The studies on microfluidics have advanced with the use of microchannels in micro bioreactors, applications in biomedical engineering, bioengineering and pharmaceuticals, fuel cells and compact heat exchangers for heating and cooling of micro electro mechanical systems. For a wide application of microchannels, it is very important to elucidate the effect of the tube diameters on the flow phenomena of gas liquid two-phase flow in microchannels. The flow phenomena and the frictional pressure drop of gas-liquid two-phase flow were investigated experimentally by using three kinds of circular microchannels made of fused silica tubes, with inner diameters of 0.1 mm, 0.15 mm and 0.25mm. In these channels, bubbly flow, slug-churn flow, and annular flow were observed. In annular flow, two characteristic flows of ring film flow and disturbed ring film flow were observed. For the 0.25 mm diameter microchannel, several conductance probe sensors were flush mounted to the channel wall in order to measure the very thin mean film thickness or liquid holdup in a microchannel. The flow patterns maps were also made by using the high speed video images and the holdup wave signals. The effects of tube diameter on the flow patterns were investigated.

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