Optimization of SCR inflow uniformity based on CFD simulation

Ke Sun; Haiyang Zhao; Kui Zhao; Da Li; Shuzhan Bai

doi:10.1515/phys-2020-0221

Open Access Published by De Gruyter Open Access December 31, 2020

Optimization of SCR inflow uniformity based on CFD simulation

Ke Sun , Haiyang Zhao , Kui Zhao , Da Li and Shuzhan Bai

From the journal Open Physics

https://doi.org/10.1515/phys-2020-0221

Abstract

The inflow uniformity before selective catalytic reduction (SCR) catalyst carrier is a major issue for DeNO_x capability of diesel engine after-treatment. Through the construction of the numerical model and CFD simulation of six perforated plate variations with different structural and positional characteristics, the influence of perforated plates on the uniformity of the airflow velocity at the inlet of the SCR catalyst carrier was analyzed. Comparison of different perforated plate variations shows that the encircling flow is a major hindrance to achieve higher inflow uniformity. Enclosed flow passage can remove the encircling flow and increase inflow uniformity at the cost of increased pressure drop. Rational layout of the perforated plate can achieve uniformity increase, while decrease pressure drop. High-velocity exhaust coupled with larger holes can improve both uniformity and pressure drop. The uniformity index increased from 97.6% of the original design to 98.7% of the optimized design, while pressure drop increased from 11.20 to 12.09 kPa. Weighing the relationship between inflow uniformity and pressure drop is an issue worthy of attention.

Keywords: inflow uniformity; SCR; perforated plate; encircling flow

Nomenclature

SCR: selective catalytic reduction
DOC: diesel oxidation catalyst
NO_x: nitrogen oxide
CFD: computational fluid dynamics
ρ: density
t: time
u i , u j: velocity (the value range of i and j is 1, 2 and 3)
ϕ: scalar
k: turbulent kinetic energy
ε: turbulent dissipation rate
l ε: length scale function
DPF: diesel particulate filter
ASC: ammonia slip catalyst
EGR: exhaust gas recirculation
DOE: design of experiment
Re d: wall-distance reynolds number
Re y ∗ , Δ Re y: model coefficient
μ t: turbulent viscosity
v f: velocity vector
v fn: normal velocity
φ: uniformity index

1 Introduction

In order to meet the increasingly stringent requirements of emission regulations, various technologies have been developed and optimized to decrease engine NO_x emission [1,2]. Inner engine clarification such as exhaust gas recirculation [3], coupled with after-treatment devices such as selective catalytic reduction (SCR) [4,5], is a widely applied method for DeNO_x. To obtain the necessary reductants for SCR reaction, ammonia is generally produced by 32.5% urea–water solution through a series of physical and chemical processes, i.e., atomization and evaporation process, urea pyrolysis, and isocyanate hydrolysis. The related processes are depicted in Figure 1.

Figure 1

Schematic diagram of producing NH₃ by urea–water solution. (a) Spray of urea–water solution, (b) urea pyrolysis, (c) isocyanate hydrolysis.

In terms of catalysts, some researchers enhanced the efficiency of NO_x conversion and broadened operation temperature window of high-efficiency reaction zone [6,7,8,9]. In terms of producing ammonia from urea–water solution, some researchers investigated the generation process of ammonia [10,11] and optimized the control strategy of spraying urea–water solution [12,13], which is of great significance to the enhancement of NO_x conversion, prevention of ammonia slip, and abatement of crystallization risk. In terms of design and optimization of SCR packaging, researchers widely combined CFD (Computational fluid dynamics) method with experiment methodology to carry out investigations. It was commonly believed by researchers that excellent mixing of NH₃ species and diesel engine exhaust is the precondition to ensure high NO_x conversion efficiency [14]. Capetillo et al. [15], using CFD code, carried out the DOE experiment of the mixer of SCR converter. Through comprehensive assessment of pressure drop, uniformity of ammonia, and liquid film thickness, the structure and layout of the mixer scheme were improved. Cho et al. [16] systematically studied the reductants distribution situation and NO_x conversion efficiency in thirteen sorts of mixer layouts, and the results showed that there were significant effects of mixer structure and layout on SCR performance. Hu et al. [17] established the three-dimensional numerical model of straight tube type after-treatment device consisting of diesel oxidation catalysts (DOC) and SCR. The distribution of ammonia was analyzed and a better scheme was proposed, and apparent improvement of pressure drop and NO_x conversion efficiency was found in the results of the optimized scheme. From these researches that have been done [15,18,19], it can be concluded that the CFD method can be adopted in predicting the crystallization risk and supporting guidance for structure optimization.

