1,721,069 research outputs found
Soot modeling in large eddy simulation of fires
No full textRookdeeltjes, voornamelijk bestaand uit koolstof, worden gevormd in brandstofrijke, hoogtemperatuurvlamzones en spelen een cruciale rol bij warmtetransport, het verminderen van zichtbaarheid en het vormen van gezondheidsrisico’s bij branden. Het begrijpen van roetvorming is uitdagend vanwege de complexe chemische en fysieke mechanismen zoals initiatie, oppervlaktegroei, oxidatie, agglomeratie en fragmentatie. Vooruitgang in Computational Fluid Dynamics (CFD) maakt gedetailleerd brandonderzoek mogelijk, maar het modelleren van roetvorming blijft computationeel intensief en beperkt door realistische beperkingen, zoals de complexiteit van brandbare materialen en de noodzaak van efficiënte berekeningen.
Dit onderzoek ontwikkelt een kader voor een roetmodellering platform voor CFD-brand simulaties, met de nadruk op een LSP-gebaseerd (Laminar Smoke Point) roetkinetisch model. Een turbulentie-roet-interactiemodel (TSI) werd ook geïntroduceerd om het roetmodel aan te passen voor turbulente vlammen, wat de nauwkeurigheid verbetert en de computationele eisen beheert met behulp van een in-situ adaptieve tabulatie (ISAT) schema. Vergelijkende evaluaties tonen de nauwkeurigheid van het platform aan in diverse vlamomstandigheden. Verdere verbeteringen omvatten een snelle chemie-verbrandingsmodel voor laminaire vlammen en vooruitgangen in roetdynamica, waardoor een betere weergave van de deeltjesgrootteverdeling en polydispersiteit van roet mogelijk wordt gemaakt
Experimental and numerical study of fire dynamics in air-tight buildings
No full textAir-tight compartments are encountered in both residential and industrial
buildings. The demand for energy-saving promotes the use of air-tight residential
buildings to limit the energy consumption. Besides, a high confinement level is
also applied in industrial constructions such as nuclear power plant buildings to
prevent the radioactive substance from leaving the building. However, from the
standpoint of fire safety, the airtightness will introduce new fire risks. Unlike in
compartments with sufficiently large openings, the fire-induced pressure can rise
substantially in air-tight compartments due to thermal gas expansion and the
difficulty in releasing hot combustion products through small openings.
The overall objective of the study is to investigate fire dynamics in air-tight
compartments, with a specific focus on fire-induced pressure variations, which is
generally less considered a concern in fire research. The thesis contains
experimental research, theoretical analysis and numerical studies.
In terms of the reduced-scale NYX experiments: an extensive experimental
campaign was designed and conducted at IRSN (French Institut de
radioprotection et de Sûreté Nucléaire) to systematically study the influence of
heat release rate (HRR) evolutions, mechanical ventilation configurations and
boundary conditions on fire dynamics, focusing on the pressure variation. In
general, the overall fire-induced pressure evolution trends have been shown to be
similar in the tests. Qualitatively, the pressure follows the evolution trends of the
fire HRR. The pressure evolution is characterized by an over-pressure peak
followed by an evolution to a quasi-steady state, then an under-pressure peak and
a relaxation to the initial pressure level.
Four fire HRR evolution parameters are considered in the experiments,
namely the fire growth/decay rate coefficient, the maximum steady state heat
release rate, the fire growth/decay exponent and the steady state fire duration. In
general, the higher the fire growth/decay rate coefficient, the higher the obtained
over-pressure peak and under-pressure peak. For a given maximum fire HRR, the
magnitudes of the over-pressure peak and under-pressure peak increase with
increasing fire growth/decay rate coefficient, until a plateau value is reached. The
magnitudes of the over-pressure peak and under-pressure peak increase linearly
with increasing maximum steady state HRR. For a set fire growth/decay rate
coefficient, the higher the maximum steady state HRR, the longer the time for the
pressure to reach the peak value. For cases with fire growth/decay exponent below
1, the pressure reaches peak values before the HRR reaches a steady state
(maximum value or zero) because the net heat gained in the gas phase already
reaches its peak values then. In cases with fire growth/decay exponent above 1,
the pressure reaches peak values at the moment where the HRR reaches the steady
state. The fire growth/decay exponent has an impact on the under-pressure peak,
while it hardly affects the over-pressure peak. The over-pressure is not affected
by the steady state fire duration because the temperature, and hence heat loss, is
small at the fire growth phase and the HRR evolution is the dominant influencing
factor. The under-pressure does depend on the fire duration, through the heat
losses, but becomes almost independent of the steady state fire duration when the
fire lasts long enough.
