In this example, the authors show how CFD helped redesign a return duct for a dryer. The figure shows a section of duct that connects two chambers of a dryer. Flow moves vertically upward through a return duct on the right, and then vertically downward through a heating coil in the second chamber on the left. The CFD analysis of this duct (image B in the figure) shows a number of opportunities for improvement:
- Because the entry to the duct is off-center in the
previous chamber, the air enters the duct with a horizontal velocity component that
pushes the majority of the flow toward the left side of the duct. The
non-uniformity increases pressure drop and sets the flow up for further
- At the elbow, the momentum of the flow causes most of the air to
crash into the outside and flow across the top of the next chamber. Again,
non-uniformity increases pressure drop and sets up further problems.
- Most of the airflow passes through the left side of the heat
exchanger. This increases pressure drop, has an adverse impact on heat
transfer, and sets up further problems in the next piece of duct.
Image D in the figure shows the CFD output for the new geometry. The inlet transition results in much more uniform flow in the duct, going vertically upward. The turning vanes deliver the air much more uniformly into the top of the heating coil. The resulting pressure drop changes from 4.14" w.c. in the original design to 1.25" w.c. in the final design.