Methodology
The investigation starts by determining the fuel type, shape, and air and fuel input velocities, all of which depend on earlier studies of Byeonghun Yu et al. [46]. In order to replicate the combustion process, the author used these parameters to develop a 2D axisymmetric model in ANSYS Fluent. In order to better represent the conditions found in many industrial combustion systems, this model uses CH₄ (methane) as the fuel in a non-premixed arrangement with air. A 2D axisymmetric model is chosen to maximise computational efficiency while maintaining important flame structure features, providing in-depth understanding of the interactions between fuel and air under controlled circumstances. The study intends to establish repeatability in the examination of emissions along with flame behaviour and test its findings against prior studies by keeping consistent input circumstances.
Examining the effects of changes in the EGR percentage on emissions, especially NOx and other greenhouse gases, is a crucial component of this study. The impact of raising EGR on reducing emissions is examined using both experimental configurations and CFD simulations. Higher CO₂ concentrations from EGR contribute to a cooling impact that can limit NOx generation, and variations in EGR percentage are predicted to alter the in-cylinder combustion temperature in CFD study.
This study uses a 2D axisymmetric model created with ANSYS Fluent to perform a CFD simulation to analyse combustion with varied EGR levels, building on the work of Byeonghun Yu et al. [30]. The fuel, methane (CH₄), is combined with air to monitor combustion behaviour, especially when EGR is present. EGR values between 0% and 10% are evaluated in 5% increments to verify the correctness of the model. This variance enables the study to investigate the effects of small increases in EGR on emissions and combustion parameters. In order to precisely depict flame behaviour and flow dynamics, the model’s physical parameters comprise a bigger domain of 200 mm of diameter as well as 400 mm in height, a 9 mm nozzle diameter enabling fuel injection, and a 34 mm burner diameter enabling air flow.
Using the axisymmetric nature of this simulation to reduce the computing needs, the geometry for this configuration is designed in CATIA software, wherein just half of the model is originally constructed. This half-model method preserves symmetry while offering computing economy, and if required, it can be expanded to represent the entire model. The goal of this CFD simulation is to determine how EGR lowers NOx emissions along with additional combustion byproducts by applying EGR at particular rates and examining the subsequent flame behaviour.
Fig: Full model. Fig: Axisymmentric 2D Model .
In CATIA, the geometry of the combustion simulation is created using a surface model. The first step in the procedure is to create a simple schematic that shows the fundamental measurements and form of the combustion domain, comprising the burner, nozzle, and wider surrounding domain. The “Fill” command, that turns the sketched profile into a surface model, is used to generate the surface once the sketch is complete.
In this study, the oxidiser (air) has a flow rate of 1 x 10⁻⁵ m³/s, while the methane fuel (CH₄) has a flow rate of 5 x 10⁻⁶ m³/s. Under these particular flow conditions, the effects of EGR levels on combustion emissions along with flame characteristics can be controlled by mixing the EGR with the fuel stream. To make sure the simulation setup appropriately depicts real-world combustion behaviour, the model can be verified using existing information or literature benchmarks by initially utilising methane being the primary fuel. This validation stage aids in ensuring that the geometry, EGR mixing strategy , and, flow rates, produce accurate outcomes.
The 2D model is imported into ANSYS for additional analysis, and meshing is done to make the model discrete for numerical simulations. To improve the mesh’s precision and provide an accurate depiction of the geometry’s curvature and behaviour under various circumstances, quadratic components are used. To get the necessary resolution, 150 pieces are proposed along the model’s length, with an edge size of 1 mm. With 14,100 elements along with 14,345 nodes in the final mesh, the model is clearly specified for the analysis. In order to correctly simulate physical processes that will be assessed in later stages, that include fluid dynamics, heat transport, and structural behaviour, this mesh model is essential. The meshing model and geometry are shown below.
Fig: Geometry model. Fig: Meshed model.
