【作者】 张钊；

【导师】 田华；

【作者基本信息】 天津大学 ， 工程（专业学位）， 2021， 硕士

【摘要】 内燃机排气由于温度高、热量多等特点,具有巨大的余热回收潜力。在众多余热回收技术中,热力循环技术因具有效率高、适应性强、运行稳定等特点,被认为是有效方案之一,而其中的烟气换热器主要用于回收排气中的热量,是直接影响整个循环热效率的关键部件,受到学术界的广泛关注。内燃机中燃料燃烧会生成碳烟颗粒,排气中的碳烟颗粒沉积在换热器表面,形成的沉积层导热系数很低,这将导致换热器的换热性能大幅降低,除此之外,还会增加制造成本、运行成本、维护费用,増加设计的不确定性。因此,为减少换热器的性能衰减,针对碳烟颗粒沉积机理和沉积特性的研究具有重要意义。本文通过探究碳烟颗粒沉积分布特性,找出颗粒沉积集中的区域,并具有针对性地开展换热器结构优化研究,具体内容如下。本文以Kern-Seaton模型为基础,建立了碳烟颗粒沉积分布的数值模型,并对管排换热器进行了CFD计算,以颗粒沉积率为衡量指标,探究了碳烟颗粒在换热管上以及不同管排处的沉积分布特性,并分析了流速、入口温度、壁面粗糙度、纵向管间距等参数对碳烟颗粒沉积的影响。结果表明:碳烟颗粒的沉积主要发生在颗粒速度较小且与换热表面碰撞几率较大的区域,沉积位置集中在第一排管正面的滞止区与各排管后的尾流区附近,由于管后尾流区的存在,不同流速与粒径下第三排管处的沉积率均为最大,沉积占比最高达57.8%;颗粒沉积率与流速呈负相关、与入口温度和壁面粗糙度呈正相关变化;当纵向管间距增大时,颗粒沉积率先稳定后上升,针对顺排管排换热器,综合考虑沉积率和换热压降性能,纵向管间距为1.75时最优。基于碳烟颗粒沉积分布特性,进一步设计了不同换热管形状(圆管、椭圆管)以及不同管排布置方式(顺排、叉排),探究了不同结构对其沉积性能的影响以及沉积前后换热、压降性能的变化,并提出了一种最佳结构。结果表明:在换热性能方面,沉积前叉排结构的换热性能普遍更优,但其沉积后性能下降明显,使得大部分顺排结构的换热性能更好;在压降方面,沉积前后顺排结构性能均更优;综合换热压降性能,在沉积前叉排结构更优,沉积后顺排结构更优,性能最优的结构为顺排轴长比r/R=0.8的椭圆管结构;在不同入口温度下,顺排轴长比r/R=0.8的椭圆管结构的平均换热压降性能相较于顺排圆管和叉排圆管,在沉积前分别提高了23.28%和9.94%,在沉积后分别提高了51.63%和223.31%,故在本文的工况与条件下,此结构为最佳结构。更多还原

【Abstract】 The exhaust gas of internal combustion engine(ICE)has great potential for waste heat recovery due to its high temperature and high heat.Among the many waste heat recovery technologies,the thermal cycle technology is considered to be one of the effective solutions due to its high efficiency,strong adaptability,and stable operation.The exhaust gas heat exchanger is mainly used to recover the heat in the exhaust gas.It is a key component that directly affects the thermal efficiency of the entire cycle and has received extensive attention from academia.However,the combustion of fuel in an internal combustion engine will generate soot particles,and the soot particles in the exhaust will be deposited on the surface of the heat exchanger.The deposited layer formed has a very low thermal conductivity,which will cause the heat transfer performance of the heat exchanger to be greatly reduced.In addition,there will bring problems such as increased manufacturing costs,operating costs,maintenance costs and increased design uncertainty.Therefore,in order to reduce the performance degradation of heat exchangers,research on the deposition mechanism and characteristics of soot particles is of great significanceThis paper establishes a numerical model that can calculate the distribution of soot particles based on the Kern-Seaton model.The CFD two-dimensional simulation calculation was then conducted on the tube heat exchanger.Taking the particle deposition rate as a measure,the particle deposition distribution on the heat exchange tube were examined,and the influence of parameters such as flow velocity,inlet temperature,wall surface roughness,and longitudinal tube spacing on the deposition of soot particles was explored.The results show that: The deposition of soot particles mainly occurs in areas with low particle velocity and high probability of collision with heat exchange surface.The particle deposits accumulate primarily in the stagnation region of the first-row tubes and the wake region of each tubes.The deposition rate at the third row of tubes is the largest,with a deposition rate of up to 57.8%.The particle deposition rate is negatively correlated with flow velocity,and positively correlated with inlet temperature and wall surface roughness.When the longitudinal tube spacing increases,the particle deposition rate stabilizes firstly and then increases.For the inline tube heat exchanger,considering the particle deposition and heat transfer pressure drop performance,a tube spacing value of 1.75 is recommended.Based on the deposition and distribution characteristics of soot particles,we further designed different heat exchange tube shapes(round or elliptical)and different tube row arrangements(in-line or staggered),and explored the effects of different structures on their deposition performance,and the changes in heat transfer and pressure drop performance before and after deposition.The results show that: In terms of heat transfer performance,the heat transfer performance of the deposited front fork row structure is generally better,but its performance decreases significantly after deposition,making most of the in-line structure better heat transfer performance;In terms of pressure drop,it is better to arrange in-line before and after deposition;Considering heat transfer and pressure drop performance,before deposition,the staggered arrangement is better than in-line arrangement.After deposition,it is better when arranged in line.The structure with the best performance is the elliptical tube row structure with the axial length ratio r/R = 0.8 arranged in line.At different inlet temperatures,compared with in-line round tubes and staggered round tubes,the average performance of the elliptical tube structure with the axial length ratio r/R = 0.8 in the in-line increases by 23.28% and 9.94% respectively before deposition,and increases by51.63% and 223.31% respectively after deposition.Therefore,this structure is the best structure under the working conditions and conditions of this article.更多还原