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Citation: Wang, K.; Qiu, R.; Ming, Y.;
Xu, H. Experimental Study on the Hot
Surface Ignition Characteristics and a
Predictive Model of Marine Diesel in a
Ship Engine Room. J. Mar. Sci. Eng.
2024,12, 798. https://doi.org/
10.3390/jmse12050798
Received: 15 April 2024
Revised: 2 May 2024
Accepted: 7 May 2024
Published: 10 May 2024
Copyright: © 2024 by the authors.
Licensee MDPI, Basel, Switzerland.
This article is an open access article
distributed under the terms and
conditions of the Creative Commons
Attribution (CC BY) license (https://
creativecommons.org/licenses/by/
4.0/).
Journal of
Marine Science
and Engineering
Article
Experimental Study on the Hot Surface Ignition Characteristics
and a Predictive Model of Marine Diesel in a Ship Engine Room
Kan Wang * , Rui Qiu, Yang Ming and Hang Xu
College of Ocean Science and Engineering, Shanghai Maritime University, Shanghai 201306, China;
202230410076@stu.shmtu.edu.cn (R.Q.); 202130410092@stu.shmtu.edu.cn (Y.M.);
202330410075@stu.shmtu.edu.cn (H.X.)
*Correspondence: wangk@shmtu.edu.cn; Tel.: +86-(021)-38282517
Abstract: To ensure the safe protection of marine engine systems, it is necessary to explore the hot
surface ignition (HSI) characteristics of marine diesel in ship environments. However, an accurate
model describing these complex characteristics is still not available. In this work, a new experimental
method is proposed in order to enhance prediction performance by integrating testing data of the
characteristics of HSI of marine diesel. The sensitivity of HSI is determined by various factors such as
surface parameters, flow state, and the ship’s environment. According to variations in the HSI status
of marine diesel in an engine room, the HSI probability is distributed in three phases. It is essential
to determine whether the presence of marine diesel or surrounding items can intensify the risk of
an initial fire beginning in the engine room. A vapor plume model was developed to describe the
relationship between HSI height and initial specific buoyancy flux in vertical space. Further, field
distribution revealed significant variation in the increase in temperature between 200 and 300 mm
of vertical height, indicating a region of initial HSI. In addition, increasing surface temperature did
not result in a significant change in ignition delay time. After reaching a temperature of 773 K, the
ignition delay time remained around 0.48 s, regardless of how much the hot surface temperature
increased. This study reveals the HSI evolution of marine diesel in a ship engine room and develops
data-based predictive models for evaluating the safety of HSI parameters during initial accident
assessments. The results show that the goodness of fit of the predictive models reached above 0.964.
On the basis of the predicted results, the HSI characteristics of marine diesel in engine rooms could
be gleaned by actively determining the parameters of risk.
Keywords: ship engine room; marine diesel; ignition characteristics; surface temperature; predictive
model
1. Introduction
When a ship is sailing or working near a port, a failure in the marine fuel container
or oil supply pipeline of an engine system can cause combustible liquid leaks and flows.
Marine fuel can leak and make contact with typical high-temperature machinery surfaces
and then easily trigger hot surface ignition (HSI). Ship fires are a serious and dangerous
type of maritime accident that can result in injuries and fatalities. According to statistical
data from 2017–2022 by Det Norske Veritas, approximately 63% of fires and explosions
originate in the engine room. The leakage and dispersion of operating fuels onto hot
surfaces are a common cause of accidental fires. The piping system, piping connections,
and associated components such as O-rings pose a higher risk of fire in some cases. For
instance, marine diesel can leak when the piping components are not tightened to the
required torque. Sometimes, piping connections may loosen due to vibrations caused by
the ship’s motion on the sea. The oil filter covers of marine diesel may also come loose, and
the rotary spindle may become displaced from the top cover for various reasons. These
issues can result in minor leaks in the engine room, which can cause oil to soak into the
J. Mar. Sci. Eng. 2024,12, 798. https://doi.org/10.3390/jmse12050798 https://www.mdpi.com/journal/jmse
J. Mar. Sci. Eng. 2024,12, 798 2 of 20
surrounding area over time. Rubber hoses can be vulnerable to rupture due to thermal
damage from the ship’s mechanical equipment. In addition, bolts for flanges or filters may
break due to fatigue caused by overtightening over time, and securing bolts may be found
loose or missing altogether. Other potential causes of failure include incorrect assembly
after maintenance in the ship engine room. However, the aforementioned common causes
of marine diesel leakage and dispersion often result in fires. These incidents are primarily
caused by energy system failures, most commonly in low-pressure marine diesel piping,
which allows for the leakage of diesel, allowing it to come into contact with an unprotected
hot surface. The current study highlights three main aspects: the diesel’s properties, the
high-temperature ignition source, and a reasonable guide for preventing highly flammable
marine diesel from contacting a high-temperature ignition source.
The incidence of accidents caused by engine room fires is much higher than that in
passenger cabins, cargo storage, and other ship areas. Some researchers have analyzed
ship engine system fires and the damage they cause [
1
–
3
]. The Le Boreal incident [
4
]
demonstrated that safer alternative arrangements for the ship engine room were not
considered, and the fire scenario was deemed too unlikely for cost-effective safety measures.
Spyrou et al. [
5
] discovered that ignition probabilities can be calculated using the present
empirical formula. However, larger ships may have lower probabilities of fire occurring in
the engine room. The engine system contains many combustible materials, such as fuels or
chemicals, which can potentially be ignited. Any thermal source that ignites these materials
can cause fire accidents [
6
–
8
]. Shao et al. [
9
] identified hazard identification as an essential
stage in the unique development of a ship. It was found that the most significant risk
factors in the engine system were fire accidents caused by fuel leakage from connecting
valves or flanges. These accidents occur when combustible materials reach their ignition
temperature and enter into a chemical reaction with an oxidizer. The ignition of leaked fuel
continued in the engine room due to the presence of an ignition source and combustible
materials [10].
Numerous studies have been conducted on fireproofing and fire evaluation using
numerical tools. Numerical simulations of some cases indicate that in both the engine
room and the vicinity of the exit, the position of the fire exerts the greatest influence on the
distribution of temperature [
11
]. Ship fire accidents in engine rooms with different types of
low-pressure fuel gas supply systems have been compared, and the potential risk of a high
concentration of leaked fuel vapor has been identified [
12
,
13
]. The simulation database
is crucial in developing a ship fire safety plan that enhances safety management in the
early design stage [
14
]. Additionally, fatality estimation is beneficial for improving design
safety and ensuring an effective response to ship engine room fires [
15
,
16
]. Lan produced a
full-scale model of the engine room and found that ventilation conditions had a significant
effect on combustion behavior [
17
]. While numerical methods have certain advantages [
18
],
it is important to constantly improve results and models by comparing them with real
experimental data. In their study, Wang et al. conducted experiments in an engine room
to investigate the vertical temperature distribution profile during a fire [
19
]. They found
that the development of the fire was primarily influenced by the concentration of oxygen.
