Hydrodynamic transition delay in rectangular channels under high heat flux

When upgrading a research nuclear reactor for a higher power output it can be expected that the cooling flow rate has to be increased. In the case of a reactor designed with a laminar cooling flow this upgrade may take the flow into the transition hydrodynamic regime.In this work we explore experimentally the laminar to turbulent transition in a rectangular channel with similar characteristics as the refrigerating passages between the fuel plates in a research reactor core. The measurements were performed under heat fluxes up to 10 W/cm2, which is similar to the heat flux found in pool type research reactors. Special care has been taken in the test section to minimize the entrance turbulence and to reproduce the superficial characteristics of the aluminium cladding of the nuclear fuel.We have measured the transition Reynolds number as the bulk Reynolds number at the point of minimum Nusselt number for each condition studied. It was observed that the bulk Reynolds number at which the transition to turbulence is detected depends on the heat flux at the walls. This result is in agreement with a recent theoretical analysis, which shows that property variations near the wall, particularly the reduction of viscosity, have a stabilizing effect on the flow.