ISSN 1674-8484 CN 11-5904/U 汽车安全与节能学报, 第 8 卷 第 3 期 , 2017 年 J Automotive Safety and Energy, Vol. 8 No. 3, 2017 07/13 268 — 278 Simulation of Hydrocarbon Injection used for Catalyzed Diesel Particulate Filter during Active Regeneration Process ABDALLA Aniseh, WANG Guoyang, ZHANG Jun, SHUAI Shijin (State Key Laboratory of Automotive Safety and Energy, Tsinghua University, Beijing 100084, China) Abstract: Diesel oxidation catalyst (DOC) upstream of catalyzed diesel particulate filter (CDPF) with additional diesel injection (hydrocarbon injection, HCI) were used to control the particulate matter (PM) emissions to meet stringent emission regulations for heavy-duty diesel vehicles. This study optimized the location of injector upstream of DOC by evaluating the uniformity of HC distribution on the entrance of DOC. An HCI was used in active regeneration to enhance the exothermic heat of CDPF to initiate the soot regeneration. Numerical analysis was established for the combination of after-treatment technology DOC-CDPF, and a spray model for HCI was implemented into a commercial CFD code named AVL FIRE. Different locations and orientations of the injector were tested. The results show that injector location has a significant effect on the optimal mixing between the exhaust gas and injected hydrocarbon (HC). Optimum injector location is the injector located farthest from DOC, while the long distance between injector and DOC allows enough time for injected HC to form secondary droplets, as it mixes well with exhaust gas downstream. Using HCI upstream of DOC causes the exhaust gas temperature to increase and reaches 550 ℃ at the entrance of CDPF, which is enough to initiate active DPF regeneration. Key words: vehicle exhaust emission; particulate matter (PM); hydrocarbon injection (HCI); catalyzed diesel particulate filter (CDPF); diesel oxidation catalyst (DOC); active regeneration 柴油机颗粒过滤器再生过程的碳氢喷射模拟 ( 英文 ) 艾尼塞，王国仰，张 俊，帅石金 （清华大学 汽车安全与节能国家重点实验室，北京 100084，中国） 摘 要：为满足严格的重型柴油机排放法规，柴油催化氧化器 (DOC) 和催化型柴油颗粒捕集器 (CDPF) 得到了广泛的应用。碳氢喷射 (HCI) 是一种实现 DPF 主动再生的有效方法。该文通过改变柴 油喷嘴位置来优化 DOC 入口的碳氢化合物 (HC) 分布。使用计算流体力学软件 AVL-FIRE，建立了 DOC-CDPF 模型，用软件内置 HCI 喷雾模型，对喷射器的安装位置和喷射方向进行了数值模拟。模 拟结果显示：喷射器的位置对 HC 分布有很大的影响，喷射器位置距离 DOC 越远，DOC 入口处的 HC 混合均匀性越高，这是因为喷雾在 DOC 前有更长的距离和时间形成均匀混合气；排气管喷射柴油 可以使 DPF 入口温度达到 550 ℃，满足了CDPF 主动再生的需求。 收稿日期 / Received ：2017-06-05。 基金项目 / Supported by ：国家“十三五”规划重点研发项目 / National Key R&D Program of China (2017YFC0211105) 。 第一作者 / First author ：艾尼塞 / ABDALLA Aniseh (1985 － ) ，女 / female，约旦 /Jordan，博士后 /Postdoctoral research fellow. E-mail: [email protected]。 269 艾尼塞，等：柴油机颗粒过滤器再生过程的碳氢喷射模拟 关键词：汽车废气排放；颗粒物 (PM) ；碳氢喷射 (HCI) ；催化型柴油颗粒捕集器 (CDPF) ；柴油催 化氧化器 (DOC) ；主动再生 中图分类号：TK 411+.5 文献标识码：A DOI: 10.3969/j.issn.1674-8484.2017.03.007 Introduction In general, the soot particles are divided into ﬁve categories according to its size in aerodynamic diameter : 1) Large particles represents particles diameter, Dp > 10 μm; 2) Coarse particles represents, Dp = 2.5~10 μm; 3) Fine particles represents, Dp = 0.1~2.5 μm; 4) Ultra- ﬁne particles represents, Dp = 50~100 nm; Burtscher reported that the small size particles that make up nuclei mode as shown in Figure 2 are present in the greatest number, but contribute little to the particle mass of diesel engine PM emissions. On the other hand, larger size particles are found in small numbers that make up accumulation mode and contribute more to the particle mass [1-2, 5]. In general, the particulate emission regulations are being legislated mainly in a particulate mass (PM). Last emission regulations (Euro VI & Beijing VI) are legislated not only in particulate mass (PM) but also included the particulate number (PN). PN is used to control the small size particles since the Ash and other 13% Unburnt oil SOF 25% Fine particles DP< 2.5 μm PN Nanoparticle DP< 50 nm PM 108 Ultra fine particles DP< 100 nm 106 104 102 DP< 10 μm Nuclei Mode 10-3 10-2 Accumulation Mode 10-1 DP / μm 1 10 Fig 2 Concentration of PN and PM vs. Particle Size Distribution from Diesel Exhaust  small size particles are more harmful to human health than relatively large size particles [3, 9]. Table 1 compares emission standards for Europe, U.S., and China. The legislation for China III-V are based on Euro III-V which was adopted in 2005 . Table 1 Exhaust Emission Standards for Heavy-Duty 5) Nanoparticles represents, Dp < 50 nm. Sulfate/water 14% 1010 Normalized conccntration [dN / d lg(DP / μm)] /cm-3 Emission control