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The Moderate or Intense Low-oxygen Dilution (MILD) combustion is a promising technique to reduce pollutant emissions of combustion processes, especially nitrogen oxides. This combustion mode involves Turbulence-Chemistry Interaction (TCI), which constitutes a challenge in terms of numerical simulation since it must be properly captured. Up to now, the TCI has been modelled and the corresponding models generally involve coefficients, leading to epistemic uncertainties and, therefore, to different numerical results depending on the used model. The study presented in this paper aims to assess the relevance of performing Large Eddy Simulation of a typical Jet-in-Hot-Coflow flame, simulating diluted combustion, assuming that the TCI is directly resolved, given the grid and the chemical kinetics resolutions. Avoiding TCI modelling allows for lower numerical uncertainties. However, simulations without TCI modelling will normally fail for other types of flames and for higher Reynolds-numbers, so that such simulations can normally only be conducted using TCI modelling. Here, the simulations are performed using Finite Rate Chemistry without TCI model on the Adelaide Jet-in-Hot-Coflow flame. First, the proposed methodology was experimentally validated, highlighting that the obtained reacting results are consistent in terms of temperature and mass fractions with the measurements. Additionally, the results obtained with the “TCI-resolved” assumption are compared to the results obtained using a classical TCI model, the Partially Stirred Reactor model. Moreover, the validity of the approach, consisting in directly resolving the TCI, is assessed based on an analysis of the local Damkohler number A large part of the mesh cells presents a very low Damkohler number, confirming that TCI modelling is not required for the burner under consideration.

The growing share of renewable energies in our electricity production, together with the still lacking storage capacity, strongly reinforces the need for more flexible electricity production units. In this context, combined cycle gas turbines (CCGTs) have a role to play, both in the current and future electricity production system due to their high efficiency, high load flexibility, and low CO2 emissions compared to other conventional thermal power plants. Nevertheless, bearing in mind our current challenges concerning climate change, the CO2 emissions of these CCGTs need to be reduced drastically. The amine-based absorption carbon capture (CC) process is currently the most mature and applicable CC technology. This process is known to require a considerable amount of thermal energy, degrading plant performance. However, to back-up renewable production, CCGTs will operate most of the time under part-load conditions. The impact of these part-load operations on the CC is still relatively unknown. Within this framework, this study aims to assess the performance of the CC process applied to a typical CCGT under part-load operation using specific simulation models. The CC plant model has been successfully validated against experimental data from a pilot-scale capture facility. Then, the CC plant has been scaled-up to the CCGT scale and the process has been optimized for each operating condition. The simulation results show that the specific reboiler duty increases for part-load operation, while the specific cooling requirements decrease. Moreover, the analysis of the yearly CCGT operation highlights a relative increase in CC energy penalty of 21% for an annual CCGT load factor of 0.5, impacting significantly plant performance. The next step will involve reducing this energy penalty.

Ammonia and hydrogen are promoted as potential energy carriers for centralized energy restitution. This article investigates ammonia/hydrogen/air premixed turbulent combustion, using Large-Eddy Simulations, in an academic atmospheric gas turbine swirled burner. A one-dimensional flame analysis demonstrates the existence of a trade-off in NOX and NH3 emissions for ammonia/hydrogen blends, and the possibility to obtain 1D flame propagation characteristics close to that of a lean methane flame by adjusting the amount of H2. Large-Eddy Simulations of the PRECCINSTA burner exhibit stable combustion, while the optimized trade-off equivalence ratio is pinpointed at φ=1.46 for XH2Fuel=0.46. Corresponding emissions are XNOX≈XNH3≈300 ppmv. Large amounts of hydrogen are found in the exhaust gases, inducing a low combustion efficiency. The flame structure, combustion dynamics, influence of kinetics modelling and mesh resolution are discussed. This work paves the way for future studies, in the perspective of applications to industrial systems., SCOPUS: ar.j, info:eu-repo/semantics/published

As a distributed energy production technology, micro gas turbines (mGTs) offer a great potential for small-scale combined heat and power (CHP) production, adding flexibility to the electricity system. Nevertheless, due to the evident climate change, a very low greenhouse gas (GHG) emission is a fundamental requirement for our future energy systems. For this purpose, combining an mGT with a carbon capture (CC) plant might offer an effective carbon clean energy production. In the literature, several studies are available, addressing individual aspects of this attractive energy solution; however, an in-depth analysis focusing on the energy integration and strategy optimisation of an mGT working in CHP mode with CC has never been performed. This work is the continuation of the previous thermodynamic analysis in which an mGT and a micro Humid Air Turbine (mHAT) have been directly connected with a CC unit without heat recovery. In the current study, the aim is to find the best plant layout and the best operating strategy based on the electrical, thermal and global cycle performance. Results show that the full CHP operation of the mGT offers the highest global efficiency between all plant layouts. Contrary to what may be expected from previous analyses on cycle humidification, the mHAT does not entail a better performance when the turbine cycle and the CC unit are energetically integrated. Direct heat recovery, which reduces the CC thermal demand, is a preferable measure which involves a lower energy degradation. Sankey and Grassmann diagrams are presented to support the numerical results.

