Experimental and Numerical
Study on Mixture Fractions
and Flame Radiation of
Ethanol-Heptane Pool Fires
Chapter 1: Introduction
1.1 Research Background
brief subject (blended fuels as a means of reducing emissions) > specific subject (production, storage
and transportation of blended fuels tend to cause pool fires) > focused subject (characteristics of
blended fuel pool fires)
Pool fires have been extensively studied over the recent decades, owing to their
hazardousness during fuel transport and storage Ditch BD et al (2013). The burning rate, heat
release rate, thermal radiation to the surroundings and more are some of the most vital
combustion characteristics for evaluating the hazards of pool fires. Nonetheless, singlecomponent fuel pool fire is the main element of concern owing to the steady properties of
fuel. However, in practice, common flammable liquids generally do not consist of a single
chemical species. For example, most transportation fuels like gasoline and diesel are mixtures
of many components. These fuels are also commonly blended with other chemicals that have
significantly different boiling points and properties. A common example is the blending of
ethanol with diesel and gasoline in vehicles to reduce soot and NOx emissions. Most
processes in the chemical and petrochemical industries employ flammable solvents that are
either produced or used as mixtures. For example, industrial alcohol production involves
distillation of an ethanol-water mixture, and commercial production of benzene, toluene, and
xylene as chemical precursors requires many processes involving mixtures containing these
flammable components. Fuel blends exhibit different combustion behaviour than pure fuels
due to the difference in volatility between compounds, which causes the fuel to take on
flammability and combustion properties that are similar to the more volatile component.
Additionally, additives are often incorporated into fuels to improve their performance.
Therefore, studies on blended pool fires are also gaining increasing traction.
D. Fernández-Rodríguez, M. Lapuerta and L. German, Progress in the use of biobutanol
blends in diesel engines, Energies, 14 (11) (2021) reports that in 2020, 1100 papers were
published on alternative fuels used in diesel engines, which reflects the significance of
blended fuels. Liquid fuel blends are being widely used in industrial applications and
vehicles. Countries like Brazil, the European Union, the United States, China, and India have
popularized the usage of alcohol additives in hydrocarbon fuels to achieve carbon reduction
in the environment. Over the last few years, due to the particular urgency on reducing
emissions, blends are becoming top alternatives as vehicle fuels. Hence, a large number of
storage farms are required to contain the blended fuels, and their transportation and storage
may incur fire safety problems. Accidents involving leakage of blended fuels are more prone
to occur, which can be easily ignited, and formation of pool fires take place as a result.
Therefore, it is of utmost importance that the combustion properties of blended fuels are
comprehensively investigated.
1.2 Problem Statement
1. Why blended fuels?
2. What are characteristics of blended fuel for the occurrence of the pool fires
3. What hazard could be posed when blended pool fire occurs?
1.3 Objective
1. to quantify the fire burning behavior of the blended pool fire in term of heat release rate, mass burning
rate, flame height and flame temperature in the influence of fuel mixing ratio experimentally and
numerically using FDS model
2. to determine the heat radiation rate of the blended pool fire to the target in the influence of fuel mixing
ratio
1.4 Scope of Study
1. Two fuels blending with 40:60, 50:50, 80:20
2. The analysis of burning rate, flame height and flame temperature at pan size of 20, 40, 80
and 150mm
3. The experimental data will be compared with numerical analysis using FDS model
1.5 Significance of Study
Numerous research has been done on pure fuel pool fires, the knowledge on the burning behaviours
of blended fuels is rather insufficient.
Chapter 2: Literature review
2.1 Pool Fires
Among the three broad categories of accidents that occur in chemical processes industries
(CPI)—fire, explosion and toxic release—fire is the most common type of accident
encountered consequentially with production, transportation and/or storage of flammable
substances. Amidst the different types of fire accidents—pool fire, jet fire, flash fire and fire
ball—the most common is pool fire. In a survey of accidents taken mainly from major
hazards incident data from home and abroad, it can be concluded that the majority
(approximately 42%) of all accidents in CPI involve pool fires. It so often happens that a fire
may potentially spread from one pool to create other pool fires, or an explosion may set off
more than one pool fires. The resulting multiple pool fires can then exert a cooperative effect,
enhancing each other’s heat load and destructive potential.
