本文關(guān)鍵詞： SCR系統(tǒng) 尿素沉積物 過渡態(tài) 混合器 混合算法 優(yōu)化設(shè)計(jì)　出處：《廣西大學(xué)》2016年碩士論文　論文類型：學(xué)位論文

【摘要】：為了應(yīng)對柴油機(jī)國家排放法規(guī)中對氮氧化物排放量的嚴(yán)格要求,機(jī)外凈化選擇性催化還原(selective catalystic reduction,SCR)成為了必不可少的關(guān)鍵技術(shù),然而此系統(tǒng)也帶來了新的問題-尿素沉積物風(fēng)險(xiǎn)。尿素沉積物的存在會影響催化器進(jìn)口處NH3摩爾濃度的分布均勻性,從而影響SCR系統(tǒng)的NOx轉(zhuǎn)化效率。本文運(yùn)用CFD仿真手段建立了詳細(xì)的沉積物機(jī)理模型,研究沉積物生成規(guī)律,尋求影響沉積物產(chǎn)生的因素�；谶@些因素,運(yùn)用先進(jìn)的混合算法對SCR系統(tǒng)進(jìn)行多參數(shù)結(jié)構(gòu)優(yōu)化,實(shí)現(xiàn)減少甚至避免SCR系統(tǒng)中沉積物生成的目的,以此來降低發(fā)動機(jī)背壓和提高SCR的NOx的轉(zhuǎn)化效率。構(gòu)建了尿素分解化學(xué)反應(yīng)動力學(xué)模型包含縮二脲、三聚氰酸和三聚氰酸一酰胺的生成機(jī)理,涉及15種組分和11步反應(yīng),彌補(bǔ)了液膜模型現(xiàn)象級分析的不足。同時(shí)采用分子貢獻(xiàn)基團(tuán)的方法,建立三聚氰酸一酰胺的比熱容與溫度的關(guān)系式,獲取了機(jī)理模型所需要的熱力學(xué)參數(shù)。提出了基于量子化學(xué)計(jì)算方法、利用過渡態(tài)求解化學(xué)反應(yīng)速率的思路,解決了在無法通過文獻(xiàn)和實(shí)驗(yàn)獲取反應(yīng)式C3H3N3O3+ NH3→C3H4N4O2+H2O的活化能和指前因子的難題。開展了幾何結(jié)構(gòu)和噴射參數(shù)對尿素沉積物和NH3均勻性的影響研究。研究表明:管路長度和管路半徑的增加,都會增加尿素液滴在排氣管路中的停留時(shí)間,提升催化器入口處NH3均勻系數(shù),雖然管路長度對NH3均勻系數(shù)的影響低于管路半徑的,但對尿素分解效率的影響會比管路半徑的顯著。增加混合器后,增加了管路內(nèi)的湍流和渦流強(qiáng)度,能夠大幅度提升NH3均勻系數(shù)和降低尿素沉積物的生成。噴霧錐角和噴射角度的改變對NH3均勻系數(shù)和尿素沉積物的濃度的影響并不明顯。運(yùn)用先進(jìn)的SHERPA算法,設(shè)立管路長度、管路半徑、混合器安裝位置、混合器葉片數(shù)目、葉片長度、葉片傾斜角度、噴霧錐角以及噴射角度這8個(gè)設(shè)計(jì)變量,并以此8個(gè)變量構(gòu)建優(yōu)化搜索空間,對整個(gè)SCR模型進(jìn)行優(yōu)化研究。結(jié)果表明:當(dāng)管路長為1.2m、直徑為0.15m、混合器安裝位置為0.6m、葉片數(shù)為3、葉片長為0.02m、葉片傾斜角為30度、噴射角度為45度以及噴霧錐角為80度時(shí)NH3均勻系數(shù)最好,為0.816,提高了 39%。
[Abstract]:In order to meet the strict requirements of the national emission regulations for NOx emissions from diesel engines, selective catalytic reduction of selective catalystic reduction (scr) has become an indispensable and key technology. However, this system also brings about a new problem-urea sediment risk, the presence of urea sediment will affect the distribution uniformity of NH3 molar concentration at the catalyst inlet. In order to affect the efficiency of NOx transformation in SCR system, a detailed sediment mechanism model is established by CFD simulation method, and the rules of sediment generation are studied, and the factors affecting sediment generation are sought. The advanced hybrid algorithm is used to optimize the multi-parameter structure of SCR system, so as to reduce or even avoid sediment generation in SCR system. In order to reduce the back pressure of the engine and improve the conversion efficiency of NOx of SCR, a kinetic model of urea decomposition chemical reaction including biuret, cyanuric acid and monoamide tripolycyanate was constructed, which involves 15 components and 11 steps. The method of molecular contribution group was used to establish the relationship between specific heat capacity and temperature. The thermodynamic parameters needed for the mechanism model are obtained. Based on the quantum chemistry calculation method, the idea of solving the chemical reaction rate by using the transition state is presented, which solves the problem of obtaining the reactive C _ 3H _ 3N _ 3O _ 3 NH3 in the paper and in the experiment. 鈫扵he effects of geometric structure and jet parameters on the uniformity of urea sediment and NH3 were studied. The results showed that the length and radius of the pipeline increased. Both increase the residence time of urea droplets in the exhaust pipe and increase the NH3 uniformity coefficient at the entrance of the catalytic converter, although the influence of the length of the pipeline on the uniformity coefficient of the NH3 is lower than that of the radius of the pipeline. However, the effect on urea decomposition efficiency is more significant than the radius of the pipe. When the mixer is added, the turbulence and eddy current intensity in the pipe are increased. The uniform coefficient of NH3 can be greatly increased and the formation of urea sediment can be reduced. The change of spray coning angle and injection angle has no obvious effect on the uniformity coefficient of NH3 and the concentration of urea sediment. Using advanced SHERPA algorithm, the length of pipeline is established. The eight design variables, such as pipe radius, mixer installation position, mixer blade number, blade length, blade tilt angle, spray cone angle and spray angle, are used to construct the optimal search space. The optimization of the whole SCR model shows that when the pipe length is 1.2 m, the diameter is 0.15 m, the installation position of the mixer is 0.6 m, the number of blades is 3, the blade length is 0.02 m, the blade inclination angle is 30 degrees. When the injection angle is 45 degrees and the spray cone angle is 80 degrees, the NH3 uniformity coefficient is the best (0.816), which increases 39%.
【學(xué)位授予單位】：廣西大學(xué)
【學(xué)位級別】：碩士
【學(xué)位授予年份】：2016
【分類號】：TK423

【參考文獻(xiàn)】

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本文編號：1496985