# Experimental and Numerical Study on Fire Suppression via Water Mist in a Mechanically Ventilated Compartment

Lahna Acherar, Hui-Ying Wang, Bruno Coudour, Jean-Pierre Garo

► To cite this version:

Lahna Acherar, Hui-Ying Wang, Bruno Coudour, Jean-Pierre Garo. Experimental and Numerical Study on Fire Suppression via Water Mist in a Mechanically Ventilated Compartment. Journal of Physics: Conference Series, 2024, 2885 (1), pp.012046. 10.1088/1742-6596/2885/1/012046. hal-05314365

HAL Id: hal-05314365

https://normandie-univ.hal.science/hal-05314365v1

Submitted on 14 Oct 2025

HAL is a multi-disciplinary open access
archive for the deposit and dissemination of sci-
entific research documents, whether they are pub-
lished or not. The documents may come from
teaching and research institutions in France or
abroad, or from public or private research centers.

L'archive ouverte pluridisciplinaire HAL, est
destinée au dépôt et à la diffusion de documents
scientifiques de niveau recherche, publiés ou non,
émanant des établissements d'enseignement et de
recherche français ou étrangers, des laboratoires
publics ou privés.

<figure><img src="image_1.png" /></figure>

Distributed under a Creative Commons Attribution 4.0 International License
# Experimental and Numerical Study on Fire Suppression via Water Mist in a Mechanically Ventilated Compartment

Lahna ACHERAR, Hui-Ying WANG, Bruno COUDOUR and Jean Pierre GARO

Institut P', Fluides-Thermique-Combustion, CNRS, ENSMA, Université de Poitiers, 86961 Futuroscope Chasseneuil Cedex, France

wang@ensma.fr

**Abstract.** The present work provides a detailed experimental and numerical study on suppression phenomena via water mist of a buoyant, turbulent fire over liquid (heptane) surface. It is found that water mist in a hot confined environment allows to achieve maximum droplet evaporation in the smoke layer, and however, it does not conduct to suppression of a fully developed fire due to re-ignition of unburnt fuel. Through the analysis on HRR, gas temperature, heat flux and chemical species, it is concluded that in a hot environment, a consecutive efficient cooling of smoke layer leads to the fire suppression only at lower ACPH (Air Change Per Hour) via lack of oxygen.

## 1. Introduction

Fire suppression is controlled by many factors including mist application time, flow rate, initial water discharge, and water transport to the fire source [1]. Activation of fire suppression system at the early stage of fire growth allows to reduce the flame temperature and surface heat flux during a long time [2]. Airflows induced by natural or forced ventilation in water mist significantly affect the fire suppression efficiency [3]. Droplets evaporation removes about 6-11% of the combustion heat from a fire as a function of fire scale [4]. When water mist spray splashes into an enclosed space, the drag force becomes significant due to presence of the walls [5]. Heat radiation from the fire source is effectively reduced when the entire fire extinguishing space is filled by the water mist [6]. Suppression performance of large-scale, turbulent fires via water mist was investigated [7] without the detailed characterization and controlled conditions necessary for model validation. The present work provides a detailed experimental and numerical study on impact of ACPH (Air Change Per Hour) on fire suppression via water mist in a highly confined enclosure, such as nuclear facilities. Only at low ACPH, both increased heat absorption and reduction in oxygen concentration below 10% in the smoke layer via dilution by increased vaporization of water mist can significantly reduce HRR up to global fire extinguishment. An accuracy of prediction on the enclosure fire suppression via water mist depends on the studied fire scenario even by using the measured burning rate curves as an input in the numerical simulation.

## 2. Experimental set-up and methodology

A laboratory-scale compartment is a cube (1 m³) to which an external ventilation system is connected (Fig.1, left). By adapting several scaling laws and preserving the ratio between HRR, Q² and length scale, L⁵, a good similitude in the physical phenomenon can be obtained between the reduced scale experimental device with a volume of 1 m³ and a full scale 100 m³ compartment for HRR below 1 MW. The air intake is located in low position at 0.15 m height. There is a baffle close to the exit of the admission duct to direct the inlet flow away from the fire source for avoiding impact of the ventilation flow on the fuel pan. Before activating the fire, ACPH (Air Change Per Hour) is determined from the
inlet volume flow rate at the admission duct. The smoke extraction duct is located at a height of 0.7 m with a length of 1.1 m. The extraction duct is connected to a dilution duct at the same height with a length of 2.5 m. Ventilation ducts are made of smooth galvanized steel and have a circular section of 0.1 m in diameter. The rectangular door with a surface of 0.8x0.55 m² is located in the middle of the front wall. The fuel pan containing heptane is placed at the center of the compartment on a metallic support, at a height of 30 cm. A Canon digital camera was used for recording the experimental images of the fire scenarios through the porthole with a size of 16x16 cm².

