Challenge on Combustion

Aim

To simulate axisymmetric 2D combustion of methane fuel in presence of air and study the characteristics of species. The study is further extended to investigate the influence of water content in the fuel on emissions of CO₂, NO_x and soot formation.

1. Introduction

Combustion of a fuel in presence of air results in release of energy which is used to carry out mechanical work. With the growing energy demands, there is extensive usage of combustion engines, propulsion systems, burners etc. However, the current generation faces challenges of limiting fuel consumption and emissions released by the combustion process. With limited availability of the petroleum sources, heavy demand for it calls for research studies to optimize the process by improving thermal efficiency and in turn, reduce the fuel consumption. To reduce the testing and development expenditure, CFD models were developed and the combustion process was simulated using numerical integration. The complexity of numerous species and their interactions were tackled with separating the combustion process into different types. Based on mixing, it can be classified into non-premixed, premixed and partially pre-mixed combustion. Based on phase, it can be classified into fluid phase or volumetric reactions, surface reactions and porous region combustion reactions. The CFD models solve the coupled partial differential equations for mass, momentum in general. Additionally, a species transport equation is solved with the laminar or turbulent diffusion models. The source term is calculated depending on the case and the various models used are finite rate chemistry models, mixing rate models or the finite rate eddy dissipation model.

With the growing concerns of global warming, the emission norms for all combustion devices have become stringent. The norms limit the emission of greenhouse gases such as CO₂, CO; toxic gases such as SO_x, NO_x and particulate matter such as soot particles in the exhaust gases. Due to this, design of an engine is reliant on the emission concentrations in the exhaust gases simulated using CFD models. Modeling of NOx and soot models is highly complex and accurate prediction of their concentration is still a challenge for the Original Equipment Manufacturers.

In this study, a 2D model with air and fuel inlet is defined and used to simulate combustion of methane CH₄ in presence of air to determine the temperature characteristics, species mass fractions and study the effect of water content in fuel on the emissions.

2. Theory

Mass fraction calculation for air and fuel

The combustion reaction can be written as follows:

CH₄ + a (O₂ + 3.76 N₂) ó C0₂ + bH₂O + cN₂

Solving the stoichiometric equation, we get the coefficients a= 2 moles. That is for 1 mol CH₄ combustion, 2 mol of air is required. Considering atomic weight of O= 16 and N=14,

Mass fraction of O₂ in 2 mol of air = Mass of O₂ / Mass of air = 32/137.28 = 0.23

Similarly, Mass fraction of N₂ in 2 mol of air = Mass of N₂ / Mass of air = 105.28/137.28 = 0.77

3. Objective

The objectives are as follows:

• To perform a combustion simulation on the combustor model and plot the variation of the mass fraction of the different species- CO2, H2O, CH4, N2, O2, NOx emissions & Soot formation- in the simulation using line probes at different locations of the combustor
• To add the water content in the fuel from 5% to 30% by mole and observe the effect of it on the results

4. Solving & Modelling Approach

The following steps are followed for solving the case:

1. The solid model is imported and 2D section is prepared
2. The geometry is meshed with triangular mesh method and named selections are defined
3. The case is set up using turbulence models, species models and the boundary conditions are assigned
4. The solution is run for several iterations till convergence of residuals is observed
5. The results are interpreted and contour & line plots are generated
6. The case is simulated again for different fuel inlet boundary conditions for the range of percentage of water content defined for the case.

5. Pre-processing & Solver setting

5.1 Geometry

Fig 1- 3D solid model of combustor

The 3D model is imported in Spaceclaim. The sectional view below describes the fuel inlet and inlet.

Fig 2- Sectional view of combustor

For the simplicity of the case under study, a 2D section is extracted by splitting the component by X-Z plane and X-Y plane. From the generated geometry, the faces are selected and pasted into a new design file. The component surfaces are combined to generate a 2D combustor model.

It is to be noted as the given case is changed from 3D to 2D axisymmetric, the axis of revolution must be along x-axis and the 2D surface must be in X-Y plane for ANSYS Fluent to successfully simulate the case.

Fig 3- 2D combustor geometry

Fig 4- Zoomed in view of fuel inlet

5.2 Mesh

The geometry is meshed for an element size of 5 mm and a triangular mesh method is applied considering two dimensional nature of the case.