A pool fire is a turbulent diffusion fire burning above a horizontal pool of vaporising
hydrocarbon fuel where the fuel has zero or low initial momentum. Fires in the open will be
well ventilated or in other terms fuel-controlled, but fires within enclosures may become
under-ventilated or ventilation-controlled. Pool fires may be static, when a pool is contained,
or 'running' fires. Pool fires represent a significant element of the risk associated with major
accidents on offshore installations, particularly for Northern North Sea (NNS) installations
that may have large liquid hydrocarbon inventories. The occurrence of a pool fire takes place
when a flammable liquid is accidentally released on ground or water, and ignites. When this
happens, a buoyancy-driven, turbulent non-premixed flame is formed above the pool. The
resulting fire is distinguished from other types of fires by a very low initial momentum and
the propensity to be strongly influenced by buoyancy effects. Based on the medium on which
the pool is formed, presence or absence of confinement, and the type of location, pool fires
can be classified as below:
2.1.1 Prominent Pool Fire Accidents
Denver, Colorado, 1990 On November 25, 1990, seven pool fires occurred at a tank farm
containing about 61,000 KL of jet feed. The multiple pool fires are suspected to have been
initiated by the ignition of fuel leaking from an operating feed pump. The electric motor for
the pump served as the ignition source. A cracked supply pipe in the valve pit was found to
be providing additional fuel to the pool fires. As the fires continued, coupling gaskets in the
piping worsened in quality and more fuel flowed out of the storage tanks, substantially
feeding the fires. Furthermore, the valve controlling fuel flow in the supply line to the airport
sporadically released fuel in the valve pit. Flame heights of up to 100 m were observed at the
time of the accident. The damages ran up to US $ 30.6 million.
Saint Herblain, France. On October 7, 1991 the remote-controlled opening of a valve located
at the base of a 6,500 m³ tank containing 4,525 m³ of lead-free premium-grade gasoline
coincides with the appearance of a cloud of fuel vapour in the sector, forming a milky-white
fog. Alerted by one of the drivers, the 2 employees rushed to the scene. The fog that had
topped the 2 m-high earthen barricades of the catchpit expanded across the parking area,
covering it with a cloud approximately 1.5 m thick. The cloud, having an estimated volume
of 25,000 m³, ignited 20 minutes later. The explosion, an Unconfined Vapour Cloud
Explosion (UVCE), fatally injured a driver and seriously injured 2 employees. Thrown into
the catchpit, these 2 employees were nevertheless able to return to the control station,
attempted in vain to close the valves (the remote-control system had been destroyed in the
explosion) and sounded the alarm. Two pool fires started as a result of a vapor cloud
explosion caused by overfilling of gasoline. The tank farm consisted of 11 storage tanks
containing 1500–15,000 KL of fuel, adding up to 80,000 KL. Two of the tanks were
completely burned out. The accident damaged another 3 hydrocarbon tanks in a nearby tank
farm. Flames went up to 50 m in air. Besides the loss of a life, serious burn injuries were
sustained by two employees and minor injuries by three others.
Cilacap, Indonesia. On October 24, 1995, lightning struck a floating roof storage tank
containing naphtha, initiating a pool fire at the 48,000 KL per-day refinery. The explosion
took place in in a fuel tank storage area in Indonesia’s heavily populated central Java
province which led to the pool fire then multiplying, enveloping six additional storage tanks.
