In order to separate gas and solid two phases completely, the velocity of particles at the exit of Laval nozzle should be high enough, therefore the Laval nozzle should be long enough.

• ??了更好地实现气、固两相分离，颗粒在缩放喷管出口处的速度应该足够大，因而缩放喷管就应该足够长。