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• The heat transfer characteristics was investigated that are exhibited with the occurrence of misalignment at flat and profiled endwall using naphthalene sublimation method. • The occurrence of misalignment in the flat endwall significantly increases the thermal load in the region upstream of the endwall. • There was no significant difference in heat transfer when a misalignment occurred in the profiled endwall. • A profiled endwall had a lower thermal load than a flat endwall, and is a robust design because the thermal load changes less even if misalignment occurred. Misalignment of gas turbine engines is caused by the difference in thermal expansion from the design point when starting or stopping the engine, or when operating at partial load. This study evaluated the differences in heat transfer characteristics between a flat endwall and a profiled endwall with misalignment. To ensure consistency with the flow conditions, experiments were conducted at an outlet Reynolds number of 300,000 for both geometries. The experiment was performed under three different conditions: a case without step, a case with a forward-facing step (h / C x = -0.1), and a case with a backward-facing step (h / C x = 0.1). In the flat endwall, the thermal load of the upstream region increases significantly with both step circumstances compared to the case without a step. The misalignment causes a recirculation flow to occur between the interface of the combustor and turbine. This leads to the mainstream reattaching upstream of the endwall. Nevertheless, the heat transfer characteristics of the profiled endwall differ from those of the flat endwall under step conditions. Most locations do not experience significant changes, with only a slight increase observed in the upstream region. The recirculation flow is crucially reduced due to the intensity of the secondary vortex in the profiled shape. Hence, the increase in the thermal load on the profiled endwall with step is considerably lower than that on the flat endwall. As a result, the thermal load on the profiled endwall has decreased by 25 % for the forward-facing step and 20.7 % for the backward-facing step compared to the flat endwall. [ABSTRACT FROM AUTHOR]

Recently, domestic gas turbines are primarily operated under partial loads. Hence, thermal expansion of components decreases, leading to a discontinuity between the 1st blade and 2nd vane. In this study, the local heat transfer coefficients are derived using the naphthalene sublimation method to analyze thermal characteristics based on step height, and flow characteristics are analyzed using numerical simulation. An extreme heat load of the flat endwall appears around the leading edge of the vane due to the secondary vortex. Under the step conditions, the heat load increases in the upstream region as the main flow near the endwall is reattached to the endwall surface. In addition, two high heat transfer distributions appear due to the recirculation flow, and a step-induced vortex is formed. As the step-induced vortex moves to the suction side within the vane flow path, a high thermal load appears in the corresponding region. Consequently, we found that the occurrence of a step causes severe thermal damage in the upstream region of the vane endwall. The area-averaged heat transfer of the stepped endwall is increased by about 10.6 % and 17.3 % at hs/Cx = 0.05 and 0.1, respectively, as compared to that of the flat endwall. [ABSTRACT FROM AUTHOR]

The efficiency of the gas turbine engine has been enhanced by improving aerodynamic performance and reducing heat load through control of secondary vortices according to the first-stage vane endwall configurations. In this study, the heat transfer features of flat and profiled endwalls were investigated under different turbulence intensities. Experiments were conducted under a fixed turbine-vane exit Reynolds number of 300,000 and Mach number of 0.15 on both geometries. The test specimen was scaled up 3.23 times based on the actual gas turbine vane geometry. The profiled endwall configuration was optimized considering the combustor outlet/exit geometry. Five control points were set for the vane geometry, and the minimum aerodynamic loss at the vane exit plane was determined as an objective function. As the turbulence intensity is increased, the heat transfer is increased on both endwall geometries. However, the intensity of the horseshoe vortex was significantly reduced with the profiled endwall compared with the flat endwall. Consequently, the area-averaged Sherwood number on the profiled endwall was about 25.8% (under the low turbulence intensity) and 19% (under the high turbulence intensity) lower than those of a flat endwall case, respectively. [ABSTRACT FROM AUTHOR]

