In addition, flow and fluid-structure interaction (FSI) analysis is performed according to the position of the catalyst bed installed in the chamber. In the flow analysis, three models are investigated according to the position of the first catalyst bed. In the FSI analysis, the internal pressure field obtained as the result of the flow analysis is applied to the solid domain.
INTRODUCTION
Background of a research
On the other hand, there is the advantage that installation and maintenance costs are low because replacement of the catalyst bed is not necessary.[2] Taking into account the advantages and disadvantages of each system, the SCR system is applied to an installation that continuously operates or emits a large amount of exhaust gas, and the SNCR system is applied to an installation that emits a small amount of exhaust gas. Therefore, the SCR system is mainly used as an after-treatment system for purifying exhaust gases from large marine diesel engines. The other is the studies on safety evaluation and improvement of structural components according to different designs of SCR systems.
Study plan
In addition, the root mean square (RMS) value of the velocities is calculated, which is the standard for measuring the flow uniformity which is highly dependent on the efficiency of the catalyst bed, and the pressure drop and flow uniformity are calculated and compared with each other for the three cases by changing the location of the first catalyst bed. In the Fluid-Structure-Interaction (FSI) analysis, the pressure field resulting from the fluid analysis for the three cases is applied to the boundary condition of the solid domain, and the structural analysis is performed. Finally, by comparing the result of fluid analysis with the result of FSI analysis according to the locations of the first catalyst bed, the design with the lowest pressure drop and the highest flow uniformity is derived.
Structural Analysis
- Fundamentals of a structural analysis
- Finite elements method
- Stress-strain curve
- Failure criterion
- Simulation model
- Convergence study
- Result and discussion
The stress-strain curve describes the properties of the tested material and provides important information on the mechanical properties and behavior of the materials. Starting at this point, significant elongation of the test specimen occurs without appreciable increase in tensile strength. This phenomenon is known as material yielding and point B is called the yield point.
Elongation of the specimen in this range requires an increase in the tensile load, and therefore the stress-strain curve has a positive slope from C to D. The load eventually reaches its maximum value, and the corresponding stress is called the ultimate stress. Further stretching of the rod is actually accompanied by a reduction in the load and breakage. finally occurs at a point such as E in fig. The length of the SCR chamber is 3700 mm and the thickness of the main body is 8 mm.
The von Mises stress and the displacement of the y-axis direction are calculated by using only the internal pressure, which is the most influential load mode, and the convergence is determined (Fig. As a result, the maximum von Mises stress of 40Mpa or more) is shown, when it 1 layer in. The value decreases and remains constant when the 2 layers of the thickness direction elements are stacked (5mm~3mm).
The offset of the y-axis direction also maintains a constant value and converges from an element size of 5 mm. The final analysis is performed on a model with three layers of thickness direction elements for accuracy. In addition, the maximum displacement of the y-axis direction due to the weight of the catalyst bed and chamber is about 0.06 mm in the supporting part of the catalyst bed.
Fluid Analysis
Fundamentals of a fluid analysis
- Governing equations and turbulent model
- Flow uniformity
- Mathematical modeling of catalyst system
Devices such as SCR chamber used in this study are required to calculate the flow uniformity in the internal catalyst bed. This is because the flow uniformity at the front of the catalyst bed is an important index for the purification efficiency of the exhaust gas. Flow uniformity represented by Weltens and Velocity RMS (%) are typical methods to calculate the flow uniformity.
The root value represents the standard deviation of the speed value and finally the RMS value is calculated by dividing the standard deviation by the mean value. If the RMS value is 0%, this indicates that the flow is uniform throughout the cross section as shown in Figure 3. When performing the computational analysis of the structure with this shape, it is impossible to generate meshes considering the order of dimension difference of about 103~104 due to lack of memory.
The analysis is carried out using a porous model, which is an equivalent system that takes into account the physical properties of the catalyst layer, which causes a pressure drop in the direction of the flow (Fig. Equivalent system of the catalyst layer. The flow inside the catalyst layer flows in the longitudinal direction). direction and not in the other direction. -13) is expressed as a term proportional to the velocity and a term proportional to the square of the velocity.
Therefore, by measuring the velocity and pressure drop in the catalyst bed, the values of the first permeability coefficient (Eq. 3-14)) and the second permeability coefficient (Eq.
Simulation model
In the governing equation, porous media are modeled by adding a momentum source term to the standard fluid flow equations in Eq. The first way is to measure velocity and pressure drop by experiments,[15] and the second way is to measure velocity and pressure drop by CFD.[16]. The catalyst bed is modeled by applying the porosity coefficient to the simplified cell area instead of applying the real geometry.
