During the last decades, many efforts have been invested by the scientific community in minimising exhaust emissions from aeronautical gas turbine engines. In this context, many advanced ultra-low NOx combustion concepts, such as the Lean Direct Injection treated in the present study, are being developed to abide by future regulations. Numerical simulations of these devices are usually computationally expensive since they imply a multi-scale problem. In this work, a non-reactive Large Eddy Simulation of a gaseous-fuelled, radial-swirled Lean-Direct Injection (LDI) combustor has been carried out through the OpenFOAM Computational Fluid Dynamics (CFD) code by solving the complete inlet flow path through the swirl vanes and the combustor. The geometry considered is the gaseous configuration of the CORIA LDI combustor, for which detailed measurements are available. Macroscopical analysis of the main turbulent features related to the swirling flow and the generated Central Recirculation Zone (CRZ) are well established in the literature. Nevertheless, a more in-depth characterization is still required in this area of active research since theory and experimental data are not yet able to predict which unstable mode dominates the flow. This work aims at using Large Eddy Simulation for a complete characterisation of the unsteady flow structures generated within the combustion chamber of a gaseous methane injection immersed in a strong non-reactive swirling flow field. To do so, a spectral analysis of the flow field is performed to identify the frequency, intensity and instabilities associated to the phenomena occurring at the swirler outlet region. A coherent structure known as Precessing Vortex Core (PVC) is identified both at the inner and the outer shear layers, resulting in a periodic disturbance of the pressure and velocity fields. The pressure and velocity fluctuations predicted by the CFD code are used to compute the spectral signatures through the Sound Pressure Level (SPL) amplitude at multiple locations. This allows investigating both the complex behaviour of the PVC and its associated acoustic phenomena. The acoustic characteristics computed by the numerical model are first validated qualitatively by comparing the spectrum with available experimental data. In this way, the use of dimensionless numbers to characterise the most energetic structures is coherent with the experimental observations and the characteristics of the PVC. Then, the numerical identification of the main acoustic modes in the chamber through Dynamic Mode Decomposition (DMD) allows overcoming the Fast Fourier Transform (FFT) shortcomings and better understanding the propagation of the hydrodynamic instability perturbations. This investigation on the main non-reacting swirling flow structures inside the combustor provides a suitable background for further studies on combustion instability mechanisms.

Thermoacoustic instabilities can occur in a wide range of combustors and are prejudicial since they can lead to increased mechanical fatigue or even catastrophic failure. A well-established formalism to predict the onset, growth and saturation of such instabilities is based on acoustic network models. This approach has been successfully employed to predict the frequency and amplitude of limit cycle oscillations in a variety of combustors. However, it does not provide any physical insight in terms of the acoustic energy balance of the system. On the other hand, Rayleigh’s criterion may be used to quantify the losses, sources and transfers of acoustic energy within and at the boundaries of a combustor. However, this approach is cumbersome for most applications because it requires computing volume and surface integrals and averaging over an oscillation cycle. In this work, a new methodology for studying the acoustic energy balance of a combustor during the onset, growth and saturation of thermoacoustic instabilities is proposed. The two cornerstones of this new framework are the acoustic absorption coefficient Δ and the cycle-to-cycle acoustic energy ratio λ, both of which do not require computing integrals. Used along with a suitable acoustic network model, where the flame frequency response is described using the weakly nonlinear Flame Describing Function (FDF) formalism, these two dimensionless numbers are shown to characterize: 1) the variation of acoustic energy stored within the combustor between two consecutive cycles, 2) the acoustic energy transfers occurring at the combustor’s boundaries and 3) the sources and sinks of acoustic energy located within the combustor. The acoustic energy balance of the well-documented Palies burner is then analyzed during the onset, growth and saturation of thermoacoustic instabilities using this new methodology. It is demonstrated that this new approach allows a deeper understanding of the physical mechanisms at play. For instance, it is possible to determine when the flame acts as an acoustic energy source or sink, where acoustic damping is generated, and if acoustic energy is transmitted through the boundaries of the burner.

