Direct Numerical Simulation Of Plume In Fire

Fire is a kind of catastrophic combustion phenomenon, which causes casualties and property loss tremendously, and brings greatly hazards to society. Thus the mechanism and regularity of fire phenomenon and process are needed to be researched. Plume in fire is one of important aspects in these researches including experiments and numerical simulation. Numerical simulation based on mathematical model is popular among the researchers for many advantages, such as flexible parameters setting, good repeatability, low test cost, etc. DNS(Direct Numerical Simulation ) which involves the numerical solution of the equations that govern fluid flows is a research tool that provides us with an extremely detailed description of the flow field. Therefore, the article use DNS to simulate plume in fire, including plume simulate fire plume characters in this paper, including thermal plume and plume involved reaction.The simulation of thermal plume involves in the fourth part, containing the comparative analysis between planar plume and axisymmetric plume. The results show that the averaged radial velocity and vorticity of axisymmetric plume is lower than that of planar plume, while frequency is higher than that of planar plume. The vorticity transport equation is adopted to analyze this phenomenon, the results imply that the vortex stretching term vωr of axisymmetric plume is negative, and causes vorticity descend and mixing intensity reduce.The fifth part simulates axisymmetric plume involved chemical reaction, studies the influence on plume by non-dimensional parameter Qh,Reynolds,Froude,Da,Ze. Combustion model use a one-step chemical reaction with the Arrhenius kinetics. The influence on vortex structure by heat released caused by chemical reaction is analyzed first. The results show that, vortex structure occurs in the case without buoyancy and chemical reaction, which caused by the instability of Kelvin–Helmholtz. Budgets of the vorticity transport are examined to understand the mechanism. It can be seen that the leading term is the vortex stretching term which depends on the local vorticity and radial velocity. No large vortical structures can be observed in the flow field of a non-buoyant case with combustion. This implies that the chemical heat release in the non-buoyant reactive case tends to destroy vorticity without the gravitational effect. This is caused by volumetric expansion. Considering buoyancy, the birth of varicose is an absolute instability effect initiated by the torque of the Archimedes force. With the increasing of Froude number, vorticity, air entrainment by plume and frequency descends, flame is longer and deviates from axis gradually. Besides axial velocity of plume also decreases for buoyancy attenuation. This is also analyzed from vorticity transport equation, with the acceleration of gravity g increases, Froude decreases and gravity term of vorticity transport equation enhances remarkably. These change not only increases the vorticity, but also enhances the entraining intensity. With the decline of Froude/Reynolds number, the vorticity of plume and entraining intensity descend. Because flame is mainly in the vicinity of the axis, heat release rate becomes larger, leading to the larger of temperature and axial velocity. Because the plume is controlled by the flow, the small change of chemical reaction parameter does not affect the plume, changes in Da,Ze lead to vortex structure change a few little.