(Dr. Said I. Abdel-Khalik, advisor)
"Theoretical Modeling of Onset of Ledinegg Flow Instability in Heated Channel"
Microchannels subject to forced convective cooling by subcooled liquids are utilized in high performance cooling systems of various modern industrial applications. The occurrence of subcooled boiling can lead to the onset of significant void (OSV), and eventually may lead to the onset of flow instability (OFI) which is an important safety threshold for systems cooled with liquids. The subcooled boiling phenomena in microchannels are not well understood. The objective of this investigation was to theoretically assess the applicability and relevance of the well-known bubble ebullition phenomena that lead to the onset of nucleate boiling (ONB), OSV, and OFI, to microchannels.
The well accepted mechanistic models for ONB and OSV were utilized for parametric studies, and the mechanistic models for hydrodynamically controlled OSV were modified for the inclusion of the surface tension forces on bubble internal pressure. The parametric studies indicated that, due to their extremely small diameters, bubbles generated in microchannels may be strongly influenced by the Marangoni and Saffman forces. The well accepted models for ONB and OSV may thus be inadequate for microchannels.
A mechanistic model for OFI which is based on the numerical solution of one-dimensional conservation equations, accounts for thermodynamic non-equilibrium (i.e., the presence of subcooled liquid and saturated vapor) by applying an empirical quality profile fit, and assumes that significant voidage occurs only beyond the OSV point in heated channels, was validated against recently generated experimental data dealing with OFI in heated thin annular channels. The model well predicted the conditions that led to OFI in the experiments.