In this paper, we propose a two-step methodology to evaluate the convective heat flux along the rotor casing using an engine-scalable approach based on Discrete Green’s Functions. The first step consists in the use of an inverse heat transfer technique to retrieve the heat flux distribution on the shroud inner wall by measuring the temperature of the outside wall; the second step is the calculation of the convective heat flux at engine conditions, using the experimental heat flux and the Green Functions engine-scalable technique. Inverse methodologies allow the determination of boundary conditions, in this case the inner casing surface heat flux, based on measurements from outside of the system, which prevents aerothermal distortion caused by routing the instrumentation into the test article. The heat flux, retrieved from the inverse heat transfer methodology, is related to the rotor tip gap. Therefore, for a given geometry and tip gap, the pressure and temperature can also be retrieved. In this work, the Digital Filter Method is applied in order to take advantage of the response of the temperature to heat flux pulses. The Discrete Green’s Function approach employs a matrix to relate an arbitrary temperature distribution to a series of pulses of heat flux. In the present procedure, the terms of the Green’s Function matrix are evaluated with the output of the inverse heat transfer method. Given that key dimensionless numbers are conserved, the Green’s Functions matrix can be extrapolated to engine-like conditions. A validation of the methodology is performed by imposing different arbitrary heat flux distributions, to finally demonstrate the scalability of the Green’s Function Method to engine conditions. A detailed uncertainty analysis of the two-step routine is included based on the value of the pulse of heat flux, the temperature measurement uncertainty, the thermal properties of the material and the physical properties of the rotor casing.

The power output of steam turbines is controlled by steam turbine inlet valves. These valves have a large flow capacity and dissipate a huge amount of energy in throttled operation mode. The dissipation process generates strong pressure fluctuations and leads to high dynamic forces potentially causing valve vibrations. A brief survey of the literature dealing with valve vibrations reveals that vibrational problems and damages mostly occur in throttled operation when jets, shocks and shear layers are present. Previous investigations of the authors reveal a feedback mechanism between the dynamic flow field and the vibrating valve plug. Depending on the flow topologies either axial or lateral forces will dominate the force spectrum. In this paper the design of a test rig including a scaled model of a steam valve is described. As it is difficult to analyse lateral forces in conventional experiments the model is designed not only to study the flow conditions but also the lateral and axial movement of the valve plug. To investigate and model the dynamic characteristic of the valve the entire periphery including the mechanical drive, sealing etc. needs to be considered. To ensure that fluid–structure-interactions are correctly scaled, dimensionless numbers derived with the Buckingham Pi Theorem are used as design criteria. Positions of the transmitters are selected based on results of numerical simulations.

Lean premixed single-stage combustion is state of the art for low pollution combustion in heavy-duty gas turbines with gaseous fuels. The application of premixed jets in multi-stage combustion to lower nitric oxide emissions and enhance turndown ratio is a novel promising approach. At the Lehrstuhl für Thermodynamik, Technische Universität München, a large scale atmospheric combustion test rig has been set up for studying staged combustion. The understanding of lift-off behavior is crucial for determining the amount of mixing before ignition and for avoiding flames anchoring at the combustor walls. This experiment studies jet lift-off depending on jet equivalence ratio (0.58–0.82), jet preheat temperature (288–673 K), cross flow temperature (1634–1821 K) and jet momentum ratio (6–210). The differences to existing lift-off studies are the high cross flow temperature and applying a premixed jet. The lift-off height of the jet flame is determined by OH* chemiluminescence images, and subsequently, the data is used to analyze the influence of each parameter and to develop a model that predicts the lift-off height for similar staged combustion systems. A main outcome of this work is that the lift-off height in a high temperature cross flow cannot be described by one dimensionless number like Damköhler- or Karlovitz number. Furthermore, the ignition delay time scale τ ign also misses part of the lift-off height mechanism. The presented model uses turbulent time scales, the ignition delay and a chemical time scale based on the laminar flame speed. An analysis of the model reveals flame stabilization mechanisms and explains the importance of different time scale.

