Fire Engineering 4 Assignment: Analysis and Solutions

This document presents solutions to a fire engineering assignment, addressing key concepts and calculations. The first question focuses on mass burning rate, heat release rate, and radiative fraction, including relevant formulas and calculations. The second question explores the use of the neutral plane in fire location and ventilation strategies. The third question examines the combustion process and fire extinguishing mechanisms, particularly the role of water. The fourth question defines and analyzes flashover, non-flashover, and backdraft scenarios, including graphical representations and influencing factors. Finally, the fifth question discusses computer fire models, differentiating between zone and field models and their applications in predicting fire development. The assignment covers diverse aspects of fire engineering, providing a comprehensive understanding of fire dynamics, safety, and modeling techniques.

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Assignment
Name
Institution Affiliation

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Question 1
a)
Mass burning rate is the mass loss per unit of the burning material within specific condition.
But,
M`` = 1.27× 106 ῥl ( ΔHc
ΔH∗v ) . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . ( i)
Where; ῥl is the density of fuel at boiling point, ΔHc is the heat of combustion and ΔH∗v is the
modified heat of combustion and M`` is the mass burning rate density
Also, ΔH∗v is given by ΔHc+∫
Ta
Tb
Cp dT
The density of kerosene ῥl is 0.0008kg/m3
And heat of combustion of kerosene= 418kJ/mole
Modified heat of combustion of kerosene is given by 178kJ/mole
Therefore M`` = 1.27× 106 ῥl ( ΔHc
ΔH∗v )
= 0.010106 ( 418
178 )
Thus, 0.0237kgs-1m-2
b.
Heat release rate
Heat rate = Ws ×C×ΔT ……………………………………………………………………….. (ii)

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Where Rh is the heat rate in btu/hr
Ws is steam flow in btu/hr
C is the specific heat capacity
And ΔT is the change in temperature in 0F
But the heat capacity is given by 0.48btu/1b 0F
Hence Rh is 0.48Ws
c.
Radiative fraction is heat produced during combustion and is distributed to the surrounding
mainly the radiation and the convection. The most significant parameters of radiation is the
fraction of the heat produced from the fire.
q= δT4A . . . . . . . . . . . . …………………………………………………………………… (iii)
Where, q is the radiative fraction, δ is 5.6703 ×108 (W/m2k4), T is the absolute temperature in K
and A is the area of the body.
T^4 this is the absolute temperature near the surface of the flame that is dependent.
q= 5.6703 ×108×T4×Πr2
q= 5.6703 ×108×T4×3.142 ×2.52
= 1.1135× 1010 T4 mC/kg.
d. What is the necessary separation of the tanks

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To avoid wastage as when more than one tank catches fire the other do not hence no
wastages. This can also lead to the contamination of the fuel in the other tank.
To reduce more accident as when more than one tank catches fire more accident can be seen.
And the formula between these two burning tanks can be obtained by the following formula;
D= (δT 4 ×Πr2) / (ρ1V1 +ρ2V2 )
Where ρ1 is the density of the kerosene in tank 1 and ρ2 is the density of kerosene in tank 2,
V1 is the volume in tank 1 and V2 is the volume in tank 2 and D is the distance between the
tanks.
Question two
The neutral plane can be used in the determination of the on what area the fire is
located. Below the neutral plane the air is drawn into the structural and then above the plane
the exhaust gases from combustion are removed. The identification of the plane is made once
the opening is done. This identification aids in the indication of the location of fire. When the
fire is detected in particular area in the first floor, the opening of door at the front is done that
will allow the smock to come out of the floor from the top part whereas the fresh air will be
allowed to come in from the lower part of the floor. This process is where the opening is the
vent and is created with regards to the fire in the room but in case where the location of fire is
at the basement there will be creation of the opening in any area of the surface hence leading to
the removal of the smock in any direction thus the flow is unidirectional.
But is the making of the opening is done under the neutral plane then the smock that
flows out from the new vent might not be seen, hence the heat amount that flows is higher but

