Study On Heat Transfer Characteristics Of Combustion Process In Porous Media With Variable Pore Structure

The combustion temperature of porous media combustion is high due to the heat feedback.If no measures are taken to reduce the combustion temperature,the high NOx emission of high calorific value gas combustion will be caused.How to control the heat flux by changing the pore arrangement of the porous media to control flame temperature is one of the important issues this paper concerned.A porous media burner with cooling boundary was designed and the temperature distribution and pollutant emission of stable combustion under different variable pore structures and cooling conditions were measured in this paper.The effective thermal conductivity measurement test of ceramic foams was conducted.The effect of the pore density,combination thickness of the ceramic foams,and heat source temperature on the effective thermal conductivity of ceramic foam was studied.Then the variation of the effective thermal conductivity of the porous media with variable pore structure,which combined by many ceramic foam blocks with different pore density,was investigated.In this paper,the three-dimensional reconstruction of the real structure of the ceramic foams was also conducted.The thermal radiation at different heat source temperatures was numerically studied,and the contribution ratio of radiation to effective thermal conductivity was analyzed.By changing the arrangement of the pores in the core structure of the burner and the flow rate of cooling air,the combustion test in porous media with variable pore structure was performed,and the changes in combustion temperature and pollutant emissions were analyzed.The results show that: when the density of the outer ring holes of the building block structure is uniformly arranged,it is not conducive to the heat transfer in the radial direction of the burner,and when the density of the outer ring holes of the building block structure is stepwise,it is beneficial to the heat transfer in the radial direction of the burner.When the density of the inner core holes of the building block structure is arranged in a stepwise,it is beneficial to increase the temperature downstream of the combustion chamber.As the cooling air flow rate increases,the heat exchange coefficient between the cooling air and the wall of the combustion chamber increases,the cooling air outlet temperature increases,and the overall temperature of the combustion chamber decreases.The results of the test of the effective thermal conductivity of ceramic foams show that:the effective thermal conductivity of ceramic foams decreases with the increase of pore density and decreases with the increase of the combination thickness of the ceramic foams.When the heat source temperature is less than 200 ℃,the effective thermal conductivity does not change much.When the heat source temperature is more than 200 ℃,the effective thermal conductivity significantly increases with the increase of the heat source temperature.The effective thermal conductivity of the uniformly arranged ceramic foams is greater than that of the building block arrangement.The CT scan of the solid structure of the ceramic foams was used to obtain slice photos of its real structure.The real structure of the ceramic foams 10 PPI was reconstructed by Simpleware software,and the effective thermal conductivity of the ceramic foams was numerically simulated using Fluent software.The results show that the effective thermal conductivity of ceramic foams increases with the increase of the heat source temperature.At low temperature,the increase is smoothly with temperature,and at high temperature,the increase is rapidly with temperature.The proportion of the contribution of radiation heat transfer to effective thermal conductivity increases with the increase of the heat source temperature.When the heat source temperature is lower than750 ℃,the entire heat transfer process is mainly heat transfer.When the temperature is higher than 750 ℃,the entire heat transfer process is mainly radiation heat transfer.