<figure><img src="image_2.png" /><figcaption>Figure 1. 3D view of the experimental set up with a mechanical ventilation system (left) and 2D side view with the measurement location (right)</figcaption></figure>

Molar fraction of oxygen was measured at the extraction duct thanks to a paramagnetic gas analyzer. The gas temperature was measured with three profiles of thermocouples type K of chromel-alumel and 0.5 mm wire, their arrangement is given in Fig.1(right). At left (TL) and right (TR) sides, 10 thermocouples are positioned at regular intervals with a distance of 10 cm. These two thermocouple trees are located at 0.275 m from the centreline. The thermocouple tree at the center (TC) comprises 20 thermocouples which arranged at intervals of 2.5 cm nearby the fire source and 5 cm close to the ceiling. The heat flux was measured by using flux meter and the wall temperature by using K thermocouple at the extraction side with a distance of 10 cm in relation to the central axis of the right wall at a height of z=36 cm (see Fig.1, left). Heat Release Rate (HRR) was estimated by measurements of oxygen concentration within the exhaust system through the species-based calorimetry technique [8]. The experimental set up comprises a fire suppression system via water mist by using one nozzle with a hole diameter of 3.5 mm, and it is fixed in the left corner of the box (see Fig.1, left). This nozzle is located at x=10 cm, y=90 cm, z=90 cm, and oriented to fire source. Flow rate of water was fixed at 3.5 g/s as used in the previous study [9] for a small-scale liquid pool fire and relative pressure of ejected air at 1 bar.

### 3. Numerical modelling

The hydrodynamic model in gas phase consists of the fully three-dimensional, transient equations of mass, momentum, energy and species conservation. A soot model such as LSP (Laminar Smoke Point) is implemented in FDS6.7 [10]. A dedicated module for modelling Heating, Ventilation and Air-Conditioning (HVAC) systems, consisting of mass, energy and momentum equations, is coupled with the gas phase solver [10]. The liquid phase is modeled using a Lagrangian approach. The droplet model consists of the droplet evaporation and the rates of change in vapor mass/in the droplet temperature/in the gas temperature and in the wall temperature. The governing equations for both the gas and the liquid phases are discretized and iteratively solved, and a detailed description is given in FDS6 user guide [10]. With a mesh size of 2 cm, the CPU time for a simulation during a physical time of 1500 s is approximately 150 h. With each halving of the grid size, the time required for a simulation increases by a factor of $2^4=16$, making numerical simulations difficult.

### 4. Results and discussions

The local speed and Sauter Mean Diameter were measured at a distance of 70 cm from the nozzle exit by using a laser anemometry system as PDPA (Phase Doppler Particle Analyser). Thanks to the comparison with the experimental data in Fig.2, droplet diameter of about 50µm and an injection
velocity of 10 m/s at the nozzle exit were numerically calibrated. Such calibrated droplet diameter and injection velocity are used at the nozzle exit as an input in the simulation by using FDS6 [10].

<figure><img src="image_3.png" /><figcaption>Figure 2. Calibration of Sauter Mean Diameter (left) and injection velocity (right)</figcaption></figure>

A number of ACPH as 4.2, 7, 28, 38, 48, 58 and opening condition by removing the door (see Fig.1, left) is achieved. In no water spray case, analyse of the HRR shows that the fully developed fire becomes progressively ventilation-controlled for ACPH below 38, and a further increase in ACPH from 38 to 58, conducts to a fuel-controlled fire. Fire is suppressed as soon as the spray-plume interfaces approaches the flame sheet for ACPH below 28 above which only the flame structure changes completely with fluctuations. Here, histories of the measured and calculated HRRs are indicated in Fig.3 solely for ACPH of 4.2, 38 and no door case to illustrate the suppression and combustion-supporting phenomena. Water mist is activated at the time, $t_0$, when HRR reaches a peak in fire fully developed stage. In the numerical simulation, the measured Mass Loss Rate (MLR) curves are used as an input in FDS6 [10]. The moment $t_0$ is identified at $t_0=600$ s of experiment time for opening condition, $t_0=750$ s at 38 ACPH and $t_0=1000$ s at 4.2 ACPH. This allows to analyse the radiant heat feedback over the pyrolysis surface during the perturbation influence of a mist addition on interaction between flame destabilization and extinction. The HRR increases by a factor of 50% with a rise of ACPH from 4.2 to 38. At 38 ACPH, an interplay of the gas-phase combustion within the mist cloud lowers the HRR, but doesn't extinguish the fire. The water mist has no significant suppression effects on HRR in no door case because cooling by buoyancy-induced air entrainment towards the fire source maintains a fuel-controlled fire with a HRR lower than that with 38 ACPH.

