Characteristics of Micro-Liquid-Layer Behavior in a Mini-Channel Vaporizer for Water and Ethanol

With growing demands for a cleaner global environment, there is a need for automotive powertrains that can provide higher efficiency and contribute to lower C02 emissions. From this perspective, fuel cell vehicles are expected to gain a share of the automotive market in the 21st century. The reformer type fuel cell vehicle requires a heat exchanger with high efficiency and a low heat capacity to meet powertrain requirements for quick response and compactness. The use of a mini-channel vaporizer for the reformer is one possible way of meeting these requirements. In a mini-channel, the bulk liquid, bubbles and thin film that form between the heating surface and the bubbles affect the heat transfer characteristics in complex ways, just as in the case of pool boiling. However, as reported in the literature, the characteristics of evaporation in a mini-channel are completely different from those of pool boiling, because of the important role played by the superheated thin film that forms in the process of bubble expansion. For example, although deterioration of heat transfer characteristics has been reported for an extremely small gap, the mechanism involved has not yet been analyzed. In order to research and develop a mini-channel vaporizer efficiently, it is necessary to elucidate the mechanism of these phenomena, and clarify measures for improving heat transfer characteristics. Then a simple method may be devised, based on the related findings, to predict the heat transfer characteristics of a mini-channel vaporizer with sufficient accuracy at the development stage. In this study, by using ethanol instead of water as used in past research, the thickness of the thin film that plays an important role in the heat transfer characteristics was measured by application of the laser extinction method for channel gap sizes of 0.3 and 0.15 mm. The process of bubble growth was simultaneously recorded with a high-speed camera. These results were then compared with those obtained for water. Similar trends as those obtained for water were observed for ethanol. That is, observations show that the thin film thickness is essentially influenced by the growth speed of the interface between the bubble and the bulk liquid and that it approaches a certain value above a certain speed. At the same time, the thickness of the thin film decreases with a decrease in gap size.