The influence of mechanical ventilation configurations is studied with
different ventilation flow resistances and initial ventilation flow rates. The
magnitudes of pressure peaks are strongly affected by the mechanical ventilation
settings. Correspondingly, the fire-induced pressure has an influence on the
ventilation flow rates. Higher pressure peaks are obtained with increased
ventilation flow resistance. It has been illustrated that not only the total, or
‘equivalent’, flow resistance is important, but also the individual flow resistances
in the admission and extraction ducts separately. A higher imposed initial flow
rate leads to a higher equivalent flow resistance and hence higher over-pressure
and under-pressure peak values. The enhancement and reduction of ventilation
flow rates depend on both the fire-induced pressure and ventilation flow
resistances. Experimental results show that, with the same HRR evolution, the
ventilation configurations do not strongly affect gas temperatures.
The change of boundary conditions (i.e., wall properties) in the current
experiments affects the temperature evolution but hardly influences the pressure
variation and ventilation flow rate.
Full-scale experiments were conducted at RPA (Régie Provinciale Autonome)
in Belgium in cooperation with the University of Mons. The leakage area of the
compartment is measured under different pressure conditions and is proved to
increase with pressure rise. In the methanol pool fire tests with different square
fuel pan sizes (i.e., 0.3 m × 0.3 m and 0.5 m × 0.5 m), the pressure inside the fullscale compartment reaches peak values of 133 Pa and 697 Pa for the small and
the large pool fire cases, respectively. The pressure is high enough to prevent
evacuation.
In order to better interpret the fire-induced pressure variation in air-tight
compartment fires, the energy balance equation is discussed starting from a
commonly used form, taking into account the mutual influence between
compartment pressure and ventilation behavior. The obtained pressure
formulation, under certain assumptions, can explain the observed linear
relationship between pressure variation and fire evolution. Based on the
mathematical development, the difference between fire-induced pressure and
initial compartment pressure is expected to depend on the net heat gained in the
gas phase. It is also found from the theoretical reasoning that the mass flow rate
difference between admission and extraction ducts depends on the net heat gained
in the gas phase, and is not strongly related to the ventilation resistance. These are
subsequently validated by experimental results.
Numerical studies were conducted using FDS (Fire Dynamics Simulator).
Different versions were employed, i.e., 6.5.2, 6.5.3 and 6.7.5. The quantitative
assessments of the simulation results (compared with reduced-scale experiments)
indicate that the fire-induced pressure and ventilation flow rates are less sensitive
to mesh cell size and can well be reproduced, while the temperature is strongly
dependent on mesh cell size. The predictions of CO2 and O2 are better than those
of CO. In the full-scale methanol pool fire simulations, the discrepancies between
experimental and numerical over-pressure (under-pressure) peaks are 6.5 % (40 %)
and 1.6 % (21.7 %) for the small pool fire case and large pool fire case,
respectively. The magnitudes of the simulated leakage flow rate reach a maximum
of 65 m3/h for the small pool fire case and a maximum of 200 m3/h for the large
pool fire case, which are comparable to mechanical ventilation flow rates. The
parametric numerical study based on full-scale experiments (which have been
conducted by UMons) with mechanical ventilation indicates that the combination
of the real fan curve and dampers, as in the set-up of the full-scale tests, provides
better results than a surrogate. The settings of reference leakage area and leakage
pressure exponent have an impact on the pressure peaks. The adjacent room
pressure also reaches a high level although the door between rooms is closed.