<figure><img src="image_4.png" /><figcaption>Figure 3. Impact of water mist on HRR at 4.2 (left), 38 ACPH (middle) and no door case (right)</figcaption></figure>

<figure><img src="image_5.png" /><figcaption>Figure 4. Instantaneous flame images inside enclosure fire after application of water mist at 4.2 ACPH (a, b, c) and at 38 ACPH (d)</figcaption></figure>

Only at the low ACPH of 4.2, the heptane fire weakens earlier and finally stops after activation of water mist, as illustrated in Fig.4 on instantaneous images at the different times. A rapid deterioration in the flame structure is coupled with periods of flame-base destabilization at $t=t_0+5$ s, and followed
by significant decrease in combustion intensity with a weak flame luminosity at t= t₀+10 s. This immediately precedes global fire extinguishment after about 10.5 s of water mist application. Similar phenomenon was observed for ACPH below 28. A high ACPH of 38 (picture d) has a combustion-supporting effect with a flame expansion due to the thick flammable vapour layer near the liquid surface when the water spray is applied.

Fig.5 presents impact of water mist on the history of oxygen concentration in the smoke layer near the extraction duct for various ACPH. After applying water mist at 4.2 ACPH, the oxygen-lean/fuel-rich layer is formed in the smoke layer, a rapid fall of oxygen concentration below 10% leads to global fire extinguishment. The predicted oxygen curve is far away from the measured one with an over-prediction by a factor of two times as long as the fire becomes under-ventilated. For a high ACPH of 38 and opening condition, the oxygen concentration above 12% in the smoke layer allows to sustain combustion, and a confined environment is hot enough to result in a strong droplet evaporation. As a result, most of the water mist with lower spray momentum than that of the buoyancy-induced plume can't reduce the burning rate via wetting. Before activating water mist, in comparison with the experimental data, difference in the predicted oxygen concentration remains within 5%, but after mist application, there is a tendency to over-predict oxygen concentration by a factor of 40%. Under opening condition, the enclosure fire seems fuel limited with oxygen volume fraction of 20%, and the predicted oxygen concentration follows closely the experimental trend. Fire suppression conducts to a rise of oxygen concentration regardless of ventilation condition.

<figure><img src="image_6.png" /><figcaption>Figure 5. Impact of mist on oxygen concentration in the smoke layer at 4.2 (left) and 38 ACPH (right)</figcaption></figure>

The characteristic dependence between the drop in pressure (P₀-P) inside the compartment and the inlet velocity from the admission duct is illustrated in Figs.6 and 7 at 4.2 and 38 ACPH. Here, P₀ denotes atmospheric pressure and P enclosure pressure. Application of water mist does not affect a lot the peak of drop in pressure which is mainly controlled by the fire dynamics. As shown in Fig.7, at 4.2 ACPH, air supply velocity from the admission duct towards the compartment is roughly 1 m/s. A high ACPH of 38 favours an increase of inlet velocity to 7 m/s. Thanks to a baffle close to the exit of the admission duct, such a strong crossflow of air in the admission duct does not prevent the water mist from effectively reaching the fire, negating the suppression effects. The deviation of the calculated inlet velocity from the HVAC module [10] is within 5% in relation to experimental data.

<figure><img src="image_7.png" /><figcaption>Figure 6. Evolution of drop in pressure inside the enclosure at 4.2 (left) and 38 ACPH (right)</figcaption></figure>
<figure><img src="image_8.png" /><figcaption>Figure 7. Evolution versus time of inlet velocity at 4.2 (left) and 38 ACPH (right)</figcaption></figure>

Profiles of temperature along the three thermocouple trees TL, TC and TR at 90 s after application of water mist ($t_0+90$ s) are presented in Fig.8. For 38 ACPH and opening condition, the water droplets evaporate well inside the smoke layer, and a consecutive efficient cooling of the smoke can't reduce the flame temperature below a critical value necessary to sustain combustion. A hot environment with a temperature level above 700°C is maintained at the fire base, high enough to trigger re-ignition of unburnt fuel even when water mist system was activated. At 4.2 ACPH, all droplets evaporate well inside the flame sheet, and a larger spray surface area available for heat absorption leads to a strong heat sink, yielding reduction in gas temperature below 250°C inside the enclosure. There is a good fit between the predicted and measured gas temperature inside the enclosure at 4.2 ACPH. Regarding the gas temperature field at the flame base, the profiles, in turn, are not captured as well with a deviation of 20% in relation to the measurement for 38 ACPH and opening condition.