Additionally, the mass loss rate of marine fuel was identified as an important parameter that
characterizes the combustion process in engine room fires [
20
]. Marine diesel is a commonly
used fuel in ship engine systems. Ignition can be triggered by equipment failure or thermal
sources. Liu et al. [
21
] conducted an experiment using a rectangular pool to study the
ignition zone and flame behavior after initial fire growth. The ventilation structure and
mode of the engine room were different from those in other cabin rooms. The mass loss
rates of leaked fuel increased as the ventilation velocities increased in the ship engine
room [
22
]. During the initial stage of the fire, air entrainment was greatly limited [
23
],
resulting in the formation of a broken fire merge above the obstacle in the ship engine room.
It was found that the overall heat loss factor was the critical variable that significantly
determined the increase in pressure and smoke filling [
24
]. Additionally, the ignition
sources during the initial stage of an engine room fire were affected by various factors,
J. Mar. Sci. Eng. 2024,12, 798 3 of 20
including the properties of the fuel, geometry of the ignition source, and environmental
boundaries. A significant heat transfer mechanism was observed with a continuous fuel
dispersion process and ignition [
25
]. In ship engine rooms, the conductive heat transfer
term is not applicable, and heat loss to the substrate cannot be ignored in thin layers of
marine fuel. One study suggested that fire prevention measures could be improved by
detecting proximate events that occur immediately before the HSI [26].
Many engine room fires are caused by failures in marine oil systems, which allow
leaked fuel to come into contact with unprotected hot surfaces. Recent research has been
conducted to clarify the relevance of the evaporation process and ignition mechanism.
Mizomoto et al. focused on measuring the mass of n-cetane evaporated a short time after
the droplet’s contact with a hot surface [
27
]. They found that ignition occurred when the
stoichiometric n-cetane/air mixture reached a droplet diameter of approximately 4 mm
to 5 mm. Bennett et al. proposed a one-dimensional model for the airborne HSI event,
which revealed that diffused fuel vapor buoyancy and surrounding entrained air limited
ignition over a wider elevation range [
28
]. Statistical measurements could provide the
critical temperature at which the ignition probability was 50% [
29
]. The materials of the
hot surface affected the fuel ignition process. It was found that the ignition temperature
on a hot surface made from a stainless steel heat shield was generally lower than the
temperature of one made from 409 stainless steel [
30
]. One study utilizing high-speed
video data records revealed that the temperature of the combustible fuel was higher during
the initial stage, resulting in a lower ignition delay time [
31
]. The ignition was affected
by the flammable liquid’s properties, the hot surface temperature, hot surface type, and
environmental parameters [
32
]. However, there is currently no widely applicable model
for HSI, so individual experiments must be carried out for different liquids in ship engine
rooms.
This study focuses on experimentally investigating the ignition characteristics of
leaked marine diesel on a hot surface in a ship engine room. The purpose of determining
the phase transition, initial ignition, and flame propagation of marine diesel after leakage
onto a high-temperature surface is to predict the characteristics and parameters of the
HSI process. To achieve this, HSI equipment was set up in ship laboratory to test the HSI
parameters of marine diesel. This study analyzes high-speed camera images to determine
the spatial locations of the hot surface-driven initial ignition behavior of marine diesel. A
predictive model for ignition height is proposed by combining experimental data with heat
transfer mechanisms. The temperature field’s distribution and radiant heat transfer mode
from diesel contacting a hot surface until ignition are investigated using data collected by
ten thermocouples. A predictive model of the HSI delay time of marine diesel and surface
temperature is developed, taking into account a hot surface characterization system (e.g.,
temperature field, ignition occurrence, hot surface temperature, etc.) that covers key factors
in flow field and combustion. This study expands the existing limitations of assessing
ignition sources with predictive models, aiming to provide more support for controlling
and evaluating marine diesel leakage in ship engine rooms.
2. Materials and Methods
2.1. Experimental Method
The purpose of this study is to investigate the initial ignition and combustion process
of marine fuel on hot surfaces. To achieve this, an effective experimental platform and data
acquisition system must be constructed. The ship utilized in the current study is a deep sea
double-deck trawler, a type of fishing vessel that is primarily employed for single-bottom
trawling in the West African sea. The length of pelagic trawl is 33.2 m, with a design draft
and a displacement of 3.5 m and 510.81 t, respectively. The main engine is a model of
a diesel engine, with a rated power of 551 kW. Figure 1shows a schematic diagram of
the complete experimental test system located in the ship’s engine room. Additionally,
a hot surface simulation device featuring temperature regulation was designed for this
experiment. The hot surface device can accurately adjust the increase in temperature to
J. Mar. Sci. Eng. 2024,12, 798 4 of 20
within 1.0 K, with a maximum temperature of 1100.0 K. To accommodate its high power
usage, the device’s power cord is connected to the outside through an opening on the side
of the ship engine room. It is capable of simulating local high-temperature areas generated
by equipment or pipe housings in the ship engine room. The hot surface device has an
effective surface size of 0.4 m
×
0.4 m, and its structure can be modified by adding an
edge structure. The device is controlled by an external controller located outside the ship
engine room, connected through an extension cord, and set by researchers for each group
of experimental tests. An electronic balance with high measurement precision of up to 0.01
g is placed underneath the hot surface. The mass loss rate of the marine diesel leaking
onto the hot surface can therefore be obtained. The hot surface device is placed on a three-
degrees-of-freedom sloshing simulator to produce effective simulations of hull tilt and
movement for study in the stationary ship engine room. A device for controlling marine
diesel leakage is positioned above the hot surface and connected to a peristaltic pump. This
device ensures that marine diesel consumption during each experiment remains within
the range of 5 mL to 20 mL, which is similar to the ignition of fuel droplets on a hot
surface. Following a brief interval, a marine diesel droplet comes into contact with the
hot surface device. According to the height set by the nozzle of the flow controller in this
experiment, the distance between the droplet touching the hot surface after flowing out of
flow controller is approximately 0.045 m. Once the fuel vapor concentration in the vicinity
of the droplet reaches a specific threshold and the temperature is sufficiently elevated, the
ignition process is initiated.
