Various mathematical models in literature conclude that the porosity profile varies in an oscillatory fashion and the amplitude decreases with increasing distance from the wall. However, scrutiny of the literature indicates a distinct deviation in the models that a strong porosity peak at the centerline is not amenable to the predictions for packed beds with small tube to particle diameter ratios. In this study, the radial porosity peak at the centerline of packed bed is investigated by experiments. According to the structure of packed beds at D/d < 337, the bed is divided into two parts-the near wall part and the central part. The mean porosity value of different parts is obtained by counting method. In the experimental result, high porosity value at the central part is observed. Then, the reasons that lead to the porosity peak-the highly ordered ring structure and the internal channel similar to the bed at D/d < 2-are identified. Moreover, the channel at the centerline promises that significant bypass flow would occur in the packing arrangement, which should be taken into account for the analysis of transport phenomena in packed beds. (C) 2017 Elsevier B.V. All rights reserved.