The resonant coupling between flames and acoustics is a growing issue for gas turbine manufacturers. They can be reduced by adding acoustic dampers on the combustion chamber walls. Nonetheless, if the engine is operated out of the stable window, the damper may be exposed to high-amplitude acoustic levels, which may trigger unwanted nonlinear effects. This work aims at providing an overview of the dynamics associated with those limit cycles using a simple analytical model, where a perfectly tuned damper is coupled to the combustion chamber. The damper, crossed by a purge flow in order to prevent hot gas ingestion, is modeled as a non-linearly damped harmonic oscillator, with vortex shedding as the main dissipation mechanism. The combustion chamber featuring a linearly unstable thermoacoustic mode is modelled as a Van der Pol oscillator. The fixed points of the coupled system and their stability can be determined by analyzing the averaged amplitude equations. This allows the computation of a fixed point topology map as function of the growth rate of the unstable mode and the mean velocity through the damper neck. Simulink simulations are also performed and compared to the analytical predictions. Finally, experiments are performed on a simple rectangular cavity, where the thermoacoustic instability resulting from the interaction between heat release and acoustic pressure is mimicked by an electro-acoustic instability. A feedback loop is built, where the signal from a microphone is filtered, delayed, and amplified before being sent to a loudspeaker placed inside the rectangular cavity. The delay and gain of the feedback loop can be modified to change the growth rate of the instability. One Helmholtz damper can be added to the cavity and tuned to the unstable mode of interest. The growth rate reduction capabilities of the damper and the amplitude of the limit cycle in the unstable cases are in good agreement with the analytical and numerical predictions, underlining the potentially dangerous behavior of the limit cycles which should be taken into account for real engine cases.

The need to study flow and heat transfer in turbine blade cooling design calls to develop appropriate modelling approaches able to return accurate predictions at a reduced computational costs. Here we propose and scrutinize a quadratic version of the well-known k-ε-ζ-f RANS turbulence models, aiming at sensitizing the model to the effect of rotation in configurations mimicking the flow in turbine internal cooling. Starting from the evidence that rotation modified turbulent flow through a turbulence suppression (enhancement) on the stabilized (destabilized) surface, we modified the Cμ coefficient present in the formulation of turbulent viscosity introducing a dependence on the strain and vorticity tensors, the latter explicitly including solid body rotation. The proposed model was tested on plane channel and square-sectioned duct flows, and then used for simulating a rib-duct rotating channel. Results are assessed against DNS literature data and properly developed LES computations, by examining flow variables, heat transfer and turbulence budgets. We demonstrate that, as for the channel flows, the proposed quadratic model is able to accurately reproduce velocity, temperature and turbulent variables at various angular velocity regimes. In the duct flow the flow is subjected to the mutual influence vorticity induced by rotation and turbulence anisotropy developing close the walls. In particular, the non-linear rotation-sensitized model is able to reproduce the near-wall turbulent kinetic energy distribution close to the suction side, returning a zero value in the mid-span and a small peak close to the wall on the suction side. Turbulent kinetic energy and temperature budgets analysis demonstrates the capabilities of the model in describing all the terms in the equations. Also if some tuning of the model is required, these analysis showed very encouraging results. In fact if the basic mechanisms of turbulence and heat transfer are properly predicted, then it can be expected that the model can be successfully applied to a set of different cases. For such reason, the model was applied to the analysis of flow and heat transfer in a rotating ribduct with reasonably results.

Truncating the exit of a discrete passage centrifugal compressor diffuser is observed to enhance a research compressor’s stall line. By interrogating the experimental data along with a set of well-designed Reynolds-Averaged Navier Stokes computations, this improvement is traced to reduced impact of secondary flows on the truncated diffuser’s boundary layer growth. The secondary flow system is characterized by counter-rotating streamwise vortex pairs that persist throughout the diffuser passage. The vortices are traced to two sources: background vortices resulting from impeller exit flow non-uniformity, and incidence vortices resulting from flow separation off the leading edge cusps unique to a discrete passage diffuser. The incidence vortices detrimentally impact the diffuser pressure rise capability by accumulating high loss flow along the diffuser wall near the plane of symmetry between the vortices. This contributes to a large passage separation in the baseline diffuser. Using reduced order flow modeling, the impact of the vortices on the boundary layer growth is shown to scale inversely with diffuser aspect ratio, and thus the separation extent is reduced for the higher aspect ratio truncated diffuser. Because the diffuser incidence angle influences the strength and location of the vortices, this mechanism can affect the slope of the compressor’s pressure rise characteristic and impact its stall line. Stall onset for the baseline diffuser configuration is initiated by the transition of the vortex location and corresponding passage separation between diffuser pressure and suction sides with increased cusp incidence. Conversely, because the extent of the passage separation in the truncated diffuser is diminished, the switch in separation from pressure to suction side does not immediately initiate instability.

