CFD Simulation of Premixed Flame in Counter Burner under the Influence of a Magnetic Field

2.1 CFD simulation of counter flame burner

CFD simulation techniques are efficient tool for representing a mechanical problem and analyzing their physical phenomena for engineering applications. In the present simulation, the commercial CFD software ANSYS 17.0 FLUENT software with premixed flame-let model and the dynamics of premixed flame through counter vertical burner under influencing of magnetic field, Soldwork ver. 2019, are used to model and simulate the counter flame burner, three models at different conditions. There are several steps that carried out in order to perform the simulations, these steps are presented in Figure 1.

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Figure 1. Simulation steps in ANSYS 17

The modelling is carried out in sold work and subsequently the mesh is imported FLUENT Premixed Flame-let Module for solving and post processing as combustion problem. This simulation is divided into three combustion models according to the research objectives. A benchmark comprehensive numerical model to simulate the counter flame burner under influencing magnetic field was developed to predict the Magneto-hydrodynamic behavior of the system. The solution procedure of the model was achieved by computational coding in ANSYS environment. And assumptions were; the flow steady, the simulation analyzed 1D, a uniform transverse magnetic field is applied normal to the flow direction, and it was assumed that both gases are treated as two interacting continua and densities of the mixtures remain constant.

2.2 Modeler for premixed double counter flame

A three dimensional model of premixed gas fuel-air vertical counter burners are prepared in SolidWorks software. A computational fluid dynamics (CFD) model is used to investigate the premixed stationary flame under influence of magnetic field at different parameters, such as burning velocity, quenching diameter, stability limits, and dynamical premixed gas fuel/air flow. The initial step in CFD simulation is to prepare a computational domain. A three dimensional model of premixed gas fuel-air burner is prepared in SolidWorks software. The dimensions of premixed counter-flow burner geometry are taken from experimental setup and is shown in Table 1. The geometrical model of counter-flow burner is shown in Figure 2. The counter-flow burner system includes two counter tubes at 26 mm diameter, with each one has an end edge of 16 mm diameter, and the distance between burners edges was fixed on 30 mm. The air/LPG mixture is flowing through top and bottom a cross-sectional area of the burner’s edges. There are three schemes (Model-D1, Model-D2, and Model-D3) with different distance between magnetic poles 150mm, 180 mm, 220 mm are used.

Table 1. Dimensions for the counter flow burners geometry

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Figure 2. Computational domain prepared of three schemes of counter-flow burners

One of the important steps in CFD is to create the computational grid, which consists of computational cells. The governing equations are solved in computational cells. The air/LPG flow and combustion field domain are meshed with tetrahedral elements using unstructured meshing. The unstructured tetrahedral meshing is selected because of complex geometry. A recommended approach to eliminate the influence of mesh size is to seek the mesh independent solution has been followed. Optimum grid size is selected where solution is not affected by the grid size. Figure 3 shows the 3D meshed geometrical model of counter flame burner. The most accurate and comparable results for the counter flame front between the burner edges under influencing the magnetic field; burning velocity, quenching diameter and behavior of the flame stability were obtained by fine meshing scheme. Thus, the fine mesh was selected to perform the simulations and result interpretation. Nodes and elements of fine mesh for all schemes represented in Table 2. For all domains, number of cells in Table 2 selected for all simulation according to grid dependency as shown in Figure 4.

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Figure 3. Geometrical mesh model of counter flame burner

Table 2. Nodes and elements for counter flame burners

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Figure 4. Cells selected for all simulation models

2.3 Governing equations

The solution technique follows the finite volume method to solve the system of governing partial differential equations by using ANSYS 17.0 Fluent FGMs Module. Transient numerical simulation for the laminar counter flow premixed flame is carried out using Premixed FGMs module [2, 3]. While the only possible configuration for a diffusion flame let in 1D is opposed flow, 1D steady premixed flame lets can have several configuration. These include unstrained adiabatic freely propagating, unstrained non-adiabatic burner-stabilized. As a strain opposed flow premixed flames. The 1D adiabatic premixed flame equations can be transformed from physical-space to reaction – progress space [5, 6]. Neglecting different-diffusion, these equations are:

