Experimental study of the effective thermal conductivity in the near-wall region of a packed pebble bed

•A short description of the newly built NWTCTF test facility is provided.•Experimental investigation of effective thermal conductivity in the near-wall region.•Effects of conduction and radiation heat transfer are included.•Results are presented for structured and randomly packed beds of graphite spheres.•Near-wall effects have a notable influence on the effective thermal conductivity. The effective thermal conductivity is an important parameter that represents the overall heat transfer in packed beds, including packed pebble beds. Experimental test facilities have been used to determine the effective thermal conductivity of packed pebble beds; however, the experimental results are often case specific and do not account for wall effects in the near-wall region. Recently the Near-Wall Thermal Conductivity Test Facility (NWTCTF) was developed at the North-West University in Potchefstroom, South Africa to perform detailed investigations of the conduction and radiation heat transfer phenomena in the near-wall region of a packed bed of spheres. The experimental test facility was introduced in a previous paper and some results were presented. This paper provides a short description of the test facility and then presents extensive experimental results obtained for structured and randomly packed beds consisting of 60 mm diameter graphite spheres at temperatures of up to 800 °C, together with associated uncertainties. Results obtained demonstrate the effect of the near-wall region, the packing structure and the presence of the wall on the effective thermal conductivity.