Effect of unheated segment on heat transfer over a thermally thick material under forced flow

Abstract

Emmons' solution does not include a development length in front of the flame, which causes the thermal and velocity boundary layers to have different starting locations. In this paper, the effect of the unheated starting segment on solid burning was first experimentally investigated over a horizontal 10 cm × 10 cm × 1.5 cm Poly (methyl methacrylate) sample under 0.75 m/s concurrent airflow with varying unheated lengths L0 (5, 10, 20 cm). The flow field, flame structure, and gas-phase temperature profile were experimentally recorded, and the subsequently produced flame standoff distance and dimensionless temperature gradient were combined to reveal the effect of the unheated segment on the boundary layer combustion. The parameters directly related to fire hazard, like heat transfer coefficient (burning rate), and mass loss rate were also measured and theoretically deduced, to obtain the effect of the unheated segment on wind-driven fire development. Results demonstrated that owing to the presence of the unheated segment, the pressure gradient is misaligned with the density gradient to be prone to produce vortices and form streaky flames. The streak structure distributed in the flame is associated with a stronger heat transfer, resulting in a larger convective heat transfer coefficient than the theoretical ones. Due to the viscous drag effect, with increasing unheated length, the flame standoff distance increases while the convective heat transfer coefficient decreases. Additionally, coupled effects of the unheated length and forced airflow, solid burning is a transient process. As time progresses, with the valley and vortex effect, the flame standoff distance increases and decreases in the leading and trailing section, respectively, and the differences in flame standoff distance and heat transfer coefficient for varying unheated lengths are enlarged. Eventually, the mass-loss rate has an increasing tendency accompanied by a decreasing burning contribution of the leading section.