Drag Coefficient

Drag is the force offered by the liquid on the solid in the direction of flow. This drag is the function of the resistive force offered by the dynamic liquid and dynamic gas on their interaction and also resistive force offered by the stationary solid phase. The drag coefficient is related to the pressure drop over a bed of particles. Drag coefficient should be applicable to a bed with a voidage ranging from the packed bed condition to the fully expanded state. Under turbulent flow conditions the dominant contribution to energy dissipation in a bed of particles comes from the numerous expansions and contractions of the fluid streams in their passage around the particles. As fluid streams do not follow the surface of the particles strictly in the turbulent regime, we believe that in this regime the surface structure does not influence the energy dissipation. The large discrepancy between measured and predicted pressure drops can be elucidated by considering the drag force on a single particle in the bed. the drag coefficient of packed bed might be inuenced by wall effects.A good measure for this drag force is the drag coefficient Cd. This drag coefficient can be related to the pressure drop Pper unit bed height Drag coefficient can be determined byDrag coefficient = It is observed that the drag coefficient decrease as the Reynolds no. Increases the drag coefficient decreases sharply for the particular value of Reynolds no. and then its stay almost constant.

Fig.- Effect of Reynolds no. on drag coefficient