Gas injection into saturated porous media has a high practical relevance. It is applied in groundwater remediation (air sparging), in CO2 sequestration into saline aquifers, and in enhanced oil recovery of petroleum reservoirs. This wide range of application necessitates a comprehensive understanding of gas flow patterns that may develop within the porous media and required modeling of multi-phase flow. There is an ongoing controversy in literature, if continuum models are able to describe the complex flow pattern observed in heterogeneous porous media, especially the channelized stochastic flow pattern. Based on Selker’s stochastic hypothesis, a gas channel is caused by a Brownian-motion process during gas injection. Therefore, the pore-scale heterogeneity will determine the shape of the single stochastic gas channels. On the other hand there are many studies on air sparging, which are based on continuum modeling. Up to date it is not clear under which conditions a continuum model can describe the essential features of the complex gas flow pattern. The aim of this study is to investigate the gas flow pattern on bench-scale and field scale using the continuum model TOUGH2. Based on a comprehensive data set of bench-scale experiments and field-scale experiments, we conduct for the first time a systematic study and evaluate the prediction ability of the continuum model. A second focus of this study is the development of a “real world”-continuum model, since on all scales – pore-scale, bench scale, field scale – heterogeneity is a key driver for the stochastic gas flow pattern. Therefore, we use different geostatistical programs to include stochastic conditioned and unconditioned parameter fields. Our main conclusion from bench-scale experiments is that a continuum model, which is calibrated by different independent measurements, has excellent prediction ability for the average flow behavior (e.g. the gas volume-injection rate relation). Moreover, we investigate the impact of both weak and strong heterogeneous parameter fields (permeability and capillary pressure) on gas flow pattern. The results show that a continuum model with weak stochastic heterogeneity cannot represent the essential features of the experimental gas flow pattern (e.g., the single stochastic gas channels). Contrary, applying a strong heterogeneity the continuum model can represent the channelized flow. This observation supports Stauffer’s statement that a so-called subscale continuum model with strong heterogeneity is able to describe the channelized flow behavior. On the other hand, we compare the theoretical integral gas volumes with our experiments and found that strong heterogeneity always yields too large gas volumes. At field-scale the 3D continuum model is used to design and optimize the direct gas injection technology. The field-scale study is based on the working hypotheses that the key parameters are the same as at bench-scale. Therefore, we assume that grain size and injection rate will determine whether coherent channelized flow or incoherent bubbly flow will develop at field-scale. The results of four different injection regimes were compared with the data of the corresponding field experiments. The main conclusion is that because of the buoyancy driven gas flow the vertical permeability has a crucial impact. Hence, the vertical and horizontal permeability should be implemented independently in numerical modeling by conditioned parameter fields.

In-cylinder fuel-air mixing in a compressed natural gas (CNG)-fuelled, single-cylinder, spark-ignited engine is analysed using a transient three-dimensional computational fluid dynamic model built and run using STAR-CD, a commercial CFD software. This work is motivated by the need for strategies to achieve improved performance in engines utilizing gaseous fuels such as CNG. The transient in-cylinder fuel-air mixing is evaluated for a port gas injection fuelling system and compared with that of conventional gas carburetor system. In this work pure methane is used as gaseous fuel for all the computational studies. It is observed that compared to the premixed gas carburetor system, a substantial level of in-cylinder stratification can be achieved with the port gas injection system. The difference of more than 20% in mass fraction between the rich and lean zones in the combustion chamber is observed for the port gas injection system compared to less than 1% for the conventional premixed system. The phenomenon of stratification observed is very close to the “barrel stratification” mode. A detailed parametric study is undertaken to understand the effect of various injection parameters such as injection location, injection orientation, start of injection, duration of injection and rate of injection. Furthermore, the optimum injection timing is evaluated for various load-speed conditions of the engine. It is also observed that the level of stratification is highest at 50% engine load with a reduced level at 100% load. For low engine loads, the level of stratification is observed to be very low. To analyse the effect of stratification on engine performance, the in-cylinder combustion is modeled using the extended coherent flame model(ECFM). For simulating the ignition process, the arc and kernel tracking ignition model(AKTIM) is used. The combustion model is first validated with measured in-cylinder pressure data and other derived quantities such as heat release rate and mass burn fraction. It is observed that there is a good agreement between measured and simulated values. Subsequently, this model is use to simulate both premixed and stratified cases. It is observed that there is a marginal improvement in terms of overall engine efficiency when the stoichiometric premixed case is compared with the lean stratified condition. However, a major improvement in performance is observed when the lean stratified case is compared with lean premixed condition. The stratified case shows a faster heat release rate which could potentially translate to lower cycle-to-cycle variations in actual engine operation. Also, the stratified cases show as much as 20% lower in-cylinder NOx emissions when compared with the conventional premixed case at the same engine load and speed, underscoring the potential of in-cylinder stratification to achieve improved performance and lower NOx emissions.

