CFD simulation of methane circulation dynamics in a ventilation installation's tubing (cid:3)

. Reducing the risk of occurrence of phenomena such as explosions, as a result of the untimely penetration of a certain amount of explosive gases, inside a ventilation installation, is a priority, regarding the health and safety of workers, material assets and the negative influence on the environment, due to the possible problems caused by the occurrence of explosive atmospheres, formed by mixing air with gases, vapors, mists and combustible dusts. The main measure to prevent explosive atmospheres is to provide adequate ventilation in closed or semi-closed industrial and non-industrial premises. In order to avoid dangerous situations at the level of poorly ventilated ventilation installations where explosive atmospheres can occur, it is necessary to know the behavior of explosive gases as well as their dispersion method. CFD technique was used to study the dynamics of methane circulation inside the piping of a ventilation installation with the help of the ANSYS MULTIPHISICS software package. The paper presents the CFD analysis regarding the influence of the state parameters on the methane flow dynamics inside the ventilation systems. Thus, the state parameters determined in the experimentation area and their introduction into CFD modeling are presented, namely the establishment of the methane flow and dispersion model in ventilation ducts with high complexity. (cid:3)


Introduction
In order to simulate the dynamics of gas dispersion, the CFD technique is used.The purpose of the integrated tools that are part of the CFD technique is to reproduce experimental tests by using specific mathematical relationships.Especially in engineering, CFD is at the stage where "problems with complex geometries can be solved with simple physical models and those with simple geometry can be solved with complex physical models".[1,2].
To perform CFD simulation of the dynamics of circulation and dispersion of methane inside the tubing of a ventilation installation under the influence of air condition parameters, was used as a model "experimental system with variable structure for the study of complex industrial ventilation networks".Ventilation systems are complex and can serve several closed enclosures.In the conditions in which variable concentrations of methane appear untimely in the suction area of the ventilation system, the flow will have a distinct regime, being influenced both by the amount of explosive gas and by air condition parameters.Through the CFD simulation, the virtual image of the complex flow process is obtained with the highlighting of the areas in the tubing with higher and lower methane concentrations, respectively.The obtained results can be used to determine the preventive measures of explosion phenomena, respectively the identification of the necessary measures to ensure the safety of workers

CFD technique
CFD technique was used to study the dispersion dynamics of explosive gases with the help of the ANSYS MULTIPHISICS software package.
The ANSYS package allows practically parametric realization and then optimization of any problem, regardless of the type of parameters, the types of finite elements used, the type of analysis, etc. and even using user-declared optimization algorithms.This package contains a series of solvers dedicated to the particularities of analyzing and solving the systems taken into account: mechanical, thermal and stability for solving structural and thermal problems, linear or not; Separate task

Experimental conditions
The experimental system with variable structure for the study of complex industrial ventilation networks consists of a centrifugal fan-motor aggregate and a complex structure of rectangular tubing with dimensions of 300/400 mm.The ductwork is located on the southern, western and northern walls, fresh air intake and air exhaust respectively on the eastern wall.[3,4,8,9] In order to carry out the simulation in a manner as close as possible to the experimental conditions, the topographic elevation of the ventilation installation was carried out within the experimental laboratory, where the experiments were carried out in the laboratory, and the computer model was made, fig.1-4.[5,7,8,11] Fig. 1 Modeling of the ventilation installation in 3D system

Modeling of methane dispersion inside the ventilation installation
The following aspects were established for modeling: The introduced methane gas has a concentration of 100% Vol.;The introduction of the gas is carried out at the level of the suction mouth; The methane gas is discharged through a hose with an internal diameter of 8 mm; The flow of gas pumped into the ventilation pipe is 223 l/min; The phenomenon of methane dispersion occurs at constant pressure; Atmospheric pressure, temperature and humidity are those currently encountered at the place of experimentation; Simulation time set at 5 minutes; Methane concentration 1000 ppm.
Following the modeling of the methane dispersion inside the ventilation duct, the results of the gas concentration in the right-side chain, frontal area, left side chain and exhaust area were obtained, which are presented in table 1.For the dispersion of methane inside the ventilation pipe, the variation diagram of methane gas concentration was obtained, fig. 5.

Fig. 5 -Variation of methane dispersion in the vent pipe
The result of the computer modeling for a simulation time of 5 minutes is shown graphically in fig.6-9.

DISCUSSIONS
From the modeling regarding the variation of air condition parameters inside industrial ventilation installations by using methane, CH4, the following can be deduced: o The dispersion process at the level of the industrial ventilation installation in the right-side chain is characterized by a variable evolution.Thus, a variation of methane concentrations along the direction of flow was found.A preferential flow was also found in the middle area, while the upper and lower parts of the right lateral chain do not present significant methane concentrations; o The dispersion process at the level of the industrial ventilation installation in the frontal area is characterized by a variable evolution.Thus, a variation of methane concentrations was found throughout the structure of the frontal area.
A preferential flow was also found on the upper area with high methane concentrations, while on the lower part of the frontal area it shows moderate methane concentrations; o The dispersion process at the level of the industrial ventilation installation in the left side chain is characterized by a variable evolution.Thus, a variation of methane concentrations along the direction of flow was found.A preferential flow was also found on the middle area at high concentrations, while the upper and lower parts of the left lateral chain show moderate methane concentrations.At the same time, an area with insignificant concentrations of methane was found at the level of the anterior upper and lower anterior ventilation ducts; o The dispersion process at the level of the industrial ventilation installation in the exhaust area is characterized by a constant evolution.Thus, a uniform methane concentration along the direction of flow, at a high level, was found.

CONCLUSIONS
The present work allowed highlighting the following findings: 1. Inside a ventilation installation, the air flow speed must be strong enough both to drive and direct particles and explosive gases such as methane to the suction mouth and to ensure their continuous transport to the discharge mouth; 2. At the international level, the CFD -computational fluid dynamics technique is used as a top tool in the research field; 3. To study the influence of air condition parameters on potentially explosive atmospheres, the CFD technique was used with the help of the ANSYS MULTIPHISICS software package.4. The methane dispersion process is characterized by a variable evolution; 5. From the modeling regarding the variation of the air state parameters inside the industrial ventilation installations using CH4 methane -the methane concentration tends asymptotically towards the value of 620 ppm which it reaches after 100 seconds from the start of the modeling; 6.The methane gas dispersion process inside the ventilation installation was controlled in 4 areas, namely: -The industrial ventilation installation on the right-side chain, which also includes the suction mouth; -Industrial ventilation installation in the front area; -Industrial ventilation installation left side chain; -Industrial ventilation installation discharge area.; 7. Modeling regarding the variation of air condition parameters inside industrial ventilation installations by using methane CH4, presented a phenomenon of dispersion oriented along the direction of flow of the introduced gas.The flow of methane

Fig. 2 Fig. 3
Fig. 2 Modeling of the ventilation installation in 2D system with focus on the suction area