technologies are required to achieve stringent emission regulations such as Beijing VI (equivalent to Euro VI) to overcome air pollution problem caused by diesel engine vehicles since diesel engine vehicles are a major source of particulate matter (PM) emissions. Figure 1 represents the PM composition emitted from a heavy-duty diesel engine [1, 2]. The composition of PM of the exhaust gas emitted from diesel engines is mainly characterized as a mixture of soot, soluble organic fraction (SOF), sulfates, and inorganic materials (ash) [3-5]. Figure 1 shows that the soot concentration is the highest in the composition of PM. Those PM compositions are depending on several factors, such as engine design, operating conditions, and fuel used [6-8]. Soot 41% Fuel SOF 7% Fig 1 PM Composition Emitted from a Heavy-Duty Diesel Engine. Engine Test Bench has been Carried out in a Transient Cycle Test  on-Road Diesel Engines [1, 10-11] Emission Legislation, Year e(NOx) g·kWh-1 e(PM) mg·kWh-1 PN 1011·(kWh) -1 Euro IV, 2005 3.50 20, (ESC) 30, (ETC) - Euro V, 2008 2.00 20, (ESC) 30, (ETC) - Euro VI, 2013 0.40 0.46 10, (ESC) 10, (ETC) 8, (WHSC) 6, (WHTC) US 2007, 2007 1.50 13 - US 2010, 2010 0.26 13 - China IV, 2015 3.50 20 30 (ESC+ELR) (ETC) China V, 2015 2.00 20 30 (ESC+ELR) (ETC) China VI b , 2017 0.40 0.46 10 10 8, (WHSC) 6, (WHTC) VI b — Proposed Beijing VI limits, ESC — European steady cycle, ETC — European transient cycle, ELR — European load response test, WHSC — world harmonious steady cycle, WHTC — world harmonious transient cycle. 270 第 8 卷 第 3 期 2017 年 汽车安全与节能学报 Current emission regulation in Beijing is Beijing V, Beijing V was implemented from 1st of June 2015. It is reported that Beijing will implement Beijing VI emission regulation in December 2017, and China in 2020. The bad quality air becomes signiﬁcant public health problem not only in Beijing but also in many cities around the world. Therefore, strict regulations for air pollutant emissions are necessary. Diesel particulate ﬁlter (DPF) is the most efﬁcient system that is used to trap the PM emissions. Figure 3 illustrates a conﬁguration of an after-treatment system that consists of catalyzed ﬁlter (DPF and DOC). A wall-ﬂow monolith is used as the most common type of DPF. Wall-ﬂow ﬁlter structure commonly made of silicon carbide (SiC) or Cordierite (Cd) ceramic material coated with a catalyst which is widely used in heavy-duty engine applications . The main design criteria of DPF’s wall-ﬂow ﬁlters that contribute to optimum ﬁltration and regeneration performance are high ﬁltration efﬁciency, low-pressure drop, high maximum operating temperature, low thermal expansion, high resistance to the thermal stress, low reactivity with ash and high durability . Inside the DPF there are many parallel channels, these channels are divided into inlet and outlet channels. The inlet channel inside the DPF is plugged at the end of the channel to force the exhaust gas to pass through the wall of the ﬁlters into outlet channel . Therefore, the exhaust gas is allowed to enter the inlet channels which are open toward the inlet side. The soot accumulates on the walls of the inlet channels; the process is called soot loading. As the ﬁlter accumulate more soot, it builds up backup pressure that may cause engine/ﬁlter failure and decreases fuel economy . Therefore, the DPF have to remove the accumulated soot from the wall ﬁlter to avoid the high backpressure due to soot accumulation; the process is called regeneration [13-14]. On heavy-duty diesel engines, the combination of passive regeneration (continuous regeneration during regular operation) and active regeneration (periodic regeneration starts when the soot loading of DPF reaches a threshold) are necessary to achieve reliable DPF regeneration at all driven conditions . Passive regeneration does not involve additional energy source for soot oxidation. In contrast, active regeneration relies on external energy sources such as additional diesel injection (HCI). Soot particle A wide range of studies on passive and active regeneration indicate that DOC-CDPF after-treatment system is very efﬁcient in controlling the soot, carbon monoxide (CO), and hydrocarbon (HC) emissions. CDPF is suitable not only for light-duty engines but also for heavy-duty diesel engines in highway and non-road applications [15-18]. The scheme of the after-treatment system is shown in Figure3. The combination of passive and active regeneration is required