Int. J. Turbomach. Propuls. Power, Volume 6, Issue 2 (June 2021) – 15 articles

This paper presents an adjoint-based shape optimization framework and its demonstration in a conjugate heat transfer problem in a turbine blading. The gradient of the objective function is computed based on the continuous adjoint method, which also includes the adjoint to the turbulence model. Differences in the gradient resulting from making the frozen turbulence assumption are discussed. The developed software was used to optimize both the blade shape of the internally cooled linear C3X turbine blade and the position of cooling channels aiming at (a) minimum total pressure drop of the hot gas flow and (b) minimum highest temperature within the blade. A two-step optimization procedure was used. A free-form parameterization tool, based on volumetric NURBS, controls the blade airfoil contour, while the cooling channels are free to move following changes in the coordinates of their centers. Geometric and flow constraints are included in the performed optimizations, keeping the cooling channels away from the airfoil sides and retaining the turbine inlet capacity and flow turning. Full article

This study investigates the aerodynamic and aeroelastic characteristics of a transonic axial compressor, focusing on blade count reduced rotor behavior. The analysis is based on experiments, conducted at the Transonic Compressor Darmstadt test rig at Technical University of Darmstadt and compulsory simulations. In order to obtain measurement data for the detailed aerodynamic and aeroelastic investigation, extensive steady and unsteady instrumentation was applied. Besides transient measurements at the stability limit to determine the operating range and limiting phenomena, performance measurements were performed, presenting promising results with respect to the capabilities of blade count reduced rotors. Close to the stability limit, aerodynamic disturbances like radial vortices were detected for both rotors, varying in size, count, speed and trajectory. Comparing the rotor configurations results in different stability limits along the compressor map as well as varying aeromechanical behavior. Those effects can partially be traced to the variation in blade pitch and associated aerodynamics. Full article

We quantify the sensitivity of turbine acoustic impedance to aerodynamic design parameters. Impedance boundary conditions are an influential yet uncertain parameter in predicting the thermoacoustic stability of gas turbine combustors. We extend the semi-actuator disk model to cambered blades, using non-linear time-domain computations of turbine vane and stage cascades with acoustic forcing for validation data. Discretising cambered aerofoils into multiple disks improves reflection coefficient predictions, reducing error by up to an order of magnitude compared to a flat plate assumption. A parametric study of turbine stage designs using the analytical model shows acoustic impedance is a weak function of degree of reaction and polytropic efficiency. The design parameter with the strongest influence is flow coefficient, followed by axial velocity ratio and Mach number. We provide the combustion engineer with improved tools to predict impedance boundary conditions, and suggest thermoacoustic stability is most likely to be compromised by change in turbine flow coefficient. Full article

In this paper, we investigate the feasibility of using DNS data and machine learning algorithms to assist RANS turbulence model development. High-fidelity DNS data are generated with the incompressible Navier–Stokes solver implemented in the spectral/hp element software framework Nektar++. Two test cases are considered: a turbulent channel flow and a stationary serpentine passage, representative of internal turbo-machinery cooling flow. The Python framework TensorFlow is chosen to train neural networks in order to address the known limitations of the Boussinesq approximation and a clustering based on flow features is run upfront to enable training on selected areas. The resulting models are implemented in the Rolls-Royce solver HYDRA and a posteriori predictions of velocity field and wall shear stress are compared to baseline RANS. The paper presents the fundamental elements of procedure applied, including a brief description of the tools and methods and improvements achieved. Full article

Panels were tested at different locations around the turbine blade, on both suction and pressure surfaces. Three different surface porosities were also tested. Results demonstrated that the approach can be very successful with high levels of film cooling effectiveness, exceeding 95%, achieved using low coolant mass flow rates. Increasing the surface porosity also proved to be an important parameter in the panel’s performance. Additionally, staggering the film holes lead to significant positive interactions between individual films, resulting in much improved panel performance. Full article

This paper presents a new improved approach to calculation of polytropic performance of centrifugal compressors. This rapid solution technique is based upon a constant efficiency, temperature-entropy polytropic path represented by cubic polynomials. New thermodynamic path slope constraints have been developed that yield highly accurate results while requiring fewer computing resources and reducing computing elapsed time. Applying this thermodynamically sound cubic polynomial model would improve accuracy and shorten compressor performance test duration at a vendor’s shop. A broad range of example case results verify the accuracy and ease of use of the method. The example cases confirm the cubic polynomial methods result in lower calculation uncertainty than other methods. Full article

