Heat and mass transfer to, from, and within cellulosic solids burning in air

Abstract

In this paper, the problem of cellulosic material burning in air is explored in the following three ways:o1.The free-convection heat-and-mass-transfer coefficients for fuel-soaked wicks burning in air are determined for a number of wick geometries and orientations. In brief, orientation, shape, and size have little effect on the coefficients for turbulent flames.2.Gross characteristics of cellulose cylinders burning in air are examined. X-ray photographs show similarities between the burning process and drying of porous materials. The burning rate is found to depend on time and on the initial diameter of the specimen. Examination of the data suggests that the burning rate depends on the rate at which an isotherm propagates into the solid.3.Detailed studies of temperature-time histories of pyrolyzing cylinders are employed to determine local heat source and sink strengths. These are in turn compared with Differential Thermal Analysis (DTA) data for cellulose. The decomposition appears to take place endothermically at 300°–400°C, and exothermically, above 500°C. The temperature-time history is strongly influenced by the movement of vapor in two distinct ways. Moisture resulting from the thermal decomposition diffuses, and re-evaporates to yield a thermostatic effect in the neighborhood of 100°C. Vapors produced by the endothermic pyrolysis of the solid in the interior of the specimen undergo exothermic pyrolysis in the hot char near the surface. The free-convection heat-and-mass-transfer coefficients for fuel-soaked wicks burning in air are determined for a number of wick geometries and orientations. In brief, orientation, shape, and size have little effect on the coefficients for turbulent flames. Gross characteristics of cellulose cylinders burning in air are examined. X-ray photographs show similarities between the burning process and drying of porous materials. The burning rate is found to depend on time and on the initial diameter of the specimen. Examination of the data suggests that the burning rate depends on the rate at which an isotherm propagates into the solid. Detailed studies of temperature-time histories of pyrolyzing cylinders are employed to determine local heat source and sink strengths. These are in turn compared with Differential Thermal Analysis (DTA) data for cellulose. The decomposition appears to take place endothermically at 300°–400°C, and exothermically, above 500°C. The temperature-time history is strongly influenced by the movement of vapor in two distinct ways. Moisture resulting from the thermal decomposition diffuses, and re-evaporates to yield a thermostatic effect in the neighborhood of 100°C. Vapors produced by the endothermic pyrolysis of the solid in the interior of the specimen undergo exothermic pyrolysis in the hot char near the surface.