**2.5. CFD simulation**

diesel engine, the flame is intermittent non-premixed reaction while the flame in gas turbine is more lean and premixed reaction. The study done on this gas turbine shows that biodiesel can be used for operation [33]. The structure of a gas turbine with injectors are placed at designated location. Fuel spray is injected adjacent to the combustion air in a confined area. The presence of the preheated combustion and swirling air is critical in promoting droplet

The atomization test rig was designed to achieve the atomization characteristics spray of biodiesel and diesel blends. The equipment comprise of a compressor, a timing control panel, pressure tank, solenoid valve, spray gun, test rig, and a high speed camera. Figure 3 shows the schematic diagram of the test rig. The fuel is injected into the atomizer under pressurized conditions channeled through the air compressor. An air-assist automatic spray gun connected to a high pressure pump or solenoid valve is used to atomize the fuel, using different tip size to achieve the desired atomization and spray pattern size. As the droplets are sprayed, the high speed camera is used to capture the images of the spray pattern. The atomization test was conducted for five blends of biodiesel and diesel fuel, under various pressure ranging from

evaporation and minimizing fuel impingement on the injector walls.

**2.4. Atomization test rig**

**Figure 2.** Simple actual flow in gas turbine

220 Advances in Internal Combustion Engines and Fuel Technologies

0.1MPa to 0.5MPa.

In order to simulate the atomization process, the Computational Fluid Dynamics (CFD) model has to be constructed. The CFD model will be simulating the spray region of the fuel atomi‐ zation. Figure 4 shows a sample of simulated CFD model with square shape of the spray region using CFD preprocessor tools and Figure 5 is the axis symmetry constructed using the cylindrical shape of spray region. Both figures become the samples of the CFD model geometry simulating on spray region. Study in atomization and spray characteristic give an idea where the meshs of the atomization spray region can be created as cylinder shape or square shape [21]. Figure 4 is the computational grid for the numerical analysis and it also shows the size of the grid as modeled for atomizer [8,21]. Furthermore, different injection pressure will affect the spray length and angle. Figure 5 show the measuring points for analyzing the atomization characteristic and the calculation meshes. [8,21].

Chemical properties and ambient pressure will affect the pattern and SMD of the spray. It is proved in Figure 6 and Figure 7 whereby after the injection the velocity increased due to droplet [34]. Thus, it is a high velocity and the relative velocity of droplets injected at later stage is decreased. Pressure and temperature can also affect the spray flow. In addition, chemical characteristics also will affect the spray length, spray angle, spray pattern and SMD. It depends on the various blend of the fuel whereby every blend of fuel consist of different amount of chemical characteristic such as density, viscosity, surface tension and others. Figure 6 shows the effect of pressure and Figure 7 shows the ambient pressure with different blend of fuel.

**Figure 3.** Experimental testing set up

**Figure 4.** Measuring points for analyzing the atomization characteristics

**Figure 6.** Spray length and spray pattern for different injection pressure

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**Figure 7.** The contour plot of biodiesel and DME fuels at various ambient pressures

**Figure 5.** Computational grid for the numerical analysis

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**Figure 6.** Spray length and spray pattern for different injection pressure

**Figure 4.** Measuring points for analyzing the atomization characteristics

222 Advances in Internal Combustion Engines and Fuel Technologies

**Figure 5.** Computational grid for the numerical analysis

**Figure 7.** The contour plot of biodiesel and DME fuels at various ambient pressures

Commercial ANSYS CFD software which consists of Gambit software creates the geometry and Fluent software which solve and run the simulation of the model analysis. The CFD model was created using the specification of the real equipment used for the atomization testing experiment. It needs to be created for the atomization testing where the spray injector will produce fuel atomization in the spray region. The specification needed to create the CFD model in Gambit are the spray tip diameter and spray region of the fuel atomization. The spray tip diameter is to be 0.04 mm according to the real atomization testing equipment. Meanwhile geometry is set to be 0.5m in height and 0.5m in diameter. The smaller region of CFD model is already sufficient to generate the fuel atomization and it is much easier for the Fluent software to analyze the simulation. The CFD model was created as a cylindrical spray region with a spray tip.

