Experimental and Numerical Modelling of Gravity Currents Preceding Backdrafts

This study investigates the turbulent mixing within gravity currents preceding backdrafts and validates the ability of the computational fluid dynamics (CFD) software Fire Dynamics Simulator version 4 (FDS) to simulate these flows. Backdrafts are rapid deflagrations, which occur after the introduction of oxygen into compartments containing unburned gaseous fuel. They may form large fireballs out of the compartment opening and present a significant hazard to the safety of fire-fighters. Gravity currents which precede backdrafts are responsible for the formation of flammable gas mixtures required for ignition. Scale saltwater modelling is used to generate Boussinesq, fully turbulent gravity currents for five different opening geometries, typical of fire compartments. Width-integrated concentration fields and two-dimensional velocity fields are generated using the non-intrusive light attenuation (LA) and particle tracking velocimetry (PTV) flow visualisation techniques respectively. Numerical simulations are carried out with FDS to replicate these flows. The experimental and numerical results are compared directly. Front velocities are shown to be governed directly by local buoyancy conditions, in the later stages of the flows, and therefore the initial conditions associated with the opening geometries only influence the front velocities indirectly. The internal concentration structure, internal velocity structure and location of potential flammable regions are found to be highly opening geometry dependent. In general, the results of the numerical simulations are quantitatively similar to those from experiment, which suggests that the numerical model realistically predicted the experimental flows. However, the numerical concentration fields appear slightly lumpier than those from the experiments, possibly due to unresolved turbulence on scales smaller than the numerical grid (0.01H, where H = compartment height)

Introducing Virtual Reality for Firefighter Skills Training: Opinions from Sweden and Brazil

The emergence of immersive virtual reality (IVR) technologies has raised interest in the use of fire and rescue services (FRS) as a supplement to the established practice-based hot fire-live simulation (HF-LS) training. This is due to features such as time efficiency, portable technologies, and training in scenarios not possible in HF-LS. However, whether IVR provides realistic firefighter training situations has been called into question. Previous studies have revealed differences regarding perceived presence in, and attitudes toward IVR training between novice firefighters (who can only relate to HF-LS training) and experienced firefighters (who can relate to both HF-LS and real fires). In the present study, two groups of experienced full-time employed firefighters, 53 from Brazil and 18 from Sweden tested the same IVR technology. The hypothesis was that differences in national education and training programs and real fire experiences might influence experiences in IVR technology. This study examines the differences and similarities in experienced presence, opinions on whether the graphical representations and tasks performed convey realism, and attitudes toward the IVR-supported training format. Data were collected via systematic post-training presence questionnaires and observations. The results revealed a highly experienced presence and perceived realism of the representations by the participants from both countries. However, attitudes toward using IVR technologies differed. The motivation to utilize currently available IVR training tools was higher in Brazil than in Sweden. This may be partly explained by less frequent HF-LS training opportunities in Brazil. Nevertheless, further research is needed to investigate the training transfer of IVR technologies and how these can better support skills training

Análisis del comportamiento del fenómeno Backdraft en recintos confinados

[ES] El Backdraft consiste en la reavivación de un incendio confinado, el cual estaba falto de  comburente y se le aporta una fuente suficiente para alcanzar el límite superior de  inflamabilidad, provocando una bola de fuego consecuencia de una deflagración en el interior  del recinto. Esta situación puede generar graves accidentes en incendios que aparentemente  estaban apagados. En ese contexto, en este trabajo se pretende mediante el uso de las nuevas  tecnologías de diseño y simulación, analizar este fenómeno de combustión en recintos  confinados denominado Backdraft, para estudiar su comportamiento, su potencia y su  peligrosidad, con la finalidad de encontrar medidas preventivas tanto de detección como de  extinción. [CA] El Backdraft consisteix en la reanimació de un incendi confinat, el qual estava mancat de  comburent i se li aporta una font suficient com per arribar el límit superior de inflamabilitat,  provocant una bola de foc a conseqüència de una deflagració en el interior del recinte.  Aquesta situació pot generar accidents graus als incendis que aparentment estaven apagats.  En aquest context, en aquest treball es pretén mitjançant l’ús de les noves tecnologies de  disseny i simulació, analitzar aquest fenomen de combustió en recintes confinats denominat  Backdraft, per a estudiar el seu comportament, la seua potencia i perillositat, amb la finalitat  de trobar mides preventives tant de detecció com de extinció. [EN] Backdraft phenomena consist on the revival of a confined fire, which was devoid of oxidizer  and a source of fresh air is provided, reaching the upper limit of flammability, causing a fireball  resulting from a deflagration inside the enclosure. This situation can lead to serious fire  accidents that would were apparently extinguished. In this context, the aim of this work is to  analyze that combustion phenomena in confined spaces called Backdraft, through the use of  new design and simulation technologies to study its behavior, its potential and its  dangerousness, in order to find preventive measures both detection and extinction. Basset Tomás, D. (2017). Análisis del comportamiento del fenómeno Backdraft en recintos confinados. http://hdl.handle.net/10251/78517.TFG