J. Mar. Sci. Eng. 2024, 12, x FOR PEER REVIEW 4 of 20
for this experiment. The hot surface device can accurately adjust the increase in tempera-
ture to within 1.0 K, with a maximum temperature of 1100.0 K. To accommodate its high
power usage, the device’s power cord is connected to the outside through an opening on
the side of the ship engine room. It is capable of simulating local high-temperature areas
generated by equipment or pipe housings in the ship engine room. The hot surface device
has an effective surface size of 0.4 m × 0.4 m, and its structure can be modified by adding
an edge structure. The device is controlled by an external controller located outside the
ship engine room, connected through an extension cord, and set by researchers for each
group of experimental tests. An electronic balance with high measurement precision of up
to 0.01 g is placed underneath the hot surface. The mass loss rate of the marine diesel
leaking onto the hot surface can therefore be obtained. The hot surface device is placed on
a three-degrees-of-freedom sloshing simulator to produce effective simulations of hull tilt
and movement for study in the stationary ship engine room. A device for controlling ma-
rine diesel leakage is positioned above the hot surface and connected to a peristaltic pump.
This device ensures that marine diesel consumption during each experiment remains
within the range of 5 mL to 20 mL, which is similar to the ignition of fuel droplets on a
hot surface. Following a brief interval, a marine diesel droplet comes into contact with the
hot surface device. According to the height set by the nozzle of the flow controller in this
experiment, the distance between the droplet touching the hot surface after flowing out of
flow controller is approximately 0.045 m. Once the fuel vapor concentration in the vicinity
of the droplet reaches a specific threshold and the temperature is sufficiently elevated, the
ignition process is initiated.
Figure 1. Configuration of experimental apparatus and system utilized in HSI tests.
The temperature data are collected during the ignition of marine diesel using K-type
thermocouples to determine the distribution of the air temperature field. K-type thermo-
couples typically consist of temperature-sensing elements, mounting fixtures, and junc-
tion boxes. For this study, a bundle of thermocouple test rods are formed by installing ten
K-type thermocouples on a unified test rod. The thermocouple test rod is positioned above
the hot surface, with the thermocouples arranged in sequence and labeled T1 to T10. The
spacing between the first and second thermocouples and the hot surface is 20 mm, while
the distance between the third and fourth thermocouples and the previous thermocouple
reaches 30 mm. The spacing between the fifth and tenth thermocouples is 50 mm. The
real-time data obtained from thermocouple acquisition are connected to and stored in the
TP-700 data acquisition device via a connection cable. In this experiment, two high-speed
Figure 1. Configuration of experimental apparatus and system utilized in HSI tests.
The temperature data are collected during the ignition of marine diesel using K-
type thermocouples to determine the distribution of the air temperature field. K-type
thermocouples typically consist of temperature-sensing elements, mounting fixtures, and
junction boxes. For this study, a bundle of thermocouple test rods are formed by installing
ten K-type thermocouples on a unified test rod. The thermocouple test rod is positioned
above the hot surface, with the thermocouples arranged in sequence and labeled T1 to T10.
The spacing between the first and second thermocouples and the hot surface is 20 mm, while
the distance between the third and fourth thermocouples and the previous thermocouple
reaches 30 mm. The spacing between the fifth and tenth thermocouples is 50 mm. The
real-time data obtained from thermocouple acquisition are connected to and stored in the
TP-700 data acquisition device via a connection cable. In this experiment, two high-speed
cameras are positioned at the observation port and inside the ship engine room to capture
the initial moment of ignition of the marine fuel on the hot surface. A SONY AX700
J. Mar. Sci. Eng. 2024,12, 798 5 of 20
high-speed camera (Sony, Tokyo, Japan) is used, which is capable of capturing images
up to 1000 fps/s. The laboratory is equipped with humidity controllers to simulate the
humidity conditions in the ship engine room during offshore operations. The laboratory’s
humidity is regulated and maintained between 70% and 90%, based on monitoring data
from the ship nacelle. The temperature and humidity monitoring instruments are installed
on the opposite side of the wall to ensure consistency between the experimental scenario
and the real operating environment of the ship engine room. It is important to adjust the
temperature and humidity accordingly.
2.2. Experimental Materials
According to accident cases, this experiment identifies three types of marine fuel as
experimental tests and determines the physical and chemical properties of experimental
materials, as shown in Table 1. The marine diesel used in this experiment consists of various
components, including alkanes, cycloalkanes, alkenes, cycloalkenes, and aromatic hydro-
carbons. The heavy molecular combustion of marine diesel is based on the hydrocarbon
composition characteristics of typical blended raw materials. Among them, the asphaltene
molecule has an average molecular structure of asphaltene in crude oil, whose average
molecular formula is C
47
H
41
NOS
2
and whose relative molecular mass is 699. The main
raw materials of marine diesel are light cycle oil (LCO), and the saturated hydrocarbon
components in LCO are mainly alkanes. The origin of marine diesel used by the main
engine of multifarious ships displays differences, which leads to differences in marine
diesel components. Using the numerical method, it is difficult to directly present the
ignition characteristics of actual marine diesel. Therefore, the ignition characteristics of
marine diesel are usually represented by n-heptane, whose cetane number is close to that
of marine diesel. According to the experimental tests in the current study, the experimental
amount of marine fuel ranges from 5 mL to 20 mL. Table 1presents the marine diesel fuels
selected for this experimental test of ignition on heated surfaces. A total of 30 tests in
different conditions are performed to determine the ignition and combustion characteristics
of marine fuels on hot surfaces.
Table 1. Physical properties of marine diesel in the current experimental work.
Items Density at
288.15 K (kg/m3)
Viscosity at
313.15 K (mm2/s)
Flash Point
(K)
Pour Point
(K)
Total Base
Number
(mg KOH/g)
Marine diesel 864.0 4.52 375 264 13.5
3. Results and Discussion
3.1. Initial Position of HSI Occurrence with an Elevated Hot Surface Temperature
The fuel leaked in the ship engine room evaporates through heating on a hot surface,
forms combustible vapor above the hot surface, and mixes with the surrounding air. The
combustible vapor mixture ignites over the hot surface. In the process of heat evaporation,
the leaking marine fuel also causes heat loss to air and the surface housing the equipment
due to the action of the heat source. In the process of fuel his, the leaking fuel is affected
by air flow organization, environmental humidity, and wall thermal feedback. As a result,
the leaking marine fuel’s HSI characteristics, ignition time and ignition probability are
changed. The heat loss of combustible liquid and the effect of environmental thermal
feedback are affected by the specific design of the top opening in the engine room and
changes in humidity in the operating environment. NFPA 921 suggests that ventilation can
influence ignition. If too much air blows across a hot surface, it can hinder the ability of
the combustible liquid to volatilize quickly enough to allow ignition. Conversely, if too
little air blows across the hot surface, the atmosphere can become too rich. According to
Equation (1) [
33
], the heat received by the surface of marine fuel may be the sum of heat
J. Mar. Sci. Eng. 2024,12, 798 6 of 20
conducted by the hot surface, heat transferred to the liquid surface by thermal convection,
and heat transferred to the liquid surface by thermal radiation.