Fotonická krystalická vlákna (PCF) představují slibný nástroj pro spojení technik známých z mikroskopie rastrovací sondou, elektronové mikroskopie a systémů pro vstřikování plynu. Přivedení světla a přenos pracovního plynu současně do blízkosti vzorku umístěného uvnitř elektronového mikroskopu přináší nové možnosti experimentů. PCF by mohly být použity nejen k charakterizaci nebo modifikaci struktur na mikroskopické úrovni, ale také k jejich výrobě. Tato práce se zabývá výzkumem literárních zdrojů s tématikou fotonických krystalů s důrazem na PCF. Leptání PCF pomocí kyseliny fluorovodíkové bez poškození jejich vnitřní struktury je experimentálně studováno s velkou precizností. Optické vlastnosti závislé na geometrii PCF jsou testovány pro různé modifikace PCF. Dále se práce zabývá spojením PCF s mikroskopií atomárních sil a následnou integrací do elektronového mikroskopu.

Gas injection into saturated porous media has a high practical relevance. It is applied in groundwater remediation (air sparging), in CO2 sequestration into saline aquifers, and in enhanced oil recovery of petroleum reservoirs. This wide range of application necessitates a comprehensive understanding of gas flow patterns that may develop within the porous media and required modeling of multi-phase flow. There is an ongoing controversy in literature, if continuum models are able to describe the complex flow pattern observed in heterogeneous porous media, especially the channelized stochastic flow pattern. Based on Selker’s stochastic hypothesis, a gas channel is caused by a Brownian-motion process during gas injection. Therefore, the pore-scale heterogeneity will determine the shape of the single stochastic gas channels. On the other hand there are many studies on air sparging, which are based on continuum modeling. Up to date it is not clear under which conditions a continuum model can describe the essential features of the complex gas flow pattern. The aim of this study is to investigate the gas flow pattern on bench-scale and field scale using the continuum model TOUGH2. Based on a comprehensive data set of bench-scale experiments and field-scale experiments, we conduct for the first time a systematic study and evaluate the prediction ability of the continuum model. A second focus of this study is the development of a “real world”-continuum model, since on all scales – pore-scale, bench scale, field scale – heterogeneity is a key driver for the stochastic gas flow pattern. Therefore, we use different geostatistical programs to include stochastic conditioned and unconditioned parameter fields. Our main conclusion from bench-scale experiments is that a continuum model, which is calibrated by different independent measurements, has excellent prediction ability for the average flow behavior (e.g. the gas volume-injection rate relation). Moreover, we investigate the impact of both weak and strong heterogeneous parameter fields (permeability and capillary pressure) on gas flow pattern. The results show that a continuum model with weak stochastic heterogeneity cannot represent the essential features of the experimental gas flow pattern (e.g., the single stochastic gas channels). Contrary, applying a strong heterogeneity the continuum model can represent the channelized flow. This observation supports Stauffer’s statement that a so-called subscale continuum model with strong heterogeneity is able to describe the channelized flow behavior. On the other hand, we compare the theoretical integral gas volumes with our experiments and found that strong heterogeneity always yields too large gas volumes. At field-scale the 3D continuum model is used to design and optimize the direct gas injection technology. The field-scale study is based on the working hypotheses that the key parameters are the same as at bench-scale. Therefore, we assume that grain size and injection rate will determine whether coherent channelized flow or incoherent bubbly flow will develop at field-scale. The results of four different injection regimes were compared with the data of the corresponding field experiments. The main conclusion is that because of the buoyancy driven gas flow the vertical permeability has a crucial impact. Hence, the vertical and horizontal permeability should be implemented independently in numerical modeling by conditioned parameter fields.