Preliminary Experimental And Numerical Study Of Premixed Combustion In Porous Inert Media

Development of porous burners has been encouraged by high combustion efficiency and low emission standards as well as the advantages these burners offer, such as fuel flexibility, the ability to operate at low equivalence ratios,high turn-down ratio etc.The objective of the present work is to theoretically and experimentally investigate the performance characteristics of premixed inert porous burners.The stability of premixed flame has been studied, and the levels of pollutants,the distribution of temperature have been measured. A method also has been developed to let lower heat value gas fuels stably burn within inert porous media.The experimental results indicate that, the porous media burner, which consists of an upstream section of porous ceramic foam with small pores as the preheating layer and a downstream section of porous ceramic foam with big pores as the flame supporting layer, can not only work for premixed liquefied petroleum gas and air mixture,but also for diluted premixed mixture with nitrogen. Therefore,it is possible for the present designed porous burner to work with gas fuels which have lower heat value.The premixed gas can burn stably above the surface of metal fibers under some rigorous conditions,which are stacked on the upstream section of porous ceramic foam with small pores.When the mixture of liquefied petroleum gas and air is used, the flame stabilizes within the porous ceramic inserts,and it is near the interface between the upstream section and the downstream section.The distribution of temperature near the chemical reacting zone is uniform. The measured levels of pollutants are very low, CO level is lower than 30 ppm, while NOx level is lower than 10 ppm.When the diluted mixtures are used, the same conclusion can be drawn. NOx level is about 10 ppm, while the CO level is something higher, but still less than 100 ppm.For the sake of guiding for our continuous experimental research and improving the performance of inert porous burners,a two-dimensional steady state model also has been developed to predict the thermal performance of premixed combustion within the inert porous ceramic burner. The governing equations are solved by the commercial software-FLUENT, and the flow field, temperature field and concentration fields of species are obtained. In the end, the effects of solid matrix properties on the temperature field are also studied. The numerical results indicate that the flow field, temperature field and concentration field of species appear to be approximately one dimensional.The flame stabilizes near the interface between the upstream section and the downstream section.It coincides with the experimental result, besides the gas temperature from numerical solution is somewhat higher. Compared with traditional burner, the chemical reacting zone is extended in porous media burner, while the gas temperature gradient near the flame zone is reduced.The numerical simulations also imply the following conclusions.Solid matrix properties, such as conductivity, radiative extinction coefficient and volumetric heat transfer coefficient, can significantly affect the distributions of the gas phase temperature and the solid phase temperature.In order to realize more stable,more safe and higher efficient burning, the upstream section should have an intermediate conductivity, a large radiative extinction coefficient and high volumetric heat transfer coefficient; while,the downstream section should have a high conductivity, a small radiative heat transfer coefficient and an intermediate volumetric heat transfer coefficient.