Qex =Qcond +Qconv +Qrad (1)
where Q
conv
is the heat generated by thermal convection, kW/m
2
;Q
rad
is heat transferred
by thermal radiation, kW/m
2
;Q
ex
is heating rate of external heat source per unit area
of liquid surface, kW/m
2
; and Q
loss
is heat loss rate per unit area of combustible liquid,
kW/m2.
According to the experimental tests in the previous section, the process of the ignition
of leaking marine diesel above a hot surface is accompanied by significant and complex
evaporation behavior. In this study, the evaporation and ignition characteristics of marine
diesel in the vertical space of a hot surface are analyzed. The data are collected by a high-
speed camera, and each image is processed according to a time axis. The experimental
amount of marine diesel is 10 mL and the hot surface temperature is set at 748 K as an
example. Figure 2presents the process of diesel leaking onto a high-temperature surface,
causing evaporation and initial ignition. When marine diesel leaks onto a hot surface
in the initial stage, the rapid phase transition of marine diesel can be clearly observed.
Due to the high temperature of the hot surface, marine diesel is almost always in a state
of film boiling. This creates a gaseous layer between the diesel and high-temperature
surface, which changes the mode of heat transfer from the high-temperature surface to
liquid. At 3.675 s, a significant amount of white vapor mixture is produced when marine
diesel comes into contact with a hot surface. The marine diesel produces a wider range of
combustible vapor mixtures above hot surfaces when compared to marine lubricant. This
is due to the flow characteristics of marine diesel. The evaporation product of diesel on
the high-temperature surface mixes fully with fresh air, forming a combustible mixture in
the vertical space. The heat continually transfers from the high-temperature surface to the
liquid surface and, coupled with thermal feedback, causes the initial ignition behavior of
the high-temperature surface; ignition therefore occurs instantaneously when diesel leaks
at 3.725 s. Once the initial ignition forms a core in the air, it gradually grows and generates
flame propagation, as depicted at 3.733 s. This reveals that the initial ignition takes place at
a higher position above the high-temperature surface.
J. Mar. Sci. Eng. 2024, 12, x FOR PEER REVIEW 6 of 20
air blows across the hot surface, the atmosphere can become too rich. According to Equa-
tion (1) [33], the heat received by the surface of marine fuel may be the sum of heat con-
ducted by the hot surface, heat transferred to the liquid surface by thermal convection,
and heat transferred to the liquid surface by thermal radiation.
Qex = Qcond+
Qconv+
Qrad
(1)
where Qconv is the heat generated by thermal convection, kW/m2; Qrad is heat transferred
by thermal radiation, kW/m2; Qex is heating rate of external heat source per unit area of
liquid surface, kW/m2; and Qloss is heat loss rate per unit area of combustible liquid,
kW/m2.
According to the experimental tests in the previous section, the process of the ignition
of leaking marine diesel above a hot surface is accompanied by significant and complex
evaporation behavior. In this study, the evaporation and ignition characteristics of marine
diesel in the vertical space of a hot surface are analyzed. The data are collected by a high-
speed camera, and each image is processed according to a time axis. The experimental
amount of marine diesel is 10 mL and the hot surface temperature is set at 748 K as an
example. Figure 2 presents the process of diesel leaking onto a high-temperature surface,
causing evaporation and initial ignition. When marine diesel leaks onto a hot surface in
the initial stage, the rapid phase transition of marine diesel can be clearly observed. Due
to the high temperature of the hot surface, marine diesel is almost always in a state of film
boiling. This creates a gaseous layer between the diesel and high-temperature surface,
which changes the mode of heat transfer from the high-temperature surface to liquid. At
3.675 s, a significant amount of white vapor mixture is produced when marine diesel
comes into contact with a hot surface. The marine diesel produces a wider range of com-
bustible vapor mixtures above hot surfaces when compared to marine lubricant. This is
due to the flow characteristics of marine diesel. The evaporation product of diesel on the
high-temperature surface mixes fully with fresh air, forming a combustible mixture in the
vertical space. The heat continually transfers from the high-temperature surface to the
liquid surface and, coupled with thermal feedback, causes the initial ignition behavior of
the high-temperature surface; ignition therefore occurs instantaneously when diesel leaks
at 3.725 s. Once the initial ignition forms a core in the air, it gradually grows and generates
flame propagation, as depicted at 3.733 s. This reveals that the initial ignition takes place
at a higher position above the high-temperature surface.
(a)
(b)
(c)
(d)
Figure 2. The initial ignition of marine diesel leaking onto the hot surface of a ship engine system.
(a) t = 3.108 s; (b) t = 3.675 s; (c) t = 3.725 s; (d) t = 3.733 s.
The interface between the leaked fuel and high-temperature hot surface is a solid–
liquid-state heat transfer interface. At this interface, the high-temperature surface of the
ship engine system transfers heat to leaking oil by means of heat conduction. The hybrid
Figure 2. The initial ignition of marine diesel leaking onto the hot surface of a ship engine system.
(a)t= 3.108 s; (b)t= 3.675 s; (c)t= 3.725 s; (d)t= 3.733 s.
The interface between the leaked fuel and high-temperature hot surface is a solid–
liquid-state heat transfer interface. At this interface, the high-temperature surface of the
ship engine system transfers heat to leaking oil by means of heat conduction. The hybrid
scenario shows faster movement of liquid marine fuel due to the formation of vapor
underneath and the curvature of the leading edge of leaked marine fuel caused by surface
J. Mar. Sci. Eng. 2024,12, 798 7 of 20
tension. If the high-temperature surface is significantly elevated, the marine fuel will boil
violently on the high-temperature surface, and a film of vapor will form on the surface of
the liquid fuel. The film’s boiling mode can cause the pool of marine fuel to break up into
smaller globules due to surface tension. When film boiling occurs in the marine fuel above
the hot surface, combustible vapor is generated at different distances from the marine fuel’s
surface. The film boiling regime of leaked marine fuel starts at a specific value of excess T
s
over Tsat and can be given by Equation (2) [34].
Ts-Tsat =0.127 3
√ghvρv
λvrσ
ρl
3
rµv
ρl
(2)
where
ρv
is the density of fuel released into ambient air from the hot surface, kg/m
3
;
ρl
is the density of marine fuel, kg/m
3
;
λv
is the thermal conductivity of vapor in the film
between the marine fuel and hot surface, W/(m
·
K); h
v
is the latent heat of vaporization,
kJ/g;
σ
is the surface tension of marine fuel; and N/m;
µv
is the viscosity of the vapor in
the film between the surface and fuel, mm2/s.
