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Journal of Advanced Research in Fluid Mechanics and Thermal Sciences 126, Issue 2 (2025) 62-72
62
Journal of Advanced Research in Fluid
Mechanics and Thermal Sciences
Journal homepage:
https://semarakilmu.com.my/journals/index.php/fluid_mechanics_thermal_sciences/index
ISSN: 2289-7879
Simulation of Liquid NH3 Spray Characteristics for Gasoline Direct
Injection (GDI) under Engine-Relevant Conditions
Nuraqilah Zahari1, Bukhari Manshoor1,*, Mohamad Hafizuddin Roslan1, Azwan Sapit1, Izzuddin
Zaman1, Djamal Hissein Didane1, Reazul Haq Abdul Haq1, Kamarul-Azhar Kamarudin1, Abdul Rafeq
Saleman2, Rio Marco Rathje3, Christin Rothe3, Mohd Nizam Ibrahim4
1
Faculty of Mechanical and Manufacturing Engineering, Universiti Tun Hussein Onn Malaysia, Batu Pahat, Johor, Malaysia
2
Faculty of Mechanical Engineering, Universiti Teknikal Malaysia Melaka, Durian Tunggal, Melaka, Malaysia
3
Institute for Regenerative Energy Technology (in.RET), Nordhausen University of Applied Sciences, Nordhausen, Germany
4
Maxpirations (M) Sdn Bhd, Sura Gate Commercial Centre, Dungun, Terengganu, Malaysia
ARTICLE INFO
ABSTRACT
Article history:
Received 29 August 2024
Received in revised form 18 December 2024
Accepted 29 December 2024
Available online 20 January 2025
Fuel/air mixing characteristics of liquid ammonia under direct injection engines have
gained significant attention in the automotive industry due to their potential for
improving fuel efficiency and reducing emissions. This study employed a simulation
approach to investigate the spray characteristics of liquid ammonia for Gasoline Direct
Injection (GDI) under Engine-Relevant Conditions. Conversely, lower injection pressures
resulted in shorter spray penetration due to reduced fuel momentum and weaker
atomisation. The main variable in this study is injection pressure, which is 50 bar, 80 bar
and 110 bar. Meanwhile, the parameters for the orifice diameter used in the CFD
simulation by Ansys Fluent are 0.3 mm and the ambient pressures, which are
atmosphere pressure and the other hand, remain constant throughout the research. The
simulation results showed a clear relationship between injection pressure and spray
penetration. Higher injection pressures increased spray penetration, indicating improved
fuel dispersion and atomisation. This is attributed to the higher kinetic energy of the fuel,
leading to enhanced breakup and smaller droplet sizes.
Keywords:
Liquid ammonia spray; GDI injector;
fuel dispersion; computational fluid
dynamics (CFD); spray penetration
1. Introduction
On average, a standard passenger vehicle emits about 4.6 metric tonnes of carbon dioxide
annually. Sustainable alternative fuels can help reduce emissions and make transportation more
environmentally friendly. One promising option is Gasoline Direct Injection (GDI) technology, which
improves fuel efficiency and control [1-3]. Ammonia and hydrogen are highly promising clean fuel
alternatives as they do not produce carbon dioxide emissions when burned and have other desirable
properties [4-7]. These advancements align to minimise the negative effects of combustion on
climate change and ensure a sustainable transportation future [8-10].
* Corresponding author.
E-mail address: bukhari@uthm.edu.my
https://doi.org/10.37934/arfmts.126.2.6272
Journal of Advanced Research in Fluid Mechanics and Thermal Sciences
Volume 126, Issue 2 (2025) 62-72
63
Burning petroleum fuels releases harmful gases and particles like carbon monoxide, carbon
dioxide, nitrogen oxides, and solid particles, posing environmental and human health risks. Ammonia
shows potential as a fuel alternative, but challenges, such as its toxicity, flammability, and high
production costs from renewable sources, need to be addressed [11-13]. Establishing infrastructure
for ammonia storage, distribution, and utilisation is lacking and would require significant investment.
Ammonia has a lower energy density of 12.7 MJ/L, requiring larger quantities for the same distance,
leading to increased costs and logistical complexities [14-16]. Additionally, engines would need
modifications to run on ammonia. While ammonia combustion avoids carbon dioxide emissions, it
produces nitrogen oxides, harmful pollutants with environmental consequences [17-19]. Overcoming
these obstacles and conducting research and development efforts are crucial for practically adopting
ammonia as a sustainable fuel option.
