We report an experimental study on the enhancement of heat transfer efficiency in turbulent thermal convection via manipulation of the bulk flow, which is achieved by inserting artificial structures into the convection domain. These structures are designed to alter the bulk flow without perturbing the boundary layer. Two types of convective domains are designed and fabricated. The Type I domain has an extruded square grid structure in the bulk and the Type II domain can be viewed as a modification to the first one that devides the domain into parallel connected dual-channel structures. The working fluid is water at a Prandtl number of Pr = 4.3 and the Rayleigh number covers a range of 6.4×10⁷ ≤ Ra ≤ 8.5×10⁹. It is found that structuring the bulk flow can enhance the Nusselt number by a factor of up to 14% for the Type I domain and by 30% for the Type II domain, respectively. Local temperature and velocity field measurements reveal that the enhancement in Nusselt number results from both an increase in plume coherency and an enhanced flow strength. The latter feature is in sharp contrast with the significantly reduced flow strength found in geometrical confinement induced heat transfer enhancement reported in previous studies. Our results thus shed new light on flow manipulation and inspire future research on optimization of thermal control.