[en] The thermo fluid dynamic characteristics of natural convection flow depend strongly on thermal boundary condition such as the spatial and temporal variation of heat flux on the pool wall boundaries. In general the natural convection heat transfer phenomena involving the bottom heat generation are represented by the Rayleigh number, Ra, which quantifies the bottom heat source and hence the strength of the buoyancy force. This work focuses on natural convection in which the density gradient is due to a temperature gradient and the body force is gravitational. The presence of a fluid density gradient in a gravitational field does not ensure the existence of natural convection currents, however, in an apparatus enclosed by two horizontal plates of different temperature. The temperature of the lower plate exceeds that of the upper plate, and the density decreases in the direction of the gravitational force. The LIDO (Liquid Internal Dynamics Operation) tests are conducted in a horizontal circular layer 500 mm in diameter and 220 mm in height using fluid, whose thermophysical properties are typified by the Prandtl number, Pr. The tests cover the range of 3x10⁵ < Ra < 1x10¹⁰ and 0.02 < Pr < 2.22 Tests are conducted with air, water and Wood's metal (Pb-Bi-Sn-Cd) as simulant to determine the Nusselt number, Nu. The upper and side walls are cooled, while the lower wall is heated at uniform temperatures