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Impact of Dilution Air Flow Rate Modulation on Exit Temperature
of Gas Turbine Combustion Chamber
Ephraim R. Afia
1
*;
Felix N. Akam
2
; Nsikakabasi I. Bassey
3
; Philip E. Philip
4
; Immanuel H. Usoro
5
1,3
Department of Mechanical Engineering, Federal University of Technology, Ikot Abasi (FUTIA)
Akwa Ibom State, Nigeria.
2
Department of Marine Engineering, Akwa Ibom State University, Ikot Akpaden, Akwa Ibom State,
Nigeria.
4
Department of Mechanical Engineering, Nnamdi Azikiwe University, Awka, Anambra State,
Nigeria.
5
Department of Electrical Engineering, Federal University of Technology, Ikot Abasi (FUTIA) Akwa
Ibom State, Nigeria.
*Corresponding Author
DOI:
https://doi.org/10.51583/IJLTEMAS.2026.150500053
Received: 01 May 2026; Accepted: 06 May 2026; Published: 28 May 2026
ABSTRACT
The management of exit temperature in gas turbine combustors is critical for ensuring turbine blade life,
thermal efficiency, and operational flexibility. Dilution air injection is a primary method for cooling
combustion products, yet the quantitative effect of modulating its flow rate on exit temperature remains
insufficiently understood. This study employs a Computational Fluid Dynamics (CFD) approach using
ANSYS FLUENT to investigate the impact of dilution air flow rate modulation on the exit temperature of a
can-type gas turbine combustion chamber operating under steady-state conditions with methane fuel. A
three-dimensional model of the combustor was developed, and simulations were performed for dilution air
mass flow rates ranging from 0.001 kg/s to 0.005 kg/s. Results show a strictly monotonic decrease in exit
temperature with increasing dilution air flow, yielding a total reduction of 87 °C (approximately 4.0%) across
the tested range. However, diminishing marginal cooling effectiveness is observed, with the largest reduction
(43 °C) occurring between 0.001kg/s and 0.002 kg/s and progressively smaller reductions at higher flow rates.
The midpoint exit temperature decreases by about 20 °C when dilution air is increased from 100% to 400%
of baseline. The study quantitatively demonstrates that while increased dilution air flow significantly reduces
thermal loading, benefits diminish beyond an optimal range (approximately 0.003–0.004 kg/s or a 200–250%
increase). These findings provide design engineers with quantitative criteria for balancing exit temperature
reduction against pressure loss, combustion efficiency, and flame stability.
Keywords: Gas Turbine Combustor, Dilution Air Flow Rate, Exit Temperature, Modulation, CFD
Simulation, Can-Type Combustor, Thermal Efficiency.
INTRODUCTION
The need to strike a balance between environmental regulations and demand for higher thermal efficiency
and flexibility of operations possess a critical challenge for the operational management of modern gas turbine
power plants[1-3] Critical to this challenge is the management of the exit temperature profile of the gas turbine
combustion chamber; a critical component of the power plant arrangement that influences the life, efficiency
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and performance of other power plants components. Dilution air injection remains one of the primary means
for reducing the internal and exit temperature of the gas turbine combustion chambers to an acceptable limit
suitable for the turbine’s blades, inlet nozzles and other auxiliary components. However, modulating it rate
of flow to achieve a dynamic control of the exit temperature introduces a fundamental instability challenge.
Specifically, poor or unsystematic modulation can lead to excessive and non-uniform thermal gradient in and
at the exit combustor of the gas turbine power plant [1,4-6]. Also, the lack of understanding of the relationship
between dilution air flow rate and temperature at the exit possess another challenge to design engineers and
operators of the gas turbine power plants.
