The Ignition of Damp Combustible Material where the Fibres are Entirely Covered with H₂O

ABSTRACT

A model taking account of condensation and evaporation within damp combustible material is considered where the fibres are considered completely covered by water liquid so that the evaporation term is not dependent on the amount of liquid present This leads to a simpler equation set and highlights the important findings that (a) greater dampness can induce an ignition event (even in the absence of a wet hygroscopic reaction) and (b) there can be situations where ignition takes place after quite a sharp (initial) temperature decrease.

The model ignores diffusive effects and considers both the heat transfer and mass transfer in a lumped approach extending the classical Semenov theory to include extra heat losses due to the evaporative term and heat gains from the condensation. The mass transfer equation describing the conservation of water vapour is then also added in parallel to the energy equation. Even though the dependence on water liquid is removed, the model demonstrates that the thermal behaviour of a stack of combustible material can drastically change when the humidity of the ambient atmosphere alters.

Key Words:

• Ignition
• damp combustion
• condensation