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the heat loss is smaller in the lateral surface thus the thermal continuity plane is determined.
The measurement of the heat conductivity in the room is the thermal conductivity and is given
by the thermal heat transmission over a given length and the area of the room. This continuity
dependents on the temperature of the room. When there is hot air in the room and cold air is
allowed in the rom the ventilation properly managed, hence thermal continuity plane is
achieved.
Question three
Combustion is a process that brought about by the chemical reaction of substances like
oxygen.it comprises three conditions for its occurrence. One of the condition for combustion is
availability of the substances that are compatible, like fuel and oxygen. The other conditions,
availability of the substances that supports the combustion for example, the supply of adequate
oxygen .Lastly, the attainability of temperature for ignition, catching of fire by the substances
cannot happen if the required temperature is not attained.
Mechanisms of fire extinguishing for water.
Fire suspension is much done by water due to its physical properties that are favorable. The
absorption of the heat and fuel is significantly done due to it high latent heat and heat capacity.
When evaporation takes place, the expansion of water is up-to about 1700 times this make the
oxygen and the fuel to be diluted three is farther increase in the fire suspension that is brought
by the by the formation of the fine droplets. The summary of the extinguishing for water is
indicated below.
Extraction of heat (cooling)

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Water as cooling mechanisms for fire suspension is categorized as fire plume and fuel surface
cooling. The conversion of the water to stream occurring when high percentage of water
entered the fire plume is an attribute of the flame cooling by water. When the adiabatic flame
temperature is reduce to temperature lower that the required for burning, the fire is
extinguished. This will result into the reaction of oxygen and fuel for combustion to be
terminated.
Displacement of oxygen
The displacement of the oxygen is occurring either in space that is localized or in
compartmental. The fine water droplets in the stream can lead to the elimination of the oxygen
concentration in the compartments. When addition of water is done into the hot comportment
there is absorption of heat due to the latent heat of vaporization of water. The increase in the
depletion of the oxygen and the displacement of the oxygen as a result of the formation of
water vapor is directly proportional to the increase in the size of the fire and the pre-burn length
of the fire. But when sprays of water taken through the fire plume there is conversion of water
into vapour whose expansion is about 1700 times the volume of the liquid hence disrupts the
mixing of air into the flame.
Question 4
1. Flashover
On the definition of the flashover, it is abrupt involvement of a location in flame from may
be the floor of the room to the ceiling board of the roof which can be as a result of feedback of

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the thermal radiation (Grant, 2012). This radiation feedback is the energy of the fire radiated
back to the room and its content (hence the name feedback). The energy radiation to the
contents of the room will actually increase entirely the content in the room to their respective
ignition temperature. The flashover will at that point when the entire content of the room
simultaneously and abruptly ignite (Abu, 2017). Therefore the flashover is a temperature driven
act. On the analysis of the flashover as a fire scenario will entail the scrutiny with a clear graph
of the flashover (Zhang, 2015). Such graph can be shown as below;
Fig 1: showing the graph of flashover
From the figure one above, it can be clearly seen that the flashover shows that the fire has a
growth stage which will move until it rejects the fully developed stage. From the fully developed
stage, the flashover will start decaying (declining stage) (Wang, 2013). There are some major
important factors which affect the flashover. These factors affect whether a room will have a

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flashover or not (Beard, 2013). They include the following; air supply to the room, insulation of
the room and the size of the room which all combined will determine whether a room can have a
flashover or not. Application of the sprinkler will make the full developed stage to be very
shorter. It will also make the growth stage to reach full developed within a very low temperature
(Lentini, 2012).
2. Non-flashover
For this kind of fire scenario, the below graph best describes it;
Fig 2: Showing the graph for non-flashover
Non-flashover is a unique graph as it has almost a flat graph, that is to say, it has a very short
fully developed stage (Yung, 2011). Unlike in flashover stage, in non-flashover, the three stages
–growth stage, fully developed stage and decay stage are almost equal and the same in height.
Non-flashover can be well illustrated as an environment where there are insufficient burning
gases hence temperature will always remain low (Materials, 2011). On application of sprinkler,

