A New Experimental Method to Predict the Development of a Fire

M. Laouisset Centre de Recherches RhBne Poulenc Industries, 12 rue des Gardinoux, Aubervilliers, France

Among the various methods for predicting the development of a fire in real conditions we have chosen the one which works on a small-scale model. The differences in gas concentrations, available fuel, and convection and heat transfer are recognized and discussed. A small-scale model has been built in which any oxygen concentration, heat flux and air supply can be reproduced simultaneously. The method makes it possible to simulate a good range of conditions, and by controlling these conditions a better knowledge of the geometric reduction which is to be used in the small- scale model can be obtained.

INTRODUCTION

The main steps in the development of a fire are ignition of a material, flame spread, and flash-over. The pro- bability of the full development of each of these steps depends on the air supply and on the nature, the amount and the relative position of the materials. These factors have a considerable effect upon radiation, convection, pyrolysis and combustion. To predict the resulting temperatures and gas concentrations, two methods of investigation are possible:

The first method is to predict the development of the fire by calculation. This is a long-term and rather difficult method. It entails determining the set of differential equations governing the fire. (Calculations are based on the laws of thermodynamics and fluid mechanics.) A compromise has to be made between the model’s func- tional simplicity, its accuracy and its development cost. To simplify the calculations, it may be helpful to base the model on experimental tests which give values that can be used directly in the equations.

The second method is shorter. It involves predicting the development of a fire from a small-scale model. This means that a reproduction of the room involved has to be built and the conditions arranged so that the development of the fire is the same as in the full-scale model-gas concentrations, flame spread and flash-over, etc. This method of investigation is the subject of this paper. The experimental procedure which is being developed in our laboratories is described and it is shown how the results of a theoretical model can be used.

DEFINITION OF THE PROBLEM

During the combustion of a given amount of fuel (such as a piece of furniture), a quantity of heat is released and convected to the upper part of the room. Part of this heat is lost through ventilation; another part is lost

through convection and radiation. The combustion and the pyrolysis are increased by the heat flux. Flash-over occurs when the concentrations of pyrolysis gases and oxygen in the upper part of the room or near the surface of a material make a flammable mixture. During such a fire the behaviour of the materials other than the main heat source is strongly affected by the resulting heat flux, convection currents and oxygen depletion.

If the same experiment is carried out on a small-scale model, large differences will appear:

1. The gas concentrations will not be the same because the geometric reduction will not have the same effect on volumes as on areas.

2. The fire will not last so long because there is less fuel. 3. Heat transfers through convection and radiation will

be greater because the distances are shorter.

Three solutions emerge. The first one is a general method (known in engineering as ‘dimensional analysis’). There is a set of theoretical equations which describe the development of the fire. These equations are then rewritten so that as many of the parameters as possible are in a dimensionless form. These dimensionless parameters are the constants of the model and give the dimension reduction ratios to be used in the small-scale model. Unfortunately, this method is unsatisfactory because the above relationships are often contradictory and therefore compromises have to be made.

The second solution is based on statistical methods. Full- and small-scale trials are carried out and the results of both are compared. Through statistical analysis, relationships between the two modes of behaviour are found. A major disadvantage is the required number of trials and the resulting cost.

The third solution, based on a simulation technique, has beeii chosen. This technique is based on the following assumption: a material, other than the main heat source, subjected to a given set of conditions, can be expected to behave in the same way in the full-scale model as in the small-scale model.

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A NEW EXPERIMENTAL METHOD TO PREDICT THE DEVELOPMENT OF A FIRE

EXPERIMENTAL

An instrumented small-scale model has been built in which the combustion conditions from the main heat source can be simulated, namely, oxygen depletion, heat flux and air supply. For this purpose the following experimental equipment is needed : a small computer, a numerical voltmeter, three digital/analogue (D/A) converters, and three power modulators.

The computer and the numerical voltmeter are used both as accumulation and regulation systems. The D/A converters change the digital value stored in the computer into a 0-1OV analogue signal. The power modulators then convert this signal into a 0-220 V signal.

Heat flux simulation The heat flux is produced by a radiant panel and/or a gas burner. The radiant panel is placed on the ceiling of the small-scale model to simulate the radiation of the hot gas layer. The gas burner replaces the piece of burning furniture.

