2. Key technologies for ultrahigh-temperature gas turbines
2.1 Development of boundary layer controlled high-performance compressor
To improve the cycle efficiency by increasing the turbine inlet temperature, the compressor
is required to have a larger pressure ratio than the current one. To achieve an increase in thepressure ratio with the number of stages and an axial length equivalent to those of the conventional
compressor, a higher aerodynamic loading needs to be applied to the compressor blade rows, which
may cause a reduction of the efficiency, surge margin and stability at gas turbine start up.
Therefore, a highly accurate numerical flow simulation method with the actual geometry being
modeled in detail was developed, and we have been studying the clarification of the mechanism toincrease the growth of the boundary layer associated with the increase in pressure ratio, as well as
an improved design for controlling the boundary layer. In addition, a scale model of an actualmachine was manufactured, and the performance verification of the improved design and theaccuracy evaluation of the numerical flow simulation have been conducted, and the results havebeen reflected in the design of an actual machine to develop an even more reliable compressor.3
Figure 3 shows the verification results of the compressor instability (surge) point under the
low-cycle condition, using test equipment that is a scale model of the front 8 stages of thecompressor. It demonstrated that through a change of the blade shape, the target flow rate wassatisfied as expected. Figure 4 shows the verification of the number of rotating stall cells and theamplitude of pressure fluctuation that is generated under the startup condition. Stable starting
characteristics were obtained as expected.
Using these technologies, we have promoted the development of a further
high-performance/high-reliability gas turbine compressor.

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)25

Figure 3 Verification of 8-stage compressor surge test and simulation

Figure 4 Comparison of measurement and simulation for rotating stall phenomena at
startup condition of 8-stage compressor

2.2 Development of unsteady combustion control technology
Competition in the development of gas turbines has intensified, and we have been trying toincrease the combustion temperature of gas turbines for an increase in efficiency. In addition, thereduction of NOx emissions, which increase as the temperature rises, is an urgent necessity. The
lean premix combustion method is effective in reducing NOx, but in premix combustion, the flame
position is unstable and combustion oscillation due to fluctuations in the heating value tends tooccur. Therefore, in the development of a premix combustor, the reduction of combustion
oscillation together with the rarefaction of premixed gas and the reduction of NOx through uniform
premix are also important issues in design. To deal with these issues, it is necessary to identify the
specific position of the flame inside an actual combustor and reflect it in the design of the
combustor. However, it is difficult to determine it under high-temperature and high-pressure
conditions, and the state of the inside of the combustor under actual conditions has been unknown
so far. Therefore, we developed flame position evaluation technology through optical measurement
and large-scale unsteady numerical simulation.
Figure 5 shows an overview of the optical measurement technology. In this measurement
technology, the light emission from the flame is collected by the optical probe installed on the
combustor wall face and detected by the photomultiplier, thereby estimating the heating value ofthe local flame. Furthermore, by installing multiple probes, the distribution of the heat release ratein the axial direction of the combustor can be measured. We developed a pressure-resistant andheat-resistant probe, which enabled the identification of the flame distribution for an actual-scale

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)26

combustor under actual conditions.
At the same time, we developed large-scale unsteady numerical simulation technology using
Large Eddy Simulation (LES). With this analysis technology, the application of unsteadysimulation enabled flame estimation with high precision (Figure 6).

Figure 5 Overview of the opticalmeasurement for the inside of a
combustor

Figure 6 Measurement of the combustor
heat release rate in the high-temperatureand high-pressure field

These technologies enabled the evaluation of flame position under actual conditions from
two aspects of measurement, and resulted in the analysis shown in the comparison results in Figure
6. The conventional steady simulation results show a peak on the downstream side, while the newly
developed large-scale unsteady simulation results show a peak on the upstream side. Thesesimulation results are in good agreement with the measured values, and we were able to verify that
this analysis technology enables high-precision prediction. These technologies have been put topractical use in the development of a low-NOx combustor.

2.3 Development of high-performance turbine
The turbine exhaust diffuser (Figure 7) is positioned downstream of the turbine blades andhas the function of reducing the turbine blade exit velocity and increasing the static pressure toatmospheric pressure. As a result, the exit static pressure of the last stage blade located upstream ofthe exhaust gas diffuser becomes lower than atmospheric pressure, and the substantial turbinepressure ratio (expansion ratio) and output are increased, resulting in the improvement of turbine
efficiency. We studied an improvement concept through optimization by integrating the turbine anddiffuser, to which the advanced simulation technology was applied, in order to increase theperformance improvement of the exhaust gas diffuser, and verified the performance improvements
under the rated load condition through the turbine exhaust diffuser rig test (Figure 8).