This project aims to model the injector tip and chamber geometry and simulate the spray
injection fuel process using computational fluid dynamics (CFD). The objectives are to determine
ammonia's nozzle flow and spray characteristics at various conditions, focusing on mixture
formation. The scope of the study involves conducting CFD simulations to analyse the combustion
processes of these fuels. The nozzle parameters are kept constant, while the choice of fuel, inlet
pressure (100 bar), and chamber pressure (10 bar) are the main variables. The project's primary goal
is to investigate a spark ignition injector's spray pattern and atomisation characteristics under
different conditions using computational analysis compared with the experimental approaches.
ANSYS Fluent software is utilised for simulations, comprehensively analysing the combustion process.
By combining computational and empirical methods, the project aims to enhance understanding of
the spray pattern and atomisation process in spark ignition injectors, contributing to knowledge
about the combustion dynamics of ammonia fuels.
2. Methodology
The simulation was carried out on a direct injection fuel using fluid simulation software, Ansys
Fluent 2022 R1. For the simulation, iso-octane was used as a fuel. As discussed previously, the
simulation of spray characteristics of a Gasoline Direct Injection (GDI) injector is a useful tool for
predicting and understanding spray behaviour and interaction with the combustion chamber. CFD
modelling may offer detailed information on the spray pattern, droplet size, velocity, temperature,
and fuel vaporisation, which can be used to optimise the design and performance of GDI injectors.
CFD consists of three major stages: pre-processing, solver, and post-processing [20-22]. Each level is
linked. It began with pre-processing created output that would later be utilised as input in other
software.
The geometry modelling required to execute a simulation to meet the finite different analysis
was done at the pre-processing step. The simulation is carried out by defining the boundary
conditions based on the research. Meshing can also be selected between coarse and fine-level
meshing. Meanwhile, during the processing step, the governing equation is solved using a solver in
ANSYS Fluent CFD software, and data and results are gathered and shown. Transport equations of
mass, momentum, and energy-based are shown in Eq. (1), Eq. (2) and Eq. (3), respectively, on the
conservation law derived in the Eulerian frame [23].
imass
i
uS
tx
+=
(1)
Journal of Advanced Research in Fluid Mechanics and Thermal Sciences
Volume 126, Issue 2 (2025) 62-72
64
,
i j ij
imom i
j i j
uu
uS
t x x x
+ = − + +
(2)
jj
im
ij t m energy
j j j j j j j
m
u e u uY
eT
p K D h S
t x x x x x x x
+ = − + + + +
(3)
where 𝜌, 𝑝, 𝑇, and 𝑒 are the carrier phase's density, pressure, temperature, and internal energy,
respectively, and 𝑢𝑖 is the velocity component along 𝑥𝑖 direction. 𝜎𝑖𝑗 is the viscous stress tensor. 𝐾𝑡 is
the turbulent conductivity, 𝐷 is the mass diffusion coefficient. 𝑌𝑚 and ℎ𝑚 denote the mass fraction
and enthalpy of species 𝑚, respectively. 𝑆mass, 𝑆mom,
𝑖
and 𝑆energy are, respectively, the mass,
momentum and energy source terms attributed to the gas-liquid interaction. The simulation process
was summarised in the flowchart shown in Figure 1.
Problem statement
Literature review
Geometry modelling
Meshing
Boundary condition
Simulation of Iso-Octane
Spray
Validation
Data validation
Plotting the graph
Satisfy
Data
analysis
Discussion &
Recomendation
End
Start
Yes
Yes
No
No
Fig. 1. Flowchart for the research of liquid NH3 spray
characteristics for GDI
2.1 Simulation Model and Meshing
The simulation involved modelling the injector tip in the combustion chamber using various mesh
sizes ranging from 4mm to 10mm. Ammonia was used as the injector fluids. The purpose of this
simulation was to calculate the length and width of the penetration, so the injector nozzle was
designed to match the actual geometry of the injector. The injector was represented as a hollow cone
injector in the model. Figure 2 shows the design of the injector proposed for the simulation works
and the meshing applied. For the meshing, tetrahedral meshing was used to suit the complexity of
the simulation model. Figure 2(a) is the simulation model, including the injector and the combustion
chamber. Figure 2(b) and Figure 2(c) are the details of meshing at the critical location in the model,
which is at the tip of the injector area.
