The current research focuses on the thermoacoustic instability of lean premixed combustion, which is a promising technique to inhibit Nitrogen Oxides (NOx) emission. Thermoacoustic instability describes the condition that the pressure oscillation is unusually high in the combustion device. It results from the coupling between pressure fluctuation and heat release oscillation, which experiences significant temporal and spatial variations. These variations are closely related to the flame shape deformation and critical in determining the trend of the global instability. Therefore, the current study aims to examine both the global and local flame features created by thermoacoustic instability. The first part of the work is studying the unstable flame induced by artificial acoustic perturbation. The particular focus is on the global and local heat release rate oscillation. In the experiment, the global heat release rate oscillation was indicated by the hydroxyl (OH*) chemiluminescence captured with a photomultiplier tube (PMT). On the other hand, the flame shape and the local mean heat release rate were examined with flame surface density (FSD), which was calculated with the images captured with the planar laser-induced fluorescence of the hydroxide radical (OH-PLIF) method. The main analysis methods used in the current research are Rayleigh criterion and proper orthogonal decomposition (POD), which can efficiently capture the dominant oscillation mode of the flame. The acoustic perturbation study first examined the effect of pressure variation (0.1 - 0.4 MPa) on the flame response to the acoustic perturbation. Results show that the elevated pressure intensifies the fundamental mode of heat release oscillation when the heat release oscillation is in phase with the pressure fluctuation; otherwise, the fundamental oscillation tends to be inhibited. The pressure affects both the strength and the distribution of the local fundamental and the first harmonic oscillations. Furthermore, the effect of the pressure on the distribution is larger than that on the strength. The study also investigated the role of Strouhal numbers in characterizing the flame oscillation induced by acoustic perturbation. Results show that the Strouhal number can characterize the changing trend of the oscillation amplitude, whereas the oscillation phase-delay is less dependent on the Strouhal number. The local analysis reveals that the nonlinear flame behavior results from the flame rollup induced by acoustic perturbation. Furthermore, the reconstruction of the global heat release shows that the cancellation of out-of-phase local oscillations can cause a low-level global oscillation. Results also demonstrate that the local heat release oscillation contains intense harmonic oscillations, which are closely associated with the flame rollup. However, the harmonic oscillation is less likely the main reason causing nonlinear flame behavior. Besides the study with acoustic perturbation, the current study also conducted experimental and modeling studies on the self-excited thermoacoustic instability. The particular focus is examining the effects of hydrogen addition on the instability trend. Results demonstrate that the hydrogen concentration can affect both the oscillation frequency and amplitude. Pressure analysis shows that the low-frequency mode is triggered when the hydrogen concentration is low, whereas a high hydrogen concentration tends to excite a high-frequency mode. Moreover, the frequency tends to increase with an increasing hydrogen concentration. Modeling results illustrate that the change of the oscillation mode, which is determined by the turbulent flame speed, is mainly affected by the delay time between the heat release oscillation and the velocity fluctuation. The modeling work shows that the one-dimensional model is not very efficient in capture the instability trend of the high-frequency mode. It may result from the lack of the knowledge of the mechanism of acoustic damping and flame dynamics.