Fig 5- Mesh

The next step involves defining the edges for air-inlet, fuel-inlet, outlet and walls using named selections.

Fig 6- Named selections

5.3 Models

For the given case, the turbulence model k-Ɛ Standard is selected. The species model is turned by selecting species transport with volumetric reaction and Eddy dissipation source calculation. The diffusion is also calculated at the inlet.

In order to map the NO_x and soot formation, the species models are turned on with fuel as CH₄ and oxidant as O₂. For soot prediction, one factor model is selected.

5.4 Boundary conditions

The following boundary conditions are defined:

Air-Inlet velocity = 0.5 m/s

Air-inlet mass fraction: O₂ = 0.23

Fuel-inlet velocity = 80 m/s

Fuel-inlet mass fraction: CH₄ = 1.00

Outlet: Pressure-outlet

5.5 Solution

Steady state, pressure-based, axi-symmetric solver is selected and the case is simulated for 1000 nos.

6. Results

6.1 Convergence

Fig 7- Plot of residuals against iteration

From the figure, it can be noted that the solution achieves convergence at iteration- 800.

6.2 Temperature contour

Fig 8- Temperature contour

From the figure, it can be seen that the fuel combustion starts at the intermixing and spreads in the direction of flow. The temperature increases as we move away from the inlet towards the outlet. At the outlet, the temperature distribution is fairly uniform indicating complete combustion of the fuel inside the combustor. The maximum temperature attained inside the combustor is 2310.39 K

6.3 Characteristics of species

In order to study the characteristics, mass fractions at different locations are calculated. Since the flame formed is not uniform in the direction perpendicular to the direction of flow, we define line along the y-direction at x= 0.03, 0.06, 0.15, 0.3, 0.6 and outlet. The contour plot and line plot are generated for the individual species.

Fig 9- Lines created along y-direction

6.3.1 CO₂ Mass fraction

A contour plot of the CO₂ mass fraction shows that CO₂, a product of combustion is formed at the intermixing of the fuel and air. As the combustion flames advances, the concentration of CO₂ increases in the combustor. As shown in the line plot, the mass fraction of CO₂ increases along the direction of flow. Along the y-direction, the concentration decreases due to the flame concentration or combustion occurring close to the lower surface. However, diffusion occurs as we move towards the outlet and the distribution along y-direction becomes uniform at the outlet.

Fig 10- Contour plot of CO₂ mass fractions

Fig 11- Line plot of CO₂ mass fractions at different locations

6.3.2 H₂O mass fractions

Fig 12- Contour plot of H₂O mass fraction

Fig 13- Line plot of H₂O mass fractions at different locations

The characteristics of H₂O species are very similar to that of CO₂ as both are products of combustion process.

6.3.3 CH₄ mass fractions

The fuel CH₄ concentration can be found only in the inlet region and intermixing region with air. As the fuel is injected into the air stream and combustion occurs, the fuel is converted to intermediary species and products. Hence, the mass fractions at the other regions is found to be zero.

Fig 14- Contour plot of CH₄ mass fraction

Fig 15- Line plot of CH₄ mass fractions at different locations

6.3.4 N₂ mass fractions

Fig 16- Contour plot of N₂ mass fraction

Fig 17- Line plot of CH₄ mass fractions at different locations

Nitrogen is inert in nature up to certain temperature. This is reason why the concentration of N₂ measured using mass fractions does not change significantly as is evident from the contour and line plots.

6.3.5 O₂ mass fractions

As we know, for the combustion of fuel, oxygen concentration is very crucial. As the combustion takes places, the fuel burns by consuming oxygen resulting in reduction of O₂ mass fraction as we move away from inlet. This can be observed from the contour plot. However, the consumption of O₂ depends on the richness of the fuel mixture and the temperature distribution inside the combustor. Due to uneven temperature distribution, some O₂ are not burnt or are formed as an intermediary species. This explains the relatively low mass fractions of oxygen at the outlet region.