Flame heights of up to 50 m were observed at the time of accident. The damage was worth
over US $ 38 million
Buncefield, UK. On December 11, 2005, a gasoline storage tank was being filled from a
pipeline at a fuel terminal at Buncefield. Safety systems fitted to prevent the tank overfilling
failed and gasoline began to spill from the vents on the tank roof. A low-lying cloud of
heavy, flammable vapour accumulated and spread out for about 250m in all directions around
the tank. Upon coming in contact with an ignition source, a powerful vapour cloud explosion
devastated the fuel depot. The ensuing fire spread to other tanks and was not fully
extinguished for several days. The overfilling of the 300 KL of unleaded gasoline over a
period of 40 min at one of the tanks caused a simultaneous initiation of 22 pool fires in
various tanks storing gasoline in the facility. The pool fires lasted for about 4.5 days and
impacted an area of 80,000 m2. The vegetation around the storage facility was completely
consumed by the fire. Flame heights of up to 100 m were observed at the time of the accident.
Besides injuring 43, the accident costs an equivalent of US $ 13 billion in losses.
Jaipur, India, 2009. During the evening of the 29th October 2009, preparations were being
made for the transfer of kerosene and gasoline to a neighbouring terminal. At approximately
6:10 pm, during the process of preparing Tank 401-A for pumping, a large leak occurred
from a ‘Hammer Blind Valve’ on the tank outlet. The leak resulted in a jet of gasoline
directed upwards from the valve. The leak continued for some 75 minutes in calm, low wind
speed, conditions. The nature of the release is likely to have assisted in the production of
vapour and post incident analysis indicates that a flammable vapour cloud appears to have
covered much of the IOC site. An independent inquiry committee (IIC) estimated that of the
order of 1000 Tonnes of gasoline were released from the tank prior to ignition. The VCE
caused by the leak initiated 11 pool fires which generated flames high enough to be visible
from a distance of 30 km. Over time, the flames leapt higher and wider into the air and raged
for about 2 weeks until all the fuel had been burnt off. The accident killed 12 persons and
injured another 200. More than half a million people had to be evacuated from the area
during the 2-week period that the fire raged. The damages ran into US $ 32 million.
San Juan, Puerto Rico. On October 23, 2009, the blast and blaze at the Caribbean Petroleum
Corp facility, a major supplier of oil products including gasoline in the U.S. Caribbean Island
territory, sent up a huge plume of black smoke over the seaside capital. 17 pool fires started
simultaneously at the Caribbean Petroleum Corporation after a massive VCE. Another four
storage tanks burned before the blaze was extinguished 72 h later. The combined effect of the
pool fires sent huge flames and smoke plumes into the air. Flames reaching more than 100 m
into the air could be seen from several kilometres away.
Sharjah, United Arab Emirates (UAE), 2011 On March 24, 2011, a VCE started a fire which
ignited up a 30-KL pool of diesel. The fire then moved to a neighbouring oil depot which
contained 37 KL of diesel, and converted it to another pool fire. The combined effect of the
two pool fires led to flames which rose up to 25 m. Four lorries parked in the compound at
the time of accident were burned to scrap. The damage was estimated at US $ 0.2 million
(IFW 2012).
2.1.2 Pool Fire Characteristics
2.1.2.1 Heat Release Rate
Heat release rate (HRR) is the rate at which heat energy is generated by burning and is also
known by the term firepower. Babruskas and Peacick (1992) states that the rate of which
energy is released in a fire (ܳ̇ Qc ) is a significant factor in characterizing its behaviour. If the
HRR is known, it can among other things be used to estimate the flame size and radiation to
surroundings, and assess likely flame behaviour in practical situations. However, the HRR is
not simply a variable used to characterize fire. In fact, it is the single most important variable
in defining phenomena such as a fire hazard. Heat release rate is the driving force in terms of
fire. Hence, the production of undesirable effects of fire and its products also elevates with
increased HRR. This means that toxic gases, smoke and other types of fire hazards increase
in parallel with the heat release rate.
