Flow characteristics of coaxial-nozzle ejector for PEMFC hydrogen recirculation system

Abstract

Multi-nozzle ejectors are an effective method to overcome the conflicts between the ejector fixed geometry and various system operating conditions in the ejector-driven proton exchange membrane fuel cell (PEMFC) system. However, conventional studies usually employ the Reynolds-Averaged Navier-Stokes (RANS) numerical simulation methods to investigate flow field information in multi-nozzle ejectors, neglecting the transient characteristics of turbulent eddies, which will hinder the design and performance improvement of the ejector due to an ambiguous understanding of turbulent characteristics. To overcome this problem, this study adopts the Large Eddy Simulation (LES) method to investigate the transient information of turbulent vortexes and disclose the relationships between operating conditions, fluid flow evolution characteristics and structural parameters of coaxial-nozzle ejector (CNE) in PEMFC system, and then proposes a variable mixing chamber length design to enhance the CNE performance. A three-dimensional simulation model is established and validated by experimental results. The results show that: The fluid velocity self-similarity position (VSSP) inside CNE decreases initially and then increases as the primary pressure increases, and the CNE continuously works under critical mode when the velocity self-similarity appears; The fluid VSSP decreases with the growth of back pressure as the CNE works from critical mode to subcritical mode; The mixing chamber length has a greater impact on the ejector performance in subcritical mode than in the critical mode; The obtained functions between the mixing chamber length and ejector operating conditions can improve the entrainment ratio of CNE about 17.48%; The CNE has more ordered turbulent structures compared with typical ejector under variable operating conditions. This study can effectively guide the design of variable structure ejectors and provide an effective solution for improving the PEMFC system performance under the whole operation conditions.