A comparative study of the influence of different means of turbine cooling on the thermodynamic efficiency and specific work of gas turbines is presented. A common general model of a simple open cycle gas turbine is used to compare the performance of turbines using different types of cooling; internal convection and impingement by air, film cooling by air, internal convection and impingement by steam, film cooling by steam and closed loop cooling by water. The results are also compared to the previously published results of the analysis of open loop water cooled gas turbines. The model evaluates the efficiency and specific work of simple cycle gas turbines as it is influenced by mixing losses of coolant with combustion gases, pumping work of coolant and heat transfer from the expanding gas. The study is performed in terms of dimensionless variables in order to achieve generality and to provide useful design guidelines and insights. Blades internally cooled by convection and impingement are treated as heat exchangers operating at constant metal temperature and the coolant exit temperature is simply expressed as a function of a heat exchanger effectiveness, an independent parameter which is normally a function of the intricacy of the layout of the cooling passages. The coolant requirements and heat transfer with film cooling are determined using a dimensionless correlation derived experimentally at M.I.T. Sample calculations give the optimum turbine inlet temperature of thermodynamic efficiency and specific work for different pressure ratios and typical dimensionless numbers. The data on specific work are significant because they can be readily used in evaluations of a given type of gas turbine in a combined cycle. The sensitivity of the efficiency and specific work to each key input parameter is reported. The use of superheated steam as a coolant can provide some performance advantages since the steam raised in a waste heat boiler expands with the combustion gases, increases the turbine mass flow and also provides a certain amount of heat regeneration. Performance results are also reported for this steam cooled gas turbine operating with mixed working fluid.

Hydraulic seals are used in aero engines because of their excellent sealing properties. Sealing of oil inside bearing chambers is extremely important as leakage of oil into internal spaces of the engine increases the oil consumption and can result in undesirable effects, ranging from cosmetic to mechanical. A robust dimensioning of the seal is therefore essential. However, the maximum pressure capacity of the hydraulic seal is not always determined accurately enough with many of the existing design approaches, so a high safety factor must be used. It is desirable to keep improving the accuracy of these methods, in particular to handle ever larger pressure differences. A new dimensionless design method is therefore introduced here to improve the determination of the maximum pressure capacity. This paper reports on a numerical CFD investigation using an axisymmetric Volume-of-Fluid (VOF) method building on the work of Young and Chew [1]. The numerical results are validated with the results of a two-shaft test rig, alongside analytical calculation results. Additionally, a parametric study based on CFD simulations is performed to identify dominant influence quantities. The parameters include the fluid properties of oil, the shaft speeds and the geometry parameters of the seal. Employing a data reduction approach, a new dimensionless number is introduced which allows the presentation of experimental and numerical results of the hydraulic seal in a dimensionless form. Based on this representation, a correlation is proposed, which shows a very promising trend. This validated CFD investigation and subsequent correlation introduced here show significant potential for the dimensionless description of hydraulic seals and their maximum pressure capacity.

To characterize contactless seals in turbo machinery, their discharge behavior, the development of the circumferential velocity (swirl) and the loss induced total temperature increase (windage heating) are of special interest for the designer. For the discharge behavior of non-rotating labyrinth seals, a well established set of non-dimensional numbers already exists: the discharge coefficient of two seals with different sizes but similar geometry is identical, if pressure ratio, axial Reynolds number, fluid properties and turbulence level are also identical. In this paper, the set of non-dimensional numbers is extended to cover swirl and windage heating using the well established Buckingham-π theorem to derive possible candidates. First, as a proof of concept, the known set of numbers for the non-rotating case was redeveloped and subsequently the influence of rotation was included. To validate the candidates, a comprehensive numerical parametric study was conducted. A variety of convergent and divergent stepped labyrinth seals was scaled from laboratory to typical engine conditions such that the dimensionless numbers stayed constant. Then, simulations at different rotational speeds, radii, and inlet circumferential velocities were performed to investigate the effects of rotation while maintaining nearly constant discharge behavior. The numerical data were used to validate the new non-dimensional numbers and to derive laws for the scaling of labyrinth seals. The non-dimensional numbers can also be applied to other seal types, such as brush or finger seals, because their theoretical deduction does not imply a specific geometry.