A point is selected at which the heat flux follows a programmed output (the corresponding curve is stored in the computer). During each time increment the heat flux is measured and compared with the theoretical value. Then, if necessary, the computer modifies the power of the radiant panel or the combustion rate of the gas burner using a D/A converter and a power modulator.

Oxygen simulation The oxygen depletion is obtained by the gas burner. For each time increment the oxygen concentration is com- pared with the theoretical value. The combustion rate of the gas burner is then adjusted, if necessary, to give the theoretical oxygen values.

Air supply simulation The ventilation rate is altered using a fan whose rotational speed for each time increment is predetermined by the computer. This ventilation system makes it possible to simulate any air supply conditions.

Simulation technique For each time increment these three simulations are carried out in a stepwise manner. First the air supply is determined by a D/A converter. Then the combustion rate of the gas burner is corrected so that the oxygen concentration is equal to the theoretical value. Finally the power of the radiant panel is adjusted to make the actual and the theoretical heat flux values identical.

APPLICATIONS ~

The experimental equipment described above makes it possible to simulate the combustion conditions from any given heat source. The advantage of this method is that even though the experiments are carried out on a small scale, the duration of the experiment is identical to the large-scale test. This is fundamental for all thermal and chemical phenomena.

Two observations can be made at this stage:

1. There is still a problem in the selection of the geometric reduction of the other materials so that the development of the fire is quantitatively the same as in the full-scale model.

2. The experimental equipment makes it possible to simulate any oxygen depletion level, any air supply and any heat flux, but these parameters cannot be chosen independently.

Geometric ratio determination Let V be the volume of the full-scale room and S the area of a given material (where v, s are the same parameters for the small-scale model), and let x be the geometric reduction ratio. If d M is the amount of gas released by the material during the increment of time dt (dm for the small-scale model), this quantity is directly proportional to the surface of the material. With all conditions equal, the proportion coefficient is the same for the small-scale model :

d M = a . S.dt

dm= a,s.dt

The gas concentrations will show the same variations on both scales if and only if

or s v - - -x3 S - V -

This relationship has to be supplemented by an appropriate thickness reduction ratio. Since the experi- mental equipment makes it possible to keep the same time scale, the pyrolysis and the combustion will last as long as in the full-scale model if the thicknesses E and e are equal, or

e E= 1

Furthermore, the vertical heat flux gradient which is reproduced in the small-scale model makes it necessary to use the geometric reduction ratio for the vertical dimensions L and I :

I 1 --x _. - L

These relationships do not overcome all the problems, but they are a good first approximation.

Determination of the surrounding conditions to be simulated There are two possible choices. The first one consists in reproducing the evolution of air supply, oxygen depletion and heat flux level, as measured in a full-scale experiment. Then, by comparing the development of the fire with the results obtained during the full-scale test, the quanti- ties of the materials and their relative position can be determined more precisely so that the development of the fire will be the same in both small-scale and full-scale tests.

The second choice consists in using the theoretical values of the above parameters. First a combustion curve of the heat source is chosen. The theoretical model

0 Heyden & Son Ltd, 1980 F I R E AND MATERIALS, VOL. 4, NO. 1, 1980 43

M. LAOUlSSET

determines, without taking the other materials into consideration, the average resulting heat flux, oxygen depletion and air supply. A two-zone model is satisfactory. Then these conditions are reproduced in the small-scale model with all materials. By using different combustion rates, various heat sources can be simulated for the same arrangement of materials.

CONCLUSIONS

and, furthermore, the gas burner combustion products are different from those of the simulated combustion. On the other hand, the experimental equipment makes it possible to work in the same conditions as in a real fire. The practical experience acquired shows this to be the major factor of behaviour similarity. Finally, it has been shown how the actual simulation technique is applied and how a theoretical model can be used together to simulate various fire situations for a given choice and arrangement of materials. Applications of this method will be described in later papers.

This paper describes only the method and how it may be applied to small-scale fire tests.

Of course, some objections can be made. The radiant panel creates a high temperature gradient at the ceiling

9 FIRE AND MATERIALS, VOL. 4, NO. 1,1980

Paper presented a t lnterflam '79, Guildford, UK, March 1979

0 Heyden & Son Ltd, 1980

0 Heyden & Son Ltd, 1980