Journal of Advanced Research in Fluid Mechanics and Thermal Sciences
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Grid Independent Test (GIT) was performed to ensure a precise and valid result in the CFD
processor by perpetuating the GIT as low as possible without disturbing the result, as shown in Table
1. There grids with different base sizes are employed for comparison. The first grid, considered as the
coarse mesh, is an isotropic mesh with element size of 15 mm. The second grid, denoted as the
medium mesh, is an isotropic grid with element size of 7 mm while the third grid, called as the fine
mesh, is an isotropic grid with element size of 2 mm. The last grid is the finest mesh where it is an
isotropic grid with element size of 0.2 mm. For all the grids, three level of AMR based on velocity
gradient is adopted. The ammonia liquid and vapor spray average velocity predicted by the four grids
are compared to the experimental results [x]. As can be seen, the differences of the results predicted
by the four grids are small, especially for the finest meshes. Thus, the finest mesh with a base cell
size of 0.2 mm is adopted in the present study in compromise of accuracy and efficiency.
(a)
(b)
(c)
Fig. 2. Simulation model for injector and meshing; (a) Injector and chamber, (b) The
meshing of the model, (c) Meshing at the nozzle injector
Table 1
Grit independent test
Element
size
No. of
nodes
No. of
elements
Skewness
Average
velocity (m/s)
Absolute
error (%)
Relative
error (%)
15
628691
1839141
0.89
253.6828
0.014
0.027
7
628914
1840927
0.85
257.6310
0.009
0.017
2
1208892
3411718
0.85
260.2362
0.008
0.015
0.9
5307504
15004101
0.88
254.1956
0.002
0.003
Combustion
chamber
Injector
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2.2 Boundary Conditions
Boundary conditions are crucial specifications that define the entry or exit conditions for fluid and
air in an injector. These conditions include parameters such as the inlet pressure, fluid flow velocity,
and temperature. Other boundaries, such as the injector walls, must be defined. In some cases, heat
transfer on the injector wall may also need to be considered and accounted for in the boundary
conditions [24-26]. In fluid flow, a solid wall is a fundamental boundary condition. The walls of the
injector nozzle and spray chamber are designed to remain stationary because there is no wall motion
in this case. Furthermore, the injector may experience the prescribed shear friction for the fluid flow.
Figure 3(a) depicts the wall described for this investigation.
The spray injector's inlet selection is shown in Figure 3(b). In this case, the inlet selection is the
mass flow rate, and after the mass flow enters the domain, the fluid velocity, pressure, or mass flow
rate can be computed. The outlet boundary's pressure output shown in Figure 3(c) will be chosen as
the bottom of the chamber and the surface at the end of the injector nozzle exit. The fluid flow will
penetrate the domain after entering through the nozzle hole of the injector.
(a)
(b)
(c)
Fig. 3. Computational domain and boundary condition; (a) Wall, (b) Inlet section, (c) Pressure
outlet section
2.3 Method of Solution
The CFD code package Ansys Fluent was used in this study. A density-based solver coupled with
a modified pressure implicit with splitting of operators (PISO) method is used to solve compressible
flow equations. A second-order central difference method is applied for temporal discretisation,
while a second-order upwind scheme is used for spatial discretisation. The RNG 𝑘 − 𝜀 turbulence
model is used to predict turbulent flow characteristics for the turbulent model. The physical
properties of the injected fuel influence the flow and cavitation characteristics in the injector nozzle,
which is reflected in the mass flow rate, effective nozzle diameter, and velocity at the nozzle exit. For
spray simulations, the effect of in-nozzle flow on near-spray behaviour can be determined by the
nozzle discharge coefficient (Cd), injection flow rate profile, and initial droplet size distribution, which
are widely employed in similar spray studies and show reasonable results [27-29]. In addition, a
Combustion
chamber
Injector
Wall section
Combustion
chamber
Inlet
Pressure
outlet
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variable time step methodology based on the Courant–Friedrichs–Lewy (CFL) number is adopted to
accelerate the computations while maintaining good accuracy [30].
