Experimental and analytical investigation of flow loop induced instabilities in micro-channel heat sinks

Abstract

Two-phase flow instabilities, both static and dynamic, are commonplace in closed two-phase loops. For two-phase micro-channel heat sinks, past studies have reported different types of instability, including density wave oscillation (DWO), pressure drop oscillation (PDO), and parallel channel instability (PCI). In the present study, emphasis is placed on identifying and suppressing dominant instabilities for flow boiling of R134a in a micro-channel heat sink having 609.6-mm long and 203.2-mm wide base area, and containing 100 of 1 × 1 mm2 channels. A dominant charge transition instability (CTI) is identified as causing appreciable oscillations in both mass velocity and pressure drop. Discussed is how PID control could be used to suppress CTI. But PCI was also prevalent within individual CTI oscillation cycles, taking the form of comparatively small amplitude, high frequency oscillations. It is shown CTI and PDO are fundamentally different in that ensuing fluctuations in mass velocity and pressure drop are synchronized for the former, but out of phase for the latter. Additionally, CTI occurs in the positive slope region of the heat sink’s pressure drop versus mass velocity characteristics, while PDO occurs in the negative slope region. A detailed analytical model is also presented to predict onset of CTI. Overall, this study shows how loop instrumentation signals may be used in the model to both predict and suppress instability.