Numerical Simulation and Mechanism Study of Liquid Nitrogen Flow Instability in a Sudden Expansion–Contraction Channel

Sudden expansion–contraction channels are critical components in engineering systems. Understanding fluid flow within this geometry is essential for predicting jet mixing characteristics, holding significant scientific and practical value. This paper employs numerical simulation methods to investigate the flow and heat transfer characteristics of liquid nitrogen jets. The analysis of flow and heat transfer evolution under varying structural parameters reveals that the internal flow transitions from a steady state to periodic oscillations and, ultimately, to chaos as the Re increases. Further investigation of the mechanism behind this nonlinear phenomenon, through velocity spectrum analysis, velocity phase diagrams, and Poincaré section analysis, verifies the existence of multiple solutions in nonlinear systems. The results provide theoretical foundations for studying similar nonlinear systems and for the design and application of engineering installations.