# The importance of the GER definition in the analysis of combustion regimes in a closed mechanically ventilated compartment

Hugues Pretrel, Jeri At Thabari, Georgios Maragkos, Youk Moorthamers, Alexander Snegirev, Bart Merci

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Hugues Pretrel, Jeri At Thabari, Georgios Maragkos, Youk Moorthamers, Alexander Snegirev, et al. The importance of the GER definition in the analysis of combustion regimes in a closed mechanically ventilated compartment. 5th European Symposium on Fire Safety Science (ESFSS2025), Slovenian National Building and Civil Engineering Institute (ZAG), Sep 2025, Ljubljana, Slovenia. pp.012015, 10.1088/1742-6596/3121/1/012015. hal-05443947

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# The importance of the GER definition in the analysis of combustion regimes in a closed mechanically ventilated compartment

Hugues Pretrel¹*, Jeri At Thabari², Georgios Maragkos², Youk Moorthamers², Alexander Snegirev², Bart Merci²

¹Autorité de Sûreté Nucléaire et de Radioprotection (ASNR), PSN-RES/SA2I/LEF, F-13115, Saint Paul lez Durance, France

²Ghent University – UGent, Dept. of Structural Engineering and Building Materials, Technologiepark 60, Zwijnaarde, B-9052, Belgium

*E-mail: hugues.pretrel@asnr.fr

**Abstract.** Buoyant fires with gaseous fuel are discussed in the context of a closed, mechanically ventilated enclosure. Focus is on the ventilation flow rates (admission and extraction) and on the effect of leakages. The impact of the combustion is discussed. Multiple definitions of the global equivalence ratio (GER) are considered and the consequences therefore on the combustion regime is quantified. In the experimental campaign, these regimes varied from steady well-ventilated combustion to rapid extinction. These new results shed light on the fire scenario in a closed, mechanically ventilated enclosure and identify the most complex and critical configurations in terms of risk assessment.

## Introduction

Compartment fires in mechanically ventilated enclosure is a topic of growing interest in the field of fire risk assessment. This configuration is widely encountered in the industrial, transport and nuclear energy sectors. One important phenomenon concerns pressure variations induced by fire due to the tightness of the enclosure and the air resistance of the ventilation ducts [1], [2], [3]. The fire size in terms of heat release rate (HRR) plays an important role. In its turn, it is strongly influenced by the oxygen content and temperature of the environment [4]. Hence it is useful to quantify the effect of oxygen depletion on combustion [5], [6]. Therefore, global analyses based on large and small-scale pool fire experiments have been recently attempted to define combustion regimes within a compartment using a global equivalence ratio (GER), also named ventilation factor parameter in [7], [8], [9]. It is well-known that the position of air admission and smoke extraction also affect the conditions inside the enclosure, but that is not the focus of the present study, carried out jointly by the team of the ASNR (Authority for Nuclear Safety and Radioprotection) Fire Experiment Laboratory (LEF) and Ghent University. The analysis of the main findings, as well as the complete description of the experimental set-up, can be found in [10]. An important aspect, highlighted in [10], is the definition for GER in case of forced ventilation compartment. This is not trivial and is the scope of the present paper. This also requires discussion and quantification of the leakage flow rate. The aim of this present study is to discuss the use of the GER for interpreting the combustion regimes, given a certain set-up of admission and extraction ducts.

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# 1. Materials and methods

## 1.1 Definition of GER for forced ventilated compartment

The GER, $\phi_v$, is derived from the well stirred reactor (WSR) model, originally developed for premixed combustion, but adapted to fires in a mechanically ventilated enclosure [7]. It is defined as the ratio between the rate of oxygen required for complete combustion of the fuel and the rate of oxygen supplied by the ventilation system, $\phi_v = (s \cdot \dot{m}_F)/(Y_{O_2}^o \cdot \dot{m}_v)$. In this definition, s denotes the stoichiometric mass oxygen/fuel ratio, $Y_{O_2}^o$ is the initial value of oxygen mass fraction while $\dot{m}_F$ and $\dot{m}_v$ are the fuel supply rate and ventilation flow rate, respectively. The GER distinguishes between fuel-controlled ($\phi_v \ll 1$) and ventilation-controlled ($\phi_v \gg 1$) combustion regimes. In the event of a fire in a building, the oxygen brought into the room may not be completely consumed by the combustion reaction due to the fire scenario, such as: the localised character of the fire in the room (combustion does not take place throughout the room); the position of the admission and exhaust ducts (oxygen can be extracted without reaching the fire zone); leakage air entrance. In this case, the GER, as defined above, does not reflect the effective amount of oxygen consumed by combustion in the denominator. The aim of this study is to discuss the change in the GER definition. In this study, the fuel mass flow rate is fixed during the test at a certain level.