Prediction of flow and combustion in IC engines remains a challenging task. Traditional Reynolds Averaged Navier Stokes (RANS) methods and emerging Large Eddy Simulation (LES) techniques are being used as reliable mathematical tools for such predictions. However, RANS models have to be further refined to make them more predictive by eliminating or reducing the requirement for application based fine tuning. LES holds a great potential for more accurate predictions in engine related unsteady combustion and associated cycle-tocycle variations. Accordingly, in the present work, new advanced CFD based flow models were developed and validated for RANS and LES modelling of turbulent premixed combustion in SI engines. In the research undertaken for RANS modelling, theoretical and experimental based modifications have been investigated, such that the Bray-Moss-Libby (BML) model can be applied to wall-bounded combustion modelling, eliminating its inherent wall flame acceleration problem. Estimation of integral length scale of turbulence has been made dynamic providing allowances for spatial inhomogeneity of turbulence. A new dynamic formulation has been proposed to evaluate the mean flame wrinkling scale based on the Kolmogorov Pertovsky Piskunow (KPP) analysis and fractal geometry. In addition, a novel empirical correlation to quantify the quenching rates in the influenced zone of the quenching region near solid boundaries has been derived based on experimentally estimated flame image data. Moreover, to model the spark ignition and early stage of flame kernel formation, an improved version of the Discrete Particle Ignition Kernel (DPIK) model was developed, accounting for local bulk flow convection effects. These models were first verified against published benchmark test cases. Subsequently, full cycle combustion in a Ricardo E6 engine for different operating conditions was simulated. An experimental programme was conducted to obtain engine data and operating conditions of the Ricardo E6 engine and the formulated model was validated using the obtained experimental data. Results show that, the present improvements have been successful in eliminating the wall flame acceleration problem, while accurately predicting the in-cylinder pressure rise and flame propagation characteristics throughout the combustion period. In the LES work carried out in this research, the KIVA-4 RANS code was modified to incorporate the LES capability. Various turbulence models were implemented and validated in engine applications. The flame surface density approach was implemented to model the combustion process. A new ignition and flame kernel formation model was also developed to simulate the early stage of flame propagation in the context of LES. A dynamic procedure was formulated, where all model coefficients were locally evaluated using the resolved and test filtered flow properties during the fully turbulent phase of combustion. A test filtering technique was adopted to use in wall bounded systems. The developed methodology was then applied to simulate the combustion and associated unsteady effects in Ricardo E6 spark ignition engine at different operating conditions. Results show that, present LES model has been able to resolve the evolution of a large number of in-cylinder flow structures, which are more influential for engine performance. Predicted heat release rates, flame propagation characteristics, in-cylinder pressure rise and their cyclic variations are also in good agreement with measurements.

Turbulent premixed propane/air and methane/air flames were studied using planar Rayleigh scattering and particle image velocimetry on a stabilized Bunsen type burner. The fuel-air equivalence ratio was varied from Φ=0.7 to 1.0 for propane flames, and from Φ=0.6 to 1.0 for methane flames. The non-dimensional turbulence intensity, u'/SL (ratio of fluctuation velocity to laminar burning velocity), covered the range from 3 to 24, equivalent to conditions of corrugated flamelets and thin reaction zones regimes. Temperature gradients decreased with the increasing u'/SL and levelled off beyond u'/SL > 10 for both propane and methane flames. Flame front thickness increased slightly as u'/SL increased for both mixtures, although the thickness increase was more noticeable for propane flames, which meant the thermal flame front structure was being thickened. A zone of higher temperature was observed on the average temperature profile in the preheat zone of the flame front as well as some instantaneous temperature profiles at the highest u'/SL. Curvature probability density functions were similar to the Gaussian distribution at all u'/SL for both mixtures and for all the flame sections. The mean curvature values decreased as a function of u'/SL and approached zero. Flame front thickness was smaller when evaluated at flame front locations with zero curvature than that with curvature. Temperature gradients and FSD were larger when the flame curvature was zero. The combined thickness and FSD data suggest that the curvature effect is more dominant than that of the stretch by turbulent eddies during flame propagation. Integrated flame surface density for both propane and methane flames exhibited no dependance on u'/SL regardless of the FSD method used for evaluation. This observation implies that flame surface area may not be the dominant factor in increasing the turbulent burning velocity and the flamelet assumption may not be valid under the conditions studied. Dκ term, the product of diffusivity evaluated at conditions studied and the flame front curvature, was a magnitude smaller than or the same magnitude as the laminar burning velocity.