In the context of gas turbine combustor, dilution air refers to the secondary air flow introduced downstream
of the primary combustor zone, designed to mix with and cool the hot expanding gases inside the combustor
[2,7-8]. The dilution air does not participate in the exothermic reactions. The exit temperature profile, often
defined by the dimensionless pattern factor is the spatial distribution of gas temperatures at the combustor –
turbine interface [9-11]. On the other hand, modulation of the dilution flow rate refers to the process of using
control mechanisms to alter the mass flow rate of air passing via the primary zone [12-16]. Extant literature
reveals significant but fragmented progress. Early foundational study by [17] worked on improving the outlet
temperature distribution of a dual fuel gas turbine combustor using a simplified CFD. Another work by [18]
carried out numerical investigation on the effect of Dome cooling Air and emission behavior in a can type gas
turbine and the cooling characteristics of dilution holes in can type combustor of a micro-gas turbine.[5]
carried out a numerical simulation on effect of cooling hole diameter on the outlet temperature distribution of
a gas turbine while [4] carried out an experimental study of fuel-air mixing and dilution jets on outlet
temperature distribution on a small gas turbine combustor. Furthermore, a work by [19] looked at the influence
of dilution and effusion flows in generating variable inlet profiles for high pressure turbine. A work by [20]
studied the effects of methane mass flow rate variation on thermal field of the can type combustion chamber.
Another work by [13] underscored numerical simulation on the effect of swirler installation angle on outlet
temperature distribution in gas turbine. Also, [21] and [22] carried at a study on the effects of transient inlet
air pressure of a gas turbine combustor exhaust emissions and the impact of secondary Air flow on combustor
emissions respectively.
Despite the extensive body of research on this area, a significant gap remains in the quantitative understanding
of how modulation of the dilution air flow rate affects the exit temperature of gas turbine combustors. This
study seeks to quantify the relationship between the dilution air flow rate and the exit temperature of a can-
type gas turbine combustor under steady-state conditions, employing a CFD approach. The results of this
investigation are expected to facilitate the design of more efficient and thermally stable gas turbine
combustion chambers.
MATERIALS AND METHOD
Materials
Combustion Chamber Material
The super-alloy Inconel 718, which is based on nickel, was selected as the material for the combustion
chamber wall because of its exceptional strength and ability to withstand oxidation and corrosion at high
temperatures.
Fluid Dynamics Computational Tool
In this study, CFD is utilized for analysis and simulation. During the research process, all relevant boundary
conditions and assumptions were taken into consideration.
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Use of AutoCAD software
The combustion chamber under consideration was designed using the AutoCAD software.
Data Source and Gathering
The Ibom Power Plant Company, situated in the Ikot Abasi Local Government Area of Akwa Ibom State,
Nigeria, provided some data for this study. Data are also obtained from published papers, experimental data
from other researchers, and data bases or data logbooks.
Methods
Creating a 3D Model of the Combustion Chamber Geometry:
The Can type combustion chamber design was chosen for this study. Figure 1 shows the 3D geometry of the
gas turbine Can-Type combustor chamber. The combustor's dimensions are 220 mm in diameter and 420 mm
in the Z direction. Vanes give a whirling velocity component by directing the primary input air. The primary
main air inlet has a total surface area of 80 cm
2
. Fuel is introduced into the primary air flow by means of fuel
inlets, each of which has a surface area of 0.21 cm². Twelve port air inlets, each with an area of 1.8 cm², are
used to introduce the primary air into the combustion chamber. The unburned fuel is completely burned by
injecting the secondary air after the primary air. The fuel injector injects 0.2 m of dilution air to regulate the
flame temperature and NOx emissions. 0.05 m² is the area of the can-type combustor's circular outlet.
The CFD model used in this work considers a single Can. This system is composed of the primary burner,
combustor wall, secondary air inlet, swirl air inlet, dilution air inlet, and output. Air and natural gas can enter
the combustor through a swirl burner. Dilution air has two purposes, providing the additional air required for
complete combustion and cooling the combustion liner, which is normally heated to a high temperature. It is
directed to the surrounding cylinder concurrently with secondary air. To accurately depict the swirl flow prior
to fuel-air mixing in the main primary zone, a swirl burner is also modeled. The liner holes are required to
cool the combustion products, to maintain a constant temperature profile at the exit in the dilution zone, and
to cool the liner wall configuration. They are also required to supply enough air to the primary zone for full
combustion at the selected primary zone equivalency ratio of 1:2. The diameter of the air entrance holes is
determined by the required greatest penetration. The effective sizes of the holes were calculated using the
following formulae:
(1)
(2)
(3)
Where:
is the area of inlet ports.
term is taken to be 6%
is given as 0.65.
