High-porosity open-cell metal foams are well known to enhance the heat transfer mechanism in rectangular or circular channels. Their high surface area to volume ratio makes them a great candidate for manufacturing high-performance small-scale heat exchangers. This two-part experimental study investigated the two-phase flow boiling inside a circular copper mini tube. In Part I, the flow pattern was visualized by high-speed imaging in glass tubes. Flow pattern maps, the heat transfer coefficient, and pressure drop are presented for mean vapor quality of 0.1-0.7, heat flux of 20-40 kW/m(2), and mass flux of 400-700 kg/m(2) s. The experiments were also performed without the metal foam in the mini tube for comparison to the original data. In this range of experimental conditions, the metal foam increased the heat transfer coefficient up to 3.2 times. Also, as expected, the metal foams adversely affected the pressure drop inside the tubes. (C) 2016 Elsevier Ltd. All rights reserved.