Dans les moteurs à combustion interne, l'injection de carburant est une phase essentielle pour la préparation du mélange et la combustion. En effet, la structure du jet liquide joue un rôle essentiel pour la qualité du mélange du combustible avec le gaz. Le présent travail porte sur les phénomènes d'atomisation de jet liquides dans les conditions opératoires des moteurs diesel. Dans ces conditions, la morphologie du jet liquide comprend une phase liquide séparée (c'est à dire un noyau liquide) et une phase liquide dispersée (c'est à dire un spray). Ce manuscrit décrit les étapes de développement d'un nouveau modèle d'atomisation, pour un jet liquide à grande vitesse, basée sur une approche eulérienne diphasique. Le phénomène d'atomisation est modélisée par des équations définissant une densité de surface pour le noyau liquide en plus de celle des gouttelettes du spray. Ce nouveau modèle a été couplé avec un système d'équations diphasique et turbulent de type Baer-Nunziato. Le processus de rupture des ligaments et son éclatement subséquent en gouttelettes sont modélisés en utilisant des connaissances rassemblées à partir des expériences disponibles et des simulations numériques précises. Dans la région dense du jet de liquide, l'atomisation primaire est modélisée comme un processus de dispersion en raison de l'étirement turbulent de l'interface, à partir du côté du liquide en plus du côté du gaz. Différents cas tests académiques ont été effectués afin de vérifier la mise en œuvre numérique du modèle dans le code IFP-C3D. Enfin, le modèle est validé avec les résultats DNS récemment publiés dans des conditions typiques de moteurs Diesel à injection directe. / In internal combustion engines, the liquid fuel injection is an essential step for the air/fuel mixture preparation and the combustion process. Indeed, the structure of the liquid jet coming out from the injector plays a key role in the proper mixing of the fuel with the gas in the combustion chamber. The present work focuses on the liquid jet atomization phenomena under Diesel engine conditions. Under these conditions, liquid jet morphology includes a separate liquid phase (i.e. a liquid core) and a dispersed liquid phase (i.e. a spray). This manuscript describes the development stages of a new atomization model, for a high speed liquid jet, based on an eulerian two-phase approach. The atomization phenomenon is modeled by defining different surface density equations, for the liquid core and the spray droplets. This new model has been coupled with a turbulent two-phase system of equations of Baer-Nunziato type. The process of ligament breakup and its subsequent breakup into droplets are handled with respect to available experiments and high fidelity numerical simulations. In the dense region of the liquid jet, the atomization is modeled as a dispersion process due to the turbulent stretching of the interface, from the side of liquid in addition to the gas side. Different academic test cases have been performed in order to verify the numerical implementation of the model in the IFP-C3D software. Finally, the model is validated with the recently published DNS results under typical conditions of direct injection Diesel engines.

Reduction of the pollutant emissions is a challenge for the gas turbine industry. A solution to this problem is to employ the low swirl burner which can operate at lower equivalence ratios than a conventional swirl burner. However, flames in the lean regime of combustion are susceptible to flow perturbations and combustion instability. Combustion instability is the coupling between unsteady heat release and combustor acoustic modes where one amplifies the other in a feedback loop. The other method for significantly reducing NOx and CO2 is increasing fuel reactivity, typically done through the addition of hydrogen. This helps to improve the flammability limit and also reduces the pollutants in products by decreasing thermal NOx and reducing CO2 by displacing carbon. In this work, the flammability limits of a low swirl burner at various operating conditions, is studied and the effect of pressure, bulk velocity, burner shape and percent of hydrogen (added to the fuel) is investigated. Also, the flame structure for these test conditions is measured using OH planar laser induced fluorescence and assessed. Also, the OH PLIF data is used to calculate Rayleigh index maps and to construct averaged OH PLIF intensity fields at different acoustic excitation frequencies (45-155, and 195Hz). Based on the Rayleigh index maps, two different modes of coupling between the heat release and the pressure fluctuation were observed: the first mode, which occurs at 44Hz and 55Hz, shows coupling to the flame base (due to the bulk velocity) while the second mode shows coupling to the sides of the flame. In the first mode, the flame becomes wider and the flame base moves with the acoustic frequency. In the second mode, imposed pressure oscillations induce vortex shedding in the flame shear layer. These vortices distort the flame front and generate locally compact and sparse flame areas. The local flame structure resulting from these two distinct modes was markedly different.

