Article,
Autoignition of Combustible Fluids in Porous Insulation Materials
Combustion and Flame, 99 (3-4): 541--550 (December 1994)
DOI: http://dx.doi.org/10.1016/0010-2180(94)90047-7
Abstract
The leakage of combustible fluids into the lagging of pipe-work in
the process engineering industry can be very hazardous because of
the increased residence time for oxidation as the liquid resides
in the porous medium and also the substantially modified heat and
mass transfer rates when compared with ignition at hot surfaces.
The exothermic reaction can lead to ignition or at least severe self-heating
with the consequent damage of pipe-work, etc. Experiments have been
performed to simulate this hazard. The thermal behavior of a number
of combustible liquids placed in porous material has been monitored
and evidence is presented in this work that self-heating can indeed
take place. It has been found that auto-ignition occurs at an important
watershed oven temperature that is related to the volatility
of the combustible fluid. A mathematical model for the auto-ignition
of combustible liquid in an inert porous material is presented. The
simple model takes a spatially uniform approach to both the energy
equation and the liquid equation for the fluid and predicts a watershed
temperature such that for a given concentration of fluid in the porous
material, the thermal behavior of the system alters abruptly. For
all practical purposes, thermal runaway is predicted beyond this
watershed condition even though the classical Semenov theory simply
predicts an eventual decay to a stable steady state, with no strict
criticality prediction. The watershed temperature is shown
to depend on volatility and reactivity.
