R.BORCHIELLINI, M.CALI and G.MUTANI Dipartimento di Energetica, Politecnico di Torino, Italy Abstract
In this paper, two mathematical models are compared regarding their predictive ability to perform fire hazard analysis connected with smoke and toxic gases diffusion. After a classification of the computer programs used to estimate fire and smoke behaviour into buildings, the attention is turned on differentiation between two types of models: the “transient”
models which are able to describe in detail fire behaviour and the “steady state” models, less detailed but easier to use. For instance, the “transient”
models provide the temperature profile in the fire origin room, while the
“steady state” models need this profile as an input.
In this work a comparison of the results obtained with three computer programs which simulate smoke movement represent the building with a uni-dimensional fluid network is exposed. The parameters varied in this study are the wind effect on the external facades of the building, the thermal gradients between the nodes of the network and finally the type of building examined.
The “steady state” models are expected to be used more frequently owing to their simplicity while the “transient” ones analyse in particular the event. This paper focuses on application limits of both models.
1. Introduction
In these last years fire simulation with computer programs is gaining an important part in fire safety analysis. Actually, using these tools it is possible to estimate the diffusion of fire into a building with low expense of time and money.
The development of a fire is a really complex phenomenon and computer programs use different keys to identify the problem. There are two main ways to classify models describing the fire course into a building; those classifications depend on the way the
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building and the fire behaviour are considered. The first classification is based on building representation in which the phenomenon develops. It can be used a uni- or bidirectional network with nodes, representing zones or compartments, connected by branches that are windows or doors. Along this network there is fire and smoke spread;
those models, very flexible and easy to use for big structures with high number of compartments and connections are called “zone models”. Besides, “field models” work on a tri-dimensional dominion and are used to describe fire spread with much more detail into a few compartments.
Independently from first classification, a second one stands, which differently selects the models: the ones that describe the event giving time dependent results and others that give a steady fire analysis; the first are called “transient” and the second are steady state models. The comparison between these two last types of models could give the right conditions in which a transient model is useful and whereas should be used a steady state one. The utilisation of these two types of models involves different costs in time and money, so, for a detailed analysis it is sometimes required to use a transient model while in other cases the steady-state one is sufficient. The aim of this work consists in the determination of the application limits of these two types of models and for this reason there were selected three different but still comparable, computer programs.
2. The examined programs for the comparison
The comparison is made between three programs: CFAST 2.0 [1], ASCOS [2] and CONTAM [3]. These programs were used to estimate, the first the complete fire course, the second and the third the smoke diffusion and airflow into confined spaces. Those three programs utilise a “zone model” representation; CFAST 2.0 uses a “transient”, while ASCOS with CONTAM a “steady-state” approach to study the event. This work compares the smoke net flow across the building which results from the three programs changing the data that mainly influence the smoke spread into a building: wind velocities thermal gradients and types of building.
2.1. CFAST 2.0: simulation of fire behaviour with a transient model
CFAST 2.0 has been developed to improve in FAST [4] (Fire And Smoke Transport) the fire characteristics and to introduce the vertical connections between compartments.
CFAST 2.O is a zone model with two homogeneous volumes in every zone: in the lower there is mainly air, in the upper the hot smoke and gases coming out from the burning objects, and in the burn room there is a fire plume convecting mass and energy between the zones.
The solution is obtained by solving the following set of equations over small increments of time: conservation of enthalpy, mass, and the Bernoulli equation for momentum coupled with the ideal gas law.
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This program calculates the evolving distribution of the time-dependant characteristics that vary with the fire behaviour.
2.2. ASCOS: estimate of smoke propagation with a steady-state model ASCOS (Analysis of Smoke Control Systems) is a program for steady air flow analysis of smoke control systems along a network. The purpose of this program is to verify how the ventilation system can be utilised to evacuate smoke which is due to fire or how it limits the smoke movement into the building.
ASCOS is a uni-directional network model that connects several nodes (every one with homogeneous characteristics) through vents located at reference heights for every zone level.
