A numerical model based on an analytical lumped vapor assumption was proposed for highly efficient simulation of transient performances of heat pipes. The wick is modeled as fully thawed porous medium in which both the Darcian and non-Darcian effects are considered. The evaporation and condensation rates of the working substance are calculated locally as a function of not only the liquid-vapor interface temperature but also the vapor state properties by the kinetic theory. The coupled equations for liquid flow and heat conduction inibetween components of the heat pipe are solved by a thermal lattice Boltzmann algorithm. Validation of the model is conducted by reproducing representative cases from the literature and then comparing the present results with their experimental and theoretical data. It turns out that both the transient temperature variation and the steady-state temperature and pressure profiles are in accordance with the literature results. The vapor velocity profile inferred from the evaporation rates is also found to be sufficiently accurate, which even gives a more reasonable estimate than the reference in comparison. In order to further improve the simulation efficiency of the code, nonuniform lattice and parallel algorithm are incorporated, based upon which the lumped vapor model achieves a speed over 50 times faster than the plain model with complete vapor consideration. The present model could serve as an efficient tool for quick evaluation of transient heat pipe behaviors and for assisting parametric studies of heat pipes. (C) 2016 Elsevier Ltd. All rights reserved.