The scaling analysis indicates that the scaled-up fires corresponding to the t2
fires in reduced-scale NYX experiments are classified as medium fires to fires
that grow faster than the ultra-fast classification. A preliminary scaling analysis of
pressure based on energy balance illustrates that the fire-induced pressure should
be scaled as the geometry scale, which is confirmed by simulations of cases in
different scales.
The present study is expected to: 1) give more insight into the fire dynamics,
especially fire-induced pressure variations, in air-tight compartments; 2) provide
an assessment of numerical modeling of the fire-induced pressure with FDS and
to provide extended analyses using FDS; 3) raise attention in considering the
potential risk associated with fire-induced pressure during fire safety design.
Besides, the experimental results presented in this thesis serve to provide data for
further model validations
Numerical study on under-ventilated enclosure fires and fire spread on building façades
Full text linkNumerical study on oscillatory fire behaviour in confined and mechanically-ventilated enclosures
Full text linkSpray modeling for medium speed diesel engines
No full textInternal combustion engines (ICEs) have served human beings in a broad scope of our daily life since their invention in the late 19th century. In recent decades, global warming and air pollution have drawn increasing criticism. With the development of electrification, the internal combustion engine, as a contributor to greenhouse gases and noxious emissions, is not favored by many people.
From published literature, in the foreseeable future, land transportation is likely going to be characterized by a mix of solutions in battery electric vehicles, hybrid electric vehicles, and ICE vehicles, according to the specific application and cost. However, there are less options for marine transportation. In fact, more than 90% of global cargo transportation is carried out by ships, which are mainly powered by diesel engines. To confirm its commitment to reduce pollutant emissions, the International Maritime Organization introduced stringent emission legislation, regulations and standards for engine emissions.
As a tried and tested tool for the powertrain, the internal combustion engine is also becoming fuel-flexible in recent years. Different gaseous (e.g., hydrogen, natural gas) and liquid (e.g., methanol) alternative fuels, which can be produced in green and sustainable ways, are gaining gradual interest due to their clean and low-carbon combustion characteristics. Compression ignition remains the reliable and efficient way for marine engines to initialize the combustion for traditional and emerging alternative fuels. In compression ignition engines, the liquid fuel is injected from the high-pressure injection system and undergoes a series of processes, such as atomization, evaporation, fuel-vapor/air mixing before the ignition or combustion occurs. It is widely acknowledged that the spray or the performance of the injection system directly affects engine combustion efficiency, fuel consumption, and pollutant emissions.
Optimization of the fuel injection, spray formation and the subsequent combustion process is seen as one of the most effective means for diesel engines to meet emission regulations without a loss in engine performance. Extensive research on diesel sprays has been performed for the automotive and truck engines used for land transportation. However, due to high technical requirements and expense, spray research targeting medium speed four-stroke engines is still rare. Based on the previous experimental studies in the Ghent University Combustion Chamber I (GUCCI) setup, this Ph.D. focuses on the modeling work of evaporating sprays under engine-like conditions.
Computational fluid dynamics (CFD) simulation has been an essential tool for engine design and optimization. In this work, the OpenFOAM code was employed to study the spray process in diesel engines based on the Reynolds -averaged Navier-Stokes method. This code was first validated using the highly reliable spray data provided by the Engine Combustion Network (ECN). A satisfactory agreement with the ECN data demonstrated that the simulation can correctly capture the spray processes. However, a discrepancy was found when simulating the marine engine sprays measured in the GUCCI setup. After summarizing and analyzing the values of the turbulence model constant (C1 of the standard k − e model) used from published literature, a lower value of C1 was adopted, and good agreement under a wide range of ambient conditions (density varying from 7.6 to 22.5 kg/m3, and temperature varying from 700 to 950 K) was achieved. Also, the disagreement that was noted for the liquid penetration for a low-temperature case could be explained by the ligament detachment phenomenon which was captured by the simulation.
