Conjugate Calculation of Effusion Cooling With Realistic Cooling Hole Geometries

Within Collaborative Research Center 561 “Thermally Highly Loaded, Porous and Cooled Multi-Layer Systems for Combined Cycle Power Plants” at RWTH Aachen University an effusion-cooled multi-layer plate configuration with seven staggered effusion cooling holes is investigated numerically by application of a 3-D in-house fluid flow and heat transfer solver, CHTflow. The effusion-cooling is realized by finest drilled holes with a diameter of 0.2 mm that are shaped in the region of the thermal barrier coating. The geometry of the cooling holes is not symmetrical, due to the manufacturing process. Furthermore, the inlet of the cooling holes in the plenum shows no sharp edges, which has a significant influence on the formation of the kidney-vortices within the cooling hole. The impact of this geometry on the cooling effectiveness has to be quantified prior to application. A hot gas Mach-number of 0.25 and blowing ratios of approximately 0.28 and 0.48 will be considered. The numerical grid contains the coolant supply (plenum), the solid body for the conjugate calculations and the main flow area on the plate. The form of the hole, especially that of the diffuser, leads to a skewered mass flow from the hole and a non-symmetric temperature distribution on the plate surface. Therefore, neither the flow field on the hot gas surface nor the temperature distribution can be compared with the usually investigated idealised hole geometries.

Conjugate Flow and Heat Transfer Investigation of a Turbo Charger

In this paper a three-dimensional conjugate calculation has been performed for a passenger car turbo charger. The scope of this work is to investigate the heat fluxes in the radial compressor, which can be strongly influenced by the hot turbine. As a result of this, the compressor efficiency may deteriorate. Consequently, the heat fluxes have to be taken into account for the determination of the efficiency. To overcome this problem a complex three-dimensional model has been developed. It contains the compressor, the oil cooled center housing, and the turbine. Twelve operating points have been numerically simulated composed of three different turbine inlet temperatures and four different mass flows. The boundary conditions for the flow and for the outer casing were derived from experimental test data (Bohn et al.). Resulting from these conjugate calculations various one-dimensional calculation specifications have been developed. They describe the heat transfer phenomena inside the compressor with the help of a Nusselt number, which is a function of an artificial Reynolds number and the turbine inlet temperature.

Improvement of a Film-Cooled Blade by Application of the Conjugate Calculation Technique

The conjugate calculation technique has been used for the three-dimensional thermal load prediction of a film-cooled test blade of a modern gas turbine. Thus, it becomes possible to take into account the interaction of internal flows, external flow, and heat transfer without the prescription of heat transfer coefficients. The numerical models consist of all internal flow passages and cooling hole rows, including shaped holes. Based on the results, deficiencies of the test configuration close to the leading edge region and in the blade tip region have been detected, which lead to hot spots and surface areas of high thermal load. These regions of high thermal load have been confirmed by thermal index paint measurements in good agreement to the conjugate calculation results. Based on the experimental and numerical results, recommendations for the improvement of the blade cooling were derived and an improved blade-cooling configuration has been designed. The conjugate calculation results, as well as new measurement data, show that the changes in the cooling design have been successful with respect to cooling performance. Regions of high thermal load have vanished, and effective cooling is reached for all critical parts of the test blade.

Systematic Investigation on Conjugate Heat Transfer Rates of Film Cooling Configurations

For the determination of the film-cooling heat transfer, the design of a turbine blade relies on the conventional determination of the adiabatic film-cooling effectiveness and heat transfer conditions for test configurations. Thus, additional influences by the interaction of fluid flow and heat transfer and influences by additional convective heat transfer cannot be taken into account with sufficient accuracy. Within this paper, calculations of a film-cooled duct wall and a film-cooled real blade with application of the adiabatic and a conjugate heat transfer condition have been performed for different configurations. It can be shown that the application of the conjugate calculation method comprises the influence of heat transfer within the cooling film. The local heat transfer rate varies significantly depending on the local position.

Improvement of a Film-Cooled Blade by Application of the Conjugate Calculation Technique

The conjugate calculation technique has been used for the three-dimensional thermal load prediction of a film-cooled test blade of a modern gas turbine. Thus, it becomes possible to take into account the interaction of internal flows, external flow, and heat transfer without the prescription of heat transfer coefficients. The numerical models consist of all internal flow passages and cooling hole rows including shaped holes. Based on the results, deficiencies of the test configuration close to the leading edge region and in the blade tip region have been detected, which lead to hot spots and surface areas of high thermal load. These regions of high thermal load have been confirmed by thermal index paint measurements in good agreement to the conjugate calculation results. Based on the experimental and numerical results, recommendations for the improvement of the blade cooling were derived and an improved blade cooling configuration has been designed. The conjugate calculation results as well as new measurement data show that the changes in the cooling design have been successful with respect to cooling performance. Regions of high thermal load have vanished and effective cooling is reached for all critical parts of the test blade.

