The turbine center frame (TCF) is an inherent component of turbofan aircraft engines and is used for connecting the high-pressure turbine (HPT) to the low-pressure turbine (LPT). Its position immediately downstream of the HPT makes it susceptible to the extremely high temperatures of future engines. Despite this, fundamental knowledge of heat transfer in TCFs and the influencing factors is still missing. This paper presents a new 45° sector-cascade test rig specifically designed for fundamental studies of film cooling effectiveness and heat transfer coefficient in TCFs and for the development and validation of a measurement technique involving infrared thermography and heating foils. Measurements of heat transfer coefficient in the TCF were taken for two purge-to-mainstream mass flow ratios corresponding to the case of no purge and nominal (to engine operation) purge. The magnitude of the heat transfer coefficients on the hub and strut surfaces was highly influenced by the various flow structures in the passage and by the velocity variation of the mainstream flow due to the “aggressive” design of the TCF. Heat transfer on the surface of the strut was mainly governed by boundary layer behavior (laminar near the leading edge and turbulent for the rest of the strut) augmented by the effect of the secondary flow structures. Measurements of film cooling effectiveness were also taken for the single case of nominal purge. A region of high film cooling effectiveness was observed, extending from the purge cavity exit to about 40% of the passage axial length. In this region, the effectiveness decreased with increasing axial length. On the surface of the struts and fillet radii the film cooling effectiveness was found to be zero. This was attributed to the effect of the horse-shoe vortex which sweeps the purge flow away from the strut surface and dilutes it by continuously entraining hot mainstream flow.

Due to stringent environmental legislation and increasing fuel costs, the efficiencies of modern turbofan engines have to be further improved. Commonly, this is facilitated by increasing the turbine inlet temperatures in excess of the melting point of the turbine components. This trend has reached a point where not only the high-pressure turbine has to be adequately cooled, but also components further downstream in the engine. Such a component is the turbine center frame (TCF), having a complex aerodynamic flow field that is also highly influenced by purge-mainstream interactions. The purge air, being injected through the wheelspace cavities of the upstream high-pressure turbine, bears a significant cooling potential for the TCF. Despite this, fundamental knowledge of the influencing parameters on heat transfer and film cooling in the TCF is still missing. This paper examines the influence of purge-to-mainstream blowing ratio, purge-to-mainstream density ratio and purge flow swirl angle on the convective heat transfer coefficient and the film cooling effectiveness in the TCF. The experiments are conducted in a sector-cascade test rig specifically designed for such heat transfer studies using infrared thermography and tailor-made flexible heating foils with constant heat flux. The inlet flow is characterized by radially traversing a five-hole-probe. Three purge-to-mainstream blowing ratios and an additional no purge case are investigated. The purge flow is injected without swirl and also with engine-similar swirl angles. The purge swirl and blowing ratio significantly impact the magnitude and the spread of film cooling in the TCF. Increasing blowing ratios lead to an intensification of heat transfer. By cooling the purge flow, a moderate variation in purge-to-mainstream density ratio is investigated, and the influence is found to be negligible.

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High-bypass ratio turbofan engines are commonly employed in aircrafts. Their usage is essential to guarantee low specific fuel consumption, reduced CO2 emissions and low noise levels. Such modern aero-engines benefit from high efficiencies by operating at turbine inlet temperatures in excess of the melting point of the turbine components. To enable this, compressor air is supplied to the turbine for cooling and purging purposes. The re-introduction of the cooling air back into the mainstream flow is known to alter the flow field and to affect the aerodynamic performance of the turbine components. A component especially susceptible to the interaction between the mainstream and purge flow is the Turbine Center Frame, located between high-pressure turbine (HP) and low-pressure (LP) turbine. For ever higher bypass ratios, this turbine transition ducts need to be designed with axial lengths as short as possible and larger radial offsets to avoid engine weight penalties while at the same time maintaining aerodynamic performance. More detailed experience in the field of intermediate turbine ducts is needed to identify further opportunities to improve turbofan engine performance, including an in-depth understanding of the interaction between mainstream and purge flows. This paper presents a Computational Fluid Dynamics (CFD) study of the effect of the purge flow temperature, and hence density, on the aerodynamic performance of an engine representative Turbine Center Frame (TCF). Several steady-state Reynolds-averaged Navier–Stokes (RANS) simulations were conducted for varying purge flow temperatures using an in-house code called LINARS. Time-averaged five-hole-probe measurements acquired in the Transonic Test Turbine Facility (TTTF) at Graz University of Technology were used as inlet boundary conditions to impose an engine-relevant flow field. The results obtained from two reduced and two increased purge flow temperature conditions were compared to a reference case. The reference case results showed agreement with static wall pressure measurements, hence validating the simulation. Changing the purge flow temperature significantly affected the main flow locally as well as overall. The position and size of vortices in the TCF were changed under the presence of hotter or cooler purge flows. Additionally, a flow separation on the outer duct wall observed in the baseline case was suppressed in the cold-purged flow case. The cold-purged TCF showed a 28.8% lower total pressure loss than the hot-purged one. This indicates that a more aggressive TCF design may be feasible in a cold-purged operation.