## 1.2 Experimental data

To discuss the GER definition, a set of experiments described in detail in [10] has been considered. Only the elements that are essential for the present study are mentioned here. A rectangular chamber (height = 1 m, width = 1.25 m, length = 1.5 m) is connected to a ventilation network (see Fig. 1). The ventilation network consists of an admission line (natural supply) and an exhaust line, with an extraction fan (mechanical extraction) that could be adjusted in the range from 0 to 50 m³/h. The ducts cause a pressure drop in both intake and exhaust lines, measured by pressure gauges along the network. The fuel is propane (s = 3.63 combustion enthalpy $\Delta H = 46.4$ MJ/kg), issued from a square gas burner placed on the floor in the centre of the enclosure (height = 0.06 m, side = 0.18 m). The burner has no lip. The enclosure is characterised by the leakage rate induced by the pressure difference between the room and the outside, which has been evaluated as: $\dot{q}_v = 0.012\Delta P$, where $\dot{q}_v$ is in m³/h and $\Delta P$ is in Pa as recommended by ventilation standards (here with the exponent to the pressure equal to 1) [11]. In the range of flow rates studied, the pressures in the room vary from -700 to 0 Pa, which leads to leakage in-flow rates within the interval [0;8.4] m³/h. The experimental set-up is instrumented to characterise the burner, the flame, the temperature, the molar fraction of chemical species in the gas, and the flow rates in the ventilation network. The propane flow rate is measured by a mass flow meter. The volumetric air flow rates are measured in the supply and exhaust ducts upstream and downstream of the enclosure using bi-directional McCaffrey-type sensor. The molar fraction of oxygen is measured using paramagnetic gas analysers connected to sampling lines distributed throughout the facility. To investigate different combustion regime, the effect of two parameters, propane flow rates and ventilation flow rates, were considered in the tests grid. Four pre-set propane flow values (0.10, 0.20, 0.25 and 0.30 g/s) were applied. The ventilation volume flow rate values are adjusted to have GERs in the two regimes, fuel and ventilation controlled. The test procedure is as follows. First, the enclosure is closed, and the ventilation air flow is applied by adjusting the fan speed. The burner is then ignited and regulated to the set value. The propane flow is stopped after 1300 s for all tests where combustion is maintained. This duration has been set to achieve a steady state for well-ventilated conditions. In certain configurations in ventilation-controlled regime, the fire is self-extinguished before 1300 s because the oxygen concentration drops to a level too low to
ensure combustion. In this case, as soon as the operator sees on the video that there is no flame, the propane line is shut off. For these tests, extinction can be reached before a steady state is achieved.

<figure><img src="image_3.png" /><figcaption>**Figure 1.** Description of the facility and locations of the measurements.</figcaption></figure>

## 2. Results

### 2.1 Typical behaviour

The time evolutions of the main variables of a test are shown in Figure 2 for two combustion regimes fuel-controlled on the first line (a)-(c) and ventilation control on the second line (d)-(f). For the fuel-controlled regime test, the propane flow rate is fixed at a constant value over time, $\dot{m}_F = 0.25$ g/s and the ventilation flow rate is also set to a fixed value of 40 m³/h before ignition. A difference between the air supply and gas exhaust values is observed because of the leakage inflow induced by the negative pressure in the enclosure. The mass conservation balance has been evaluated for all tests by calculating the sum of the inflow and outflow ventilation rates and the leakage flow rate. This results in a difference of less than 10% of the largest flow rate, the one at the exhaust. After ignition (t = 0 s), the exhaust ventilation flow rate quickly increases, whereas there is a significant sudden drop in the air supply flow rate. This is induced by pressure variation in the enclosure which is completely in line with the previous work [8]. After this transient phenomenon, the air supply flow rate relaxes back to a constant value (in about 150 s). Meanwhile, the combustion reaction leads to a decrease in the molar fraction of oxygen. The time scale can be approximated from the tangent to the initial drop in oxygen molar fraction. This oxygen reduction time scale is also about 150 s. It corresponds to the accumulation effect of combustion products in the gas phase, as can be deduced from the volume of the compartment (1.875 m³) and the air supply flow rate (40 m³/h). This gives a time scale of about $1.875 \times 3600/40 = 169$ s. Clearly, both time scales are of the same order of magnitude. For this test, the molar fractions become steady after about 600 s. This confirms that the above-mentioned time scale is linked to accumulation of mass in the gas phase or dm/dt (i.e., steady state after about 3 times the related time scale). For this test in fuel-controlled regime, the limiting oxygen index (LOI, assessed to be about 14% vol) is not reached and extinction occurs after 1300 s of combustion when the fuel supply is manually terminated.

For the ventilation-controlled regime test (Figure 2, d-e-f), the propane flow rate is also fixed at a constant value over time, $\dot{m}_F = 0.25$ g/s but the ventilation flow rate is set to a lower fixed value of 25 m³/h before ignition. Again, a difference between the air supply and gas exhaust values
is observed but of lower amplitude. This is due to the lower pressure inside the enclosure because of a lower ventilation flow rate. The time evolution of molar fraction of oxygen shows a faster decrease because of a lower ventilation flow rate. The molar fraction of oxygen reaches the lower limit below which combustion cannot be sustained. Extinction occurs approximately after 300 s, i.e., before the steady state is reached.

