A numerical and experimental comparison of heat transfer in a quasi two-dimensional packed bed

Heat transfer plays a major role in many industrial processes taking place in packed beds.An accurate and reliable simulation of the heat exchange between particles is therefore crucial for a reliable operation and to optimize the processes in the bed.The discrete ordinates method(DOM)provides an established numerical technique to model radiative heat transfer in granular media that offers the possibility to consider the directional dependence of the radiation propagation.In this work,DOM is compared with Monte Carlo ray tracing,which provides an alternative method for heat transfer simulations.Geomet-rically simple configurations are used to investigate the influence of the angular discretization on the accuracy of the results and the computation time in both methods.The obtained insights are then transferred to a more complex configuration of a quasi two-dimensional test rig consisting of metal rods for which also experimental results are available.Our results show that both DOM and Monte Carlo ray tracing allow for an accurate simulation of heat transfer in packed beds.Monte Carlo ray tracing requires thereby computation times that are surprisingly competitive(although still somewhat slower)compared to DOM and allows for an easier computation of highly accurate reference solutions.In our preliminary comparison to the experimental test rig,Monte Carlo ray tracing also provides the advantage that it can more easily model highly specular materials such as stainless steel.Both methods are comparable for diffuse materials such as magnesium oxide.

Experimental investigation of radiative heat propagation in a simplified generic packed bed

A novel test rig for the investigation of radiative heat transfer in packed beds has been developed and is introduced with representative experimental results.The individual components and the calibration are discussed.The generic packed bed is realized in a simplified way by an arrangement of parallel rods,which represent particles in pseudo-2D.In this arrangement,electrically heated rods provide the radiation propagating through the rod array to heat the passive counterparts.A sophisticated temperature-control scheme with a large number of thermocouples and infrared-imaging provides in-depth information about heat transfer in the system.Spectral radiation intensities are determined with a Fourier-transform infrared spectroscopy,which has been modified and validated for this specific application.In order to compare the influence of different surface properties of particles on the heat propagation and surface reflections,rod samples made of stainless steel and magnesium oxide are used.The influence of material properties becomes clearly visible by comparing the high radiation intensities resulting from a stainless steel rod array to the same geometry built from magnesium oxide rods.In addition,the influence of the surface properties is particularly evident in the infrared images since the reflections are significantly higher for the stainless steel samples than for the magnesium oxide samples.The experimental results in the current work demonstrate the ability of the test rig to provide data with a well-defined accuracy as a validation base for numerical radiation simulations in packed beds.