When marine fuel on a high-temperature surface is in a film boiling state, h
f
can be
presented [35] as follows.
hf=0.425 4
v
u
u
u
t
λ3
vhvρlρvg
(Ts-Tsat)µvσ
ρl1
2
(3)
where hfis the heat transfer coefficient of film boiling, W/m2·K.
In the film boiling state, the evaporation rate of fuel on a high-temperature surface can
be obtained by Equation (4) [36], as shown:
mf=hfS
cpln(B+1)(4)
where m
f
is the evaporation rate of leaked marine fuel within unit area, g/s; Sis the surface
cross-sectional area from leaking marine fuel on the hot surface, m
2
; and Bis the mass
transfer number.
The mass transfer number is also called the D. B. Spalding number, which represents
the ratio of chemical energy to the evaporation energy of fuel. Bcan be found using
Equation (5) [37]:
B=cp(Ts-Tb)
hv(5)
where Tbis the boiling temperature, K, and cpis the specific heat, J/(kg·K).
Figure 2illustrates that when the marine diesel leaks and is ignited above a hot
surface, the initial flame entrains air in the engine system. The experimental phenomenon
demonstrates that without mechanical ventilation, ignition always occurs vertically upward
from the center line of the high-temperature surface. Marine diesel HSI typically occurs
between the heated surface and the ceiling of the ship engine system. The initial flame
appears on the hot surface and further spreads. This shows that the accumulation of marine
diesel vapor above the heating surface, combined with the hot surface temperature, affects
the entrainment of the vapor/air mixture. Equation (6) shows the relationship between the
entrainment velocity of the high-temperature surface ignition flame and the rising velocity
of the gaseous mixture in the initial fire core [38].
ue=ηub(6)
where u
e
is flame velocity at which the vapor/air mixture is entrained in the horizontal
direction, m/s; u
b
is the rising velocity, which is attributed to vapor and buoyancy effect,
m/s; and ηis the correction coefficient.
J. Mar. Sci. Eng. 2024,12, 798 8 of 20
Equation (7) expresses that the rising velocity decreases as the distance from the high-
temperature surface increases in the vertical space in which a gaseous mixture is formed by
the marine diesel vapor and air existing in the engine room [39].
ub=7.3qsgH
cpTaρ01
3(7)
where q
s
is the heat flux density of the solid–liquid surface, W/m
2
; g is the gravitational
acceleration, m/s
2
;His the marine diesel ignition height, m;
ρ0
is the ambient air density,
kg/m3;Tais the initial temperature of ambient air, K; and cpis specific heat, J/(kg·K).
As a result, by obtaining the correction coefficient, we can further determine the
expression of the entrainment velocity at which the initial ignition flame is on the hot
surface. Figure 3presents the difference between the initial ignition height of marine
diesel and the heated surface’s temperature. The ignition height of marine diesel exhibits a
declining trend with an increase in heated surface temperature. However, this downward
trend does not follow a simple linear relationship, as can be seen in the experimental results
when the probability of HSI is 50%, for instance. When the heated surface temperature is
close to 748 K, the ignition height of marine diesel above the hot surface is about 0.51 m.
If the surface is heated and reaches a temperature of 758 K, the ignition height drops to
approximately 0.42 m, with a change amplitude of 0.009 m/K. If the surface reaches a
temperature of 778 K, the diesel will be ignited on the hot surface with 100% probability. At
this high surface temperature, the height of the HSI for marine diesel is 0.22 m. The increase
in surface temperature causes the marine diesel that leaks onto the hot surface to evaporate
rapidly, forming combustible vapor that mixes with the fresh air. The high-temperature
surface has a great thermal effect in a short time, leading to HSI behavior in the area near
the high-temperature surface. As the heated surface’s temperature goes up, the ignition
height remains around 0.18 m above the hot surface instead of fluctuating greatly. In a
previous work [
40
], a vapor plume model was developed. This model is presented in
Equation (8) and can be used to describe the relationship between the ignition height of
marine diesel and the initial specific buoyancy flux [41] in a certain vertical space.
(ρ0-ρv)
ρ2
0
=H-5
35Bi
6aπg9Bia
10π-1
3(8)
Bi=Qvg(ρ0-ρv)
ρv(9)
where
ρ0
is the density of ambient air, kg/m
3
;
ρv
is the density of marine diesel released into
the ambient air from the hot surface, kg/m
3
;His the ignition height of marine diesel, m; B
i
is the initial specific buoyancy flux, m
4
/s
3
;ais the correction coefficient, dimensionless;
and Qvis the volumetric flow rate of marine diesel, m3/s.
Using the experimental data, a relationship between initial ignition height and hot
surface temperature is developed in the current study, as proposed in Equation (10). This
model is expected to predict the initial location of ignition given various heated surface
temperatures when marine diesel leaks into a ship’s engine room.
H=H0+0.401ex p-Ts-Tsat
πgT0(10)
where His height at which the ignition of marine diesel initially occurs, m; H
0
is the
surface height of marine diesel, m; T
s
is the hot surface temperature, K; and T
0
is the initial
temperature of marine diesel, K.
J. Mar. Sci. Eng. 2024,12, 798 9 of 20
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where H is height at which the ignition of marine diesel initially occurs, m; H0 is the sur-
face height of marine diesel, m; Ts is the hot surface temperature, K; and T0 is the initial
temperature of marine diesel, K.
Figure 3. Heights at which marine diesel ignites with various heated surface temperatures.
To alleviate the issue of increasing hot surface-driven initial ignition height, the fol-
lowing steps can be taken:
(a) The on-site safety manager can regularly inspect the vicinity of pipelines, paying spe-
cial aention to thermal insulation measures to protect the hot surfaces of equipment
and thus preventing the ignition of accidentally leaked marine diesel by a hot surface;
(b) An existing monitoring gauge in ship engine room may be used. The research results
of this section suggest installing an area covering the vertical space of the high-tem-
perature surface of the equipment in the ship engine room in order to monitor the
possible initial ignition of a fire core in different positions.
3.2. Temperature Field Distribution according to HSI-Driven Flame Propagation
After the marine diesel is ignited on a high-temperature surface, the size of the fire
core rapidly increases, and flame propagation forms. According to the relationship be-
tween the initial temperature of combustible fuel and its flash point, flame propagation
on the liquid surface can be classified into liquid-phase controlled fire spreading and gas-
phase controlled fire spreading. If ignition occurs on a hot surface, the flame propagates
rapidly through premixed combustion. The size of the marine diesel leakage area on the
high-temperature surface has a small influence on the flame propagation. Figure 4 shows
the transmission of combustion flames after the marine diesel is ignited above the hot
surface. When marine diesel leakage occurs at 3.741 s, the fire nucleus generated in the air
begins to grow larger. Because the flame consumes more combustible vapor mixture, it
needs more fuel. The flame gradually begins to propagate in the direction of the hot sur-
face. This reflects the energy transfer from the flame in the chemical reaction zone to the
unburned gas mixture in the preheating zone. The flame trajectory of the marine diesel
vapor in the vertical space can be observed in Figure 4. When the flame is transmied, it
is not in the form of a small core but instead in the form of a larger fireball. The color of
the HSI-driven flame as it passes is a bright red–yellow, which is different from the blue
core produced during initial ignition. The HIS-driven flame surface is accompanied by the
consumption of components of marine diesel vapor and the generation of combustion
Figure 3. Heights at which marine diesel ignites with various heated surface temperatures.