Considering the growing interest in Power-to-Fuel, i.e. production of H2 using electrolysis to store excess renewable electricity, combustion-based technologies still have a role to play in the future of power generation. Especially in a decentralized production with small-scale cogeneration, micro Gas Turbines (mGTs) offer great advantages related to their high adaptability and flexibility, in terms of operation and fuel. Hydrogen (or hydrogen enriched methane) combustion is well-known to lead to flame and combustion instabilities. The high temperatures and reaction rates reached in the combustor can potentially lead to flashback. In the past, combustion air humidification (i.e. water addition) has proven effective to reduce temperatures and reaction rates, leading to significant NOx emission reductions. Therefore, combustion air humidification can open a path to stabilize hydrogen combustion in a classical mGT combustor. However accurate data assessing the impact of humidification on the combustion is still missing for real mGT combustor geometries and operating conditions. In this framework, this paper presents a comparison between pure methane and hydrogen enriched methane/air combustions, with and without combustion air humidification, in a typical mGT combustion chamber (Turbec T100) using Large Eddy Simulations (LES) analysis. In a first step, the necessary minimal water dilution, to reach stable and low emissions combustion with hydrogen, was assessed using a 1D approach. The one-dimensional unstretched laminar flame is computed for both pure methane (reference case) and hydrogen enriched methane/air combustion cases. The results of this comparison show that, for the hydrogen enriched combustion, the same level of flame speed as in the reference case can be reached by adding 10% (in mass fraction) of water. In a second step, the feasibility and flexibility of humidified hydrogen enriched methane/air combustion in an industrial mGT combustor have been demonstrated by performing high fidelity LES on a 3D geometry. Results show that steam dilution helped to lower the reactivity of hydrogen, and thus prevents flashback, enabling the use of hydrogen blends in the mGT at similar CO levels, compared to the reference case. These results will help to design future combustor towards more stability.

Computational fluid dynamics represent a powerful tool to assess the performance of a combustor and identify possible issues/instabilities, helping thus e.g. to investigate the impact of advanced cycle modifications on the combustion in mGTs. The steady Reynolds-averaged Navier-Stokes (RANS) approach is still mostly used in this framework. With growing computational power, Large Eddy Simulation (LES) has gained more interest. LES provides higher details concerning flow structures and can better predict possible instabilities, specifically needed for advanced cycle modelling. On the other hand, LES remain rather challenging for real industrial applications. This work aims at providing an answer whether the advantages of LES justify the much higher computational costs. The objective of the present study is thus to assess the combustion performance and emissions of a typical small-scale 3.2 kWe micro gas turbine (mGT), using steady RANS and LES for various fuels. In this framework, a comparison of RANS and LES approaches (two levels of fidelity) is performed on a typical industrial case, to point out the strengths and weakness of each method with regard to industrial and research needs. The results show that both RANS (at a reduced cost) and LES can accurately predict the time-averaged trends of the main performance parameters, like temperature levels and emissions, also using various non-conventional inlet conditions. For the accurate prediction of the instabilities, the LES approach stands out as this approach takes into account the time-variation of the different quantities. Finally, a significant discrepancy has been observed between the CO levels provided by RANS and LES approaches where LES is overestimating the level of CO in the exhaust gases. Whereas it is difficult for LES to compete with convincing results provided by RANS, especially in the prediction of global emissions at reduced simulation cost, the LES strengths come out especially in flame and combustion stability analysis.

The aim of this chapter is twofold. On the one hand, it describes a complete simulation method to evaluate the performance of a Doppler LIDAR for aircraft wake vortices detection near the ground. The principal interest of the simulation presented here is to combine a LASER beam propagation methodwith fluid dynamics simulations. On the other hand, it proposes effective wind velocity map reconstruction algorithms.

Considering the growing interest in Power-to-Fuel, i.e., production of H2 using electrolysis to store excess renewable electricity, combustion-based technologies still have a role to play in the future of power generation. Especially in a decentralized production with small-scale cogeneration, micro-Gas Turbines (mGTs) offer great advantages related to their high adaptability and flexibility, in terms of operation and fuel. Hydrogen (or hydrogen enriched methane) combustion is well-known to lead to flame and combustion instabilities. The high temperatures and reaction rates reached in the combustor can potentially lead to flashback. In the past, combustion air humidification (i.e., water addition) has proven effective to reduce temperatures and reaction rates, leading to significant NOx emission reductions. Therefore, combustion air humidification can open a path to stabilize hydrogen combustion in a classical mGT combustor. However accurate data assessing the impact of humidification on the combustion is still missing for real mGT combustor geometries and operating conditions. In this framework, this paper presents a comparison between pure methane and hydrogen enriched methane/air combustions, with and without combustion air humidification, in a typical mGT combustion chamber (Turbec T100) using Large Eddy Simulations (LES) analysis. In a first step, the necessary minimal water dilution, to reach stable and low emissions combustion with hydrogen, was assessed using a one-dimensional (1D) approach. The one-dimensional unstretched laminar flame is computed for both pure methane (reference case) and hydrogen enriched methane/air combustion cases. The results of this comparison show that, for the hydrogen enriched combustion, the same level of flame speed as in the reference case can be reached by adding 10% (in mass fraction) of water. In a second step, the feasibility and flexibility of humidified hydrogen enriched methane/air combustion in an industrial mGT combustor have been demonstrated by performing high fidelity LES on a 3D geometry. Results show that steam dilution helped to lower the reactivity of hydrogen, and thus prevents flashback, enabling the use of hydrogen blends in the mGT at similar CO levels, compared to the reference case. These results will help to design future combustor toward more stability.

