Interaction of density wave oscillations and flow maldistribution for two-phase flow boiling parallel channels

Abstract

Density wave oscillations (DWO) and flow maldistribution (FMD) are two important instabilities among others in two-phase flow boiling systems. As both the phenomena can lead to undesirable consequences such as channel burnout, the design intent is to avoid the same. DWO is caused by the interaction between single-phase and two-phase region as perturbation propagates slowly in the later region. Whereas in case of FMD, it leads to one channel receiving higher flowrate and other channel receiving lower flowrate for a twin parallel channels system under forced flow condition. Thus, channels subjected to similar thermal hydraulic and geometric condition receive non-identical flowrate. Bifurcation analysis is used to identify DWO and FMD instabilities in this work. DWO is identified by Hopf bifurcation, thus Hopf curve which constitutes Hopf bifurcations represents the DWO boundary. FMD has been observed earlier through numerical simulation but the region which depicts the same in a parametric space has not been demarcated in previous studies. In this work, FMD is identified by a pitchfork bifurcation. This leads to straight-forward identification of FMD boundary as a pitchfork curve. A combined analysis of multiple instabilities using a single stability map can demarcate regions with the dominance of particular instability. It can thus provide a better perspective for system design and system instabilities in terms of suitable operating region selection. A stability map which can capture and demarcate both the phenomena simultaneously i.e, DWO and FMD in a parametric space is the interest of present work. The boundary of DWO and FMD splits the parametric space into three distinct regions. These are DWO region, a region of symmetrical stable solutions and FMD region which contains non-identical (asymmetric) stable solutions. This breakdown of symmetry of the system is observed by non-identical flowrate in the channels. In some cases, boundary depicting DWO and FMD intersects. Hence, a fourth region is also observed which possess the characteristics of both the instabilities. The stability map shows the transition between these regions.