$\rho \frac{\partial Y_k}{\partial t}+\frac{\partial Y_k}{\partial c} \dot{\omega}_c=\rho x_c \frac{\partial^2 Y_k}{\partial c^2}+\dot{\omega}_c$         (1)

$\begin{aligned} \rho \frac{\partial T}{\partial t}+\frac{\partial T}{\partial c} \dot{\omega}_c= & \rho x_c \frac{\partial^2 T}{\partial c^2}-\frac{1}{c p} \sum_k h_k \dot{\omega}_k +\frac{\rho x_c}{c p}\left(\frac{\partial c p}{\partial c}+\sum_k c p \cdot k \frac{\partial Y_k}{\partial c}\right) \frac{\partial T}{\partial c}\end{aligned}$      (2)

where, Y_k is the Kth species mass fraction, T is the temperature, ρ is the fluid density, t is time, ὡ_k is the Kth species mass reaction rate, h_k is the total enthalpy, and cpk is the Kth species specific heat at constant pressure. The scalar-dissipation rate, X_c in Eq. (1) and Eq. (2) is defined as:

$x_c=\frac{\lambda}{\rho c p}|\nabla c|^2$       (3)

where, λ is the thermal conductivity. Note that X_c varies with c and is an input to the equation set.

2.4 Initial and boundary conditions

The values of the input parameters, which are the fuel gas-air flow velocities, A/F percentage values, and magnetic field intensity, to diffusion and premixed flame-let cases, are obtained from the mixture measured data from the experimental test. Experimental parameters were measured at entry counter burner of all modeller’s schemes and modeller for fuel gas-air mixtures, as shown in Table 3.

Table 3. Simulation parameters and variables

Then, the numerical simulation has also been carried out for the same experimental parameters. Numerical simulation of all modellers were carried out successfully by results compared with experimental measurements. Further evaluation was achieved by applying considerable values of same parameters to study effect of increasing air-fuel gas ratio, magnetic intensity and combustion models types on burning velocity, flame stability limits. Moreover, changing distance between magnetic poles was carried out by using three values (150 mm, 180 mm, and 220 mm) when distance between burner edges is 30 mm, increased air-fuel gas ratio of the entry parameter was represented a new evaluation variable. Whilst output parameters connected to volume fraction distribution of flame front, burning velocity, temperature distribution.

The mechanism for magnet–flame interaction is due to the magnetic para-magnetism of oxygen in the air, which is diffused into the flame. However, the combustion characteristics of the flame subject to the influence of magnetic field are not fully expansion yet. In this side some of questions must answering on it. First, which any parameter influencing on flame front is most sensitive to gradient magnetic field. Second, can flame temperature be varied by gradient magnetic field? This paper describes a numerical study of influence of magnetic field on premixed flame on the counter burner. Laminar premixed flames for different LPG gas flow rates propagating in counter burner of a different magnetic field intensities 1000 to 5000 gauss have been numerically investigated. An influence of the changing a distance between magnetic poles and magnetic force on the flame behavior, combustion velocity and flame temperature has been analyzed. The simulation was carried out using ANSYS Fluent software version 17.0, with premixed flame-let model and the dynamics of premixed flame through counter vertical burner under influencing of magnetic field. CFD results were appeared for behavior laminar flame in counter burner:

• Flame disc diameter in the counter burner is decreased gradually with increase magnetic field intensity and it affects positively on the combustion velocity of fuel/ air mixtures, and this behavior due to probably caused by effect magnetic force on oxygen zone.
• While, the results CFD results were shown decrease in the combustion velocity with increasing the distance between magnetic poles.
• The results have been demonstrated by an increase in the distance between magnetic poles on the combustion for LPG mixtures with air at 150, 180 to 220 mm leads to a significant decrease in both flame temperature with 3.7% and 4.7%.
• Finally, there was no significant effect on the flame temperature in the middle of the anti-flame disc with effect magnetic field.

Further analysis of the influence of magnetic field on combustion characteristics for another gas fuel such as methane, propane to study effect number carbon atoms and knowing the extent of its effect on the stability limits. Also, study the effect of magnetic field on the propagation turbulent flame.