<figure><img src="image_9.png" /><figcaption>Figure 8. Profiles of gas temperature at $t=t_0+90$ s along the height at 4.2 (left), 38 ACPH (middle) and under opening condition (right)</figcaption></figure>

<figure><img src="image_10.png" /><figcaption>Figure 9. Impact of water mist on the wall heat flux at 4.2 (left) and 38 ACPH (right)</figcaption></figure>

The radiation heat flux from the fire to enclosure's panels and wall temperature at a height of 36 cm (see Fig.1, left) are shown in Figs.9 and 10. Heat feedback from the fire and hot smoke gives a wall heat flux of about 3 kW/m² at 38 ACPH, and application of water mist conducts to a rapid decrease of heat flux. In no door case, the wall radiation heat flux is lower compared to that at 38 ACPH due to weaker HRR. The heat flux allows the wall temperature to increase monotonously to a peak of 140°C at 38 ACPH and of 80°C at 4.2 ACPH before activation of water mist. An opening condition encourages intimate flame interaction with the droplet cooled oxidizer, maintaining also a low wall temperature of about 80°C. Similar to wall heat flux, a decrease of the wall temperature occurs once starting application of water mist. Although the wall temperature is high enough to enhance water vaporization, its contribution to emitting a radiation heat flux over the liquid surface seems moderate.
The deviation of the numerical results in relation to the measured wall heat flux and temperature is within 25% mainly due to over-prediction of the gas temperature level around the fire source (see Fig. 8, middle).

<figure><img src="image_11.png" /><figcaption>Figure 10. Impact of water mist on the wall temperature at 4.2 (left) and 38 ACPH (right)</figcaption></figure>

## 5. Conclusions

Impact of ventilation conditions in a range of ACPH from 4.2 to 38 on the compartment fire suppression by applying water mist is clearly shown. Limiting the supply of fresh air into the compartment at low ACPH restricts the available oxygen below 10% in concentration, and a rapid deterioration in the flame structure immediately precedes fire extinguishment after water mist application. A high ACPH including an opening condition has a combustion-supporting effect with preceding fire suppression by application of water mist. Lack of oxygen below 10% in the smoke layer is identified as the main mechanism leading to fire suppression via water mist in a highly confined environment which is hot enough to result in a strong burning rate of liquid fuel. In comparison with the experimental data, the uncertainty in the prediction of fire suppression by water mist is estimated within 20-30% by taking into account the shortcoming of a fast Eddy Dissipation Concept combustion model which may induce a multitude of potential errors in the numerical simulation, especially for the ventilation-controlled fire.

## References

[1] Z. Liu, A.K. Kim, Review of water mist fire suppression systems – fundamental studies, J. Fire Prot. Eng. 10(2000) 32-50.

[2] G. Grant, J. Brenton, D. Drysdale, Fire suppression by water sprays, Progress. Energy Combust. Sci. 26 (2000) 79-130.

[3] James P. White, Salman Verma, Elizabeth Keller, Ailing Hao, Arnaud Trouvé, André W. Marshall, Water mist suppression of a turbulent line fire, Fire Safety J. 91 (2017) 705-713

[4] R. Mehaddi, A. Collin, P. Boulet, Z. Acem, et al., Use of a water mist for smoke confinement and radiation shielding in case of fire during tunnel construction, International J. of Thermal Sciences 148 (2020) 106156

[5] Liu, Y., Beji, T., Thielens, M., Tang, Z., Fang, Z., Merci, B., Numerical analysis of a water mist spray: The importance of various numerical and physical parameters, including the drag force. Fire Safety J. (2021) 127, 103515.

[6] Yao, Y., Li, Y.Z., Ingason, H., Cheng, X., Zhang, H., Theoretical and numerical study on influence of wind on mass loss rates of heptane pool fires at different scales. Fire Saf. J. (2020) 120, 103048.

[7] A. Jenft, A. Collin, P. Boulet, G. Pianet, A. Breton, A. Muller, Experimental and numerical study of pool fire suppression using water mist, Fire Safety J. 67(2014) 1-12.

[8] Marc L. Janssens, Measuring rate of heat release by oxygen consumption, Fire Technology (1991) 234-249.

[9] Richard J., Garo J.P., Souil J.M. and Vantelon J.P., On the determination of soot and droplet concentration and velocity fields during the addition of a water mist on a liquid pool fire at laboratory scale, Seventh International Symposium on Fire Safety Science, Worcester Polytechnic Institute, USA, June 16-21 (2003) 7: 507-518.

[10] K. Mcgrattan, R. Mcdermott, S. Hostikka and J. Floyd, Fire Dynamics Simulator (Version 6), User's guide (2023), NIST Special Publication.