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Water-mists are emerging as an effective agent for the suppression of fires. However, the mechanisms of suppression are complex and the behaviour of individual water droplets in a smoke layer generated by fires must be quantified. This study investigates the behaviour of individual droplets injected from a nozzle into a hot air environment induced...
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... A defining feature of water-mist nozzles is that they produce fine mists consisting of tiny droplets with diameters of less than 1000 µm. The fine mists exhibit fog-like behaviour that renders their fire suppression mechanism quite different from conventional water sprays that comprise larger droplets [11]. Studies of the interaction of traditional sprinkler spray with hot air or smoke layers [12][13][14][15][16][17][18][19] have focused mainly on the convective heat transfer phenomena between the large water droplets and the layer of hot air. ...
... In addition, the suspension time in the air and the evaporation rate of the droplets are explored. A previously developed water droplet evaporation model (WDEM) by the same authors [11] is used to facilitate this investigation. ...
... The tiny droplets of water that comprise fine mists have a higher surface area/volume ratio. This results in their rapid evaporation, and their movement is highly responsive to their local environment [11]. As a result, water droplets emanating from a nozzle, the diameter and velocity change continuously due to evaporation, and this affects the drag coefficient. ...
This paper presents work on investigating the effect of the initial size of water mist droplets on the evaporation and removal of heat from the fire-induced hot gas layer while travelling through the air in a compartment. The histories of the temperature, diameter and position of droplets with different initial diameters (varied from 100 µm to 1000 µm) are determined considering surrounding air temperatures of 75 °C and 150 °C and a room height of 3.0 m. A water droplet evaporation model (WDEM) developed in a previous study (Fire and Materials 2016; 40:190–205) is employed to navigate this work. The study reveals that tiny droplets (for example, 100 µm) have disappeared within a very short time due to evaporation and travelled a very small distance from the spray nozzle because of their tiny size. In contrast, droplets with a larger diameter (for example, 1000 µm) reached the floor with much less evaporation. In the case of this study, the relative tiny droplets (≤200 µm) have absorbed the highest amount of energy from their surroundings due to their complete evaporation, whereas the larger droplets have extracted less energy due to their smaller area/volume ratios, and their traverse times are shorter. One of the key findings of this study is that the smaller droplets of spray effectively cool the environment due to their rapid evaporation and extraction of heat from the surroundings, and the larger droplets are effective in traversing the hot air or smoke layer and reaching the floor of the compartment in a fire environment. The findings of this study might help in understanding the behaviour of water-mist droplets with different initial diameters in designing a water-mist nozzle.
... Water-mist fire suppression systems (WMFSS) represent a promising technology for various applications within the field of fire protection due to their low demand for water and their highly effective fire suppression capability [1]. The water-mist spray comprises fine mists with a higher surface area/volume ratio, resulting in rapid evaporation [2]. However, the efficacy of water-mist sprays in suppressing fires is strongly influenced by the characteristics of the sprays [3,4]. ...
... A second experiment was conducted with a different flow pressure (P 2 ), and the corresponding data of the distribution of flux density of water spray was collected. Using Equation (2), the median diameter of droplets (dm 2 ) of the spray was calculated for P 2 . The flow rate of water and the angle of the spray was also measured in the experiment. ...
... As mentioned in the methodology, a second experiment was carried out using a pressure, P 2 , of 75.8 bar, and the spray's corresponding flux density distribution data was measured. Afterwards, the expected dm 2 of the spray was calculated for P 2 by using Equation (2). The calculated median diameter of droplets for the spray in the second experiment was 275 µm. ...
