Real scale experimentations have been conducted in order to investigate interactions that may occur in a compartment where sprinklers and Smoke and Heat Exhaust Vent Systems coexist. Fuel oil spray was used as fire source with steady heat release rate of 400 and 800 kW. Both sprinkler system and natural Smoke and Heat Exhaust Vent Systems were designed according to the French standards. Effect of vent on sprinkler activation time was studied. An analysis of the temperature field inside the compartment was conducted while vent was opened prior or after sprinkler activation. Simulations of the experiments were also carried out with Fire Dynamics Simulator v.6 for validation purpose and also to provide supplementary data regarding soot flow rate and energy extracted at the vent.

Fire suppression with water spray was investigated, focusing on cases where fuel cooling is the dominant suppression mechanism, with the aim to add a specific suppression model addressing this mechanism in Fire Dynamics Simulator (FDS), which already involves a suppression model addressing effects related to flame cooling. A series of experiments was selected, involving round pools of either 25 or 35cm diameter and using both diesel and fuel oil, in a well-ventilated room. The fire suppression system is designed with four nozzles delivering a total flow rate of 25l/min and injecting droplets with mean Sauter diameter 112μm. Among the 74 tests conducted in various conditions, 12 cases with early spray activation were especially considered, as suppression was observed to require a longer time to cool the fuel surface below the ignition temperature. This was quantified with fuel surface temperature measurements and flame video recordings in particular. A model was introduced simulating the reduction of the pyrolysis rate during the water spray application, in relation to the decrease of the fuel local temperature. The numerical implementation uses the free-burn step of the fire to identify the relationship between pyrolysis rate and fuel surface temperature, assuming that the same relationship is kept during the fire suppression step. As expected, numerical simulations reproduced a sharp HRR decrease following the spray activation in all tests and the suppression was predicted in all cases where it was observed experimentally. One specific case involving a water flow rate reduced such that it is too weak to allow complete suppression was successfully simulated. Indeed, the simulation showed a reduced HRR but a fire not yet suppressed. However, most of the tests showed an under-estimated duration before fire suppression (discrepancy up to 26s for a spray activation lasting 73s), which demonstrates the need for model improvement. In particular the simulation of the surface temperature should require a dedicated attention. Finally, when spray activation occurred in hotter environments, probably requiring a combination of fuel cooling and flame cooling effects, fire suppression was predicted but with an over-estimated duration. These results show the need for further modeling efforts to combine in a satisfactory manner the flame cooling model of FDS and the present suggested model for fuel cooling.

Experiments in a real-scale room were done on water mist application to a pool fire. A fire produced with fuel oil in a 35cm cylindrical pool was used, with a heat release rate reaching 75kW in stationary conditions. Water application was studied with a nominal flow rate equal to 25l/min provided by a set of four nozzles, injecting droplets with mean Sauter diameter equal to 112μm. Observations of fire suppression in these conditions showed two behaviors, which were analyzed and detailed with the help of numerical simulations conducted with FDS.v5. On one hand, a fast suppression (about 10s required) was observed when water mist was applied to a developed fire. In this case, droplets were injected into a hot environment and thus evaporated strongly, generating a significant vapor concentration and resulting in a fast gas cooling and in an inerting effect. On the other hand, when the mist was applied early, fire growth was controlled, but its suppression required a longer application (about 1min) and only occurred after a significant cooling of the flame and the liquid pool. These two mechanisms were detailed numerically through mass and energy balances for both the gas and the liquid phases and could help to derive suppression model improvements.

Among the primary phenomena observed when studying fire suppression are fuel surface cooling, fire plume cooling and inerting effects. The last two result from water evaporation generating a significant vapor concentration, thus leading to an important heat sink as well as displacement and dilution of both oxygen and fuel vapor. Fire Dynamics Simulator (FDS. v6) is expected to be able to reproduce these effects. Extinguishment criterion focusing on plume cooling and inerting effects is based on a dedicated heat balance, whereas suppression model related to fuel surface cooling evaluates the burning rate decrease according to an exponential law taking into account local water mass reaching the fuel surface per unit area and an empirical constant which penalizes the prediction ability. Therefore, a new model derived from an Arrhenius equation has been implemented, which links the burning rate to the fuel surface temperature. Numerical simulations are conducted and compared with experimental data for all extinguishing mechanisms.

The work described here is a part of a research campaign aiming at improving the understanding of the interactions between water mist spraying and tunnel fire. Several results obtained by present authors during the last five years are reported at various scales. It relies on an extensive use of numerical simulations using the CFD code Fire Dynamics Simulator (FDS, NIST). First, the computational tool is verified and validated on the basis of comparisons with other computational codes and experimentations of increasing complexity: from the laboratory scale for assessing one particular part of the water spray model up to the tunnel scale. For the last case, the code validation makes use of the results of a reduced scale (1/3 rd ) tunnel fire test campaign conducted between 2005 and 2008. Once the validation is achieved, the computational tool is used intensively in order to improve the understanding of the interaction phenomena between water mist, tunnel longitudinal ventilation and fire. In particular, the water mist influence on the tunnel air flow is studied, the water mist heat contribution is quantified and the heat transferred to the droplets is identified.