Low-frequency oscillation flow in nitrogen jet condensation

Abstract

Jet condensation is a complicated two-phase flow phenomenon that occurs in pipe flow, especially in cryogenic fluids. In this study, the condensation flow dynamics of a stream of nitrogen (N2) injected into a subcooled liquid nitrogen (LN2) flow was photographically investigated using a high-speed camera. The pulsating pressure was measured at different observation points using dynamic pressure transducers. Three typical condensation flow patterns were observed and measured under different nitrogen volume fluxes: suck-back flow, transition flow, and oscillation flow. Suck-back flow occurs at a smaller N2 volume flux, where the LN2 is periodically sucked back into the gas chamber. Oscillation flow is encountered at larger N2 volume flux, along with intense pressure fluctuations. Using fast Fourier transform analysis, the frequency of these pressure oscillations was found to be in the range 16–18 Hz with a maximum amplitude of about 10 kPa. An image-processing method was developed to track the fluid motion in the dynamic photographs recorded by the high-speed camera according to the local changes of pixels. The results show the presence of a periodic shrinking of the vapor-plume trail with the same frequency as the pressure oscillation. The mechanism for the low-frequency oscillation flow is, therefore, identified as periodic interfacial condensation. The findings of this study provide strong evidence for the origin of the low-frequency pressure oscillations observed in cryogenic liquid rocket engines. This information can be used to improve the design and operation of such propulsion systems.