Experimental study of flow transitions in random packed beds with low tube to particle diameter ratios

In the present study, the electrochemical technique is used to test flow transitions in random packed beds with five low tube to particle diameter ratios (N=d_t/d_p), including N=2.6, 5.3, 8.1, 9.9 and 12.5. The microelectrodes are placed at the tube wall and inner particle surfaces to test the local flow at the pore level, with particle Reynolds number (Re) ranging from 20 to 2200. The critical Reynolds numbers corresponding to the end of laminar flow and onset of turbulent flow are obtained according to Fluctuating Rate (FR) of current signals. The results of tube wall probes and inner probes are compared in detail to analyze the influences of the tube to particle diameter ratio N on flow transitions. It is found that, the critical Reynolds numbers corresponding to the end of laminar flow regime of packed beds with different tube to particle diameter ratios are at about 110 for packed beds with N=5.3, 8.1, 9.9 and 12.5. Meanwhile, as the tube to particle diameter ratio N increases, the onset of turbulence would take place earlier for inner probes in different packed beds. Furthermore, the critical Reynolds numbers corresponding to both the end of laminar flow and onset of turbulent flow in the packed bed with N=9.9 are quite close to those in the packed bed with N=12.5, which would indicate that, when N≥9.9, the wall effects on flow transitions in the packed beds would be unremarkable.