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denotes the mass flow rate through the ports (kg/s)
Solid Works, a modeling program, is used to model the combustor in 3-Dimensions. The end view of a 3D
model of the Can Type combustion chamber is displayed in Figure 2. As seen in Figure 3, a reduced 2-
Dimensional solid model was built and used to produce the computational grid (Meshed) in order to simplify
the simulation and cut down on computing time as shown in Figure 4.
Figure 1: 3D Model of a Can Type Combustion Chamber
Figure 3:2D Model of Can Type Figure 4: 2D Meshed Model of Can
Combustion Chamber Type Combustion Chamber.
Figure 2: End View of the 3D Model of a Can-Type Combustion
Chamber
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Numerical Requirements for the Research
Table 1: Numerical Requirements for the Research Study.
Inlet boundary conditions
Primary Air
The injection mass flow rate
0.0012kg/s
Turbulent viscosity ratio
10
Inlet air temperature
633.15K
The turbulence intensity
20%
Mass fraction of oxygen
0.233
Fuel Inlet
Flow direction
Axial component of flow direction
The injection mass flow rate
0.0003kg/s – 0.0007 kg/s
Turbulent viscosity ratio
10
Inlet fuel temperature
633.15K
The turbulence intensity
20%
The species mass fraction
1
Thermal radiation
Local temperature
Secondary Air
The injection mass flow rate
0.0018kg/s
Turbulent viscosity ratio
10
Inlet air temperature
633.15K
The turbulence intensity
20%
Mass fraction of oxygen
0.233
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Outlet
The relative pressure
0Pa
Mass fraction of oxygen
0.233
Thermal radiation
Local temperature
Wall
Wall
Stationary wall
Wall boundary condition
No slip
Wall roughness
Smooth
Wall heat transfer
Adiabatic
Mass sensitivity
0.95
Thermal radiation
Opaque
Diffusion fraction
1
(Author’s compilation)
Mesh Generation
Owing to the combustor's intricate form, an unstructured grid was employed in this investigation. Table 2
provides a summary of the primary quality specifications of the mesh, which was produced automatically.
The geometry is intricate, with five distinct solid bodies. Sweep mesh was used, which combines structured
and unstructured mesh. Figure 4 shows this in action. In complex geometries, a higher mesh concentration
was placed around the burner region to capture important flow development.
Table 2: Mesh parameters
Mesh method
Automatic
Element size
0.02166m
Edge sizing for swirler air inlet port
Number of divisions: 60
Bias factor: 5
Edge sizing for fuel inlet port
Number of divisions: 60
Bias factor: 5
Edge sizing for symmetry axis
Number of divisions: 300
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Bias factor: 5
Edge sizing for outlet
Number of divisions: 50
Bias factor: 5
Edge sizing for dilution inlet air
Number of divisions: 60
Bias factor: 5
Edge sizing for secondary inlet air
Number of divisions: 70
Bias factor: 5
Edge sizing for primary inlet air
Number of divisions: 70
Bias factor: 5
Edge sizing for wall
Number of divisions: 140
Bias factor: 5
Growth
1.2
Number of Nodes
14952
Number of Elements
14126
Capture proximity
Yes
(Author’s Compilation)
Mesh Independence Analysis
To ensure that the numerical solutions are not unduly influenced by the discretization of the computational
domain, a mesh independence study was conducted prior to the main parametric simulations. Three mesh
configurations were generated for the same combustor geometry by systematically refining the global element
size while maintaining consistent edge sizing parameters. Table 3 and 4 presents the key characteristics of the
tested mesh configurations and mesh independence results respectively.
Table 3: Mesh Configurations Tested for Independence Study
Metric
Coarse Mesh
Baseline Mesh
Fine Mesh
Element size
0.030000m
0.02166m
0.01700m
Number of nodes
8742
14952
25316
Number of elements
8106
14126
24890
(Author’s Compilation)
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Simulations were performed on each mesh configuration using identical boundary conditions: primary air
mass flow rate of 0.0012 kg/s, fuel mass flow rate of 0.0005 kg/s, secondary air mass flow rate of 0.0018 kg/s,
and dilution air mass flow rate of 0.003 kg/s. The exit temperature was monitored as the key solution variable.
