【作者】 刘丽萍；

【导师】 田茂诚；

【作者基本信息】 山东大学 ， 热能工程， 2009， 博士

【摘要】 为满足日益苛刻的排放法规,仅凭机内净化措施来减少柴油机的微粒排放是极其困难的。另外,尽管机内净化技术使微粒物的质量排放总量得以削减,但微粒的个数却没有减少,而且生成了粒径更小的排放物。越来越多的研究表明,柴油车排放的小微粒对人体健康危害更大,而未来的法规很可能会对柴油机微粒物排放的数量进行限制。因此越来越多的国家关注机动车的小微粒排放,而微粒过滤器是未来解决小微粒排放问题的最有效措施之一。基于此点,本文围绕柴油机微粒过滤系统进行了如下研究:1.柴油机微粒过滤器对柴油机微粒组分特性影响的研究。运用自行设计、制作的柴油机微粒取样器,对未安装过滤器、安装袋滤器、安装陶瓷过滤器三种情况下的柴油机排气微粒进行采集,分析柴油机工况对微粒组分特性的影响,以及微粒中可溶有机组分(SOF-Soluable Organic Fraction)中烷烃和芳香烃等成分的排放特性等。研究结果表明:在相同转速下微粒排放量随负荷的增加而增大。随着负荷的增加,微粒中SOF的百分含量在逐渐减少,不可溶有机成分(IOF-Insoluble Organic Fraction)的百分含量在逐渐增加。SOF的含量从低负荷时的15%到高负荷时的45%左右,其对微粒特性的影响不能忽略。微粒中可溶性有机物SOF中正烷烃的总含量占到了70%～80%;支链烷烃的总含量在2%～22%之间;多环芳香烃的总含量在1%～13%之间。柴油机在不同运行工况下,其排出SOF碳原子数分布稍有不同。另外,袋滤器过滤后微粒中SOF的百分含量比陶瓷过滤器过滤后的略低:袋滤器过滤后微粒SOF中各组分百分含量均在20%以下,各组分过滤比较均匀;陶瓷过滤器过滤后个别工况个别组分的相对含量达到近40%;袋滤器对多环芳香烃的过滤效率要高于陶瓷过滤器。综合来看,袋滤器是一种过滤效果较好的后处理装置。2.柴油机排气微粒燃烧特性的分析研究。对采集的柴油机排气微粒,利用热重分析技术,研究了微粒在氧气浓度分别为10%、20%、30%,升温速率分别为10℃/min、20℃/min、30℃/min、40℃/min,微粒质量分别为＞5mg、3.5mg～3mg、＜1.5mg的燃烧性能。结果表明:微粒的燃烧明显分为挥发份析出(低温段)和固定碳(高温段)的燃烧两个阶段。低温段的失重占总失重的20%左右;高温段的失重占总失重的70%左右,说明微粒燃烧过程主要是固定碳的燃烧。氧气浓度较低的时候,燃烧需要较高的温度;反应温度较低的时候,需要较高的氧气含量。综合起来看,在氧浓度较低时,随着氧气浓度增大,燃烧性能呈现良好的趋势。但同时也表现出,氧气浓度为20%、30%时的情况比较接近,与氧气浓度为10%时的差距比较明显。说明提高空气中氧气浓度对微粒燃烧性能影响不大。另外研究还发现,当氧气浓度在20%、30%时的反应平均活化能为60kJ/mol左右。目前采用燃油添加剂可以使微粒的活化能降低到这个数值,也就是说如果用空气或含氧量更高的气体来做燃烧反应气氛的话,可以满足过滤器被动再生时对微粒活化能的要求。3.根据理想流体质点的声传播控制方程,建立了考虑流速、热传导和粘性时,微粒过滤器内的声传播控制方程。以此为基础,利用达西定律求解微粒过滤器相邻单元孔壁的压降,进而建立了微粒过滤器的传播常数和传递矩阵的计算模型。以此模型计算了发动机在特定工况下的插入损失,并和试验测量的数值进行了对比分析,验证了理论分析的正确性。4.建立了微粒过滤器气体流动的三维数值模型。以κ-ε紊流模型理论为基础,把过滤单元作为多孔介质,模拟了袋滤器内的排气流动特性,并用FLUENT软件进行求解。最后将数值计算值与试验值进行对比,验证了所建模型的正确性。计算结果表明:在袋滤器内部,压力是层层递减的,而在过滤单元处突然降低,说明过滤单元是产生压力损失的主要原因。另外,入口壁面附近也是产生压力损失比较集中的一个位置。袋滤器结构参数会影响整个袋滤器的压力损失。5.设计了适用于固定式柴油机以及公交车等对场地要求不严的设备,利用袋滤器技术来控制微粒排放的固定式卸灰系统,解决了再生难的问题。更多还原

