Izar, an innovative smoke testing system by using premixed
flames for a new smoke testing strategy
Juan Jose Blond Hernandez
Basler & Hofmann AG, Zurich, Switzerland, [email protected]
Abstract:
This paper focuses on a new generation of hot smoke tests, using premixed propane flames as
a fire source and the possibilities associated with this system to carry out smoke tests.
Premixed flames burn in a highly efficient manner, reducing the flame height and the radiative
heat released by the flame. These two characteristics allow reducing the necessary structural
protection measurements around the test place.
The smoke testing system is equipped with an automatic control system, which allows the
user to control the heat release rate of the apparatus in real time, enabling to stop the test in
case untenable conditions were reached. Pre-defined ‘fire’ curves can be reproduced to study
the reaction of the detection systems or to evaluate smoke filling times and the system
guarantees the repeatability of the tests. Thanks to these properties a new smoke testing
strategy is introduced to create more realistic testing conditions.
The system is described through real tests and validated computer models. The first ones
confirm that the system is capable of producing realistic smoke layers and the second ones
make it possible to describe the main plume properties and generate validated geometries for
further simulations.
1. Introduction
The aim of this paper is to present and innovative smoke testing system developed by the
Swiss company Basler & Hofmann and patented under the name Izar. This system can be
classified under the hot smoke testing methods, but it overcomes many of the problems
associated with these methods and provides a new and innovative tool to test performance
based designs.
There is nowadays an increasing interest on building singular constructions. This trend can be
observed in shopping malls, airports or residential buildings. However these modern
constructions must comply with the highest safety standards playing the smoke management a
key factor.
The design of smoke management system for these buildings is a demanding activity which
requires imaginative engineering approaches which may be included within the performance
based design. The main aim of these systems is to guarantee the egress and to reduce the
consequences in the affected compartment. The safety solutions must be tested both to obtain
the authorities conformity and regularly to check the state of maintenance of the smoke
management system during the building service life.
The current smoke testing methods have different associated problems to correctly assess the
performance of these smoke management systems. On the one hand, the cold smoke
generators are not capable of creating buoyancy smoke columns representative of a fire
scenario. On the other hand, hot smoke testing systems such as pool fires release a constant
power rate, which makes it impossible to study a vital phase of the evacuation such as the
growing phase of the fire. The most realistic options are the full-scale tests, but it is normally
impossible to carry out this kind of test both due to economic reasons and the damages which
may produce these high energy tests.
The smoke testing system Izar is a portable and easy using system capable of testing and
replicating fires from 50 kW till 1’800 kW per burning unit and reproducing different fire
growing curves, during long periods of time. It takes advantage of the properties of the
premixed flames to create the heat source: the flames are short, avoiding the flame
impingement on the ceiling and the smoke is soot-free avoiding the post cleaning activities.
2. System Description
Three main elements compose the smoke testing system Izar: a propane burner, a gas supplier
and the fog generators. More burning units can be added if a higher testing power is required.
The burner has a V-shape open combustion chamber where the gas and air cross streams are
mixed and burnt. The ratio fuel/air can be tuned in order to obtain different combustion
conditions. Several gas valves guarantee that the required amount of gas is supplied at a
constant pressure and a ventilator blows in the combustion chamber the corresponding air
volume. The following figure shows an overview of the burner and a schema of the chamber.
Figure 1.- Burner schema showing the V-shape burning chamber. The gas is at the bottom and the air through
the side plates injected. The cross streams intimate mix the fuel and the air in order to create premixed flames.
Propane gas is burnt by the system. The high power output produced requires an especial
supply system. This gas supply system can accommodate eight propane bottles capable of
supplying gas to run the system around 46 minutes at full power, equivalent to a fire of 1’800
kW, without disruption. The gas bottles can be changed during the operation increasing the
test duration.
The high efficient combustion process produces no soot. Therefore the combustion products
column, the smoke, is transparent. In order to track the plume and the smoke layer, it is
seeded with an aerosol. This fog has only a visual effect and the characteristics of the
combustion products plume are the responsible for the smoke layer properties.
3. The use of premixed flames for a smoke test
Premixed flames occur when the fuel and the oxidant agent, air, are well-mixed before the
combustion takes place.
Two mechanisms transfer the energy produced during the combustion: convection and
radiation. The combustion products bring the convection energy into the smoke layer, while
the particles presented in the flames radiate energy to the surrounding volume.
Premixed flames reach very high combustion efficiency, and consequently high flame
temperatures and very low soot yield. This low particle emission feature makes these flames
very suitable to carry out smoke tests because it reduces the energy radiated which may lead
to secondary ignitions.
A natural flame release, considering standard values, around 2/3 of the energy by convection
and 1/3 by radiation. This ratio can be calculated by a premixed flame as a function of the
partial pressures of the combustion products [1]. Both the combustion equation and the fuel
properties, propane, are well known and can be found in the literature.
The calculation confirms that only a fraction around 3% is radiated to the surrounding area. A
more detailed description of these calculations can be found under the reference [5].