The present work aims to investigate cavitating flows of water, liquid hydrogen and nitrogen on hydrofoils numerically, using the open source code OpenFOAM. The Eulerian homogeneous mixture approach has been used, consisting in a mass transfer model which is based on the combination of a two-phase incompressible unsteady solver with a Volume of Fluid (VOF) interface tracking method. Thermal effects have been introduced by means of the activation of energy equation and latent heat source terms plus convective heat source term. The dependency of the saturation conditions to the temperature has been defined using Antoine-like equations. An extended Schnerr-Sauer model based on the Classical Nucleation Theory (CNT) has been implemented for the computation of the interfacial mass transfer rates. In order to investigate the nucleation effects, an extension of the Classical Nucleation Theory has been considered by coupling the Population Balance Equation/Extended Quadrature-Based Method of Moments (PBE-EQBMM) with the CFD model, which has been defined in combination with a transport equation for the nuclei density. Results showed that nucleation determined a non-uniform field of nuclei density so as to produce a reduction of the temperature drop inside the vapor bubbles, as well as a warmed wake downstream the vapor cavity. Unsteady computations also revealed an influence of the nucleation on the dynamics of the vapor cavity and the bubble detachment.

In an attempt to validate a Large Eddy Simulation (LES) approach, computations of a transonic centrifugal compressor with a backswept, unshrouded impeller followed by radial and axial vaned diffusers are performed. A sector composed of one main blade and one splitter blade, two radial diffuser vanes and six axial diffuser vanes is simulated including all the technological effects of the experimental rig. The LES methodology to simulate the rotor/stator configuration is introduced. Emphasis is put on the best trade-off between accuracy of the simulation and affordable CPU cost. A law-of-the-wall boundary condition is used to reduce the mesh size, with a target of y+ around a hundred for all walls except in the tip leakage with y+ around five. Computation of one entire characteristic line is obtained continuously in time: the transient from the flow at rest to the converged points at blockage, peak efficiency, near surge and path to deep surge is computed increasing progressively the outlet pressure as in the experiments. First, LES results are compared to experiments and show excellent agreement both in terms of overall performance and time-averaged internal flow fields previously obtained by Laser Doppler Anemometry. Then, a focus is proposed on the complementary information LES provide in the rotor. The key findings are that contrary to previous URANS studies in this centrifugal compressor, LES capture influential details of the flow structures in the rotor: secondary structures, shock/boundary layer interaction and boundary layer separation at the tip of the impeller. Moreover, it is clearly shown that the tip leakage vortex increases in size and intensity from peak efficiency to surge and becomes much more erratic. Emphasis is put on the causes and consequences of the tip leakage spillage in the neighbouring rotor channels. Pressure fluctuations were also found to increase from peak efficiency to surge downstream the splitter blade leading edge. The whole results finally show that LES with a law-of-the-wall provides excellent results in such a complex case.