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the graph will be a perfectly straight in the fully developed stage with a very short growth and
decay stages.
3. Backdraft
The backdraft graph is shown by the graph below;
Fig 3: Showing the backdraft
This involves a rapid combustion or an explosion of hot flammable product and hot pyrolysis
products on mixing with air (Hirschler, 2012). The following will contribute to the existence of
backdraft;
i. The low oxygen concentration in the compartment up to a point where flaming
combustion is limited.
ii. There must be a fire which proceeds to a ventilation controlled state having a high
concentration of pyrolysis flammable products of combustion (Yuen, 2013).
iii. There should be enough temperature to enable the fuel to be ignited when mixed with

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air.
On the application of the sprinkler, the fully developed stage of the graph will reduce in height
but not level (Wakatsuki, 2016).
Question 5
The computer fire models are series of mathematical equations obtained from experimental data.
The use of a computer is employed to help solve a question which is very tiresome to be solved
by hand (Ohnstad, 2012). The complex models constitute hundreds of thousands of equations
while the simplest models are those with single equations to be solved. These fire models are
employed to help predict the development of a fire in a structure or room. There are two major
group of the computer fire models include field model, zone model;
i. Zone Models
These models split the enclosure or a room into several zones but in most cases, the zone is split
into two zones. These are the lower zones and the upper zones, the upper layer including of
heated combustion product while the lower layer has a cooler air that is free from the combustion
product. In two zone models, these lower and upper zones are connected. This model constitutes
openings to the outside or to other rooms and in some cases heat loss via ceiling and walls. The
input to the model includes building materials and room dimension, location and size of the room
opening and fire heat release rate. And the output of this model includes the time to flashover,
upper and lower temperature layers, prediction of sprinkler and the combustion gas concentration
(Mehaffey, 2012).

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ii. Field Models
These models are also referred to as a computational fluid dynamic model. In this model, the
splits or enclosure are in a large number of small 3 D boxes known as cells. This enclosure may
have hundreds of thousands of cells whose dimension may be in centimeters or meters. These
models are basically on the physical principle s of energy, momentum conservation, and mass. In
this model, the computer will help calculate the flow of heat and smoke between the cells over
duration of time (Madrzykowski, 2012). At any given moment, it is likely to find the velocity,
temperature, concentration of gas within the cells. Since there are a large amount of cells, the
situations in the room can be anticipated in more details. This model is capable to predict the
condition in a very small and large spaces. The complexities which are encountered in the zone
models are bedeviled using this field model due to a large number of cells.
Importance of Computer fire Models
 These models help the fire department personnel to be able to help individuals to exit the
building safely.
 It increases time for evacuating building by the occupants before a fire spreads out of
control
 With the computer fire models, emergency medical can be sent immediately to those who
are in need.
Limitation of computer fire models
 For the computer fire models, there is lack of numerical and theoretical assumptions
 There is lack of fidelity of numerical solutions process

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 There are direct mistakes in software
 There are mistakes in the application of computer fire models.
Trends in future development
Some of the expected future trends in the computer fire development includes the following;
 Movement from augmented reality to the virtual World.
 There is expected facility planning in the virtual World.
 There is also a simulating Crowd Dynamics.
 There will be immersive Training with
Question 6
i.
Mean thermal inertia β = √kpc = 1.7 kWs ½ . The following equation can be employed to
estimate the temperature of the skin subjected to a perpetual heat flux.
T= T0+ 2Q
β √ t
π . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . (iv)
Where T is the thermal penetration depth, T0 is initial temperature Q is the heat flux t is the time
of exposure and β is the mean thermal inertia.
Taking the arbitrary increasing time as5 seconds,10 seconds and 15 seconds.
T0=370C
Q=1Kjm-2s-1