Conjugate Calculations for a Film-Cooled Blade Under Different Operating Conditions

Conjugate heat transfer and flow calculation techniques (CCT: Conjugate Calculation Technique) developed by several numerical groups have been applied to more and more complex three-dimensional cooling configurations. With respect to gas turbine blade cooling, conjugate calculation codes are turning out as useful tools for the support of the thermal design process. Thus, the main focus of the present study is to investigate the applicability of the CCT on a realistic film-cooling configuration of a modern gas turbine blade under hot gas operating conditions. Thermal index paint measurements for the investigated configuration have been performed at KHI Gas Turbine R&D Center in order to provide thermal load data for comparison to results of conjugate blade analysis. The comparison shows that with respect to regions with high thermal load a qualitatively good agreement of the conjugate results and the measurements can be found although the calculation models contain several simplifications for the internal cooling configuration particularly. The tip region of the blade trailing edge is exposed to a high thermal load. This result can be found in the measurement data as well as in the numerical analysis. The influence of off-design flow conditions on the film cooling flow at the blade leading edge is also investigated. Despite the model simplification, the Conjugate Calculation Technique turns out to be applicable for the numerical testing of the cooling configuration investigated. With the numerical results, useful information for further improvement of the investigated cooling configuration can be provided.

Conjugate Flow and Heat Transfer Investigation of a Turbo Charger: Part I — Numerical Results

In this paper a three-dimensional conjugate calculation has been performed for a passenger car turbo charger. The scope of this work is to investigate the heat fluxes in the radial compressor which can be strongly influenced by the hot turbine. As a result of this, the compressor efficiency may deteriorate. Consequently, the heat fluxes have to be taken into account for the determination of the efficiency. To overcome this problem a complex three-dimensional model has been developed. It contains the compressor, the oil cooled center housing, and the turbine. 12 operating points have been numerically simulated composed of three different turbine inlet temperatures and four different mass flows. The boundary conditions for the flow and for the outer casing were derived from experimental test data (part II of the paper). Resulting from these conjugate calculations various one-dimensional calculation specifications have been developed. They describe the heat transfer phenomena inside the compressor with the help of a Nusselt number which is a function of an artificial Reynolds number and the turbine inlet temperature.

Analysis of the Influence of Cooling Steam Conditions on the Cooling Efficiency of a Steam-Cooled Vane Using the Conjugate Calculation Technique

Closed circuit steam cooling of blades and vanes in modern gas turbines is an promising alternative instead of film-cooling using compressor air. The temperature drop across the first-stage nozzle, which is convectively steam-cooled, is reduced significantly in comparison to an intensive film-cooled vane using compressor air. Thus, the firing temperature (temperature in front of the first-stage blade row) can be increased while the combustion temperature can remain as low as necessary for low-Nox purpose. In this paper, a steam-cooled test configuration consisting of a 3-vane cascade is numerically analysed. A computer code using a Conjugate Calculation Technique is applied. The CHTflow code has been developed at the Institute of Steam and Gas Turbines in Aachen. Due to the direct coupling of fluid flow and solid body, heat transfer boundary conditions at the external and internal surfaces become unnecessary. Validation of the code for a similar convection-cooled configuration is also given here. The presented investigations focus on the thermal load analysis and the cooling efficiency analysis of the test configuration. It consists of a planar cascade with a convection-cooled central vane where cooling fluid can be supplied to 22 radial passages. One main aspect of the paper is to show the influence of cooling steam conditions (low-, medium & high-pressure steam supply) on the local and global cooling efficiencies. The results show that, for reaching a defined cooling efficiency level, medium steam pressure supply might be advantageous in comparison to a high-pressure level in supply. Although a lower pressure level demands an increase in steam mass flow, the overall effect on the thermal efficiency of the whole process is acceptable if one keeps in mind the advantages of handling steam at lower pressure levels. For further comparison, convective air-cooling with reasonable cooling conditions and comparable flow and heat transfer characteristics is analysed. For the given geometry of the configuration, sufficient cooling of the trailing edge becomes problematic for steam- and air-cooling application.