Modern trend in installation design is moving towards very high-bypass ratio turbofans. Very high-bypass turbofans represent an effective way of improving the propulsive efficiency of civil aero-engines. Such engines require larger and heavier nacelles, which partially offset the gains in specific fuel consumption. The penalty associated with a larger installation can be mitigated by adopting thinner walls for the nacelle and by shortening the intake section. Such inlet sections are characterized by more restrictive operation condition because they are more prone to separation at high incidence flight conditions. Moreover, in short nacelle installations the by-pass guide vanes and pylon are closer to the fan blades and consequently the distortion due to potential effects induced by the presence of the pylon and non-axisymmetric OGV stage play a significant role in terms of unsteady interaction in the entire system. It is mandatory to consider the inlet, fan, bypass and pylon as a unique coupled system also at the design stage, for assessment of fan force. This kind of assessment is usually carried on by expensive URANS calculation. The factors leading to high computational demands are the spatial resolution required in the fan domain and the time resolution required to sample the fan blade passing frequency. Large savings are therefore possible if simplifications are introduced which relax the resolution requirements in the fan passages and change the nature of the computation into a steady-state computation for the ducts. The present contribution documents a simplified fan model for fan-intake computations based on the solution of the double linearization problem for unsteady, transonic flow past a cascade of thin aerofoils with finite mean load. The coupling with the intake flow and the bypass is performed by using the flow patterns at fan face and fan exit as boundary conditions for the fan model and computing circumferentially non-uniform boundary conditions for the intake and the bypass from the fan model. The computation of the flow in the intake, bypass and pylon is therefore reduced to a steady problem, whereas the computation of the flow in the fan is reduced to one steady problem and a set of linearised models in the frequency domain. The model is applied to a well-documented test case and compares favourably with experimental data and much more expensive three-dimensional, time domain computations.

The aim of the present work is to facilitate insight into the modeling errors in the context of blade row coupling approaches which capture unsteady flow phenomena at different levels of detail. The focus is on RANS-based steady mixing plane computations as well as time domain and frequency domain unsteady computations. The concept of mixing loss is revisited to quantify the amount of unsteadiness in a flow field. Following an idea by Fritsch and Giles, we compute a second order approximation of the mixing losses which are generated at blade row interfaces. The resulting formula decomposes the entropy jump into contributions of circumferential and temporal fluctuations. The mathematical derivation, however, is based upon simpler arguments. It is shown that Fritsch and Giles’ main result can be extended to non-ideal gases. Moreover, the second order mixing loss formula is applied to time and frequency domain unsteady simulations. It is shown that an additional term has to be computed which accounts for the interaction of evanescent acoustic modes if the method is applied to unsteady flows. The methodology decomposes the overall mixing entropy into contributions of single perturbation types and harmonics. This may be used to assess whether unsteady flow phenomena of interest are adequately resolved and, in particular, to quantify the unsteadiness contained in the unresolved harmonics. A detailed investigation of the transonic IGV-rotor configuration of DLR’s Rig 250 compressor demonstrates the approach.

The paper presents the results of measurements of flow parameters behind the last stage of a 1090 MW nominal power steam turbine in a nuclear power plant. The results were obtained by traversing a pneumatic probe at a distance of about 100 mm from the trailing edges of the LSB (Last Stage Blade). Furthermore, both side walls as well as the front wall of one flow of the LP (Low Pressure) exhaust hood were fitted with a dense net of static pressure taps at the level of the flange of the turbine. A total of 26 static pressures were measured on the wall at the output from the LP exhaust hood. Another 14 pressures were measured at the output from the condenser neck. The distribution of static pressures in both cross sections for full power and 600 and 800 MW power is shown. Another experiment was measured pressure and angle distribution using a ball pneumatic probe in the condenser neck area in a total of four holes at a distance up to 5 metres from the neck wall. The turbine condenser is two-flow design. In one direction perpendicular to the axis of the turbine cold cooling water comes, it heats partially. It then reverses and it heats to the maximum temperature again. The different temperature of cooling water in the different parts of the output cross section should influence the distribution of the output static pressure. Differences in pressures may cause problems with uneven load of the tube bundles of the condenser as well as problems with defining the influential edge output condition in CFD simulations of the flow of the cold end of the steam turbine Due to these reasons an extensive 3D CFD computation, which includes one stator blade as well as all moving blades of the last stage, a complete diffuser, the exhaust hood and the condenser neck, has been carried out. Geometry includes all reinforcing elements, pipes and heaters which could influence the flow behaviour in the exhaust hood and its pressure loss. Inlet boundary conditions were assumed for the case of both computations from the measurement of the flow field behind the penultimate stage. The outlet boundary condition was defined in the first case by an uneven value of the static pressure determined by the change of the temperature of cooling water. In the second case the boundary condition in accordance with the measurement was defined by a constant value of the static pressure along all the cross section of the output from the condenser neck. Results of both CFD computations are compared with experimental measurement by the distribution of pressures and other parameters behind the last stage.

