In this online-only sidebar to "Using CFD Analysis to Reduce Energy Consumption," CFD analysis for a return duct is shown. In this dryer, flow moves vertically up through a return duct, and then vertically down toward a heating coil.

The return duct connects two chambers of a dryer. (A) 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. (B) CFD analysis of the duct showed a number of opportunities for improvement. (C) To improve the airflow through the return duct, a transition was added at the duct inlet, and turning vanes were positioned in the elbow and above the heat exchanger. (D) CFD analysis of the revised duct shows the inlet transition results in more uniform flow in the duct.


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 problems.
  • 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.
To improve the situation, a transition was added at the duct inlet and turning vanes were strategically positioned in the elbow and above the heat exchanger (image C in the figure). The cost impact of these changes is very small.

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.

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