Due to its simplicity and widespread adoption in engine simulation codes, empirical spray penetration models or spray correlations, used to predict the spray tip penetration, are also an interest of this work. Two classical empirical penetration models (i.e., Dent’s model and Arai’s model) were utilized to predict the penetration results of the ECN experimental data before using them for the marine engine sprays obtained in the GUCCI setup. Considering the transient characteristics of the pump-line-nozzle injection system in the target engines, a time-dependent injection pressure profile is suggested for the calculation of spray penetration. The spray tip penetration at a large distance under low density (7.6 and 15.2 kg~m3) conditions was expected to be proportional to t^2/3, which is supported by a previous theoretical investigation. The classical model of t^1/2 law, is still valid under high density (22.5 kg~m3) conditions.
This work compares the difference between diesel sprays for land transportation engines and marine transportation engines in terms of modeling approaches. The conclusions drawn from this work lay a foundation for future research on marine engine sprays
Numerical modelling of combustion instabilities induced by a liquid pool fire in a well-confined and mechanically ventilated compartment
No full textThis thesis utilizes Fire Dynamics Simulator (FDS 6.7.5) to explore the fire characteristics in an airtight and mechanically-ventilated enclosure. The goal is to enhance the understanding of liquid pool fire modelling in a forced-ventilated enclosure under various ventilation scenarios. New evaporation approaches are implemented in the source code of FDS and tested in various ventilation conditions. Key modelling factors are identified for modelling the fire behaviours which are sensitive to the coupling of fuel MLR and ventilation flow rates.
Industrial facilities such as Nuclear Power Plants (NPPs) are typically designed to be well-confined and mechanically ventilated to meet safety standards. However, this airtight environment can lead to significant pressure fluctuations during a fire incident. Thermal expansion within the compartment can trigger a pressure build-up, which can compromise the confinement and lead to the release of harmful substances. Moreover, pressure variations can induce a reverse flow in the ventilation ducts, causing the release of radioactive and unburnt gasses. These gases can spread to other compartments via the ventilation system and reignite with the introduction of fresh air, increasing the risk of fire spread. In nuclear installations, liquid fuels such as Hydrogenated TetraPropylene (HTP) is used in nuclear fuel reprocessing control laboratories to isolate specific radionuclides, e.g., plutonium. To this end, liquid pool fires can cause the above-mentioned pressure variations and fire risks in mechanically-ventilated enclosures. Therefore, this thesis focuses on fire behaviour induced by liquid fuels in such well-confined and forced-ventilated conditions.
In this research, numerical simulations have been carried out using a Computational Fluid Dynamics (CFD) tool, i.e., FDS (version 6.7.5). The aim is to enhance our understanding of fire behaviour and the fire modelling for a wide range of ventilation conditions. The study focuses on evaporation modelling of liquid fuels, a critical aspect in capturing fire behaviour in ventilation sensitive phenomena. New approaches based on natural convection correlations for evaporation have been developed and implemented into the FDS source code. Key parameters, such as extinction modelling based on Auto-Ignition Temperature (AIT) and effective absorption coefficient for modelling in-depth radiation in the liquid phase, are thoroughly examined.
The study begins with unrestricted open atmosphere conditions, providing ample air supply for a liquid pool fire to develop and burn stably. The goal is to test and examine the newly developed approach for several fuels and pool sizes. This includes a 1 m diameter methanol pool fire, a 0.70 m × 0.81 m ethanol pool fire, an 18 cm diameter heptane pool fire, and a 72 cm diameter (0.4 m2) heptane pool fire.
The new approaches incorporate natural convection correlations in mass and heat transfer, and more specifically, the calculation of the Sherwood and Nusselt numbers. Two new methods based on natural convection correlations, one based on modified correlation from hot surface tests and the other on cold plate correlation, are compared with the forced convection correlations used in FDS 6.7.5.
The natural convection approaches yield improved surface temperature predictions, regardless of fuel type or pool size. These new methods predict higher liquid surface temperatures, approaching the boiling point. In contrast, the forced convection significantly under-predicts the fuel surface temperature. This is because the high lower limit for the Reynolds number in the mass transfer calculation leads to a high minimum mass flux and an overestimation of evaporative cooling. Additionally, the calculated actual Reynolds number is found to be almost two orders of magnitude lower than the mentioned Reynolds lower bound. A slight improvement for the transient fire growth is also observed in the numerical results calculated with the natural convection correlations.