Though influences of mixer’s geometry characteristics on SCR performance have already been abundantly discussed in previous studies, the influences of the perforated plate geometry on inflow uniformity of SCR catalyst carrier were less investigated. Therefore, in this paper, a kind of understanding of adjusting geometry characteristics of perforated plate to affect the exhaust inflow uniformity of SCR catalyst carrier is exhibited. The mathematical model, including the turbulence model and porous medium model, was established, describing the flow process of exhaust before SCR converter and generating the mesh model. The original model and five perforated plate modification models were simulated to analyze the effect of perforated plate position and layout with different geometry characteristics on the exhaust flow before SCR catalyst carrier, namely inflow uniformity and pressure drop. The effect of encircling flow and perforated plate layout was studied through comparing velocity distribution and flow streamlines of different variations. The new understanding presented by this paper concentrated on a less discussed structure, which may help the industry and other researchers optimize the SCR after-treatment system from the viewpoint of fluid dynamics.

2 Numerical model

The compact after-treatment device includes DOC, DPF, SCR, ASC, and outside packaging. Of which, the SCR module is composed of injector, mixer, baffle, perforated plate, catalysts, and support of catalysts. The geometry is shown in Figure 2. Exhaust, originating from the diesel engine, gets into the after-treatment system from the device inlet and inflows the SCR module after flow through the two porous medium space of DOC and DPF. The exhaust velocity exhibits uneven distribution inside SCR due to the existence of the mixer and other complex structures. To acquire an excellent state of exhaust inflow uniformity of SCR catalyst carrier, the perforated plate is used to organize and coordinate the fluid.

Figure 2

Layout of after-treatment system.

The operation process of diesel engine after-treatment is pretty complex, and it refers to at least three processes, like exhaust flow, ammonia generation, and SCR reaction. Since this research mostly concentrates on the influence of the geometry characteristics and layout characteristics of SCR perforated plate on the exhaust inflow uniformity of SCR catalyst carrier, the numerical model is mainly used to simulate the exhaust flowing inside of the SCR module. And the computational domain for this research is surrounded by the red dotted line in Figure 2.

Due to the complexity of the fluid field structure, the core zone of the fluid domain is discretized based on an advanced polyhedral mesh generation algorithm. Meanwhile, the boundary layer zone is divided into prism mesh. The discretized computational domain is shown in Figure 3.

Figure 3

Discretization of computational domain.

2.1 Turbulence model

2.1.1 Fundamental equations

Theoretically, the unsteady continuity equation and the Navier–Stokes equation are adaptable for the instantaneous motion of most complex turbulence [20]. And the simulation of a three-dimensional transient turbulent flow system can be achieved through setting or calculated parameters of control equations. Time-averaged fundamental equations, including continuity equation, Navier–Stokes equation, and transport equation of scalar ϕ , are expressed as index form of tensor, and they are listed below:

Time-averaged continuity equation:

(1) ∂ ρ ∂ t + ∂ ( ρ u i ) ∂ x i = 0

Time-averaged Navier–Stokes equation:

(2) ∂ ∂ t ( ρ u i ) + ∂ ∂ x j ( ρ u i u j ) = ∂ ∂ x j μ ∂ u i ∂ x j − ρ u i ′ u j ′ ¯ − ∂ p ∂ x i + S i

Time-averaged transport equation of scalar ϕ :

(3) ∂ ρ ϕ ∂ x i + ∂ ρ u j ϕ ∂ x j = ∂ ∂ x j Γ ∂ ϕ ∂ x j − ρ u j ′ ϕ ′ ¯ + S

Due to the additional occurrence of Reynolds stress terms in time-averaged Navier–Stokes equation and transport equation of scalar ϕ , appending a new turbulence model is the prerequisite to ensure these equations are closed [21]. Taking into account calculation accuracy and convergence problem, the two-layer model is regarded as the eddy viscosity to solve realizable k – ε model.

