Large eddy simulation of spray combustion in an HSDI engine is carried out in this thesis. The implementation of the code was performed in logical steps that allowed both assessment of the performance of the existing KIVA-LES and later development. The analysis of the liquid annular jet confirmed existence of typical, annular jet exclusive structures like head vortices, stagnation point and recirculation in the inner zone. The influence of the swirl in the ambient domain was found to have profound impact on the development, penetration and radial spreading of the jet. Detailed results were reported in Jagus et al. (2008). The code was further validated by performing an extensive study of large eddy simulation of diesel fuel mixing in an engine environment. The reaction models were switched off in order to isolate the effects of both swirl and the different numerical treatment of LES. Reference RANS simulation was performed and significant differences were found. LES was found to capture much better the influence of the swirl on the liquid and vapour jets, a feature essentially absent in RANS results. Moreover, the predicted penetration of the liquid was much higher for the LES case and more in accordance with experimental measurements. Liquid penetration and subsequent evaporation are very important for prediction of heat release rates and encouraging results formed a good basis to performing a full simulation with models for ignition and combustion employed. The findings were analyzed in the paper by Jagus et al. (2009). Further modifications were introduced into the LES code, among them changes to the combustion model that was originally developed for RANS and calculation of the filter width. A new way of estimating the turbulent timescale (eddy turnover time) assured that the incompatibilities in the numerical treatment were eliminated and benefits of LES maximized. The new combustion model proved to give a very good agreement with experimental data, especially with regard to pressure and accumulated heat release rates. Both qualitative and quantitative results presented a significant improvement with respect to RANS results and old LES formulation. The new LES model was proved to give a very good performance on a spectrum of mesh resolutions. Encouraging results were obtained on a coarse mesh sets therefore proving that the new LES code is able to give good prediction even on mesh sizes more suitable for RANS. Overall, LES was found to be a worthy alternative to the well established RANS methods, surpassing it in many areas, such as liquid penetration prediction, temperature-turbulence coupling and prediction of volume-averaged data. It was also discovered that the improved LES code is capable of producing very good results on under-resolved mesh resolutions, a feature that is especially important in industrial applications and on serial code structure.

Reproduction de : Thèse de doctorat : Dynamique des fluides : Toulouse, INPT : 2005. / Texte en anglais. Titre provenant de l'écran-titre. Bibliogr. 108 réf. Index.

Producer gas derived from biomass is one of the most environment friendly substitutes to the fossil fuels. Usage of producer gas for power generation has effect of zero net addition of CO2 in atmosphere. The engines working on producer gas have potential to decrease the dependence on conventional fuels for power generation. However, the combustion process is governed by complex interactions between chemistry and fluid dynamics, some of which are not completely understood. Improved knowledge of combustion is, therefore, of vital importance for both direct use in the design of engines, and for the evolution of reliable simulation tools for engine development. The present work is related to the turbulent combustion of producer gas in closed vessels and engine cylinders. The main objective of the work was multi-dimensional simulation of turbulent combustion in the bowl-in-piston engine operating on producer gas fuel and to observe the flame and flow field interaction. First, the combustion model was validated in constant volume combustion chamber with experimental results. Experimental turbulent combustion data of producer gas (composition matching with engine operating conditions) was presented. The required data of laminar burning velocity of producer gas was computed and used in the simulation of turbulent combustion in closed vessel. The effect of squish and reverse squish flow on flame propagation in the bowl-in-piston engine cylinder was described. Laminar burning velocity of unstretched flame was computed using flame code which was developed earlier in this laboratory. One dimensional computations of unstretched planar flame were made to calculate laminar burning velocity of the producer gas-air mixture at pressures (1-10 bar) and temperatures (300-600 K). A correlation of laminar burning velocity of producer gas as a function of pressure and temperature was fitted and compared with experiments. A fixed composition and equivalence ratio of producer gas-air mixture, typical of the engine operating conditions, was considered. The correlation was used in simulation of turbulent combustion in closed vessel. The turbulent combustion experiments with producer gas-air mixture were conducted in a closed vessel. The