Heat and mass transfer and chemical kinetics in the combustion of polymethyl methacrylate under free convection in the air

Heat and mass transfer processes and the rate of fuel oxidation are the determining parameters of combustion of pre-mixed fuel gas streams and an oxidizer and combustion of condensed fuels in a gaseous oxidizer. A correct description of these processes is of both scientific and practical interest. The influence of the kinetics of chemical reactions and diffusion of fuel molecules on the thermal and chemical structure of the flame forming around a polymethyl methacrylate (PMMA) sphere under f natural convection in the air has been studied by numerical simulation. The three-dimensional gas flow around the solid body was calculated on the basis of the system of full Navier–Stokes equations for a multicomponent mixture taking into account diffusion and heat exchange between the surface and gas, convection, and radiation heat transfer. The kinetic model represents conjugate reactions both on the surface of the condensed material and in the gas phase. The formation of gaseous fuel methyl methacrylate (MMA) on the surface is described by a one-step efficient PMMA pyrolysis reaction. The oxidation of MMA in the gas phase is described by the global reaction C$_5$H$_8$O$_2$ + 6O$_2$ $\to$ 5CO$_2$ + 4H$_2$O. It has been found that the temperature and flame species concentration profiles practically do not depend on the rate constant of this reaction provided that the characteristic reaction time is much less than the characteristic time of MMA diffusion. It has been shown that varying the diffusion coefficient of MMA has a significant effect on the thermal and chemical structure of the flame. An increase in the diffusion coefficient of MMA leads to an increase in the maximum flame temperature. The results of the study show that the transport properties of compounds required to calculate their transpot coefficients are one of the most important parameters for accurate CFD simulation.