The development of turbulent vortical wakes released downstream of wind turbines is a key physical phenomenon as it presents many technological implications for windfarm design and exploitation. The numerical prediction of these wakes constitutes a challenging problem as they involve the shedding of fine vortical structures, their instabilities, and interactions with an ambient turbulent flow. The capture of these complex, three dimensional, unsteady flow phenomena calls for a Large Eddy Simulation (LES) approach. Yet, the computational cost of a scale resolved LES can be huge and the mesh generation process is not obvious when the zones of interest are not known a-priori. Adaptive mesh refinement (AMR) allows generating Eulerian elements only in the regions of interest of the flow, where an action takes place. The AMR strategy proposed here uses the MMG3D library coupled with the YALES2 unstructured finite volume solver. The method is successfully demonstrated on two test cases, the NTNU blind test case for which experimental data exist and the reference NREL 5MW under dynamic yaw conditions.

International audience; The constant growth of computational resources allows performing Large-Eddy Simulation (LES) on realistic flow configurations. In this context, it is important to properly model boundary conditions and particularly inflow turbulence. This study addresses wind tunnel applications where the object under investigation is downstream of a turbulence grid. This grid aims at generating a highly turbulent sheared flow for further use in wind turbines applications. In this work, an original strategy based on the actuator line method (ALM) is proposed to emulate the grid and generate realistic flow structures downstream at a moderate computational cost. As a first validation test of the proposed method, the LES of a single square rod is performed and the results are compared to a scale resolving simulation. In a second phase, the method is applied to a passive grid with non-homogeneous spacing, which generates fully-developed turbulence. Results are compared to experimental data and other state-of-the-art turbulence injection strategies such as homogeneous isotropic turbulence injection and also scale resolving simulations.

This study addresses turbulence modelling issues related to the simulation of flow over complex terrain using a coupling between NWP (numerical weather prediction) code and a classical CFD (computational fluid dynamics) code. A new planetary boundary layer (PBL) parameterization scheme, which is specifically formulated for coupling purposes, is presented. The capability of the developed closure models to be applied to the whole atmospheric boundary layer, not only to the surface layer is assessed by performing idealized simulations and comparing the results with the Leipzig dataset. CFD simulations sustained with inflow conditions acquired from mesoscale simulations are performed with different turbulence closures for the Askervein Hill case and their performances are evaluated in comparison with wind speed and turbulence kinetic energy measurements. Finally, the proposed turbulence modelling methodology is applied to the Ria de Ferrol Case. The new PBL scheme is implemented and tested with the open source mesoscale code, Weather Research and Forecasting (WRF) model. Microscale simulations and the turbulence model implementation are performed with the open source computational fluid dynamics (CFD) toolbox, OpenFOAM v. 2.3.1.

This study focuses on bridging the gap between the turbulence modelling methodologies of meterological and engineering codes by proposing a novel methodology to define the closure coefficients of Reynolds-Averaged Navier-Stokes turbulence models consistently with the physics of the atmospheric boundary layer. In this framework, different turbulence closures have been developed and tested on different full-scale test cases corresponding to different atmospheric stability conditions by performing microscale simulations with the inflow conditions provided by a numerical weather prediction (NWP) code. Developed turbulence models have been implemented into the open source computational fluid dynamics (CFD) toolbox, OpenFOAM and the inflow conditions have been acquired with another open source code, the Weather Research and Forecasting (WRF) model.

The phenomenon of meandering of the wind-turbine wake comprises the motion of the wake as a whole in both horizontal and vertical directions as it is advected downstream. The oscillatory motion of the wake is a crucial factor in wind farms, because it increases the fatigue loads, and, in particular, the yaw loads on downstream turbines. To address this phenomenon, experimental investigations are carried out in a wind-tunnel flow simulating an atmospheric boundary layer with the Coriolis effect neglected. A $$3 \times 3$$ scaled wind farm composed of three-bladed rotating wind-turbine models is subject to a neutral boundary layer over a slightly-rough surface, i.e. corresponding to offshore conditions. Particle-image-velocimetry measurements are performed in a horizontal plane at hub height in the wakes of the three wind turbines occupying the wind-farm centreline. These measurements allow determination of the wake centrelines, with spectral analysis indicating the characteristic wavelength of the wake-meandering phenomenon. In addition, measurements with hot-wire anemometry are performed along a vertical line in the wakes of the same wind turbines, with both techniques revealing the presence of wake meandering behind all three turbines. The spectral analysis performed with the spatial and temporal signals obtained from these two measurement techniques indicates a Strouhal number of $$\approx 0.20 - 0.22$$ based on the characteristic wake-meandering frequency, the rotor diameter and the flow speed at hub height.