Figure 7

Turbine exhaust diffuser

Especially under the low-load condition where the flow swirl angle increases, a large amountof separation occurs on the inner wall (ID) of the diffuser, resulting in a substantial reduction in theperformance of the exhaust diffuser (Figure 8). Concerning this issue, we clarified from the resultsof simulations and measurements, that the large separation is associated with the vortex structure
which appears in the vicinity of the inner wall face (ID wall face) of the flow path between theMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)27
4

struts (Figure 9). In addition, it was verified through flow analysis and measurements that evenunder the same partial load condition, the large amount of separation can be effectively reduced bycontrolling the vortex structure between the struts (Figure 10).

Figure 8 Verification of performance of the improved turbine exhaust diffuser

Figure 9

Flow structure in the diffuser strut inner wall (low-load condition)

Figure 10

Effects in cases where the vortex structure is controlled (low-load condition)

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)28

2.4 Development of high-performance cooling system
A turbine blade leading edge has a cooling structure called a showerhead film hole, and a
relatively large amount of cooling airflow is used. Therefore, it is expected that there is room forthe reduction of airflow. By measuring the local cooling efficiency at this point, a more appropriate
cooling system can be adopted depending on the cooling structure, resulting in the reduction of thecooling airflow and the improvement of performance.
As such, using a test rig that simulates the combustor and a single-stage stationary blade ofan actual gas turbine (Figure 11), we obtained the efficiency distribution of the film around thestationary blade leading edge (Figure 12). We will pursue the study of further cooling airflow
reduction based on these measurement results in the future.

Figure 11

Schematic diagram of the device

Figure 12 Test results(Film efficiency
distribution)

2.5 Development of strength evaluation technology at ultrahigh-temperature
In a 1700°C-class gas turbine, the metal temperature rises, and therefore, DS (Directionally
Solidified) materials, which have already been applied, and SC (Single Crystal) materials need to
be used, from the viewpoint of high strength at high temperature. For DS and SC materials, thecrystals of these materials have grown in a certain direction during casting. Therefore, in thestrength evaluation, the anisotropy needs to be considered. On the other hand, as the metal
temperature rises, the cooling performance is intensified, and especially at stress concentration
areas such as cooling holes, a high amount of stress is generated. Therefore, a highly reliable andreasonable thermal fatigue evaluation method is required. To this end, through the thermal fatigue
test using a test piece which simulated the stress concentration at the thermal stress field (Figure13), we have identified the effects of the crystal growing direction and the cooling hole shape on
the crack initiation life5 and we will reflect the results in the thermal fatigue life evaluation method(Figure 14).
In addition, when the metal temperature is high, the material strength inevitably changes.
Therefore, long-term reliability at such a high temperature is required. To this end, we havechecked the material strength using a deteriorated simulation material prepared assuming that it
was used in an actual unit at a high temperature for a long time, and we are developing a method ofquantitatively evaluating the deterioration state after operation.

Figure 13 Apparatus of the thermal fatigue test

Figure 14 Thermal fatigue test results

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)29

2.6 Development of advanced thermal barrier coating
Toward the development of 1700°C-class gas turbines, we have consistently conductedmaterial development, the optimization of thermal spraying process parameters, the development ofa program for application to actual blades, and verification with an actual unit, and have also
developed an advanced thermal barrier coating (hereinafter referred to as TBC) and implemented afurther improvement in performance. Using the material calculation system based on the electronic
structure, we conducted a computational study of candidate materials with low-thermal
conductivity and superior high-temperature stability and tried to manufacture the sintered bodies
with the compositions of the candidate materials and evaluated them. As a result, materials with
low-thermal conductivity and superior high-temperature stability were extracted(6) (Figure 15).

