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This paper presents the results of the flow boiling performances in an open microchannels heat sink under different spatial orientations with a wide range of mass fluxes. Subcooled flow boiling experiments were conducted with deionized water as the coolant in an open microchannels heat sink with 31 parallel microchannels and 400 µm uniform gap over the channel top. The microchannels were 305 µm in width and 312 µm in depth and having a fin thickness of 300 µm. Flow boiling experiments were conducted with the heat sink positioned in four different orientations such as horizontal upward facing (HU), horizontal downward facing (HD), vertical downflow (VDF) and vertical up flow orientation (VUF) orientations with mass flux ranging from low, 77 kg/m² s to high, 911 kg/m² s. The performance of the open microchannel configuration was also compared with the closed microchannel performance …