(Dr. S. Mostafa Ghiaasiaan , advisor)
"Transport Phenomena in Microchannels and Proton Exchange Membrane Assemblies of Fuel Cells"
In this study forced-flow subcooled boiling phenomena in microchannels (channels with hydraulic diameters in the 100 Ám ~ 1 mm range) and the transport of hydrogen in proton conducting membrane assemblies are addressed. Proton Exchange Membrane (PEM) fuel cells have a large number of applications, and are the focus of current research. This thesis contains a one-dimensional model of the transport of hydrogen as the single working fluid in a PEM assembly. Mathematical models are presented for both a hydrated and an anhydrated PEM electrode assembly. A parametric study is conducted on the basic operating characteristics of the anhydrated assembly.
Data and analysis have shown that bubble nucleation and ebullition phenomena in microchannels are likely to be different than in large channels. The macroscale models and correlations often fail to correctly predict data representing the aforementioned processes in microchannels. The available data dealing with boiling incipience of water in microchannels are analyzed. It is suggested that boiling incipience in microchannels may be controlled by the thermocapillary force that tends to suppress the microbubbles that form on wall cavities. Accordingly, a semi-empirical method is proposed for predicting the onset of nucleate boiling (ONB) in microchannels, and is validated against the available data. Parametric calculations are also made.
It is also hypothesized that hydrodynamically-controlled onset of significant void (OSV) in microchannels is due to bubble departure from wall cavities, and the latter process is controlled by thermocapillary and aerodynamic forces that act on the bubble. Accordingly, the limited available relevant experimental data are semi-analytically modeled, and the soundness of the proposed hypothesis is shown. Some parametric calculations are also performed.
Available experimental data dealing with turbulent flow in microchannels have shown that, due to reasons not fully understood turbulence characteristics in microchannels may be different than large channels. In both the ONB and OSV cases the effect of the turbulence characteristics of the microchannel on the developed mechanistic models are parametrically examined.
The role of the thermocapillary force in the bubble ebullition processes in microchannels is not fully understood. A theoretical study was therefore performed to that end. Quasi-steady bubbles, attached to a heated microchannel surface supporting subcooled nucleate boiling, were modeled, where the effect of surface tension non-uniformity, itself resulting from a non-uniform bubble surface temperature distribution, was considered. The result shows that the thermocapillary effect tends to slightly distort and elongate the bubbles in the direction perpendicular to the heated surface, and leads to a thermocapillary force that resists bubble detachment. The geometric distortion of the bubble also leads to an increase in drag force in comparison with a chopped-spherical bubble with the same volume.