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Coupled Aero-Thermo-Mechanical Simulation for a Turbine Disc Through a Full Transient Cycle
Abstract
Use of computational fluid dynamics (CFD) to model the complex, 3D disc cavity flow and heat transfer in conjunction with an industrial finite element analysis (FEA) of turbine disc thermo-mechanical response during a full transient cycle is demonstrated. The FEA and CFD solutions were coupled using a previously proposed efficient coupling procedure. This iterates between FEA and CFD calculations at each time step of the transient solution to ensure consistency of temperature and heat flux on the appropriate component surfaces. The FEA model is a 2D representation of high pressure (HP) and intermediate pressure (IP) turbine discs with surrounding structures. The front IP disc cavity flow is calculated using 45° sector CFD models with up to 2.8 million mesh cells. Three CFD models were initially defined for idle, maximum take-off (MTO) and cruise conditions, and these are updated by the automatic coupling procedure through the 13000 seconds full transient cycle from standstill, to idle, maximum take-off, and cruise conditions. The obtained disc temperatures and displacements are compared with an earlier standalone FEA model which used established methods for convective heat transfer modelling. It was demonstrated that the coupling could be completed using a computer cluster with 60 cores, within about two weeks. This turn around time is considered fast enough to meet design phase requirements and in validation it also compares favorably to that required to hand-match an FEA model to engine test data, which is typically several months.
... Final results were validated by thermocouple results of tests. Similarly, Sun et al. [11,12,13] studied on cavities of gas turbine engine with complicated cooling flows. They solved both steady state and transient heat transfer problems with certain convergence criteria. ...
In this study jet impingement cooling method is investigated with coupled analysis. Total cooling rate is observed for the specific jet impingement configuration using both finite volume and finite element methods. The specific configuration contains single row of jets of separate four rowed impingement cooling system. This single row is placed at the suction side of vane near trailing edge. For the observation, finite volume analysis is carried out via Fluent program. CFD model, which uses constant hot wall (target surface) temperature, is validated using the test case available in the literature. Constant wall temperature is 1250 K and hot gas of system is at 1500 K with 800 kPa. Moreover, conditions of cooling air are 500 K and 400 kPa. All conditions are determined to simulate specifications of a vane of middle class engine. The coupled solution is performed to calculate realistic heat transfer coefficient (htc) values. It involves concurrent execution of finite element analysis and finite volume analysis for aero-thermal optimization. Iterations are carried out via exchanging heat transfer coefficient values for finite element analysis and metal temperature values for finite volume analysis. At the end of three iterations, 8.1% decrease of htc values is obtained and optimum metal temperature values for the specified cooling configuration are calculated.
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... Their study also showed the important role played by TBC in reducing the maximum temperature in the turbine material. Sun, et al. [10,11] examined the different ways to compute the unsteady flow and heat transfer of gas-turbine components through a full transient cycle from shut down to idle to maximum take-off, and to cruise. The emphasis was on how to efficiently couple the fluid phase and the solid phase to ensure continuity in temperature and heat flux at every instant of time at all solid-fluid interface, the convergence criteria needed to ensure meaningful solutions, and assess what is feasible as a design tool. ...
The temperature in a material could exceed the maximum allowable during transients when the heat load is suddenly increased despite a corresponding increase in cooling. This is because there is a time lag in the response of the material. Unsteady RANS based on the shear-stress transport model with conjugate analysis were used to study the unsteady heating and cooling of a superalloy flat plate with a thickness of 1 mm. The flat plate was initially at steady-state conditions, heated on one side by a constant heat flux of 10 W/cm2 and cooled on the other side by impingement of air jets such that the maximum temperature in the plate was just below 900 °C, where 900 °C was taken to be the maximum allowable material temperature. Suddenly, the heating load was increased from 10 W/cm2 to 68 W/cm2 with a corresponding increase in the cooling such that the maximum temperature in the plate remains just below 900 °C when steady state is reached. Results obtained show that though the maximum temperatures at the two steady states are just below 900 °C, the highest temperature in the material can exceed 900 °C by up to 14 seconds during the transient from one steady state to the other. Thus, the minimum cooling flow based on steady-state conditions are inadequate during the transient process if the sudden heating and cooling occurred simultaneously. However, if the increase in cooling preceded the sudden increase in heating by a sufficient amount of time (i.e., pre-cooling), then over temperature was found to not occur during the transient. This paper presents results that show the unsteady flow and heat transfer in the fluid phase and the transients in the solid phase with and without pre-cooling.
