2004 ME Graduate Student Conference

April 17, 2004

latticework cooling with rotation and film extraction

Jonathan LaGrone
M.S.
Faculty Advisor: Dr. Sumanta Acharya

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Abstract

One of the primary modes for increasing turbine efficiencies involves increasing the turbine inlet temperatures. However, operating temperatures of modern gas turbines are well beyond the material limits of turbine components. These high temperature demands therefore require advanced cooling strategies. The cooling strategies employed in modern turbine blades often involve serpentine passages with rib-turbulators or pin fins along with film cooling (Fig. 1).

The current research focuses attention on cooling through the internal serpentine passages, and aims to explore a lattice-cooling configuration (Fig. 2). The latticework cooling structure was first introduced by researchers in the former Soviet Union over 25 years ago; however, as pointed by Bunker [1], the western world has very few published investigations into the characteristics of the lattice cooling structure. Further, as discussed by Bunker [1], all studies with this configuration have been done in a stationary environment, and without film-coolant extraction. However, under rotational conditions, the Coriolis forces and the centrifugal-buoyancy forces generated have been shown to significantly affect the characteristics of the flow through smooth and ribbed serpentine passages. Therefore, for turbine blade designers to accurately predict the life and reliability of the blade these effects must be quantified experimentally in a rotating frame. The first goal of the research is to examine latticework cooling under rotational condition. These measurements will confirm if rotation adversely affects the leading surface heat transfer as in conventionally turbulated passages. A second goal of the research is to explore the effectiveness of lattice cooling with film coolant extraction. Work done toward achieving both of the above-stated goals is described here.

Figure 1: Typical cooling strategy for turbine blades (Wagner et al. [5])

Latticework Cooling with rotation

Stationary studies with the latticework configuration have shown potential advantages including spatially-uniform streamwise distributions of the heat transfer coefficient, greater blade strength, and enhancement levels comparable to conventional rib turbulators [1]. The promising stationary results prompted the current study in which the heat transfer and pressure drop characteristics of latticework coolant blade passages have been investigated experimentally under conditions of rotation. In the present study, a latticework coolant passage, with orthogonal-ribs, is studied in a rotating heat transfer test-rig for a range of Reynolds numbers (Res), Rotation numbers (Ros), and density ratios.

Figure 2: Flow through latticework geometry

Measurements [3] indicate that for Reynolds number, Re20,000, the latticework coolant passage provides very uniform streamwise distributions of the Nusselts number, Nu, with enhancement levels (relative to smooth-channel values) in the range of 2.0 to 2.5. The Nusselt number had no observable dependence on rotation number or density ratio except at lower Re values (10,000). In contrast, for the conventional rib turbulator design, rotation number and density ratio significantly affects heat transfer characteristics, with an increase in heat transfer of up to 350 percent on the pressure-side (at a Rotation number of 0.35), and a corresponding decrease of nearly 40 percent on the suction side of the blade as compared to the stationary case. [2,4,5]. The thermal performance factors, (Nu – Nus)/(f-fs) where f is friction factor and subscript s signifies smooth channel), for the lattice geometry are comparable to those of a ribbed channel.

Latticework Cooling with rotation

As shown in Figure 1, film cooling is also used to protect turbine blade from the harsh high temperature environment. The coolant flow for film cooling is supplied by a series of bleed holes in the serpentine passages. The presence of these bleed holes have a significant effect on the internal cooling performance of these passages. In the present work, stationary tests are currently being performed on the latticework geometry to determine the effects of coolant bleed holes on the pressure or suction side of the blades. These tests will determine the effects of film cooling on the heat transfer characteristic, and friction factors of an internal passage with lattice geometry. Heat transfer measurements will be made using the steady state liquid crystal technique. Various spanwise bleed-hole locations and various ratios of bleed flow to mainstream coolant flow will be investigated.

HIGH ROTATION NUMBER MEASUREMENTS

Some modern gas turbines (closed-loop with steam cooling) operate at Rotation numbers of the order of 1 and at Reynolds numbers order of 100,000. Parameters of this magnitude can be achieved individually by the current facilities; however, they cannot be achieved concurrently. Recent tests by Zhou at el. [2] have shown interesting patterns in Nusselt number with increasing rotation number. For example, for a smooth rectangular passage with an aspect ratio of 4:1, Zhou et al. reported that at Reynolds numbers of 10,000 and 20,000 the Nusselt number initially decreases with rotation dropping by 20% and 40% at rotation numbers of 0.1 and 0.2 respectfully before beginning to increase again with rotation number. This has prompted us to manufacture a new rotating liquid crystal test section. For a given Reynolds’s number, the new test section will allow us to achieve rotation numbers four times higher than currently obtainable. Additionally, the new test section will provide detailed Nusselt number distributions of the entire channel, instead the area average values of discrete points along the channel as currently available.

Acknowledgments

This work was supported by a grant from General Electric Global Research. I appreciate the help and support from my colleagues Fuguo Zhou who took a lot of the data under rotational conditions, and Dr. Gazi Mahmood, who designed the latticework test facility. Their support is gratefully acknowledged.

References

1.  Bunker, R.S., 2004, “Latticework (Vortex) Cooling Effectiveness Part 1: Stationary Channel Experiments”, Paper No. GT-2004-54157, IGTI Turbo Expo, Vienna.

2.  Zhou, F., LaGrone J., Acharya S., 2004, “Internal Cooling in 4:1 AR Passages at High Rotation Numbers”, IGTI Paper No. GT2004-53501, IGTI Turbo Expo, Vienna

3.  Acharya, S., Zhou, F., Bunker, R.S., LaGrone, J., Gazi, M., “Latticework (Vortex) Cooling Effectiveness Part 2: Rotating Channel Experiments” IGTI Paper No. GT2004-53983, IGTI Turbo Expo, Vienna

4.  Wagner, J. H., Johnson, B.V., Kopper, F.C., 1991, “Heat Transfer in Rotating Serpentine Passages With Smooth Walls”, Journal of Turbo Machinery, 113, pp 321:330

5. Wagner, J.H., Johnson, B.V., Graziani, R.A. and Yeh, F.C. 1992, Heat transfer in rotating serpentine passages with trips normal to the flow, J. Turbomachinery, 114, pp. 847-857.

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