Abstract: An experimental study of flow structure and dynamics and their relationship with the heat transport in turbulent thermal convection is reported. It is shown that the instantaneous large-scale flow structure in a cylindrical convection cell consists of an elliptical circulatory roll and two smaller counter-rotating rolls. As the quasi-2D instantaneous flow structure rotates azimuthally, it generates a time-averaged 3D flow pattern featuring two toroidal rings near the top and bottom plates respectively. It is found that the apparently stochastic azimuthal motion of the flow structure, which generates a net rotation on average, has the characters of a Brownian ratchet. We also provide convincing evidence that a particular value of the heat transport efficiency Nu can be associated with a specific internal flow mode of the system.