Boiling heat transfer in micro-geometries has been studied extensively in the recent years as an efficient heat removal mechanism. Numerical simulations have been traditionally employed to resolve the small spatial and temporal scales of the flow, thus accessing many details of the flow unattainable experimentally. However, existing fundamental analyses mainly have focused on circular channels that could be modeled with 2D axisymmetric geometries, while flows in more complex cross-sections are still seldom explored. Since geometries for microfluidic applications have typically non-circular shapes with sharp corners, this constitutes an important gap in the literature. Built on this consideration, the present work presents a fundamental investigation of the fluid flow and heat transfer features associated with slug flow boiling in a square microchannel. Numerical simulations are conducted with the open-source finite-volume solver OpenFOAM 2.3.1. The built-in VOF method is adopted to capture the interface dynamics, while phase change has been introduced through external subroutines. Flow configurations that differ in terms of geometry of the channel cross-section (circular and square) and liquid flow rate are analyzed. It is observed that the bubble dynamics and heat transfer change considerably depending on the channel geometry. In the square channel, the bubble always travels faster compared to the circular tube, while the liquid film thickness reaches values which are one order of magnitude smaller given the same capillary and Reynolds numbers due to the thickening at the corners and thinning on the faces. The flow configuration in the square channel undergoes a substantial transition as the capillary number is reduced, as the bubble shape becomes non-axisymmetric, the interface curvature changes sign along the sides of the cross-section, and the liquid film thickness varies considerably along the perimeter of the cross-section. This has a profound impact on the heat transfer performance, with extremely high values of the heat transfer coefficient along the faces while the corners of the square cross-section give a negligible contribution to heat removal. Importantly, draining flows which are transversal to the main stream direction make the liquid film thinner as the bubble rear is approached, so that in a square channel the liquid film will always eventually dry out when the bubble becomes sufficiently long. (C) 2018 Elsevier Ltd. All rights reserved.