Assessment of the Fire Dynamics Simulator Modeling for the Heating and Evaporation of a Single Water Droplet at Moderate and High Temperatures

Abstract

The work described in this paper is undertaken with the purpose of providing a detailed assessment of the current modelling capabilities of the effects of fire suppression systems (e.g., sprinklers) in fire-driven flows. Such assessment will allow identifying key modelling issues and, ultimately, improving the reliability of the numerical tools in fire safety design studies. More specifically, we studied herein the heating and evaporation of a single water droplet. This rather ‘simple’ configuration represents the first step in a tedious and rigorous verification and validation process, as advocated in the MaCFP (Measurement and Computation of Fire Phenomena) working group (see https://iafss.org/macfp/). Such process starts ideally with single-physics ‘unit tests’ and then more elaborate benchmark cases and sub-systems, before addressing ‘real-life’ application tests. In this paper, we are considering the recently published comprehensive and well-documented experimental data of Volkov and Strizhak (Applied Thermal Engineering, 2017) where a single suspended water droplet of a diameter between 2.6 and 3.4 mm is heated up by a convective hot air flow with a velocity between 3 and 4.5 m/s and a temperature between 100 and 800°C. The high temperatures considered therein represent a strong element of novelty, since previous experimental studies on single droplets were limited to rather relatively ‘moderate’ temperatures, up to around 350°C. Furthermore, the monitoring of the time history of the droplet temperature field, in addition to the droplet lifetime, provides very useful information for model development and validation purposes. In this numerical study, 36 experimental tests have been simulated with the Fire Dynamics Simulator (FDS 6.6.0). The results show that the droplet lifetime is overpredicted with an overall accuracy of 31%. The accuracy in the range 300 to 800°C is even better, i.e., 7 %, whilst the cases of 200 and, more so 100°C, showed much stronger deviations. The measured droplet saturation temperatures did not exceed 70°C, even for high air temperatures of around 800°C, whereas the predicted values approached 100°C. Based on the current findings, further analysis is required on the modelling of the heat and mass transfer coefficients, and more specifically the sub-models for the Nusselt and Sherwood numbers.