to ensure the CDPF regeneration at all driven conditions , For example, if the vehicle is driven at low speed or suddenly stops during uncontrolled regeneration, it could lead to overheating and damaging the CDPF. Passive regeneration does not involve additional energy source for soot oxidation. In contrast, active regeneration relies on external energy sources. Active regeneration of DPF with direct injection of diesel (Hydrocarbon injection, HCI) into the exhaust pipe is the most widely used regeneration method for heavy-duty diesel engines [41-43]. As mentioned before, hydrocarbon injection (HCI) is also called secondary fuel (diesel) injection is used to increase the exhaust gas temperature to initiate the soot regeneration within Honeycomb wall-flow filter HCI 15.25 cm DPF 45 cm Pressure sensor CDPF R=13.5 cm In order to meet the latest Euro VI emission regulation, diesel injection (hydrocarbon injection, HCI) was used with DOC upstream of CDPF which raises the exhaust gas temperature by injecting a small amount of diesel into the exhaust stream in order to enhance thermal oxidation of soot during active regeneration (also called periodic regeneration). DOC is used not only to raises the exhaust gas temperature but also to produce more Nitrogen Dioxide (NO2) by converting Nitrogen Monoxide (NO) to NO2. NO2 assisted oxidation of soot in the CDPF during continuous regeneration (also called passive regeneration) . Engine Temperature sensor 27.97 cm After-treatment technologies such as DPF, becomes mandatory to control soot emissions, while heavy-duty diesel engines are a major source of soot emissions. Kuwahara et al. investigated catalyzed DPF (CDPF), and he found that CDPF enhances soot regeneration efﬁciency and oxidation activity . Therefore, CDPF is applied in this study. There are many studies investigated the CDPF and developed coated catalysts in order to enhance ﬁltration efﬁciency of soot. The demand and consequently the development of after-treatment technologies are expected to increase over the coming few years. DOC lnlet lnlet Outlet Soot Particles Ash Particles 20 cm Soot removal HC,CO removal Fig 3 Configuration of an After-Treatment System (DOC-CDPF) with Secondary Fuel Injection (Hydrocarbon Injection, HCI) [14, 19-20] 271 艾尼塞，等：柴油机颗粒过滤器再生过程的碳氢喷射模拟 The injected HC is burnt in DOC. The injector is mounted in the exhaust pipe upstream of DOC, to increase the exhaust gas temperature to a level suitable for soot oxidation (e. g. from 339℃ to 550 ℃ ). Thermal oxidation of soot at regeneration process is enhanced when the CDPF temperature is greater than 500 ℃ , while optimum temperature for soot regeneration is 600 ℃ [21-22]. The mixture of exhaust gas contains nitric monoxide (NO), carbon monoxide (CO), and HC emissions which are oxidized inside the DOC, resulting in thermal load over DOC and CDPF, which may lead to damage to the DOC or/CDPF components due to the thermal stress . Therefore, it is important to control the exhaust gas temperature by controlling several factors including the amount of diesel fuel injected into the exhaust pipe (upstream of DOC), diesel injector (HCI) location and exhaust mass ﬂow rate  (in this study, mass ﬂow rate is kept constant). Exothermic reactions in DOC are described by the following reactions : , (1) , . (2) (3) The amount of liquid diesel fuel (hydrocarbons (HC)) added by the injector to provide the heat required to initiate the CDPF regeneration was calculated by the following equation : . (4) Where, is the mass ﬂow rate of diesel injection (HCI), is the mass ﬂow rate of exhaust gas, ∆Texh is the temperature difference between target temperature and actual exhaust gas temperature at DOC outlet, cp is the speciﬁc heat of the exhaust gas, and Hu is the lower heating value of the diesel fuel. The lower heating value of the diesel fuel Hu is set to 42.76 MJ/kg. Dongfeng commercial diesel engine X7 is used in this study, and the exhaust after-treatment system consisted of DOC and CDPF are used to control the soot emission. The main objective of this study is to optimize the injector location for active regeneration process of a CDPF. However, the main challenges for CDPF systems in automotive applications are controlling the regeneration process and ensuring the safety of catalyzed ﬁlter (CDPF and DOC) materials. To control the regeneration process of CDPF, several issues are considered in the study: the amount of diesel fuel injected into the