Flow instability of supercritical hydrocarbon fuel in a multi-channel cooling system

Abstract

Thermally-induced flow instabilities are a critical issue in multi-channel regenerative cooling systems. In particular, the interactions between Density-Wave Oscillations (DWO) and Flow Maldistribution (FMD) can result in complex and disastrous instability phenomena. This study investigates the instability behaviors of hydrocarbon fluid in a four-channel system with a constant heat flux ratio using both frequency- and time-domain methods. As the heat flux increases, the in-tube flow sequentially destabilizes in each channel and converges to new equilibrium states, leading to the emergence of FMD phenomena. This also causes the system eigenvalue to change repeatedly from negative to positive rather than increasing monotonically. Additionally, the system eigenvalues are between those of the two most unstable channels, indicating that the stability behavior of the entire system is dictated by the most unstable channel. After FMD occurs, flow oscillations are activated in channels with weak stability, and the in-tube flow is observed to evolve into various flow patterns, including stable flow, self-sustained oscillation, oscillation divergence, quasi-periodic oscillation, and oscillation excursion. The novel instability mode of oscillation excursion involves a spontaneous transition of operating states. It oscillates from an equilibrium state and then stabilizes at a new operational state after oscillation-induced redistribution. However, the newfound stable state may also be only temporary, with the in-tube flow regressing to the initial state, resulting in quasi-periodic oscillation.