2.1.2 Viscosity model

The two-layer approach, originally proposed by Rodi [22], has the capability to take place of the low-Renolds number approach to ensure the applicability of the k – ε model in the viscous-affected layer, containing the viscous sub-layer and the buffer layer. The realizable two-layer k – ε model combines the realizable k – ε model with the two-layer approach, and the all-y ⁺ wall treatment is more flexible.

In the two-layer model, the dissipation rate near the wall is defined as:

(4) ε = k 3 / 2 l ε

where the l ε is a length scale function that is calculated by model variant of two-layer formulation suggested by Wolfstein [23].

The combination of two-layer formulation and the full two-equation model is accomplished by the following formulation [24]:

(5) λ = 1 2 1 + tanh Re d − Re y ∗ A

where Re d is the wall-distance Reynolds number, which is defined as: Re d = k d v . Re y ∗ is a model coefficient, and value is 60. A determines the width of the wall-proximity indicator, which is defined as: A = ΔRe y a tanh 0.98 , where ΔRe y is a model coefficient, and value is 10.

The correlation between turbulent viscosity μ t from the k – ε model and the two-layer value is established by the following formulation:

μ t = λ μ t + 1 − λ μ μ t μ 2layer ,

where μ t μ 2layer is obtained by Wolfstein’s model variant [23].

2.2 Porous medium model

In this research, the type of SCR catalyst carrier is a wall-flow honeycomb ceramic monolith that is a kind of anisotropic porous structure. The relevant parameters are listed in Table 1. Besides, the inertial resistance coefficient and viscous coefficient of SCR catalyst carrier are obtained by binomial fitting of the pressure drop curve [25].

Table 1

Relative parameters of SCR catalyst carrier

Material	Cordierite
Mesh number	400
Size (diameter × length)/inch	12 × 8
Wall thickness/mil	4
Porosity	0.4952
Inertial resistance coefficient	2.32
Viscous resistance coefficient	1528.0

Due to the complicated porous structure, it is unrealizable to solve the porous domain using a full three-dimensional model. Therefore, the pressure drop of the porous medium field is calculated by adding a source term into the momentum equation. The source term contains a viscous loss term and an inertial loss term [26]. The resistance of a porous medium is defined as the point multiplication of superficial velocity v s and the porous resistance tensor P , which is shown below:

(6) f p = − P ⋅ v s

where the porous resistance tensor P is defined as: P = P v + P i v s , P v is viscous term, P i is inertial resistance term.

2.3 Uniformity index

The distribution state of a kind of scalar on a certain surface can be described by the correlated functions. As regard the quantitative description of velocity uniformity, the functions for evaluating uniformity are defined by statistical methods. In this paper, the functions of the uniformity index of velocity distribution are shown in the following formula:

(7) v fn = v f ⋅ cos θ

(8) ϕ = 1 − ∑ f v fn − v fn ¯ A f 2 v fn ¯ ∑ f A f

where v fn is the normal component of vector v f , i.e., the direction of vector v fn is perpendicular to the front surface of SCR catalyst carrier, v f is the velocity vector in the front surface of SCR catalyst carrier, θ is the angle between v f and v fn , ϕ is the uniformity index, v ¯ fn is the average of v fn , and A f is the area of a surface.

Uniformity index can quantitatively demonstrate the distribution state of velocity in the selected area. The closer the value of ϕ is to 1, the closer the velocity distribution is to the ideal state. When the value of ϕ is 1, the airflow velocity on the surface is completely uniform.

3 Results and discussion

The diesel engine exhaust at the standard working condition is selected as the boundary condition for simulation, which is shown in Table 2.

Table 2

Boundary conditions

Mass flow rate/(kg h⁻¹)	1,672
Temperature/K	659.15
Inlet pressure/kPa	122.80
Outlet pressure/kPa	100.00

3.1 Effect of encircling flow

To investigate the effect of encircling flow on inflow uniformity before SCR catalyst carrier, three types of perforated plates are analyzed. Perforated plate of variation A is an incomplete round plate, with 6 mm holes distributed on the incomplete half and 10 mm on the complete half. Exhaust can either flow through the hole of the plate or around the incomplete plate, before converging into the SCR catalyst carrier. Variation B has a complete round plate; while the perforated plate of variation C moved up 15 mm along its central axis, the flow passage is completely enclosed and removed encircling flow completely. Related geometry details are shown in Table 3.