Turbulent premixed propane/air and methane/air flames were studied using planar Rayleigh scattering and particle image velocimetry on a stabilized Bunsen type burner. The fuel-air equivalence ratio was varied from Φ=0.7 to 1.0 for propane flames, and from Φ=0.6 to 1.0 for methane flames. The non-dimensional turbulence intensity, u'/SL (ratio of fluctuation velocity to laminar burning velocity), covered the range from 3 to 24, equivalent to conditions of corrugated flamelets and thin reaction zones regimes. Temperature gradients decreased with the increasing u'/SL and levelled off beyond u'/SL > 10 for both propane and methane flames. Flame front thickness increased slightly as u'/SL increased for both mixtures, although the thickness increase was more noticeable for propane flames, which meant the thermal flame front structure was being thickened. A zone of higher temperature was observed on the average temperature profile in the preheat zone of the flame front as well as some instantaneous temperature profiles at the highest u'/SL. Curvature probability density functions were similar to the Gaussian distribution at all u'/SL for both mixtures and for all the flame sections. The mean curvature values decreased as a function of u'/SL and approached zero. Flame front thickness was smaller when evaluated at flame front locations with zero curvature than that with curvature. Temperature gradients and FSD were larger when the flame curvature was zero. The combined thickness and FSD data suggest that the curvature effect is more dominant than that of the stretch by turbulent eddies during flame propagation. Integrated flame surface density for both propane and methane flames exhibited no dependance on u'/SL regardless of the FSD method used for evaluation. This observation implies that flame surface area may not be the dominant factor in increasing the turbulent burning velocity and the flamelet assumption may not be valid under the conditions studied. Dκ term, the product of diffusivity evaluated at conditions studied and the flame front curvature, was a magnitude smaller than or the same magnitude as the laminar burning velocity.

The search for clean and efficient combustors is motivated by the increasingly stringent emissions regulations. New gas turbine engines are designed to operate under lean conditions with inhomogeneous reactants to ensure cleanliness and stability of the combustion. This ushers in a new mode of combustion, called the inhomogeneous reactants premixed combustion. The present study investigates the effects of inhomogeneous reactants on premixed combustion, specifically on the interactions of an initially planar flame with field of inhomogeneous reactants. Unsteady and unstrained laminar methane-air flames are studied in one- and two-dimensional simulations to investigate the effects of normally and tangentially (to the flame surface) stratified reactants. A three-dimensional DNS of turbulent inhomogeneous reactants premixed combustion is performed to extend the investigation into turbulent flames. The methaneair combustion is represented by a complex chemical reaction mechanism with 18 species and 68 steps. The flame surface density (FSD) and displacement speed S_d have been used as the framework to analyse the inhomogeneous reactants premixed flame. The flames are characterised by an isosurface of reaction progress variable. The unsteady flames are compared to the steady laminar unstrained reference case. An equivalence ratio dip is observed in all simulations and it can serve as a marker for the premixed flame. The dip is attributed to the preferential diffusion of carbon- and hydrogen- containing species. Hysteresis of S_d is observed in the unsteady and unstrained laminar flames that propagate into normally stratified reactants. Stoichiometric flames propagating into lean mixture have a larger S_d than lean flames propagating into stoichiometric mixtures. The cross-dissipation term contribution to S_d is small (~~10%) but its contribution to the hysteresis of S_d is not (~~50%). Differential propagation of the flame surface is observed in the laminar flame that propagates into tangentially stratified reactants. Stretch on the flame surface is induced by the differential propagation, which in turn increases the flame surface area.

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.

