This thesis presents a systematic investigation on the characteristics of adiabatic gas-liquid two-phase flow and boiling heat transfer in miniature noncircular channels. It has been shown that the capillary action resulted from both the channel size reduction and the channel sharp comers has significant impacts on the characteristics of both adiabatic two-phase flow and boiling heat transfer....[ Read more ]

This thesis presents a systematic investigation on the characteristics of adiabatic gas-liquid two-phase flow and boiling heat transfer in miniature noncircular channels. It has been shown that the capillary action resulted from both the channel size reduction and the channel sharp comers has significant impacts on the characteristics of both adiabatic two-phase flow and boiling heat transfer.

The study of Taylor bubble behaviors in vertical miniature circular and noncircular (triangular and rectangular) channels shows that Taylor bubbles in the noncircular channels always rose upward even though the hydraulic diameter was as small as 0.866 mm. The visual study on the adiabatic two-phase flow patterns indicates that a so-called capillary bubbly flow existed in the mini triangular channels when their size was sufficiently small. It is also found that flow pattern transition boundaries are substantially different from those in large-sized circular channels. In this work, we have also developed a correlation for predicting the frictional pressure drops of single-phase laminar and turbulent flow in the mini triangular channels. With this newly proposed friction factor correlation for single-phase flow, it is found that two-phase frictional pressure drops can be well predicted by the Lockhart-Martinelli's correlation.

An experimental study of pool boiling of water around an electrically-heated platinum wire placed in a miniaturized channel is also reported in this work. The experimental results show that the CHF decreased drastically as the channel size shrank from the unconfined space. The boiling curves further demonstrate that both the CHF and the heat transfer rate for the triangular channel are higher than those for the square channel with same hydraulic diameter. Subsequently, an approximate analysis of boiling heat transfer in a heated channel submerged in a liquid pool is presented. The analysis shows that the critical heat flux for triangular channel is higher than for a circular tube with same hydraulic diameter. The experimental results of pool boiling of FC-72 in heated vertical mini circular and triangular channels submerged in a liquid container show that the channel geometry has a significant effect on the boiling characteristics. For instance, vapor blocking in the smaller circular channel with a diameter of 1.1 mm caused severe boiling hysteresis phenomena. The boiling curves further demonstrate that both the CHF and the heat transfer rate for the triangular channel are higher than those for a circular tube with a similar diameter.

Keywords: two-phase flow, bubble rising velocity, flow pattern, frictional pressure drop, pool boiling, critical heat flux, noncircular channel, and mini/micro channel.