The solution is obtained by solving the following set of equations: conservation of mass, and the Bernoulli equation for momentum coupled with ideal gas law. The output gives the steady state pressures and net flows along the examined network.
2.3. CONTAM (version 94): a program for the airflow analysis
CONTAM represents the building as a mono-dimensional network in which every node is an enclosed region (as a room), with uniform air, temperature and contaminant concentration. CONTAM was developed to have a contaminant diffusion and airflow analysis program utilising a mouse-driven graphic interface. CONTAM combines most of the capabilities of many previous programs in which there is ASCOS.
The airflows calculation may be made in three simulation modes: steady, transient (up to 24 hours) and cyclic (24-hours steady-periodic). In this paper the airflows analysis is made in the steady state mode.
3. Tests cases
The correspondence among CFAST 2.0, ASCOS and CONTAM has been found by testing three cases significant to understand which are the main variables causing and influencing the smoke flow.
The selection about the test cases was made considering at first a simple case where there are only two zones connected by a vertical vent. Then, in the second case, the network becomes more complex with addition of two levels and a shaft. These two first buildings are paper cases because of compartments and vents dimensions which allow an easy description of the network with all the programs. Finally the last case takes place into an existing apartment.
In this comparison the first step is to sketch a network representing the building and the path along which the fire develops with a zone model. Then, the resulting smoke flow from CFAST 2.0, ASCOS and CONTAM is analysed by changing the following variables:
1. the wind velocity working on the outside facades of the building; the considered values are: 0, 1, 3 and 9m/s. In CFAST and ASCOS it must be described as a wind power law; the values needed to describe this law are: an exponent of 0.2 and a reference height
Results comparison of smoke movement analysis in buildings 63
of 10m for wind speed, instead CONTAM requires a wind pressure profile for every opening connected with outside;
2. the fire power used in CFAST is as follows:
type A: the material heat of combustion is 34,3MJ/kg, the fire area is 3m2 and the heat release is 1, 6 MW (after 15 minutes from the ignition both are constant) and the mass ratio of hydrogen to carbon produced in pyrolysis is 0,333;
type B: the material heat of combustion is 17kJ/kg, the fire area is 3m2 and the heat release is 784W (after 15 minutes from the ignition both are constant) and the mass ratio of hydrogen to carbon produced in pyrolysis is 0,333.
With these two types of fire CFAST 2.0 gives for every test case a different temperatures distribution; the first with higher and the second with lower temperature gradients.
The outside ambient conditions are always the following: temperature 21°C, barometer absolute pressure 101325 Pa, relative humidity 60% and altitude meteo station 0m.
To compare the three programs the relation of the temperature of every zone is the following:
(1)
where: T is the temperature and V is the volume (because CFAST divides the zone in two layers, while ASCOS and CONTAM have only one zone with homogeneous characteristics).
The comparison of the resulting mass flows (kg/s) is made calculating the relative difference in percent by the term δ calculated as follows:
(2)
3.1. Case 1: two connected zones on different levels
This first case is a paper test case very simple and analytically easy to solve. With these characteristics it is possible to control how the Codes treat the data.
The building with the relative network used by the computer programs in Case 1 is shown in Figure 1. The particular shape is made to consider the stack-effect having a simple network. The dimension of the two nodes are: width 3m, depth 5m and height 3 m. All the vents are opened having a flow area of 1m2 at a relative height of 1,5m from the floor of the considered node. The fire is into the node 1.
3.2. Case 2: four zones on different levels connected with a shaft
This second case is a paper test case with two more levels and a shaft. The building with the relative network used by the computer programs in Case 2 is shown in Figure 2. The dimensions of the four nodes are: width 3m, depth 5m and height 3m and the horizontal
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area of the shaft has the same dimensions of the area four zones. All the vents connected with the outside are partially closed with a flow area of 0,01m2 at a relative height of 1,5 m from the floor of the considered node; the inside vents are opened doors with an area of 2m2. The fire is into the node 1.
3.3. Case 3: house test floor
This last case is a real apartment located at the ground floor of a house. This floor with the relative network, the dimensions of the six nodes and the relative connections are