The large employment of renewable energies requires flexible, efficient and low-carbon production from fossil fuels. From all non-renewable power production routes, the electricity produced with micro Gas Turbines (mGTs) running on natural gas has a very high load flexibility and the lowest CO2 emissions. However, if we want to move towards full carbon clean power production, then CO2 in the exhaust must be captured. In this scenario, mGTs coupled with a Carbon Capture (CC) plant could be a suitable option, but only a few numerical and experimental analyses are available which assess their real potential. The low concentration of CO2 in the mGT exhaust gas is disadvantageous from a CC point of view, however the concentration can be increased by performing Exhaust Gas Recirculation (EGR). Furthermore, the efficiency loss introduced by the CC plant can also be reduced by using the concept of micro Humid Air Turbine (mHAT). In this study, both the dry and the wet operations of the Turbec T100 coupled with a chemical-absorption plant are simulated and compared using the software Aspen Plus®. The goal of these simulations was to investigate state-of-the-art measures which could be applied to small-scale generation to assess what the energy impact of a carbon clean production would be. Simulation results show that applying EGR to a mGT or mHAT can reduce the exhaust gas mass flow rate by 50% and it can increase the concentration of CO2 by three times the traditional cycle. The humidification of the mGT can entirely compensate for the efficiency losses of the EGR application. Both cycle performances are strongly affected by the thermal input from the stripping process, decreasing the global electric efficiency by around 7.9 absolute percentage points in the mGT case and by 8.3 absolute percentage points in the mHAT case in full load conditions. The results of this paper can be used as a starting point for new cycle concepts between the mGT and CC plant.

This paper quantifies the changes in turbine performance due to manufacturing tolerances and profile degradation of the blade-tip region during engine operation. An extensive numerical study was co...

From all fossil fuel power production routes, the electricity produced with Gas Turbines (GTs) running on natural gas has the lowest CO2 emissions. However, if we want to move towards full carbon clean power production, the CO2 in the exhaust must be captured. The energy impact of a Carbon Capture plant (CC) applied to the micro Gas Turbine (mGT) still remains unclear because few quantitative analyses are available. The low concentration of CO2 in the GT exhaust gas is disadvantageous from a CC point of view. Exhaust Gas Recirculation (EGR) is one of the technologies used to increase the CO2 concentration in the GT flue gas. It is potentially an effective method to reduce the high energy-penalty caused by the carbon capture. A typical capture method is an absorber–stripper system where the absorbent is commonly a 30wt% aqueous monoethanolamine (MEA) solution. In this work, a Turbec T100 mGT coupled with a chemical-absorption plant is considered. The entire plant has been simulated using Aspen Plus®. Simulation results show that the specific reboiler duty is rather constant (around 4.3 MJ/kgCO2) when varying the electrical power output of the mGT. The cycle performance is strongly affected by the thermal energy requirement for the stripping process, decreasing the global electric efficiency around 6.2 absolute percentage points. These results could be a starting point for future energy integrations between the mGT and the CC plant.

The growing share of wind and solar power in the total energy mix has caused severe problems in balancing the electrical power production. Consequently, in the future, all fossil fuel-based electricity generation will need to be run on a completely flexible basis. Micro Gas Turbines (mGTs) constitutes a mature technology which can offer such flexibility. Even though their greenhouse gas emissions are already very low, stringent carbon reduction targets will require them to be completely carbon neutral: this constraint implies the adoption of post-combustion Carbon Capture (CC) on these energy systems. To reduce the CC energy penalty, Exhaust Gas Recirculation (EGR) can be applied to the mGTs increasing the CO2 content in the exhaust gas and reducing the mass flow rate of flue gas to be treated. As a result, a lower investment and operational cost of the CC unit can be achieved. In spite of this attractive solution, an in-depth study along with a robust optimization of this system has not yet been carried out. Hence, in this paper, a typical mGT with EGR has been coupled with an amine-based CC plant and simulated using the software Aspen Plus®. A rigorous rate-based simulation of the CO2 absorption and desorption in the CC unit offers an accurate prediction; however, its time complexity and convergence difficulty are severe limitations for a stochastic optimization. Therefore, a surrogate-based optimization approach has been used, which makes use of a Gaussian Process Regression (GPR) model, trained using the Aspen Plus® data, to quickly find operating points of the plant at a very low computational cost. Using the validated surrogate model, a robust optimization using a Non-dominated Sorting Genetic Algorithm II (NSGA II) has been carried out, assessing the influence of each input uncertainty and varying several design variables. As a general result, the analysed power plant proves to be intrinsically very robust, even when the input variables are affected by strong uncertainties.

