Abstract
A new draft equation involving swirling motions for natural draft dry cooling towers (NDDCTs) is derived and solved analytically. A 2-D axisymmetric model for a short NDDCT is built and computational fluid dynamics (CFD) simulations are carried out to verify the theoretical predictions. Results show that the optimized location for swirl input is at the tower outlet to avoid the swirl decay. It is noted that the swirl influence on the air flow draft velocity gradually becomes significant especially when the dimensionless input swirl ratio exceeds 2. The theoretical predictions generally agree with the numerical results, but deviate when: (1) vortices adjacent to the wall occur in the presence of excessively strong swirl, in the case with swirl created by a thin source zone; (2) cold air inflow penetrates due to the significant heat exchanger resistance coefficient; (3) vortex breakdown appears, in the case with swirl filling the whole tower. All the aforementioned unfavourable phenomena are local effects and thus cannot be predicted by the draft equation unless proper local resistance terms are added.
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