aim of experiments was to generate pressure-time data, in closed vessel during turbulent flame propagation, which was required to validate turbulent combustion models. Determination of (ST /SL) was made from pressure-time data which requires corresponding laminar combustion data with same initial conditions. For this purpose a set of laminar combustion experiments was conducted. Experimental setup consists of a constant volume combustion chamber of cubical shape and size 80 x 80 x 80 mm3 . The initial mixtures pressure and temperature were 1 bar and 300 K respectively. A fixed composition and equivalence ratio of producer gas-air mixture, typical of the engine operating conditions, was used. The composition of producer gas was H2 -19.61%, CO2 -19.68%, CH4 -2.52%, CO2 -12.55% and N2 -45.64% on volume basis. Fuel-air mixture was ignited with electric spark at the center of the cube. Initial turbulence in the chamber was created by moving a perforated plate with specified velocity. Perforated plate was placed in chamber so that the central hole in the plate passes over the spark electrodes as it sweeps across the chamber. Two geometrically similar plates with hole diameter of 5 and 10 mm were used. The new experimental setup constructed as a part of this work was first tested with one set of experiments each with methane and propane data of SL and ST /SL from the literature. Maximum turbulent intensity (u’) achieved was 1.092 ms−1 . The ratios of turbulent to laminar burning velocity (ST /SL) values were determined at six different turbulence intensity levels. Laminar combustion experiments were extended to elevated initial pressures 2-5 bar and temperature 300 K. The value of SL was calculated from the pressure-time history recorded during laminar stretching flame propagation inside closed vessel. These SL values were compared with computed SL,∞ after accounting for stretch. Turbulent combustion simulations were carried out to validate combustion models suitable for multi-dimensional CFD simulation of combustion in constant volume closed chamber. Two models proposed by Choi and Huh, based on Flame Surface Density (FSD) were tested with the present experimental results. User FORTRAN code for the source terms in transport equation of FSD was implemented in ANSYS-CFX 10.0 software. First model called CFM1, grossly under-predicted the rate of combustion. The second model called CFM2, predicted the results satisfactorily after replacing the arbitrary length scale with turbulent integral length scale (lt) having a limiting value near the wall. The modified CFM2 model was able to predict the propagation phase of the developed flame satisfactorily, though the duration for initial flame development was over-predicted by the model. CFD simulation of producer gas engine combustion process was carried out using ANSYSCFX software. Mesh deformation option was used to take care of moving boundaries such as piston and valve surfaces. The fluid domain expands during suction process and contracts during compression process. In order to avoid excessive distortion of the mesh elements, a series of meshes at different crank angle positions were generated and checked for their quality during mesh motion in the solver. For suction process simulation, unstructured meshes having 0.1 to 0.3 million cells were used. During the compression and combustion process simulations, structured meshes having 40,000 to 0.1 million cells were used. k-ε model was used for turbulence simulation. The suction, compression and combustion processes of an SI engine were simulated. Initial flame kernel was given by providing high flame surface density in a small volume comparable to the spark size at the time of ignition. The flame surface density model, CFM-2, was adapted with the modification of length scale tested against constant volume experiments. A suitable limiting value was used to avoid abnormal flame propagation near the wall. The limiting value of integral length scale (lt) near the wall was determined by linear extrapolation of the integral length scale in the domain to the wall. Engine p - θ curves of three different ignition timings 26°, 12° and 6°before top dead center (TDC) were simulated and compared with earlier experimental results. The effects of flow field on flame propagation have been observed. A comparison of the simulated and experimental p - θ diagram of the engine for all above cases gave mixed results. For the ignition timing at 26° before TDC case, predicted peak pressure value was 17% higher and at 3° earlier than those of the experimental peak. For the other two cases, the predicted peak pressure value was 28% lower and 5° later than those of the experimental peak. The reason for under-prediction of the pressure values could be due to the delay in development of initial flame kernel. Simulated pressure curves have offset about 3-4° compared to the experimental pressure curves. It was observed that in all predicted p - θ cases, there was a delay in the initial flame development. It is evident from the under-prediction of pressure values, especially in the initial flame kernel development phase and it also affects the p - θ curve at later stage. The delay was about 3-4° of crank angle rotation in various cases. The delay in predicting the initial