- Author
-
Sauer, F. M.
- Title
- Charring of Wood During Exposure to Thermal Radiation: Correlation Analysis for Semi-Infinite Solids.
- Coporate
- Department of Agriculture, Washington, DC
- Report
-
Interim Technical Report; AFSWP-868
August 1956
71 p.
- Keywords
-
wood
|
charring
|
exposure
|
thermal radiation
|
solids
|
weight loss
|
radiant energy
|
thermal decomposition
- Abstract
- Available data on the behavior of wood undergoing charring and weight loss are summarized, both for isothermal heating and exposure to radiant energy. On the basis that measured temperatures during radiant heating behave as predicted by opaque semi-infinite solid theory and that thermal degradation of wood is governed by a first order kinetic reaction, a differential equation is postulated for the charring process. Schematic integration of this equation yields dimensionless groups which are independent of a "charring temperature". Analysis of Naval Radiological Defense Laboratory data leads to the conclusion that during radiant exposure the charring process is prtiarily controlled by the diffusion of heat into the interior of the solid, i.e., the effect of rate-controlling variables 1s small. Comparison of NRDL square wave and field pulse data leads to the relationship for the equivalent square wave pulse: H (square wave) = 0.60 Hm (field pulse), t (square wave) = 4.0 tm (second maximum). This relationship holds over the entire range of NRDL investigations within the confidence level of data, and provides a satisfactory check against National Bureau of Standards BUSTER field data. Massachusetts Institute of Technology and NBS charring data are compared against the derived dimensionless correlations. These data yield char depths which are 25 percent lower than NRDL results. This differential is attributed to different heat flow directions relative to the grain of the wood. This conclusion, however, requires clarification through additional experimentation. Naval Material Laboratory static exposure depth of char data show large relative scatter and appear to be influenced by the finite solid boundary. Dynamic exposure data yield correlations unlike those of square wave (static) exposures due to the pulse shape imposed by the method of exposure. The presented correlations are believed directly applicable to a wide variety of woods other than those tested, provided these woods do not have a marked grain structure such as in Douglas fir. Exposure of the latter type of wood produces a corrugated effect due to the difference in density between spring and summer growth. The rate of charring in each case must then be treated separately.