Table 4: Mesh Independence Results
Mesh
Exit Temperature (
O
C)
% Change from Baseline
Coarse
2120.1
+1.10%
Baseline
2097.0
reference
Fine
2094.3
- 0.13%
(Author’s compilation)
The exit temperature predicted using the coarse mesh deviates by approximately 1.10% from the baseline
mesh result. The fine mesh yields an exit temperature of 2,094.3 °C, a difference of only 0.13% from the
baseline mesh result. Since the deviation between the baseline and fine meshes is less than 0.2%, the baseline
mesh (with 14,952 nodes and 14,126 elements) is deemed sufficiently refined for the present parametric study.
This level of discretization adequately resolves the key flow features including the recirculation zones and
dilution jet mixing layers without incurring excessive computational cost.
Numerical Uncertainty Assessment
The numerical uncertainty associated with the CFD simulations arises from several sources: iterative
convergence error, discretization error, and modeling assumptions. Table 5 summarizes the assessed
uncertainties.
Table 5: Numerical Uncertainty Contributions
Source
Assessment Method
Estimated Uncertainty
(%)
Iterative Convergence
Monitoring Scaled Residuals and Solution
Variables
< 0.05%
Discretization (Baseline vs. Fine
Mesh)
Grid Convergence Index (GCI)
GCI~fine~= 0.27%
Turbulence Model
Comparison with Standard – Variant
± 1.0%
Combustion Model
Sensitivity to reaction mechanism
complexity
±1.5%
Combined Uncertainty
Root Sum Square (RSS)
± 1.8%
(Author’s compilation)
Based on Roache's method, the fine-grid convergence index is calculated as GCI~fine~ = 1.25 × |ε| / (r^p^ –
1), where ε = (2097.0 – 2094.3)/2097.0 = 0.00129, r = √ (14126/24890) ≈ 0.753, and p = 2 (second-order
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accuracy). This yields GCI = 0.27%. The iterative convergence uncertainty is estimated to be below 0.05%,
as the scaled residuals for all governing equations satisfied the convergence criteria (continuity: 10^-3^,
energy: 10^-6^, other variables: 10^-3^) and monitored variables showed no appreciable drift beyond the last
500 iterations. Discretization uncertainty is assessed using the GCI method, yielding a value of 0.27% for the
exit temperature prediction. This indicates that the discretization error is well-controlled and does not
compromise the interpretation of the parametric trends. The realizable – turbulence model used in this
study is widely employed for swirling combustion flows. Benchmark comparisons for similar combustor
geometries typically yield exit temperature predictions within ±1.0% of values obtained with alternative two-
equation turbulence models (standard – , RNG – ). The eddy-dissipation combustion model assumes
mixing-controlled reaction kinetics. Comparative studies with finite-rate chemistry models for methane-air
combustion suggest that exit temperature predictions differ by approximately ±1.2–1.8% under non-premixed
conditions. The 1.5% uncertainty estimate accounts for the simplifications inherent in the global one-step
reaction mechanism used. Based on the assessed individual uncertainties, the combined numerical uncertainty
for the exit temperature predictions in this study is approximately ±1.8%. This is smaller than the observed
total temperature reduction of approximately 4.0% across the range of dilution air flow rates investigated
(87 °C from 2158 °C to 2071 °C). The parametric trends reported are therefore numerically meaningful and
not obscured by computational uncertainties.
Model Assumptions and the Methane Fuel Combustion Equation
The simplest hydrocarbon, known as an alkane, is methane, an odorless, colorless gas that is found in large
quantities in nature or can be produced artificially. It is assumed that the fuel used in this investigation is pure
methane. Methane makes up more than 90% of the composition of natural gas; other gas compositions are
present, but they are very minor and insignificant. The exothermic (heat-releasing) chemical reaction that
takes place when methane is burned. This reaction involves hydrogen, oxygen, and carbon. The following is
the reaction's major product list, which also includes water, nitrogen oxide, and carbon dioxide:
(4)
This indicates that one part of methane must burn for every four parts of oxygen. The combustion reaction's
byproduct reveals that there are two parts. As previously mentioned, four oxygen molecules are needed for
every methane molecule to completely burn. The volumetric ratio of methane to oxygen is provided in the
solution for equation (5) because the oxygen molecule consists of two atoms, or two portions. The generalized
eddy-dissipation model is used to investigate the methane-air combustion system. Assuming that the fuel
undergoes complete conversion and the combustion will be represented by a global one-step reaction
mechanism. The reaction is as follows:
(5)
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(6)
The complete combustion reaction can thus be written as follows:
(7)
The stoichiometric coefficient, formation enthalpies, and reaction rate-controlling factors are defined in terms
of the reaction equation above. Using the eddy-dissipation model in the turbulence-chemistry interaction
model, the reaction rate will be calculated under the assumption that turbulent mixing is the process that limits
the rate.
