Transient laminar natural convection regimes occurring in a thermal convection loop heated from below and cooled from above are investigated numerically for a wide range of Rayleigh numbers spanning the interval from 10(3) to 2.6 x 10(7). In the model system, the lower half of the loop is heated and maintained at a constant high temperature, while the upper half is cooled and maintained at a constant low temperature. A three-dimensional numerical model based on the finite volume method is used to solve the system of governing flow equations. Simulations are performed using water as the working fluid (Pr = 5.83) and detailed numerical results are presented and discussed for conduction, steady convection, and unsteady flow regimes. Although this subject has attracted researchers for decades, there have been no detailed three-dimensional numerical simulations of the dynamics of flow in the thermal convection loop. The objective of the present study is to fill this gap by presenting the temporal evolution of the velocity and temperature fields at key locations within the system. Emphasis is given to the analysis of dynamical behavior of the flow during the unsteady regime. The complexity of flow in the loop, which is characterized by vertical structures and flow recirculation, is visualized for the first time by performing detailed 3-D numerical simulations. (C) 2011 Elsevier Ltd. All rights reserved.