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Effect of under-oxygenation on combustion parameters

​David Alibert has defended his thesis on 22th November 2017 at IUSTI, in Marseille (France).

Document type > *Mémoire/HDR/Thesis

Keywords >

Research Unit > IRSN/PSN-RES/SA2I/LEF

Authors > ALIBERT David

Publication Date > 22/11/2017

Summary

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.
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