The theoretical mechanism for combustion is the vaporization of fuel, the
multiplication of mass flow rate of fuel vapor, mf with the complete heat of
combustion, ∆hc, hence giving HRR it’s equation:
The calculation of HRR is highly beneficial in various industries, especially in commercial
aircraft. In the 1970s, the first method to measure HRR was discovered. Today it is possible
to determine the rate of heat release experimentally by using the method of oxygen
consumption calorimeter (cone calorimeter). This method is widely used throughout the
world and it is recognized as the most accurate and practical technique for measuring heat
release rate from experimental fires. Unfortunately, there are limitations in fire sizes when
using the cone calorimeter. As an alternative method, HRR is measured based on mass loss
rate using a simple scale (weight equipment). The time dependent mass loss rate measured
from the fire experiment can then be used in equation below:
2.1.2.2 Mass Burning Rate
Burning rate is an essential parameter used in defining the characteristics of pool fires.
Drysdale 1998 defines burning rate, ܳ̇′′, as the ratio of the difference of heat flux supplied by
the flame, ܳ̇ܳ′′, and the heat losses, ܳ̇ܳ′′, to the heat required to produce the volatiles, ܳv.
QF’’ = Heat flux supplied by the flame (kW/m2)
Q’’L = Losses expressed as a heat flux through the fuel surface (kW/m2)
Lv = Heat required to produce the volatiles (kJ/kg)
Skarsbo (2006) states that Q’’F has to be related to the rate of energy released within the
flame and the mechanism of heat transfer involved. Apart from that, it has also been stated
that liquids have a lower Lv value than solids where chemical decompositions are involved.
Varghese, S., Renjith, V. R. (2022) carried out laboratory-scale pool fire experimental tests
on n-propanol, diesel, and mixtures in an attempt to understand the fire behaviour of blended
alcohol biofuels. The burning rate of pure and blended fuels was estimated by two methods,
namely the Babraukus method and the time-average method, and compared. Their results
concluded that the time-average method is a lot more well grounded than the Babraukus
model for estimating the mass burning rate of pure and blended pool fires. The mass burning
rate of a pool fire by the time-average method was estimated by averaging the mass loss with
pool area and burning time. It was found that n-propanol had the highest mass burning rates
for all pool diameters than diesel and blends due to the higher oxygen content of n-propanol
compared with other fuels. Apart from that, it was also recorded that the addition of npropanol into diesel increases the burning rate of blended fuels. This phenomenon was further
elaborated that the partial vapor pressure of diesel and n-propanol is proportional to the mole
fraction of diesel and n-propanol at a given pool fire temperature due to a homogenous
mixture of diesel and n-propanol blends serves as an ideal mixture that obeys Raoult’s law,
hence justifying the linear variation of the burning rate of diesel-n-propapnol mixture with
vapor compositions.
Chong et al (2020) investigated the characteristics of pool fire burning methyl
esters/biodiesels of palm, soybean, coconut and their blends with diesel in comparison to
baseline diesel. It was discovered that the mass burning rate increased with the increase of
crucible size for all tested fuel due to the larger surface area available for burning. Higher
heat transfer rate occurs from the flame to the liquid fuel pool surface, encouraging fuel
vaporisation on the pool surface which leads to higher burning rate. Additionally, it was
recorded that biodiesel displayed higher mass burning rates when compared to diesel.
Biodiesels exhibit higher mass burning rates partly due to the oxygen content in the fuel that
assists in the combustion process, higher density and lower radiative heat loss. From the
experiment, it was observed that the neat biodiesel (B100) displayed overall higher burning
rates than B20 (20/80 biodiesel/diesel blend). The mass burning rate of the liquid fuel was
calculated based on the equation:
where mi is the initial mass of the fuel, mf is the final mass of the fuel and ∆t is the
time interval. The mass of the fuel was measured by using a weighing scale, and the
mbr were determined after a period of time. This method would be applied in this fyp.