La situation en Antarctique est complexe. Continent peu connu et isolégéographiquement,les processus qui contrôlent son bilan de masse et son bilan d'énergie sont encore mal compris. Dans ce contexte, l'étude de la température de surface connaît un intérêt grandissant de la part de la communauté scientifique. En effet, en contrôlant fortement la température de la neige jusqu'à des dizaines, voire des centaines, de mètres sous la surface, la température de surface influence l'état thermique de la calotte du plateau antarctique, sa dynamique et, par conséquent, son bilan de masse. De plus, en agissant sur les émissions de flux thermiques infrarouges et sur les flux turbulents de chaleurs sensibles et latents, la température de surface est directement liée au bilan énergétique de surface du plateau antarctique. L'analyse de la température de surface et l'étude des processus physiques à l'origine de sa variabilité participent à l'amélioration de la compréhension du bilan énergétique de surface, étape nécessaire pour déterminer l'état actuel de sa calotte et faire des prévisions sur sa potentielle contribution à l'élévation du niveau des mers. Ce travail de thèse participe à cet effort en s'intéressant au cycle diurne de la température de surface et aux différents facteurs contribuant à sa variabilité spatiale et temporelle sur le plateau antarctique. Il débute par une évaluation de différentes données entre 2000 et 2012 montrant le bon potentiel de la température de surface MODIS qui peut dès lors être utilisée comme donnée de référence pour l'évaluation des modèles et réanalyses. Un biais chaud systématique de 3 à 6°C dans la réanalyse ERA-interim de la température de surface est ainsi mis en évidence sur le plateau antarctique. L'observation du cycle diurne de la température de surface a, quant à elle, permis d'identifier la densité superficielle parmi ses facteurs de variabilité. Sur les premiers centimètres du manteau neigeux où se concentrent la majorité des échanges de masse et d'énergie entre l'atmosphère et la calotte antarctique, la densité de la neige est une donnée cruciale car elle agit sur l'absorption du rayonnement solaire dans le manteau neigeux mais également sur la conductivité thermique du manteau et donc sur la propagation de la chaleur entre la surface et les couches en profondeur. La densité superficielle de la neige présente cependant de nombreuses incertitudes sur sa variabilité spatio-temporelle et sur les processus qui la contrôlent. De plus, ne pouvant être mesurées qu'in situ, les données de densité superficielle en Antarctique sont restreintes géographiquement. Cette thèse explore une nouvelle application de la température de surface consistant à estimer la densité superficielle de la neige via une méthode d'inversion de simulations numériques. Une carte de la densité superficielle en Antarctique a ainsi pu être produite en minimisant l'erreur de simulation sur l'amplitude diurne. / The antarctic ice sheet is a key element in the climate system and an archive of past climate variations. However, given the scarcity of observations due to the geographical remoteness of Antarctica and its harsh conditions, little is known about the processes that control its mass balance and energy. In this context, several studies focus on the surface temperature which controls the snow temperature up to tens, if not hundreds, of meters beneath the surface. It also influences the thermal state of the antarctic ice sheet, its dynamics, and thus, its mass balance. Surface temperature is also directly linked to the surface energy balance through its impact on thermal and surface turbulent heat flux emissions. Thus, surface temperature analysis and the study of physical processes that control surface temperature variability contribute to the better understanding of the surface energy balance, which is a necessary step to identify the actual state of the antarctic ice sheet and forecast its impact on sea level rise. This thesis work contributes to this effort by focusing on the surface temperature diurnal cycle and various factors impacting spatial and temporal surface temperature variability on the Antarctic Plateau. First, an evaluation of MODIS data, done by comparison with in situ measurements, shows MODIS great potential in the observation of the surface temperature of the Antarctic Plateau under clear-sky conditions. Hourly MODIS surface temperature data from 2000 to 2011 were then used to evaluate the accuracy of snow surface temperature in the ERA-Interim reanalysis and the temperature produced by a stand-alone simulation with the Crocus snowpack model using ERA-Interim forcing. It reveals that ERA-Interim has a widespread warm bias on the Antarctic Plateau ranging from +3 to +6°C depending on the location. Afterwards, observations of the surface temperature diurnal cycle allow an identification of the surface density as a factor of surface temperature variability. On the topmost centimeters of the snowpack where most mass and energy exchanges between the surface and atmosphere happen, density is critical for the energy budget because it impacts both the effective thermal conductivity and the penetration depth of light. However, there are considerable uncertainties around surface density spatio-temporal variability and the processes that control it. Besides, since surface density can only be measured in situ, surface density measurements in Antarctica are restricted to limited geographical areas. Thus, this thesis also explores a new application of surface temperature by estimating surface density in Antarctica based on the monotonic relation between surface density and surface temperature diurnal amplitude. A map of surface density is obtained by minimising the simulation error related to diurnal amplitude of the surface temperature.

