|
Ingo Cremer, Joachim Luy, Jens Elmers, and Albrecht Gill, Fluent Germany
An overview of the buildings at Terminal 1
In Germany, the occurrence of one severe fire accident at an airport
has led officials to review the existing fire protection strategies for
airport buildings as well as those for renovated terminals. Thus when
the renovation of Terminal 1 at Frankfurt Airport was planned, fire protection
scenarios had to be checked and possibly optimized. In order to compare
the performances of different concepts, FLUENT simulations of the original
geometry of the terminal were ordered by the airport authorities. For
validation purposes, experiments using a 1:20 model were performed.
The effort began with simulations of the external air flow around the
buildings that make up the terminal. The results were used to predict
static pressures along the outer surfaces of the buildings and at several
potential building openings. A second set of simulations focused on fire
management inside the departure hall of Terminal 1. Of particular interest
was the time-dependent dispersion of smoke using different combinations
of ventilation fans and openings. Results from the external simulations
were used to identify the optimum locations for fresh air supplies for
the fire scenarios.
For the external flow simulations, GAMBIT and TGrid were used to build
a hybrid mesh of about 4.9 million cells, based on engineering drawings
of the airport buildings. This model spans a geometric region of 2830
x 2830 x 500m3. Surrounding the building of interest, a typical mesh
size of 0.9m was used. An exponential profile for the wind velocity as
a function of height above the ground was used as a boundary condition.
Two wind conditions were considered: one blowing from the Northeast at
3.7 m/s (8 mph), and one from the Southwest at 5.4 m/s (12 mph). The simulations
were performed using the parallel version of FLUENT. All of the external
flow results, even the pressure levels on the building surfaces, were
successfully validated through measurements on the scaled structure.
Because of a tall building adjacent to (and south of) the departure hall,
the pressures on the roof of the departure hall were found to be different
for the different wind conditions. This important realization made it
clear that the smoke management had to be based on a combination of fans
and natural smoke outlets, rather than on outlets alone. Fans ensure consistent
smoke extraction, independent of exterior weather conditions that might
compromise the efficacy of the outlets.
The second phase of the project involved an examination of the flow field
inside Terminal 1 itself, with the primary goal being the optimization
of the smoke management system in the departure hall. A mixed concept
of mechanical and natural ventilation systems was tested. The internal
geometry was again created in GAMBIT based on engineering drawings of
Terminal 1. Most of the meshing was done in GAMBIT as well, while TGrid
was used to assemble the meshed parts into a whole. The resulting mesh
had 1.3 million cells. To have the flexibility of placing trial outlets
where needed, this model was equipped with openings in many locations.
For each simulation, the inactive outlets were switched to walls in FLUENT.
The calculations were again performed using the parallel solver.
Static pressure on the outer surfaces of the buildings during northeast
(above) and southwest (below) wind conditions. The tall structure at the
center alters the pressure on the roof of the adjacent departure hall
for the different wind conditions.
All fire simulations are inherently unsteady. Taking into account the
flow physics, safety requirements, and flow handling devices typically
used for fire prevention tasks, a sophisticated time dependent control
system was developed. At t=0, the fire is assumed to begin. After one
minute, it is detected, and after another minute, the smoke outlets are
activated. Three minutes after the fire begins the extinguishing system
is activated and after ten minutes, the fire fighters arrive on the scene.
For the indoor simulations, fires at five different locations were set
up following the guidelines of a fire pro-Static tection expert. The fires
were modeled as transient sources of hot smoke in FLUENT with a number
of simplifying assumptions. Most of the fire simulations were run for
a physical time of 8 minutes, using a time step that ranged from 0.2 to
4 seconds. In spite of the simplifications made, all of the simulations
showed good agreement with experimental measurements from the scaled 1:20
model.
The geometry of the internal model (top) showing some of the grid detail
(bottom). The departure hall is the area colored green in the geometry.
Several optimization runs were performed for the different fire locations.
During this phase of the project, it became evident that dividing the
hall volume into active smoke management segments had a very positive
effect on the smoke exhaust, because the fans were loaded with the nearby
smoke and not air. In contrast, attempts to dilute the smoke with air
had a negative effect. The contaminated volume merely grew more rapidly
and, as a consequence, more fans with a given volume flow were needed
to carry the smoke-air mixture out of the hall.
In addition to segmenting the hall, attempting to create a layer of smoke
in the upper region while keeping air in the lower region of the hall
was found to be advantageous, especially near the escape routes. In order
to achieve this, the mixing of smoke and air had to be suppressed and
a stable stratification of gases had to be achieved with a well-chosen
combination of ventilation fans and building openings. To achieve this
goal, it was found that windows should not be opened in the wrong places,
and that fresh air supplies in general should be large enough and far
away enough to avoid unwanted mixing.
In the course of the project, several parameters were modified as the
five different fire locations were independently studied. Special care
was taken for regions with low ceilings, where it was more difficult to
create and maintain a thin smoke layer well above the floor. Properly
positioned fans and smoke outlets were critical for keeping a nearly
smoke-free layer, about 2m thick, on the floor, to allow people to escape
safely.
Based on the experimental and CFD results, the airport management is
able to judge renovation measures beforehand in order to maintain a high
level of airport security.
|