2.1.2.3 Flame Height
J. L. Consalvi, Y. Pizzo, B. Porterie and J. L. Torero, On the flame height definition for
upward flame spread, Fire Saf. J., 42 (2007) describes flame height as the average position of
a luminous flame. Flame height of a pool fire is a crucial factor that should not be overlooked
when working with fire safety design as it can potentially affect fire suppression systems,
escape possibilities, fire ventilation and the fire heating of building structures. Generally, the
flame height is dependent on the mass burning rate and ventilation conditions. There are
several methods in defining the height of a flame; i.e. based on visible height criteria or
temperature criteria. Extracted from (NFPA, 2002), two formulas denied by Heskestad and
Thomas can be used to calculated the flame height of pool fires under the condition that there
is no cross-wind:
The Heskestad correlation is only valid for normal atmospheric conditions (293K and
1atm) and where the heat liberated per unit mass of air entering the combustion
reactions ∆Hc/r is around 3000kJ/kg. However, this only applies for a large number
of gaseous and liquid fuels.
Chong et al (2020) also mentioned that the flame height increased as the crucible diameter
increases. This finding has been further justified by Drysdale D (2011), elaborating that
larger crucibles enable more fuel to be burned. As a consequence, a greater amount of air is
entrained into the flame to react with the volatiles. The interaction between ambient air
entrainment and lower density within the flame produces buoyancy-driven flows that
inherently elevate the flame to a higher vertical position.
In a study carried out by P.Zhu (2019), the burning rates of blending fuels were investigated.
A 20cm circular pool was filled with fuel at ethanol ratio ranging from 0% to 100%. The
mass burning rate, flame temperature, fuel temperature and flame image were measured and
analysed. It was recorded that as the ethanol blending ratio increased from 0% to 100%, the
stoichiometric air fuel decreased. Consequently, the visible flame height decreased. Liu FS et
al (2017) explained this occurrence by stating that the diffusion flame height is mostly
associated to the mass burning rate and stoichiometric ratio at a specific geometry, where the
lower the air/fuel ratio, the shorter the height of the flame produced.
2.1.2.4 Flame Temperature
Flame temperature is found to be dependent on the diameter of the pool fires. It increases
with an increase in the diameter of the pool fire. The flame temperature influences the heat
transfer from flame to fuel surface, which is predominantly dominated by combustion heat of
the fuel, oxygen concentration and soot products. Varghese, S., Renjith, V. R. (2022) reports
that pure propanol had the highest flame temperature, followed by diesel and other blends for
all diameters, due to the complete combustion process of propanol. P.Zhu (2019) further
confirms this as it was observed that the flame temperatures of high ethanol fuels were
relatively larger than that of low ethanol fuels, this being on account to the more radiant heat
loss of high sooty gasoline fires.
2.2 Blended Fuels
Blended fuels or in short blends, can be thought of as transitional fuels. The lowestpercentage blends are being marketed and introduced to work with current technologies while
paving the way for future integration. For example, B5 and B20 (biodiesel) can be pumped
directly into the tank of any diesel car or truck. Ethanol is also blended (about 10 percent)
into much of the gasoline dispensed in the United States, especially in metropolitan areas, to
reduce emissions.
The term biofuels usually apply to liquid fuels and blending components produced from
biomass materials called feedstocks. Most biofuels are used as transportation fuels, but they
may also be used for heating and electricity generation. Gaseous fuels produced from
biomass that are used directly as a gas or converted to liquid fuels may qualify for use in
government programs that promote or require use of biofuels. The terminology for different
types of biofuels used in government legislation and incentive programs and in industry
branding and marketing efforts varies. For example, the names of biofuels may include
preceding the type or use of the fuel with bio (such as biodiesel or bio jet) or with the terms
advanced, alternative, clean, green, low-carbon, renewable or sustainable (like sustainable
aviation fuel). Most biofuel consumption occurs as a blend with refined petroleum products
such as gasoline, diesel fuel, heating oil, and kerosene-type jet fuel. However, some biofuels
do not require blending with their petroleum counterparts and are referred to as dropin biofuels. The U.S. Energy Information Administration (EIA) publishes data on four major
categories of biofuels as stated below:
Biofuels
Description
Ethanol
An alcohol fuel that is blended with petroleum gasoline for use in
vehicles and accounted for the largest shares of U.S. biofuel
production (85%) and consumption (82%) in 2021.
