Characterization of the velocity field and thermal in a mixed convection context

A new sealed wind tunnel has been constructed at VKI to perform measurements in low Prandtl gas mixture; a Helium-Xenon (HeXe) gas mixture. The aim of this facility is twofold: provide a better understanding of thermal turbulence in low Prandtl number fluids and produce data for non-confined flows representative of pool type configurations. The objective of this new measurement campaign is to extend the existing database in forced and natural convection to mixed convection. This particular regime is of high interest for MYRRHA given the mixed circulation flow patterns that are found in some areas of the upper plenum, or during transition between operational to natural circulation mode. A mixed convection boundary layer (MCBL) is analyzed in He-Xe, measuring velocity and temperature fluctuations up to several kHz by means of a probe that combines hot-wire and cold-wire sensors. The He-Xe mixture is characterized by a Prandtl number (Pr=0.2) that is an order of magnitude lower compared to air. These differences are found to be sufficient to trigger a change in fluid behavior as expected in low Prandtl fluids, therefore proving very useful for the validation of new heat transfer models better suited for thermally-driven flow regimes. Potential improvements of the facility could include in the future stronger heating of the plate so as to reach Reynolds numbers closer to real scale, while still maintaining the Richardson similarity on the thermal effects.