Simulation of Gas Flow Instability in a Spray Dryer

In this paper, turbulent flow behind a sudden pipe expansion followed by a contraction is simulated numerically, in order to investigate the effect of the downstream contraction on the flow instability in non-swirling flows, as well as in weakly swirling flows. Calculations are carried out using a commercial CFD code (CFX4.4), in which the transient Reynolds averaged N–S equation approach and the standard k–ɛ model are implemented. The first case investigated is the effect of a sudden contraction with varying length. The diameter ratio for both the expansion and the contraction is 5. The length of the large pipe (normalized by its diameter D) is varied in the range from 1.0 to 4.0, and the results are compared with those for the case without a contraction. The current results show that a sudden contraction tends to stabilize the non-swirling flow when the large pipe length is reduced. This stabilizing effect does not apply to swirling flow, although the precessing direction and frequency are affected significantly. The second case simulated is the gas flow in a simple short-form spray dryer configuration, for which some measured data are available in the literature. A self-sustained quasi-flapping oscillation is predicted, which behaves in a similar way to the phenomenon observed in experimental measurements.