Experimental study of turbulent supercritical open channel water flow as applied to the CLiFF concept

An experimental study of turbulent open channel water flows was conducted that simulated basic features of the flow of molten salt in the convective liquid flow first-wall (CLiFF) concept, which is a part of the Advanced Power Extraction (APEX) study. Unlike many other studies of open channel flows, the present one concentrates on a supercritical flow regime, in which surface waviness and wave–turbulence interaction are the most important processes that determine the heat transfer rate in CLiFF flows. The current study covers the Reynolds number and Froude number range of 1×104–6×104 and 150–250, respectively, with a fixed chute inclination angle of 30°. The statistical characteristics of the wavy interface were obtained with an ultrasound transducer. A spectral analysis of the oscillating flow thickness shows that a major part of the spectrum is presented by long finite-amplitude waves (f=10–50 Hz), which carry a significant part of the volumetric flux. Based on dye technique observations, short waves are mostly responsible for mixing the liquid at the surface. The surface waviness can be characterized by a parameter built through the mean flow thickness, h, and its standard deviation (S.D.), σ, as 0.5σ/h, which is almost constant, 0.1, in all experiments. The mean flow thickness variations are predicted well with the ‘K–ε’ model of turbulence [Int. J. Eng. Sci. 40/6 (2002) 693], but the fluctuations are not resolved. Thermal images of the free surface measured by an infrared (IR) camera are very non-uniform and show the ‘strike’ structures in the form of elongated strips of ‘hotter’ and ‘cooler’ liquid. The present observations are the first steps to better understanding and quantitative predictions of liquid wall flows in the CLiFF design.