Aerodynamic Drag Optimization for Commercial HVAC Units
CFD-driven external aerodynamic study reducing fan power consumption by 18% on a commercial rooftop packaged air conditioning unit through systematic geometry modification.
Project Brief
A leading HVAC equipment manufacturer approached Shirsh TechnoSolutions to investigate unexpectedly high fan power consumption in their latest 15-ton rooftop packaged unit. Field measurements showed the unit was drawing 12% more fan power than predicted by catalog-based hand calculations, leading to failure against the target EER (Energy Efficiency Ratio) specification.
The manufacturer needed to quantify the root cause of the excess pressure drop and identify geometry modifications that could recover the performance deficit without altering the unit footprint or coil specifications.
Problem Statement
The packaged rooftop unit exhibited excessive external static pressure loss across the condenser section and discharge plenum. The recirculating airflow pattern at the condenser coil face was non-uniform, with measured face velocity variation exceeding ±35% — well beyond the ±15% target. This maldistribution degraded coil heat rejection capacity and forced the condenser fans to operate at higher speed to compensate, directly increasing energy consumption.
Hand calculations based on uniform flow assumptions could not capture the three-dimensional flow separation and recirculation zones responsible for the performance shortfall.
Approach & Solution
- Resolving flow separation at the condenser inlet with complex 3D geometry requiring 14 million cell mesh
- Validating CFD predictions against limited field data (static pressure taps at only 4 locations)
- Balancing mesh resolution requirements against OpenFOAM solution turnaround time for 12 design variants
- Maintaining manufacturing feasibility of guide vane geometry within sheet metal forming constraints
Shirsh TechnoSolutions delivered a full external aerodynamic CFD study using OpenFOAM (steady-state RANS with k-ω SST turbulence model) for baseline flow characterization, followed by parametric design sweeps in ANSYS Fluent for geometry optimization with its adjoint solver capability.
The optimized design incorporated revised inlet bell-mouth geometry, condenser coil face angle adjustment, and discharge plenum guide vanes — achieving an 18% reduction in total fan power while maintaining the same heat rejection capacity.
- 1CAD preparation in SolidWorks: defeatured full unit geometry, extracted external fluid domain with atmospheric boundary
- 2Baseline CFD in OpenFOAM: snappyHexMesh with boundary layer refinement, steady-state simpleFoam with k-ω SST
- 3Validation: compared simulated static pressures at measurement tap locations against field data (within 8% agreement)
- 4Root cause analysis: identified flow separation at condenser inlet and recirculation in discharge plenum via streamline visualization
- 5Optimization in ANSYS Fluent: used adjoint solver to compute surface sensitivity, iterated on inlet and plenum geometry modifications
- 6Final design verification: confirmed 18% fan power reduction with maintained coil face velocity uniformity within ±12%
Project Outcome
- Fan power consumption reduced by 18%, recovering the EER target with margin
- Condenser coil face velocity uniformity improved from ±35% to ±12%
- Design changes implemented within existing unit footprint — no tooling changes required for the cabinet
- CFD methodology established as standard practice for all future product development in the manufacturer's engineering team
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