Numerical investigation of radial mixing capabilities in strongly buoyancy-influenced vertical, turbulent channel flows

The flow behavior in the HDR downcomer during setting of the initial conditions for blowdown tests is investigated with the numerical simulation program for turbulent channel flows, TURBIT-3. This computer code is based on the complete 3-dimensional non-stationary basic equations for mass, momentum and heat. The subgrid scale models used for the turbulence structures not directly resolved by the grid are extended to take into account the buoyancy in the case of turbulent channel flow. The extended computer code is used to investigate how fast differences in temperature can be reduced, which are caused by inadequate mixing in the lower plenum during upward flow in the downcomer under conditions of mixed convection. It appears that, contrary to the computations neglecting the influences of buoyancy, the temperature differences are rapidly reduced already in the entrance zone of the downcomer. In this zone, local recirculation takes place in the cold region, which is quickly suppressed with increasing distance from the entrance by the intensification of the turbulence effects. A hot chimney extending through the whole downcomer cannot develop. Already at half level, the influence of buoyancy can be considered to be negligible in the downcomer which is assumed adiabatic. Under these conditions it should be possible in principle to set the enthalpy stratification by the planned layout of the experiment in the HDR-pressure vessel.