Research Areas

- Tip leakage flow

In the gas turbine industry, there is a continuous effort to achieve higher efficiency. The inevitable gap between the rotor tip of the turbine blade and the stationary casing produces a tip leakage flow (TLF), which is a major source of aerodynamic loss because it mixes with the mainstream flow. [1] This flow is generated by the pressure difference between the pressure side and the suction side of the blade (Fig. 1). Because it mixes with the main flow passing between the blades, it becomes a major source of turbine aerodynamic loss.

Fig. 1. Tip leakage loss generation inside the tip gap and by mixing with the passage flow [2].

- Tip aerothermal study

The present work provides a combined analysis of aerodynamics and the heat transfer of the blade tip. The analysis is not limited to the observation of aerodynamic or thermal results. Rather, it is centered on the flow physics that connects the two. Attention is given to the tip cavity. In this region, the tip leakage flow interacts with the cooling flows injected from the tip and the pressure side. This interaction forms vortical structures such as the counter-rotating vortex pair (CRVP) and alters the local heat transfer on the tip surface. It also determines where the leakage flow leaves the tip clearance. These coupled aerothermal features are examined through combined experiments and CFD. The aim is to explain not only how much the loss and the heat load change, but why they change.

Fig. 2. Tip leakage loss generation inside the tip gap and by mixing with the passage flow [3].

- Experiment facility

To simulate the flow field and condition in the turbine, we built a custom wind tunnel as shown in Fig. 3 (a). A mesh heater is installed in the wind tunnel. It raises the mainstream temperature within a short time. This enables heat transfer experiments in the same facility, including the measurement of the heat transfer coefficient (HTC) on the tip surface. In the cascade, experiment results will be obtained by a 5-hole probe before and after the blade, which will provide information on flow structure and aerodynamic losses, as shown in Fig. 3 (b). In the upstream boundary layer bleed region, a slight reduction of the cascade height removed the boundary layer of the inlet flow. Thus, we can obtain a uniform flow in the cascade.

Fig. 3. Experiment facility (a) Wind tunnel, (b) Cascade.

- Tip cooling flow

It is important to understand how flow structure and aerodynamic performance change under the conditions in which the real turbine operates. So, we analyze the dust and cooling holes from the perspective of aerodynamics. We aim to find the optimal tip geometry in terms of aerodynamics by changing the arrangement, number, and shape of holes. (Fig. 4)

1. 

    Figure 4. Tip hole research facility. (a) Blower and manifold, (b) Cascade blade with injection facility.

    

   - Results of aerothermal study

   A five-hole probe is traversed at several planes, from inside the passage to downstream of the cascade, as shown in Fig. 4(a). These data quantify the flow field and the development of tip leakage vortex. Inside the passage, the tip leakage vortex forms near the suction side (SS) and develops downstream, as presented in Fig. 4(b). The heat transfer coefficient (HTC) distribution on the tip surface is presented in Fig. 4(c). In this way, the aerodynamic and thermal results are analyzed simultaneously.

    

   Fig. 5. Experiment results of aerothermal study. (a) Passage measurement region, (b) Total pressure loss coefficient blade passage, (c) Tip surface heat transfer coefficient (HTC).

    

    

   [1]  Denton, J., 1993, “Loss Mechanisms in Turbomachines,” ASME J. Turbomach., 115, pp. 621–656.
   [2] Pátý, M., Cernat, B. C., Maesschalck, C. D., and Lavagnoli, S., 2019, “Experimental and Numerical Investigation of Optimized Blade Tip Shapes—Part II: Tip Flow Analysis and Loss Mechanisms,” ASME J. Turbomach., 141(1), p. 011007.
   [3] Ma, H., Zhang, Q., He, L., Wang, Z., and Wang, L., 2017, "Cooling Injection Effect on a Transonic Squealer Tip—Part II: Analysis of Aerothermal Relationship," ASME J. Eng. Gas Turbines Power, 139(5), p. 052507