In a second stage, the numerical study has been carried out for a well-confined and mechanically ventilated enclosure, in two different sizes (1.50 m × 1.25 m × 1.00 m in height vs. 5 m × 6 m × 4 m in height). Fire scenarios are examined under a wide range of ventilation conditions. The newly developed approach and experience gained from the open atmosphere tests are further scrutinized in the forced-ventilated enclosure. Given the similar predictions produced by the two natural convection approaches, the decision is made to continue the study in the enclosure using only the cold surface correlations, where modifications to the correlations are not required.
When the ventilation flow rate is sufficiently high relative to the burning rate in the enclosure, e.g., Air Renewal Rate (ARR) equals to 22.5 h-1 in the small-scale enclosure, combustion leads to stable steady-state burning. A lower but steady fuel mass loss rate (MLR) is observed in this regime, compared to open atmosphere conditions. The lowered MLR is due to the reduced oxygen concentration because of combustion. The simulation results indicate that current CFD codes are able to capture the quasi-steady burning of a liquid pool fire under relatively well-ventilated conditions, and both the forced and natural convection approaches predict fuel MLR profiles in close agreement with experimental data. Moreover, it is observed that the predictions are not significantly influenced by the prescribed AIT. The default low AIT assumption (set at -273°C) generates results that are closely aligned with the experimental data.
Unstable oscillatory fire behaviour can occur when the ventilation rate is not sufficient to sustain a stable burning. For example, ARR = 15 h-1 for the small enclosure or ARR = 12 h-1 in the large enclosure. In the unstable oscillatory burning regime, combustion-induced thermal expansion in the airtight compartment increases the room pressure, reducing air inflow and leading to near-extinction. As pressure decreases due to the weakened combustion, the inlet air flow increases, revitalising the flame and raising room pressure again, creating a cyclic behaviour. This results in low-frequency oscillations in room pressure, ventilation flow rates, gas temperature, and fuel MLR. The numerical results confirm that the prediction (or not) of the oscillatory fire behaviour is sensitive to the evaporation modelling approach. The study demonstrates that the natural convection approach can qualitatively replicate fire oscillations and reproduce the transient profiles of the fuel MLR, room pressure, gas temperatures, and ventilation rates in both small and large-scale enclosures. Nonetheless, the oscillatory frequencies and amplitudes are not fully captured. On the contrary, the forced convection approach fails to reproduce significant oscillatory behaviour in the small-scale enclosure on fine meshes.
The study also identifies key modelling factors for predicting fire oscillations such as extinction modelling in the gas phase and in-depth radiation absorption in the liquid phase. The results show that extinction modelling based solely on Critical Flame Temperature (CFT) is insufficient for capturing fire oscillation and flame displacement. The default assumption of a low AIT (set at -273°C) fails to predict (local) flame quenching in the small-scale enclosure. Prescribing a physical AIT = 215°C captures near-extinction, flame displacement, and oscillatory combustion in the small enclosure. In the large-scale compartment, a higher AIT = 300°C is needed for better reproducing the oscillatory burning behaviour, where higher gas temperature predictions are observed in the simulations. The predictions of fuel in-depth temperatures and MLR are found to be significantly influenced by the modelling of radiative heat absorption in the liquid phase. The concept of effective absorption coefficient (κ) is found to be crucial as it determines the heat absorbed in the liquid phase, thereby affecting the in-depth temperature distribution and transient fire growth rate. Prescribing a low κ value can cause the in-depth liquid temperature to surpass the boiling point, resulting in excessive mass flux production. This increased burning rate impacts the prediction of fire oscillations due to the phenomenon’s sensitivity to the burning rate prediction.