Fig 18- Contour plot of O₂ mass fraction

Fig 19- Line plot of O₂ mass fractions at different locations

6.3.6 NO_x emissions

Nitrogen molecule is inert at low temperatures. However, due to the high temperatures attained during combustion, N₂ reacts with oxygen to form different compounds such as NO, NO₂ and N₂O. However, the predominantly observed NO_x compound is NO. A comparison of the contour plot of NO and N₂O shows that maximum concentration of NO is 460 ppm by mass whereas NO₂ is found to be merely 0.33 ppm. Hence, for this study we focus only on studying NO. From the contour plot, it is can be seen that NO concentration increases towards the outlet. This is the region where flame concentration occurs and the high temperature leads to NO formation. NO, thus formed diffuses into the mixture and is emitted through the outlet.

Fig 20- Contour plot of N₂O mass fraction

Fig 21- Contour plot of NO mass fraction

Fig 22- Line plot of O₂ mass fractions at different locations

From the line plot, it can be observed that NO mass fraction is very low till mid region where the combustion flame is developing. After complete development of the flame, NO concentration increases and is diffused into the outlet mixture. Along y-direction, the concentration decreases in line with the decrease in temperature in the direction. However, at the outlet, the gradient is reduced due to diffusion.

6.3.7 Soot formation

Fig 23- Contour plot of Soot mass fraction

Soot is the unburnt carbon particulate matter formed due to incomplete combustion inside the combustion chamber. From the contour plot, it can be seen that soot formation occurs at the mid region after complete development of the flame and is concentrated at the end of combustion. These soot particles are carried through the air flow via the outlet. In accordance with the flame distribution along y-direction, the soot mass fraction decreases along that direction.

Fig 24- Line plot of O₂ mass fractions at different locations

7. Influence of water content in fuel on emission reduction

From the results, it can be seen that the exhaust gases coming out of the combustor through the outlet contains emissions of pollutants, namely, CO₂, NO and soot. To reduce the concentration of these pollutants, an effective measure incorporated by several manufacturers is to inject water molecules along with fuel into the combustor. To study the influence of water content on the emissions, we use a range of 5% to 30% water content by moles. This can be defined using mole fractions options in the boundary conditions of fuel-inlet in ANSYS Fluent Setup.

The characteristics are studied by plotting the contour and line plots for the different cases.

Fig 25- Contour plot of temperature for different concentration of water in fuel mixture

A brief study of the temperature contours for combustion of fuel mixture having different water concentrations show that the temperature distribution follows a similar pattern. However, the peak temperature drops with an increase in water content. For 0% water content, the peak temperature is found as 2310 K whereas for 30% water content, the peak temperature is reduced to 2214 K. Also, the region with high temperature concentration is found to decrease with water content. This contributes in the emission reduction as the intermediary pollutant species are formed mainly at high temperatures.

Fig 26- Contour plot of CO₂ mass fraction for different concentration of water in fuel mixture

Fig 27- Contour plot of NO mass fraction for different concentration of water in fuel mixture

Fig 28- Contour plot of Soot mass fraction for different concentration of water in fuel mixture

The above figures show the contour plots of mass concentrations for emission CO₂, NO and soot. It is observed that the characteristics for all cases are very similar and the distributions inside the combustor are not significantly affected by the water content of the fuel mixture. However, the maximum concentrations for the species are found to decrease with an increase in water content. This can be explained with line plots.

Fig 29- Line plot of CO₂ mass fraction for different concentration of water in fuel mixture

Fig 30- Line plot of NO mass fraction for different concentration of water in fuel mixture

Fig 31- Line plot of Soot mass fraction for different concentration of water in fuel mixture

From the line plots, it can be seen that the mass concentration for CO₂, NO and soot decreases with an increase in water content of the fuel mixture. This can be explained by considering the reduction in peak temperature due to the water content. The reduction in temperature affects negatively the reaction of inert N₂ gas with O₂ resulting in reduced concentrations at the outlet. In case of CO₂, a plausible explanation is the shift in equilibrium towards the reactant side due to presence of water concentration (product). This results in reduced mass fractions of CO₂ at the exhaust.

8. Conclusion

The study successfully simulates the combustion of methane in presence of air for a 2D axisymmetric combustor using CFD. The species concentrations are plotted and the influences of water content in the fuel mixture on their concentrations are examined. It is concluded that an increase in water content in the fuel mixture results in reduced level of emissions of CO₂, NO and soot.