exhaust pipe; the injector location and orientation; hydrocarbon (HC) concentration ﬁeld on the DOC upstream, and temperature distribution at CDPF. The commercial CFD code named AVL FIRE is used for numerical analysis . It is essential to estimate the HC distribution on the entrance of DOC and optimize the location of the injector by enhancing the uniformity of HC distribution on the entrance of DOC. We observed that high concentration of injected diesel could cause hot spots in the entrance of DOC. Therefore, this phenomenon could be avoided by optimizing the location of the injector when possible or using mixers to enhance the mixing efﬁciency. Uniformity indices have been used to rank the uniformity of the catalyst ﬂow. The uniformity index γ of φ (in this case φ represent HC concentration) at a catalyst crosssection is deﬁned as follows : , . (5) (6) Here, ADOC is the front area of DOC, N is total number of computational cell. Uniformity index γ is mainly evaluated for assessing the performance of the catalyst (DOC/CDPF) for those parameters which have an inﬂuence on the performance of the after-treatment systems, such as HC distribution, temperature distribution, and velocity distribution. This study investigated HCI which is used for the heavyduty diesel engine on a stationary operating point. We also optimized the injector location by enhancing the uniformity of HC distribution in the entrance of DOC. 1 Numerical Methodology 1.1 Governing Equations The Reynolds-average governing equations for mass conservation, momentum, and energy are solved to predict the behavior of the compressible multicomponent gas. For gas mixture (exhaust gas and injected HC) distribution problems, the solution of the governing equations is necessary to be in transient form. Therefore, the governing equations conducted in this work are the unsteady average Navier-Stokes equations. The computations are initiated with 3-D transient simulations. The ﬂuid properties’ (temperature, pressure, velocity, etc.) in the multicomponent gas are solved by transport equations. The HC is injected from a nozzle (injector hole diameter is 0.5 mm) as a spray. Injected diesel fuel assigned as a vapor phase. The transport equation for multicomponent exhaust gas for each gas species is deﬁned as following [26-30]: . (7) Where, ρ is the density of the gas mixture, Yα is the mass fraction of gas species α, t is the computational time in seconds, Jα is the mass diffusion ﬂux vector, and U is the velocity component in three-directions (x, y, z). The source (S α) is the net rate of creation of the conserved mass inside the control volume; it can be converted by phase changes from one state (liquid) to other states (gas) or split into a component by Δ CDPF. HCI is one of the promising external energy sources that is used to enhance the exothermic heat of CDPF to initiate the soot regeneration . HCI is positioned in the exhaust pipe in the direction of the exhaust stream to ensure optimal mixing between injected hydrocarbon (HC) and exhaust gas. The hydrocarbon (HC) is introduced by injector as spray during the regeneration process. HCI location has a signiﬁcant inﬂuence on the mixing of HC with exhaust gas ﬂow . Therefore, it is important to optimize the injector location and orientation. 272 汽车安全与节能学报 chemical reactions. Simulation of the transport Eq. (7) for each gas species get the general continuity mass Eq. (10), because of summation source term (S α) of Eq. (7) for each gas species equal to zero. Summation of the mass fraction for each mixture component α is determined as following, here N is the number of the components α. . (8) The diffusion ﬂux for the component α is deﬁned as: . 第 8 卷 第 3 期 2017 年 purpose, k-ζ-f turbulence model has been used to close the Reynolds average equations. It is known that all quantities in Eq. (10) are considered time-averaged values for the turbulence models. To solve these time-averaged quantities, a set of transport equations of turbulence quantities (the turbulent kinetic energy (k) and, the rate of dissipation of k (ε)) is applied with the artiﬁcial turbulent viscosity (μtur) (see Eq. (20)). μtur is used to account for the additional diffusion ﬂux due to turbulent motion. Therefore, the effective diffusion coefﬁcient Γ is deﬁned as following: (9) Where, Jα is the diffusion ﬂux of gas species, ρ is the density of the gas mixture, Yα is the mass fraction of gas species α, Dα is the mass diffusion