Effects of seal installation in the mid-passage gap between turbine blade platforms on film cooling

https://doi.org/10.1016/j.applthermaleng.2021.116683Get rights and content

Highlights

  • The seal prevented ingress of the main flow through the mid-passage gap.

  • The seal improved the film cooling effectiveness of the platform.

  • The sealing effect was negligible under a high blowing ratio.

  • The seal caused the leakage flow to reattach closer to the gap than without the seal.

  • The seal distributed the heat transfer peaks closer to the gap than without the seal.

Abstract

Hot combustion gas in gas turbines can be ingested into the mid-passage gap between the blade platforms, resulting potentially a catastrophic damage. To prevent the ingress of hot gas, the blade platform underneath cavity is pressurized with purge air. Seals are typically installed in the mid-passage gap to minimize the purge air requirement. Under inadequate back flow margin, ingress of the main flow can occur especially at the upstream region of the mid-passage gap due to the pressure distribution characteristics on the platform. Excessive leakage flow can significantly reduce the turbine performance. This study investigated the effect of a seal installed in the mid-passage gap on film cooling effectiveness and heat transfer on the platform. Experiments were conducted by varying the ratio of the leakage flow to the main flow by 0.2%, 0.4%, and 0.6%, in which the mid-passage gap was unblocked (no seal; Opening 100% case) and partially blocked (seal; opening 50% case). The film cooling effectiveness of the platform was measured using the pressure sensitive paint method, and the heat/mass transfer coefficient of the platform was obtained using the naphthalene sublimation method. The seal in the mid-passage gap reduced or prevented completely the ingress of main flow under the given conditions. Under a high blowing ratio, the seal had little effect on the platform film cooling effectiveness. At the same mass flow rate of leakage flow, the heat transfer distributions were similar, regardless of whether a seal was installed. However, the platform with a partial seal in the opening 50% case showed a peak in the heat transfer closer to the gap than the platform without the seal, as the leakage flow reattached closer to the gap.

Introduction

Turbine blades are generally assembled one by one, resulting a mid-passage gap between platforms. This gap is also designed to prevent damage or failure due to thermal expansion and vibration. However, the ingress of hot combustion gas through mid-passage gaps can cause irreparable thermal damage to the turbine disk on the disk-post. The blade platform underneath cavity is typically pressurized to prevent a hot gas ingress. This is accomplished by purge air which is egested through the mid-passage gap. The resulting leakage flow provides a film cooling effect on the platforms. Many studies in the past have investigated the effect of mid-passage gaps on film cooling effectiveness and heat transfer on the platforms [1], [2], [3], [4], [5], [6], [7] as well as the heat transfer characteristic of the platforms [8], [9], [10], [11], [12], [13], [14], [15], [16].

However, excessive use of the purge flow drawn from the compressor degrades the gas turbine engine performance, and the leakage flow egested through the mid-passage gap also results in aerodynamic losses. Therefore, a seal is typically installed to block the mid-passage gap and prevent the leakage flow waste. The seal comes in various shapes, including pin-type [17] and strip-type [3], [4] seals.

Most previous studies that investigated the effect of mid-passage gaps on the platform include both an upstream slot and a mid-passage gap; Their results showed that the mid-passage gap had less of film cooling and heat transfer effects on the platform compared with the upstream slot. However, several studies [3], [4] have shown that flow ingress occurs in the upstream region of the mid-passage gap, whereas flow egress takes place in the downstream region of the gap. In addition, despite most turbines being equipped with seals in the blade mid-passage gaps, few studies have investigated the effect of seals. Therefore, it is of interest to further investigate the effect of mid-passage gaps with seals on the platform film cooling and heat transfer.

In our previous study [18], we showed that flow ingress and egress occur through the mid-passage gap if the mass flow rate of leakage flow is insufficient, i.e., if the pressure underneath the blade platform cavity is not sufficiently high. Despite an increase in the mass flow rate of the leakage flow, the film cooling effectiveness decreased due to the high blowing ratio and the vortex that formed via interference with the main flow. Here, we compare the results of the current study with those from our previous study to demonstrate the changes that occur when a seal is installed in the mid-passage gap. As in the previous study, film cooling effectiveness was measured using a pressure sensitive paint method, and the heat/mass transfer coefficients were determined using a naphthalene sublimation method. To have effective comparisons, experiments were conducted using the same experimental setup and conditions as those of our previous study, except for the seal installation. To simulate partial blockage of the mid-passage gap, a seal was installed in the mid-passage gap, which blocked 50% of the gap’s cross-section.

Section snippets

Experimental apparatus and conditions

Fig. 1 shows a schematic diagram of the linear cascade with the leakage flow chamber used in the previous study [18]. Table 1 shows the geometric information of the blade and mid-passage gap. The linear cascade was installed in a blowing-type wind tunnel, and its inlet dimensions were 300 × 200 mm. As shown in Fig. 1(a), an endwall specimen for measurements of FCE and HTC was installed between the third and fourth of six linear blades, which positioned in the middle of the flow path. A trip

Results and discussion

Fig. 3 shows a graph comparing the static pressure distribution at the endwall and the leakage flow chamber (underneath). The static pressure distribution of the platform surface was calculated by performing a numerical simulation, and the static pressure of the underneath leakage flow chamber was measured using a pressure tap on the chamber wall. The total and static pressure of the main flow passage was measured using a pitot tube and a pressure tap on the wind tunnel wall. Solid lines show

Conclusions

The leakage flow egested through a mid-passage gap helps to prevent ingress of the main flow. The leakage flow must be supplied with a sufficient mass flow rate, i.e., with enough pressure to prevent overheat to the blade platform due to the ingress of the main flow through the mid-passage gap. However, excessive leakage flow could cause aerodynamic losses and performance degradation. The seal helps to reduce a leakage flow waste and ingress of the main flow through mid-passage gap. This study

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgement

The authors wish to acknowledge support for this study by Korea Southern Power Co., Ltd. and Mitsubishi Hitachi Power Systems, Ltd. This work was supported by the Human Resources Development program (No.20174030201720) of the Korea Institute of Energy Technology Evaluation and Planning (KETEP), grant funded by the Korea government Ministry of Trade.

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