In the EU-APR1400, an ex-vessel core catcher system, a key design component of considerable interest is the engineered corium cooling system. It consists of a cooling channel in which many short columnar structures called studs are placed to support the static and dynamic loads on the core catcher body and to provide the coolant flow path underneath the core catcher body. This study investigated the effect of aligned obstacles on the inclined downward facing critical heat flux (CHF) under a low flow condition using a high-speed camera and an optical fiber microprobe measuring the local void fraction. A laboratory-scale experimental apparatus for a prototypical core catcher system was built, and experiments were conducted with various mass fluxes and obstacle shapes under atmospheric pressure. Results showed a linear proportional relation between the inlet mass flux and corresponding CHF with the existence of a transition flow rate, beyond which the proportionality becomes double. It was also confirmed that the aligned flow obstacles had a positive effect on the CHF under a relatively high mass flux, but a negative effect appeared in the pool boiling condition. Computational fluid dynamics helped in the understanding of key physical mechanisms through which the obstacles affect the CHF, on the basis of the experimental results. (C) 2018 Elsevier Ltd. All rights reserved.