This result is really important to understand the smoke testing system behaviour. The energy
is mainly transferred by convection and the radiation can be neglected to simplify the calculus. The burner is regarded as a convective heat source and correlated with a natural fire with
diffusion flames following the 2/3 and 1/3 ration for the convection and radiation heat release.
This calculation and the convection-radiation ratio from diffusions flames are the base to correlate a test with premixed flames and a natural fire: the radiation from the premixed flames is
neglected and the premixed flames must generate the same convective heat as the natural fire.
An example considering the previous mentioned standard values is: a 1’500 kW natural fire
releases around 1’000 kW convective heat and 500 kW radiation heat. The premixed flame
system must generate 1’000 kW to produce the same smoke layer.
The smoke testing system Izar can produce 1’200 kW working at full power. This HRR is
pure convective and therefore each burning unit is equivalent to a 1’800 kW natural fire.
4. Plume model
Different models are available in the literature to represent temperatures and velocities along
axisymmetric buoyant plumes created by diffusion flames. However, it cannot be found any
model to represent the buoyant plume originated by a premixed flame. The information offered by these models plays a critical role when a test must be prepared: the test cannot damage the geometry under investigation and to avoid any risk, the engineer must know before
the tests which are the expected temperature and velocity conditions in the plume on the ceiling above the heat source.
The program FDS, Fire Dynamic Simulator, has been used to model the system. The program
cannot model a premixed combustion but the recent version FDS 6 incorporates an improved
turbulent model very suitable for the purpose of these simulations. Instead of modelling the
combustion, the burner is represented as a source of combustion products at a certain temperature. This is possible because the combustion process is well defined: it is a stoichiometric
process where the necessary amount of propane and air are mixed for each energy output. The
high efficiency combustion process allows defining the adiabatic temperature as the initial
one.
A sufficient fine mesh and a measurement grid are necessary to draw reliable conclusions.
Four different HRR were tested: 100 kW, 400 kW, 800 kW and 1’200 kW. A 3.25 cm cubic
mesh has been implemented and for the lowest values while a 6.5 cm cubic mesh was used for
the highest. The same measurement grid was used for all the simulations: from the central
position a temperature and a velocity sensor was programmed each 20 cm both along the X
and the Y axis and reproduced each 19.5 cm along the Z axis covering a total high of 6 meters
above the burning unit. These measurements are necessary to build the plume profile at different heights. A Gaussian distribution is adequate to describe the plume cross section and
define the plume boundary according to the literature [2].
5. Experimental results
The results obtained during the real tests confirm the theoretical plume model developed and
validate the computer model.
The following graphs show the comparison between the temperatures obtained in the
simulations and those measured during the test for two different HRR, 400 kW and 1’200 kW
Y‐Axis X‐Axis
X‐Axis
Y‐Axis
Figure 2.- The schema shows the reference axis used during the experimental test.
Graphic 1.- The graphic shows the comparison between the temperatures predicted by the simulations and the
temperatures recorded during the real tests along the reference axis. These results correspond to the smoke
testing system working at 400 kW.
Graphic 2.- The graphic shows the comparison between the temperatures predicted by the simulations and the
temperatures recorded during the real tests along the reference axis. These results correspond to the smoke
testing system working at 1’200 kW
The temperature field across the plume section is the result of the turbulent process which
takes place around the entire plume perimeter and entrains air in the hot plume. The model
must be capable of reproducing this turbulent phenomenon.
The graphs show the accuracy of the model. The best examples are the small differences between the temperatures at the central positions. These temperatures are the result of the entire
turbulence around the plume. The narrow differences between these temperatures confirm that
the model is capable of reproducing the plume created by the smoke testing system. The differences are slightly higher at the boundary limits but this fact does not invalidate the model,
those points are at the plume boundary and the model is not capable to reproduce down to the
last detail the plume, and it is in fact unnecessary, but the central temperatures confirm that it
is capable of reproducing it with an accuracy enough to represent the whole system.
The validated model has been used to simulate the entire smoke tests scenario. These tests
took place in a big volume industrial hall. The geometry and the smoke testing system were
simulated and some of the simulations results are show in the following graphs:
Sensor 1
25 cm
75 cm
Sensor 6
150 cm
Sensor 11
225 cm
5
10 m
20 m
Figure 3.- The temperature sensors distribution in the industrial hall is schematic shown. Three sensors three
were hang on the ceiling at 5 m, 10 m and 20 m from the fire source. The sensors were placed at 25 cm, 75 cm,
150 cm and 225 cm from the ceiling to record the temperature field within the smoke layer during the test
duration
Graphic 3.- Comparison between the temperatures recorded during the smoke tests and the temperatures
recorded in the simulations. The first graphic show the temperatures from the sensor 1 placed 5 m away from the
fire source and 25 cm under the ceiling. The second one shows the sensor 6 placed 10 m away and 75 cm under
the ceiling. The third one shows the sensor 11 placed 20 m away and 125 cm under the ceiling
These results confirm that this smoke testing system is a valuable tool to validate simulation
geometries and it can be regarded as part of the testing strategy.
