Experimental study and prediction of flow instability of supercritical nitrogen in a vertical upward tube

Supercritical cryogenic fluids are widely used in thermal equipment for liquid air energy storage (LAES), hypersonic aircraft, liquid hydrogen, etc. Unstable flow of supercritical fluids can cause thermal fatigue, mechanical vibration, noise, etc. In this work, a cryogenic heat transfer system was established to study the flow instability characteristics of supercritical nitrogen (SN₂) in a vertical tube. The dynamic unstable flow of density-wave oscillation was captured in the experimental system. The oscillation amplitude of wall temperature is more than 100 K and the temperature peak can reach 150% of T_ave. Based on the pseudo-two-phase model, the effect of several key parameters was analyzed. Unstable flow is prone to occur when the outlet state is located in the two-phase-like (TPL) region with dramatic changes in physical properties. The outlet state can be kept away from the TPL region by two approaches. One is to locate the outlet in the vapor-like (VL) region by increasing the heat flux, decreasing the mass flux, and increasing the inlet temperature; the other one is to locate the outlet in the liquid-like (LL) region by decreasing the heat flux, increasing the mass flux, and decreasing the inlet temperature. When P_in is close to P_c, the TPL region is extremely narrow, and the outlet state is easily located in the VL region. The dimensionless p-h diagram is proposed to divide the unstable flow region of supercritical fluids. Unstable flow occurs in the region where the outlet dimensionless enthalpy (N_SPCO) is located between the VL boundary (N_{SPCO_V}) and the LL boundary (N_{SPCO_L}). The analysis of flow instability characteristics and the proposed prediction methods can provide a basis for the system design of supercritical cryogenic fluids.