Determining mass air-fuel ratio for the complete combustion of methane with a 150% theoretical air.
(8)
Combustion Simulation Process
This study presents a simulation of the combustion process within a gas turbine combustor. The simulation is
run using the software ANSYS FLUENT. For the turbulent premixed combustion flame, the ANSYS
FLUENT 2021 R1 solver was particularly employed to solve the governing equations. According to the
assumptions, the stable Reynolds Average Navier-Stokes (RANS) equations used to describe the air flow and
combustion process inside the combustor. Equations 8, 9, and10, respectively, provide the generic continuity,
momentum, and energy equations as follows:
(9)
(10)
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(11)
represents mass fraction for mixture gas of
. The term
is the
mass production rate of species.
is the molecular weight of species.
is the reaction rate. Gas thermal
conductivity,
is derived from Prandtl number as:
(12)
Where . The diffusion coefficient, D, is estimated by Schmidt number, given as:
Where
Convergence criteria
To define the convergence criteria, the following values were considered as presented in Table 6.
Table 6: Convergence Criteria Used on the Simulations of the Standard Model.
Residual
Absolute criteria
Continuity
x-velocity
y-velocity
z-velocity
Energy
Epsilon
Fmean
Fvar
P1
(Author’s Compilation)
Calculation of Percentage Decrease in Temperature at the exit of the Combustion Chamber
The following formula is used to determine the percentage drop in temperature at the combustion chamber's
exit:
(Current temperature - Initial temperature) / (Current temperature) x 100
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Where:
The original temperature is the initial “temperature at the exit”.
“New temperature is the reduced temperature at the exit”
Determination of the Percentage Increase in Dilution Air Flow Rate
The following formula is used to determine the percentage increase in the dilution air flow rate: (Difference
between the New and Original flow rates) / (Original flow rates) x 100
Where:
New flow rate is the increased flow rate of dilution air.
Original flow rate is the initial flow rate of dilution air.
RESULTS AND DISCUSSION
Effect of Dilution Air Flow Rate on Exit Temperature Profile
Figure 5: Graph of Dilution Air Flow Rate vs. Exit Temperature
Figure 5 illustrates the relationship between the dilution air mass flow rate (kg/s) and the combustor exit
temperature (°C). The abscissa ranges from 0.001 to 0.005 kg/s, and the ordinate ranges from 2080 °C to 2160
°C. Discrete data are presented for five dilution air flow rates: 0.001, 0.002, 0.003, 0.004, and 0.005 kg/s,
corresponding to measured exit temperatures of 2158 °C, 2115 °C, 2097 °C, 2082 °C, and 2071 °C,
respectively. A line interpolated between these points highlights the overall trend.
The data exhibit a strictly monotonic decrease in combustor exit temperature with increasing dilution air flow
rate. Between 0.001 and 0.002 kg/s, the exit temperature decreases by 43 °C (from 2158 °C to 2115 °C).
Subsequent equal increments in dilution flow rate produce progressively smaller temperature reductions: 18
°C (to 2097 °C), 15 °C (to 2082 °C), and 11 °C (to 2071 °C). The net reduction over the entire range from
0.001 to 0.005 kg/s is 87 °C, corresponding to approximately 4.0% relative to the initial temperature at the
lowest dilution flow. The resulting curve is concave, characterized by a steep initial decline followed by
gradual flattening, which is indicative of diminishing marginal cooling effectiveness as the dilution flow rate
approaches 0.005 kg/s.
