| Porous media combustion (PMC) has a lot of advantages of wide reactants flow rate, extremely lean fuel/air mixtures, high modulation range, low emission of pollutants like NOx and CO, etc. However, some combustion wave instabilities usually occur in burner, such as flame front inclination, hot spot, flame front collapsing, combustion wave jumping, etc. These kinds of instablites may give rise to the safety problems for the application of PMC. Therefore, it is necessary to investigate the dynamical mechanism of PMC instabilities to improve the development of PMC technique. In this paper, the means of experimental measurements, numerical simulation, and theoretical analysis are adopted to study the PMC characteristics and the dynamical characteristics of combustion wave instabilities. The contents are as follows.1. Numerical investigation. A two-temperature model is presented using Fluent6.3 software package together with the user-defined function (UDF). The two-dimensional unsteady mathematical model is used to simulate respectively the submerged standing combustion and the combustion wave propagating in porous media burner.The temperature field and the component distribution in submerged porous media combustion are compared for the different combustion models (one step global reaction model and more detail multi-step reaction (17 species,58 reaction steps) Peters model). And the calculated results are compared with those of the literature. It's verified that the mathematical model and the combustion model are valid. It is found that the reaction rate of one-step combustion model is faster and reaction zone is smaller, whereas the reaction rate of multi-step is slower because of both the differences of reaction time and different reaction rate for each reaction step. In addition, the calculated results using two-dimensional model more intuitively visualizes the combustion situation and observes the flame shape and the pollutants distribution in the burner. It is a good visualization tool to the design of porous media burner.The prediction of dynamical factors of flame front inclination instability is numerically taken for the low-velocity filtration comsbution. The effects of experimental case parameters (such as equivalence ratio, inlet filtration velocity) on the evolution of flame front inclination instability are studied. Moreover, the local non-uniformity of porosity and the initial preheating non-uniformity in the burner are verified to perturb the combustion wave, and to lead to the occurrence of the flame front inclination instability. The main conclusions are as follows:(1) The equivalence ratio and the filtration velocity significantly affect on the evolution of flame front inclination instability. For each experimental case, the inclinational angle of flame front develops faster in the early stage of the combustion wave propagatiing, the inclinational angle increases slower in the later stage. The effect of equivalence ratio on the stability of flame shows that, the smaller equivalence ratio gives rise to the faster inclinational angle-growth rate of the flame front, vice versa. Besides, the higher filtration velocity also leads to the faster inclinational angle-growth rate of flame front; conversely, the inclinational angle-growth rate is slower.(2) The non-uniformity of initial preheating is confirmed as a dynamical factor, which leads to the occurrence of flame front inclination instability. For all conditions, the development of flame inclination instability is slower in the early stage of combustion wave propagation, and the development is faster in the later stage. Furthermore, it is found that, the more non-uniformly the preheating section is preheated, the faster the flame front inclination instability develops.(3) The porosity of porous media burner is also confirmed as a dynamical factor, which leads to the occurrence of flame front inclination instability. The non-uniformity of porosity affects the stability of combustion wave significantly. This shows that, the more non-uniformly the porosity distributes in the burner, the more rapidly the flame inclination instability develops.2. Experimental investigation. The investigations of lean hydrogen/air low-velocity filtration combustion instabilities in packed bed with alumina balls are conducted experimentally. These instabilities include the flame front inclination and hot-spot appearing during the concurrent propagating of filtration combustion wave, the jumping propagation instability of combustion wave during the countercurrent propagating.For the experiments of the concurrent filtration combustion, the evolution of flame front inclination instability and the cellular flame, which consisted of hot spots, are observed at different experimental parameters. The critical experimental parameters (filtration velocity ug and hydrogen concentration YH2) are determined for the occurrence of hot-spot instability. In addition, it is verified that the porosity is an important factor, which gives rise to the flame front inclination instability. When the hot spots emerge in the burner, they will grow and connect with each other and form the cellular structure. The cellular structure dominates the stability of the main combustion wave, and depresses the wave propagation velocity. The experimental results also expand the scope of experimental parameters in the literature, which presented the occurrence of the hot-spot instability phenomenon. Meanwhile, as hot-spot distribution and the hot-spot size are very sensitive to the porosity, the effect of porosity on the combustion wave stability can not be neglected. It shows that, the quantity of hot spots is larger and the volume of a single hot spot is smaller in the case of smaller porosity; on the contrary, the quantity is still less and the volume is bigger in the case of bigger porosity, and the massive structure is even formed.For the experiment of the countercurrent filtration combustion, the jumping propagation instability phenomenon of combustion wave is firstly observed. The critical parameters are determined for the occurrence of the jumping propagation instability under various experimental conditions. Under the condition of YH2=13%, the critical value of filtration velocity is ug;cr=0.9?1.0m/s for the occurrence of the jumping propagation instability. For YH2=14% and YH2=15%, the critical value is respectively ug,cr=0.8m/s, ug,Cr=0.5m/s. Based on this parameters, when the filtration velocity is successively improved, the wave jumping distance of combustion wave increases, and the jumping propagation instability changes sharper.3. Theoretical analysis. The combustion wave inclination instability is theoretically analyzed on the basis of hydrodynamics and heat transfer. Thus the evolution mechanism of combustion wave inclination instability is understood. Besides, the hot-spot existing mechanism is also theoretically analyzed using the theory of heat and mass transfer. It indicates that the conventional mathematical model of volume-averaged treatment is invalid to interpret the hot-spot phenomenon. |
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