Dependence of kinetic sensitivity direction in premixed flames

© 2020 The sensitivities of turbulent combustion simulations to chemical kinetic parameters can be analyzed to understand the controlling reactions in turbulent flames and to quantify the uncertainties in simulations. However, computing the sensitivity of turbulent combustion simulations to a large number of kinetic parameters is still challenging. A promising approach is to estimate the sensitivity from laminar flames, especially for cases where the flamelet model is applicable. Under these conditions, the underlying hypothesis is that the sensitivity direction of the flamelet profiles is independent of the strain rate and the flame coordinate, which is the progress variable for premixed flames. In the present work, this hypothesis was tested in laminar premixed counterflow flames. We first studied the sensitivity directions of two extreme cases, the near-extinction strained flames and the freely propagating unstretched flames. It was found that the sensitivity directions of the extinction strain rate and the laminar flame speed are aligned with each other for various fuels, equivalence ratios, and pressures. We then studied the dependence of the sensitivity direction of the maximum flame temperature on the strain rate as well as the dependence of the sensitivity direction of the species profiles on the progress variable. It was found that the sensitivity direction of maximum temperature was largely independent of the strain rate. Moreover, the sensitivity directions of the temperature and species profiles were independent of the progress variable, and they were all similar to the sensitivity direction of the extinction strain rate. These findings suggest that there is a universal sensitivity direction for turbulent premixed flames and the direction can be estimated by the sensitivity direction of extinction strain rate. These conclusions will enable efficient sensitivity analysis of turbulent combustion simulations when the hypothesis is valid

Molecular Dynamics Simulation of Self-Aggregation of Asphaltenes at an Oil/Water Interface: Formation and Destruction of the Asphaltene Protective Film

It is well known that asphaltene molecules play a significant role in stabilizing emulsions of water in crude oil or diluted bitumen solutions. Molecular dynamics simulations were employed to investigate the aggregation and orientation behaviors of asphaltene molecules in a vacuum and at various water surfaces. Two different continental model asphaltene molecules were employed in this work. It was found that the initially disordered asphaltenes quickly self-assembled into ordered nano­aggregates consisting of several molecules, in which the aromatic rings in asphaltenes were reoriented to form a face-to-face stacked structure. More importantly, statistical analysis indicates that most of the stacked polycyclic aromatic planes of asphaltene nano­aggregates tend to be perpendicular to the water surface. If the asphaltene molecules are considered as “stakes”, then the asphaltene nano­aggregate can be regarded as a “fence”. All the fence-like nano­aggregates were twined and knitted together, which pinned them perpendicularly on the water surface to form a steady protective film wrapping the water droplets. The mechanism of stabilization of the water/oil emulsions is thereby well understood. Demulsification processes using a chemical demulsifier were also studied. It was observed that the asphaltene protective film was destroyed by a demulsifier of ethyl cellulose molecules, leading to exposure of the water droplet. The results obtained in this work will be of significance in guiding the development of demulsification technology

Molecular Dynamics Simulation of Self-Aggregation of Asphaltenes at an Oil/Water Interface: Formation and Destruction of the Asphaltene Protective Film

It is well known that asphaltene molecules play a significant role in stabilizing emulsions of water in crude oil or diluted bitumen solutions. Molecular dynamics simulations were employed to investigate the aggregation and orientation behaviors of asphaltene molecules in a vacuum and at various water surfaces. Two different continental model asphaltene molecules were employed in this work. It was found that the initially disordered asphaltenes quickly self-assembled into ordered nano­aggregates consisting of several molecules, in which the aromatic rings in asphaltenes were reoriented to form a face-to-face stacked structure. More importantly, statistical analysis indicates that most of the stacked polycyclic aromatic planes of asphaltene nano­aggregates tend to be perpendicular to the water surface. If the asphaltene molecules are considered as “stakes”, then the asphaltene nano­aggregate can be regarded as a “fence”. All the fence-like nano­aggregates were twined and knitted together, which pinned them perpendicularly on the water surface to form a steady protective film wrapping the water droplets. The mechanism of stabilization of the water/oil emulsions is thereby well understood. Demulsification processes using a chemical demulsifier were also studied. It was observed that the asphaltene protective film was destroyed by a demulsifier of ethyl cellulose molecules, leading to exposure of the water droplet. The results obtained in this work will be of significance in guiding the development of demulsification technology

Functionalized carbon black nanoparticles used for separation of emulsified oil from oily wastewater

<p>Functionalized carbon black (F-CB) nanoparticles were synthesized by covalently grafting the polyvinyl alcohol on carbon black (CB) surfaces and used as demulsifier to separate the oil from the emulsified oily wastewater. The bottle test showed that the residual oil content in the separated water was as low as ∼50 mg/L corresponding to a demulsification efficiency of about 99.90% at an optimal condition within a few minutes. It was believed that the surface wettability of the carbon black could be tuned by modifying with the PVA molecules, which enables the F-CB nanoparticles to be readily migrated to the oil/water interface and have the opportunity to interact with and/or displace the stabilizers of the emulsion. As a result, the demulsification process was accomplished with the coalescence of the oil droplets promoted by the F-CB nanoparticles. The interaction behavior between F-CB nanoparticles and asphaltenes was investigated by quantum chemical calculations. The results showed that the F-CB nanoparticles have strong interaction with the asphaltene molecules in form of π−π and θ−π forces. The findings in present study are significant for understanding the demulsification mechanism and also provide a novel demulsifier for the demulsification of emulsified oily wastewater.</p