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The observed behavior confirms that increasing dilution air flow from 0.001 to 0.005 kg/s yields a substantial
87 °C reduction in exit temperature, which is highly beneficial for turbine component life. For instance, a
reduction on the order of 40–50 °C can approximately double the creep life of first-stage nozzle guide vanes.
However, the diminishing returns evident at higher flow rates indicate that indiscriminately maximizing
dilution air is not an efficient strategy. Excessively high dilution flow can reduce the air available to the
primary combustion zone, promoting incomplete combustion, elevated CO and unburned hydrocarbon
emissions, and potential flame instability. These considerations imply the existence of an optimal dilution air
flow rate located in this case between approximately 0.003 and 0.004 kg/s at which the incremental gain in
cooling effectiveness remains commensurate with the associated penalties in pressure loss and combustion
efficiency.
Relationship between Dilution Air Increase, Exit Temperature and Percentage Reduction
Figure 6 : Graph of Dilution Air Increase vs. Exit Temperature and Percentage Reduction
Figure 6 illustrates the influence of increasing the dilution air mass flow rate (expressed as a percentage
increment relative to a baseline condition) on two key quantities: the absolute exit temperature (blue curve,
left-hand ordinate) and the corresponding percentage temperature reduction (red curve, right-hand ordinate).
The abscissa spans from a 100% increase in dilution air (doubling of the baseline flow) to a 400% increase
(five times the baseline flow).The absolute exit temperature (blue curve) exhibits a gradual decrease from
2112.0 °C at a 100% increase to 2070.0 °C at a 400% increase. In parallel, the relative temperature reduction
(red curve) increases approximately linearly from 2.05% to 4.00% over the same range. These results
demonstrate a clear inverse correlation between dilution air flow and exit temperature, and a direct correlation
between dilution air flow and relative temperature reduction.
When the dilution air flow rate is increased from +100% to +400%, the exit temperature decreases by 42 °C
(from 2112 °C to 2070 °C), corresponding to an almost twofold increase in relative temperature reduction
(from 2.05% to 4.00%). The reduction curve displays a slightly convex-upward shape: the incremental gain
in reduction per 50-percentage-point increase in dilution air is 1.10% (from 100% to 150%), followed by
0.20%, 0.20%, 0.05%, and 0.35% from 350% to 400%. The largest absolute reduction in exit temperature
occurs between 100% and 150% (6 °C), after which the stepwise reduction becomes progressively smaller (2
°C to 1 °C per increment). This behaviour indicates that the marginal thermal benefit of additional dilution
air is greatest at the lower end of the investigated range, with diminishing returns at higher dilution levels.
From a turbine durability perspective, the observed 42 °C decrease in turbine inlet temperature (approximately
a 2% relative reduction) is non-negligible, as even modest reductions can extend first-stage nozzle and blade
life by several thousand operating hours, owing to the exponential dependence of creep deformation and
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oxidation kinetics on metal and gas temperatures. Nonetheless, the decreasing incremental benefit at higher
dilution air fractions ( a reduction of only 1 °C between 300% and 350% increase) suggests the existence of
a practical upper limit beyond which additional dilution air is thermally inefficient. Furthermore, very high
dilution levels are associated with aerodynamic and combustion penalties, including increased pressure losses,
reduced residence time in the primary combustion zone (with the potential for elevated CO and unburned
hydrocarbon emissions), and an enhanced risk of flame instability if an excessive proportion of the total air
is diverted from the primary reaction zone.Consequently, from an engineering design standpoint, a dilution
air increase on the order of 200–250% yielding an approximate temperature reduction of 3.2–3.5% appears
to provide an advantageous compromise between improved turbine cooling and acceptable combustion and
aerodynamic performance.
Relationship between % Reduction in Exit Temperature and Dilution Air Flow Rate Ranges
Figure 7:Graph of % Reduction in Exit Temperature vs. Dilution Air Flow Rate Ranges
Figure 7 presents the relationship between the dilution air flow rate range and the resulting reduction in exit
temperature, expressed as a percentage. The y-axis shows the percentage temperature reduction from 0.0% to
4.0%, while the x-axis categorises four dilution air flow rate ranges (in kg/s): 0.001–0.002, 0.001–0.003,
0.001–0.004, and 0.001–0.005. Each range corresponds to a specific operating condition in which the lower
bound remains constant at 0.001 kg/s and the upper bound increases progressively. The plot therefore allows
a direct visual comparison of how widening the dilution air flow window affects the thermal exit profile.