【Abstract】 Nowadays,it is very difficult to resort to in-cylinder control strategies only to meet the increasingly rigorous laws and regulations for diesel emissions control.In addition,even though the in-cylinder control technology could reduce the total mass of particulate emissions,the number of particles remains almost the same and smaller diesel particles are generated.More and more studies have shown that those smaller diesel particulates are more harmful to human health.And it is quite probable that future laws and regulations will set a limit to the quantity of diesel particulates emissions.Therefore,more and more countries have focused their attention on the control of the number of small particulates emissions.The particulate filter is one of the most effective methods to solve such problems.In this paper,a series of research has been carried out concerning the diesel particulate filter system.1.Research on influences of diesel particulate components of diesel particulate filters.A particulate sampling device has been designed to sample the diesel engine particulates in three cases:base line engine without any filters,engine with bag filter,engine with ceramic filter.The influence of diesel engine operating conditions on the characteristics of particulate components is analyzed,as well as the emission characteristics of alkane and aromatic hydrocarbons in SOF(Soluable Organic Fraction).The research results show that at the same engine speed particulate emissions increase with the increasing engine load.The results also show that with the increase of engine load,the percentage of SOF in particulate decreases gradually,while the percentage of IOF(IOF- Insoluble Organic Fraction) increases gradually.The percentage of SOF increases from 15%to 45%,its influence on particulate characteristic can not be ignored.The total content of n-paraffin hydrocarbon in SOF of particulate ranges from 70%to 80%,and that of branched paraffin is between 2%and 22%;multiring hydrocarbon takes up from 1%to 13%.Under different operating conditions,the number of carbon atom in SOF is distributed slightly different.In addition,the percentage of SOF in particulate is a little smaller with the bag filter than with the ceramic filter.With the bag filter,the percentage of each component in SOF after filtration is all below 20%,relatively evenly filtered.The relatively content of individual component in certain operating mode is nearly 40%after filtered by the ceramic filter.The filtration efficiency of bag filter to multiring hydrocarbon is higher than that of the ceramic filter.In summary,bag filter is a better post-treatment device.2.Research on combustion characteristics of diesel particulates. By the use of the thermal gravimetric analysis technique,this paper studies the combustion performances of those collected diesel exhaust particulates under the following conditions:the oxygen content is 10%,20%,30%;heating rate is 10℃/min,20℃/min, 30℃/min,40℃/min;and the particulate mass is more than 5mg, from 3.5mg to 3mg,less than 1.5mg.The experimental results show that the particulate combustion is clearly divided into two stages, volatile matter burning(low-temperature stage) and fixed carbon burning(high-temperature stage ).The weight loss at low-temperature stage takes up about 20%of the total weight loss, and the high-temperature stage about 70%.It shows that the combustion of carbon is the main part of the combustion process. When the oxygen content was lower,the combustion of the particulates needs higher temperature.When reaction temperature is lower,it requires higher oxygen content.In summary,the combustion performance tends upwards with the increase of the oxygen content at low oxygen content.Meanwhile,the results also show that the combustion performance is almost the same when the oxygen content is 20%and 30%.While compared with the performance at 10%oxygen content,the difference is obvious.This indicates that increasing the oxygen content has little effect on the combustion performance of particulates when oxygen content changes from 20%to 30%.The research also finds that average reaction activation energy is about 60kJ/mol when the oxygen content is 20%and 30%,which is equivalent to the value caused by the addition of fuel additives used in ceramic filter.At present,the use of fuel additives can reduce the activation energy of particles to this figure,that is to say,when setting a combustion reaction atmosphere with air or higher-oxygen-content gas,the requirements of passive filter regeneration to the activation energy of particles can be met.3.Based on the sound propagation control equation of ideal fluid particle,the equation used in particulate filter is deduced taking flow velocity,thermal conductivity and viscosity into account. On this basis,by using Darcy’s law to solve the pressure drop problem of hole wall at neighboring filter unit,the filter propagation constant and the computation model of transfer matrix are set up. Based on this,the insertion loss of engine under specific operating mode is calculated and compared with the measured value,verifying the correctness of the theoretical analysis.4.The three-dimensional numerical model of gas flow of particulate filter is set up.On the basis ofκ-εturbulent flow theory,the filtration unit is assumed to be the porous medium and the exhaust flow characteristics in bag filter is simulated and solved with the FLUENT software.The comparison of numerical values and experimental values verifies the correctness of the established model. The calculating results show that the pressure decreases progressively in the bag filter,while decreases abruptly in filtration unit.This indicates that the filtration unit is the main cause of pressure loss.In addition,the neighborhood of the entrance wall surface is another concentrated position of pressure loss.The structural parameters will also affect the whole pressure loss of bag filter.5.The stationary particulate-discharging system is designed.It can be applied to stationary diesel engines as well as buses and other equipment with little request to location and which use bag filter to control particulate emissions.The difficult problem of regeneration is solved.更多还原