Modernization of steam turbine components can extend the life of a power plant, decrease maintenance costs, increase service intervals and improve operational flexibility. However, this can also lead to challenging demands for existing components such as bearings, e.g., due to increased rotor weights. Therefore, a careful design and evaluation process of bearings is of major importance. This paper describes the advanced modeling methods applied for the optimization of a novel 900 mm three-pad tilting pad journal bearing followed by validation results that showed a high bearing temperature sensitivity to the fresh oil supply temperature during operation. The bearing was especially developed to cope with increased rotor weights within the framework of low pressure steam turbine modernizations at two similar 1000 MW nuclear power plants. With a static bearing load of approximately 2.7 MN at a rotor speed of 1500 rpm, it represents one of the highest loaded applications for tilting pad journal bearings in turbomachinery worldwide. After identification of the reasons for the sensitivity, advanced modeling methods were applied to optimize the bearing. For this purpose, a more comprehensive bearing model was developed taking into account the direct lubrication at the leading edge of the pads and the thermo-mechanical pad deformation. For the latter, a co-simulation between the bearing computation code and structural mechanics software was performed. The results of the entire analyses indicated modifications of bearing and pad clearance, pad pivot position, circumferential and axial pad length as well as pad thickness. Furthermore, the oil distribution into the pads was optimized by modifying the orifices within the bearing. The optimized bearing was then implemented on both units and proved its excellent operational behavior at increased fresh oil supply temperatures of up to 55°C. In addition, inspections during scheduled outages after 18 months of operation and subsequent restarts with reproducible bearing behavior confirmed the robustness of the optimized bearing. In conclusion, the application of advanced modeling methods proved to be the key success factor in the optimization of this bearing, which represents an optimal solution for large steam turbine and generator rotor train applications.

This paper introduces an improvement of the stress calibration methodology of stator blades to consider a nodal diameter of interest of the structure. The proposed calibration procedure and the search for optimal excitation set-up are detailed. To this purpose, the following points are addressed. Experimental modal analyses are performed using both accelerometers and strain gages. Post-processing techniques are developed to determine the nodal diameters of the identified modes. SAFE (Singh’s Advanced Frequency Evaluation) diagrams are computed from the experimental data and compared with the diagrams obtained numerically by finite elements computations. Multiple excitations are used to appropriate the targeted modes. A comparative study of different shaker types, of the number of excitation points and of their location is performed. Calibration is achieved by comparing strain measurements taken on one gage installed on the engine and velocities measured using laser vibrometers. It allows reducing the impact of the instrumentation on the modal content. The calibration factors obtained by the novel proposed procedure are compared to the ones given by the currently used methodology.

Today, Vortex Generators (VGs) are becoming an integral part of a Wind Turbine blade design. However, the challenges that are involved in the computation of the flow around VGs are yet to be dealt with in a satisfactory manner. A large number of VG models for flow solvers have been proposed and among them, the BAY model is one of the most popular for its ease of use and relatively low requirements for user input. In the present paper, a thorough investigation on the performance and application of the BAY model for aerodynamic Vortex Generator flows is presented. A Fully Resolved Reynolds Averaged Navier Stokes simulation is validated against experiments and then used as the benchmark for the BAY model simulations. The Benchmark case is the flow past a wind turbine airfoil at Reynolds number 0.87e6. When the grid related errors are excluded, it is found that in the model simulations, the generated vortices are weaker than in the fully resolved computation. The latter finding is linked to an inherent deficiency of the model, which is explained in detail. As the vortex generation mechanism is different between the fully resolved and the BAY model simulation, so is the vortex evolution and interaction, even on the same numerical mesh. With regards to grid dependence, the integral BAY force depends on both grid density and grid architecture.

