Oxygen supply has a
leading role in fire growth in confined spaces. The oxygen quantity available
for combustion depends on the oxygen consumption by the fire and on the air
supply from the mechanical ventilation system or openings. A decrease of oxygen
concentration of the oxidant flow will lead to a decrease of the heat flux
feedback from the flame to the fuel surface, which in turn will lead to a
decrease in mass loss rate. This is accompanied by changes in other properties,
such as the regression rate of the slab, gas temperature and composition, and
total and radiative heat fluxes. The present experimental study has a dual
objective: understand the effects of an under-oxygenated atmosphere on the
combustion of solids and liquids and collect data for model validation. The
Controlled Atmosphere Device for Unburnt and Carbon Emission Evaluation
(CADUCEE) of IRSN has been used to conduct experiments at oxygen concentrations
ranging from the limiting oxygen concentration for extinction to 21%vol. Fuels
used are polymethylmethacrylate (PMMA) and heptane at different scales. For
both fuels, data analysis shows a linear decrease of mass loss rate, heat flux,
gas temperature and CO2 yield, but an increase of CO yield with the oxygen
concentration. For PMMA, the radiative and convective fractions of the total
heat flux and the soot yield are nearly constant regardless of the scale and
oxygen concentration. On the contrary, for heptane, the soot yield decreases
with the oxygen concentration and shows a threshold curve versus the pool
diameter. A measurement of the liquid temperature highlights a decrease of the
flame feedback to the surface of the pool with the oxygen concentration. A good
agreement with literature data for various fuels and scales is found. Assuming
chemical equilibrium, it is also found that the global equivalence ratio,
deduced from the concentration of CO2 in the extracted gases, is close to
unity, which reveals a weakly reductive incomplete combustion. This suggests
that the flame, and thus the mass loss rate, adapt themselves to the available
concentration of oxygen in the oxidant flow, to stay close to stoichiometry.