Cue-centric model of the fireground incident commander's decision making process

Pattern recognition based models propose that in highly routine situations, the FireGround Incident Commanders (FGC) make decisions using experiences of the past similar incidents (Klein et al, 1986), which are stored in memory as schemas (Klein et al, 2006). Due to the nonsystematic development of schemas that guide pattern recognition (Beach & Mitchell, 1978) and the biases attached with pattern recognition (Tversky & Kahnmen, 1974), this approach is least favorable candidate for decision making in nonroutine situations. The nonroutine situations are characterized by: failure to clearly recognize relevant past episodes (Bousfield & Sedgewick, 1944), deliberate avoiding of recalling the past episodes (Jacoby et al, 1989) or time constraint and ambiguity of available information for decision making. This research proposes that in nonroutine situations, the FGCs rely on thorough search and assessment of diagnostic, relevant, and important cues. Therefore, one aim of this research is to propose a model of the FGCs' decision making process for nonroutine situations; the model will base on the use of cues rather than the pattern recognition approach. This research also aims to provide a robust and coherent definition of the FGC’s decision making process and will subsequently specify the structure and the underlying phases of it. The context of the research is the decisions made by the FGCs during large fires, involving at least 5 fire appliances. 20 FGCs from 2 of the UK’s large firebrigades with at least 7 years of experience in command position participated in a fieldwork carried over a period of 1 year. For the data collection, multiple case studies in the form of critical incident reports are obtained from the participants. Each critical incident is explored further through semi-structured interviews. For the data analysis, theoretical or deductive thematic approach and process reconstruction method (Nutt, 1983) are used. Results indicate that the current definition of the term ‘FGC’s decision making process’ is incomplete. The definition of the FGC’s decision making process proposed in this research now, recognizes that each process of selection and evaluation of a course of action to solve a problem (Klein et al, 1986) is preceded by a process of identification of a problem. This definition commensurate with the widely acceptable definition of decision making process proposed in Nutt (1984). This research also found that the FGCs make decisions in 2 cyclic and distinguishable phases, which are the ‘problem recognition’ phase, and the ‘solution generation’ phase. Finally, a cue-centric model of the FGC's decision making process is proposed. The model showed that in nonroutine situations, when pattern recognition fails to guide the decision making process, the FGCs develop a mental model of a situation through thorough search and assessment of the valuable cues based on their diagnosticity, importance and relevance. The mental model assists in identifying problems and selecting a course of action to solve that problem. This research fulfills the need of developing descriptive models for clarifying issues arising in the areas of training, selection, and in developing decision support systems (Klein et al, 1986)

Experimental study on the effect of a heated plume within vented enclosures

This thesis presents an experimental study on the effect of a heated plume within vented enclosures. The characteristics of a heated plume within enclosures are studied using controlled and systematic experiments, simulating the movement of heated air due to the development of fire within a room.A short enclosure (Case CS1) and a tall enclosure (Case CS2) were subjected to two heating configurations representing natural and forced convection heat transfer. The cross-sectional dimension of the enclosures, and the heating power were maintained the same for both cases in order to compare the plume effects due to the change of height. For each case, different combinations of top ventilation were also considered for the present experiments.The air temperature and pressure within the enclosure were measured with suitable thermocouples and pressure probes, specially fabricated and mounted on racks that were positioned at three locations: (i) at 100 mm above the base of the enclosure; (ii) middle of the enclosure; and (iii) at 100 mm below the ceiling of the enclosure. The ceiling temperature profile was also obtained to determine the ceiling jet impingement.The experimental results show that for both case studies – CS1 and CS2, for the same heat transfer mechanism (natural or forced convection), the hot air temperature within the enclosure is decreased due to an increase in the number and size of vents. Hot air is exhausted quicker into the ambient and the exchange rate of hot air inside the enclosure and cool air from the ambient is increased. It was found that natural convection mechanism led to a higher ceiling and air temperature distribution than forced convection. The short enclosure had a higher ceiling temperature compared to the tall enclosure. Correlations for ceiling temperature distribution for short and tall enclosures are also provided. The study has resulted in obtaining valuable conclusions for the effect of a heated plume within a vented enclosure