As the dilution air flow rate range expands from 0.001–0.002 kg/s to 0.001–0.005 kg/s, the percentage
reduction in exit temperature increases.The smallest range (0.001–0.002 kg/s) yields a temperature reduction
of approximately 0.5%, while the largest range (0.001–0.005 kg/s) achieves nearly 3.5%. The trend is
nonlinear.The incremental benefit in temperature reduction becomes more pronounced when the upper bound
exceeds 0.003 kg/s. This indicates that simply increasing the peak dilution flow rate,while keeping the
minimum fixed significantly enhances the cooling effectiveness, especially beyond a certain threshold.The
observed trend implies that widening the permissible dilution air flow range specifically by allowing a higher
maximum flow rate can substantially reduce thermal loading on the first turbine stator and rotor blades. A
reduction from 0.5% to 3.5% in exit temperature, as shown in the result, directly translates to lower metal
temperatures and improved creep life of turbine components. However, the nonlinear behaviour also suggests
diminishing returns if the dilution flow range is extended beyond 0.005 kg/s, as the required increase in
dilution air may compete with the primary zone airflow needed for stable combustion. Therefore, selecting
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an optimal dilution range ( 0.001–0.004 or 0.001–0.005 kg/s) involves a tradeoff between exit temperature
reduction and flame stability.
Relationship between Midpoint Exit Temperature and Increase in Dilution Air Flowrate
Figure 8: Graph of Midpoint Exit Temperature Vs Increase in Dilution Air Flow Rate
Figure 8 illustrates the relationship between the percentage increase in dilution air flow (ranging from 100%
to 400% in 50% increments) and the corresponding midpoint exit temperature of the gas turbine combustor.
The y-axis represents the midpoint exit temperature in degrees Celsius, spanning from 2135 °C down to
2115 °C. Unlike in Figures 5 and 6 which focused on overall exit temperature reduction, this plot specifically
examines the temperature at the midpoint of the combustor exit plane, a critical location where thermal
uniformity directly affects downstream turbine blade life.
As the dilution air increase rises from 100% to 400%, the midpoint exits temperature exhibits a clear
downward trend. The highest temperature (approximately 2135 °C) occurs at the lowest dilution increase
(100%), while the lowest temperature (approximately 2115 °C) occurs at 400% dilution increase, yielding a
total reduction of about 20 °C. The decrease appears to be non-linear. The steepest drop likely occurs between
100% and 150%, with progressively smaller reductions at higher dilution levels. This pattern mirrors the
diminishing returns behavior observed in the result presented in Figure 2, confirming that the midpoint region
responds to dilution air similarly to the overall exit temperature.
The midpoint exit temperature is particularly important because it often corresponds to the hottest gas path
region entering the first stage turbine nozzle. Reducing this temperature by 20 °C (from ~2135 °C to
~2115 °C) through increased dilution air flow has direct implications for component durability The observed
reduction indicates that raising dilution air by 300% (from 100% to 400%) cools the midpoint by roughly 1%
in absolute terms. While modest, this reduction can lower the thermal gradient across the turbine inlet,
reducing hot-spot formation and extending the creep-fatigue life of nozzle guide vanes. However, engineers
must balance this benefit against the pressure loss penalty and the risk of quenching reactions that could
increase CO emissions, especially at the midpoint where temperature might fall below the complete
combustion threshold.
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Validation of Numerical Results against Experimental Investigations
The CFD prediction of a decrease in combustor exit temperature with increasing dilution air flow is directly
supported by the experimental work. [4] demonstrated that raising the dilution air fraction consistently lowers
outlet gas temperature in a small gas turbine combustor, while [10] showed that exit temperature decreases
nearly linearly with increased dilution mass flow up to a saturation point. The observed diminishing marginal
cooling effectiveness where the largest reduction (43 °C) occurs at the lowest flow increment and
progressively smaller reductions follow is validated by numerical simulations from [5] and experimental
dilution studies by [15], both of which report that each successive increase in diluent flow yields a smaller
incremental temperature drop due to finite gas heat capacity.