flame development needs to be corrected in order to predict the combustion process properly. The proposed FSD model seems to have capability to predict p - θ values fairly in the propagation phase of developed flame. Reasonably good match was obtained by advancing the ignition timing in the computation by about 3-4° compared to the experimental setting. In the bowl-in-piston engine cylinders, the flow in the cylinder is characterised by squish and reverse squish when the piston is moving towards and away from the top dead center (TDC) respectively. The effect of squish and reverse squish flow on flame propagation has been assessed. For the more advanced ignition case, i.e., 26° before TDC, The flame propagation did not have favorable effect by the flow field. The direction of flame propagation was against the squish and reverse squish flow. This resulted in suppressed peak velocities in the cylinder compared the motoring process. Hence the burning rate was not augmented by the turbulence inside the cylinder. For the ignition 12° before TDC case, the flame propagation did have favorable effect by the flow field. During the reverse squish period, the flame had reached the bowl wall. At this stage, the flame was pushing the reactants out and this augments the reverse-squish flow, and hence the maximum reverse-squish velocity was increased to 2.03 times the peak reverse-squish velocity of motoring case. The reverse-squish flow was distorting the flame from spherical shape and the flame gets stretched. Flame surface enters the cylindrical region faster compared to the previous case. The stretched flame in the reverse-squish flow may be considered as reverse squish flame, as was proposed earlier by Sridhar G. The burn rate during the reverse squish period may be 2 to 2.5 times the normal burn rate. For the ignition 6° before TDC case, the flame was very small in size and it did not affect the flow in squish period. During the reverse squish period, the flame radius was moderate compared to the bowl radius. The flame was pushing the reactants out and it increased the maximum reverse-squish velocity to 1.3 times by the flame. In this case, the reverse-squish flow moderately affecting the flame shapes. The results of this study could give an idea of what ignition timing must be kept for favorable use of flow field inside the engine cylinder. Main contributions from the present work are: Multi-dimensional simulation of combustion process inside the engine cylinder operating on producer gas was carried out to examine flame/flow field interactions. Two models based on FSD were first tested against present experimental results in constant volume combustion chamber. In CFM2 model; a modification of replacing the arbitrary length scale by integral length scale with a limiting value near the wall was suggested to avoid prediction of abnormally large turbulent burning velocity near the wall. This combustion model has been implemented in ANSYS-CFX10. The required data of laminar and turbulent burning velocities of producer gas-air mixture has been determined by experiments and computations at varied initial pressures and turbulent intensities. Finally, the simulated engine pressure data has been compared with earlier experimental data of the engine operating on producer gas. The proposed FSD model has the capability to match well with the experimental results except for the initial flame kernel development phase. Even though this issue needs to be resolved, the work has brought out the important interaction between the flame propagation and flow field within the bowl-in-piston engine cylinder.

Le développement des turbines à gaz d’aviation actuelles et futures est principalement axé sur la sécurité, la performance, la minimisation de la consommation de l’énergie, et de plus en plus sur la réduction des émissions d’espèces polluantes. Ainsi, les phases de design de moteurs sont soumises auxaméliorations continues par des études expérimentales et numériques. La présente thèse se consacre à l’étude numérique des phases transitoires et stationnaires de la combustion au sein d’une turbine à gaz d’aviation opérant à divers modes de combustion. Une attention particulière est accordée à la précision des résultats, aux coûts de calcul, et à la facilité de manipulation de l’outil numérique d’un point de vue industriel. Un code de calcul commercial largement utilisé en industrie est donc choisi comme outil numérique. Une méthodologie de Mécanique des Fluides Numériques (MFN) constituée de modèles avancés de turbulence et de combustion jumelés avec un modèle d’allumage sous-maille, est formulé pour prédire les différentes phases de la séquence d’allumage sous différentes conditions d’allumage par temps froid et de rallumage en altitude, ainsi que les propriétés de la flamme en régime stationnaire. Dans un premier temps, l’attention est focalisée sur le régime de combustion stationnaire. Trois méthodologies MFN sont formulées en exploitant trois modèles de turbulence, notamment, le modèle basé sur les équations moyennées de Navier-Stokes instationnaires (URANS), l’adaptation aux échelles de l’écoulement (SAS), et sur la simulation aux grandes échelles (LES). Pour évaluer la pertinence de l’incorporation d’un modèle de chimie détaillée ainsi que celle des