Unique characteristics of adiabatic gas-liquied flows in microchannels: Diameter and shape effects on flow patterns, void fraction and pressure drop

Microfluidics is a rapidly developing area of research with great potential for a wide range of applications in many fields. One area of microfluidics is gas-liquid two-phase flow in microchannels, which is important for the development of microreactors, lab-on-a-chip systems, micro heat exchangers and micro-heat pipes, among others, that are highly relevant to industry. Recently, much interest has also been shown toward studying the two-phase flow in micro fuel cells. This keynote paper presents a state-of-the-art review of past and present research on adiabatic two-phase flow in minichannels and microchannels, which are considered to have channel diameters between 250 mum - 6 mm, and less than 250 mum, respectively. From this review, certain differences between minichannels and microchannels are identified. These notable differences are also explained, based on some of our recent experiments on two-phase flow in microchannels. Our experiments have been performed using several microchannels to determine the effects of the microchannel diameter and shape on the adiabatic two-phase flow of nitrogen gas and de-ionized water. The effect of channel geometry was examined by characterizing the two-phase flow in a circular and square microchannel of similar hydraulic diameter. A video camera was used to capture images of the gas-liquid interfacial structure. From the video recordings, it became clear that the channel size strongly influences the two-phase flow patterns occurring in the circular microchannel. The flow pattern was predominantly intermittent, exhibiting alternating sequences of liquid and gas slugs. Only slug flow was observed in the microchannel for all flow conditions tested. There were no instances of bubbly flow, chum flow, slug-annular or annular flow, as reported for minichannels. Instead, four new sub-classes of slug flow were defined to better describe the interfacial structure in the time average sense: slug-ring flow, ring-slug flow, semi-annular flow and multiple flow. The time-averaged void fraction was estimated from the recorded images of the two-phase flow structure. It was found that as the channel diameter decreased, the void fraction data deviated more from those obtained for minichannels. A new void fraction correlation was developed for both the circular and square microchannels, which differs significantly from those developed for minichannels. In both microchannels, the two-phase pressure drop was best predicted by treating the two phases as being nonhomogeneous and having a large velocity difference. This result was consistent with the occurrence of slug flow and significant departure of the average void fraction from those in minichannels. A possible explanation for the strong deviation of void fraction data in microchannels from the correlations applicable to minichannels is offered based on a phenomenological examination of the flow structure. Regarding the effect of microchannel geometry, the experimental results showed little difference in the void fraction and pressure drop data. However, the two-phase flow regime maps were not the same between the circular and square microchannels. The transition boundaries of the sub-categories of slug flow were noticeably shifted. The region of ring-slug flow in the circular microchannel disappeared in the square microchannel, which can be attributed to the suppression of the liquid-ring film due to the accumulation of liquid in the corners of the square microchannel.