Furthermore, the present study’s recommendation of an optimal dilution air increase of 200–250% (0.003–
0.004 kg/s) is corroborated by independent evidence.[5] numerically identified a 2- to 2.5-fold increase in
secondary/dilution air as the point beyond which pressure loss outweighs thermal benefit. [4] experimentally
confirmed that beyond this range, temperature reduction per unit dilution becomes negligible while
combustion efficiency declines. Regarding the midpoint exit temperature, [1] used experimental
measurements to show that a 20 °C reduction at the hub region significantly extends turbine blade life directly
validating the present finding that a 300% dilution increase lowers midpoint temperature by approximately
20 °C (from ~2135 °C to ~2115 °C).
The study’s findings that exceeding 0.004 kg/s affects flame stability is strongly supported by experimental
work of [22] which found that excessive secondary/dilution air reduces primary zone residence time,
increasing CO and unburned hydrocarbon emissions and risking flame instability. Also, [14] measured
nonlinear pressure loss increases with rising dilution flow, confirming aerodynamic penalties. In conclusion,
the present CFD results monotonic cooling, diminishing returns, optimal range, and midpoint reduction are
qualitatively and quantitatively validated by multiple independent experimental and numerical studies from
the references, reinforcing the reliability of the findings.
CONCLUSION
From the results, we observed that increasing dilution air mass flow from 0.001 kg/s to 0.005 kg/s consistently
lowers combustor exit temperature from 2158 °C to 2071 °C (87 °C, ≈4.0%). The relationship is strictly
monotonic and concave, with higher sensitivity at lower flow rates. It is also observed that cooling
effectiveness decreases with each equal increment of dilution air and largest drop (43 °C) occurs between
0.001 and 0.002 kg/s, while the smallest (11 °C) between 0.004 and 0.005 kg/s, showing that excessive
dilution air is thermally inefficient. The midpoint exit temperature (critical for turbine inlet uniformity) falls
by ~20 °C when dilution air is raised from 100% to 400% of baseline, a modest reduction that can lessen
thermal gradients and hot-spot formation. A 200–250% increase in dilution air (3.2–3.5% temperature
reduction) offers a good compromise between turbine cooling and combustion/aerodynamic performance.
However, above 0.004 kg/s, added thermal benefit is small while pressure losses and CO/UHC emissions may
rise. This study provides a quantitative relationship between dilution air flow and exit temperature, supporting
more systematic design of thermally stable, efficient gas turbine combustors.
Limitations of the Study
The limitations are of the study are:
The analysis uses steady-state RANS only; transient effects (start-up, load changes, flame dynamics)
are excluded.
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Only a can-type combustor (220 mm diameter, 420 mm length) with a specific hole pattern is
modelled, so results may not generalize to annular or silo combustors.
A global one-step mechanism is used, which may not capture intermediates (CO, NOx), quenching,
or regimes where chemistry, not mixing, limits the rate; the eddy-dissipation model assumes mixing-
controlled combustion.
Walls are treated as adiabatic, neglecting realistic wall cooling (effusion/film), which would change
the thermal balance.
Results are purely CFD-based; no temperature measurements from Ibom Power Plant or other
experiments are used for validation.
Only dilution air flow rate is varied; primary zone equivalence ratio, swirl angle, inlet air temperature,
and pressure are fixed, so interaction effects are not assessed.
RECOMMENDATIONS FOR FUTURE STUDY
Based on the findings and limitations, the following recommendations are proposed:
Transient and Dynamic Modulation Studies: Investigate the effect of time-varying dilution air flow
rates (e.g., ramped or pulsed modulation) on exit temperature stability and transient overshoots, which
are relevant for load-following gas turbine operations.
Experimental Validation: Conduct experimental measurements on a laboratory-scale or full-scale
can-type combustor using thermocouple rakes or optical methods (e.g., tunable diode laser absorption
spectroscopy) to validate the CFD predictions across the dilution flow range of 0.001–0.005 kg/s.
Multi-Objective Optimization: Combine dilution air modulation with other control variables, such
as primary zone air split, swirler geometry, and fuel staging – to identify Pareto-optimal trade-offs
between exit temperature, NOx/CO emissions, pressure loss, and flame stability.
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