Experimental investigation of a confined heated sodium jet in a co-flow

Low-Prandtl-number convection is investigated in vertical axisymmetric turbulent buoyant sodium jets discharging into a slowly moving ambient. Measurements of mean velocity, mean temperature and temperature fluctuations are performed simultaneously using a miniature permanent-magnet flowmeter probe. By varying the ratio of momentum to buoyancy flux, or the densimetric Froude number, different intensities of buoyancy are obtained giving a range of conditions encompassing forced-convection jets, buoyant jets and plumes. In line with the classical properties of jets the radial velocity and temperature profiles can be described by the Gaussian function, independent of the flow regime, at all axial measuring positions. The axial decay of the centreline mean velocity for sodium is the same as for fluids of higher Prandtl number, governed by power laws with indices of -1 for forced convection, -2/3 for the transitional buoyant region and -1/3 for plume flow. In contrast, the centreline mean temperatures for sodium plumes decrease with a power of -1 compared with the -5/3 decay for fluids of higher Prandtl number. The different behaviour in sodium is due to the dominance of molecular diffusion in heat transport, while momentum transport is dictated by turbulent diffusion, which gives a similarity solution for forced-convection jets but not for buoyant jets or plumes. The radial profiles of the temperature r.m.s. values can be described by an axisymmetric curve with two maxima, independent of the flow regime, at all axial measuring positions and the two maxima are more pronounced than in conventional fluids. The temperature fluctuations are analysed to give statistical parameters such as minimum and maximum values, skewness, flatness, probability density functions and spectral distribution. The spectral distributions display both a convective subrange and the conductive subrange predicted for fluids of low Prandtl number. Integral length scales of the temperature fluctuations are evaluated and found to be significantly smaller than turbulent velocity scales.