A measurement technique for recording convective heat transfer coefficient and adiabatic film cooling effectiveness in demanding environments with highly curved surfaces and limited optical access, such as turbomachinery, is presented. Thermography and tailor-made flexible heating foils are used in conjunction with a novel multistep calibration and data reduction method. This method compensates for sensor drift, angle dependence of surface emissivity and window transmissivity, heat flux inhomogeneity, and conductive losses. The 2D infrared images are mapped onto the 3D curved surfaces and overlapped, creating surface maps of heat transfer coefficient and film cooling effectiveness covering areas significantly larger than the window size. The measurement technique’s capability is demonstrated in a sector-cascade test rig of a turbine center frame (TCF), an inherent component of modern two-spool turbofan engines. The horseshoe vortices were found to play a major role for the thermal integrity of turbine center frames, as they lead to a local increase in heat transfer, and at the same instance, to a reduction of film cooling effectiveness. It was also found that the horseshoe vortices lift off from the curved surface at 50% hub length, resulting in a pair of counter-rotating vortices. The measurement technique was validated by comparing the data against flat plate correlations and also by the linear relation between temperature difference and heat flux. This study is complemented with an extensive error and uncertainty analysis. Article highlights This paper presents an accurate measurement technique for heat transfer and film cooling on 3D curved surfaces with limited optical access using flexible tailor-made heating foils, infrared thermography and a high-fidelity multistep calibration process. Graphical abstract

This paper presents a new 45° sector-cascade test rig specifically designed for fundamental studies of film cooling effectiveness and heat transfer coefficient in Turbine Center Frames (TCFs) and for the development and validation of a measurement technique involving infrared thermography and heating foils. Measurements of heat transfer coefficient in the TCF were taken for two purge-to-mainstream mass flow ratios corresponding to the case of no purge and nominal (to engine operation) purge. The magnitude of the heat transfer coefficients on the hub and strut surfaces was highly influenced by the various flow structures in the passage and by the velocity variation of the mainstream flow due to the “aggressive” design of the TCF. Heat transfer on the surface of the strut was mainly governed by boundary layer behavior (laminar near the leading edge and turbulent for the rest of the strut) augmented by the effect of the secondary flow structures. Measurements of film cooling effectiveness were also taken for the single case of nominal purge. A region of high film cooling effectiveness was observed, extending from the purge cavity exit to about 40% of the passage axial length. In this region, the effectiveness decreased with increasing axial length. On the surface of the struts and fillet radii the film cooling effectiveness was found to be zero. This was attributed to the effect of the horse-shoe vortex which sweeps the purge flow away from the strut surface and dilutes it by continuously entraining hot mainstream flow.

The imperative improvement in the efficiency of turbofan engines is commonly facilitated by increasing the turbine inlet temperature. This development has reached a point where also components downstream of the high-pressure turbine have to be adequately cooled. Such a component is the turbine center frame (TCF), known for a complex aerodynamic flow highly influenced by purge-mainstream interactions. The purge air, being injected through the wheelspace cavities of the upstream high-pressure turbine, bears a significant cooling potential for the TCF. Despite this, fundamental knowledge of the influencing parameters on heat transfer and film cooling in the TCF is still missing. This paper examines the influence of purge-to-mainstream blowing ratio, density ratio and purge swirl angle on heat transfer and film cooling in the TCF. The experiments are conducted in a sector-cascade test rig specifically designed for such heat transfer studies using infrared thermography and tailor-made flexible heating foils with constant heat flux. Three purge-to-mainstream blowing ratios and an additional no purge case are investigated. The purge flow is injected without swirl and also with engine-similar swirl angles. The purge swirl and blowing ratio significantly impact the magnitude and the spread of film cooling in the TCF. Increasing blowing ratios lead to an intensification of heat transfer. By cooling the purge flow, a moderate variation in purge-to-mainstream density ratio is investigated, and the influence is found to be negligible.