Direct numerical simulation of turbulent natural convection in riser of passive cooling system

The database of turbulent flow by using direct numerical simulation can support the development of more powerful turbulent models. In the paper, the characteristics of averaged velocity, averaged temperature, intensities of fluctuating velocity and temperature, Reynolds stress, turbulent heat fluxes and turbulent structures of natural convection for different Prandtl numbers are studied by using direct numerical simulation. The results show that: (1) The profiles of averaged velocity and averaged temperature of turbulent natural convection are anti-symmetrical to the center-line of the vertical channel, and the locations of the maximum value of averaged velocity and temperature are near the heated wall. With the Prandtl number, the velocity decrease but the temperature gradient increase near the wall. (2) The intensities of fluctuating velocity and temperature, Reynolds stress, turbulent heat fluxes are all decrease with the increase of Prandtl number, moreover, turbulent heat fluxes are negative near the heated wall. (3) There are no regular structures for fluctuating velocity and temperature fields in turbulent natural convection, but the turbulent structures of mainstream velocity keep at constant angle to the wall. (authors)