Chapter 7
Summary and conclusions
An integrated research program comprising the development of a semi-empirical model of the evaporation of water droplets, an experimental study to validate FDS, a state-of-the-art CFD based model, validation of FDS in predicting the growth of fire and suppressing it by water-mist spray and, finally, a parametric study of the effect of different factors on the suppression of fires has been carried out. The overarching objective is to determine the efficacy of water mists in suppressing fires.
The semi-empirical model development is aimed at developing a detailed understanding of the science of droplet evaporation. Against this semi-empirical model, FDS is validated in terms of single droplet evaporation. Experiments have been conducted to obtain a set of benchmarked results for validation of the numerical modelling of the distribution of flux densities of water-mist sprays. Validation of FDS in relation to the evaporation of a single droplet and the behaviour of water-mist sprays has been carried out in four stages:
The rates of evaporation of a single droplet predicted by FDS are compared with those produced by this semi-empirical model.
Distributions of flux densities of water-mist sprays predicted by FDS have been compared with benchmark experiments conducted in this study.
The predicted rates of pyrolysis and the combustion of a solid fuel (PMMA) have been compared with a set of literature data.
The pyrolysis and combustion of PMMA fire combined with water-mist suppression have been validated against a set of literature data.
The semi-empirical model developed in this study can also be used to evaluate the performance of the different size of droplets at different air condition and to find an appropriate median size of droplets in a particular scenario. The experimental data on distribution of flux densities can be used to validate any other CFD based model in simulating sprays. As the overarching objective is to investigate the capability of a CFD based model in simulating the suppression of fires using water mists, this study has identified the ability of FDS in simulating this.
The validation study has led to a parametric study using FDS. However, due to the enormous computational requirement, this part is qualitative rather than quantitative. The results of this research are discussed below.
7.1 Development of a semi-empirical model for evaporation
of water droplets
7.1.1 Model development and validation
The behaviour of individual water droplets in the hot air induced by a fire is examined using the ‘semi-empirical water droplet evaporation model’ developed in this study. The proposed model is validated against experimental and analytical data and the performance of FDS is evaluated and validated against the proposed model developed in this study. The analysis and FDS are found to be in close agreement when predicting the behaviour of droplets in hot air. The terminal velocities of droplets of different sizes are calculated using the proposed model and FDS. It is observed that estimated values by FDS are very close to the calculated results of the proposed model within a variation less than 8%. The proposed model and FDS are also used to calculate the saturation temperature of droplets at a different temperature
Chapter 7: Summary and conclusions
to the surrounding air. The FDS prediction does not differ by more than 10% of the calculated values of the proposed model.
7.1.2 Evaluation of the characteristics of the different sizes of droplets The proposed ‘water droplet evaporation model’ is also used to evaluate the characteristics of the different sizes of droplets in a layer of hot air i.e. whether a particular size of droplet is suitable for the cooling of hot gas by heat extraction or whether it is suitable for the wetting and cooling of the fuel surface by reaching there before complete evaporation. The results show:
(i) The smaller droplets have a higher rate of evaporation and longer suspension time in the air, which enables them to extract heat more effectively from the hot gases; whereas the larger droplets have a higher terminal velocity, which results in a higher penetration capability through the hot air and enables them to reach the burning surfaces.
(ii) The effect of the high mass transfer rate on the evaporation of a droplet is insignificant for droplets when the temperature of the air is in the range 0– 100 .
7.2 Spray distribution – a benchmark experiment and
validation of FDS
A set of experiments is conducted using a single and multi-orifice water-mist nozzle and when the distribution of flux densities on a horizontal surface is measured. A set of numerical simulations is also conducted to mimic the experiment. The distributions of flux densities are also estimated in the simulation and compared with the experimental measurements. It is observed that the numerical results are in good agreement with the experimental data. The distribution of the sprays in both cases of
eccentricities of the ellipses are less pronounced in the numerical model in both of the cases of the sprays.
The distribution of the sprays is found to be influenced by the presence of a solid wall in the vicinity of the spray. The experimental and numerical results indicate that the flux distribution is closer to the wall than would otherwise be the case. However the effect of the boundary wall on the distribution of the spray in the numerical model is less pronounced compared to that of the experimental results. The overall results from FDS are in close agreement with a variation of less than 20% with the experimentally determined distribution produced by the spray nozzles.
7.3 Validation of FDS for the burning of PMMA and the
suppression of fires by water-mist spray
7.3.1 Simulation of PMMA fire without the presence of a water-mist system
The burning rates of PMMA fire are numerically simulated using FDS and the numerical results are compared with the published experimental data by Magee and
Reitz [1974]. The FDS results of the steady state burning rates of the PMMA slab are
in reasonable agreement with the experimental measurements, with a difference of not more than 23%. The orientation of the specimen (vertical or horizontal) has a profound effect on the burning rate and time required to be ignited and attain the steady state burning rate of the material. In this study, the vertically oriented slab exhibits higher burning rates and shorter times to reach the steady state burning rate compared to that of the horizontally oriented PMMA slab.
Chapter 7: Summary and conclusions
7.3.2 Simulation of PMMA fire with the presence of a water-mist system
Validation of FDS in the scenarios involving the interaction of fire with a spray is conducted. The numerical results of the burning rate of the PMMA slab after activation of the water spray are in reasonable agreement with the experimental measurements of Magee and Reitz [1974]. The results show that FDS has predicted the burning rates of PMMA with an error not exceeding 15% of the experimental data.
7.4 Parametric study using FDS to assess the efficacy of
water-mist sprays
FDS is used to qualitatively examine a range of parameters that affects the performance of the suppression of fires by water-mist spray. This study is based on a qualitative analysis as the quantitative analysis demands enormous computational resources and time. The parameters are the effect of the obstruction and location of the fire, number of nozzles and size of the droplets of a spray. Suppression of a fire is considered when it is lowered to the level of 65% of HRR of its quasi-steady-state by the spray [Zhao et al. 1998; AFAC 2004]. The outcomes of this study are summarised below:
i) Obstruction: The spray is less effective in suppressing a horizontally obstructed fire. However unobstructed fires are suppressed in shorter times and to a greater extent than obstructed fires in all cases except for fires that are vertically obstructed and located directly underneath the spray.
ii) Location: The spray is found to be most effective for unobstructed fires that are located directly underneath the nozzle. Notably, the opposite phenomenon is observed for horizontally obstructed fires.