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型接头广泛应用于管道系统中，每当需要将两条流合并入单一管道时，便会采用T型接头。在某些应用中，流体的混合作用对于下游元件性能的确定至关重要。对于活塞发动机中低压EGR（废气再循环）接头而言，该情况尤为引人注目，因为其中暖湿的排气气体与冷鲜空气相遇，这种流体的混合可能产生水蒸气凝结。这种凝结会损害相邻压缩机叶片的完整性和性能。本研究通过特定气体测试平台对T型接头处的横截面进行表征，并利用3D CFD模拟探索了上述流动配置。研究采用了两种光学技术：激光粒子图像测速技术，用于表征横截面的次级流动；平面激光诱导可视化技术，用于获取凝结模式。实验测量在两个不同的工作点以及两种不同的T型接头设计下进行，与嵌入先前开发的凝结模型的3D CFD模拟结果一致。研究结果表明，T型接头的设计决定了空气-EGR混合是在狭窄的界面处发生，还是在整个横截面上进行，这会加剧产生的凝结。因此，所展示的技术可用于优化T型接头设计，从而在发动机预热期间减少NOX、CO2和颗粒物的排放。