A Multi-scale Numerical Study On The Filtration Combustion Of Ultra-low Calorific Value Premixed Gas

There are many advantages of porous media combustion technology,such as expanding combustion limitation,improving combustion efficiency and reducing emission,etc.Porous media combustion technology has a good industrial application prospect.However,the application of porous media combustion technology in industry is limited due to its instability,such as flame inclination and breakup phenomenon.In this paper,based on the filtration combustion technology of premixed gas with ultra-low calorific value,a multi-scale model is established to study the flame inclination instability phenomenon.The data exchange between different scale models and the adaptive movement of the mesh with the flame are realized.The multi-scale model combines the advantages of volume average method and pore scale method to improve the accuracy of calculation near the flame front while saving the calculation cost.Comparing with the pore-scale model and volume-average model,the established multi-scale model provides a numerical model with accurate calculation results and less calculation time for the study of low-speed filtration combustion.The research on the occurrence and development mechanism of flame inclination provides a theoretical basis for the stable combustion of premixed gas in porous media.Meanwhile this study contributes to the wide application of porous media combustion technology in the industrial field.The main research contents and conclusions of this paper are as follows:（1）For the filtration combustion of premixed gas with extra-low calorific value,a spatial and temporal multi-scale model is established.The packed bed is divided into preheating zone（Case1）,reaction zone（Case2）and exhaust zone（Case3）.By using the User Define Function（UDF）function in FLUENT software,the real-time data exchange between different regions,the adaptive movement of the mesh along with the flame,and the multi-scale of time are realized.（2）The flame dynamics of methane-air premixed gas combustion in porous media are studied by multi-scale model.The effects of concentration(X_CH4=0.2%-1%),flow velocity（0.46-1.85 m/s）,peak value of initial temperature field（1250-1400 K）and width of high temperature zone（accounting for 30%-75%of the total height of the packed bed）on flame propagation rate and flame thickness are analyzed.The results show that the lower the concentration of methane,the faster flame propagation rate and the larger flame thickness.The larger the flow rate,the faster flame propagation rate and smaller flame thickness.When the peak value of the initial temperature field is larger and the width of the high temperature zone is larger,the thickness of the flame is smaller and the flame propagation rate is faster.（3）The influences of the perturbation of intake parameters and the physical properties of the packed bed on the flame inclination instability are studied by the multi-scale model.Besides,the influences of the perturbation range（0%-60%）and the perturbation amplitude(X_CH4=0.4%-1%,q_m=0.008-0.02 kg/s,C_s=1.6-4 k J/（kg·K）,ε=0.38-0.7)on the flame inclination angle are studied.The results show that the flame inclination in the packed bed is caused by the perturbation of the air inlet parameters and the physical properties of the packed bed materials.Taking the flame inclination angle as the evaluation standard,the influences of the four factors on the flame inclination angle are ranked as follows:porosity,flow rate,specific heat capacity and concentration.Meanwhile the larger amplitude of the perturbation will lead to greater influence on the flame inclination.The larger perturbation range will generate smaller flame inclination angle,but the perturbation range has little effect on the flame inclination angle.（4）The reason leading to flame inclination are analyzed by Method of Flow Competition（MFC）.The horizontal heat radiation and conduction when the flame inclines will inhibit the intensification of the inclined flame.Due to the influences of heat radiation and conduction,there are three phenomena in the development of flame inclination angle:a.the flame inclination angle increases to the maximum and then keeps stable;b.the flame inclination angle increases until flame breaks up;c.the flame inclination angle increases to the peak and then decreases.（5）The inhibition of flame inclination is studied under the two conditions,preserving the initial perturbation and removing the initial perturbation.Under the condition that the initial perturbation is the specific heat capacity perturbation of level 1,the flame inclination angle can be reduced by increasing the concentration perturbation of level 2.Besides,the larger amplitude of the increasing concentration perturbation will lead to faster decline of the flame inclination angle.In addition,the larger amplitude of the initial specific heat capacity perturbation will require larger amplitude of the concentration perturbation.After removing the initial flow perturbation,the flame inclination angle will decrease to 0°slowly without imposing the concentration perturbation.Meanwhile the rate of decline of the flame inclination angle will increase after imposing the concentration perturbation.