Table 3

Perforated plate variations (A, B and C)

Variation	Geometry	Description
A		Original design
B		Complete round plate
C		Enclosed (moved up by 15 mm)

Velocity distribution of variation A and flow streamlines before perforated plate are shown in Figure 4. Roughly 80% of the area has little difference in the airflow velocity, the distribution is relatively uniform, and the average velocity is about 17 m/s. However, a significant velocity gradient, with the highest velocity at about 46 m/s, occurred near the center. As shown in the flow streamlines, the encircling flow bypassed the plate with high velocity and leads to the high-velocity area before SCR catalyst carrier. Also, the velocity of the holes with a diameter of 10 mm is higher than that of the holes with a diameter of 6 mm, which showed the effect of flow inertia and obstruction of perforated plate. In addition, the mass flow rate of 6 mm holes is apparently smaller than that of 10 mm holes, and the size of holes should be adjusted for a better flow situation. The velocity uniformity index before SCR catalyst carrier is 97.6%, while the total pressure drop before SCR is 11.15 kPa. The encircling flow through the incomplete part of the perforated plate largely resulted in the high-velocity area, which is contained in variation B.

Figure 4

Velocity distribution of variation A (left) and flow streamlines before (right).

Velocity distribution of variation B and flow streamlines before perforated plate are shown in Figure 5. The change in velocity distribution is not significant, with the central high-velocity area having decreased compared to variation A. Yet the highest velocity occurred at the center is still 46 m/s. As shown in the flow streamlines, the effect of encircling flow is not completely removed by a complete round plate. The exhaust flow around the perforated plate retained high velocity, and an angle of about 30–45° exists between the velocity vector of the encircling flow and the main flow through the perforated plate. The velocity uniformity index before SCR catalyst carrier is 98.2%, while the total pressure drop before SCR increased to 11.66 kPa. In order to achieve better uniformity, it is necessary to completely contain encircling flow.

Figure 5

Velocity distribution of variation B (left) and flow streamlines before (right).

Velocity distribution of variation C and flow streamlines before perforated plate are shown in Figure 6. Since the exhaust completely flow through the perforated plate, the central high-velocity area disappeared. The flow streamlines tend to develop along the normal direction without obvious vortex. Even so, as visible in the flow streamlines, after passing the perforated plate, the exhaust flow rotates around its central axis, causing visible velocity increase at the edge. The velocity uniformity index is 98.3% and the increase is insignificant compared to the pressure drop of 12.31 kPa, which increased by 5.21%. The divergence of uniformity and pressure drop indicates that there is a kind of “trade-off” relation [27], especially when effect of encircling flow is removed completely. The significant pressure drop increase without much velocity uniformity increase calls for further analysis of the perforated plate layout.

Figure 6

Velocity distribution of variation C (left) and flow streamlines before (right).

3.2 Effect of perforated plate layout

To investigate the effect of perforated plate layout on inflow uniformity and pressure drop before SCR catalyst carrier, another three types of perforated plates are analyzed. Variation D rotated the perforated plate of variation C by 180° around its central axis. Perforated plate of variation E has 6 mm holes only, while variation F has only 10 mm holes. Related geometry details are shown in Table 4.

Table 4

Perforated plate variations (D, E, and F)

Variation	Geometry	Description
D		180° rotation
E		6 mm holes only
F		10 mm holes only

Velocity distribution of variation D and flow streamlines before perforated plate are shown in Figure 7. Compared to variation C, the rotate flow around the central axis is less significant and the velocity increased slightly near the center rather than the edge. The restriction of 6 mm holes decreased the velocity difference of the high-velocity area, resulting in better velocity uniformity index of 98.6%. However, the restriction of 6 mm holes also further increased the pressure drop to 12.60 kPa. The “trade-off” relation between velocity uniformity and pressure drop is further proved, while the perforated plate layout needs more specific analysis.