To alleviate the issue of increasing hot surface-driven initial ignition height, the fol-
lowing steps can be taken:
(a)
The on-site safety manager can regularly inspect the vicinity of pipelines, paying spe-
cial attention to thermal insulation measures to protect the hot surfaces of equipment
and thus preventing the ignition of accidentally leaked marine diesel by a hot surface;
(b)
An existing monitoring gauge in ship engine room may be used. The research results
of this section suggest installing an area covering the vertical space of the high-
temperature surface of the equipment in the ship engine room in order to monitor the
possible initial ignition of a fire core in different positions.
3.2. Temperature Field Distribution according to HSI-Driven Flame Propagation
After the marine diesel is ignited on a high-temperature surface, the size of the fire
core rapidly increases, and flame propagation forms. According to the relationship between
the initial temperature of combustible fuel and its flash point, flame propagation on the
liquid surface can be classified into liquid-phase controlled fire spreading and gas-phase
controlled fire spreading. If ignition occurs on a hot surface, the flame propagates rapidly
through premixed combustion. The size of the marine diesel leakage area on the high-
temperature surface has a small influence on the flame propagation. Figure 4shows the
transmission of combustion flames after the marine diesel is ignited above the hot surface.
When marine diesel leakage occurs at 3.741 s, the fire nucleus generated in the air begins
to grow larger. Because the flame consumes more combustible vapor mixture, it needs
more fuel. The flame gradually begins to propagate in the direction of the hot surface. This
reflects the energy transfer from the flame in the chemical reaction zone to the unburned
gas mixture in the preheating zone. The flame trajectory of the marine diesel vapor in the
vertical space can be observed in Figure 4. When the flame is transmitted, it is not in the
form of a small core but instead in the form of a larger fireball. The color of the HSI-driven
flame as it passes is a bright red–yellow, which is different from the blue core produced
during initial ignition. The HIS-driven flame surface is accompanied by the consumption
of components of marine diesel vapor and the generation of combustion products and heat.
As the flame passes to the hot surface, the remaining marine diesel is further ignited.
J. Mar. Sci. Eng. 2024,12, 798 10 of 20
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products and heat. As the flame passes to the hot surface, the remaining marine diesel is
further ignited.
(a)
(b)
(c)
(d)
(e)
(f)
(g)
(h)
(i)
(j)
Figure 4. Flame propagation from ignition position to the hot surface. (a) t = 3.741 s; (b) t = 3.758 s;
(c) t = 3.933 s; (d) t = 4.091 s; (e) t = 4.174 s; (f) t = 4.299 s; (g) t = 4.333 s; (h) t = 4.424 s; (i) t = 4.541 s;
(j) t = 4.673 s.
During the thermal oxygen degradation process, it can be observed that the light-
weight components—such as water, trace lipids, and ethers—evaporate first as the hot
surface temperature increases. Figure 5 presents temperature field variation during ma-
rine diesel HSI as the temperature of the surface continues to increase. The specified ma-
rine diesel has 10 mL leakage, and the heated surface temperature ranges from 753 K to
778 K. The thermocouples are positioned above the center of the high-temperature surface
to measure temperatures at different heights ranging from 20 mm to 400 mm above the
hot surface. Figure 5 shows that as the heated surface temperature increases, the temper-
ature field distribution in the vertical space increases accordingly. The temperature field
distribution mainly changes between 3.011 s and 9.036 s after the marine diesel contacts
the hot surface, as indicated by the temperatures obtained at different times.
Figure 5a indicates the temperature field changes in the vertical space as ignition oc-
curs at 3.011 s. The thermocouple located 20 mm above the high-temperature surface ex-
hibits the fastest response to the temperature increase. As the surface is heated to temper-
atures from 753 K to 778 K, the air temperature at 20 mm changes from 532.81 K to 570.89
K. This indicates that the increase in air temperature is not the highest closest to the hot
surface. When the leaked marine diesel is evaporated by the hot surface, it diffuses to the
top of the hot surface to form a certain distribution of concentrated liquid and gas. Mean-
while, due to the temperature transfer generated by thermal diffusion and thermal con-
duction of high-temperature marine diesel and gas, a certain spatial temperature field is
formed. It can be seen from Figure 5a that because marine diesel is a large-mass molecule,
the heat transfer rate is greater than the mass transfer rate in the process of marine diesel
evaporation when the leakage is from 5 mL to 20 mL. As a result, the temperature in the
Figure 4. Flame propagation from ignition position to the hot surface. (a) t = 3.741 s; (b) t = 3.758 s;
(c) t = 3.933 s; (d) t = 4.091 s; (e) t = 4.174 s; (f) t = 4.299 s; (g) t = 4.333 s; (h) t = 4.424 s; (i) t = 4.541 s;
(j) t = 4.673 s.
During the thermal oxygen degradation process, it can be observed that the lightweight
components—such as water, trace lipids, and ethers—evaporate first as the hot surface
temperature increases. Figure 5presents temperature field variation during marine diesel
HSI as the temperature of the surface continues to increase. The specified marine diesel
has 10 mL leakage, and the heated surface temperature ranges from 753 K to 778 K. The
thermocouples are positioned above the center of the high-temperature surface to measure
temperatures at different heights ranging from 20 mm to 400 mm above the hot surface.
Figure 5shows that as the heated surface temperature increases, the temperature field
distribution in the vertical space increases accordingly. The temperature field distribution
mainly changes between 3.011 s and 9.036 s after the marine diesel contacts the hot surface,
as indicated by the temperatures obtained at different times.
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J. Mar. Sci. Eng. 2024, 12, x FOR PEER REVIEW 12 of 20
(a)
(b)
Figure 5. Cont.
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(c)
(d)
Figure 5. Temperature field distribution induced by marine diesel HSI. (a) t = 3.011 s; (b) t = 5.132 s;
(c) t = 7.312 s; (d) t = 9.036 s.