effets de chimie hors-équilibre, deux différentes hypothèses sont considérées : l’hypothèse de chimie-infiniment-rapide à travers le modèle d’équilibre-partiel, et l’hypothèse de chimie-finie via le modèle de flammelettes de diffusion. Pour chacune des deux hypothèses, un carburant à une composante, et un autre à deux composantes sont utilisés comme substituts du kérosène (Jet A-1). Les méthodologies MFN résultantes sont appliquées à une chambre de combustion dont l’écoulement est stabilisé par l’effet swirl afin d’évaluer l’aptitude de chacune d’elle à prédire les propriétés de combustion en régime stationnaire. Par la suite, les rapports entre le coût de calcul et la précision des résultats pour les trois méthodologies MFN formulées sont explicitement comparés. La deuxième étude intermédiaire est dédiée au régime de combustion transitoire, notamment à la séquence d’allumage précédant le régime de combustion stationnaire. Un brûleur de combustibles gazeux, muni d’une bougie d’allumage, et dont la flamme est stabilisée par un accroche-flamme, est utilisé pour calibrer le modèle MFN formulé. Ce brûleur, de géométrie relativement simple, peut aider à la compréhension des caractéristiques d’écoulements réactifs complexes, en l’occurrence l’allumabilité et la stabilité. La méthodologie MFN la plus robuste issue de la précédente étude est reconsidérée. Puisque le brûleur fonctionne en mode partiellement pré-mélangé, le modèle de combustion paramétré par la fraction de mélange et la variable de progrès est adopté avec les hypothèses de chimie-infiniment-rapide et de chimie-finie, respectivement à travers le modèle de Bray-Moss-Libby (BML) et un modèle de flammelettes multidimensionnel (FGM). Le modèle d’allumage sous-maille est préalablement ajusté via l’implémentation des propriétés de la flamme considérée. Par la suite, le modèle d’allumage est couplé au solveur LES, puis successivement aux modèles BML et FGM. Pour évaluer les capacités prédictives des méthodologies résultantes, ces dernières sont utilisées pour prédire les évènements d’allumage résultant d’un dépôt d’énergie par étincelles à diverses positions du brûleur, et les résultats sont qualitativement et quantitativement validés en comparant ceux-ci à leurs homologues expérimentaux. Finalement, la méthodologie MFN validée en configuration gazeuse est étendue à la combustion diphasique en la couplant au module de la phase liquide, et en incorporant les propriétés de la flamme de kérosène dans le modèle d’allumage. La méthodologie MFN résultant de cette adaptation, est préalablement appliquée à la chambre de combustion étudiée antérieurement, pour prédire la séquence d’allumage et améliorer les prédictions antérieures des propriétés de la flamme en régime stationnaire. Par la suite, elle est appliquée à une chambre de combustion plus réaliste pour prédire des évènements d’allumage sous différentes conditions d’allumage par temps froid, et de rallumage en altitude. L’aptitude de la nouvelle méthodologie MFN à prédire les deux types d’allumage considérés est mesurée quantitativement et qualitativement en confrontant les résultats des simulations numériques avec les enveloppes d’allumage expérimentales et les images d’une séquence d’allumage enregistrée avec une caméra infrarouge. / The development of current and future aero gas turbine engines is mainly focused on the safety, the performance, the energy consumption, and increasingly on the reduction of pollutants and noise level. To this end, the engine’s design phases are subjected to improving processes continuously through experimental and numerical investigations. The present thesis is concerned with the simulation of transient and steady combustion regimes in an aircraft gas turbine operating under various combustion modes. Particular attention is paid to the accuracy of the results, the computational cost, and the ease of handling the numerical tool from an industrial standpoint. Thus, a commercial Computational Fluid Dynamics (CFD) code widely used in industry is selected as the numerical tool. A CFD methodology consisting of its advanced turbulence and combustion models, coupled with a subgrid spark-based ignition model, is formulated with the final goal of predicting the whole ignition sequence under cold start and altitude relight conditions, and the main flame trends in the steady combustion regime. At first, attention is focused on the steady combustion regime. Various CFD methodologies are formulated using three turbulence models, namely, the Unsteady Reynolds-Averaged Navier-Stokes (URANS), the Scale-Adaptive Simulation (SAS), and the Large Eddy Simulation (LES) models. To appraise the relevance of incorporating a realistic chemistry model and chemical non-equilibrium effects, two different assumptions are considered, namely, the infinitely-fast chemistry through the partial equilibrium model, and the finite-rate chemistry through the diffusion flamelet model. For each of the two assumptions, both one-component and two-component fuels are considered as surrogates for kerosene (Jet A-1). The resulting CFD models are applied to a swirl-stabilized combustion chamber to assess their ability to retrieve the spray flow and combustion properties in the steady combustion regime. Subsequently, the ratios between the accuracy