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Abstract: A visualization study is conducted to investigate condensation flow in wide rectangular silicon microchannels with the hydraulic diameter of 90.6 μm and width/depth ratio of 9.668. Droplet-annular compound flow, injection flow, and vapor slug-bubbly flow are observed along the channel, which differ from that in other cross-sectional shape microchannels. In the droplet-annular compound flow region, the vertical walls (short side) of the channel are completely covered by the condensate, while droplet condensation still exists on the horizontal wall (long side) of the channel. The location of the injection flow will be postponed with the increasing inlet vapor Reynolds number. The injection frequency will increase with the increasing inlet vapor Reynolds number and condensate Weber number. More specifically, the frequency in the wide rectangular microchannels is lower than that in triangular microchannels having the same hydraulic diameter. It is confirmed that the cross-sectional shape of the microchannel plays a significant role on the instability of condensation flow. In addition, the correlation of Nusselt number is also presented. [Copyright &y& Elsevier]