<figure><img src="image_4.png" /><figcaption>**Figure 2.** Temporal evolution of the major variables((a, d) propane flow rate, (b, e) mass flow rates in the extraction and admission ducts and of leakages, (c, f) molar fractions of O₂) during two tests: first line, (a-b-c) with $\dot{q}_v$ = 40 m³/h and second line, (d-e-f) with $\dot{q}_v$ = 25 m³/h, (for both tests $\dot{m}_F$ = 0.25 g/s).</figcaption></figure>

## 2.2 Combustion regime and GER

All the tests can be divided into two groups according to the extinction mode (extinction after 1300 s imposed by the manual termination of fuel supply or self-extinction due to lack of oxygen). This distinction, illustrated in Figure 3-a, makes it possible to identify the two combustion regimes, ventilation- or fuel-controlled. A critical ratio between the fuel flow rate and the ventilation flow rate (the latter is measured at the exhaust and prior to ignition) $\dot{m}_F/\dot{q}_v$ can be identified to distinguish these two regimes. This ratio is made dimensionless by considering the ratio of (mass flow rates of oxygen with the relation $(s.\dot{m}_F)/(Y_{O_2}^o.\rho_o\dot{q}_v)$ which corresponds to the GER identified in the WSR model. The critical GER value for this experiment is equal to 0.36 as indicated in Figure 3-a. A value less than 1, which can be surprising at first sight, is explained by the fact that not all the oxygen introduced into the enclosure is consumed, but a significant part bypasses the flame zone and is extracted without being used as an oxidant in combustion. This is due to the inhomogeneity of the gases in the enclosure, which is determined by the location of the burner (and thus the reactive flame zones) in the enclosure. The relative positions of the ventilation lines and of the flame affect this inhomogeneity. A GER equal to $(s×UFL/0.21×M_{C3H8}/M_{air}) = 2.5$ would correspond to the ideal premixed situation with uniform spatial distribution of the reactants in stoichiometric proportions in the entire volume, considering 9.5% for the upper flammability limit (UFL) of propane. This demonstrates the need to modify the WSR model for fire applications in which the flame occupies just a fraction of enclosure volume. Indeed, the ratio 0.36/2.5=0.14 indicates that the reactive zone occupies approximately 14% of the total volume of the room, which is consistent with the observations made during the experiments.
<figure><img src="image_5.png" /><figcaption>**Figure 3.** Illustration of the two combustion regimes, ventilation and fuel controlled with three definitions of the reference ventilation flow rate based on (a) the exhaust flow rate before ignition (b) the sum of inlet and leakage flow rates during the combustion phase and (c) the inlet flow rate only during the combustion phase.</figcaption></figure>

In order to determine experimentally the value of the GER corresponding to the transition between the two regimes, it is necessary to define the fuel flow rate and ventilation flow rate. In the present case, the fuel flow rate is constant whereas the ventilation flow rate varies with time and thus can be evaluated in different ways, resulting in several possible values of the GER. An evaluation of the GER with three definitions of the reference ventilation flow rate has been carried out: (1) the exhaust flow rate before ignition (use in Figure 3-a); (2) the sum of inlet and leakage flow rates during the combustion phase; and (3) the inlet flow rate only during the combustion phase. These definitions lead to the following values of the critical GER: 0.36, 0.41 and 0.47 respectively as illustrated in Figure 3. With respect to the first definition, an increase in the GER of up to 50% can be obtained, especially for values corresponding to under-ventilated and transient scenario (GER greater than about 0.5). The value of the critical GER obtained here is close to, albeit lower than the range from 0.5 to 0.58 obtained for similar configurations with pool fires [9], [10].

This parametric study of the choice of reference ventilation flow rate shows its influence on the assessment of the GER and the importance of using an identical definition when the different experimental datasets are compared.

<figure><img src="image_6.png" /><figcaption>**Figure 4.** Variation of the dimensionless fire duration as a function of the GER.</figcaption></figure>

The analysis of the duration of the combustion phase, text, versus the GER allows to illustrate how important is the GER to distinguish the two regimes as illustrated in Figure 4. For the fuel-
controlled mode, the maximum duration of 1300 s is determined by fuel termination; for the ventilation-controlled mode, the duration depends on the GER and decreases as the GER increases. In the ventilation-controlled regime, the higher the GER, the shorter the duration of the combustion phase, tending towards a limit corresponding to the time required to consume only the oxygen initially present in the room. The present study shows that the choice of the reference ventilation flow rate can influence the value of the critical ventilation flow rate. In addition, the contribution of leakages may be important.

## Conclusion

The aim of this study was to highlight the importance of the GER in characterising fire scenarios in a closed and mechanically ventilated compartment. Based on reduced scale propane gas fire experiments, the results show that the GER is a key parameter in differentiating between fuel and ventilation-controlled combustion regimes. The definition of the reference ventilation flow rate is of importance. Depending on the definition, variations in the GER of up to 50% can be reported. This will affect the determination of the critical GER that characterises the transition between the two regimes. The results also show the importance of considering leakage when assessing the GER.

## Acknowledgements

This research is funded by The Research Foundation-Flanders (FWO-Vlaanderen) via PhD fellowship 1104125N and projects G023221N and G034725N, and through the OFFERR FUND fast-track project.

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