of the results and the computational cost of the three CFD methodologies are explicitly compared. The second intermediate study is devoted to the ignition sequence preceding the steady combustion regime. A bluff-body stabilized burner based on gaseous fuel, and employing a spark-based igniter, is considered to calibrate the CFD model formulated. This burner of relatively simple geometry can provide greater understanding of complex reactive flow features, especially with regard to ignitability and stability. The most robust of the CFD methodologies formulated in the previous configuration is reconsidered. As this burner involves a partially-premixed combustion mode, a combustion model based on the mixture fraction-progress variable formulation is adopted with the assumptions of infinitely-fast chemistry and finite-rate chemistry through the Bray-Moss-Libby (BML) and Flamelet Generated Manifold (FGM) models, respectively. The ignition model is first customized by implementing the properties of the flame considered. Thereafter, the customized ignition model is coupled to the LES solver and combustion models based on the two above-listed assumptions. To assess the predictive capabilities of the resulting CFD methodologies, the latter are used to predict ignition events resulting from the spark deposition at various locations of the burner, and the results are quantitatively and qualitatively validated by comparing the latter to their experimental counterparts. Finally, the CFD methodology validated in the gaseous configuration is extended to spray combustion by first coupling the latter to the spray module, and by implementing the flame properties of kerosene in the ignition model. The resulting CFD model is first applied to the swirl-stabilized combustor investigated previously, with the aim of predicting the whole ignition sequence and improving the previous predictions of the combustion properties in the resulting steady regime. Subsequently, the CFD methodology is applied to a scaled can combustor with the aim of predicting ignition events under cold start and altitude relight operating conditions. The ability of the CFD methodology to predict ignition events under the two operating conditions is assessed by contrasting the numerical predictions to the corresponding experimental ignition envelopes. A qualitative validation of the ignition sequence is also done by comparing the numerical ignition sequence to the high-speed camera images of the corresponding ignition event.

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Growing environmental concerns are causing a large transformation within the energy industry. Within the gas turbine industry, there is a large drive to develop improved modern dry-low emission combustion systems. The aim is to enable gas turbines to run on green fuels like hydrogen, while still keeping emission as NOx down. To design these systems, a thorough understanding of the aerothermal and kinetic processes within the combustion system of a gas turbine is essential. The goal of the thesis was to develop a one-dimensional general network model of the combustion system of Siemens Energy SGT-700, which accurately could predict pressure losses, mass flows, key temperatures, and emissions. Three models were evaluated and a code that emulated some aspects of the control system was developed. The models and the code were evaluated and compared to each other and to test data from earlier test campaigns performed on SGT-700 and SGT-600. Simulations were also carried out with hydrogen as the fuel.  In the end, a model of the SGT-700 combustion chamber was developed and delivered to Siemens Energy. The model had been verified against test data and predictions made by other Siemens Energy thermodynamic calculation software, for a range of load conditions. The preforms of the model, when hydrogen was introduced into the fuel mixture, were also tested and compared to test data / En växande medvetenhet kring klimatfrågan, har medfört stora förändringar i energibranschen. I och med detta behöver även gasturbinindustrin förbättra de nuvarande dry-low emissions systemen och göra det möjligt för gasturbiner att förbränna gröna bränslen som väte. Samtidigt måste också utsläppen av NOx hållas nere. För att kunna utforma dessa system behövs en fullständig förståelse för de aerotermiska och kinetiska processerna i en gasturbins förbränningskammare. Målet med detta examensarbete var att utveckla en endimensionell generell nätverksmodell för förbränningssystemet i Siemens Energys SGT-700. Modellen skulle noggrant kunna förutsäga tryckförluster, massflöden, viktiga temperaturer samt utsläpp. Tre modeller utvärderades och en kod som emulerade vissa aspekter av styrsystemet utvecklades också. Modellerna och koden utvärderades och jämfördes mot varandra och även mot testdata från tidigare testserier som utfördes på SGT-700 och SGT-600. Simuleringar utfördes också med väte som bränsle. Slutligen levererades en modell av SGT-700 förbränningskammaren till Siemens Energy. Modellen har verifierats för en rad olika lastfall, mot testdata och data som genererats av andra termodynamisk beräkningsprogram som utvecklats av Siemens Energy. Hur modellen uppförde sig när väte var introducerat in i olika lastfall jämfördes också mot testdata