The physical characteristics of water sprays profoundly influence the efficacy with which fires are extinguished. One of the most important physical characteristics of water sprays is the median diameter of the water droplets. However, this parameter is difficult to measure without resorting to the use of specialised equipment. Furthermore, the distribution of the size of water droplets and their initial velocity are profoundly sensitive to the pressure at the nozzle head. This paper presents a simple technique to determine the median droplet size of a water spray produced by a nozzle. The method required only two experiments to determine the mass flux distribution generated by a nozzle operating at two known pressures. A computational fluid dynamics (CFD) program was then used to estimate the median diameter of the water spray under these conditions. The median droplets generated when the nozzle was operating under a different pressure can be calculated using an established empirical relationship. The approach advocated in this paper is supported by invoking Whewell’s principle of consilience of inductions. This was achieved by observing that the CFD software accurately predicts the mass flux distribution when the new pressure and estimated median diameter of the droplets were used as inputs. This provides independent evidence that the proposed approach has some merit. The findings of this research may contribute to establish a technique in calculating the median diameter of droplets when direct measurement of droplet diameter is not available.
... A Lagrangian particle model, one of the sub-models of FDS, is used to simulate the transport of particles in the flow field. The model is extensively verified and validated for liquid particles like droplets and mist (Mahmud et al., 2016). However, verification and validation for the transport of solid particles are very limited. ...
Firebrands play a crucial role in increasing the severity of wildfires by driving fire growth, damaging structures, and starting new fires. Predicting the transport of firebrands and their propensity to ignite new fires is of significant interest to fire communities. Developing an operational firebrand transport sub-model from the field studies is cumbersome, expensive, and has significant associated risks to equipment, community and firefighters. Physics-based models have the potential to assist in the development of such firebrand transport sub-models, which can be utilised to improve the efficacy of existing operational fire models. The present study showcases one of the initial works carried out in the development of such a physics-based firebrand model. The work utilises Fire Dynamics Simulator (FDS), a commonly used open-source physics-based fire model. The Lagrangian particle sub-model of FDS is used to simulate the transport of firebrand particles. The Lagrangian sub-model is generally used to model the transport of droplets and mist, and has been extensively validated. However, the validation of this sub-model for the transport of solid particles such as firebrands is limited. The issue is exacerbated when particles are of a non-spherical shape and can undergo complex reactions over their transport, such as burning.
In this work, we utilise a firebrand generator prototype that produces a uniform Lagrangian shower of non-burning idealised firebrands. A set of in-house experiments are conducted to study the transport of three isometric shapes of non-burning firebrands i.e. cubiform, cylindrical and square-disc. These sets of experiments are used to quantify the efficacy of the inbuilt particle drag model of FDS and suggest potential alternative drag models that can be employed without loss of computational speed, major amendment in the fire model, are applicable to a wide range of particles shapes, and potentially improved prediction. In general, it is found that the suggested alternative Haider and Levenspiel drag model improves the estimation of firebrand distribution in terms of peak location, maximum and minimum longitudinal distribution with exception to cubiform particles for peak location. The exception is mainly due to inherent error association with the alternative drag model in overestimating the drag coefficient. For other situations, Haider and Levenspiel drag model shows either an improvement or stays the same. However, the study found that the existing point particle assumption to represent particles in FDS is not suited to estimate the lateral spread of firebrands, especially when the secondary motion of a particle on its axis is involved, such as cylindrical and square-disc particles. Our studies found, the lateral spread is found to be in the range of ~5-15% thinner compared to its experimental width for cylindrical particle distribution. For the square disc, it is not possible to quantify such differences due to the computational limit associated with our present study. It can be qualitatively suggested that it is found to be more than cylindrical particles. A further set of experiments and their numerical validation is required to ascertain the above finding, especially with different sizes, isometric and non-isometric shape, the speed of firebrand particles and burning process to establish the efficacy of a particular drag model for firebrand transport.
... A Lagrangian particle model, one of the sub-models of FDS, is used to simulate the transport of particles in the flow field. The model is extensively verified and validated for liquid particles like droplets and mist [30,31]. However, the verification and validation for solid particles are very limited. ...
... The rate, q r , at which heat is transferred to the droplet by radiation is estimated using the following equation (see for example [8,9]): ...
