Biofuel Combustion Comparison Modelling With ANSYS FLUENT

Modelling methodology and simulation setup

The use of biofuel is set to play an increasingly important role in the provision of sustainable renewable energy, heating and transportation systems. Nowadays, release of harmful gases to atmosphere has become an environmental concern. There are many anthropogenic sources like: - Electricity generation, Transportation, Industrial, Residential, Commercial, and Agriculture. Out of these Transportation sectors is a major user of energy, and burns most of the world’s petroleum.

This report presents the comparison modelling of biofuel combustion i.e., the combustion of methane and air vs the combustion of hydrogen and air in search of better renewable source of energy with less pollutants and more efficiency.

We have proposed the simulation of biofuel combustion by using ANSYS FLUENT using CFD technique. 

The technology selected to analyse the flow simulation of combustion of methane and air and the combustion of hydrogen and air is CFD (Computational Fluid Dynamics) using the aid of tool ANSYS FLUENT (Fluid Flow).

So, initially for the combustion the basic geometry was build as per the given model of combustion chamber, then the various inlet, outlet, wall and axis was defined and the model was meshed. After meshing the setup for analysis was prepared by defining the boundary conditions and finally the model was simulated and the results were obtained.

This report compares the combustion properties of methane vs hydrogen with air. Comparing both the gases Hydrogen gives 63% improved efficiency and Methane gives 39% in normal city driving conditions. But methane is a cleanest burning fossil fuel as when hydrogen is burned with air there are no carbon atoms for the oxygen atoms to combine with, so a higher proportion combines with nitrogen from the air to form NOx and thus the amount of NOx formation is more in case of hydrogen and air combustion.   

The methodology used for the simulation of combustion of methane + air and hydrogen + air was species transport method in Ansys Fluent using the mixture material properties present inside the Ansys library.

The main method used was the formulation of NOx emission which is present in the species transport model and by using the custom field function to compute NO parts per million (ppm)

NO ppm can be computed from the Eqn. (1), after adding it to Ansys fluent by using custom field function: -

Eqn. (1) 

The main technique was to include the computation of NOx formation after the combustion by turning the NOx button on in Ansys Fluent. Also as mentioned above the NO ppm was calculated by the Eqn. (1). By this way the comparison between the emission of methane + air and hydrogen + air can be computed.

The model for the analysis was build using Ansys Design Modeler, by taking the Fig. 1 as an example as per the given conditions.

Fig. 1: Schematic of the gaseous flow combustion domain. 

In Fig. 1, biofuel and air are mixed in a gaseous combustion system. Biofuel is introduced at velocity ????_???? and temperature ????_???? into a shock tube of air via a smaller and 10 times shorter tube of radius ????. The air is entrained inside the large cylindrical tube of radius ???? at velocity ????_???? and temperature ????_????. The outer wall is regulated a fixed temperature ????_???? and insulated from outside. A complete combustion, with conversion of the fuel/air mixture to chemical reaction products is assumed.

CFD Analysis

So, in Ansys Design Modeler a rectangle shape was drawn considering the air inlet

radius and fuel inlet radius (r + R) on one side and the other side’s length was taken as the radius of air outlet (R). 

Table 1: Boundary conditions for the CFD Analysis.

The dimensions for the rectangle were taken from the given boundary conditions as shown in Table 1.

The model built in Ansys Design Modeler is as shown in Fig. 2 below: -

Fig. 2: Geometry model for the CFD of combustion. 

This is a 2D model designed for 2D flow problem with axisymmetric condition.After the modelling and defining the surface, the model is meshed in Ansys as shown in Fig. 3 and various named selections are created for the CFD analysis as shown in Table 2.

Fig. 3: Meshed model for the CFD of combustion. 

Table 2: Named part and their given conditions.

After the meshing of the model and updating the model, the setup for the CFD analysis was prepared by starting with switching on the Energy on and setting up the Species model with NOx and soot as the by-products of the combustion. Then the materials for the analysis were selected, first the methane-air mixture was selected and then the hydrogen-air mixture was selected for the combustion respectively. Then after specifying the boundary conditions as per the given data in Table 1 and as shown in the Fig. 4, Fig. 5, Fig. 6, Fig. 7 and Fig. 8 respectively. The CFD analysis was performed 

for both methane-air mixture and hydrogen-air mixture respectively.  

Fig. 4 and Fig. 5: Air inlet boundary conditions applied for the CFD.  