-5), the catalyst bed pressure drop is calculated by adjusting the inlet velocity at the internal 0.5 m/s from 4.5 to 8 m/s. Before using the coefficients, the reliability of the approximate model was checked. The error is the result of ignoring the constant term of the second order polynomial in the process of obtaining the first and second order permeability coefficients.
The coefficients derived from the above apply to the pore area constants, and. The inlet boundary conditions are mass flow 9,216 kg/hour and temperature 290℃, and the outlet boundary conditions are pressure 1atm (atmospheric condition) temperature 290℃. In the governing equations, the continuity equation and momentum equation are used, and the energy equation is used because the high-temperature gas flows into the SCR chamber.
3-8), the analysis is performed for three cases with the first catalyst bed position, and the pressure drop and flow uniformity are compared for each case.
Convergence study
The total pressure drop and the pressure drop in sections 2 and 3 through the catalyst bed are shown in Figure. The total pressure drop changes slightly as the element becomes finer, but the convergence can be confirmed with a constant value regardless of the number of the element. Then the velocity distribution on the symmetry plane and on the front part of the catalyst bed is shown in Fig.
In addition, the vector field and streamline on the symmetry plane are shown in Fig. Velocity distribution on the front part of the catalyst bed (a) case 1 (b) case 2 (c) case 3. Velocity RMS by sections and cases . Considering the convergence of the pressure drop and the value of the velocity RMS and the turbulent flow characteristics, the model, which has the finest mesh (Case 3), is used for the reliability of the calculation.
Result and discussion
In general, the pressure drop for each section is constant regardless of the catalyst bed position. The velocity distribution in the plane of symmetry and the velocity distribution at the front of the catalyst bed are shown in Fig. Velocity distribution at the front of the first catalyst (a) case 1 (b) case 2 (c) case 3. RMS velocity by sections and cases.
The velocity distribution in the front section of the catalyst bed shows that the velocity area at the inlet section decreases as the distance between the first catalyst bed and the inlet increases. The value of the total velocity RMS flowing into the front of the catalyst bed is also shown in case. on the plane of symmetry and on the front part of the catalyst bed for each case, and the value of the turbulence kinetic energy is measured as shown in Table (3-6).
Kinetic energy of turbulence at the front of the catalyst bed by cases Turbulence Kinetic Energy. The kinetic energy of turbulence in section 1.5 shows a high value regardless of the position of the catalyst bed. For sections 2,3 and 6, the kinetic energy of turbulence increases as the distance between the inlet and the position of the first catalyst bed increases.
Similar to the turbulence kinetic energy results, there is more turbulence in case 2 and case 3.
Analysis of Fluid-Structure Interaction
Fundamentals of a fluid-structural interaction analysis
Simulation model
The boundary conditions are the same in the structural analysis, and the only internal pressure condition for normal directions is applied to the result of the fluid analysis because there is the difference in the order of dimensions of about 103 between the pressure for the normal direction and the pressure for the shear direction as shown in Fig.
Result and discussion
In the result of the existing structural analysis, the maximum stress appears in the external stiffening part. This is because the maximum internal pressure occurs before passing through the first catalyst bed. A maximum deflection of approximately 0.034 mm occurs in the catalyst bed support due to the weight of the catalyst bed and chamber.
The deflection caused by the weight of the chamber and the weight of the catalyst layer has a negligible effect on the overall stress distribution. Compared with the allowable value of 100Mpa, it has a high safety factor of 10 or more, which is about 4 times the result of structural analysis. The reason is that the internal pressure in the structural analysis is applied to 500 mmaq, which is about 5 times the allowable pressure drop of 100 mmaq.
As a result of the liquid analysis, the maximum internal pressure is about 30mmaq before the section of the first catalyst bed.
Conclusion
Finally, when it is impossible to perform repeated testing of the SCR system at an expensive cost, the safety design of the SCR system was made possible by applying the approximate model and sufficient elements from the computational analysis.
Yi, C., Jeong, Y., Suh, J., Park, C., Jeong, K., 2012, "A numerical analysis of the power uniformity of the SCR reactor for 5000 hp marine engines", The Korean Society of Manufacturing Process Engineers, vol. Park, Y., Song, H., Ahn, G., Shim, C., 2016, “Preliminary study on factor technology of selective catalytic reduction system in marine diesel engines”, Korean Institute of Navigation and Port Research, Vol. Tsingoglou, D., N., Koltsakis, G., C., Missirlis, D., K., Yakinthos, K., J., 2004, “Transient modeling of current distribution in automotive catalysts”, Applied Mathematical Modeling, Vol.
![Fig. 2-14. Linear vibration modes (a) mode 1 [67.601Hz, scale (x367)] (b) mode 2 [75.406Hz, scale (x367)]](https://thumb-ap.123doks.com/thumbv2/123dokinfo/10518773.0/29.892.125.784.143.606/fig-linear-vibration-modes-601hz-scale-406hz-scale.webp)
