A novel design of a GPM-filled pipe structure was proposed to improve the heat transfer and reduce pressure drop of fluid flowing through the pipes filled with gradient porous materials. The pore-size gradient and porosity gradient were studied for both partially and fully filled configurations. The effects of GPMs on the fluid flow and heat transfer in the pipes were investigated and compared with the those under the conditions of non-porous materials and homogeneous porous materials (HPMs) serving as controls. Some typical GPM configurations were studied with Rp = 0.6 and Rp = 1.0, showing an enhanced heat transfer and a relatively low friction factor can be reached in comparison with the controls. An attempt was made to illustrate the mechanism of heat transfer enhancement with the field synergy theory. Velocity-based average pore-size was introduced to explain the reduction in friction factors in GPM configurations. A tradeoff analysis between pressure drop and heat transfer enhancement was made based on performance evaluation criteria (PEC). (C) 2015 Elsevier Ltd. All rights reserved.