Abstract: Based on kinetic model of single particle, forces  upon the single particle in vertical spiral rectangular duct are analyzed. The results indicate that the whole motion of the particle is mainly affected by the centrifugal force, Coriolis force, effective gravity and drag force. There is an obvious exponential relation between λ and ζ , of which λ stands for the ratio of tangential velocity of particle to water velocity, and ζ  represents the ratio of Coriolis force to centrifugal force. When λ =1,the Coriolis force acting upon particle equals zero; when λ >1, the Coriolis force and centrifugal force are in the same direction, both deviating from the center; while when λ <1, the Coriolis force and centrifugal force are in the opposite direction. The probability of Coriolis force being in the same order of magnitude with centrifugal force is 3/4. It's concluded that the Coriolis force should not be ignored in analyzing the force upon particle in small-scale and low-speed rotational flow.

Key words: spiral duct, kinetic model of single particle, Coriolis force, centrifugal force