Numerical Evaluation of Hot Air Recirculation in Server Rack
Applied Sciences (MDPI AG) · Nelson Bafana Madihlaba, Thokozani Justin Kunene
A detailed Star-CCM+ model of a 2U Dell 2950 server built from a 3D scan studies how hot-aisle containment back pressure drives hot air back through the server, and the effect of adding four fans in series behind the original parallel fan set.
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Overview
In air-cooled data centres, hot-aisle containment prevents hot and cold air mixing but creates a pressure imbalance between cold and hot aisles, with the hot side high. That back pressure pushes against the server fan system, causing hot recirculation and insufficient cooling. The study examines whether series-configured fans can overcome the back pressure and eliminate recirculation. A three-grid GCI study (1.22 to 1.99 million cells) selected a 1,799,871-cell trimmed hexahedral mesh with seven prism layers, giving near-wall y+ of about 5.
Against published experiments, the CFD server inlet temperature under varying back pressure (about 20 °C supply, -0.15 to 0.02 inch w.c.) was within 5%, and chassis static pressure versus airflow was within 15%. Server inlet temperature begins to rise once back pressure reaches the fans' free-delivery point, and from there it rises roughly in proportion to back pressure. Four fans were added in series behind the original parallel fan set, and back pressures from -10 to 20 Pa were tested at a fixed 6800 rpm.
Series fans raised available system pressure against a 10 Pa hot-aisle back pressure but did not improve intake distribution; a dead zone persisted beside the drive bay. With series fans, server inlet temperature was higher than with the original parallel fans: about 5% higher from -20 to -5 Pa back pressure and 19% higher at free delivery (0 Pa), while outlet temperature fell by 1%. Series fans raise system pressure but do not remove recirculation, and inlet temperatures rise.
The authors conclude that fan sizing should follow component temperature needs rather than containment pressure. Limits: a single server model (Dell 2950 2U), a fixed 6800 rpm and a specific back-pressure range. The paper is useful mainly as evidence that pressure differences in a contained hot aisle feed back on server airflow.
Key points
- A three-grid GCI study (1.22 to 1.99 million cells) selected a 1,799,871-cell trimmed hexahedral mesh with seven prism layers, near-wall y+ of about 5.
- Against published experiments, CFD server inlet temperature under varying back pressure (about 20 °C supply, -0.15 to 0.02 inch w.c.) was within 5%, and chassis static pressure versus airflow within 15%.
- Server inlet temperature begins to rise once back pressure reaches the fans' free-delivery point, then rises roughly in proportion to back pressure.
- Adding fans in series raised available system pressure against a 10 Pa hot-aisle back pressure but did not improve intake distribution; a dead zone persisted beside the drive bay.
- With series fans, server inlet temperature was about 5% higher from -20 to -5 Pa back pressure and 19% higher at free delivery (0 Pa) than with the original parallel fans, while outlet temperature fell by 1%.
Using it in design
- Make hot-aisle back pressure (hot-aisle pressure above cold aisle) a design input in contained aisles; server inlet temperature rises once back pressure reaches the fans' free-delivery point.
- Do not assume adding series server fans fixes recirculation; system pressure rose but inlet temperature went up.
- Size fans and hot-aisle pressure balance by component temperature needs, and consider relief or pressure control to limit hot-aisle pressure.
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Who should read it
Cooling engineers and server designers studying hot-aisle containment pressure and server fan interaction.
Document contents
- 1. Introduction
- 2. Materials and Methods (2.1 Geometrical Configuration, 2.2 Fan)
- 3. Numerical Method (3.1 Governing Equations: SGS modeling, virtual; 3.2 Mesh Sensitivity Study; 3.3 Boundary Conditions and Model Validation Results; 3.4 Solver and Numerical Parameters)
- 4. Results and Discussion (4.2 Inlet Air Temperature, 4.3 Airflow Distribution: Fan Configuration)
- 5. Conclusions
Related standards
- ASHRAE TC 9.9 Thermal Guidelines for Data Processing Environments
Read the document
server-rack-hot-air-recirculation-2024.pdf
CC BY 4.0 — share with attribution
The document is provided unaltered because its licence permits redistribution.
Publication details
- Authors
- Nelson Bafana Madihlaba; Thokozani Justin Kunene
- Publisher
- Applied Sciences (MDPI AG)
- Year
- 2024
- Volume
- 14
- Issue
- 17
- Pages
- 7904
- Document language
- English
- Source type
- Scientific
- Access
- Open licence — downloadable
- Licence
- CC BY 4.0 — share with attribution
- Licence evidence
- www.mdpi.com
Original licence note
PDF p.1: Licensee MDPI; open access article distributed under the terms of the Creative Commons Attribution (CC BY) license. The deposited PDF contains overlapping review-layer text, and the abstract and conclusions report different headline percentages.
Citation
Nelson Bafana Madihlaba; Thokozani Justin Kunene (2024). Numerical Evaluation of Hot Air Recirculation in Server Rack. Applied Sciences (MDPI AG). 14(17), 7904. https://doi.org/10.3390/app14177904
Events on this subject
- SC26 – International Conference for High Performance Computing, Networking, Storage and Analysis
- ASHRAE Winter Conference
- AHR Expo
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