Experimental investigation of subcooled density-wave oscillations in concentric annulus

Abstract

Although flow boiling instability has been extensively studied, the effects of thermal nonequilibrium in confined annular geometries remain obscure, highlighting the need for systematic experimental investigation. This study investigates the onset of flow instabilities in subcooled flow boiling within confined concentric annuli where boiling and condensation coexist. High-speed visualization and time-resolved signal analysis reveal that volumetric fluctuations induced by periodic bubble expansion and contraction lead to oscillations in flow rate, pressure, and surface temperature. For unstable cases, the characteristic oscillation frequency corresponds to a Strouhal number of 1.4-5.9 based on the heated-section residence time, indicating that the instability is governed primarily by local phase-change dynamics. Unlike density-wave oscillations in saturated flow boiling, which are primarily driven by momentum changes from bubble expansion, the observed instability arises from volumetric fluctuations induced by competition between boiling and condensation. A flow regime map constructed using the Boiling number Bo, modified Jakob number Ja**, and subcooling to surface superheat ratio Delta Tsub/Delta Tsat shows that flow instability occurs when excessive vapor generation indicated by high Bo and low Delta Tsub/Delta Tsat coincides with active condensation indicated by high Ja**. The observed flow instability is associated with an elevated level of thermal nonequilibrium, typically in the range of (Ts - Tf)/(Tsat - Tf) approximate to 1.2-2.0, compared to stable cases. The Saha-Zuber net vapor generation criterion successfully captures over 95% of instability conditions owing to its capability to represent net vapor generation, where the coexistence of bubble expansion and contraction induces volumetric fluctuations that trigger flow instability. However, it fails to predict re-stabilization near saturation because it does not account for the removal of condensation from the instability cycle. The proposed stability map provides physical insight into instability mechanisms in confined annular geometries and offers guidance for the design of high-power thermal management systems.