The Numerical Simulation And Optimization Of Automotive Catalytic Converter

The numerical simulation and optimization of automotive catalytic converterWith the increase of the population and the development of industry, the amount of automotive is getting more and more. At the same time the destruction of the natural becomes more serious by the automobile exhaust included the CO, HC, NOX and PM, and the survival of mankind is facing a big challenge. So it is concerned as an important topic to find more effective ways to reduce environmental pollution caused by emissions of pollutants. The ways to control automobile exhaust are mainly included three types:"green" power, machine cleaning and the decontamination outside the machine. The purification technology outside the machine is to install a variety of cleaning devices in the automotive exhaust system (catalytic converter, thermal reactor and DPF. etc.), and the physical and chemical methods are applied to reduce emissions of pollutants.Catalytic converter is considered as the most effective way for reducing vehicle exhaust emissions of toxic gases, and, it has been developing rapidly since its inception in 1975. At the same time. in order to meet increasingly stringent emissions regulations, the vehicle manufacturers around the world are investing heavily in the research of high-performance catalytic converters that the characteristics are high efficiency, high durability, low air resistance Catalytic converter is mainly composed by substrate, washcoat. catalyst, damping layer, and shell. it can convert the harmful gases included HC. CO and NOx into non-toxic CO2, H2O and N2. Four-way catalytic converter can also remove some soot. The substrate and catalysts are key parts that influence the performance of the catalytic converter, and they play a decisive role. Therefore, the investigations of catalytic converter focus on the development of substrate and catalyst in recent years. The main performance evaluations of catalytic converter include conversion efficiency, the ignition temperature characteristics, air-fuel ratio characteristics, space velocity characteristics, flow characteristics and durability, etc.The mature technologies of diesel vehicle exhaust aftertreatment are the following: oxidation catalytic converter (DOC), diesel particulate filter (DPF), selective catalytic reduction (SCR), lean burn NOx trap (LNT). In recent years, the technology of eliminating PM, NOx, HC and CO in the four-way catalytic converter has become popular. It will be a very important and promising aftertreatment technology of diesel engine to use emissions of NOx and HC, CO, PM by diesel engine itself in excess oxygen redox reaction to produce harmless N2, CO2 and H2O, and it is a good technical route to meet environmental regulatory requirements.From the 70's of last century, researches on the automotive catalytic converter have never stopped. In 40 years time, the performance of catalytic converter is improved by deeply and continuously research, which reflected by several aspects:the increase of conversion efficiency of exhaust pollutants; high durability; decrease of flow resistance and so on. According to the studies of different focus, the main research can be divided into chemical reaction kinetics, heat and mass transfer process (mainly for heat and mass transfer coefficient), the gas flow characteristics, the inactivation characteristics of catalytic converters, catalytic converter transient behavior.The mathematical model of the mass and energy conservation equations of gas phase and solid phase within substrate channel of the catalytic converter can be established based on the mass and energy conservation law of the gas phase and solid phase of the classical Navier-Stokes equations of fluid dynamics through reasonable assumptions.The software used in present study is developed by the Austrian AVL FIRE which is world-renowned in the engine area and specific to the three-dimensional flow analysis in the engine, and it is recognized one of the best engine CFD software around the world. Computational model of catalytic converter is established by FIRE, and on the basis of previous work, we established the boundary conditions that are against the light-off characteristics. The problems are solved by the FIRE CFD solver. The predictions are in good agreement with the experiment and finite difference method reported before, and it is proved that the modeling is reliable. And the investigations are focused on the light-off performance of catalytic converters, including the substrate wall thickness, substrate type (including metallic and ceramic substrate) and the inlet flow temperature on the catalytic converter performance. The conclusions are:(1) Base on the study of the radial velocity distribution along the substrate, the radial and axial temperature distribution and the CO conversion efficiency on the light-off performance of the catalytic converter, a systematic study of the influence of the ceramic substrate wall thickness on the light-off performance of catalytic converter. The present work investigates the effect of ceramic monolith wall thickness on the temporal variations of the radial solid temperature, mean solid temperature, axial flow temperature and CO conversion efficiencies in monolithic reactors during the light-off phase. It is found that the wall thickness can have a significant influence on the light off performance. When the monolith wall thickness is thinner, the light off performance of automotive catalytic converter is better.(2) Based on the study mentioned, the physical characteristics of the model are changed to investigate the gas and solid temperature distribution in the metal and ceramic substrate. By comparison, the gas temperature inside the metal carrier changes become more and more uniform, while the center of ceramic solid temperature is always much higher than the edge region. It can be proved that the distribution is more uniform and the thermal stress is small in the metallic substrate. The performance of metallic substrate is more superior.(3) The flow temperature field in the catalytic converter is investigated to reflect the inlet temperature on the catalytic converter performance, when the inlet temperature changed in different ways. In the present simulation work, the inlet temperature changes by four kinds of typical ways:linear up; fluctuated in the course of the rise of temperature; showed a ladder up; fluctuated in the steady state. The numerical simulation results showed that the change of the inlet flow temperature can significantly affect the flow temperature field in the light-off process. When the inlet temperature increases linearly, the flow temperature is higher inside the substrate, especially the central region. The temperature is low. while the inlet flow temperature fluctuates or rise like a ladder. In addition, CO conversion efficiencies were investigated under different conditions, and it is found the CO conversion efficiency is higher than the other conditions, when the inlet temperature increases linearly. In the steady state, when the temperature fluctuated, CO conversion efficiency is low. Thus, the inlet flow temperature has a certain influence on the catalytic converter performance.Reference three-way catalytic technology of gasoline cars, in recent years, four-way catalytic converter has been reported. PM. CO, HC and NOx are oxidant and reductant for each other, and they will be removed to achieve the purification of diesel exhaust, so as to achieve long-term environmental protection regulations. The key of four-way catalytic technology is the optimization of catalytic converter and the development of four-way catalyst. This paper introduced the making process and bench test of diesel catalytic converter which substrate is metallic. It is included the pretreatment of metallic substrate surface; preparation and the coating process of sol-gel glass coating; supported mode of three-metal substrate catalyst; the structure optimization of diesel catalytic converter based on the computer simulation; evaluation of its performance by bench tests. The contents include:(1)The Ni-Cr-Fe metallic substrates which are divided into three parts are selected, and the surface pretreatment of the Ni-Cr-Fe is completed. The surfaces become rough and uniform. Every part of metallic substrates is coated by the sol-gel glass coating. The catalyst is coated on the glass coating by dipping.(2)The optimized scheme about the catalytic converter is promoted after the numerical simulation and the analysis about the flow distribution in the inlet header. It is evaluated that it provides a large improvement in flow uniformity. The conversion efficient of poisonous gas would increase, and the durability of the catalytic converter would be improved by means of diffusers.(3) A device that's fixed in the inlet header of catalytic converter is used to diffuse the flow distribution. The performance of diesel 4-way catalytic converter which is coated catalyst and optimized is tested by 13 benches. It found that the removal of PM is up to 85.8%, the removal of the CO is 90%, removal of NOx is 37% from the test results, and the removal of PM, HC, CO, NOx reached to the Euro IV standards. Finally, the results of the exhaust back pressure showed that the pressure loss is within the permitted range, and the influence on the economy property and dynamic property of engine is small. The performance of diesel catalytic converter used in present study is favourable.