Analysis of condensation and secondary flows at T-junctions using optical visualization techniques and Computational Fluid Dynamics

T-junctions are employed in almost all piping systems, whenever two streams need to be merged into one duct.In some applications, the mixing between streams plays an important role for determining the performance of down-stream elements. The situation is particularly interesting for low pressure EGR junctions featured in piston engines,where warm humid exhaust gases meet cold fresh air, since the mixing of both streams can produce water conden-sation. This condensation deteriorates the integrity and performance of the adjacent compressor wheel. This workexplores the aforementioned flow configuration in a T-junction by means of a specific gas test bench that allows thecharacterization of the transversal section at the T-junction outlet, and 3D CFD simulations. Two optical techniquesare employed: laser particle image velocimetry, which is used to characterize the cross-section secondary flows, andthe planar laser-induced visualization, which is employed to obtain the condensation pattern. The experimental mea-surements are conducted at two different working points and for two different T-junction designs, being in agreementwith 3D CFD simulation embedded with a previously-developed condensation model. The work shows that the T-junction design dictates whether the air-EGR mixing is either located at a narrow interface or is performed across thewhole cross-section, which boosts the produced condensation. The techniques shown can be therefore employed tooptimize the T-junction design, which would entail a reduction ofNOX,CO2and particulate matter emissions duringengine warm-ups.T型三通（T-junctions）几乎应用于所有管道系统中，用于将两股流体汇集成单管路。在部分应用场景中，两股流体的混合效果对下游组件的运行性能有着关键影响。 活塞发动机中采用的低压废气再循环（EGR）三通是极具研究价值的典型场景：温热湿润的排气与低温新鲜空气在此交汇，二者的混合过程会产生水凝结现象。该凝结现象会劣化相邻压气机叶轮的结构完整性与运行性能。 本研究针对上述T型三通内的流场构型，通过专用气体试验台（可实现T型三通出口横截面特性表征）与三维计算流体动力学（CFD）模拟开展探究。研究采用两种光学测试技术：其一为激光粒子图像测速法（laser particle image velocimetry），用于表征截面二次流场；其二为平面激光诱导可视化技术，用于获取凝结流场分布形态。 实验测量设置了两种不同工况与两款T型三通设计方案，其结果与嵌入了此前开发的凝结模型的三维CFD模拟结果高度吻合。 研究表明，T型三通的结构设计决定了空气与EGR的混合是局限于窄界面内还是贯穿整个横截面，而后者会加剧凝结现象的产生。因此，本研究采用的测试技术可用于优化T型三通的结构设计，从而在发动机暖机阶段降低氮氧化物（NOₓ）、二氧化碳（CO₂）与颗粒物的排放。