Large Eddy simulation of tracer gas dispersion in a cavity

This paper assesses the prediction of inert tracer gas dispersion within a cavity of height (H) 1.0 m, and unity aspect ratio, using large Eddy simulation (LES). The flow Reynolds number was 67 000, based on the freestream velocity and cavity height. The flow upstream of the cavity was laminar, producing a cavity shear layer which underwent a transition to turbulence over the cavity. Three distinct meshes are used, with grid spacings of H / 100 (coarse), H / 200 (intermediate), and H / 400 (fine) respectively. The Smagorinsky, WALE, and Germano-Lilly subgrid-scale models are used on each grid to quantify the effects of subgrid-scale modelling on the simulated flow. Coarsening the grid led to small changes in the predicted velocity field, and to substantial over-prediction of the tracer gas concentration statistics. Quantitative metric analysis of the tracer gas statistics showed that the coarse grid simulations yielded results outside of acceptable tolerances, while the intermediate and fine grids produced acceptable output. Interrogation of the fluid dynamics present in each simulation showed that the evolution of the cavity shear layer is heavily influenced by the grid and subgrid scale model. On the coarse and intermediate grids the development of the shear layer is delayed, inhibiting the entrainment and mixing of the tracer gas into the shear layer, reducing the removal of the tracer gas from the cavity. On the fine grid, the shear layer developed more rapidly, resulting in enhanced removal of the tracer gas from the cavity. Concentration probability density functions showed that the fine grid simulations accurately predicted the range, and the most probable value, of the tracer gas concentration towards both walls of the cavity. The results presented in this paper show that the WALE and Germano-Lilly models may be advantageous over the standard Smagorinsky model for simulations of pollutant dispersion in the urban environment.

STUDY OF GAS FUEL LEAKAGE AND EXPLOSION IN THE ENGINE ROOM OF A SMALL LNG-FUELED SHIP

The engine fuel piping in LNG-fuelled ships’ engine room presents potential gas explosion risks due to possible gas fuel leakage and dispersion. A 3D CFD model with chemical reaction was described, validated and then used to simulate the possible gas dispersion and the consequent explosions in an engine room with regulations commanded ventilations. The results show that, with the given minor leaking of a fuel pipe, no more than 1kg of methane would accumulate in the engine room. The flammable gas clouds only exit in limited region and could lead to explosions with an overpressure about 12 mbar, presenting no injury risk to personnel. With the given major leaking, large region in the engine room would be filled with flammable gas cloud within tens of seconds. The gas cloud might lead to an explosion pressure of about 1 bar or higher, which might result in serious casualties in the engine room.

Methane Fluxes at the Water–Atmosphere Interface and Gas-Geochemical Anomalies in the Bottom Sediments in the Northwestern Part of the Sea of Japan

—We present results of an integrated research into the spatial distribution of methane in the area of the northern closure of the Central Basin of the Sea of Japan and in the southern part of the Tatar Trough. Methane emissions have been revealed in the study area. The methane fluxes are distributed unevenly within the area (1 to 23 mol/(km2·day)). The discrete high-frequency measurements and calculation of methane fluxes at the water–atmosphere interface, combined with the study of the content of natural gases and microbiologic parameters in sediment cores, allow us to explain the formation of local methane emission zones in the water area. Despite the great sea depths, there are sources and fluid-conducting zones that determine methane concentrations (exceeding the equilibrium ones) and high methane emissions from the water area. The data obtained provide new information and suggest the presence of deep gas sources, which determine gas dispersion in the bottom sediments, the methane content in the surface water layer, and the distribution of methane fluxes at the water–atmosphere interface in the study area. This study is part of the integrated program of geological and geophysical expeditionary research performed by V.I. Il’ichev Pacific Oceanological Institute (Vladivostok) in the northern part of the Sea of Japan.

Gas-phase crosslinking of the lignin on the nanoscale fumed silica surface

A method for the polymerization modification of nanoscale fumed silica by crosslinking a lignin layer adsorbed on a nanosilica surface under a gas dispersion medium is described. A mixture of phenol and formaldehyde in the presence of HCl proved to be the most effective crosslinking agent. It has been suggested that the crosslinking of lignin molecules occurs by a mechanism similar to the production of phenol-formaldehyde resins.

CFD analysis on the effect of temperature on gas distribution in indoor environment

There are times when people are required to spend time indoors, especially due to unhealthy outdoor air quality as well during a pandemic when lockdowns are imposed. However, spending time indoors can also at times be dangerous due to the release of harmful gasses if left unchecked. This has very much to do with many parameters, among which is the indoor environment and its ventilation. The latter is affected by the way gases distribute inside the building. It is influenced by many factors such as temperature, wind, air circulation, and also ventilation system itself. The knowledge on how the gases spread in different conditions within the indoor environment can be utilized in many applications such as improving the smoke detector safety system and identifying as well as predicting the potential risks. This paper presents the investigation of the effect of different temperatures on gas distribution in an indoor environment. A three-dimensional simulation was performed of different temperature gas released in a closed room that has different ambient temperatures. The effect of temperature on the gas dispersion was observed. The results revealed that there is a significant effect of temperature on the way gas spread in the indoor environment support by the theoretical knowledge on the relationship between temperature and gas in the gas law.

Comprehensive Optimization of the Dispersion of Mixing Particles in an Inert-Particle Spouted-Bed Reactor (IPSBR) System

Effective gas dispersion and liquid mixing are significant parameters in the design of an inert-particle spouted-bed reactor (IPSBR) system. Solid particles can be used to ensure good mixing and an efficient rate of mass and heat transfer between the gas and liquid. In this study, computational fluid dynamics (CFD) coupled with the discrete phase model (DPM) were developed to investigate the effect of the feed gas velocity (0.5–1.5 m/s), orifice diameter (0.001–0.005 m), gas head (0.15–0.35 m), particle diameter (0.009–0.0225 m), and mixing-particle-to-reactor-volume fraction (2.0–10.0 vol.%) on the solid mass concentration, average solid velocity, and average solid volume fraction in the upper, middle, and conical regions of the reactor. Statistical analysis was performed using a second-order response surface methodology (RSM) with central composite design (CCD) to obtain the optimal operating conditions. Selected parameters were optimized to maximize the responses in the middle and upper regions, and minimize them in the conical region. Such conditions produced a high interfacial area and fewer dead zones owing to good particle dispersion. The optimal process variables were feed gas velocity of 1.5 m/s, orifice diameter of 0.001 m, gas head of 0.2025 m, a particle diameter of 0.01 m, and a particle load of 0.02 kg. The minimum average air velocity and maximum air volume fraction were observed under the same operating conditions. This confirmed the novelty of the reactor, which could work at a high feed gas velocity while maintaining a high residence time and gas volume fraction.