LOCAL MEASUREMENT OF SOOT CONCENTRATION
USING A WHITE LIGHT DIFFERENTIAL OPACIMETER
A. Bellivier, R. Delorme, S. Dupont, P. Benhaiem, Y. Chen, P. Le Bars, H. Bazin
Laboratoire Central de la Préfecture de Police, 39bis rue de Dantzig 75015 PARIS, France
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www.lcpp.fr
INTRODUCTION
Studying smoke movement is necessary to deal with fire prevention
and arson investigation. Evacuation easiness depends on smoke
opacity. Smoke nature has to be considered while investigating
about fire origin.
At the present time, these studies are performed using simulation
models, especially computational fluid dynamics (CFD) based ones.
However, these models are still under development. It is still necessary
to compare experimental measurements with the numerical data for
a validation purpose.
Simulation of Smoke movement in a public building (cinema)
Simulation of soot deposition
during fire happened in a public building
WHITE LIGHT DIFFERENTIAL OPACIMETER
RESULTS
We have been developing a white light differential opacimeter.
This device measures continuously the local smoke concentration directly in the
flow. Soot concentration is obtained from Beer-Lambert’s law.
Spreading several opacimeters allows to conduct measurements simultaneously
in different parts of the area studied. Thus, a comparison of numerical data with
experimental ones can be made.
This measurement technique is local, instantaneous and non intrusive.
Results show that the global measurement does not capture any fluctuations
because the optical path of the photometer integrates too thick smoke layer.
Those structures are linked directly to the sample which is proximately at 0,5m
below the opacimeter. At such distance, the molecular diffusion doesn’t occur yet.
When the sample finishes to burn, around 10 minutes after ignition, fluctuations
disappear : heated by the burner, the air is moved on a convective cell inside
the smokebox. The molecular diffusion occurs and we obtain homogeneous
concentrations.
At the end of the experiment, the concentrations measured with the photometer
are more important than the opacimeter ones. This may be the consequence
of a non homogeneous aggregates size distribution. New experiments will be
conducted with different opacimeter positions to check this hypothesis.
Prototype version 6-2
Non intrusive measurement principle
Thus, the photometer doesn’t allow any kind
of useful CFD validation measurement. The
fluctuation of the local measurement performed
by the opacimeter is characterized. We
demonstrate the variation of soot concentration
in flow structures. It allows to characterize the
structures of smoke from a fire as stratification
of hot gases, local smoke density etc.
Photometer optical path
and prototype optical path
EXPERIMENT
Experiments are conducted in a smoke chamber. The prototype is compared
with the smoke box photometer.
The soot concentration is obtained by the Beer-Lambert light-extinction law.
While the photometer is measuring across the global height of the chamber
(0.914m), the opacimeter measures on a local light path of 0.05m.
Zoom to see
the concentration fluctuations
Comparison of measurements.
Example of samples burned for the smoke production
The smoke box and the prototype inside
CONCLUSION AND FUTURE DEVELOPMENT
A new measurement device has been developed in order to produce experimental data to validate smoke transport model. It is based on a local, instantaneous
and non intrusive measurement. First results are promising.
Future works will mainly consist in creating heated optical windows for the opacimeter in order to avoid soot deposit. CFD simulations of the smokebox
experimentation will be done. Other full scale experiments will be conducted to compare experimental and numerical datas such as room fire ou apartment fire.
Bibliography :
Mulholland, G. W. ; Johnsson, E. L. ; Fernandez, M. G. ; Shear, D. A. (2000), Design and Testing of New Smoke Concentration Meter, Fire and Materials, Vol. 24, No. 5, 231-243.
Putorti, A. D., Jr. (1999) Design Parameters for Stack-Mounted Light Extinction Measurement Devices, NISTIR 6215, 34 p.
Acknowledgements : We would like to thank the cerhec company (www.cerhec.com) for his contribution to the achievement of the hot heads of the last generation of opacimeters.
Corresponding : [email protected]
10th International Symposium on Fire Safety Science, University of Maryland, USA, 19-24 June 2011