Biodiesel
A biofuel that is usually blended with petroleum diesel for
consumption and accounts for the second-largest shares of U.S.
biofuel production (11%) and consumption (12%) in 2021.
Renewable biodiesel
A fuel chemically similar to petroleum diesel fuel for use as a
drop-in fuel or a petroleum diesel blend with small but growing
U.S. production and consumption. Renewable diesel's percentage
shares of total U.S. biofuels production and consumption were
about 3% and 5% respectively in 2021.
Other biofuels
Includes renewable heating oil, renewable jet fuel (sustainable
aviation fuel, alternative jet fuel, bio jet), renewable naphtha and
renewable gasoline.
Ethanol is a renewable fuel made from various plant materials collectively known as
"biomass." More than 98% of U.S. gasoline contains ethanol, typically E10 (10% ethanol,
90% gasoline), to oxygenate the fuel, which reduces air pollution. Ethanol is also available as
E85 (or flex fuel), which can be used in flexible fuel vehicles, designed to operate on any
blend of gasoline and ethanol up to 83%. Another blend, E15, is approved for use in model
year 2001 and newer light-duty vehicles. Ethanol has a higher-octane number than gasoline,
providing premium blending properties. Minimum octane number requirements for gasoline
prevent engine knocking and ensure drivability. Lower-octane gasoline is blended with 10%
ethanol to attain the standard 87 octane. Ethanol contains less energy per gallon than
gasoline, to varying degrees, depending on the volume percentage of ethanol in the blend.
Denatured ethanol (98% ethanol) contains about 30% less energy than gasoline per gallon.
Ethanol’s impact on fuel economy is dependent on the ethanol content in the fuel and
whether an engine is optimized to run on gasoline or ethanol.
Optimizing the number of alternative fuels blended with conventional fuels is a method that
helps to reduce the exploitation of petroleum resources. Blended fuels improve the engine's
efficiency and decrease toxic tailpipe emissions. In 2020, 1100 papers were published on
alternative fuels used in diesel engines, which reflects the significance of blended fuels.
Liquid fuel blends are being widely used in industrial applications and vehicles. Countries
like Brazil, the European Union, the United States, China, and India have popularized the
usage of alcohol additives in hydrocarbon fuels to achieve carbon reduction in the
environment. One of the significant fire hazards associated with these blended fuels during
storage and transportation is pool fires which will be the central theme of this paper. Blended
fuels are multicomponent fuels, and their combustion behavior varies from pure fuels. A
number of commercially used fuels are complex mixtures (e.g., biodiesel, gasohol (gasoline +
alcohol), diesel (diesel + alcohol)). It is important to understand the hazardous characteristics
of blended pool fires. Nowadays, researchers are developing newly blended fuels without
knowing the fire hazards of the new fuels which leads to the objective of this paper.
2.2.1 Characteristics of Blended Fuels
In order to understand the nature of blended fuels, more specifically ethanol-blended fuels,
there is a need to understand the characteristics of polar solvents and hydrocarbons, their
differences, and how these types of products interact. Under some conditions, ethanolblended fuels will retain certain characteristics as a gasoline-type fuel, and under others it
tends to exhibit polar solvent-type characteristics.