3. Results and Discussion
The simulation results for liquid NH3 spray characteristics for Gasoline Direct Injection (GDI) under
Engine-Relevant Conditions will be discussed in this section. As the goal of this study is to analyse and
predict the simulation spray from a GDI injector into the atmosphere, with a focus on spray at short
injection periods, several characteristics will be discussed, which are the pressure on diameter and
length velocity penetration of GDI spray, an effect of spray injected into an atmosphere and the effect
of injection pressure and duration on volume fraction penetration. The velocity, volume fraction, and
spray penetration were measured at injection pressures of 50 bar, 80 bar, and 110 bar and at various
injection durations. As mentioned in the methodology section, a single-hole injector with a diameter
of 0.30 mm was used. The primary component used in the simulation was iso-octane, a commonly
used fuel surrogate. The simulation used a 3D multiphase volume of fluid (VOF) because it has three
phases: air, iso-octane liquid, and iso-octane vapour.
3.1 Diameter and Length of Velocity Penetration of GDI Spray
One of the essential characteristics that are interesting to investigate in this study is the diameter
and length of velocity penetration of GDI spray. Spray penetraon is the distance the fuel spray exits
in the injector nozzle before it spreads out or widens. The values for each injecon pressure on the
diameter and length of the velocity penetraon were ploed into graphs, as shown in Figure 4, at the
fuel injecon locaon in the chamber. The spray's distance or length from the injector nozzle before
it begins to disperse is inuenced by injecon pressure. The spray oen travels farther before it
spreads or disperses when injecon pressure is increased because greater velocies penetrate at
higher injecon pressures. However, penetraon is also inuenced by the length of the injecon. As
the gasoline is injected and propelled forward for a longer me, a longer injecon duraon enables a
larger spray penetraon distance. As a result, compared to injecon pressures of 50 bar, length
velocity penetraon tends to rise at 80 bar and 110 bar injecon pressures. Increasing injecon
pressure typically results in a narrower spray width. Higher pressures improve the atomisaon
process, producing smaller, more concentrated droplets in the core of the spray cone.
Fig. 4. Length and width velocity penetration at different injection durations and injection pressure
0
0.01
0.02
0.03
0.04
0.1 0.2 0.3 0.4 0.5
Length (m)
Time (s)
50 bar
80 bar
110 bar
0
0.002
0.004
0.006
0.008
0.01
0.012
0.014
0.1 0.2 0.3 0.4 0.5
Width (m)
Time (s)
50 bar
80 bar
110 bar
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3.2 Effect of Spray Injected into Atmosphere at Different Injection Pressure and Duration
The results for the influence of atmospheric pressure on the spray penetration at various injection
pressures and durations are shown in Figure 5 to Figure 7. All the figures obtained by the simulation
show that a shorter injection length may produce larger droplets, whereas a longer duration may
encourage finer atomisation and smaller droplets. As a result, a longer injection duration permits a
longer spray penetration distance since the fuel is injected and driven forward for a greater duration
of time.
0.1 s
0.2 s
0.3 s
0.4 s
0.5 s
Fig. 5. Velocity penetration into the atmosphere at different injection durations with Pressure = 50 bar
0.1 s
0.2 s
0.3 s
0.4 s
0.5 s
Fig. 6. Velocity penetration into the atmosphere at different injection durations with Pressure = 80 bar
0.1 s
0.2 s
0.3 s
0.4 s
0.5 s
Fig. 7. Velocity penetration into the atmosphere at different injection durations with Pressure = 110 bar
3.3 Effect of Injection Pressure and Duration on Volume Fraction Penetration
The eect of injecon pressure and its duraon on volume fracon penetraon were ploed into
graphs, as shown in Figure 8, at the fuel injecon locaon in the chamber. Again, it shows that the
Journal of Advanced Research in Fluid Mechanics and Thermal Sciences
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spray's distance or length from the injector nozzle before it begins to disperse is inuenced by
injecon pressure. Increasing injecon pressure typically results in a narrower spray width. Higher
pressures improve the atomisaon process, producing smaller, more concentrated droplets in the
core of the spray cone.
