| Turbulent filtration combustion in porous media is a new branch of combustion science, as a result of crossing and infiltrating of multidisciplinary, in which the time and length scales span over several even dozen magnitudes and all complex and difficult problems in the areas of fluid mechanics, heat transfer, chemical reaction kinetics are involved. Thus, it has a significant theoretical and practical value to conduct a numerical multi-scale study on this subject.As an import method of multi-scale investigation, Upscaling technique is often utilized in the calculation of filtration combustion. The influence of Upscaling on the intrinsic physical quantities has been studied by many academics. As a product of Upscaling, dispersion is a critical factor affecting the transport of energy and species, which has essential differences from the molecular diffusion. Dispersion is not only dependent on the fluid properties but also related with the flow field. Especially in the higher velocity case, dispersion is much stronger than molecular diffusion. It is reasonable to expect that if the chemical reaction takes place in the representative elementary volume dispersion must be affected resulting from the variation of flow field.Apart from dispersion, the evaluation of intrinsic average reaction rate is still an open problem. In fact, there is no difference between the calculations of reaction rate with and without solid matrix at the microscopic pore scale, while, this conclusion is no longer valid after the Upscaling treatment. As known, the spatial deviations of species concentration, temperature, density, pressure and so on, yield a different reaction rate at different position within the representative elementary volume. At present, most of the researchers see the intrinsic average reaction rate as that taking place in the uniform stirring reactor. This treatment is accepted widely because of the smaller flow velocity and equivalence ratio considered in lean filtration combustion. However, when the laminar-turbulent transition occurs, especially for fully developed turbulence, whether this assumption of stirring reactor is still acceptable is worth further discussing.With these problems mentioned above, in this thesis some studies on dispersion and turbulent premixed flame characteristics at the pore and system scales were done by the numerical simulation method. Key points are presented as follows.(1) Some fundamental investigations on turbulence, heat transfer, especially for thermal dispersion without heat source or sink in the porous media were conducted with the geometry model of porous media, such as diagonal face-centered cubic model and Weaire-Phelan model. The simulation results showed that the fact that microscopic eddies exist indeed in the narrow pore interstitial even for a very small flow velocity. The numerical results with diagonal face-centered cubic model revealed that from free flow zone to porous zone, turbulent intensity becomes gradually stronger but it reaches a stable level within 2 to 3 cell unit length. The convective heat transfer simulated by Weaire-Phelan model demonstrated that specific area of solid phase and surface density of representative elementary volume are import parameters affecting heat transfer in pores. In particular, the heat transfer capability has a passive relationship with the specific area of solid phase for a smaller flow velocity, while it mainly determined by the surface density of representative elementary volume for a higher flow velocity. In addition, these calculations indicated that the geometry models consisting of idealized cell units have a non-negligible drawback, i.e., the two geometry models are only applicable to some specific situations. In other words, to have an accurate description on the turbulence and heat transfer, the stochastic problems of the spatial structure of pores should be considered and statistics analysis may be a better choice.(2) A theoretical derivation and numerical simulation on thermal dispersion were conducted for the low-velocity reacting flows using a pore-scale model consisting of 2D inline and staggered cylinders. Theoretical derivation illustrated that the expression of energy conservation equation is changed after an operation of Upscaling treatment when chemical reaction takes place in the porous media, in which an additional convective transport term appears. This variation is originated from the non-equilibrium of reaction heat release due to difference of local temperatures, species concentrations. This thermal non-equilibrium is translated downstream or upstream in the form of heat conduction or convective heat transfer within the representative elementary volume. The numerical results showed that thermal dispersion is dependent on the parameters such as particle distribution, superficial velocity, thermal conductivity ratio of solid and fluid and the contribution of chemical reaction to thermal dispersion is related with the Thiele modulus and Peclet number. As the Peclet number increasing, the influence of Thiele modulus will be weakened.(3) A numerical study on turbulent premixed combustion in porous media was conducted using a pore-scale model consisting of 2D staggered cylinders. Based on the computed results, the effects of relevant length scales on the Upscaling of filtration combustion and the influence of mass dispersion on the chemical reaction rate were examined with emphasis. The results indicated that the length scale of the representative element volume should be at least six times of particle diameter under the circumstance of combustion, while the thermal non-equilibrium length scale is about 8 to 10 times of particle diameter and is mainly determined by the properties and geometry of the solid matrix. The simulation also demonstrated that the flame mainly distributes in the thin reaction zones and corrugated flamelet zones. Computed results revealed that mass dispersion has significant influence on the intrinsic average reaction rate and must be taken into account, especially for larger superficial velocity and equivalence ratio.(4) With the local volumetric average method, an Upscaling of reaction progress variable equation was implemented. The turbulent flame speed was modified through considering the effects of turbulence-flame interaction and flame quenching due to cold wall. According to the flame Peclet number, the flame propagation was divided into two patterns. This treatment made the reaction progress variable equation of the system scale can be applied to the case of turbulent premixed filtration combustion. |
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