CFD simulation of methane dispersion dynamics

. Anthropogenic activities involve the extraction, transformation and use of natural resources. During the process of extracting useful mineral substances on the surface or underground, particles or gases appear in the work environment resulting from the removal or natural release from the rock mass. The process of transforming useful mineral substances takes place on the surface in closed premises and involves the use of complex technological processes. During these, gases, fog, dust or powders of an explosive or toxic nature can be released or reach the working atmosphere. Industrial pollutants in confined spaces are released into the atmosphere for worker safety. If workers are caught in fires or explosions caused by flammable or explosive substances, morbidity is extremely high. The paper presents the CFD analysis regarding the determination of methane dispersion dynamics at the level of a closed enclosure. Through the CFD analysis, the methane flow and dispersion paths were identified, respectively the areas with minimum concentrations in order to safely evacuate the workers.


Introduction
The CFD technique is widely used to study various physical phenomena, including the dispersion of gases in closed, semi-closed or open spaces [3; 5; 9; 11; 14; 16].For the simulation of gases, the CFD technique uses specific mathematical relations such as the Euler, Gromeka -Lamb, Cauchy, Helmholtz, Navier -Stokes equations, etc. specific to the laminar flow of real fluids [10; 12; 13; 15; 18; 19; 22].For efficiency, solving algebraic equations is done using IT technique.In the case of turbulent flow of real fluids, semi-physical models are used.The CFD technique is very complex, which is why the simulation time is much longer, sometimes requiring many days to complete the modeling.The simulation of various physical phenomena, including the dispersion of gases, can be achieved both experimentally and through the CFD technique, the results obtained being sometimes different [8; 17].Due to the effectiveness of this technique, predictions of the physical effects generated by fluid flow can be obtained, even in conditions where the fluid encounters obstacles on the flow path, which is why it is extremely useful for establishing the dispersion dynamics of gases and especially methane in a closed enclosure [6; 7; 20; 21].It is useful to establish the evolution of methane dispersion dynamics both horizontally and vertically because there is a possibility that at the level of the closed enclosure there are different concentrations of gas higher or lower than the calculated average concentration [1; 2].The purpose of the work is both to establish the areas in the closed premises where the methane concentration exceeds the lower explosive limit of methane and where the risk of explosion is imminent, as well as the areas where the methane concentration is minimal for the purpose of the safe evacuation of workers.

Technique used description
For the study of the dispersion dynamics of explosive, toxic and asphyxiating gases, the CFD technique was used with the help of the ANSYS MULTIPHISICS software package.ANSYS is a finite element analysis software package widely used in industry and research, in order to simulate the response of multiphysics analyzes, offering the user the possibility to combine the effect of two or more physical phenomena (structural, thermal, electrical, magnetic , electromagnetic, electrostatic, fluid flow).The ANSYS package allows practically the parametric realization and then the optimization of any problem, regardless of the type of parameters, the types of finite elements used, the type of analysis, etc. and even the use of user-declared optimization algorithms.This package contains a series of dedicated solvers to the particularities of analysis and solving of the taken into account systems.In order to pre-process the analyzes and post-process their results, the ANSYS multiphysics package has: Fluid dynamics-specific pre-processor (CFX); Post-processor for CFX and Fluent use; Pre-and post-processor specific to the interactions of solids, fluids and gases; Pre and post-processor specific to fluid dynamics, Fluent type or equivalent; Advanced discretization application, including methods: Cartesian, hex, dominant-hex, multi-zone, prismatic, shell meshing (quad and tri, patch-based and patch-independent), tetra (Delaunay, Octree), T-Grid ; and so on.

Establishing the conditions for experimentation
In order to establish the dispersion dynamics of the explosive gases, the modeling of methane dispersion in a closed enclosure was performed with the help of CFD technique [4].Topographic map of the location within the experimental laboratory where the laboratory experiments were performed, and the computerized model was made fig.

Discussions
The following discussions can be deduced from the modeling of explosive gas dispersion indoors using methane CH4: The process of dispersing methane gas indoors was controlled in 18 points evenly distributed on three distinct levels, namely: Lower level 1 consisting of 6 control points located at 0.5 m from the floor; Medium level 2 consisting of 6 control points located at 1.5 m from the floor; Upper level 3 consisting of 6 control points located at an altitude of 3 m from the floor; The dispersion process at the lower level 1 is characterized by a variable evolution.Thus, a variation of the specific gas concentrations for turbulent flow at control points 1b and 1e was found.Also at the level of points 1a; 1c; 1d and 1f a relatively homogeneous variation specific to a laminar flow was found; The dispersion process at medium level 2 is characterized by a variable evolution.Thus, a variation of the specific gas concentrations for the turbulent flow at the control point 2e was found.Also at the level of points 2a; 2b; 2c; 2d and 2f a relatively homogeneous variation specific to a laminar flow was found; The dispersion process at the upper level 3 is characterized by a variable evolution.Thus, a variation of gas concentrations was found at all control points specific to a regime close to the laminar flow; The gradient of dispersion and progressive dilution of the gas at the level of the closed enclosure, Gd, presented a variable evolution depending on the position of the control points as follows: The gradient of dispersion and progressive dilution of the gas, Gd, at the lower level 1 showed values between 1,692 and 2,603% Vol./ h; The gradient of dispersion and progressive dilution of gas, Gd, at the average level 2 showed values between 1.816 and 2.135% Vol./ h; The gradient of dispersion and progressive dilution of gas, Gd, at the upper level 3 showed values between 1.909 and 1.988% Vol./ h; The modeling of the methane gas dispersion in the closed enclosure showed a dispersion phenomenon oriented on the flow direction of the discharged gas jet.The flow of methane gas has the shape of a flat jet glued to the floor to the middle of the enclosure in the direction of flow after which it detaches and tends to the opposite wall.The gas flow rises on the opposite wall and disperses unevenly at the ceiling.The concentration of methane gas is maximum in the source area, gradually decreases by dilution as it moves away from the source and becomes variable and reduced in the storage area at the ceiling.If methane gas were introduced continuously for a longer period of time, the atmosphere inside the enclosure would be displaced from the ceiling to the floor.

Conclusions
Modeling methane dispersion in a closed enclosure using the CFD technique at levels 1; 2 and 3 are characterized by a variable evolution.Thus, a variation of the specific gas concentrations for the turbulent flow was found to a relatively homogeneous variation specific or close to a laminar flow; The gradient of dispersion and progressive dilution of methane at the level of the closed enclosure, Gd, presented a variable evolution depending on the plan position of the control points; The CFD simulation revealed that the methane dispersion showed a pronounced variation of methane concentrations both horizontally and vertically; Methane concentrations are high in the direction of flow of the gas jet at the level of the floor in the central area, at the level of the opposite wall and at the ceiling.Reduced concentrations were obtained at the left and right side of the floor in relation to the gas jet, respectively at the side and entrance walls related to the enclosure; In the conditions of untimely releases of methane, it is necessary for the working personnel to evacuate the premises, under the protection of the security ventilation systems, along the routes where minimal concentrations of methane are recorded.Routes with high concentrations of methane where there is a risk of an explosion will be avoided.
After the evacuation of the personnel, action can be taken at the level of the premises either directly at the gas source, if possible, or by inerting the premises.

Fig. no. 3 .
Fig. no. 3. Dynamics of methane dispersion at level 1 For the dispersion of methane at the level of the plane of points 2a; 2b; 2c; 2d; 2e; 2f, the diagram of variation of the gas concentration was obtained, fig.no. 4.