Experimental analysis of natural convection in a differentially heated cavity

Turbulent heat fluxes are an important factor in the determination of heat transfer in fluids. Measurement of these fluxes is challenging as it requires simultaneous measurement of velocity and temperature fluctuations. The current work is conceived in a project aiming at developing high-fidelity instrumentation to acquire simultaneous measurements of velocity and temperature in liquid metals, mainly used as coolant in nuclear reactors. The main purpose is to get information about the turbulent thermal diffusivity, which is a fundamental term in the modelling of turbulent heat transfer in natural and forced convection. In this paper, natural convection in a differentially heated cavity is studied with water as working fluid preparing for experiments with low melting point liquid metals that will be carried out in further stages of the project. The setup consists in a cubical cavity in stainless steel 316L with an edge of 60 mm and constant temperature boundary conditions imposed on both sides of the cell by two Peltier elements used to induce the natural convection flow. Therefore, the cavity is analyzed with an imposed temperature difference of 60°C corresponding to a Grashof number of 2×108, theoretically at the limit of transition from laminar to turbulent flow. In fact, transition to turbulence is expected to appear with liquid metals in the given conditions but the same result is not obvious in water due to its higher Prandtl number. Various techniques are adopted for temperature measurements (Thermocouples and Fiber Bragg Gratings) and for velocity measurements (Ultrasound Doppler Velocimetry). The preliminary experimental results obtained underline the limitations of the measurement techniques in the case water is used as working fluid. The comparison carried out with with numerical results obtained through DNS simulations highlights the difficulty of measuring experimentally the extremely low values of velocity expected, and therefore the turbulent heat flux. At this stage of the project, limited insights can be gained from a physical standpoint. In summary, despite the current limitations in results, a robust methodology was successfully established for utilizing the same experimental setup with liquid metals in future stages of the project.