coefﬁcient of gas species α, and Sct is the turbulent Schmidt number. Thus, Jianjun XIAO pointed out that turbulent Schmidt number Sct can be selected between 0.5 and 1.0 values . The general form of conservation equations for mass, momentum, and energy in the Cartesian coordinate system have the following form: . (11) Here, the symbol“tur”represents the turbulent ﬂow, σf is the turbulence Prandtl number based on the diffusivity of variable f . The k-ζ-f turbulence model is developed by Hanjalic, Popovac and Hadziabdic (2004). The k-ζ-f turbulence model solves a transport equation for the velocity scales ratio ζ, which makes the model more robust and less sensitive to grid non-uniformities . The k, ε, ζ and transport equation of turbulence kinetic energy (k) are deﬁned by the following equations [26, 29, 33-34]: (12) ， (10) ， (13) ， (14) Here, f is the general dependent variable, u, v, and w are the velocities in the x, y, and z directions. The diffusion coefﬁcient Γf and the source term Sf are unique to each dependent variable f , as shown in Table 2. (15) Table 2 Source Terms in Cartesian Coordinate Systems Equation f Direction Γf Sf Mass 1 scalar 0 0 u x μ ρg x + Vx -R x -( P/ x) v y μ ρg y + Vy -Ry -( P/ y) w z μ ρgz + Vz -R z -( P/ z) h scalar λ dP / dt + Momentum Energy rb + +Φ Here, Vx, Vy, Vz are the balance for the viscous diffusion term, R x, Ry, R z are the distributed resistance due to solid structures in a momentum control volume, rb is the rate of heat liberated from solid structures per unite ﬂuid volume, is the rate of internal heat generation per unite ﬂuid volume, and Φ is the dissipation function. Here, ν is the kinematic molecular viscosity (μ/ρ), the terms in Eq. (15) are: A: the production of k due to mean shear, B: the production or destruction of k by buoyancy, C: a loss of k to heat through viscous dissipation, D: the diffusive transport of k and randomizing action of the pressure-strain correlation. To solve Eq. (15) for turbulent kinetic energy k, closure relations are used, then transport equation of k [see Eq. (15)] is simpliﬁed and integrated over control volume as follows: ， (16) 1.2 Turbulence Models The governing equations for turbulent ﬂow are not closed. Therefore, turbulence models are required for closing the ). For this system of non-linear Reynold stress term ( . The k-ζ-f turbulence model consists of three transport (17) 273 艾尼塞，等：柴油机颗粒过滤器再生过程的碳氢喷射模拟 equations, one for the turbulent kinetic energy k as shown in Eq. (16), one for its dissipation rate ε, see Eq. (18), and one for normalized velocity scale ζ as shown in Eq. (19). The transport equation of rate of dissipation ε, velocity scale ratio ζ, and eddy viscosity are obtained by the following equations: ， (18) (19) , (20) , (21) , (22) . The droplet breakup process is described by standard wave model , where the size of the secondary droplets is determined. Droplet wall interaction behavior is modeled by Kuhnke model . Heat transfer between wall and droplet is determined following a model proposed by Wruck 1998. The mass transfer rate during evaporation from the wall-ﬁlm is determined by using the combination of Still-Himmelsbach and Diffusion models. The combination model has a good estimation for the single component liquid ﬂuids and engine applications, while for multi-component ﬂuids a homogenous mixture is used to predict the evaporation rate . It is known that Kuhnke model is suitable for diesel spray simulations; this model distinguishes between four wall impingement regimes by dimensionless wall temperature (T*) and droplet velocity (K) parameters. These parameters are deﬁned as following : Pk in the three transport equations, see Eq. (16), Eq. (18) and Eq. (19), represents the turbulence production due to the mean velocity gradients, σζ, σk and σε are the turbulent Prandtl numbers, f is the elliptic relaxation function, T is the turbulence time scale, L is the turbulence length scale, S is the mean rate ), μ is the molecular viscosity, of strain tensor ( and μtur is the turbulent viscosity. The model coefﬁcients applied to the k-ζ-f turbulence model are listed in the below : Cμ =0.22, Cε1=1.4(1+0.012 ⁄ ζ), Cε2=1.9, C3=0.8, C2'=0.65, CT =6, CL=0.36, Cη =85, C4=0.33, σk =1.0, σε =1.3, σζ =1.2. To correctly predict the large variation in the values of turbulence properties (gradient of momentum, energy ﬂux, k, and ε) near wall, wall function treatment is used. Wall function treatment is an analytical