Combustion Mechanism And Flame Characteristics Of The Superadiabatic Combustion Of Premixed Gases In Porous Media

Superadiabatic combustion of premixed gases in porous media is a promising and advanced combustion technology, providing both low and high flammable limits and good performance regarding emission characteristics in comparison with open flame. Investigation of superadiabatic combustion mechanism and flame characteristics, including the heat recuperation effect, combustion wave propagation characteristics, the maximum combustion temperature in porous media, the lean flammable limit and the maximum half cycle in the porous burner and so on, benefits not only the development of the superadiabatic flame theory but also the design and development of porous media combustor-heator, reactors and new combustion technologies for various purposes. The thesis presents an intensive investigation on superadiabatic combustion in prous media by experimental measurements, theoretical analysis and numerical simulation.Firstly, the experimental facility is designed and set up to measure and characterize thermal wave and combustion wave in a packed bed, which consists of a combustor (quartz glass tube), a gas flow system, a measurement system and so on. Flame shapes and development is observed and recorded during the combustion wave propagating through the combustor. The temperature distributions in the packed bed are measured by thermocouples at various working parameters. Based on the experimental results, the superadiabatic combustion effect, combustion wave speed and the maximum combustion temperature and flame characteristics in the reaction zone are discussed.Secondly, theoretical analysis of low-velocity filtration combustion of lean mixture in porous media with uni-directional and reciprocating flow has been performed.(1). Combustion wave characteristics of low-velocity filtration combustion in a porous medium burner are analyzed. Based on the flame sheet assumption, a relationship between the combustion wave speed and the maximum combustion temperature is given at first. Then an approach from the laminar premixed flame theory is applied and the entire flame zone is divided into a pre-heating region and a reaction region, and treated separately. In this way, the second relationship between the two parameters is deduced. Thus a closed analytical solution for the combustion wave speed and the maximum combustion temperature is obtained. Over a wide range of working conditions, theoretical predictions show qualitative agreements with experimental data available from the literature. In addition, the results reveal that the mechanism of superadiabatic combustion is attributed to the overlapping of the thermal wave and combustion wave under certain conditions.(2). Based on the analogy with the steady countercurrent reactor, a simplified theoretical solution is presented, which is applicable to adiabatic inert porous media combustors with reciprocating flow. The model consists of two ordinary differential equations that link all major controlling parameters, which allow for a good physical understand of the process. Compared to the numerical study, the temperature profile in the reactor can be approximated very well by a piecewise linear function, in terms of the simplified model. The maximum temperatures in the porous media burner predicted by the simplified model show the same trends as those of experimental results, but are generally higher, and the deviation between the experimental data and predications is less than 20%.(3). Based on the experimental and simulation resuluts a simplified theoretical solution is further developed. The temperature profiles in the burner, the lean flammable limit and maximum half cycle are predicted by a piecewise linear function, which is applied to inert porous medium combustors with reciprocating flow in the absence of heat loss to the surroundings. The predicted lean flammable limits show the same trends as experiments when the gas velocity is less than 0.12m/s, this means that the lean flammable limits are getting lower with increasing gas velocity. However, greater gas velocities have little effect on the lean flammable limits at gas velocites greater than 0.17m/s. At the same time, results show that the lean flammable limit can be extended by using porous media of smaller pore size. In addition, it is shown that the predicted maximum half cycle is proportional to the product of the gas velocity and the ratio of the specific heats between the solid and gas. The combustor length has significant influence on the maximum half cycle and the longer combustor length permits larger half cycle. The predicted lean flammable limit and maximum half cycle provide guidelines for the design of the combustor and some indications for further improving the combustor performances.Finally, mathematical models of premixed combustion in prous media with uni-directional and reciprocating flows have been developed to improve the understanding of the superadiabatic combustion mechanism and flame characteristics.(1). Wave propagation characteristics of low-velocity filtration combustion in a porous medium burner are systemically investigated. Heat recuperation originated by the porous medium is examined by a one-dimensional numerical model. Results show that both solid conduction and radiation play important roles in the heat recuperation process. Attention is focused on the influence of solid properties such as specific heat capacities and heat conductions, heat loss, equivalence ratio etc, on the combustion wave speed and the maximum combustion temperature attained in the wave. Results show that the heat capacity of the porous media has a significant effect on combustion wave speed. At the same time it is shown that the maximum combustion temperature is almost, independent of the heat capacity of the packed bed. Over a wide range of working conditions, the numerical predictions and theoretical results show qualitative agreements with experimental data from both this paper and the available literature.(2). Superadiabatic combustion with reciprocating flow in a porous medium has been investigated through numerical calculations. The combustion wave is confined to a transient porous burner by periodically changing the direction of the flow. The one-dimensional model takes into account gas-phase transport, radiation, interphase heat exchange, and solid conduction. Attention is focused on the formation of superadiabatic combustion, the influence of gas dispersion, equivalence ratio and the material and structure of the porous media, on the major characteristics of superadiabatic combustion in the porous body. Results are validated through comparisons with available experimental data and show the same trends as the experiments. It is indicated that species dispersion has little influence on the gas temperature and reaction heat for the lean mixture. However, the gas temperature is overpredicted at the reaction zone without the effect of mixture thermal dispersion at the same condition. In addition, it is shown that the combustible limit can be extended with smaller pore size porous media and a combustible limit with the equivalence ratio of 0.092 is achieved for 30ppi cerafoam.(3). Two-dimensional numerical investigations on the structure improvement of porous inert media bumer with reciprocating flow are presented. Attention is focused on the combustion temperature and pressure loss in the burner, which is respectively packed with 10ppi ceramic foams or alumina pellets with various sizes. Results show that material and structures of porous media have significant influence on the burner performance, and that ceramic foam with a high porosity is suitable for using in the combustion region whereas alumina pellets should be placed in the heat exchange zone. According to this principle, an improved burner design is proposed and this leads to a wider high temperature plateau and moderate pressure loss for extremely dilute CH4/air mixture with an equivalence ratio of 0.1. Numerical results are validated against experiment data.