Fig. 8. Length and width volume fraction penetration at different injection durations and injection
pressure
Volume fraction penetration into the atmosphere is shown in Figure 9 to Figure 11 at various
injection pressures and injection durations. Wider spray penetration into the atmosphere is typically
encouraged by higher injection pressures. The spray can travel farther before dispersing due to
increased pressure, higher fuel velocities, and momentum. This deeper penetration may present a
larger fuel volume fraction in the surrounding air. The injection duration also affects the spray's
volume fraction penetration. A longer injection duration enables a higher fuel volume fraction to be
delivered, resulting in greater fuel dispersion and distribution in the surrounding environment.
Remembering that there can be an ideal injection duration that achieves a balance between fuel
dispersion and penetration depth is crucial. It's necessary to consider additional variables, such as
fuel characteristics, injector design, spray angle, and environmental circumstances, that may affect
the volume penetration of the spray.
0.9 s
1.0 s
1.1 s
1.2 s
1.3 s
Fig. 9. Volume fraction penetration at different injection durations with Pressure = 50 bar
0
0.001
0.002
0.003
0.004
0.005
0.006
0.007
0.9 1 1.1 1.2 1.3
Length (m)
Time (s)
50 bar
80 bar
110 bar
0
0.0002
0.0004
0.0006
0.0008
0.001
0.0012
0.0014
0.0016
0.9 1 1.1 1.2 1.3
Width (m)
Time (s)
50 bar
80 bar
110 bar
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0.9 s
1.0 s
1.1 s
1.2 s
1.3 s
Fig. 10. Volume fraction penetration at different injection durations with Pressure = 80 bar
0.9 s
1.0 s
1.1 s
1.2 s
1.3 s
Fig. 11. Volume fraction penetration at different injection durations with Pressure = 110 bar
4. Conclusions
This study's goal is to use CFD to investigate how atmospheric pressure affects the spray
characteristics of fuel under various conditions, including the injection pressure in a constant volume
chamber. The results demonstrate that the spray characteristics of the iso-octane spray are affected
by both ambient and injection pressure. The simulation flow of the fuel injection through the nozzle
spray before the combustion process that took place inside the combustion chamber is depicted in
this study. This simulation was triggered using a single-hole injector with a 0.3 mm diameter. Three
distinct injection pressures were set for the nozzle 50 bar, 80 bar, and 110 bar. The injection pressure
affects the diameter and length of the fuel's penetration. The spray quickly evaporates and disperses
as it gets larger, producing a wider spray angle. This study revealed that the 0.3 mm orifice diameter
had a broad spray cone angle. As a result, we can conclude that rising ambient pressure causes the
spray cone angle to rise. The fuel spray's lateral spread exiting the injector nozzle is called the "spray
width". You can measure variations in spray width by adjusting the injection pressure. Due to
increased atomisation and greater fuel velocities, higher injection pressures frequently lead to
narrower spray widths. The coverage and distribution of the spray can be learned by measuring its
width. Spray penetration is often increased by increasing injection pressure. In conclusion, injector
pressure impacted the spray penetration and width angle. We can conclude this investigation by
saying that
i. Ammonia has a greater saturated vapour pressure and is more sensitive to fuel temperature
than other alternative fuels. Thus, ammonia has a tendency to flash boil. A more accurate
flash boiling model that takes into account the influence of flash boiling on both primary and
secondary breakup characteristics should be developed in order to anticipate the ammonia
fuel/air mixing characteristics under a wide range of ambient circumstances.
Journal of Advanced Research in Fluid Mechanics and Thermal Sciences
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ii. The flash model, which only takes into account the enhanced evaporation effect, cannot
capture the ammonia spray characteristics, and there are no discernible differences in spray
macroscopic and microscopic properties between the results from the normal evaporation
model and the flash boiling model.
iii. Furthermore, spray included angle is an important parameter in the LPT simulation
framework that should be lowered to account for the spray collapse effect in multi-plume
sprays under intense flash boiling circumstances.
Acknowledgement
This research was supported by the Ministry of Higher Education (MOHE) through the Fundamental
Research Grant Scheme (FRGS/1/2022/TK08/UTHM/02/14), Vot K434. We also want to thank the
Universiti Tun Hussein Onn Malaysia via sabbatical leave scheme, the Institute for Regenerative
Energy Technology (in.RET), Nordhausen University of Applied Sciences and Maxpirations (M) Sdn
Bhd for supporting data and technical advice.
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