The search for water has been vital in our solar system as water indicates a potential for life. The search has been extended to satellites orbiting the outer planets. It has already been proven that water exists on Saturn’s moon, Enceladus, where the spacecraft Cassini did a flyby through a water plume. The study of the water plumes has been implemented on Jupiter’s moon, Europa, where there are signs of an underwater ocean existing. Several studies have implemented an analytical model for the gas distribution of plumes to match the detected signals with their method of plotting the distribution of water gas. In this report, we quantitatively analyzed plume distributions used in studies by Roth et al. (2014), Blöcker et al. (2016), Sparks et al. (2016), Jia et al. (2019), Arnold et al. (2019), and Huybrighs et al. (2020). In these plume studies there are 2 different plume shapes used, namely the expanding teardrop and the expanding circular shape. Different scale heights are used for the teardrop shape, leading to either shorter or longer appearance of the teardrop plume. Comparing the plume distribution of these analytical plumes, Roth’s method is the most sensitive one in this category, meaning that Roth’s plume model had the lowest quantity in terms of the density distribution values. Arnold’s plume model is the densest for altitudes below roughly 300km and Huybrigh’s plume model for altitudes above 300km. In terms of the vertical and the horizontal column densities and the total number of molecules, the most sensitive method in each category was Jia’s, Roth’s, and Blöcker’s plume model respectively. Our results suggest that the plume models are rather inconsistent with each other quantitatively. / Sökandet efter vatten har varit avgörande i vårt solsystem eftersom vatten indikerar en potential för liv. Sökandet har utvidgats till satelliter som kretsar kring de yttre planeterna. Det har redan bevisats att vatten finns på Saturnus måne, Enceladus där rymdfarkosten Cassini flyger förbi genom en vattenplym. Studiet av vattenplymer har genomförts på Jupiters måne, Europa, där det finns tecken på att ett undervattenshav existerar. Flera studier har implementerat en analytisk modell för gasfördelning av plymer för att matcha de detekterade signalerna med deras metod för att plotta fördelningen av vattengas. I den här rapporten analyserade vi kvantitativt plymfördelningar som används i studier av Roth et al. (2014), Blöcker et al. (2016), Sparks et al. (2016), Jia et al. (2019), Arnold et al. (2019) och Huybrighs et al. (2020). I dessa plymstudier används 2 olika plymformer, nämligen den expanderande tårformen och den expanderande cirkulära formen. Olika skalhöjder används för droppformen, vilket leder till antingen kortare eller längre utseende av droppplymen. Om man jämför plymens fördelning av dessa analytiska plymer, är Roths plymmodell den mest känsliga. Arnolds plymmodell är den tätaste för höjder under ungefär 300 km och Huybrighs plymmodell för höjder över 300 km. När det gäller de vertikala och horisontella kolumndensiteterna och det totala antalet molekyler var den känsligaste plymmodellen Jias, Roths respektive Blöckers plymmodell. Våra resultat tyder på att plymmodellerna är ganska inkonsekventa med varandra kvantitativt.