To reduce the emission of greenhouse gases, there is nowadays a strong focus on the electrification of transportation, especially personal vehicles. One of the main limitations of this trend is the limited range of Electrical Vehicles (EVs) due to the rather low energy density of batteries compared to liquid fossil fuels. Adding a range extender allows for a significant increase of the vehicle driving range. Typical range extenders are Internal Combustion Engine (ICE) based. Although micro Gas Turbines (mGTs) are not adequate for continuous change of load, as would be the case when used as direct power source, operating them as range extender offers large potential, especially given their higher power-to-weight ratio than ICEs, but also due to their lower emissions, vibration levels and maintenance requirements. The main drawback of the mGT compared to the ICE is its lower electrical efficiency. Humidification of the mGT cycle allows boosting its performance and by doing so making it more compatible. In this paper, we investigate the feasibility of applying a (humidified) mGT as a range extender in EVs. In this investigation, we considered a small vehicle for use in urban area. The option of using an mGT as range extender (standard and humidified) is compared to the standard ICE based range extender. Aspen plus is used to assess the engines performance, showing that the humidified mGT has the highest potential in terms of fuel consumption reduction and range extension. Moreover, the humidified mGT presents similar performance compared to an ICE, leading to the same specific energy consumption. In addition, different pathways to compensate for the water consumption (exhaust gas condensation) are studied and discussed, revealing that the vehicle does not need external input of water.

To fully embrace its opportunities in future decentralized power production, the current mGT has to become more flexible in terms of operation, i.e. decoupling heat and power production. Cycle humidification during periods with low heat demand is a possible route to handle this issue. Indeed, cycle humidification has already been proven to increase the mGT electrical efficiency. Nevertheless, even when applying the most advanced humidified cycle concept, i.e. the REgenerative EVAPoration cycle, the electrical performance increase remains rather limited. In this perspective, the more recent Maisotsenko (or M-power) cycle concept offers a larger potential for humidification, even though its potential was only proven on large-scale gas turbine cycles and never applied to the smaller mGT scale. In this paper, the concept of this M-power cycle is applied to a 100 kWe mGT (Turbec T100) to assess its performance, using Aspen Plus® simulations. Moreover, the impact of various inputs, component performance and control parameters is studied using a sensitivity analysis. The numerical results highlight that the M-power cycle has the highest waste heat recovery and thus the highest electric efficiency (up to 147 kWe electric power output with an electric efficiency of 42.1% at constant rotational speed and 41.1%, corresponding to an 8.3%point absolute increase, at constant power output). Moreover, this cycle concept allows to approach the thermodynamic limit for cycle humidification. Indeed, a large exergy destruction is avoided by not going for direct water injection, but rather using a gradual injection and evaporation. Additionally, from a technological point of view, the M-power cycle is also preferable for the small-scale mGT. Indeed, in the M-power cycle, saturation tower, aftercooler, recuperator and economizer are combined in one single component, significantly reducing the complexity of the cycle. The main limitation is the saturator, that requires a wet bulb effectiveness of up to 98% to achieve the simulated performance, which can be technological very challenging.

The aerodynamics of yawed wind turbine wakes remains a major investigation topic, especially for the use of yaw angle in control strategies. Large-Eddy Simulations are employed here to study the influence of yaw and inflow conditions on the prediction of the wind turbine wake structure. A single wind turbine setup with different yaw angles and three different inflow conditions is investigated and discussed with respect to experimental data. The wind turbine blades are modeled using the actuator line method (ALM) while tower and nacelle are represented with a body-fitted unstructured mesh. The high levels of upstream turbulence, experimentally generated by turbulence grids, are emulated here with oscillating ALM. Such approach demonstrates to be highly predictive on the turbulent flow characteristics compared to the emptied wind tunnel experimental data. Results with turbine show good agreement to the experiment data with only a slight overestimation on the magnitude of the wake deflection due to yaw. When compared to a deflection model, the confinement of the wind tunnel is highlighted. The radial and azimuthal time-averaged angle of attack exhibits a high probability of dynamic stall near the hub for a yawed turbine. These results show large discrepancies on the blade loading, highlighting the need for improved and specific models.

This paper concerns implicit large eddy simulation (ILES) of turbulent flows of industrial interest using high order discontinuous Galerkin method (DGM). DGM has a high potential for industrial applications using ILES. As dissipation is only active on very small scale features, the method mimics a subgrid scale model, while its high accuracy ensures that large scale dynamics are not contaminated by dispersive/dissipative errors. Previously DGM/ILES has been assessed on many low Mach number canonical test cases (e.g. Carton et al. Numer Methods Fluids, 78:335–354, (2015), [3]). This paper recapitulates recent validation on transonic benchmarks (Hillewaert et al. Proceedings of CTR summer program, pp. 363–372, Stanford University, (2016), [6]) and proceeds to the application on the LS89 cascade, a well-known turbomachinery benchmark.

The large adoption of renewable energies is crucial to achieve a low-carbon economy, however, in the transition period, a flexible and clean production from fossil fuels is still necessary. With the current shift towards decentralized power production, micro Gas Turbines (mGTs) appear as a promising technology for small-scale generation. The target of a carbon-clean power production calls for the implementation of Carbon Capture Use and Storage (CCUS) technologies. Compared to coal fired power production, the low CO2 concentration in the exhaust gas of a mGT makes Carbon Capture (CC) much more expensive. However, the CO2 concentration can be increased by performing Exhaust Gas Recirculation (EGR), therefore reducing the CC energy penalty. Additionally, cycle humidification can also help to increase the electrical efficiency of the turbine plant. Nevertheless, the higher CO2 content in the inlet air, in combination with the high humidity level, will affect the operation of the mGT. This paper presents a numerical study of this innovative cycle combined with preliminary experimental validation of CO2 injection. To the authors’ best knowledge, experimental analysis of EGR together with humidification applied to a mGT has never been carried out. Experimental results showed a stable turbo-machinery operation under a moderate CO2 injection. The results of this paper are a first step towards a more severe dilution conditions, with the aim of a full implementation of EGR on a micro Humid Air Turbine (mHAT).