Development of Innovative Fire Suppression Systems and Risk Mitigation Approaches with Multiphase Flow Techniques

Fire suppression and risk mitigation approaches are critical in modern life to ensure a safe living and working environment. Water-based fire suppression systems for fire suppression and waste heat powered steam ejector as a part of battery thermal management systems for fire risk mitigation were investigated. The aims of this thesis are: Developing innovative numerical tools using computational fluid dynamics (CFD) techniques to investigate the complex multiphase flow behaviour appeared and evaluate the feasibility and performance of the systems; gaining more insight into the water-based fire suppression systems by introducing a new statistical evaluation criterion and intuitive visualization of quantitative results. Exploring a potential solution for fire risk mitigation with waste heat recovering by innovative usage of steam. A novel battery thermal management design that uniquely uses recycled combustion waste heat with the steam ejector was proposed and targeted for hybrid electric vehicles (HEVs). An in-house wet-steam model considering the spontaneous condensation effect has been developed to accurately capture the complex multiphase flow behaviours. This thesis identified the key fire suppression mechanisms between conventional fire sprinkler systems and water mist systems, along with different fire suppression behaviours. Latent cooling and volumetric displacement were the major suppression mechanisms for water mist systems, and direct heat extraction dominates fire suppression for conventional sprinkler systems. The concept of water utilization rate is raised for water-based fire suppression systems due to self-developed droplet tracking and analyzing algorithms. This provided a new systematic approach for evaluating the performance of water-based fire suppression systems in any fire suppression scenario. Additionally, quantitative information such as spray pattern, accumulative mass fluxes, penetrability and number counts of water droplets were presented with an intuitive 3D visualization method. A fire risk mitigation approach for HEVs was proposed with a novel battery thermal management system. The battery management system proposed in the current thesis utilizes a steam ejector with engine waste heat as the power source; the complex transonic flow with spontaneous condensation inside the ejector was accurately captured and described by coupling an in-house wet steam model to the Eulerian-Eulerian multiphase CFD framework through user-defined functions. The current research made a successful attempt in both fire suppression and fire risk mitigation by developing and implementing numerical simulation tools

Evaluation of the Ability of Fire Dynamic Simulator to Simulate Positive Pressure Ventilation in the Laboratory and Practical Scenarios

Positive Pressure Ventilation (PPV) is a tactic that is used on fire grounds across the world everyday, both to improve tenability after the extinguishment of a fire and/or offensively during fire attack to improve firefighting conditions. PPV has proven that it can be a useful tool on the fire ground, but it can also kill or injure fire fighters and civilians if used improperly. Data from three full-scale experiments are compared with simulations completed with the computational fluid dynamic model Fire Dynamic Simulator (FDS). The full-scale experiments characterize a Positive Pressure Ventilation (PPV) fan in an open atmosphere, in a simple room geometry and in a room fire. All experiments qualify and quantify the comparison of the experimental results with the FDS results. A concluding scenario is modeled utilizing the calibration of the full-scale experiments to examine the effects of PPV on a fire in a two-story, colonial style house

Fire performance of residential shipping containers designed with a shaft wall system

seven story building made of shipping containers is planned to be built in Barcelona, Spain. This study mainly aimed to evaluate the fire performance of one of these residential shipping containers whose walls and ceiling will have a shaft wall system installed. The default assembly consisted of three fire resistant gypsum boards for vertical panels and a mineral wool layer within the framing system. This work aimed to assess if system variants (e.g. less gypsum boards, no mineral wool layer) could still be adequate considering fire resistance purposes. To determine if steel temperatures would attain a predetermined temperature of 300-350ºC (a temperature value above which mechanical properties of steel start to change significantly) the temperature evolution within the shaft wall system and the corrugated steel profile of the container was analysed under different fire conditions. Diamonds simulator (v. 2020; Buildsoft) was used to perform the heat transfer analysis from the inside surface of the container (where the fire source was present) and within the shaft wall and the corrugated profile. To do so gas temperatures near the walls and the ceiling were required, so these temperatures were obtained from two sources: (1) The standard fire curve ISO834; (2) CFD simulations performed using the Fire Dynamics Simulator (FDS). Post-flashover fire scenarios were modelled in FDS taking into account the type of fuel present in residential buildings according to international standards. The results obtained indicate that temperatures lower than 350ºC were attained on the ribbed steel sheet under all the tested heat exposure conditions. When changing the assembly by removing the mineral wool layer, fire resistance was found to still be adequate. Therefore, under the tested conditions, the structural response of the containers would comply with fire protection standards, even in the case where insulation was reduced.Postprint (published version