Experimental and numerical investigation of wall temperature in a heated pipe filled with beads

Abstract

Placing granular materials inside empty channels with coolant or heat transfer fluids flowing through is an effective way to increase the heat transfer rate to the wall. Although this approach has been used in different engineering fields, a particular goal is seldom considered in some applications, namely the temperature uniformity of the bounding wall. As temperature varies along the flow direction, a non-uniform temperature field appears on the wall. This bears consequences in some applications such as in electronics, battery cooling or composite manufacturing. In the present study, the uniformity of the wall temperature is investigated numerically for a typical cylindrical pipe filled with monodisperse ceramic beads and heated by air flow. Numerical simulations based on Reynolds-averaged Navier-Stokes (RANS) Computational Fluid Dynamics (CFD) are performed to analyze the heat transfer to the bounding wall. Because of the uncertainty mentioned in the literature, the applicability of an empirical model reflecting additional sources of production and dissipation of turbulent energy due to beads is firstly evaluated. By comparing the numerical predictions of temperature fields with experimental data, a damping function is needed to adjust these additional source terms to better fit experimental data for bulk Reynolds numbers between 100 and 700. A parametric study is then conducted to assess the effect of various parameters on the wall temperature uniformity. Results shows that wall materials of low volumetric heat capacity and a reduced wall thickness greatly shorten the duration to reach thermal uniformity. A less pronounced enhancement also exists for larger thermal conductivity of the wall material. However, using beads of lower volumetric heat capacity does not have a major influence on the uniformity of the wall temperature.