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By Emanuela Colombo, Ph.D., Energy Department, Politecnico di Milano, Italy; and Diego Donati and Marco Rossi, Fluent Italy
Engineers at Fluent Italy recently worked as external consultants in
a courtroom trial. At issue was the cause of a building fire in Milan,
Italy that claimed three lives and injured several others. The fire set
off an explosion in the apartment where it originated, causing the collapse
of some of the inner partition walls, floor slabs, and outer walls. A
poorly functioning distribution nozzle (burner) for the gas stove in the
small kitchen, that was using a mixture of propane and butane fuel, was
suspected to be the main cause of the event. The diffusion of fuel vapors
into the flat’s rooms may have caused, after a few hours of leakage,
at certain locations, the lower flammability limit (LFL) to be exceeded.
In these locations, a necessary (but hypothetical) ignition source could
have easily set off the explosion. The legal action, which involved the
insurance company and the building construction firm, is still pending.
The work performed by Fluent was done with the full support of the lawyers
and technical experts from both sides.
The building after the explosion shows the full extent of the damage
All the model details, including boundary conditions for the gas nozzle,
properties of the gaseous vapors, and indoor and outdoor temperatures
were reviewed with the lawyers from both sides prior to the start of the
project. The unsteady simulation was run using a 3D model of the apartment,
which covered roughly 90 m2. A small vent (100 cm2) in the kitchen, required
by Italian law for exhausting flue gases, was modeled in both the open
and closed position, to test its efficacy.
The FLUENT model consisted of the entire apartment and two separate volumes
of outside cold air adjacent to the walls containing windows. The small
kitchen contained a simplified stove and wall vent, used for exhaust fumes.
A few iso-surfaces of fuel vapor, colored by temperature, throughout the
apartment
Meshes of approximately 350,000 cells, most of which were hexahedral,
were used for the simulations. The fuel vapors were assigned a concentration
of 40% propane and 60% butane. Transport equations were solved for these
components as well as for oxygen and nitrogen. Flammability limits for
this
composition were calculated according to Le Chatelier’s formulation,
and found
to be in the range of 0.02 - 0.09. To ensure the development of the proper
natural convection currents at the time, two service volumes were used
outside
the apartment windows to simulate the external atmosphere of the winter
day when the accident occurred.
The simulation results for the case of a closed kitchen vent and gas
flow
rate of 0.070 kg/hr indicated that the fuel and oxygen mixture was below
the
LFL everywhere except in the area close to the gas inlet. With the vent
open,
two counterproductive effects were observed. First, fresh air was drawn
into
the room. Second, the cold outside air set up local circulation currents
that
impeded the diffusion of gas vapors throughout the remainder of the apartment.
This caused a higher concentration of vapors to be found in the kitchen
than in the scenario with the vent closed. While each scenario predicted
small
regions where the vapors were in excess of the LFL, neither was considered
to present the kind of conditions that would lead to an explosion of the
magnitude
that occurred.
It was concluded that the results were strongly dependent on the defined
scenario given by the parties, according to which the simulation was based.
The actual conditions, such as the indoor and outdoor temperatures and
degree of closure of the vent, may have been different enough to alter
the driving forces behind the air and gas flows. Indeed, small differences
might have been enough to give rise to a different explosion mixture which,
given the opportunity to ignite, could have generated the damage that
occurred.
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