VOLUME 1

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VOLUME 2

The construction of the CFD model using Gambit software whereby it is used to construct the geometry as desired. Therefore, construction of the CFD model in Gambit software is to create the spray region CFD model and insert meshes to the CFD model. Mesh can be generated on the faces or volume. The CFD model can be meshed according to the mesh element, mesh type and interval size as desired. Mesh can also be meshed by edge, face and volume. Mesh size can be set whether to create a large or small mesh element and the more meshing on the geometry the more accurate the simulation. Basically, the CFD model is divided into two volumes. The upper volume was meshed using larger interval size and the lower volume was meshed using smaller interval size. It is because the lower volume of the CFD model need more detailed CFD analysis as more droplets exists at the lower volume of the CFD model. In this project volume 1 was defined as upper volume and volume 2 is lower volume. Volume 1 and Volume 2 share the same elements and type which is Tet/Hybrid elements and TGrid type. The difference is just on the interval size whereby Volume 1 use interval size of 1 and Volume 2 uses interval size of 0.1 which is smaller interval compared to Volume 1. The reason for choosing the small interval size for Volume 2 is the fluid flow or fuel flow will go through the Volume 2 whereby the atomization process will begin. In addition, the smaller the mesh size, the more accurate the simulation and after the entire meshed step is completed, the meshed CFD model is shown as the following Figure 8 and the settings that had been made are shown

After the geometry is meshed, the geometry must be defined with boundary conditions. Then, the mesh file can be exported to the Fluent software. Table 2 shows the name and types of the boundary conditions. The set of interior is to be defined as a plane that can be considered as invisible or a plane that will not cause any blockage to the fluid flow. If the boundary is not properly defined, it will be automatically defined by the Fluent as a wall. It is important to defined as interior before exported to Fluent software. The fluid inside the CFD model must

also be defined to show that there is fluid flow in the CFD model.

**Figure 8.** Geometry of atomizer

in Table 3.

The CFD model of the spray region created will only be 1/12 of the spray region. This means 30 degree of the spray region will be created. The reason behind partial creation of the spray region as the CFD model is because the spray region can be simulated and analyzed due to the smaller size and this option uses the periodicity function in Fluent software to stitch the CFD model of 30 degree spray region to be 360 degree full CFD model spray region. The construction of the CFD model in Gambit software is to create the spray region CFD model and insert meshes to the CFD model. Before generating the meshes and the CFD model, there are two ways which is constructing the CFD model directly, creating a face or volume of the desired shape and generate a mesh on it. The other way is creating vertices (vertex in the software) and create edges by joining the vertices together. By connecting the edges, this will create faces which will then create a volume after combining all the faces together. In this process of constructing the CFD model, both steps can be used and another function was used to create the CFD model is by subtracting and uniting the volume to obtain the desired shapes of the CFD model. Table 2 show the boundary conditions that had been made and the set to the interior defined as a plane that can be considered as invisible or a plane that will not cause any blockage to the fluid flow.

In advance, setting of simulation is the most important part that have to be focused to obtain good results. Experiment result will be compared with simulation results in terms of spray angle and spray pattern for all five types of fuel. In addition, experiment results are mainly photographs of the spray angle and spray pattern but in CFD simulation, the results of SMD, spray angle and spray pattern are measured directly from simulation figures. Geometry of the spray was modeled and selected boundary condition and meshing was conducted in Gambit. Furthermore, there are few assumptions that were made such as nozzle diameter and region of the spray. Figure 8 shows the Gambit model and Table 2 shows the boundary conditions. Meanwhile, Gambit file will be exported to Fluent for simulation and injection model in Fluent is surface injection and the breakup model used is k-Epsilon model. The computations were limited to only the spray nozzle to reduce converge time. Figure 9 shows domain of the spray. When everything is already set up, the simulation will begin by running the Fluent software and Figure 18 shows the atomization process.

**Figure 8.** Geometry of atomizer

Commercial ANSYS CFD software which consists of Gambit software creates the geometry and Fluent software which solve and run the simulation of the model analysis. The CFD model was created using the specification of the real equipment used for the atomization testing experiment. It needs to be created for the atomization testing where the spray injector will produce fuel atomization in the spray region. The specification needed to create the CFD model in Gambit are the spray tip diameter and spray region of the fuel atomization. The spray tip diameter is to be 0.04 mm according to the real atomization testing equipment. Meanwhile geometry is set to be 0.5m in height and 0.5m in diameter. The smaller region of CFD model is already sufficient to generate the fuel atomization and it is much easier for the Fluent software to analyze the simulation. The CFD model was created as a cylindrical spray region

The CFD model of the spray region created will only be 1/12 of the spray region. This means 30 degree of the spray region will be created. The reason behind partial creation of the spray region as the CFD model is because the spray region can be simulated and analyzed due to the smaller size and this option uses the periodicity function in Fluent software to stitch the CFD model of 30 degree spray region to be 360 degree full CFD model spray region. The construction of the CFD model in Gambit software is to create the spray region CFD model and insert meshes to the CFD model. Before generating the meshes and the CFD model, there are two ways which is constructing the CFD model directly, creating a face or volume of the desired shape and generate a mesh on it. The other way is creating vertices (vertex in the software) and create edges by joining the vertices together. By connecting the edges, this will create faces which will then create a volume after combining all the faces together. In this process of constructing the CFD model, both steps can be used and another function was used to create the CFD model is by subtracting and uniting the volume to obtain the desired shapes of the CFD model. Table 2 show the boundary conditions that had been made and the set to the interior defined as a plane that can be considered as invisible or a plane that will not cause

