In this work, an innovative pyrolysis reactor for waste plastics pyrolysis was proposed based on phase change characteristics of waste plastics during their pyrolysis, which consisted of vertical tubes in parallel and molten polymer would fall down along the inner wall at the same time it "evaporates" to finish pyrolysis. In order to know the pyrolysis behaviors in this innovative reactor, the flow and "evaporation" processes of the molten polymer were simulated as the primary step. The pyrolysis process of molten polypropylene (PP) polymer when falling down along the inner tube wall was chosen as an example for the simulation. The effective conductivity of PP polymer was obtained through specially designed experiment. It has been found that the reactor can be modeled as a vertical falling film evaporator based on the volume of fluid (VOF) method, and the molten polymer film features a wavy structure, which is affected by the heating surface temperature. The fluid temperature rises during the pyrolysis process. Those calculation results provide guide for the reactor design. (C) 2013 Elsevier B.V. All rights reserved.