| As a extremely destructive fire phenomenon,fire whirl is characterized by a completely different flow field from a conventional fire,whose flame height is high,the burning rate is fast,and the range of fluctuation is wide.A large number of experiments and simulation researches have achieved a lot of results during past time,but there is few studies in the effect of the different subgrid models(SGS models)on the numerical simulation results.Based on the software of FDS4.0,five SGS models commonly used in fire simulation will be selected in this paper to start the research work.This paper mainly contains the following aspects:(1)Based on the small size of the natural cyclone fire whirl experiment,research on the default Smagorinsky SGS model of FDS by studying the dimensions of the grid,the way of refinement and the influence of the array value of the coupled experience system,the default model is brought to the best level.It is found that local refinement in FDS is not suitable for the simulation of fire whirlwind because it is difficult to judge the rotating shape of the flame and the data spillover when the flame swept the boundary of the region.In addition,in the coupling parameter combination,the value of the constant Smagorinsky coefficient directly affects the calculation method of the viscosity coefficient and has the greatest impact on the results.(2)Several other SGS models are programmed by modifying the source code,and the experimental conditions are simulated respectively.It is found that the Dynamic calculation of Smagorinsky coefficient and the modified method of RNG can improve the accuracy of the calculation.The dynamic model adopts the way of dynamic calculation of viscosity coefficient in time and space,which is more close to physical reality.But it needs complex shearing and average processing,and requires very high grid.For complex flows,it consumes too much computing resources.In RNG SGS model,turbulence intensity in the region of high viscosity is modified by Heaviside function model which can obtain the coefficients so that the result is reliable for fire whirl.For the problem of complex flow that is not conducive to dynamic computation,we can find the best Cs number by the method of orthogonal tests and calculate it by RNG SGS model to achieve the maximum of accuracy and efficiency.Further more,Deardorff SGS model utilizes the velocity field to solve the viscosity coefficient,which leads to the excessive underestimation of the friction,thus the rotation of the flame cannot appear.As for Vreman SGS model,the subgrid dissipation is neglectd on the mechanism.The excessive accumulation of energy on the small eddy scale is inevitable.These two models are not applicable to the simulation of the fire whirl.(3)The small scale multi pool fire cyclone experiment has been designed and completed,from which several SGS models are verified in the application of the fire whirl within the process of flame fusion.The characteristics of the flow field of the multi pool fire whirlwind are preliminarily analyzed.In addition,the classic McCaffrey plume formula is modified,and the correction results of a small scale fire whirl applicable to the different number of fire sources are obtained... |