Figure 15 Measurement results of high-temperature stability andthermal conductivity of the topcoat materials (sintered bodies)

TBC used in industrial gas turbines is generally formed by the thermal spray method.
Therefore, in cases where a new material is used, the spray process parameters need to be
optimized. We conducted particle state measurement during spraying and in situ observation of thedeposition state of the particles, evaluated TBC performances such as thermal conductivity,
spallation durability, erosion resistance and oxidation resistance, identified the correlation andreflected it in the process improvement, so that the target coating performances were achieved. The
hot parts of turbines have complex three-dimensional shapes, and so it is not easy to spray coatingon the whole blade face as ideally and homogeneously as on the test piece. Therefore, an ideal
robot program was obtained through simulation using CAD in advance, which was then reflected in
the actual TBC program. The TBC microstructure on the actual parts was evaluated by destructive
testing. Thus, we could shorten the lead time to develop the robot program for the actual blades.
For advanced TBC, we conducted long-term verification using a demonstration power generation
facility and confirmed its high thermal barrier property and reliability equivalent to the test piece7(Figure 16).

Figure 16 Verification with the actual equipment
at the demonstration power generation facility

2.7 Development of advanced manufacturing technologies(1) Welding repair technology for components subject to high-temperatures8
Whenever the turbine blades are damaged during operation, they are repaired forcontinuous use until the end of their lifetime. Accordingly, the establishment of a repair
technology for the newly developed single-crystal alloy MGA1700 is also indispensable. To
this end, as a tool to determine the welding conditions under which the repaired weld part forms

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)30

a monocrystalline structure, MHPS and MHI considered the use of numerical simulation toestimate the geometry of a weld bead and visualize the area of the monocrystalline structure
within the bead.
LMD (Laser Metal Deposition; Figure 17) by which dilution with the base material iseasily controlled, was used for welding, and an analysis was conducted using general-purpose
thermal fluid analysis software (Flow-3D). The welding consumable used was a powder withthe same composition as the base material (MGA1700).
To produce a weld metal structure with the same single-crystal orientation as the basematerial, it is necessary to appropriately control the temperature gradient (G) and the rate ofsolidification (R) in the fusion boundary and results in a large G/R value during solidification.
Accordingly, through the analysis, the area where a single crystal structure produced on thefusion boundary face to the base metal can normally grow was estimated from the G/R value.
The estimation results for the height of the sound growth area under each welding condition are
shown in Figure 18. In Figure 18, welding is not applicable in the lower right zone because thewelding heat input is too little, and welding is not applicable in the upper left zone because thewelding heat input is excessive. It was found that the welding operation should be controlled sothat the laser output is increased and the welding speed is reduced to increase the area where theweld metal is made into a single crystal structure with the same crystal orientation as the basemetal.
The structure of the cross section of the bead in the case of LMD applied under theproper conditions obtained by the analysis is shown in Figure 19. Since the weld metal portion
exhibits a sound single crystal structure, it was confirmed that good results were obtained.

Figure 17 Schematic diagram of LMD

Figure 18 Relationship between the LMD
condition and range of monocrystalline growth

Figure 19 Cross section of the weld
bead microstructure

Figure 20 High-temperature creep
rupture test results(2) Additive manufacturing technology for hot parts
With the aim of improving the performance of the gas turbine by the reduction of thecooling air volume of the gas turbine hot parts (blade, vane, ring segment), we developed
additive manufacturing technology for complex cooling structures through which the existingprocessing technologies cannot be manufactured. To ensure the material strength of additive
manufactured products, which is one of the problems, we adjusted the material composition andoptimized the building conditions and heat treatment processes. As a result, we ensured theMitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)31

required strength of the additive manufactured products in a high-temperature environment(Figure 20). We plan to develop high-precision building technologies for hot parts withcomplex cooling structures in the future.
(3) Turbine blade material and casting technology1, 9
We developed a super heat-resistant material for single crystal blades in collaboration
research with the National Institute for Materials Science (NIMS). The new single crystal alloy
named MGA1700 has excellent creep strength and dwell thermo-mechanical fatigue strength(Figure 21). MGA1700 is a superior alloy in terms of its good mechanical properties without
the expensive element rhenium.

Figure 21 Micro structure and high-temperature mechanical properties of the developed alloy

In addition to the material development, we developed various simulation technologies
for casting processes of directionally solidified blades (include uni-directionally solidified
blades and single crystal blades) (Figure 22). By using these simulation technologies, we canpredict not only casting defect generation, but also phenomena in each casting process, such as
the plastic flow behavior in the core injection molding process. As a result, the condition
optimization of the overall casting process and the improvement of casting quality wererealized.

