This article describes a comparison between a packed-sand geometric model derived from experimental digitization of a small volume of sand and from a mathematical model (Yet Another Development Engine, YADE) that produces a random packing of spherical particles. As the application of the model is towards smoldering combustion, the comparison focuses on the pressure drop and convective heat transfer at very low Reynolds numbers and a wide range of high temperatures. The comparison shows that the YADE geometric model gives results that are in good agreement with the digitized packed-sand model in the range of Reynolds numbers considered, which, given the cost and time saving of using a mathematical model, is an extremely useful result. The geometric model was also used to explore the influence of particle size variation and temperature dependence of air properties affects the flow and convective exchange at temperatures relevant to smouldering combustion. A correlation is presented to demonstrate how the heat transfer results can be put into a compact form for use in computations using a porous continuum approach. (C) 2018 Elsevier Ltd. All rights reserved.