The tip leakage flow between rotating turbine blades and the stationary turbine casing causes aerodynamic losses and high thermal loads. Numerous novel tip geometries have been proposed for reducing both of these effects, one of them is a shelf squealer tip. The objective of this study is to investigate the aerothermal performance of three squealer blade tip configurations: conventional, vertical shelf and inclined shelf. Heat transfer was measured to analyze heat transfer features on the blade tip using transient IR thermography method. Also, computational fluid dynamics simulation and particle image velocimetry were conducted to visualize the flow characteristics around the blade tip. The experiment was conducted under conditions of a Reynolds number of 140,000 which was based on the inlet velocity in a wind tunnel. Thermal loads with vertical and inclined shelf squealer tip configurations are lower than with the conventional squealer tip. Due to the recessed pressure side rim of the vertical shelf squealer tip, the tip leakage flow was less reattached comparing to conventional squealer tips. This resulted in reduced thermal load on the tip surface and a larger tip leakage vortex around the blade suction side, which increased aerodynamic losses. Especially in the inclined shelf squealer tip, a separation bubble was generated on its inclined pressure side rim, which reduced the tip leakage flow due to the vena contracta effect. Consequentially, the reduced tip leakage flow decreased both heat loads and aerodynamic losses for the inclined shelf squealer tip. Therefore, the inclined shelf squealer tip demonstrated a good performance with respect to both aerodynamic loss and heat transfer. [ABSTRACT FROM AUTHOR]

• Investigation of heat transfer characteristics on the 2nd vane endwall when the misalignment occurs without and with purge flow using Naphthalene sublimation method and CFD. • Significant increase in heat transfer coefficient in the upstream of vane endwall due to misalignment. • With purge flow, the mixing of mainstream and purge flow increases heat transfer in the upstream of vane endwall due to increase the intensity of vortex. • The area-averaged heat transfer of the stepped endwall is increased by about 11.4% and 13.7% without and with purge flow, respectively, as compared to that of the flat endwall. Since the domestic power plants are typically operated at a partial load rather than the design point, the thermal expansion decreases, resulting in misalignment between first stage rotor and second stage stator. In this study, we investigate the heat transfer characteristics of a misalignment under purge flow at the second stage vane endwall through heat/mass transfer experiments using the naphthalene sublimation method and CFD simulations. Experiments are conducted at 4 vanes in a linear cascade with an inlet Reynolds number of 120,000, based on the vane axial chord length. In the absence of purge flow, a high heat transfer occurs in upstream of the vane endwall via horseshoe vortices. When a misalignment occurs, a more severe thermal load is observed as the mainstream attaches to the endwall due to recirculation flow upstream of the leading edge. Additionally, two high heat transfer regions are observed when a step-induced vortex occurs in the vane flow path. In case of the purge flow, the mixing of the mainstream and purge flow increases the intensity of the vortex, which increases heat transfer in the region upstream of the both flat and stepped endwall. In conclusion, we have found that the step difference has a crucial effect on thermal damage upstream of the second stage vane endwall as a little misalignment occurrence. The area-averaged heat transfer of the stepped endwall is increased by about 11.4% and 13.7% without and with purge flow, respectively, as compared to that of the flat endwall. [ABSTRACT FROM AUTHOR]

In a turbine passage, the wake, which affects the heat transfer of a turbine blade, occurs periodically due to rotation of the blade. We analyzed the effect of wake on the endwall of the turbine blade according to the relative position of the turbine blade and the vane in a stationary condition. The naphthalene sublimation method was used to measure the heat transfer and detached eddy simulation (DES) was used to analyze flow characteristics. The wake from the vane was simulated by using a cylindrical rod upstream of the blade. The cylindrical rod was placed in four which positions that were aligned leading edge-to‑leading edge. The pressure and Q criterion distributions varied according to the position of the upstream wake. As the position of the upstream wake changed, the point at which the passage vortex and wake met varied. Wake and passage vortex met at x / C x  = 0.2 in position 1 and at x / C x  = 0.55 in position 2. After the wake and passage vortex had met, the secondary flow scattered. Therefore, the local and averaged heat transfer varied due to flow characteristics. Thus designers of film cooling holes on endwalls should consider these effects to ensure appropriate cooling performance. [ABSTRACT FROM AUTHOR]