Figure 7

Velocity distribution of variation D (left) and flow streamlines before (right).

Velocity distributions of variation E and F are shown in Figure 8, while flow streamlines are shown in Figure 9. Both variations retained the velocity distribution characteristic shown in variation C and D; with 6 mm holes only, high-velocity areas appeared near the edges; with 10 mm holes only, high-velocity areas appeared near the center. The exhaust flow rotation around the axis is the major cause of the remaining velocity difference, which is shown in Figure 9. Smaller holes of variation E caused pressure drop of 12.67 kPa, the largest of all six variations, with velocity uniformity index of only 98.4%. In terms of both velocity uniformity and pressure drop, variation F is the optimal design, with the highest velocity uniformity index of only 98.7% and pressure drop of 12.09 kPa, the lowest of variations without encircling flow.

Figure 8

Velocity distributions of variation E (left) and F (right).

Figure 9

Flow streamlines of variation E (left) and F (right).

Comparison of all six variations is given in Figure 10. The compact structure between DPF and SCR caused complex inflow, and the perforated plate increased the velocity uniformity before SCR, at the cost of increased pressure drop. Variations without enclosed flow passage (A, B) have lower pressure drop, yet the existence of encircling flow will inevitably cause rotating flow around the perforated plate; the high-velocity area will decrease the velocity uniformity. Enclosed flow passage (C, D, E, F) removed the effect of encircling flow, yet rotating flow around the axis still exists; further optimization of layouts of the perforated plate can increase the velocity uniformity without much increase of pressure drop. Therefore, the perforated plate before SCR needs comprehensive optimization, to achieve high SCR efficiency without hindering engine operation.

Figure 10

Uniformity and pressure drop of each variation.

4 Conclusions

This article shows a way of influencing the uniformity of airflow velocity before SCR catalyst carrier by adjusting geometry characteristics of the perforated plate. Through the construction of the numerical model and CFD simulation of six perforated plate variations with different structural and positional characteristics, the influence of perforated plates on the uniformity of the airflow velocity at the inlet of the SCR catalyst carrier was analyzed. Following conclusions can be drawn:

The structural characteristics and position characteristics of the perforated plate have a significant impact on the uniformity of the exhaust inflow at the inlet of the SCR catalyst carrier. Meanwhile, the structural characteristics and position characteristics of the perforated plate are adjusted, removing the effect of encircling flow without causing much pressure drop increase. The uniformity index increased from 97.6 to 98.7%; while pressure drop is contained to 12.09 kPa.
The encircling flow is a major hindrance to achieve higher inflow uniformity. Enclosed flow passage can remove the encircling flow, as shown in comparison between variation B and C. The high-velocity area near the center is reduced, and the velocity difference decreases significantly. However, pressure drop of enclosed flow passage will inevitably increase, and the rotation around the axis still exists, causing velocity difference.
Rational layout of the perforated plate can achieve uniformity increase while decrease pressure drop. As shown comparison of variation C, D, E, and F, high speed exhaust coupled with larger holes can improve both uniformity and pressure drop. Still, as the optimal design of the four, the pressure drop of variation F increased from 11.20 kPa of the original design to 12.09 kPa. Weighing the relationship between inflow uniformity and pressure drop is an issue worthy of attention.

Acknowledgment

This work was supported by the Shandong Provincial Natural Science Foundation, China, grant numbers ZR2019MEE041, and ZR2019QA018.

Conflict of interest: The authors declare that they have no conflicts of interest to report regarding the present study.

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Received: 2020-10-26

Revised: 2020-11-22

Accepted: 2020-11-26

Published Online: 2020-12-31

This work is licensed under the Creative Commons Attribution 4.0 International License.

Optimization of SCR inflow uniformity based on CFD simulation

Abstract

Nomenclature

1 Introduction

2 Numerical model

2.1 Turbulence model

2.1.1 Fundamental equations

2.1.2 Viscosity model

2.2 Porous medium model

2.3 Uniformity index

3 Results and discussion

3.1 Effect of encircling flow

3.2 Effect of perforated plate layout

4 Conclusions

Acknowledgment

References

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