Turbine blades and disc, which are one of the most important rotating parts of a gas turbine engine, are required to have highly efficient performance in order to minimize the total life cycle costs. Owing to these requirements, these components are exposed to severe conditions such as extreme turbine inlet temperatures, high compression ratios, and high speeds. To evaluate the structural integrity of a turbine disc under these conditions, material modeling and finite element analysis techniques are essential; furthermore, shape optimization is necessary for determining the optimal solution. This study aims to generate 2D finite element models of an axisymmetry model and a sector one and to perform thermal-structural coupled-field analysis and contact analysis. Structurally vulnerable areas such as the disc bore and disc-blade interface region are analyzed by a parametric study. Finally, an improved design is provided based on the results, and the necessity of elaborate shape optimization is confirmed.
Use of large-scale computational fluid dynamics (CFD) models in aeroengine design has grown rapidly in recent years as parallel computing hardware has become available. This has reached the point where research aimed at the development of CFD-based ‘virtual engine test cells’ is underway, with considerable debate of the subject within the industrial and research communities. The present article considers and illustrates the state-of-the art and prospects for advances in this field. Limitations to CFD model accuracy, the need for aero-thermo-mechanical analysis through an engine flight cycle, coupling of numerical solutions for solid and fluid domains, and timescales for capability development are considered. While the fidelity of large-scale CFD models will remain limited by turbulence modelling and other issues for the foreseeable future, it is clear that use of multi-scale, multi-physics modelling in engine design will expand considerably. Development of user-friendly, versatile, efficient programs and systems for use in a massively parallel computing environment is considered a key issue.
Thesis (D. Phil.)--Oxford University, 1999.
Considerable progress in development and application of computational fluid dynamics (CFD) for aeroengine internal flow systems has been made in recent years. CFD is regularly used in industry for assessment of air systems, and the performance of CFD for basic axisymmetric rotor/rotor and stator/rotor disc cavities with radial throughflow is largely understood and documented. Incorporation of three-dimensional geometrical features and calculation of unsteady flows are becoming commonplace. Automation of CFD, coupling with thermal models of the solid components, and extension of CFD models to include both air system and main gas path flows are current areas of development. CFD is also being used as a research tool to investigate a number of flow phenomena that are not yet fully understood. These include buoyancy-affected flows in rotating cavities, rim seal flows and mixed air/oil flows. Large eddy simulation has shown considerable promise for the buoyancy-driven flows and its use for air system flows is expected to expand in the future.
A fluid-solid conjugate solver has been newly developed and applied to an actual engine disk system. Most of the currently available conjugate solvers lack the special thermal modeling for turbomachinery disk system applications. In the present new code, these special models are implemented to expand the applicability of the conjugate method and to reduce the required computational resources. Most of the conjugate analysis work so far were limited to the axisymmetric framework. However, the actual disk system includes several non-axisymmetric components which inevitably affect the local heat transfer phenomena. This paper presents 3D conjugate analysis of a single stage high pressure turbine rotor-stator disk system to assess these three-dimensional effects. The predicted temperatures shows good agreement with measured data. The calculated results revealed the three-dimensional analysis is crucial to predict the correct heat transfer field which is especially important in transient situations.
The accuracy of computational fluid dynamics (CFD) for the prediction of flow and heat transfer in a direct transfer pre-swirl system is assessed through a comparison of CFD results with experimental measurements. Axisymmetric and three dimensional (3D) sector CFD models are considered. In the 3D sector models, the pre-swirl nozzles or receiver holes are represented as axisymmetric slots so that steady state solutions can be assumed. A number of commonly used turbulence models are tested in three different CFD codes, which were able to capture all of the significant features of the experiments. Reasonable quantitative agreement with experimental data for static pressure, total pressure and disc heat transfer is found for the different models, but all models gave results which differ from the experimental data in some respect. The more detailed 3D geometry did not significantly improve the comparison with experiment, which suggested deficiencies in the turbulence modelling, particularly in the complex mixing region near the pre-swirl nozzle jets. The predicted heat transfer near the receiver holes was also shown to be sensitive to near-wall turbulence modelling. Overall, the results are encouraging for the careful use of CFD in pre-swirl-system design.