When the ventilation flow rate in the compartment is further reduced to ARR = 8 h-1, the air supply rate becomes insufficient to sustain combustion, leading to a sudden fire extinction. This regime is characterised by a fast fire decay after a period of combustion, followed by total fire extinction due to oxygen vitiation. The numerical study verifies that the use of higher AIT values is crucial for predicting total flame extinction. In the small-scale compartment, an AIT of 215°C is adequate to capture complete extinction of the flame. However, for the large-scale compartment, an AIT of 300°C is required. The overall gas temperatures were presumably over-predicted, as higher gas temperatures are observed in the compartment.In kerncentrales (NPP's) zijn de compartimenten doorgaans goed afgesloten en mechanisch geventileerd om aan de veiligheidsnormen te voldoen en het vrijkomen van radioactieve materialen te voorkomen. In het geval van een brandongeval kan de luchtdichtheid echter resulteren in aanzienlijke drukvariaties, wat leidt tot de potentiële risico's van beschadiging van de ruimte en verspreiding van schadelijke producten. Het proefschrift past een Computational Fluid Dynamics (CFD) tool toe, met name Fire Dynamics Simulator, om het brandgedrag in een goed afgesloten en mechanisch geventileerd compartiment te bestuderen, met het oog op de relevantie van industriële brandveiligheid. De numerieke studie richt zich op het modelleren van vloeistofplasbranden, waarbij vloeibare brandstoffen (zoals gehydrogeneerd tetrapropyleen) worden gebruikt in controlelaboratoria voor de opwerking van nucleaire brandstoffen. Het onderzoek heeft tot doel bij te dragen aan de vooruitgang van CFD-instrumenten door nieuwe numerieke benaderingen te ontwikkelen om betere brandvoorspellingen te doen. Daarnaast omvatten de doelstellingen ook het vergroten van het inzicht in brandmodellering in luchtdichte en geforceerd geventileerde compartimenten
Experimental and numerical study of the effectiveness of water mist systems on blocking fire-induced smoke and heat in a ventilated tunnel
No full textIn recent years, the interest for fire safety issues in tunnels has increased dramatically due to a significant increase in number of tunnels worldwide and due to numerous catastrophic tunnel fires. Due to the relatively limited cross-sectional area in tunnels, hot smoke can spread rapidly, e.g., downstream with the traffic flow or due to longitudinal ventilation. Consequently, people downstream of the fire may be exposed to high temperatures and toxic gases, especially in urban tunnels that are likely to clog during rush hours. Inspired by fire compartmentation in buildings, it is worth investigating whether a tunnel can be partitioned by a water mist system into a fire zone and safety zones. If so, people can move from the fire zone into a safe zone through the water mist system. Obviously, an essential question is to examine to what extent the fire-induced heat and smoke can be blocked by the water mist system.
So far, there is still a lack of investigation of using water mist systems as a curtain to prevent smoke and heat spreading in tunnels, and there are no clear design specifications. Therefore, the main purpose of the present study is to investigate the effectiveness of water mist systems with respect to blocking smoke and heat in tunnels. Small-scale experiments and a numerical study were conducted for the abovementioned purpose.
A small-scale tunnel platform was built to investigate the effect of smoke blocking and temperature reduction by a water mist system. Two different levels of water working pressures (0.3 MPa and 0.5 MPa) and ventilation conditions (without ventilation and with a 0.8 m/s forced longitudinal ventilation) were considered. Different nozzle arrangements (2 + 2, 2 x 3 or 2 + 3 nozzles) were also considered. The experimental data mainly consists of temperature fields, extracted from thermocouple trees at different longitudinal positions in the vertical center plane of the tunnel, with measurements at 3 heights per tree.
The experimental results show that in the naturally ventilated tunnel, the installed water spray system effectively prevents smoke spreading. The water also has a strong cooling effect on the smoke in the tunnel. These observations were confirmed, regardless of the nozzle arrangement (2 + 2, 2 x 3 or 2 + 3 nozzles) or the water pressure imposed (0.3 MPa or 0.5 MPa). With longitudinal ventilation in place, with a velocity of 0.8 m/s in the reduced-scale tunnel, the smoke is no longer blocked. The smoke is still cooled down by the water, but the effect is smaller than for the naturally ventilated tunnel, because residence times are shorter and the forced ventilation also already has a cooling effect on the smoke. With longitudinal ventilation in place, the impact of the nozzle arrangement is small for a given level of water pressure. For all configurations, a higher water pressure in the water mist system leads to stronger cooling effects, due to the production of more and smaller water droplets.