Figure 22

Simulation (calculation) results for each casting process

2.8 High-precision/high-functionality inspection technologies(1) Wireless power supply technology on wireless sensing
Strain data is wirelessly transmitted to measure the vibration of the moving blade which
is rotating at high speed. We developed a module that can wirelessly receive the electric power
required for the measurement/transmission (Figure 23). High-frequency electric power isreceived from the antenna on the front face, and DC voltage is supplied to the strain measuring
amplifier and the data transmitter in the same module. Changing batteries becomes
unnecessary, and continuous measurement becomes possible. This method has the advantage

Mitsubishi Heavy Industries Technical Review Vol. 54 No. 3 (September 2017)32

that the high-frequency power transmission enables power transmission over longer distances
compared to the electromagnetic induced method.
(2) Blade inside surface defect detection technology10
In gas turbine blade defect inspection, technology for the detection of defects at invisible
positions such as the cooling passage face side is important in terms of ensuring the reliabilityof the turbine blade. Therefore, the matrix array UT probe, which has an improved focusing
property of the ultrasonic beam, was developed as a tool for accurately inspecting any defectsof the blade inner surface from the blade outer surface side (Figure 24). In an inspection of an
actual blade, the UT probe is attached to a 3D scanner and the blade face is scanned, allowing
the rapid defect inspection of the inner surface side (Figure 25).

Figure 23 Appearance of
the wireless power supply
type transmission module

Figure 24 Appearance ofthe matrix array UT
probe

Figure 25 Blade surface
scanning state|3. Conclusion
This article describes part of the high-efficiency gas turbine technology demonstration
project (the development of ultra-high efficiency 1700°C-class gas turbines) which is under
implementation as a project subsidized by New Energy and Industrial Technology Development
Organization (NEDO), and we are developing technologies to be applied to the actual equipment
toward the development and demonstration of key technologies required for actual use. Various
technologies for the improvement of performance and technologies for an increase in reliability
under severe conditions such as high temperature/high pressure and for manufacturing andinspection are under development. Some of the latest technologies, for which the effectiveness wasrecognized through technical studies and verifications, have been applied to the development of the1600°C-class J-series gas turbine and next-generation high-efficiency gas turbines. At the sametime, the obtained long-term operation data is reflected in research to improve the reliability of thedeveloped technologies. Through the diffusion of the latest combined cycle power generation
technologies, we would like to greatly contribute to the reduction of CO2 emissions at thermal
power generation plants.

References
1.
2.
3.
4.
5.
6.
7.
8.
9.
10.

Hada et al., High-efficiency Gas Turbine Development applying 1600°C class "J" Technology, Mitsubishi Heavy
Industries Technical Review Vol. 52 No. 2 (2015)
Ito et al., Key Technologies for Ultra-High Temperature Gas Turbine, Mitsubishi Heavy Industries Technical Review
Vol. 52 No. 2 (2015)
Yamashita et al., Development of Measurement Method for Verification of Multi-Stage Axial Compressor with
Improved Performance, Mitsubishi Heavy Industries Technical Review Vol. 53 No. 4 (2016)
NAGAI, N. et al., 2016, "STUDY ON REDUCTION OF BOUNDARY LAYER SEPARATION AT EXHAUSTDIFFUSER WALL", Proceedings of the Asian Congress on Gas Turbines, ACGT2016-60.
Kaneko, H. et al., “Evaluation of Low Cycle Fatigue Strength on Directionally Solidified Ni-Base Superalloys for High
Efficiency Gas Turbine”, International Gas Turbine Congress 2015, (2015)
Torigoe, T. et al., “Development and Application of the Advanced TBC for High Efficiency Gas Turbine”, 7th Tsukuba
International Coating Symposium 2016, Dec 8-9, 2016, Tsukuba, Japan
Okajima, Y. et al., “Evolution of Thermal Barrier Coatings for Land Based Gas Turbines at MHI”, International
Thermal Spray Conference & Exposition, May 21-23, 2014, Barcelona, Spain.
Tsukimoto, K. et al., Laser Welding Repair for Single Crystal Blades, Proceedings of International Gas Turbine
Congress 2015, (2015), p1354-1358.
Oguma, H. et al., Development of Ni-base Single-Crystal Superalloy and Casting Technology for High Efficiency Gas
Turbine Blade, Proceedings of International Gas Turbine Congress 2015, (2015), p173-178.
Kirihigashi, A. et al., “GT Blade Inside Crack Inspection Using Ultrasonic Matrix Phased Array Technique”,
International Gas Turbine Congress 2015 Tokyo.