The effects of heat transfer between the compressor structure and primary gas path flow on compressor stability are investigated during hot engine re-acceleration transients, or so called “Bodie” transients. A mean line analysis of an advanced, high-pressure ratio compressor is extended to include the effects of heat transfer on both stage matching and blade row flow angle deviation. A lumped capacitance model is used to compute the heat transfer of the compressor blades, hub, and casing to the primary gas path. The inputs to the compressor model with heat transfer are based on a combination of full engine data, compressor test rig measurements, and detailed heat transfer computations. Transient calculations with heat transfer show a 8.0 point reduction in stall margin from the adiabatic case, with heat transfer predominantly altering the transient stall line. 3.4 points of the total stall margin reduction are attributed to the effect of heat transfer on blade row deviation and the remainder is attributed to stage re-matching. It is found that heat transfer increases loading in the front stages and destabilizes the front block. Furthermore, the stage re-matching due to heat transfer alters the slope of the compressor characteristic and promotes modal-type stall inception. Sensitivity studies show a strong dependence of stall margin to heat transfer magnitude and flow angle deviation at low speed, due to the effects of compressibility. Computations for the same transient using current cycle models with bulk heat transfer effects, such as NPSS, only capture 1.2 points of the 8.0 point stall margin reduction. This implies that, using this new capability, opportunities exist early in the design process to address potential stability issues due to transient heat transfer.

In high-pressure turbines, cool air is purged through rim seals at the periphery of wheel-spaces between the stator and rotor discs. The purge suppresses the ingress of hot gas from the annulus but superfluous use is inefficient. In this paper the interaction between the ingress, purge and mainstream flow is studied using unsteady numerical simulations based on the DLR TRACE solver. The computations are compared to experimental measurements from a one-and-a-half stage axial-turbine rig operating with engine-representative blade and vane geometries, and overlapping rim seals. Radial traverses using a miniature CO 2 concentration probe quantified the penetration of ingress into the rim seal and the outer portion of the wheel-space. Unsteady pressure measurements from circumferentially-positioned transducers on the stator disc identified distinct frequencies in the wheel-space, and the computations reveal these are associated with large-scale flow structures near the outer periphery rotating at just less than the disc speed. It is hypothesised that the physical origin of such phenomenon is driven by Kelvin-Helmholtz instabilities caused by the tangential shear between the annulus and egress flows, as also postulated by previous authors. The presence and intensity of these rotating structures are strongly dependent on the purge flow rate. While there is general qualitative agreement between experiment and computation, it is speculated that the under-prediction by the computations of the measured levels of ingress is caused by deficiencies in the turbulence modelling.

Impinging jet arrays are typically used to cool several gas turbine parts. Some examples of such applications can be found in the internal cooling of high pressure turbine airfoils or in the turbine blade tip clearances control of aero-engines. The effect of wall-to-jets temperature ratio on heat transfer is generally neglected by the correlations available in the open literature. In present contribution, the impact of the temperature ratio on the heat transfer for a real engine Active Clearance Control system, is analyzed by means of validated CFD computations. At different jets Reynolds number and considering several impingement array arrangements, a wide range of target wall-to-jets temperature ratio (TR) is accounted for. Computational results prove that both local and averaged Nusselt numbers reduce with increasing temperature ratios. An in-depth analysis of the numerical data show that the last mentioned evidence is motivated by both the heat transfer incurring between the spent coolant flow and the fresh jets and the variation of gas properties with temperature through the boundary layer. A scaling procedure, based on TR power law, was proposed to estimate the Nusselt number at different wall temperature levels necessary to correct available open-literature correlations, typically developed with small temperature differences, for real engine applications.

Many laboratory-scale combustors are equipped with viewing windows to allow for characterization of the reactive flow. Additionally, pressure housing is used in this configuration to study confined pressurized flames. Since the flame characteristics are influenced by heat losses, the prediction of wall temperature fields becomes increasingly necessary to account for conjugate heat transfer in simulations of reactive flows. For configurations similar to this one, the pressure housing makes the use of such computations difficult in the whole system. It is therefore more appropriate to model the external heat transfer beyond the first set of quartz windows. The present study deals with the derivation of such a model which accounts for convective heat transfer from quartz windows external face cooling system, free convection on the quartz windows 2, quartz windows radiative properties, radiative transfer inside the pressure housing and heat conduction through the quartz window. The presence of semi-transparent viewing windows demands additional care in describing its effects in combustor heat transfers. Because this presence is not an issue in industrial-scale combustors with opaque enclosures, it remains hitherto unaddressed in laboratory-scale combustors. After validating the model for the selected setup, the sensitivity of several modeling choices is computed. This enables a simpler expression of the external heat transfer model that can be easily implemented in coupled simulations.

