3D measurements and numerical computations of heat transfer coefficients on spheres in an array

The focus of this paper is to investigate the heat transfer coefficient distributions on the spheres in a three-dimensional (3-D) array. This arrangement is an essential geometry in pebble bed reactors (PBRs) that are generally adopted in the chemical and the nuclear engineering. Understanding the thermal-hydraulic characteristics of the pebbles is important for the design of reactors. Using the transient liquid crystal technique, an experimental device is set up to measure the transient wall temperature on the surface of spheres arranged in an array, as heated air flows across. Based on the measured temperature distributions on the sphere surfaces, the heat transfer coefficient can be derived. A 3-D transient computational fluid dynamics (CFD) model with different turbulence models is also developed and assessed using the measured heat transfer coefficient distributions. Five turbulence models are considered in this study: the standard k-ε low-Re, AKN k-ε low-Re, standard k-ε two-layer, realizable k-ε two-layer, and v²̄-f turbulence models, respectively. Comparisons of the predicted heat transfer coefficient distributions and those in the experimental data reveal that the v²̄-f turbulence model is more suitable for simulating flow and heat transfer characteristics in a sphere array. In addition, the beneficial effect of Re_in on the heat transfer coefficient distribution is captured by both experimental measurements and CFD predictions.