Porous media burners in comparison with free flame burners have major benefits such as higher thermal efficiency, stable flame in a wider range of stoichiometric ratios and feed flow rates, capability of using low calorific fuels and low production of pollutants. In the present study, premixed and laminar combustion of hydrogen in a solid matrix with spongy lattice is simulated. The axisymmetric solid matrix is considered to be inert, isotropic and homogenous in the unsteady simulations. The burner consists of a divergent inlet followed by a constant area section. A multi-step chemical kinetics is implemented. Heat exchange between the solid and gas phases is simulated using an experimental correlation for volumetric convective heat transfer coefficient, and the diffusion approximation is used to simulate the radiation mechanism inside the solid matrix. All physical properties of the gas mixture are considered as functions of local temperature and mixture composition. The governing equations are discreted and solved by the control volume scheme and SIMPLE algorithm. The effects of certain parameters such as flow rate and physical properties of the solid matrix on the thermal/stability performance of the burner are analyzed. Increasing the feed flow rate causes upstream movement of the flame front and increase in the flame temperature and pollutant formation. The flammability limits are obtained in the range of stoichiometric ratios between 0.5 and 1.2, where the widest belongs to a stoichiometric mixture.
The one-dimensional viscoelastic fluid flow between two infinite parallel plates with oscillatory inlet condition is examined using the Johnson–Segalman model. The symmetric and antisymmetric Chandrasekhar functions in space are utilized to represent the velocity and stress fields. The non-dimensional form of the conservation laws in addition to the constitutive equations are solved numerically based on the Galerkin projection method. Two critical Weissenberg numbers (We) for various Reynolds numbers (Re) and viscosity ratios (ε) are obtained to determine the stable range of nonlinear system behavior. Moreover, for the unsteady case, the effects of Re, viscosity ratio of solvent to solution as well as We are investigated. According to the obtained results, increasing of oscillations frequency in subcritical zone, the same as low frequency case, has almost no effect on the velocity and its gradient. Nevertheless, the normal stress amplitude of oscillations is reduced. The Re number determines the number of oscillations and the needed time prior to the steady condition. For lower Re, due to higher effect of viscosity, the initial fluctuations are intensely occurred in a short time period in contrary to the high Re case.
The flow of nonlinear viscoelastic fluids between oscillating parallel plates is investigated. The investigation features time-dependent analysis of a complicated viscoelastic material modeled based on the Johnson-Segalman constitutive relation. Given the rheological parameters of certain material known from experiments, the coefficients of Johnson-Segalman constitutive equation model for the material are evaluated by fitting the data. The problem is first formulated by writing the governing equations for the flow between two independently oscillating parallel plates, i.e. oscillating Couette flow. The velocity and stress are represented by symmetric and antisymmetric Chandrasekhar functions in space. Both inertia and normal stress effects are included. A numerical scheme is applied to solve the governing equations in time domain projected by Galerkin method. For given Reynolds number and viscosity ratio, one critical Weissenberg numbers is found at which an exchange of stability occurs between the Couette and other steady flows. The model is capable of predicting the nonlinear amplitude-dependent behavior of viscoelastic flows under single and multiple-frequency excitations.
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