The regenerative cooling technology has become the most effective method to reduce the high-temperature of the scramjet engine. With physical and chemical heat sink, the endothermic hydrocarbon fuel has excellent performance in the regenerative cooling system of the scramjet engine which operates under extremely high temperature. The pyrolytic reactions not only absorb a large amount of heat, but also produce some kinds of coking precursors, mainly alkenes and aromatics. Because of the coking precursors and the coking reactions, a lot of coke would be generated on the wall and exert strong impact on the heat transfer, as the conductivity of the coke is much lower than that of the metal wall. Meanwhile, the surface coking changes the geometric parameters of the cooling tube, which leads to the flow field variations with the thickening coking layer. So, it is needed to find out the interaction between these variations. In this paper, a one-dimensional (1D) model has been developed to calculate the flow and heat transfer parameters distributions of the pyrolytically reacted RP-3 along the regenerative cooling tube with the pyrolytic coking. The 24-step pyrolytic reaction model and the coking kinetic model are applied to predict the pyrolysis and pyrolytic coking process of RP-3, with accurate computations of the physical properties of fluid mixture which undergo drastic variations during the transcritical process. Comparisons between the current predictions and the open published experimental data are carried out and good agreement is achieved. Calculations on the coupling relationships between the flow, heat transfer, pyrolysis and pyrolytic coking within 20 min in the circular tube have been conducted. With the heat flux increased, the coke mass is rising sharply and the temperature of the outer tube wall rises rapidly owing to the increasing thermal resistance of the coke layer. Moreover, the flow velocity becomes faster during the narrowing process of the tube caused by surface coking. In order to better understand the coking characteristics, further investigations on distributions of the surface coking under heat fluxes of 1.2–2.0 MW / m 2 , pressures of 2.6–7.4 MPa and with inlet velocities of 0–5 m / s have been performed. Results reveal that all these factors play an important role in the pyrolytic reactions and the coking rate distributions. The results in this paper have significant reference value in the design of the regenerative cooling system.

Axial fans are widely used in modern industry and new regulations and stringent environmental concerns are prompting manufacturer to design efficient low-noise axial fans. This paper is focused on improving the aerodynamic performances and reducing the tonal noise at BPF and its harmonics by the optimum choice of lean-swept blade and the stacking line for the low-speed axial fan. The aerodynamic characteristics of the axial fan with a shroud are explored by CFD with ANASYS CFX. A hybrid method, SST turbulence model for flow and FW-H equation for acoustics, is chosen to predict the radiated noise. The accuracy and reliability of predicted aerodynamic and aeroacoustics results are verified by comparing both computation and experimental data. A number of modified blades with different leaned angle, swept angle and the stacking lines are modeled and analyzed, and the investigation into the optimum choice of lean-swept blade and the stacking line is conducted according to aerodynamic performances and tonal noise. Q-criterion which can visualize the major flow disturbances is applied for the purpose of identification of acoustic sources. The turbulent flow structures on the leading edge, tip and suction side of the blade are main noise sources. An optimal modification is determined through the analysis of the aerodynamic performances and noise, which is to achieve the desired performances by blade sweep and lean and adjusting the stacking line. The results show that aerodynamic and acoustic performances of the optimized fan are better than that of the original fan and the improvement is more obvious to change the stacking line with centre of gravity compare to blade sweep and lean for the low-speed axial fan.