3.3. Heat Flux Intensity of the Marine Diesel HSI Process
The marine fuel surface radiant heat flux meter provides data on the thermal feed-
back of the flame to the fuel surface during flame propagation. The experimental data
quantifies the total thermal feedback received by the marine fuels. Figure 6 shows the data
collected by the heat flux meter located in different areas during the HSI of marine fuels.
In this experiment, the heat flux meter is arranged sequentially, with the hot surface at the
center. The heat flux meter closest to the hot surface is set at a height of 0.3 m, and the
horizontal distance from the hot surface center is 0.4 m. The seings of the other heat flux
meters are shown in Figure 6, which illustrates the variation of heat flux intensity with
time during the marine diesel HSI. This study analyzes the condition of 15 mL of marine
Figure 5. Temperature field distribution induced by marine diesel HSI. (a) t = 3.011 s; (b) t = 5.132 s;
(c) t = 7.312 s; (d) t = 9.036 s.
Figure 5a indicates the temperature field changes in the vertical space as ignition
occurs at 3.011 s. The thermocouple located 20 mm above the high-temperature surface
exhibits the fastest response to the temperature increase. As the surface is heated to
temperatures from 753 K to 778 K, the air temperature at 20 mm changes from 532.81 K
to 570.89 K. This indicates that the increase in air temperature is not the highest closest
to the hot surface. When the leaked marine diesel is evaporated by the hot surface, it
diffuses to the top of the hot surface to form a certain distribution of concentrated liquid
and gas. Meanwhile, due to the temperature transfer generated by thermal diffusion
and thermal conduction of high-temperature marine diesel and gas, a certain spatial
temperature field is formed. It can be seen from Figure 5a that because marine diesel is
J. Mar. Sci. Eng. 2024,12, 798 13 of 20
a large-mass molecule, the heat transfer rate is greater than the mass transfer rate in the
process of marine diesel evaporation when the leakage is from 5 mL to 20 mL. As a result,
the temperature in the vertical space of the hot surface rises very fast. At a vertical height
of 200 mm, the temperature goes up from an initial 437.07 K on the hot surface to 529.41 K
at the maximum temperature, resulting in a temperature increase of 21.14%. Based on the
data from the thermocouple farthest from the hot surface, the temperature varies from
an initial temperature of 416.39 K to a maximum of 484.46 K, presenting a range of over
16.34%. In conditions in which the hot surface temperature is below 750 K, the temperature
field’s distribution is mainly caused by the joint action of fuel vapor and hot surface heat
transfer, as there is no hot surface ignition behavior at 3.011 s. However, when the surface
is heated to a temperature over 751 K, the HSI behavior occurs within 3.0 s. Combustion
cores and flame transfer phenomena are observed in the vertical space. During flame
transfer over a hot surface, the flame continuously heats the combustible gaseous mixture
on the surface, maintaining the combustion process in the vertical space. Additionally,
the remaining liquid diesel on the hot surface is vaporized by the heat transmitted by the
flame. As a result, the temperature of the remaining diesel goes up, causing it to change
from a liquid to a gaseous fuel vapor. The vapor that becomes volatile accumulates above
the hot surface, leading to incomplete combustion with air due to the consumption of
auxiliary gas in the early combustion process. Once the flame spreads to the hot surface,
a fuel-rich core with a higher concentration and relatively lower temperature forms at
the bottom of the flame. The temperature field distribution shows significant variation in
the temperature increase between 200 mm and 300 mm in vertical height, indicating the
region of initial HSI. When marine diesel in only a small amount drops on a hot surface, the
liquid is decomposed by the action of heat generated by the hot surface to produce reactive
free radicals. Under the action of a pressure gradient and buoyance, steam containing
free radicals rises and mixes with air to form high-temperature combustible mixed gas.
Meanwhile, the concentration of combustible premixed gas gradually accumulates until it
reaches the limit of the concentration range of combustion. As the equipment surface is
heated to an elevated temperature, droplet evaporation continues. At a vertical height of 200
mm to 300 mm, when the hot surface’s temperature is lower than the critical temperature or
premixed gas does not meet the combustion limit, the ignition phenomenon will not occur.
Due to the action of air entrainment, the temperature at this height exhibits a declining
trend. However, marine diesel continues to evaporate and accumulate, leaving the fire
hazard. Once the minimum ignition temperature of the hot surface is reached, the HSI
phenomenon may occur. The change in the temperature field over time reveals higher
temperatures in the vertical space between 20 mm and 150 mm, leading the direction and
path of flame propagation.
3.3. Heat Flux Intensity of the Marine Diesel HSI Process
The marine fuel surface radiant heat flux meter provides data on the thermal feedback
of the flame to the fuel surface during flame propagation. The experimental data quantifies
the total thermal feedback received by the marine fuels. Figure 6shows the data collected
by the heat flux meter located in different areas during the HSI of marine fuels. In this
experiment, the heat flux meter is arranged sequentially, with the hot surface at the center.
The heat flux meter closest to the hot surface is set at a height of 0.3 m, and the horizontal
distance from the hot surface center is 0.4 m. The settings of the other heat flux meters are
shown in Figure 6, which illustrates the variation of heat flux intensity with time during the
marine diesel HSI. This study analyzes the condition of 15 mL of marine diesel leaked onto
a surface with a temperature of 753 K. Figure 6shows that as the contact time between the
marine diesel and the heated surface rises, the heat flux intensity shows a sudden increase
within a certain period; this is followed by decay in different forms. Heat flux intensity
begins to increase at 2.198 s in the area closest to the hot surface. During this period, the
region distant from the hot surface does not receive heat from the gas-phase medium due
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to the rapid phase transformation of the fuel on the hot surface. Between 2.6 s and 2.8 s, the
heat flux intensity near the hot surface reaches a maximum of approximately 8.42 kW/m
2
.
J. Mar. Sci. Eng. 2024, 12, x FOR PEER REVIEW 14 of 20
diesel leaked onto a surface with a temperature of 753 K. Figure 6 shows that as the contact
time between the marine diesel and the heated surface rises, the heat flux intensity shows
a sudden increase within a certain period; this is followed by decay in different forms.
Heat flux intensity begins to increase at 2.198 s in the area closest to the hot surface. During
this period, the region distant from the hot surface does not receive heat from the gas-
phase medium due to the rapid phase transformation of the fuel on the hot surface. Be-
tween 2.6 s and 2.8 s, the heat flux intensity near the hot surface reaches a maximum of
approximately 8.42 kW/m2.
Compared with the initial state, the rate of change in heat flux intensity is significant.
