Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)9Advanced Three-Dimensional Aerodynamic Design for
Next Generation Gas Turbines

EISAKU ITOYASURO SAKAMOTO

KEIZO TSUKAGOSHIAKIMASA MUYAMAJUNICHIRO MASADA|1. Introduction
Power generation using high efficiency, combined cycle gas turbines is common in thethermal power generation business. Even so, much emphasis has been placed on achieving further
efficiency improvements to reduce the cost of power generation in the face of escalating fuelprices. This has led to increasing the cycle temperature and the aerodynamic loading of eachcomponent. The development of components for the 1,700C class gas turbine is under way as anational project. This paper focuses on the turbine component. For the 1,700C class gas turbine
the load on a turbine blade in normal operating conditions is about 1.3 times greater than that of aconventional turbine. To maintain high adiabatic efficiency, which decreases as blade loading
increases, a new concept of three-dimensional design is under development.

|2. Three-dimensional end wall for the high load turbine
The increase in the secondary flow loss due to an increase in flow turning angle, a byproductof the increased load, causes a drop in aerodynamic performance. To reduce the losses due tosecondary flow, a three-dimensional end wall design (axisymmetric end wall contouring) was
compared to a conventional endwall profile. The following three items were taken into
consideration for the three-dimensional end wall:(1) Control of the static pressure distribution on the end wall to prevent the occurrence of crossflow intersecting with the primary flow leaving the trailing edge downstream of the blade.
(2) Control of the exit flow angle to reduce the passage vortex of the airfoil.
(3) Reduction of the flow rate in the high loss area near the end wall.
The title drawing shows an example of applying a three-dimensional design to the end wall
between blades. For the three-dimensional end wall, the profile consists of a combination ofconcave and convex curves. The concave and convex profiles between blades are designed toreduce the pressure difference between the pressure and suction surfaces near the endwall, as thiscauses the secondary flow and the end wall boundary layer to conform as much as possible to themainstream flow direction. At the same time, the trailing edge profile is designed such that as muchof the primary flow as possible leaves the blade smoothly in the mainstream flow direction.
In designing the three-dimensional end wall, we performed a computational fluid dynamics(CFD) analysis with a detailed model that simulated the cavity profile between the blade and thevane as shown in Figure 1. Since the unsteady effects cannot be ignored in calculating the flow
field of the cavity, it is desirable that unsteady analysis be used in the final profile determination.

Figure 1 CFD grating
Flow analysis simulating thecavity profile between the bladeand vane

Mitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)10

Figure 2 shows a comparison of the time-averaged stream lines near the end wall comparing
the conventional flat axisymmetric design profile and the three-dimensional end wall. When thethree-dimensional end is used (right-hand side of Figure 2), it was found that stream lines near theleading-edge on the pressure surface, and the stream lines in the area where the cross flow,
produced by the leg of the horseshoe vortex on the end wall, curls up onto the suction face, face theaxial direction more than in the conventional case.
Figure 3 shows a comparison of the total pressure loss coefficient distributions on thetrailing edge surface of the first-stage rotor blade. The loss was reduced for a blade height area of
5–20%, measured from the end wall. It is presumed that this effect was due to a reduction in thesecondary flow loss.

Figure 2 Time-averaged stream lines near the end wall(CFD analysis)

Figure 3 Comparison of full pressure
loss coefficient distributions offirst-stage blade (CFD analysis)|3. Cascade test
We conducted a cascade test to confirm the concept presented above. We performed the test
measurements in two steps. First, we performed detailed measurements of the flow field with alow-speed cascade test to confirm the design concept. Second, we confirmed the increase inperformance with a high-speed cascade test under transonic conditions where the velocity triangles
were analogous to, and the Mach number was equal to, the actual machine.
In the low-speed cascade test, the front three cascades were selected for testing and created ina size similar to that of the actual machine. A clear outer casing was set up to observe the flow fieldin detail. We measured the pressures at the cascade inlet and outlet, the exit flow angles, and thepressure distributions at the blade surfaces and end walls to understand the flow fieldcharacteristics. We performed a visualization of the velocity field on the blade surface using theoil-film method. At the end wall we used a laser sheet and high-speed camera located in the main
flow path to understand the flow field phenomena.
Figure 4 shows the static pressure distributions on the blade platform surfaces, measured by
the rotating system, described above, in the low-speed cascade test. It was observed that the predicted
and measured values of the effectiveness of the three-dimensional end wall are very similar to eachother. It was also observed that the effectiveness of the three-dimensional end wall reduced the staticpressure gradient reasonable for the passage vortex moving from the pressure surface to the suction
surface. This was especially true near the mid-chord in the axial direction. In the same way, weconfirmed a change in the static pressure distribution near the trailing edge.
Figure 4 Low-speed cascade test
resultThe static pressure distribution on theblade platform surface changes verticallywith respect to the mainstream at thethree-dimensional end wall (right) ascomparedwith
theconventional
axisymmetric flat end wall (left).

Figure 5 shows the first and second rotors in the high-speed cascade test, the secondexperimental step. The size of the blade used for the test was set to about one-quarter that of theactual machine due to a limitation on the capacity of the air source.
Figure 6 shows the two types of platform profile compared in the test. The left side is theMitsubishi Heavy Industries Technical Review Vol. 46 No. 1 (Mar. 2009)11

blade with the conventional axisymmetric flat end wall profile and the right side is the blade withthe three-dimensional end wall.
Figure 7 shows the efficiency distributions as a function of the blade height. This confirms
the effectiveness of the three-dimensional end wall. The efficiency became less very near the hub(less than 5% of the span) when the three-dimensional end wall was used, however the efficiency
was increased near the end wall (5%-20% of span) so that overall an efficiency improvement effectcould be expected. This is the same as the CFD estimate.

Figure 5 Rotors in high-temperature
cascade test

Figure 6 Blade under test in
high-temperature cascade test
(Left) Blade with the conventional axisymmetricflat end wall profile(Right) Blade with the three-dimensional end wall

Figure 7 Comparison of efficiency distributions
in blade height direction(High-temperature cascade test results)
Effectiveness of the three-dimensional end wall wasconfirmed within the range of 5%–20% height|4. Conclusion
Aerodynamic technology for the high-load high-performance turbine is under development
as one of the component technologies for the 1,700C class turbine. We studied the effectiveness ofa three-dimensional end wall under high-load cascade conditions and confirmed the adequacy ofthe concept. This new endwall concept led to improved efficiency in the low-speed and high-speed
cascade tests. This high-load high-performance turbine was developed as part of a project
supported by the Agency for Natural Resources and Energy of the Ministry of Economy, Trade andIndustry, and is expected to contribute significantly to increasing the level of technology for power
generation gas turbines. MHI expresses its appreciation to those who assisted with this project.

Authors
Eisaku Ito
Manager,
Turbomachinery Laboratory,
Takasago Research &Development Center,
Technical Headquarters

Yasuro Sakamoto
Turbomachinery Laboratory,
Takasago Research &Development Center,
Technical Headquarters

Keizo Tsukagoshi
Chief Engineer
Takasago Machinery Works

Akimasa Muyama
Deputy General ManagerTakasago Machinery Works

Junichiro Masada
Deputy General Manager,
Gasturbine Engineering
Department
Takasago Machinery Works