(96b) Comparison of Adiabatic Gas-Liquid Two-Phase Flows in Microchannels and Minichannels

Gas-liquid two-phase flows in small channels occur in micro heat exchangers, PEM fuel cells, micro reactors and other energy-related equipment. To better understand the flow characteristics and design more efficient devices and equipment, the adiabatic gas-liquid two-phase flow characteristics in small diameter circular and square channels have been analysed by examining the nitrogen gas-water two-phase flow data. Recent two-phase flow experiments conducted using microchannels with a diameter of 20 ~ 500 microns have shown both significant differences and some similarities with those measured in minichannels with a diameter of 500 microns ~ 5 mm. The microchannel flows have also been shown to be sensitive to the inlet geometry and the method of gas-liquid injection. In this work, the two-phase flow patterns, void fraction and two-phase pressure drop data obtained in small channels of 100 microns ~ 5 mm diameter have been analysed and compared to further clarify the effect of channel diameter on various two-phase flow parameters, including a wave velocity. In circular channels, a ring flow pattern has recently been considered to be a characteristic of microchannel two-phase flow, however, it could also be observed in minichannels with a diameter greater than 1.0 mm. This flow pattern is a type of an annular flow with a wavy interface profile with the wave peaks spaced apart at a regular interval. The wave velocity data obtained in an annular flow pattern showed that it increases steadily with a decreasing diameter from 5 mm to about 1.5 mm, and then starts to decrease as the channel diameter is further reduced. The radius of curvature of the channel affects the wave velocity as the significance of surface tension force dramatically rises as the channel diameter is reduced.

The void fraction data obtained in microchannels and minichannels showed significant differences depending on the channel cross section and inlet geometry. For the microchannel with a diameter of 100 microns, the effect of the inlet geometry on the void fraction was seen to be quite strong, while the minichannels showed a much smaller effect of inlet geometry on the void fraction. In square channels, a ring flow pattern observed in microchannels could also be observed in a 1.0 mm x 1.0 mm channel, however, the conditions under which this flow pattern could occur was limited in comparison with the circular channel. The liquid tended to flow along the four corners, reducing the liquid film thickness in both microchannels and minichannels. As a result, the void fraction data in square channels showed lower values in comparison with those obtained in circular channels. The two-phase pressure drop data also showed an increasingly stronger effect of the inlet geometry as the channel diameter was reduced.