This paper numerically investigates stator endwall designs for a mixed flow turbine. One key design parameter studied is the tilting angle of the stator endwall. By examining stator designs with different tilting angles, the aim of this paper is to improve the efficiency of the studied mixed flow turbine at low velocity ratio working conditions. The performance curve at the design speed was chosen for the comparison between the baseline design and the tilted endwall designs. First, the numerical predictions for the baseline design were validated with experimental data. Then, to understand the mechanism of the performance variation between the different designs, the internal flow field was analyzed in detail. It was found that the tilting stator endwall could form a geometric “kink” in the endwall profiles. On the shroud side, certain designs with such kink caused local flow separations upstream the rotor leading edge. This separation could have the effect of reducing the intensity of the tip leakage vortex and the exit kinetic energy losses at the rotor outlet and may also improve the performance of the exhaust diffuser. As a result, the peak of the efficiency curve shifted toward lower velocity ratio. If the turbine stage incorporated a downstream exhaust diffuser, the optimal design in this study showed a shift of the velocity ratio of the peak efficiency point from 0.62 to 0.60 compared with the baseline. The maximum efficiency improvement was 1.3% points, which occurred at low velocity ratio. Meanwhile, the peak efficiency was 0.2% points higher than the baseline. If the exhaust diffuser was removed, a similar shift of the efficiency curve was observed but less efficiency gain was achieved at the low velocity ratio condition. A preliminary unsteady simulation was also conducted for the optimal design in this study. Full article

The designers of radial turbomachinery need detailed information on the impact of the side chamber flow on axial thrust and torque. A previous paper investigated centripetal flow through narrow rotor–stator cavities and compared axial thrust, rotor torque and radial pressure distribution to the case without through-flow. Consequently, this paper extends the investigated range to centrifugal through-flow as it may occur in the hub side chamber of radial turbomachinery. The chosen operating conditions are representative of high-pressure centrifugal compressors used in, for example, carbon capture and storage applications as well as hydrogen compression. To date, only the Reynolds number range up to $Re=2·{10}^7$ has been investigated for centrifugal through-flow. This paper extends the range to Reynolds numbers of $Re=2·{10}^8$ and reports results of experimental and numerical investigations. It focuses on the radial pressure distribution in the rotor–stator cavity and shows the influence of the Reynolds number, cavity width and centrifugal mass flow rate. It therefore extends the range of available valid data that can be used to design radial turbomachinery. Additionally, this analysis compares the results to data and models from scientific literature, showing that in the higher Reynolds number range, a new correlation is required. Finally, the analysis of velocity profiles and wall shear delineates the switch from purely radial outflow in the cavity to outflow on the rotor and inflow on the stator at high Reynolds numbers in comparison to the results reported by others for Reynolds numbers up to $Re=2·{10}^7$. Full article

In the present paper, large eddy simulations are performed to study two different mechanisms of Fan/OGV broadband noise: airfoil self-noise and turbulence interaction noise. Firstly, the current study focuses on the prediction of airfoil self-noise from a thin plate with a sharp trailing edge and a chord-based Reynolds number of the order of 10${}^6$. The boundary layer is tripped to trigger transition to turbulence, and a parameter study is performed to study the influence of the near-wall modeling, grid topology and refinement in the near-wall and wake regions, the spanwise domain extent, and the tripping method. Empirical and analytical models, as well as available DNS data are used for validation purposes. Secondly, the interaction noise from a thin plate impinged by an incoming synthetic turbulent flow is studied. For both cases, far-field acoustic spectra are compared to Amiet’s models for leading and trailing edge noise showing a good agreement. Full article

A significant share of the conversion of thermal into electrical energy is realized by steam turbines. Formerly designed for continuous operation, today’s requirements include extended part load operation that can be accompanied by highly unstable flow conditions and vibrations within the control valve of the turbine. The prediction of the flow at part load conditions requires large computational efforts with advanced turbulence modeling in order to compute the flow at a reasonable accuracy. Due to the unsteadiness of the flow, the evaluation of the numerical results itself is a major challenge. The turbulent structures require statistical approaches, of which the use of Spectral Proper Orthogonal Decomposition (SPOD) has proven itself as a powerful method. Within this paper, the application of the method on a critical operating point with a temporal excitation of pressure oscillations observed in the experiments with dry air is presented. Using SPOD, the dominating flow phenomena were isolated and flow structures visualized. Full article