In advance, setting of simulation is the most important part that have to be focused to obtain good results. Experiment result will be compared with simulation results in terms of spray angle and spray pattern for all five types of fuel. In addition, experiment results are mainly photographs of the spray angle and spray pattern but in CFD simulation, the results of SMD, spray angle and spray pattern are measured directly from simulation figures. Geometry of the spray was modeled and selected boundary condition and meshing was conducted in Gambit. Furthermore, there are few assumptions that were made such as nozzle diameter and region of the spray. Figure 8 shows the Gambit model and Table 2 shows the boundary conditions. Meanwhile, Gambit file will be exported to Fluent for simulation and injection model in Fluent is surface injection and the breakup model used is k-Epsilon model. The computations were limited to only the spray nozzle to reduce converge time. Figure 9 shows domain of the spray. When everything is already set up, the simulation will begin by running the Fluent software

with a spray tip.

224 Advances in Internal Combustion Engines and Fuel Technologies

any blockage to the fluid flow.

and Figure 18 shows the atomization process.

The construction of the CFD model using Gambit software whereby it is used to construct the geometry as desired. Therefore, construction of the CFD model in Gambit software is to create the spray region CFD model and insert meshes to the CFD model. Mesh can be generated on the faces or volume. The CFD model can be meshed according to the mesh element, mesh type and interval size as desired. Mesh can also be meshed by edge, face and volume. Mesh size can be set whether to create a large or small mesh element and the more meshing on the geometry the more accurate the simulation. Basically, the CFD model is divided into two volumes. The upper volume was meshed using larger interval size and the lower volume was meshed using smaller interval size. It is because the lower volume of the CFD model need more detailed CFD analysis as more droplets exists at the lower volume of the CFD model. In this project volume 1 was defined as upper volume and volume 2 is lower volume. Volume 1 and Volume 2 share the same elements and type which is Tet/Hybrid elements and TGrid type. The difference is just on the interval size whereby Volume 1 use interval size of 1 and Volume 2 uses interval size of 0.1 which is smaller interval compared to Volume 1. The reason for choosing the small interval size for Volume 2 is the fluid flow or fuel flow will go through the Volume 2 whereby the atomization process will begin. In addition, the smaller the mesh size, the more accurate the simulation and after the entire meshed step is completed, the meshed CFD model is shown as the following Figure 8 and the settings that had been made are shown in Table 3.

After the geometry is meshed, the geometry must be defined with boundary conditions. Then, the mesh file can be exported to the Fluent software. Table 2 shows the name and types of the boundary conditions. The set of interior is to be defined as a plane that can be considered as invisible or a plane that will not cause any blockage to the fluid flow. If the boundary is not properly defined, it will be automatically defined by the Fluent as a wall. It is important to defined as interior before exported to Fluent software. The fluid inside the CFD model must also be defined to show that there is fluid flow in the CFD model.


**3. Results and discussion**

Sauter Mean Diameter (SMD) is the diameter of a sphere that has the same volume/surface area ratio as a particle of interest or can be defined as the diameter of the droplet whose ratio of volume-to-surface area is equal to that of the spray as stated [30]. The most accurate method to determine SMD of fuels is through the acquisition of a device called Phase Doppler Particle Analyzer (PDPA) system [1]. Due to cost constraints, a SMD formula generated is adopted to study the SMD size, for this research purpose [24]. The chemical properties of the fuels, namely viscosity, surface tension and density will directly affect droplet size of fuels, where viscosity is regarded to have the largest contribution to change the SMD. The correlation for SMD is:

0.385 0.737 0.737 0.06 0.54


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grr

<sup>A</sup> SMD 6156 *mm m L v P*

**3.1. Sauter mean diameter**

Where;

*νm* = mixture viscosity (m2

γm = surface tension (N/m)

*ρA* = air density (1.145 kg/m3

*ρm* = fuel density (kg/m3

/s)

)

**Figure 10.** Chart of Sauter Mean Diamater (SMD) for various fuel blends.

∆PL = liquid fuel injection pressure difference. (2 bar)

)

**Table 2.** Name of the boundary condition and types


**Table 3.** Setting for mesh

**Figure 9.** Domain of the spray