Hydrocarbon fuels (gasoline, diesel fuel, kerosene, jet fuel, etc.) generally have similar
characteristics whether they are flammable liquids or combustible liquids. Gasoline is a
hydrocarbon produced from crude oil by fractional distillation. It is non-water miscible and
has a flash point of approximately -45°F, varying with octane rating. Gasoline has a vapor
density between 3 and 4. Therefore, as with all products with a vapor density greater than 1.0,
gasoline vapors will seek low levels or remain close to ground level. Gasoline has a specific
gravity of 0.72–0.76 which indicates it will float on top of water since it is non-water
miscible or insoluble. Its auto-ignition temperature is between 536°F and 853°F, and it has a
boiling point between 100°F and 400°F depending on fuel composition. Gasoline is not
considered a poison but does have harmful effects after long-term and high-level exposure
that can lead to respiratory failure. Smoke from burning gasoline is black and has toxic
components. Gasoline’s greatest hazard is its flammability even though it has a fairly narrow
flammability range (LEL is 1.4 percent and UEL is 7.6 percent). On the other hand, Ethanol
is a polar solvent that is water-soluble and has a 55°F flash point. Ethanol has a vapor density
of 1.59, which indicates that it is heavier than air. Consequently, ethanol vapors do not rise,
similar to vapors from gasoline which seek lower altitudes. Ethanol’s specific gravity is 0.79,
which indicates it is lighter than water but since it is water-soluble it will thoroughly mix with
water. Ethanol has an auto-ignition temperature of 793°F and a boiling point of 173°F.
Ethanol is less toxic than gasoline or methanol. Carcinogenic compounds are not present in
pure ethanol; however, because gasoline is used in the blend, E-85 is considered potentially
carcinogenic. Like gasoline, ethanol’s greatest hazard as a motor fuel component is its
flammability. It has a wider flammable range than gasoline (LEL is 3.3 percent and UEL is
19 percent). However, it should be noted that ethanol for use in motor fuel blends will
generally be denatured with up to 5 percent gasoline or a similar hydrocarbon (E-95) for any
style of transport.
Blending ethanol with gasoline has multiple effects. Ethanol increases the heat output of the
unleaded gasoline, which produces more complete combustion resulting in slightly lower
emissions from unburned hydrocarbons. The higher the concentrations of ethanol, the more
the fuel has polar solvent-type characteristics with corresponding effects on conducting fire
suppression operations. However, even at high concentrations of ethanol, minimal amounts of
water will draw the ethanol out of the blend away from the gasoline. Ethanol and gasoline are
very similar in specific gravity. The two differing fuels mix readily with minimal agitation,
but the blend is more of a suspension than a true solution. Ethanol has a greater affinity for
water than it does for gasoline. Over time, without agitation, gasoline will be found floating
on a layer of an ethanol/water solution. The resulting ethanol/water solution is still flammable
since the concentration of ethanol is still fairly rich. Phase separation can occur in fuel
storage systems where water is known to be present. Consequently, blending these fuels
together alters the physical and chemical characteristics of the original fuels. For example,
the higher the content of ethanol, the less visible the black smoke content and orange flame
production.
2.3 FDS Model
When it comes to fire safety, computer tools built to predict a likely fire scenario are
becoming an important instrument. Different Computational Fluid Dynamics models, often
known as CFD models, are extensively employed in this context, with the domain of interest
divided into thousands of small control volumes. Validation against experiments is a critical
step for constructing fire models. This entails reproducing the experimental setup in CFD
models and comparing the simulation findings to the experimental results.
Full scale fire tests to verify the design are neither practical nor reasonable when working
with fire safety of buildings, offshore installations, boats, and other structures. Fire simulation
software was created in this way to test the design against genuine fire scenarios. It is critical
that the correct input data be used, that the fire model is validated and approved for use in the
given scenario, and that the personnel doing the simulations understand how to use the results
to achieve a good outcome. It's also crucial to understand why you're doing the modelling in
the first place.
Fire modelling can for instance be used to stimulate the spread of a fire. In the process
industry, flames can progress from tiny fires in buildings to big pool or jet fires. In these
situations, simulations are conducted to see if the design is good enough to avoid a minor
"uncritical fire" from becoming a major "critical fire."
Zone models and field models are the two most common types of fire models. Two control
volumes, one for the upper hot gas layer and the other for the lower cold air, are used in zone
models. The room volume is divided into numerous small cells in field models (CFD models)
(control volumes). For each control volume in space and time, differential equations are
solved (Hasib, Kumar et al., 2007). The Navier-Stokes equation is used in CFD models to
solve momentum conservation, whereas zone models employ empirical equations to calculate
velocities.