This paper presents the second validation step of a compressible discontinuous Galerkin solver with symmetric interior penalty (DGM/SIP) for the direct numerical simulation (DNS) and the large eddy simulation (LES) of complex flows. The method has already been successfully validated for DNS of an academic flow and has been applied to flows around complex geometries (e.g. airfoils and turbomachinery blades). During these studies, the advantages of the dissipation properties of the method have been highlighted, showing a natural tendency to dissipate only the under-resolved scales (i.e the smallest scales present on the mesh), leaving the larger scales unaffected. This phenomenon is further enhanced as the polynomial order is increased. Indeed, the order increases the dissipation at the largest wave numbers, while its range of impact is reduced. These properties are spectrally compatible with a subgrid-scale model, and hence DGM may be well suited to be used for an implicit LES (ILES) approach. A validation of this DGM/ILES approach is here investigated on canonical flows, allowing to study the impact of the discretisation on the turbulence for under-resolved computations. The first test case is the LES of decaying homogeneous isotropic turbulence (HIT) at very high Reynolds number. This benchmark allows to assess the spectral behaviour of the method for implicit LES. The results are in agreement with theory and are even slightly more accurate than other numerical results from literature, obtained using a pseudo-spectral (PS) method with a state-of-the-art subgrid-scale model. The second benchmark is the LES of the channel flow. Three Reynolds numbers are considered: Reτ=395, 590 and 950. The results are compared with DNS of Moser et al. and Hoyas et al., also using PS methods. Both averaged velocity and fluctuations are globally in good agreement with the reference, showing the ability of the method to predict equilibrium wall-bounded flow turbulence. To show that the method is able to perform accurate DNS, a DNS of HIT at Reλ=64 and a DNS of the channel flow at Reτ=180 are also performed. The effects of the grid refinement are investigated on the channel flow at Reτ=395, highlighting the improvement of the results when refining the mesh in the spanwise direction. Finally, the modification of the ILES parameters, that is the Riemann solver and of the SIP coefficient, is studied on both cases, showing a significant influence on the choice of the Riemann solver.

International audience; Accurate simulation of wind turbine wakes is critical for the optimization of turbine efficiency and prediction of fatigue loads. These wakes are three-dimensional, complex, unsteady and can evolve in geometrically complex environments. Modeling these flows calls thus for high-quality numerical methods that are able to capture and transport thin vortical structures on an unstructured grid. It is proposed here to assess the performances of a fourth-order finite-volume LES solver to perform massively parallel scale-resolving simulations of wind turbines wakes. In this framework, the actuator line method that takes the effect of the wind turbine blades on the flow into account is implemented. It is demonstrated that both near and far parts of the turbine wakes are accurately modeled as well as geometrical details. The methodology is assessed on two different test cases and validated with experimental results. It is demonstrated that the flow predictions are of equivalent quality on both structured and unstructured grids. The influence of the geometrical details (e.g. nacelle and tower) on the wake development as well as the influence of the discretization scheme are also investigated.

With the current shift towards more decentralized power production, micro Gas Turbines (mGTs) appear as a promising technology for small-scale Combined Heat and Power (CHP) production. Nevertheless, if we want to move towards a carbon-clean power production, the CO2 in the exhaust gas must be captured. In this context, mGTs coupled with a Carbon Capture (CC) plant might be a good candidate, however there is still an energy penalty. Especially when there is no demand for heat and the mGT is only producing electrical power, this penalty becomes a major drawback. In this case, a possible solution is waste heat recovery in the system to increase the efficiency or to reduce the energy penalty, but few analyses are available to assess the optimal solution. In this paper, we aim at identifying the optimal strategy for carbon clean mGT operation in CHP applications for different operation scenarios: full CHP mode and operation with no thermal load. The effect of different technological measures such as humidification and internal heat recoveries has been investigated. The goal of these simulations was to evaluate the most efficient strategy for each thermal scenario of the mGT by testing different combinations of the previous measures. Results show that full CHP operation of the mGT with carbon capture has still the highest performance. Nonetheless, if the thermal demand is zero, conversion into mHAT and heat recovery for the stripping process result in a sensible reduction of the CC plant energy penalty, moving closer to the efficiency of traditional mGTs. Future work would include a more detailed analysis concerning mixed operation (part-load heat demand) of the mGT and an exergy analysis to identify where more waste heat can be recovered., SCOPUS: cp.p, info:eu-repo/semantics/published

High pressure turbine (HPT) blades are cooled partly by guiding air from the compressor through internal ducts.

