Studies On Combustion And Heat Transfer Characteristics In Porous Media With Embedded Heat Exchangers

Porous media combustion is an efficient and clean energy utilization of fuel gas,featured by high flame speed,low emission,and large operation range,due to the unique mechanism of internal recirculation.In the present thesis,gas combustion characteristics and heat transfer behaviors of heat exchangers in a two-layer porous burner with embedded heat exchangers was experimentally and numerically studied,providing guidance for the design and operation of porous burner with embedded heat exchangers.The investigation work is as follows.1.Gas combustion and heat transfer characteristics in foam porous burner with embedded heat exchangers was investigated experimentally.An experimental system with porous combustion was developed,and heat exchangers was embedded in SiC foam ceramics,located in the downstream of the two-layer porous burner.The temperature profiles,stability limits,pollutant emission for methane/air premixed gas were investigated.In addition,thermal efficiency of burner and the heat transfer between heat exchangers and the porous media were analyzed.Results show that it can lower the temperature level within heat exchangers embedded,and the flame can stabilize in a relatively wide range of the inlet velocity.With heat exchangers embedded,the nitrogen oxide emission of the burner was less than that of porous burner without heat exchangers,which was lower than 35 mg/m3 under all experiment conditions.The thermal efficiency of the burner decreases with the inlet air flow rate and was kept in the range of 60%～80%.The average heat transfer coefficient between the porous media and the heat exchangers increases 150%compared with the conventional convection of flow across the pipe banks.The MATLAB software is used to get the semi-empirical formula of the heat transfer coefficient of the embedded heat exchangers,and the error between the calculated value and the test value was less than 10%.2.The effects of arrangement of heat exchangers on the performance of porous burner with embedded heat exchangers were investigated experimentally.Results show that the CO emission of the porous burner with embedded heat exchangers decreases with the increase of the horizontal and vertical pipe spacing.In order to reduce the CO emission,in the practical design of the embedded heat exchangers,too dense arrangement should be avoided.The NOx emission was lower than 20 mg/m3 under different horizontal and vertical pipe spacing.3.Metthane/air premixed gas combustion and heat transfer in porous burner with embedded heat exchangers were numerically simulated using CFD software FLUENT 6.3,mainly analyzing the heat transfer mechanism between heat exchangers and porous media.Results show that the general trends of methane/air premixed gas combustion characteristics in porous burner with embedded heat exchangers were well predicted,and the error between the predicted value of peak temperature and the experimental result are below 20%for all different inlet velocity.The heat transfer capacity of the three row heat transfer surface decreases with the increase of the distance between the location of heat exchangers and gas reaction zone.Solid radiation occupies the majority of heat transfer between the porous media and heat exchangers of the 2nd and 3rd rows,as high as 55.4%.The heat transfer between the porous media and heat exchangers of the1st row is dominated by solid conduction,which accounts for 42.6%.The thermal efficiency of the burner decreases with the increase of the extinction coefficient of the downstream material.