Based on observations, HSI of marine diesel occurs within the aforementioned time range,
leading to the formation of an initial fire nucleus. During flame propagation, the sur-
rounding area receives radiant heat from the flame. At a distance of 0.7 m and 1.0 m from
the center of the hot surface, the heat flux intensities are 5.88 kW/m2 and 1.91 kW/m2, re-
spectively. When the flame spreads to the high-temperature surface, it continues to trans-
fer heat flux to the surrounding space, resulting in stable heat flux intensity for a period
of time. As the flame spreads to the hot surface and burns the remaining marine diesel,
the heat flux intensity begins to decay. The position farthest from the center of heated
surface presents the most significant aenuation of heat flux intensity, dropping to 0.172
kW/m2 at 3.811 s. The results indicate that the heat flux intensity during fuel HSI is trans-
ferred to surrounding area. The area in close proximity to the hot surface may receive
intense radiant heat, which increases the risk of reigniting leaked fuel in adjacent areas.
Due to the properties of marine diesel, the peak value of heat flux intensity is not sustained
for a long time during the HSI process. The heat flux intensity begins to decay after the
flame propagates to heated surface of the equipment. Observing the thermal performance
of HSI-driven flame propagation, the following conclusions can be drawn:
(a) It is essential to pay aention to temperature change in the vertical space of equip-
ment surfaces (in which it is easy for a heated surface to form) in order to prevent the
accidental ignition of leakage. Safety supervisors in ship engine rooms must carry
out monitoring tasks during inspection using infrared thermal imaging techniques.
(b) According to results of this section, the heat flow generated by flames is transferred
to their surroundings in the horizontal direction, so it is particularly important to
protect inflammable and explosive substances near areas of ship engine rooms in
which incidents are likely to occur.
Figure 6. Heat flux intensities of marine diesel HSI changes with distance from the hot surface.
Figure 6. Heat flux intensities of marine diesel HSI changes with distance from the hot surface.
Compared with the initial state, the rate of change in heat flux intensity is significant.
Based on observations, HSI of marine diesel occurs within the aforementioned time range,
leading to the formation of an initial fire nucleus. During flame propagation, the surround-
ing area receives radiant heat from the flame. At a distance of 0.7 m and 1.0 m from the
center of the hot surface, the heat flux intensities are 5.88 kW/m
2
and 1.91 kW/m
2
, respec-
tively. When the flame spreads to the high-temperature surface, it continues to transfer
heat flux to the surrounding space, resulting in stable heat flux intensity for a period of
time. As the flame spreads to the hot surface and burns the remaining marine diesel, the
heat flux intensity begins to decay. The position farthest from the center of heated surface
presents the most significant attenuation of heat flux intensity, dropping to 0.172 kW/m
2
at 3.811 s. The results indicate that the heat flux intensity during fuel HSI is transferred
to surrounding area. The area in close proximity to the hot surface may receive intense
radiant heat, which increases the risk of reigniting leaked fuel in adjacent areas. Due to
the properties of marine diesel, the peak value of heat flux intensity is not sustained for a
long time during the HSI process. The heat flux intensity begins to decay after the flame
propagates to heated surface of the equipment. Observing the thermal performance of
HSI-driven flame propagation, the following conclusions can be drawn:
(a) It is essential to pay attention to temperature change in the vertical space of equipment
surfaces (in which it is easy for a heated surface to form) in order to prevent the
accidental ignition of leakage. Safety supervisors in ship engine rooms must carry out
monitoring tasks during inspection using infrared thermal imaging techniques.
(b)
According to results of this section, the heat flow generated by flames is transferred
to their surroundings in the horizontal direction, so it is particularly important to
protect inflammable and explosive substances near areas of ship engine rooms in
which incidents are likely to occur.
3.4. Effect of Hot Surface Temperature on the HSI Delay Time of Marine Diesel
The ignition delay time plays an important role in characterizing the ignition char-
acteristics of marine fuel. Various marine fuels exhibit different ignition characteristics
on high-temperature surfaces, and the time required for HSI has discrepancies. There
J. Mar. Sci. Eng. 2024,12, 798 15 of 20
are two basic prerequisites for the local ignition of marine fuel that has leaked onto a hot
surface. One is a suitable ratio of fuel vapor/air in the ignition area, which can promote
the formation of an initial ignition core. The other is a low-velocity environmental flow
field, which can ensure that the initial ignition of the fire core is stable and self-sustaining
in the local area, meaning the flame can achieve stable propagation. In a ship engine room,
there are more complex influencing factors. Previous research suggests that a vapor layer
may be formed between the fuel and the hot surface when the hot surface has a very high
temperature. This vapor layer decreases the heat transfer from the heated surface to the
fuel, resulting in a decreased evaporation rate of the liquid fuel. HSI delay time is attributed
to the interaction between the different fuels and temperatures of the equipment’s surfaces,
as shown in Equation (11) [34].
ws=Koscn
fuelcm
ox exp-Es
RTr(11)
where K
os
is the frequency factor of the combustion reaction; c
fuel
is the molar concentration
of combustible material, mol/m
3
;c
ox
is the molar concentration of oxidizer, mol/m
3
;E
s
is
the activation energy, J/mol; T
r
is the temperature of the chemical reaction, K; and nand m
are the reaction coefficients, respectively.
It is evident that the HSI delay time of combustible fuel is inversely proportional to
vapor concentration. The mathematical model of HSI delay time can be deduced using
the boiling heat transfer model and vapor plume model. Figure 7presents an evolution
of ignition delay time for marine diesel at various heated surface temperatures. The data
indicate a clear decrease in the ignition time of marine diesel vapor on heated surfaces
as the temperature increases. At lower heated surface temperatures, the ignition delay
time for diesel is longer, thus increasing the difficulty of ignition, which is consistent with
experimental observations. The reason for this is that the heated surface temperature does
not reach the critical temperature required to ignite the evaporating vapor of marine diesel.
Once the surface is heated to a temperature of 748 K, the ignition delay time of diesel vapor
is 3.692 s. A significant amount of marine diesel/air mixture gas accumulates in the vertical
space, and the probability of ignition reaches 50%. As the surface continues to be heated,
the phase transition of marine diesel absorbs a significant amount of heat, resulting in an
increase in the evaporation rate of fuel. This, in turn, reduces the time required to reach
critical ignition conditions. The experimental data reveal that the ignition delay time of
marine diesel is reduced to 2.048 s when the surface is heated to a temperature around 755 K.
If the equipment surface continues to rise up 768 K, the ignition delay time drops to 0.802 s.
Compared with the previous scenario, the ignition delay times decrease by 44.5% and
60.8%, respectively. The ignition delay time of marine diesel vapor decreases significantly
as the heated surface temperature increases from low to high. However, further increase
in the heated surface temperature does not result in a significant change in the ignition
delay time. As the surface is heated to a temperature of 773 K, the ignition delay time drops
below 0.58 s. After reaching this temperature, the ignition delay time remains around 0.48 s,
regardless of how much the heated surface temperature increases.
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