20 June, 2017
While the data center industry has long been built on the foundations of computational fluid design (CFD) through the virtual modeling of server architecture, it is only recently that it is becoming commonplace to open the scope of this scientific analysis into the wider space.
Many data center operators in the industry are now reaping the benefits of these techniques that have been honed by generations of physicists, mathematicians and engineers over the past 200 years. Whether the objective is efficiency, capacity, availability or design, science is the answer and in an ever more technically progressive world, this science is becoming invaluable.
While it has taken vast developments in the design of processors to push the use of CFD from the server to the data center space, the advantages of the continuation of such analyses into the external domain is obvious. Whether you require verification of deployment plans or a tool to aid the design process, the use of CFD can give engineers insights into situations that can result in operational issues.
Water-Sprayed Chiller Analysis
Take for instance the following rudimentary scenario: using water-sprayed chillers on the roof of a data center to reduce the coolant temperature that will be pumped down to the units in the white space. These units are placed in a recessed section of the data center wall due to structural considerations.
Figure 1: Model showing water-sprayed chillers in a recessed section of the roof of the data center.
While these units are manufactured to operate efficiently within a specific ambient temperature range, it is very difficult to ensure the actual inflow temperatures of these units remain at that of the surrounding air. If the units are not working within the specified ranges the efficiency loss can result in huge energy losses over time.
Simulating Wind Conditions
To fully understand whether the chillers are operating as expected, the facility manager can use CFD to analyze the operation of the chillers in various ambient conditions. Even with this very simple example, the addition of such real-world conditions result in issues.
Through the simulation of wind conditions, we can see the effect of changing ambient conditions. The table shows the inlet temperatures for a model without a wind condition applied:
Table 1: Inlet temperatures for a model without a wind condition applied.
Without a specified wind condition, the chillers are operating at an inlet temperature exactly the same as the ambient and only reaching a maximum increase of 1.6° C. Any facility manager who expects an ambient temperature of 26° C would be happy to operate the chillers in these conditions. However, these theoretical conditions are rarely experienced.
Figure 2: Scenario in which wind conditions are directed towards the chillers and wall.
In order to fully understand the ambient conditions, these numbers must be compared to wind conditions to ensure the chillers will be operating efficiently. As a rudimental analysis, we can look at a wind condition directed toward the wall which results in the following data:
Table 2: Inlet temperatures for a model with directional wind condition applied.
We can see that the inflow temperature of the units has increased considerably. A difference of over 4° in inlet temperature can result in a great deal of efficiency losses. The images below show that the recirculation of the hot exhaust air being caused by the wind condition is the reason for this temperature increase.
Figure 3: Recirculation of hot exhaust air caused by wind conditions.
Figure 4: Arial view of temperature plane with wind conditions.
Solution Analysis with CFD
This is where simulation can assist in a deployment design; as we can simply run various models using different chiller placements to try and eliminate the issue of increased tempeatures under these ambient conditions.
In this case it can be seen that installing a separating barrier between intake and exhaust air can solve the issue, shown below:
Figure 5: Simulation of scenario with separating barrier.
Figure 6: Temperature result plane with separating barrier, which fixes recirculation on the chillers.
We can see that placing an obstruction on top of the chillers allows for separation of the air streams. This manages to reduce the intake air temperature down to an average of 26.4°C. However, it is not often possible to install such blanking due to architectural or accessibility issues.
Figure 7: Temperature result planes with blanking, allowing separation of the air streams.
A final model with rotated chillers show that the effect of the wind can be mitigated to an acceptable level with no extra costs to the operator. It is this intelligent design that CFD can provide, allowing facilities to operate optimally for their given parameters.
Table 3: Inlet temperatures for finalized model, optimized with CFD analysis.
Whatever issue a facility may be experiencing, whatever design an engineer may be considering, external modeling using CFD does not fail to provide the user with a wealth of otherwise unattainable information.
By: Mike Eccles, Consultant Engineer
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