Fig. 6 and Fig. 7:  Fuel inlet boundary conditions applied for the CFD. 

Fig. 8:  Surface wall boundary conditions applied for the CFD.  

The results of the CFD performed are shown in the next section of the report.   

After performing the CFD analysis for the combustion of methane-air mixture following results were obtained: -

Fig. 9:  Temperature contour of methane-air combustion CFD. 

The contours displayed in Fig. 9 is the temperature contour for the methane-air combustion CFD. The temperature obtained is shown in Table 3.

Table 3: Value of Temp. after CFD of methane-air combustion.

Fig. 10:  Pressure contour of methane-air combustion CFD.

Fig. 10 shows the pressure contour for the methane-air combustion CFD. The pressure 

obtained is shown in Table 4.

Table 4: Value of Pressure after CFD of methane-air combustion.

Fig. 11:  Velocity vector of methane-air combustion CFD.

Fig. 11 shows the values of velocity of flow in case of combustion of methane-air mixture. The magnitude of velocity is shown in Table 5.

Table 4: Magnitude of Velocity after CFD of methane-air combustion.

Fig. 12:  Contour of NOx (ppm) of methane-air combustion.

Fig. 12 shows the contour of NOx - parts per million (ppm) in case of methane-air combustion. The magnitude of NOx produced after the combustion of methane-air is shown in Table 5.

Table 5: Amount of NOx (ppm) after CFD of methane-air combustion 

The results of CFD of hydrogen-air combustion mixture are shown below: -

Fig. 13:  Temperature contour of hydrogen-air combustion CF 

The contours displayed in Fig. 13 is the temperature contour for the hydrogen-air combustion CFD. The temperature obtained is shown in Table 6. 

Table 6: Amount of NOx (ppm) after CFD of methane-air combustion 

Fig. 14:  Pressure contour of hydrogen-air combustion CFD.

Fig. 14 shows the pressure contour for the hydrogen-air combustion CFD. The pressure obtained is shown in Table 7.

Table 7: Value of Pressure after CFD of methane-air combustion. 

Fig. 15 shows the values of velocity of flow in case of combustion of methane-air mixture. The magnitude of velocity is shown in Table 8.

Table 8: Velocity vector of CFD of hydrogen-air combustion

Fig. 12:  Contour of NOx (ppm) of hydrogen-air combustion.

Fig. 16 shows the contour of NOx - parts per million (ppm) in case of hydrogen-air combustion. The magnitude of NOx produced after the combustion of hydrogen-air is shown in Table 9.

Table 9: Amount of NOx (ppm) after CFD of hydrogen-air combustion.

Conclusion 

After comparing both the CFD of methane-air and hydrogen-air combustion, the following conclusions were made: -

• The magnitude of Temperature in methane-air combustion is less compared to hydrogen-air temperature keeping it cool than hydrogen-air combustion chamber. 

• The Pressure created in methane-air combustion is more compared to hydrogen-air. 

• The velocity of methane-air combustion is more compared to hydrogen-air combustion. 

• The amount of toxic gas i.e., NOx (ppm) produced in methane-air combustion is much lesser than compared to NOx (ppm) produced in hydrogen-air combustion. 

Thus, after comparing both the methane-air and hydrogen-air combustion, it can be easily seen that methane-air combustion is much less toxic and harmful as compared to hydrogen-air combustion.  

References 

[1] Frank P.Incropera, David P. Dewitt, Theodore L. Bergman, Adrienne S. Lavine “Fundamentals Of Heat And Mass Transfer”, John Wiley & Sons SIXTH EDITION, 111 River Street, Hoboken, NJ 07030-5774 2007. 

[2] Fraser, R.A,Siebers,D.L and Edwards, C.F, “Auto ignition of methane and natural gas in a simulated diesel environment”, Presented at SAE, paper SAE 2003-01-0755 

[3] https://www.cerfacs.fr/cantera/mechanisms/meth.php 

[4] https://www.sciencedirect.com/science/article/abs/pii/0010218090901228 

[5] K.M.Pandey, P. Kalita, K. Barman, A. Rajkhowa and S.N.Saikia, “CFD analysis of wall injection with large sized cavity based scramjet combustion at Mach 2” international journal of engineering and technology, vol.3,Issue.02,April 2011.

[6] A.R.Bhagat, Y.M.Jibhakate “Thermal analysis and optimization of I.C engine piston using finite element method” International journal of modern engineering research,vol.2,Issue.4,2012,pp.2919-2921