Water Mist and Cultural Heritage: can Simulation Tools help assessing its effect?

The_Blue_Boat_1892_Winslow_Homer – Museum of Fine Arts – Boston 

Watercolor images are among the most vulnerable artefacts to the effects of firefighting water  systems.

According to the NFPA 750 definition, watermist is a water spray for which the 99% of the total volume of liquid (Dv0.99) is distributed in droplets with a diameter smaller than 1000 microns at the minimum design operating pressure of the water mist nozzle.A slightly different definition has been introduced by the CEN/TS 14972,  as a water spray for which the 90% of the total volume of liquid (Dv0.90) is distributed in droplets with a diameter smaller than 1000 microns at the minimum design operating pressure of the water mist nozzle.

Given the importance of water mist systems in firefighting protection, when such resource is considered as a possible choice in the fire protection strategies, the first step to be taken before adopting them is assessing their effects on the objects to be protected. Among them, there’s a lot of objects that do not like water, even if distributed in very small droplets.

Thus, the capacity of simulating a fire and fire extinction process is critical to choosing with the required data. The study “Can we predict fire extinction by water mist with FDS?”, By A. Jenft, P. Boulet, A. Collin, G. Pianet, A. Breton and A. Muller has been recently published on Mechanics & Industry (14 , 389-393 (2013)) and deals with the general problem of simulating the suppression of a fire with water mist systems. Even if it is not focused on cultural heritage protection, the paper gives some important information about the actual possibilities of the existing simulation capacities.

The abstract of the paper starts from the consideration that, among the primary phenomena observed when studying fire suppression, there are fuel surface cooling, fire plume cooling and inerting effects. In particular, the final result of water evaporation generates significant vapor concentration, thus leading to an important heat sink as well as displacement and dilution of both oxygen and fuel vapor.

The simulation tool from NIST Fire Dynamics Simulator (FDS.v6) is expected to be able to reproduce the effects of the fuel vapor. The extinguishing criterion for focusing on plume cooling and inerting effects is based on a dedicated heat balance, while the suppression model for 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 from the Authors. Such model links the burning rate to the fuel surface temperature. Numerical simulations have been conducted and the paper illustrates the comparison with experimental data for all extinguishing mechanisms.

Why the simulation capacity of the fire spread is so important? Actually, if a fire is well modeled and is possible to reproduce the effects of the water mist suppression effects, is possible to plan a distribution of the nozzles that is compatible with the fire suppression needs and with the preservation of the artifacts vulnerable to the droplets.
Obviously, fire technologies should allow another assessment, which is as important as the simulation of the water mist operation: how and how much the smoke and dangerous gases affect the objects to be preserved. Unfortunately, in this case the problem is not in the availability of the simulation tools (we can assess in a acceptable way the quantity and the quality of gases and smoke produced by a smoke) but in the lack of data concerning the effects of fire effluents on historical objects.

The definition of an acceptable thresholds (for example, which concentration of HCN is acceptable when a XIII century oil on wood painting is exposed for 20 minutes) is still not easy for any specialist since few researches have been carried out on this specific area.

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