In this paper, we compare using computational fluid dynamics the aero-thermal performance of two candidate casing manifolds for supplying an impingement-actuated active tip clearance control system for an aero-engine high-pressure turbine. The two geometries are (a) single-entry: an annular manifold fed at one circumferential location; (b) multiple-entry: a casing manifold split into four annular sectors, with each sector supplied separately from an annular ring main. Both the single-entry and multiple-entry systems analysed in this paper are idealised versions of active clearance control systems in current production engines. Aero-thermal performance is quantitatively assessed on the basis of the heat transfer coefficient distribution, driving temperature difference for heat transfer between the jet and casing wall and total pressure loss within the high-pressure turbine active clearance control system. We predict that the mean heat transfer coefficient (defined with respect to the inlet temperature and local wall temperature) of the single-entry active clearance control system is 77% greater than the multiple-entry system, primarily because the coolant in the multiple-entry case picks up approximately 40 K of temperature from the ring main walls, and secondarily because the average jet Reynolds number of impingement holes in the single-entry system is 1.2 times greater than in the multiple-entry system. The multiple-entry system exhibits many local hot and cold spots, depending on the position of the transfer boxes, while the single-entry case has a more predictable aero-thermal field across the system. The multiple-entry feed system uses an average of 20% of the total available pressure drop, while the feed system for the single-entry geometry uses only 2% of the total available pressure drop. From the aero-thermal results of this computational study, and in consideration of holistic aero-engine design factors, we conclude that a single-entry system is closer to an optimal solution than a multiple-entry system. Full article

This article provides a summarizing account of the results obtained in the current collaborative work of four research institutes concerning near-wall flow in turbomachinery. Specific questions regarding the influences of boundary layer development on blades and endwalls as well as loss mechanisms due to secondary flow are investigated. These address skewness, periodical distortion, wake interaction and heat transfer, among others. Several test rigs with modifiable configurations are used for the experimental investigations including an axial low speed compressor, an axial high-speed wind tunnel, and an axial low-speed turbine. Approved stationary and time resolving measurements techniques are applied in combination with custom hot-film sensor-arrays. The experiments are complemented by URANS simulations, and one group focusses on turbulence-resolving simulations to elucidate the specific impact of rotation. Juxtaposing and interlacing their results the four groups provide a broad picture of the underlying phenomena, ranging from compressors to turbines, from isothermal to non-adiabatic, and from incompressible to compressible flows. Full article

Aerospike nozzles represent an interesting solution for Single-Stage-To-Orbit or clustered launchers owing to their self-adapting capability, which can lead to better performance compared to classical nozzles. Furthermore, they can provide thrust vectoring in several ways. A simple solution consists of applying differential throttling when multiple combustion chambers are used. An alternative solution is represented by fluidic thrust vectoring, which requires the injection of a secondary flow from a slot. In this work, the flow field in a linear aerospike nozzle was investigated numerically and both differential throttling and fluidic thrust vectoring were studied. The flow field was predicted by solving the Reynolds-averaged Navier–Stokes equations. The thrust vectoring performance was evaluated in terms of side force generation and axial force reduction. The effectiveness of fluidic thrust vectoring was investigated by changing the mass flow rate of secondary flow and injection location. The results show that the response of the system can be non-monotone with respect to the mass flow rate of the secondary injection. In contrast, differential throttling provides a linear behaviour but it can only be applied to configurations with multiple combustion chambers. Finally, the effects of different plug truncation levels are discussed. Full article

The use of coupled algorithms for single fluid flow simulation has proven its superiority as opposed to segregated algorithms, especially in terms of robustness and performance. In this paper, the coupled approach is extended for the simulation of multi-fluid flows, using a collocated and pressure-based finite volume discretization technique with a Eulerian–Eulerian model. In this context a key ingredient in this method is extending the Rhie–Chow interpolation technique to account for the unique flow coupling that arises from inter-phase drag. The treatment of this inter-fluid coupling and the fashion in which it interacts with the velocity-pressure solution algorithm is presented in detail and its effect on robustness and accuracy is demonstrated using 2D dilute gas–solid flow test case. The results achieved with this technique show substantial improvement in accuracy and performance when compared to a leading commercial code for a transonic nozzle configuration. Full article

The demand for increased turbomachinery performance, both, towards higher pressures and temperatures, leads to high thermal-loads of specific components and can critically affect mechanical integrity. In the particular case of rotating-disk configurations, like the back-side of wheels or in cavities, a very efficient way for cooling is jet impingement. An example for this situation are high pressure-ratio turbochargers, where cooling of the impeller disk (back wall) is introduced to achieve tolerable thermal loads. From the physical point of view, jet impingement on a rotating wall generates an unsteady heat transfer situation. On the other end, accurate values of time-averaged temperatures would be sufficient for design purposes. In general, obtaining circumferentially time-averaged solutions requires transient analysis of the conjugate heat transfer (CHT) process to account for the mean effect of jet cooling on solids. Such analysis is computationally expensive, due to the difference in information propagation time-scale for the solid and the fluid. In this paper, a new approach to directly compute circumferentially time-averaged (i.e., steady-state) temperature distributions for rotating-disk CHT problems is presented based on an adaption of the well known fluid-fluid mixing plane approach. Full article