Numerical Simulation Of Gaseous Laminar And Turbulent Combustion Within Porous Media

As a new kind of eco-friendly and efficient combustion system, porous medium burner has many advantages such as high burning rate, extended flammability limits, low emission and so on. Hence the porous medium combustion technique received wide attention all around the world in recent decades. Aiming to broaden the application scope of porous medium burner and to deeply understand the combustion mechanism in porous media, this dissertation investigates the turbulent premixed combustion and laminar non-premixed combustion in porous media using numerical method. The commercial CFD software FLUENT is selected as the computational tool. Though FLUENT has some embedded models for porous media, in order to obtain more accurate results, modification is still needed for governing equations, which is conducted by utilizing user-defined function (UDF) and user-defined scalar (UDS).Based on the k—ε turbulence model, a more specific model for turbulent flow in porous media, i.e. the N-K model is added into FLUENT and validated in cold-flow cases. By comparing the results of different turbulent and combustion models, one can easily notice the differences between them. The results obtained by using N-K turbulence model and Zimont combustion model are more reasonable than those gained from the k-ε model and LFR model in aspects of the flame front position and temperature distribution. Thus, N-K and Zimont models are chosen for the parametric study and the following conclusions are obtained:A higher operating pressure leads to higher solid and gas temperatures and drives the flame front to move towards the inlet. When the inlet velocity is increased, both the convective heat transfer and the turbulence are intensified. The flame front my hold its position within a small velocity variation range due to the rise of turbulent combustion rate. In this case, a decrease in equivalence ratio from1.0to0.8does not change the position of flame front obviously, though the temperatures of gas and solid drop slightly. But when the equivalence ratio comes to0.6, the flame is quenched. A larger particle diameter enhances the radiation in porous media and reduces the convective heat transfer between gas and solid media. This makes the correlation between the temperature distribution and particle diameter complicated.In terms of laminar non-premixed combustion, the model used in this dissertation solves additionally the species transport equations and defines mass diffusion coefficient using the kinetic-theory, which makes it more accurate and universal than the original one from the reference. Comparisons among the free flame model, one energy equation model and two energy equation model indicate their differences. The result of the two energy equation model shows lower gas temperature than others and the temperature difference between gas and solid is much higher than that in premixed combustion. To investigate the influence of dispersion, an isotropical dispersion coefficient is added into the mass diffusion coefficient. Then the flame height gets lower and the max temperature in the flame zone gets higher. Moreover, a parametric study on laminar non-premixed combustion is conducted and the conclusions are as follows:First, a rise of the inlet velocity increases the flame height and makes the max temperature in the flame zone higher due to more fuel flux. Second, larger particle diameter leads to higher gas temperature, higher flame height and lower solid temperature, because of the enhancement in radiation and weakness in convective heat transfer.