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Diabatic experiments of evaporating flows of pure methanol and ethanol are conducted to study heat transfer and pressure drop in both square and circular cross section tubes of hydraulic diameter 521um and 543um respectively, in a range of mass fluxes 60 < G < 700kg.m-2.s-1 and heat fluxes 50 < q”s < 140kW.m-2. The heat transfer coefficient is higher for low vapor qualities and shows little dependence of mass flux for G < 500kg.m-2.s-1. Nucleate boiling heat transfer decreases as the liquid film near the wall fully evaporates during long periods of vapor passage Instabilities are observed to occur under these conditions. The corners in the square cross section prevent complete evaporation and higher heat transfer coefficients are found. The experimental heat transfer data is compared to correlations developed for microchannel subcooled and nucleate flow boiling showing similar trends, but lower values than predicted. The results from visualization clearly show the transition between flow patterns with known and well defined frequency. They also present no difference between the top and bottom thin film thickness, meaning very low influence of gravity forces in the flow. Wavy plugs and fingerlike structures are observed with high-speed visualization and described. Flow pattern maps for methanol for both configurations are obtained. Results for circular cross section show discrepancies from previous studies as for square cross section the results are found to agree.