For high pressure turbine heat transfer designs, a primary requirement is to predict blade metal temperature. There has been considerable recent effort in developing coupled fluid convection and solid conduction (conjugate) heat transfer prediction methods. They are so far, however, confined to steady flows. In the present work, a new approach to conjugate analysis for periodic unsteady flows of practical interest is proposed and demonstrated. This paper starts with a simple model analysis to quantify the huge disparity in time scale between convection and conduction and the implications of this for steady and unsteady conjugate solutions. To realign the greatly mismatched time scales, a hybrid approach of coupling between time-domain fluid solution and frequency-domain solid conduction is adopted in conjunction with a continuously updated Fourier transform at the interface. A novel semi-analytical harmonic interface condition is introduced, initially for reducing the truncation error in Finite-difference discretization. More interestingly, the semianalytical interface condition enables the unsteady conjugate coupling to be achieved without simultaneously solving the unsteady temperature field. This unique feature leads to a very efficient and accurate unsteady conjugate solution approach. The fluid and solid solutions are validated against analytical solutions and experimental data. The implemented unsteady conjugate method has been demonstrated for a turbine cascade subject to inlet unsteady hot streaks.
In recognition of continuing difficulties that are experienced in performing computational analyses (CAE) on real turbine blades, this paper explores current developments in areas of integrated geometry handling, multi-domain meshing and multi-disciplinary analysis. The application of an integrated software framework first for CAD repair, then for surface and volume meshing, and finally for combined fluid, thermal and stress analysis can go some way to easing pains. The focus here is to demonstrate how it is possible in a single simulation environment, and using a finite volume methodology, to deal with real turbine blade geometries in terms of CAD handling, and then to perform conjugate-heat transfer and stress analyses using one mesh which is generated to be continuous through the fluid (external and internal passages) and blade metal. Possible gains to productivity become obvious.
A full conjugate heat transfer Computational Fluid Dynamics (CFD) study of a Biaxial rig enclosure used for the lifing of the high pressure turbine disc firtrees of a large turbofan engine has been undertaken. Initial rig tests had demonstrated challenges associated with test repeatability and unexpected thermal gradients in some components. The detailed CFD analysis was used to gain an appropriate understanding of the overall heat balance for the assembly, requiring all three modes of heat transfer as well as the fluid dynamics about the disc-blade components. Moreover the detailed modelling methodology presented for the twin short wave infrared emitters allowed the balance of radiation to convection heat transfer from the heaters, essential to the whole study, to be appropriately captured. It was observed that radiation setup the bulk assembly temperature levels (> 900K). Convection heat transfer very much influenced component temperature gradients (10K for disc front-to-back surfaces and >80K for blade front-to-back surfaces). The gap sizes between the assembly and cooling blocks associated with rig setup had a strong influence on the convection currents and flow structures about the assembly and were an important factor in test repeatability. Strong cooler inflows were entrained under the blades, which in the tests had to be compensated by asymmetric top-to-bottom heater power setting of 1.5–2 folds. Once appropriate thermal operating conditions of the assembly were achieved in the tests, the model was tuned to achieve a satisfactory match against the more reliable test thermocouple temperatures, to about -/+10K for the most important locations. Thereafter the model component thermal field was used for follow-on stress analysis, required in critical component lifing and eventual engine certification. This circumvented the need for separate thermal modelling and provided efficient utilization of man-time resource.
Recent development in commercial CFD codes offers possibilities to include the solid body in order to perform conjugate heat transfer computations for complex geometries. The current paper aims to analyse the differences between a conjugate heat transfer computation and conventional uncoupled approaches where a heat transfer coefficient is first derived from a flow solution and then taken as boundary condition for a thermal conduction analysis of the solid part. Whereas the thermal analyses are done with a Rolls-Royce in-house finite element code, the CFD as well as the conjugate heat transfer computation are done using the new version 8 of the commercial code Fine Turbo from Numeca International. The analysed geometry is a turbine cascade that was tested by VKI in Brussels within the European FP6 project AITEB 2. First, the paper presents the aerodynamic results. The pure flow solutions are validated against pressure measurements of the cascade test. Then, the heat transfer from flow computations with wall temperature boundary conditions is compared to the measured heat transfer. Once validated, the heat transfer coefficients are used as boundary condition for three uncoupled thermal analyses of the blade to predict its surface temperatures in a steady state. The results are then compared to a conjugate heat transfer method. Therefore, a mesh of the solid blade was added to the validated flow computation. The paper will present and compare the results of conventional uncoupled thermal analyses with different strategies for the wall boundary condition to results of a conjugate heat transfer computation. As it turns out, the global results are similar but especially the over-tip region with its complex geometry and flow structure and where effective cooling is crucial shows remarkable differences because the conjugate heat transfer solution predicts lower blade tip temperatures. This will be explained by the missing coupling between the fluid and the solid domain.