Most promising Generation IV nuclear reactor concepts are based on a liquid metal coolant. However, at low Prandtl (Pr ) numbers such as those of liquid metal, classical approaches derived for unity, or close to unity, Pr fail to accurately predict the heat transfer. This paper assesses the RANS modeling of forced turbulent convection at low Pr and in channel flow. Reference results at high Reynolds (Re ) number are required to ensure that the Peclet number is sufficiently high. Therefore, new reference results were obtained by performing a wall-resolved Large-Eddy Simulation of turbulent channel flows at a friction Reynolds number Reτ=2000Reτ=2000 and at Pr=0.01Pr=0.01 and 0.0250.025 (this also corresponds to the highest Re Direct Numerical Simulation (DNS) available in the literature for the flow, but without heat transfer). The LES velocity statistics are in very good agreement with those of the DNS and, as validated by the authors in previous publications, the LES approach used here accurately predicts the temperature statistics at low Pr . The LES results are used to assess RANS heat transfer modeling based on the effective turbulent Prandtl number (PrtPrt) concept. Among existing PrtPrt correlations, the correlation by Kays (1994) [10] is shown to yield the best results. Since it is also shown that the near-wall temperature profile does not follow a log-law, a new “law of the wall for temperature” is here proposed, which does not use any blending function. Its use as a wall-function is also validated in actual RANS simulations.

The aim of this paper is to verify that a modeling approach affordable at wind farm scale, in this case a fourth-order finite differences code combined with a rotating actuator disk, provides equivalent results in terms of wake meandering compared to a method with a higher fidelity level, i.e. a Vortex Particle-Mesh Method combined with immersed lifting lines. The analysis is performed on the wake centerlines obtained from Large Eddy Simulations of an isolated NREL 5-MW wind turbine subject to several synthetic turbulent inflows: 6%, 10%, and 15% turbulence intensity. A direct comparison of the wake centerlines reveals a good match between the two approaches up to five rotor diameters. Concerning the amplitude and wavelength of the wake meandering, the lower fidelity code leads to larger amplitudes and shorter wavelengths. Assuming that Taylor’s frozen turbulence hypothesis is applicable, the Strouhal number St is computed as the ratio between the rotor diameter and the wavelength. Obtained values are such that 0.22 ⩽ St ⩽ 0.4.

This study investigates the influence, on aircraft wake vortex behavior, of atmospheres that are stably stratified (neutral to very strong) and weakly turbulent, by means of large-eddy simulations at very high Reynolds number and on relatively fine grids. The atmospheric fields are first generated using large-eddy simulations of forced and stratified turbulence reaching a statistically stationary state. The obtained fields are shown to be realistic and consistent with the literature. A pair of counter-rotating vortices, with relatively tight cores, is then put in the obtained fields and evolved. The evolution of the wake vortex topology, transport, and decay is analyzed in depth by measuring the wake vortex characteristics in all cross planes. The vortex transport and deformation are related to the stratification and turbulence levels. Stratification combined with weak turbulence is seen to strongly affect the Crow instability development. Different decay mechanisms are identified, related to the interactions with the turbulence, the turbulent baroclinic vorticity, and/or between the vortices themselves. Finally, improved simplified models of vortex altitude evolution and of vortex decay (with two phases) are proposed and calibrated on the present results. They yield good agreement with the large-eddy simulation results and are usefully integrated in our operational models. Read More: http://arc.aiaa.org/doi/abs/10.2514/1.J051742

This paper deals with the issue of modeling convective turbulent heat transfer of a liquid metal with a Prandtl number down to 0.01, which is the order of magnitude of lead–bismuth eutectic in a liquid metal reactor. This work presents a DNS (direct numerical simulation) and a LES (large eddy simulation) of a channel flow at two different Reynolds numbers, and the results are analyzed in the frame of best practice guidelines for RANS (Reynolds averaged Navier–Stokes) computations used in industrial applications. They primarily show that the turbulent Prandtl number concept should be used with care and that even recent proposed correlations may not be sufficient.

In the development of accelerator driven systems, ADS, free surface lead–bismuth spallation targets are considered as promising solutions due to their possibility for compactness, their lifetime, and their ability to transport the heat deposited by the proton beam away from the spallation zone. Experiments to characterise the hydraulics of the targets are needed to allow the validation of numerical models and to improve the design. Such experiments have been performed in water on a new concept labelled “detached flow” geometry. This name was chosen because the liquid undergoes a free fall between the nozzle exit where the main free surface (that separating the void of the beam line from the liquid) is created and a second free surface downstream. The void surrounding the liquid jet plays the role of a buffer. The experiments show that a very stable main free surface with a small recirculation is obtained using this geometry thanks to the presence of the second free surface and the nozzle geometry. The experiments confirm that the level of the second free surface has no influence on the characteristics of the main free surface, improving the main free surface control. The influences of the mass flow rate and of the inlet velocity are evaluated. The free surface level rises linearly with an increase in mass flow rate. The recirculation zone is also stronger in this case. The opposite is found when the mass flow rate is decreased. For all mass flow rates studied, a stable free surface is obtained. Moreover, the outer shape of the liquid jet is similar at all mass flow rates. It is only dictated by the nozzle exit angle. Increasing slightly the inlet velocity for a given mass flow rate has a positive effect on the recirculation stability. The “detached flow” target is a promising design for ADS.