The modern compressor operation is challenged by the liquid presence in wet gas operating conditions. The liquid phase may affect the compressor stability by partially flooding the internal annular gas seals and inducing subsynchronous vibration. To improve the annular seal behavior and increase rotor stability, high-precision results of leakage flow rates and rotordynamic force coefficients are needed for annular gas seals in wet gas conditions. In order to better understand the leakage and rotordynamic characteristics of the annular gas seal in wet gas conditions, a 3D transient CFD-based perturbation method was proposed for computations of leakage flow rates and rotordynamic force coefficients of annular gas seals with liquid phase in main gas phase, based on inhomogeneous Eulerian-Eulerian multiphase flow model, mesh deformation technique and the multi-frequency rotor whirling orbit model. Numerical results of frequency-dependent rotordynamic force coefficients and leakage flow rates were presented and compared for three types of non-contact annular gas seals, which include a smooth plain annular seal (SPAS), a labyrinth seal (LABY) and a fully-partitioned pocket damper seal (FPDS). These three seals were designed to have the identical rotor diameter, sealing clearance and axial length. The accuracy and availability of the present transient CFD numerical method were demonstrated with the experiment data of leakage flow rates and frequency-dependent rotordynamic force coefficients of the smooth plain seal with four inlet liquid volume fractions (LVF) of 0%, 2%, 5% and 8%. Steady and transient numerical simulations were conducted at inlet air pressure of 62.1 bar, pressure ratio of 0.5, rotational speed of 15 000 rpm and inlet preswirl ratio of 0.3 for four inlet LVFs varying from 0% to 8% and fourteen subsynchronous and synchronous whirling frequencies up to 280 Hz. The numerical results show that inlet liquid phase has a significant influence on the leakage and rotordynamic coefficients for all three types of annular gas seals. The mixture leakage flow rate increases with the increasing inlet LVF, combining the decreasing gas-phase and linearly increasing liquid-phase leakage flow rates. The smooth plain seal leaks the most gas phase and liquid phase, followed by the pocket damper seal and then the labyrinth seal. Increasing inlet LVF significantly decreases the direct stiffness and slightly increases the effective damping of the smooth plain seal. The labyrinth seal possesses evident negative direct stiffness and shows a noticeable decreasing effective damping with the increasing inlet LVF at the subsynchronous frequency range. Increasing inlet LVF obviously increases all the force coefficients of the pocket damper seal including the positive effective damping. From a rotordynamic viewpoint, the FPDS possesses a better liquid tolerant capability and so is a better sealing scheme for the balance piston seals and center seals of the centrifugal compressor in wet gas operating condition.

Heat exchange in air-cooled condensers (ACC) is achieved by forced convection of fresh air on bundle of tubes by means of forced-draft axial-flow fans. These fans are characterized by low solidity and low hub ratio, large diameters, relatively low rotational velocity, high efficiencies. This combination usually leads to fans with non-stalling characteristics, with pressure rise continuously rising when reducing the flow rate, at least in standard (ISO or AMCA) test rigs. In real-life installations, in fact, it is quite difficult to characterize these fans, due to the practical difficulties arising in setting up a proper test rig and to control the boundary conditions of the system, in particular the fan inflow conditions. Here we focus on a real-life setting of ACC, numerically simulated with URANS. In this work the fan is simulated with a Synthetic Blade Model presented in [1]. This model is derived from actuator disk theory, and allows to simulate the unsteady movement of the blades and compute a non-constant azimuthal distribution of lift and drag forces, partially accounting for non-constant deviation in the blade-to-blade passage, while drastically reducing the mesh requirements. In this way it is possible to model the shedding of wakes behind the blades and their interaction with the heat exchanger. The flow will be assumed to be incompressible, due to the low Mach number and heat transfer will be treated assuming temperature to be a passive scalar convected by the flow. Duty point of the fan and heat exchange in the ACC will be studied while inflow conditions, in order to account for free inflow with a constant velocity distribution as well as distortions due to lateral wind. Computations will be carried out on the Virtual Test Rig of developed at Sapienza within the OpenFOAM 2.3.x library with a URANS approach and k-ε closure.

It is a challenge to simulate the flow in a Variable Speed Power Turbine (VSPT), or, for that matter, rear stages of low pressure turbines at low Reynolds numbers due to laminar flow separation or laminar/turbulent flow transition on the blades. At low Reynolds numbers, separation induced-transition is more prevalent which can result in efficiency lapse. LES has been used in recent years to simulate these types of flows with a good degree of success. In the present work, very low free stream turbulence flows at exit Reynolds number of 220k were simulated. The geometry was a cascade which was constructed with the midspan section of a VSPT design. Most LES simulations to date, have focused on the midspan region. As the endwall effect was significant in these simulations due to thick incoming boundary layer, full blade span computation was necessitated. Inlet flow angles representative of take-off and cruise conditions, dictated by the rotor speed in an actual design, were analyzed. This was done using a second order finite volume code and a high resolution grid. As is the case with Implicit-LES methods, no sub-grid scale model was used. Blade static pressure data, at various span locations, and downstream probe survey measurements of total pressure loss coefficient were used to verify the results. The comparisons showed good agreement between the simulations and the experimental data.