The main objectives of the paper are to test widely used turbulence models against selected benchmark problems identifying range of applicability and limitations of the models used and to help accurately predict flow in turbine blade passages and disc cavities. The following models are considered: the k–ε model with and without a Kato–Launder correction and a Richardson number correction, the two-layer k–ε /k–l model, and the Spalart–Allmaras model with and without correction for rotation. Weaknesses in the models are identified and suggestions made for possible improvements. Numerical implementation of the wall functions approach is also considered. The test cases considered include flat plate boundary layer, flat plate heat transfer, enclosed rotating disc, and a combined turbine blade/disc cavity model. Comparisons are made with experimental data and computations from different CFD codes.
Thermal and flow field in a rotor-stator system around a single stage HP-turbine disk are investigated. In the previous work, the authors’ group applied a newly developed conjugate analysis method to the system at a steady-state condition, compared the thermal field with the measured data, and concluded that a satisfactory level of accuracy can be achieved with the proposed method at the steady-state condition. The present paper focuses on a heat and fluid flow in the same rotor-stator system, but at a transient condition which simulates a typical accelerating and decelerating schedule expected in actual aero-engine operations. First, detailed measurement of secondary-air and metal temperatures around the turbine disk is carried out in the transient condition. Then, the 3D fluid / solid conjugate analysis is applied to the whole accelerating and decelerating schedule. The results show that the transient thermal behavior of the rotor-stator system is well captured and reproduced with the present conjugate method.
Secondary flows in the cooling jets are the main reason for the degradation of the cooling performance of a film-cooled blade. The formation of kidney vortices can significantly be reduced for shaped holes instead of cylindrical holes. 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 with application of the adiabatic and a conjugate heat transfer condition have been performed for different configurations with cylindrical and shaped holes. It can be shown that the application of the conjugate calculation method comprises the influence of heat transfer on the velocity field within the cooling film. In particular, the secondary flow velocities are affected by the local heat transfer, which varies significantly depending on the local position.
Accurate temperature predictions are essential to the optimization of gas turbine component design. This paper provides an update on the application of recent developments in heat transfer boundary condition derivation and finite element model validation. Computational fluid dynamics (CFD) is increasingly being used to determine cooling flow distributions and convective heat fluxes on a range of components. Validation of the CFD methodology for internal cavity heat transfer is also a key focus of major research programmes. In this paper, further results are presented for selected engine and rig cavities. A fully coupled CFD/finite element thermal model solution is also demonstrated. Increasingly, the application of optimization techniques to the thermal model calibration process is showing that significant savings in analysis time can be achieved for a given accuracy of ‘match’. The optimization process is described and sample results are presented from the calibration of a typical thermal model. Finally, the impact of these new analysis techniques on the derivation of thermal boundary conditions in gas turbine component cavities and the implications for compliance with Airworthiness Authority regulations are summarized with respect to offering an improved temperature prediction validation strategy.
Flow and heat transfer in the row-1 upstream rotor-stator disc cavity of a large 3600-rpm industrial gas turbine was investigated using an integrated approach. A 2D axisymmetric transient thermal analysis using aero engine-based correlations was performed to predict the steady state metal temperatures and hot running seal clearances at ISO rated power condition. The cooling mass flow and the flow pattern assumption for the thermal model were obtained from the steady state 2D axisymmetric CFD study. The CFD model with wall heat transfer was validated using cavity steady state air temperatures and static pressures measured at inlet to the labyrinth seal and four cavity radial positions in an engine test which included the mean annulus static pressure at hub radius. The predicted wall temperature distribution from the matched thermal model was used in the CFD model by incorporating wall temperature curve-fit polynomial functions. Results indicate that although the high rim seal effectiveness prevents ingestion from entering the cavity, the disc pumping flow draws air from within the cavity to satisfy entrainment leading to an inflow along the stator. The supplied cooling flow exceeds the minimum sealing flow predicted from both the rotational Reynolds number-based correlation and the annulus Reynolds number correlation. However, the minimum disc pumping flow was found to be based on a modified entrainment expression with a turbulent flow parameter of 0.08. The predicted coefficient of discharge (Cd) of the industrial labyrinth seal from CFD was confirmed by modifying the carry-over effect of a correlation reported recently in the literature. Moreover, the relative effects of seal windage and heat transfer were obtained and it was found that contrary to what was expected, the universal windage correlation was more applicable than the aero engine-based labyrinth seal windage correlation. The CFD predicted disc heat flux profile showed reasonably good agreement with the free disc calculated heat flux. The irregular cavity shape and high rotational Reynolds number (in the order of 7×107) leads to entrance effects that produce a thicker turbulent boundary layer profile compared to that predicted by the 1/7 power velocity profile assumption.
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