During two field measurement campaigns aircraft wake vortex trajectory and wind measurement data have been collected at Frankfurt airport. Three different approaches have been used to analyse the data in order to estimate crosswind threshold values supporting vortex-free corridors for departing aircraft. Although several competing effects such as wake vortex transport in and out of ground effect, temporal and spatial wind variability, and the spreading of aircraft trajectories after take-off, complicate the analyses, all three approaches lead to similar crosswind thresholds. Employing standard instrumentation at 10 m, a minimum crosswind threshold of 3.5–4.6 m s−1 has been identified to clear a safety corridor of 150 m width from wake vortices with a 95% probability within 60 s. Alternative estimations of crosswind thresholds employing different instrumentation and different height ranges are reported. Crosswind thresholds can be reduced if the wind is measured close to the air mass in which the vortices evolve. A definite crosswind threshold for operational use cannot be deduced solely from this study since critical factors such as risk and safety assessment have not yet been taken into account.

We investigate several algorithms to detect the centerline of a wind turbine wake. First, we apply these methods during Large Eddy Simulations of an isolated wind turbine subject to a uniform ( TI = 0%) and a synthetic turbulent inflow ( TI = 10%). The simulations are performed using a vortex-particle mesh method with the blades modeled using immersed lifting lines. The most robust algorithm is then applied to investigate the wakes positions inside a wind farm. At wind farm scale, the simulations are performed with a fourth-order finite difference code inside which the wind turbines are accounted for through advanced actuator disks.

The study of wind turbines requires a multidisciplinary approach that covers the electrical, mechanical and aerodynamic aspects. Most wind turbine studies focus their modelization effort on a single aspect and rely on simplified models for the other ones. The present paper aims at a complete modelization and investigation of the energy conversion chain of small wind turbines (up to 100kW), going from the turbulent wind resource to the electrical power injected into the grid. For that purpose, the following computational chain is set up: a synthetic turbulence is used as inflow to a Vortex Particle-Mesh method which computes the wind turbine coarse scale aerodynamics taking into account the rotor dynamics; the aero-mechanical results are used to feed an electrical simulation and the consistency of both simulations is ensured by using a common Maximum Power Point Tracking algorithm. We apply this methodology to compare the behavior of horizontal- and vertical-axis machines in a turbulent wind with two different turbulent intensities (low T I = 8.3% and high T I = 16%).

In this paper, we present the development of an axial aircraft wake vortex light detection and ranging (LIDAR) sensor, working in Mie scattering regime, based on pulsed 1.5-mu m high-brightness large-core fiber amplifier. An end-to-end Doppler heterodyne LIDAR simulator is used for the LIDAR design. The simulation includes the observation geometry, the wake vortex velocity image, the scanning pattern, the LIDAR instrument, the wind turbulence outside the vortex, and the signal processing. An innovative high-brightness pulsed 1.5-mum laser source is described, based on a master oscillator power fiber amplifier (MOPFA) architecture with a large-core fiber. The obtained beam quality is excellent (M 2 = 1.3), and achieved pulsed energy is 120 muJ with a pulse repetition frequency of 12 kHz and a pulse duration of 800 ns. A Doppler heterodyne LIDAR is developed based on this laser source with a high-isolation free-space circulator. The LIDAR includes a real-time display of the wind field. Wind dispersion is postprocessed. Field tests carried out at Orly airport in April 2008 are reported. Axial aircraft wake vortex signatures have been successfully observed and acquired at a range of 1.2 km with axial resolution of 75 m for the first time with fiber laser source.

Background: It is a known fact that blood flow pattern and more specifically the pulsatile time variation of shear stress on the vascular wall play a key role in atherogenesis. The paper presents the conception, the building and the control of a new in vitro test bench that mimics the pulsatile flows behavior based on in vivo measurements. Methods: An in vitro cardiovascular simulator is alimented with in vivo constraints upstream and provided with further post-processing analysis downstream in order to mimic the pulsatile in vivo blood flow quantities. This real-time controlled system is designed to perform real pulsatile in vivo blood flow signals to study endothelial cells' behavior under near physiological environment. The system is based on an internal model controller and a proportional-integral controller that controls a linear motor with customized piston pump, two proportional-integral controllers that control the mean flow rate and temperature of the medium. This configuration enables to mimic any resulting blood flow rate patterns between 40 and 700 ml/min. In order to feed the system with reliable periodic flow quantities in vivo measurements were performed. Data from five patients (1 female, 4 males; ages 44-63) were filtered and post-processed using the Newtonian Womersley's solution. These resulting flow signals were compared with 2D axisymmetric, numerical simulation using a Carreau non-Newtonian model to validate the approximation of a Newtonian behavior. Results: This in vitro test bench reproduces the measured flow rate time evolution and the complexity of in vivo hemodynamic signals within the accuracy of the relative error below 5%. Conclusions: This post-processing method is compatible with any real complex in vivo signal and demonstrates the heterogeneity of pulsatile patterns in coronary arteries among of different patients. The comparison between analytical and numerical solution demonstrate the fair quality of the Newtonian Womersley's approximation. Therefore, Womersley's solution was used to calculate input flow rate for the in vitro test bench., SCOPUS: ar.j, info:eu-repo/semantics/published

This paper presents the second step of the validation of a compressible discontinuous Galerkin method (DGM) for the direct numerical simulation (DNS) and the large eddy simulation (LES) of turbomachinery flows.