solution which is used to simplify turbulence equations. Numerical solution of turbulence equations close to the wall is impractical because the ﬁne mesh is required near a wall. Moreover, if sufﬁcient grid resolution is used near a wall, then large computational times are required. Therefore, in order to avoid the grid resolution problem and the signiﬁcant error due to steep ﬂow properties variations near a wall, special wall treatments are employed. Unfortunately, there is no single wall function that can promise good accuracy for various types of ﬂows. The generalized wall treatment, also called hybrid wall treatment can be used as low Reynolds model near the wall, which is proposed by Popovac and Hanjalic . Therefore, hybrid wall treatment is applied with k-ζ-f turbulence model in the simulations of this study. 1.3 Spray and Wall-Film Models The droplet dispersion of spray and the effect of turbulent on droplets are modeled by Gosman and Ioannidis . Droplet heat-up evaporation is predicted by using Dukowicz model . (24) ， (23) . (25) Here, Tw is the wall temperature, Ts is the particle surface temperature, ρd is the droplet density, dd is the droplet diameter, ud is the droplet velocity, σd is the surface tension of the droplet and μd is the dynamic viscosity of droplet. Kuhnke model considers four wall impingement regimes, as shown in Figure 4: Rebound, Thermal breakup, Deposition, and Splash. These regimes are modeled based on the wall temperature T* and droplet velocity K as following: A: Rebound regime formed when wall temperature higher than 1.1 value ( T*>1.1) with low droplet velocity. Existing vapor layer between wall and droplet prevent wall ﬁlm formation B: Thermal breakup regime formed when wall temperature higher than 1.1 value ( T*>1.1) with high droplet velocity. 1.4 A Rebound B Thermal breakup 1.2 T* ， This model assumes that the droplet is evaporating in a noncondensable gas. 1.0 0.8 D Splash C Deposition 0 100 K 200 Fig 4 Four Wall Impingement Regimes Based on Kuhnke Model  274 第 8 卷 第 3 期 2017 年 汽车安全与节能学报 Secondary droplets are formed, and no wall ﬁlm is formed C: Deposition regime formed when wall temperature less than 1.1 value ( T*<1.1) with low droplet velocity. Wall ﬁlm is created due to completely deposed of droplets on the wall D: Splash regime formed when wall temperature less than 1.1 value ( T*<1.1) with high droplet velocity. Secondary droplets with wall ﬁlm are formed. 2 Boundary Conditions Inlet, outlet and wall boundary conditions were speciﬁed. Gas temperature with 339 ℃ , mass ﬂow rate with 600 kg/ h of the gas and gas species’ mass fraction were applied to the inlet face. Exhaust gas was assumed to be an ideal gas. Pressure boundary with 1 bar value was used on the outlet face. Adiabatic heat transfer was estimated on the wall boundaries. The boundary conditions and the injector geometry parameters used for the HCI simulations are summarized in Table 3. In order to optimize HCI location in the exhaust pipe upstream of DOC, four different locations are settled at A, B, C and D. See Figure 6. Table 3 Inlet Boundary Conditions of CFD Model and Geometrical Data Parameters of the Injector Inlet gas ﬂow rate Inlet gas temperature Injected fuel 600 kg/h 339 ℃ diesel (HC) Fuel injection rate 65 g/min. Number of nozzle hole 1 Injector hole diameter 0.5 mm Spray cone angle 20 m/s Averaged droplet size 100 μm HCI is simulated with 339 ℃ initial gas temperature and 600 kg/h mass ﬂow rate. The regeneration process is controlled by controlling gas temperature rate ﬂowing through the aftertreatment system. While gas temperature is controlled by the HCI dosing (amount of diesel fuel) and optimizing HCI location and orientation. The amount of diesel fuel injected into the exhaust pipe upstream of DOC was 65 g/min, this amount is calculated by Eq. (4). Figure 6 is the schematic diagram of the simulated HC injector positions. A, B, C, and D indicate the location of the injector, and the arrows attached to those injectors indicate the spray direction. HC injector location and orientation are optimized to ensure optimal mixing between injected HC and exhaust gas. Figure 7 shows the HCI characteristics in the exhaust gas for the injector located at 35 cm from the end of tube bend with injection angle 45°(Design“A”). The simulation results of the HC mass fraction distribution at the entrance of DOC is shown in Figure 7. 