The inlets and outflow are defined using the proper boundary conditions in order to replicate the flow of fuel and oxidiser into the domain. Fuel enters the domain at Inlet 1, while air (oxidiser) enters the system at Inlet 2. The Outlet is where the combined combustion products from the mixing of the two streams (air and fuel) leave the domain. The temperature, species concentrations, and mass flow rate of the fuel and air are all regulated by the boundary conditions at the inlets. The circumstances at the outlet define the pressure and flow direction, guaranteeing an accurate depiction of the combustion process. The pictures below go into depth about these boundary conditions, which are essential for correctly simulating the fluid dynamics, combustion, and temperature behaviour inside the domain.
Fig: Boundary conditions.
The species transport model is used to simulate the chemical processes that take place during burning in the combustion model, which is carried out in ANSYS FLUENT. Non-premixed combustion is taken into consideration, in which the oxidiser (air) and fuel (methane) are mixed inside the combustion zone after being supplied into the domain separately. The mass fractions of these species are calculated using the fuel composition, which include nitrogen (N2), oxygen (O2), and methane (CH4). To precisely depict the combustion process and integrate the chemical reaction rates, a Probability Density Function (PDF) calculator is utilised. Predicting the temperature, velocity, as well as species distribution in the combustion domain requires an understanding of the intricacies of mixing and combustion, which this model addresses.
The boundary conditions at the inlets are precisely set to control the air and fuel flow into the area. While intake 2 supplies the air, inlet 1 is designated for the fuel (methane). To guarantee adequate fuel and oxidiser mixing, the equivalency ratio is given together with the fuel and air velocity and temperature. Because they control the temperature within the reacting species and the fuel-to-air ratio, these parameters are crucial for correctly recreating the combustion process. In order to depict the outflow condition for the air domain, the outlet boundary condition has been adjusted to a pressure of 0 Pa. The combustion process is propelled from the high-pressure areas at the inlets to the low-pressure areas at the outlet by the pressure differential between the inlets and the outlet.
The simulation starts with a hybrid initialisation to get the computational domain ready for analysis after the boundary conditions are established. In order for the solution to converge, this initialisation phase aids in establishing an initial estimate for the flow field and temperature conditions. The simulation is performed after initialisation, and the results are retrieved to show how different factors, including temperature, velocity, along with species concentrations, behave.
Fig: Temperature distribution.
The temperature distribution of the combustion region is shown in the graphic above, with the combustion process leading to a maximum temperature of 907 K. At Inlet 1, methane (CH4), the fuel, is delivered at a speed of 1 m/s and an inlet temperature of 300 K. When the mixture percentage is set to 1, the combustion zone is fully mixed with fuel and air. Air is injected at Inlet 2 at a temperature of 300 K and a significantly greater velocity of 30 m/s. The observed temperature increase of up to 907 K is the result of the high air entry velocity, which promotes effective mixing and improves the combustion process.
Fig: OH concentration
The hydroxyl radical (OH) concentration is seen in the figure above, reaching a high of 4.47e-8. During combustion, OH radicals are created at the point where the oxidiser (air) and fuel (methane) contact. Since they are essential intermediates in the creation of combustion products, these radicals are vital to the reaction kinetics of combustion. Since the highest amounts of chemical activity are seen at the interface among the fuel and oxidiser, the high concentration of OH suggests that intensive chemical reactions are taking place in the combustion zone. Because the existence and concentration of OH radicals affect the efficiency and completeness of combustion as well as the creation of pollutants like NOx, this OH distribution is crucial to comprehending the dynamics of combustion.
Fig:C0 mass fraction along the center axis. Fig:N2 mass fraction along the center axis.
Fig:C0 mass fraction along the center axis. Fig:N2 mass fraction along the center axis.
In order to analyse the emission behaviour as well as the overall performance of the combustion process, plots of species distributions such as CO, H2O, N2, CH4, and O2 are created along the centre of doamin. In order to maximise system performance and guarantee environmental compliance, these plots offer important insights into the efficiency of combustion and emission characteristics.