Stability analysis of parallel channels in flow boiling system: Flow maldistribution and density wave oscillations

Abstract

In a flow boiling thermal-hydraulic system, Density wave oscillations (DWO) and Flow maldistribution (FMD) are two important instability phenomena. These instabilities can lead to undesirable consequences of system operation. Thermal-hydraulic system with an odd number of channels behave differently as compared to an even number of channels. Following FMD for a two channel system, it leads to a new steady state operating condition. Here, one channel receives higher flowrate and another channel receives lower flowrate for a forced flow system. It may be possible that the new operating point is in a DWO zone. Post FMD, the individual flowrate of the channel starts to oscillate. It shall be noted that under constant mass flowrate condition, such local behaviour in the channels will be difficult to detect. For a twin-channel forced flow system, DWO which may be observed with/without FMD is related to out-of-phase oscillations only. It is thus interesting to study the interaction of DWO and FMD phenomena for a tri-channel flow boiling system. Symmetry breaking transition in the context of excursion instability (i.e., Ledinegg instability) is analyzed in this work. Here, a two and three channel system is used to analyze the two phenomena, FMD and DWO. The difference between the two and three channel flow boiling system is brought out in this work. Unlike two channel system, it is observed that a region in three channel system exists where both symmetrical and asymmetrical equilibrium solutions are unstable. Further, post FMD the solution starts to oscillate but the total system inflow and outflow remains constant.