50° Initial droplet velocity ESE Aftertreatment Tool of AVL FIRE. The 3D mesh created at AVL FIRE is based on sweeping a 2D mesh through space along a trajectory. Higher mesh density is used near HC injector, and very small artiﬁcial time step (1 ms) is applied to capture mass and heat transfer process of droplets. The structured grid has been used for the After-treatment system is shown in ﬁgure 5. Mesh independent study was performed in order to achieve mesh resolution that could provide independent mesh solution. Therefore, 64,000 cells and 298,100 cells were tested to examine the sensitivity of the simulation results to grid reﬁnement. We found no signiﬁcant difference in the simulation results between ﬁne and coarse mesh. The resolution of the used coarse mesh (64,000 cells) is sufﬁcient to capture the spray and exhaust gas mixture phenomena. Therefore, for HCI simulation, 64,000 cells has been used on 4 core CPU. 3.1 Optimization of Injector Location 3 Results and Discussions All the 3D simulation results presented in this paper were performed by FIRE-V2014.1 where spray and wall ﬁlm models were used. Structural type mesh was generated by Different locations and orientations of the injector have been tested to optimize the injector location. Four injectors are installed at various places as shown in Figure6. Uniformity index and HC mass fraction simulation results are compared, and the comparison of the simulations results are shown in Outlet Outlet lnlctlnlct CDPF CDPF HCl HCl DOCDOC Fig 5 Computational Model of the After-Treatment System (DOC-CDPF) with Secondary Fuel (HC) Injector 275 艾尼塞，等：柴油机颗粒过滤器再生过程的碳氢喷射模拟 1 Uniformity index of HC inlet A C 6 cm 15.25 cm 35 cm 27.97 cm B D CDPF 0.5 A B C D DOC 0 50 100 150 200 t / ms Fig 6 The Scheme of Injector Positions for Different Injectors Settled at A, B, C and D Fig 8 Uniformity Index of HC Species Distribution at the Entrance of DOC for Various Injectors HC mass fraction / 10-3 10 8 cxhaust 6 Mean HC Mass Fraction / 10-3 8 HCI 4 2 entrance of DOC 0 6 4 2 0 The simulation results in Figure 8, shows that the HC distribution is not uniform for the HC injector“D”because the distance between HC injector and DOC is very short. Therefore, a mixer is required to enhance the mixing in front of the DOC when injector“D”is used. The best injector location was found to be injector“A”(see Figure 7). The long distance between injector and DOC allows enough time for injected HC 50 100 t / ms 150 200 Fig 9 Transient HC Mass Fraction Distribution at the Entrance of DOC for Various Injectors Fig 7 HCI Characteristics in the Exhaust Gas and Contour Plot of Hydrocarbon Distribution at the Entrance of DOC ﬁgure 8 and Figure 9 respectively. A B C D to form secondary droplets, as it mixes well with exhaust gas downstream. The nozzle angle 45°of injector“A”has higher uniformity index of HC at the entrance of DOC than injector“C” . Injector“C”is positioned at the center of the exhaust pipe of the exhaust gas, and its injection direction is parallel to the exhaust gas stream. Uniformity index γ of HC at the entrance of DOC is compared for the various injectors as shown in Table 4. Figure10 shows the contour plot of HC distribution at the Uniformity Index γ / 10 -2 10 10 5 γ = 0.96 Design“A” γ = 0.88 Design“B” γ = 0.93 Design“A” γ = 0.55 Design“A” 5 0 0 Fig 10 Contour Plot of HC Distribution at the Entrance of DOC and Corresponding Gamma Values for Various Injectors 276 第 8 卷 第 3 期 2017 年 汽车安全与节能学报 entrance of DOC and corresponding gamma values γ for various injectors. We observed that the best location for the injector is position“A”. 700 600 Injector location Uniformity index of HC, γ A 0.96 B 0.88 C 0.93 D 0.55 θ/℃ Table 4 Uniformity Index Values (γ) for Each HC Injector 500 CDPF_downnstream 400 CDPF_upstrream 300 0 500 1 000 t/s CDPF_downstrcam CDPF_upstrcam T/K 870 860 850 840 830 820 Fig 11 Contour Plot of Temperature Distribution at the Entrance of CDPF (CDPF_Upstream) and the End of CDPF (CDPF_Downstream) During Active Regeneration The heat released from the catalytic reaction of injected HC in DOC causes the exhaust gas temperature to increase and reach almost 550 ℃ at the entrance of CDPF (CDPF_upstream). The 3D simulation results of temperature distribution during active regeneration on the entrance and the end of CDPF are shown in Figure 11. The temperature distributions in the CDPF are simulated with 4 g/l soot loading. Figure 12 shows the simulation results of temperature distribution on the entrance of CDPF (CDPF_upstream) and the end of CDPF (CDPF_ downstream). Therefore, thermal oxidation of soot at CDPF is enhanced when the exhaust temperature is greater than > 500 ℃ , and the temperature spreads along the CDPF and continues to increase due to the soot oxidation. The resulting heat released causes a rapid increase in the temperature at the end of CDPF and reaches regeneration temperature after 120 s from injecting HC. High regeneration temperature may lead to damage to the CDPF substrate due to thermal stress (maximum temperature limit of ﬁlter wall is 1050 ℃ ) . Therefore, it is important to control the temperature of CDPF by controlling the amount of diesel fuel injected upstream of DOC, and controlling the mass ﬂow rate of the exhaust gas. Fig 12 Simulation Results of Transient Temperature Distribution at the Entrance of CDPF (CDPF_Upstream) and the End of CDPF (CDPF_Downstream) During Active Regeneration. The CDPF Initial Soot Loading is Set to 4 g/L 3.2 Regeneration Estimations and CDPF Model Validations The CDPF regeneration simulation is performed using diesel injection upstream of DOC, while HC is injected into the exhaust pipe at 100 s. Active regeneration process in CDPF has been the focus in the following regeneration simulations. Simulation results are compared to experimental (measured) data in order to validate the numerical model. The experiment is conducted by Dongfeng commercial diesel engine X7 with steady low load engine operating condition (1 320 r/min, 320 Nm). For this investigation, CDPF soot loading is accumulated to 4 g/L, inlet mass ﬂow rate is 600 kg/h, and inlet temperature is 339 ℃ . Transient temperature, pressure drop and soot accumulation of CDPF are simulated, and the regeneration (soot oxidation) method is set as oxygen thermal regeneration with wall catalytic reaction. Regeneration reactions considered in this study are listed as follows: 2C(s)+O2 → 2CO, (26) C(s)+O2 → CO2, (27) CO+1 ⁄ 2 O2 → CO2, (28) C3 H6+9 ⁄ 2 O2 → 3CO2+3H2 O. (29) Figure 13 shows the transient temperature proﬁle at the end of CDPF during active regeneration, and maximum regeneration temperature increases and reach 600 ℃ after activating the HC injection. Figure 14 is the transient pressure drop across the CDPF during active regeneration, the pressure drop increased due to the volumetric expansion occurring after 100 s when the injected HC reacts inside the DOC. After 180 s, pressure drop stars to decrease due to the initiation of soot oxidation at CDPF. Figure 15 is the transient soot mass amount during active 277 艾尼塞，等：柴油机颗粒过滤器再生过程的碳氢喷射模拟 regeneration inside CDPF. Initial soot mass was 4 g/L and soot starts to oxidize at almost 180 s and cause pressure drop to decrease as shown in Figure 14. Entire soot is regenerated at almost 950 s. Figure 13-15 are the comparison of measured data and 700 θ/℃ 600 500 Measured data 400 3D_Simulation 300 0 500 t/ s 1 000 Fig 13 Comparison of Measured Data and Simulation Results for Temperature Profile at the End of CDPF Overall pressure drop / kPa 6 Measured data 3D_Simulation 4 500 t/ s 1 000 Fig 14 Comparison of Measured Data and Simulation Results for Pressure drop Profile along CDPF Overall soot mass /(g·L-1) 5 4 3 Measured data 3D_Simulation 2 1 0 4 Conclusion DOC-CDPF after-treatment system is very efﬁcient in controlling the PM emissions. Secondary fuel injection (Hydrocarbon injection, HCI) is used to enhance exothermic heat which is needed to raise CDPF temperature in order to initiate soot regeneration. We observed that the hydrocarbon (HC) injector location has a large inﬂuence on the HC distribution at DOC. The simulation results of different hydrocarbon injector locations are compared to optimize the injector location and orientation to ensure optimal mixing between injected HC and exhaust gas. HC uniformity index at the entrance of CDPF is used to optimize the injector location. HC species uniformity index for design“A”was around 0.96. This index indicates that HC distribution at the entrance of DOC was homogeneous since the location and orientation of injector“A”allow good mixing with exhaust gas. Therefore, no mixer is required for design“A”. Simulation results of the temperature, pressure drop and soot mass at the end of CDPF are compared with the experimental data in order to validate the numerical model, and good estimations are observed. References / 参考文献  Guan B, Zhan R, Lin H, et al. 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