A numerical study was conducted to analyze the flow field in more detail to know the mechanisms leading to the smoke blocking effect of the water mist system and the interaction between the water mist system and the longitudinal flow. Before we conduct the numerical study of the actual small-scale experiments, the impact of the longitudinal ventilation condition and the impact of the sidewalls on the flow fields induced by one single nozzle are discussed, by means of CFD simulations (FDS 6.0.1). The simulation results of a single nozzle show that: 1) an upward flow occurs near the sidewall, regardless of the distance between the nozzle and the sidewall, but its intensity will be higher when the nozzle is placed closer to the sidewall, 2) the strong upward flow could impinge onto the ceiling and deflect sideward, 3) the downward flow induced by water spray and upward flow near sidewalls are tilted backward because of the longitudinal ventilation flow, 4) a higher water working pressure makes the entrainment and the upward flow more pronounced, regardless of the ventilation condition.
Numerical studies of small-scale experiments (without longitudinal ventilation system) show the presence of a clear upward motion in the region in between the sprays and near sidewalls. Such motion is caused by spray-induced impinging jets onto the floor. These jets, induced by entrainment into the sprays, merge and create a zone of upward motion that acts as a ‘barrier’, preventing smoke spread downstream the water sprays. Different nozzle arrangements affect the temperature field, more nozzles lead to a stronger entrainment. But as the presence of an upward flow in-between water sprays and near sidewalls is not strongly affected by the nozzle arrangements, the smoke and heat blocking effects were found in each case. This is in line with the experimental observations. Besides, an analysis of the flow field near water mist system shows that more nozzles and a higher water working pressure induce stronger entrainment and an upward motion in-between sprays and near sidewalls.
For the cases with activation of longitudinal ventilation system, the impact of longitudinal mechanical ventilation has been discussed on the basis of mean flow and temperature fields. In contrast to the situation without longitudinal ventilation, the smoke is not blocked by the water mist. The upward flow in between the water sprays, which caused the smoke blocking by the water mist in the absence of longitudinal ventilation, is still observed, but it does not impinge onto the ceiling anymore: the horizontal momentum from the mechanical ventilation weakens the ‘pushing’ force of the spray-induced downward flows impinging onto the floor and moves them further downstream. Moreover, the downward momentum induced by the water mist destroyed smoke stratification. Higher water working pressure leads to a stronger cooling effect and spray-induced velocities. Different nozzle combinations have a slight impact on the temperatures downstream. However, as spray-induced momentum destroyed smoke stratification, activating fewer nozzles is better from this point of view.
In the last part, we mainly focused on applying water mist in a full-scale tunnel to block fire-induced smoke. A full-scale tunnel model is built corresponding to the small-scale model based on Froude scaling laws. Comparison of full-scale to small-scale simulation results reveals that the temperatures in the full-scale tunnel are higher than in the small-scale tunnel simulations. However, better agreement is observed for the upward flow, as well as for the entrainment by the water mist system.
As scaling up water flow rates from the small-scale model to full scale leads to unrealistically high water flow rates, a new tunnel model was simulated to investigate the application of water mist systems in a full-scale tunnel in more realistic conditions. The characteristics of the water droplets under a working pressure of 5 MPa were determined beforehand. Investigation of the impact of the nozzle combinations, with the same total water flow rate, on temperature and flow fields shows that the combination of nozzles has a slight impact on temperatures downstream, but the upward flow and the entrainment are more significant with more nozzles installed in one row (higher coverage of water mist). The study of the distance between two nozzle rows also indicates that the temperatures downstream the water mist system are not affected by this distance. For a certain level of heat release rate, activating more nozzles means lower temperature and longitudinal velocity downstream the water mist system. The entrainment caused by the water mist system plays the main role on blocking fire-induced smoke. The momentum of the water mist system was taken as the sum of all the momenta per nozzle, while the momentum of the ceiling jet flow was calculated as the product of the mass flow rate of the ceiling jet flow times its maximum velocity. The total water flow rate (number of nozzles) is proportional to the 2/3 power of the heat release rate (supposing that the thickness of the smoke layer changes only slightly from 1 MW to 3 MW), as illustrated by simulation results for 5 different values of HRR
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