Optimization Studies Of Foam Materials And Structural Parameters In A Two-Layer Porous Burner

As a novel and unique combustion technology and a new way to ameliorate energy and environmental issues,porous media combustion technology has been successfully applied in many industrial areas abroad,whereas its industrial applications in moderate and high temperature fields of our country have not yet been mature.Therefore,in the present thesis,optimization studies of the foam materials and structural parameters of a porous burner in the application fields of moderate and high temperature industries are conducted,which include the following two aspects:?1?Aiming at the strip coating process requirements,large-panel foam ceramic combustion burners containing a single-layer burner and a two-layer burner are independently developed and an experimental system of porous media premixed gas combustion is estabilished.Premixed combustion of the natural gas/air with above burners is investigated experimentally according to the drying temperature requirement of the radiation plate surface.The effects of foam materials on the temperature distributions and stable combustion characteristics of the premixed gas in the large-panel foam ceramic burner are studied.The results indicate that compared with Al₂O₃,SiC and ZrO₂ show good stability characteristics.In our experiments,we have measured many combustion parameters,and found 18 kinds of stable combustion conditions,among which the single-layer ZrO₂ material with the pore density of 35PPI and the thickness of 35mm in the firing rate of 270kW/m² and the excess air coefficient of 0.69,the single-layer SiC material with the pore density of 35PPI and the thickness of 35mm in the firing rate of 324kW/m² and the excess air coefficient of 0.67;the two-layer burner of SiC+ZrO₂ in the firing rate of324kW/m² and the excess air coefficient of 0.69 and the two-layer burner of SiC+ZrO₂in the firing rate of 270kW/m² and the excess air coefficient of 0.64,whose pore densities and thicknesses of small and big porous media are 60PPI and 25PPI,25mm and 35mm respectively,are the best stable combustion conditions.Under the same structural parameters of the combustion area medium,the two-layer porous burner can increase the heat load adjustment range,and this results in better performance.In order to obtain the widest stable combustion range,the preheat zone medium of a two-layer porous burner should have a smaller thermal conductivity,which can reduce the low heat load regulation limit.?2?Numerical studies on the effect of ceramic foam material and its micro-structure parameters on the combustion characteristics of porous media are carried out.Firstly,the two-dimensional transient mathematical model with two separate energy equations of a two-layer porous burner is proposed based on FLUENT14.5 software and conduct the grid independence test and validiation test.Then,based on the Kelvin model,the mathematical relations between microstructure parameters of ceramic foam?including strut length,strut radius and cubic node length?and its macroscopic characteristics,i.e,thermal conductivity and porosity,have been presented.The effects of the ceramic type and its microstructure parameters on the methane/air premixed combustion behaviors in a two-layer porous burner are discussed.The results indicate that when the equivalence ratio of methane/air mixture is 0.6,the stable operating range decreases in the sequence of SiC?ZrO₂ and Al₂O₃,and the SiC ceramic foam is the best material of the combustion zone.When taking the effect of temperature on the thermophysical parameters into consideration,the gas and solid temperatures of three materials in the combustion zone are lower under the same working conditions and the temperature gradients are different.And the material of the combustion zone will only affect the gas and solid temperature distributions of the combustion zone,whereas the preheating zone material will affect the temperature distribution of both the preheating zone and the combustion zone.The material of preheating zone should have smaller thermal conductivity,and the ZrO₂ ceramic foam is the best material of preheating zone.Besides,the increase of microstructure dimensionless parameter d?the ratio of cubic node length to strut length?will result in the reduction of gas and solid temperatures and also the change of flame anchoring location,whereas,the variation of e?the ratio of strut radius to strut length?has slight effect on the temperature distributions of gas and solid phases.Hence,the microstructure parameters,i.e,cubic node length and strut length,should be considered in priority for the actual selection of porous burner material.When the wall has heat loss,the gas temperature distributions of the burner shows a significant two-dimensional structure,and the two-layer burner of ZrO₂+SiC is greatly affected by the heat loss of the wall.