Read it here: doi:10.1016/j.proci.2016.07.043

Abstract

In previous work, the Reynolds analogy was used to develop a theoretical expression that allowed for the estimation of local mass burning rates in steady laminar boundary layer diffusion flames established over liquid and solid fuels. This technique was used to elucidate the mechanisms responsible for pyrolysis of both solid and liquid fuels in forced and free convective environments. These previous studies, however, focused on steady results that occur early in the combustion process, before regression of the fuel surface begins to influence results. In this work, a thorough experimental investigation of steady and transient pyrolysis of clear cast Poly Methyl Methacrylate (PMMA) is presented using both local pyrolysis rates and heat feedback to the condensed fuel surface measured at different streamwise locations in a bench-scale wind tunnel. A functional form of the Nusselt number is derived that can be readily used to identify these steady and transient regimes of PMMA burning in the form of local convective heat transfer coefficients. At early times (<150 s), a steady burning regime is identified where heat feedback properties are constant and the gas phase can be assumed to be in a steady state. At later times, a transient burning regime dominated by solid-phase effects occurs. Heat feedback from the flame and hence local mass loss rates measured at later times are transient in nature and do not correspond well with the steady state theoretical solution. Investigation under different forced-flow wind conditions reveals this transient phenomena most likely occurs due to both deformation of the surface of PMMA and solid-phase conduction into the fuel, which eventually influences the gas phase. The results presented will be useful for future modeling of transient solid-phase combustion, especially as it is applied to studies of flame spread.