# Dependence of the Flue Gas Flow on the Setting of the Separation Baffle in the Flue Gas Tract
## 2 Materials and Methods  
### The CFD Model  
The created model, which was used in CFD simulation, is shown in Figure 1. It represents the flue gas tract of the wood stove. However, the flue gas tract was modified by using three baffles, which were designed to capture particulate matter. The position of the first baffle was changed twice, and therefore the flowing was observed during three different settings. The flue gas tract was designed as a prism with a height of 500 mm, a width of 330 mm, and a thickness of 500 mm. The inlet, through which the particles entered the model, was realized as a rectangular area with a length of 500 mm and a width of 80 mm. The outlet was realized by using a rectangular area with a length of 500 mm and a width of 85 mm. The flue gas tract also contained a second baffle, which was located 170 mm from the sidewall of the model, 400 mm in height, and was divided into two parts at the end. The third baffle was placed 260 mm from the sidewall, and perpendicular to the bottom wall of the model in the shape of an extended letter \(M\). The first position of the first baffle used was 80 mm from the wall and an unchangeable height of 400 mm. At the end of the straight part of the baffle was a curvature which directs the flow of flue gases. The model settings were modified by changing the following parameters: (I) width X, (II) height Y, (III) angle \(\alpha\), (IV) angle \(\beta\), and (V) angle \(\gamma\). The values of changeable parameters are stated in Table 1.  
The meshes of the observed models are shown in Figure 2. The shape of mesh elements was mostly hexagonal in all proposed settings. The number of elements, and the average values of the orthogonal quality, the skewness, and the aspect ratio are stated in Table 2. \begin{table}
\begin{tabular}{c c c c c c} \hline \hline
**Number of Setting** & **X [mm]** & **Y [mm]** & \(\alpha\) [\({}^{\circ}\)] & \(\beta\) [\({}^{\circ}\)] & \(\gamma\) [\({}^{\circ}\)] \\ \hline \(1\) & 80 & 360 & 120 & 110 & 130 \\ \hline \(2\) & 90 & 355 & 140 & 120 & 120 \\ \hline \(3\) & 95 & 345 & 155 & 135 & 115 \\ \hline \hline \end{tabular}
\end{table} 
Table 1: The changeable parameters of the baffle. Figure 1: The model of the proposed flue gas tract: (I) X: width, (II) Y: height, (III) \(\alpha\), \(\beta\), \(\gamma\): angles. Based on the data stated in Table 2, the mesh quality was evaluated as acceptable to the criteria in all proposed settings. Wall boundary conditions were set up for individual parts of baffles. The CFD model was developed in Ansys software (ANSYS, Inc., Canonsburg, PA, USA), where the CFD solver was Ansys Fluent. Boundary conditions are stated in Table 3. The velocity of the flue gas was set up for the inlet and the pressure was set up for the outlet. Firstly, the mass flow in the inlet was evaluated by stoichiometric calculations with a consideration of 15 kW heat power. The simulation was considered with the fuel of wood pellets. Their characteristics were as follows: (I) 49.8% of carbon, (II) 6.4% of hydrogen, (III) 0.3% of nitrogen, (IV) 10% of moisture, (V) 0.8% of ash, and (VI) 15.4 MJ.kg\({}^{-1}\) of calorific value [19]. The inlet velocity was converted from the determined mass flow using the continuity equation. However, after recalculating the velocity into the flue gas tunnel leading to the model, it was not possible to achieve an experimental velocity of 1.97 m\(\cdot\)s\({}^{-1}\) resulting from the stoichiometric calculations. Therefore, the measurements were performed at the minimum velocity achievable in the experimental set-up, which was 2.5 m\(\cdot\)s\({}^{-1}\). The simulations were performed at this velocity converted to the area in the inlet of the model. They were realized at a temperature of 20 \({}^{\circ}\)C, the same as during the experimental measurement, without including the combustion process. The air density was set at 1.18 kg\(\cdot\)m\({}^{-3}\) due to a temperature of 20 \({}^{\circ}\)C. Outlet pressure was evaluated at \(-\)12 Pa according to STN EN 13240.