Figure 4.- Smoke layer created during the smoke tests in the industrial hall. It can clearly observed the smoke
stratification due to the temperature difference
6. Smoke test design
The aim of these tests is to evaluate the smoke management systems installed in a compartment to preserve the occupant’s life or minimize the economic losses The design of smoke
tests must fulfil at least two main targets: to preserve the room under investigation from any
damage and to provide reliable results to evaluate the system. These two factors are connected
with the maximum energy output which can be used during the tests.
The rooms under investigation are normally in the last phase of construction and ready to
commission or actually in use, that means that, it cannot be damaged and the different
installations such as illumination, sprinklers or decoration must be considered and set the
highest temperature that can be achieved during the tests. Those tests which take place in
mock up geometries in which the boundaries must not be preserved are out of the smoke of
this paper. It is remarkable the possibilities that the smoke testing system Izar offers to these
kind of tests, due to the repeatability of the initial testing conditions.
It is important to remark the difference between the design fire and the test fire: the design fire
is the fire which is expected to take place in the worst case scenario, while the test fire is the
energy output used during the test. The energy released by the test fire is smaller, and only in
a very few cases, equal to the energy expected from the design fire. The more similar the test
fire to the design fire is, the “more realistic” and better the test is and more reliable conclusions could be drawn.
The different building or design codes classify the buildings unlikely and prescribe fire loads
to define a design fire. The final design fire is characterised by a fire curve which describes
the duration of the fire and a maximum heat release rate. Both factors are important for the
safety strategy playing the fire curve a key role for the evacuation strategy. An egress strategy
considers that thanks to the passive and active safety elements the available safe egress time,
ASET, is longer than the required safe egress time, RSET. The ASET is closely related with
the fire curve: the faster the fire grows, the faster the room fills with smoke and the shorter the
ASET. This growth phase cannot be represent making use of the current smoke testing methods such as the pool fires because they just release a constant energy during a limited period
of time.
The Australian Standard [4] defines the maximum temperature during the smoke test at the
ceiling 10 degrees under the sprinkler activation temperature. This is a compromise value to
set the maximum HHR for the test fire test but it is also a constraint and “diverts” the test fire
from the reality represented by the design fire. However it is possible to follow the fire curve
till this maximum HRR value bringing the test closer to the reality.
Graphic 4.- Fire curves showing the difference between the design fires and the tests fires from Izar and a pool
fire
The fire simulations are an engineering tool widely spread. They are used to design the smoke
management system and even to design the evacuation strategies as a function of the smoke
filling times. The previous discussion shows that the geometries cannot be proved with the
actual design fires. An alternative is to carry out a scaled smoke test in order to validate the
simulated geometry an afterwards use this validated geometry to simulate the design fires.
The results obtained from these simulations are backed up and they conclusions are reliable.
Another key factor is the business disruption. The tests affect the normal development of the
building works or disrupt the business. This disruption time must be as short as possible. The
smoke testing installation process, the necessary protective measurements on the room and the
post cleaning activities are key factors to determinate this disruption time.
Our smoke testing system is capable of reducing all these time factors: it is a compact,
portable, and easy to move and install system; the lack of radiation allows eliminating the
protection measurements to avoid secondary ignitions and the constrained height of the
premixed flames and the live control of the system allows stopping the test instantly reducing
the protection measurements on the ceiling; finally the efficient combustion process which
does not produce any soot eliminates the post cleaning activities.
7. Conclusions
The new architectural projects represent a challenge regarding the safety of their occupants.
The smoke management systems must provide a time sufficient to complete the building
evacuation in a safe manner and they reduce the fire consequences.
Some of these smoke managements concepts are not standard concepts considered on the
standard design codes; they are instead innovative solutions developed following a
performance-based philosophy and require being proved before commissioning.
The new testing methodology suggested along this paper can only be implemented if the
necessary tools are available. The current standard testing methods are not suitable to test
those innovative solutions. It is our vision that performance-based solutions should be tested
using performance-based test instead of standard ones.
Izar, the smoke testing system presented along this work has proved to be a robust and
flexible tool to carry out smoke tests. It offers the engineers the possibility of testing their
designs under the most realistic conditions regarding both the design fire curve and the
highest HRR. In addition it reduces preparation, protection and cleaning works related with
these smoke tests.
References:
[1]
[2]
[3]
[4]
[5]
Drysdale, D., An Introduction to Fire Dynamics, John Wiley & Sons, 1992.
Karlsson, B., Quintiere, J. Enclosure Fire Dynamics. s.l. : CRC Press LLC, 2000. 08493-1300-7.
McGrattan, K., McDermott, R., Hostikka, S., Floyd, J., Weinschenk, C., Overholt, K,
"Fire Dynamics Simulator. User's Guide". National Institute of Standards and
Technology (NIST) and VTT Technical Research Centre of Finland. 2013 Australian Standard, AS-4391 Smoke management systems – hot smoke tests, 1999
Blond, J.; Validation of hot smoke tests, Master thesis submitted in the International
Master of Science in Fire and Safety, University of Edinburgh 2013