In this study, the natural convection heat transfer from chimney-based radial heat sinks was investigated numerically and experimentally. The thermal performance of a radial heat sink with chimney was estimated and compared with that of a radial heat sink without a chimney. The effects on the heat sinks' thermal performance were investigated in regard to the orientation angle with respect to gravity (0 degrees <= theta <= 180 degrees), the fin number (15 <= N <= 30), and the distance between the base plate and chimney (5 mm <= H-c <= 25 mm). The results show that there exist optimal values for and N. The orientation angles 0 degrees and 45 degrees result in slightly better thermal performance than 135 degrees and 180 degrees, while 90 degrees results in the worst thermal performance. The chimney-based heat sink with optimal structure enhances the thermal performance by up to 20% compared with the radial heat sink without a chimney. (C) 2015 Elsevier Ltd. All rights reserved.