FDS, developed by the National Institute of Standard and Technology (NIST), is a CFD
model of fire-driven fluid flow. FDS is free to download on NIST home pages. The simulator
has been under development for about 25 years, but the first public software was released in
February 2000. . It has a companion package for post-processing and visualization,
Smokeview, Forney, G.P., McGrattan, K.B., “User's Guide for Smokeview Version 3.1: A
Tool for Visualizing Fire Dynamics Simulation Data”, National Institute of Standards and
Technology NISTIR 6980, (2003), which is user friendly. Smokeview acts as the “post
processor” which produces images and animations of the results from the FDS simulation
(Kevin McGrattan, 2010). By employing FDS, it is possible to stimulate:
● Low speed transport of heat and combustion products from fire
● Pyrolysis
● Flame spread and fire growth
● Radiative and convective heat transfer between the gas and solid surfaces
2.3.1 Combustion Model and Radiation in FDS
The following information has been extracted from the FDS User Guide (Kevin McGrattan,
2010) and FDS Technical Reference Guide. In FDS, there are two types of combustion
models that can be used. Individual gas species react according to defined Arrhenius reaction
parameters in the default model, while mixture fraction (amount representing the fuel and
combustion products) is used in the second model (Only DNS simulations). For the most part,
the combustion model based on the mixture fraction notion is applied. At a particular position
in the flow field, this is a conserved scalar quantity that indicates the mass fraction of one or
more components of the gas. The mass fractions of unburned and burned fuel are estimated
explicitly by default. The most important combustion reactants and products – fuel, O2, CO2,
H2O, N2, CO, and soot – are all precalculated functions of the mixture percentage. If the
fuel is not specified on a reaction line in the input file, propane is used as default.
There are two approaches to model a fire in FDS. The most often utilised and best-predicted
method requires that the fire's mass loss rate or heat release rate be understood. The heat
release rate per unit area (HRRPUA) or mass loss rate per unit area (MLRPUA) is then
coupled to a fire obstacle or vent. This results in a gas burner with variable fuel consumption.
FDS, on the other hand, forecasts the heat release rate based on the fuel's material qualities.
In this situation, the pace at which the fuel burns is determined by the net heat feedback to the
surface. When a liquid fuel is burned, the evaporation rate is determined by the liquid
temperature and the amount of fuel vapour present above the pool surface. The ClausiusClapeyron equation governs the rate of evaporation of the fuel, with the volume fraction of
the fuel vapour above the surface given as:
hv = heat of vaporisation
wf = molecular weight
tb = boiling temperature
ts = surface temperature
When the simulations start there is no temperature and an initial guess is made for the fuel
vapour mass flux. During the simulation, the evaporation mass flux is updated based on the
difference between current close-to-the surface volume fraction of fuel vapour and the
equilibrium value given above.
Fernandez-Pello, Carlos (2004) summarises the limitations of the FDS model as follows:
validation of the transport equations is needed for special scenarios (currently is set for smoke
movement inside enclosures); the mixture fraction implies a potential excess of pyrolyzated
gases burned; finer grid size does not always imply convergence. The user has to describe the
three-dimensional geometry of the scenario in detail, including the size and location of all
objects, as well as the boundary and initial conditions. All solid surfaces need to have
assigned thermal properties, and for the burning surfaces also combustion characteristics. The
mixture fraction model needs to have assigned a particular gas reaction. One important step
when setting up the model is the size of the volumes composing the grid. The accuracy of
field models could be potentially increased using smaller volumes, which increases the
computer time as well. Usually, the smallest volume attainable is limited by the available
computer-time. Novozhilov, V., “Computational fluid dynamics modeling of compartment
fires”, Progress in Energy and Combustion Science 27 (6), 2001, pp 611-666. adds that
because of the great sophistications of field models, simulations cannot be run blindly and its
application requires the supervision of qualified individuals.
Chapter 3: Research Methodology
Chapter 4: Result and Discussion