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 a 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 initial 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. In the present numerical study, 36 experimental tests have been simulated with the Fire Dynamics Simulator (FDS 6.7.0)as well as with an in-house code. The results show that the droplet lifetime is overpredicted with an overall deviation between 26 and 31%. The deviation in the range 300–800 °C is even better, i.e., 5–8%, 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. A detailed analysis shows that the standard Ranz & Marshall modelling of the non-dimensional Nusselt and Sherwood numbers may not be appropriate in order to obtain a simultaneous good agreement for both the droplet lifetime and temperature. More specifically, the heat-mass transfer analogy (i.e., Nu = Sh)appears to be not always valid.
... Mell et al. [19,20] have discussed the applicability of a version of FDS to study grassfire propagation and tree fires. A Lagrangian particle model, one of the sub-models of FDS, has been extensively used, verified and validated for the transport of liquid particles used in sprinkler and nozzle building-fire suppression systems [21][22][23]. However, use of the Lagrangian model for the transport of Reynolds number of fluid based on the fluid velocity and pipe radius Re D Particle Reynolds number based on the velocity of the particle relative to the fluid and particle size σ Standard deviation of particles density u' ...
Firebrands are a harbinger of damage to infrastructure; their effects cause a particularly important threat to people living within the wildland-urban-interface. Short-range firebrands travel with the wind with little or no lofting, and cause spotfires. In this work, the design of a novel firebrand generator prototype is discussed to achieve a uniform shower of firebrands. The transport of short-range firebrand is studied to verify the existing Lagrangian particle model of Fire Dynamics Simulator. Uniform, non-combusting cubiform and cylindrical firebrands are projected using the firebrand generator. The experimentally observed distribution of particles on the ground is compared with a simulated distribution using the fire dynamic simulator. The results show that the existing Lagrangian model gives a good agreement with the experimental data.
The paper presents a simplified engineering model for the prediction of the rate of heat absorption by heat-up and evaporation of water droplets interacting with fire-induced smoke. The algorithm can be easily implemented in the framework of one-zone or two-zone fire models. The general methodology is based on a decoupled time scale analysis for the heating, evaporation and motion of a single droplet. Such analysis allows to determine the heat absorbed by a droplet during its residence time in the smoke layer. Under the assumption of a monodisperse spray, the injected number of droplets per second is calculated and used to estimate the rate of heat absorption (i.e., cooling) by a full spray. The assessment of the model, for single droplet as well as full spray calculations, has been carried out against results obtained with the Fire Dynamics Simulator (FDS 6.7.0). The results show that the model predicts the rate of heat absorption within 15% for droplet diameters between 0.4 mm and 0.8 mm and a surrounding gas temperature below 150°C. Larger deviations are observed under other conditions. The application of the model to a well-confined and mechanically-ventilated compartment fire (a scenario of relevance to nuclear installations and passive houses) allowed to provide a good estimate of the cooling rate of the water spray system and the subsequent average gas temperature and pressure profile within the room.
In view of the problems of low heat and mass transfer efficiency in the evaporation process of seawater desalination and wastewater treatment, high gravity direct contact evaporation (HG-DCE) technology was developed to improve efficiency and performance of heat and mass exchange. Research platform of HG-DCE was designed, in which rotating packed bed was used as evaporator to study the evaporation process and characteristics of system, and the equilibrium evaporation temperature was theoretically analyzed and experimentally verified in this paper. The results showed that the temperature of both outlet gas and circulating water have a limit value named adiabatic saturation temperature, which proved the excellent heat and mass transfer ability. Furthermore, the effects of different factors on evaporation performance were studied. The amount of water vapor carried by air reached 44.8 g/kg and evaporation efficiency reached 80–90 %, which were much higher than other works. Evaporation intensity reached 1224 kg/(m³·h), which was 1–2 orders of magnitude higher than other evaporation systems. In conclusion, HG-DCE system greatly intensified the direct contact evaporation process to obtain high evaporation capacity and evaporation intensity, providing theoretical guidance and design basis for the application of high gravity technology in seawater desalination and wastewater evaporation.
