Optimizing Air-Gapped Server Room Cooling using CFD

Abstract

High-density computing environments generate massive heat loads. In "Air-Gapped" server rooms—where no external network connection exists—physical access is restricted, making equipment reliability critical. This report details the use of Computational Fluid Dynamics (CFD) to simulate and optimize airflow, demonstrating how a minor geometric adjustment to the "Hot Aisle/Cold Aisle" containment significantly improved thermal efficiency.

1. Introduction: The Heat Density Problem

Our target facility housed 40 racks of high-performance compute nodes in a 1,200 sq. ft. secure enclosure.

Heat Load: 12 kW per rack.
Cooling Strategy: CRAC (Computer Room Air Conditioning) units with underfloor distribution.

2. Baseline Simulation (Model A)

Using Autodesk CFD (or 6SigmaRoom), we modeled the initial design:

Layout: Racks aligned perfectly parallel to the walls.
Containment: Standard cold aisle containment (roof and doors).
Result: Simulation showed "Hot Spots" at the top-rear of the racks at the ends of the row. The static pressure under the raised floor was uneven, causing insufficient airflow to the furthest tiles.

3. The 15-Degree Shift (Model B)

Fluid dynamics suggests that sharp 90-degree turns cause turbulence and pressure drop. The underfloor air had to hit the sub-floor wall and bounce back to feed the last tiles.

3.1 The Hypothesis

By rotating the entire rack assembly 15 degrees relative to the room geometry (creating a diagonal layout), we could smooth the underfloor airflow path from the CRAC units.

3.2 Simulation Setup

Boundary Conditions: Inlet velocity from CRACs set to 2.5 m/s.
Heat Sources: Racks modeled as porous blocks emitting 12 kW heat.
Solver: k-epsilon turbulence model.

4. Results & Analysis

4.1 Thermal Map Comparison

Model A (Parallel): Peak rack inlet temp: 26°C. Recirculation index: 12%.
Model B (Diagonal): Peak rack inlet temp: 22°C. Recirculation index: 3%.

The diagonal orientation reduced the "dead zones" under the floor. The air velocity became more uniform across all perforated tiles.

4.2 Energy Efficiency (PUE)

Because the intake temperatures were lower and more consistent, we could:

Raise the CRAC setpoint by 2°C.
Reduce fan speeds by 15%.

This resulted in a theoretical 20% improvement in cooling energy efficiency.

5. Conclusion

CFD provides a "virtual wind tunnel" for data center design. It allows us to test counter-intuitive geometrical solutions—like the 15-degree shift—that would be too risky or expensive to experiment with during actual construction. In a secure, air-gapped environment where hardware failure requires complex escort procedures for repair, this thermal assurance is invaluable.