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1. An Adaptive Approach for Dealing with Flow Disruption in Virtualized Water-Cooled Data Centers

   https://doi.org/10.1109/CLOUD.2019.00057  

   Puvvadi, Udaya; Desu, Anuroop; Stachecki, Tyler; Ghose, Kanad; Sammakia, Bahgat (July 2019, 2019 IEEE 12th International Conference on Cloud Computing (CLOUD))

   The recent availability of water cooling systems that can be easily retrofitted to stock servers by replacing the heatsinks with coldplates has made it possible to use such systems for non-HPC cloud/data center servers. These cooling systems use pumps to circulate water and the pumps are likely to fail in the long run. We present a technique to handle flow disruptions caused by the pump failures in a virtualized environment. The solution uses an estimation of the residual cooling capacity left in the failed cooling system to adaptively adjust the CPU clock frequency as virtual machines are migrated off the racks affected by the failure. This minimizes the degradation of the tail latencies of the served requests during the migration interval for all servers affected by the failure, as seen in the experimental results 

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2. Raising Inlet Air Temperature for a Hybrid-Cooled Server Retrofitted With Liquid Cooled Cold Plates

   https://doi.org/10.1115/IMECE2018-88497  

   Chowdhury, Uschas; Siddarth, Ashwin; Sahini, Manasa; Agonafer, Dereje (November 2018, Proceedings of the ASME 2018 International Mechanical Engineering Congress and Exposition)

   In typical data centers, the servers and IT equipment are cooled by air and almost half of total IT power is dedicated to cooling. Hybrid cooling is a combined cooling technology with both air and water, where the main heat generating components are cooled by water or water-based coolants and rest of the components are cooled by air supplied by CRAC or CRAH. Retrofitting the air-cooled servers with cold plates and pumps has the advantage over thermal management of CPUs and other high heat generating components. In a typical 1U server, the CPUs were retrofitted with cold plates and the server tested with raised coolant inlet conditions. The study showed the server can operate with maximum utilization for CPUs, DIMMs, and PCH for inlet coolant temperature from 25–45 °C following the ASHRAE guidelines. The server was also tested for failure scenarios of the pumps and fans with reducing numbers of fans and pumps. To reduce cooling power consumption at the facility level and increase air-side economizer hours, the hybrid cooled server can be operated at raised inlet air temperatures. The trade-off in energy savings at the facility level due to raising the inlet air temperatures versus the possible increase in server fan power and component temperatures is investigated. A detailed CFD analysis with a minimum number of server fans can provide a way to find an operating range of inlet air temperature for a hybrid cooled server. Changes in the model are carried out in 6SigmaET for an individual server and compared to the experimental data to validate the model. The results from this study can be helpful in determining the room level operating set points for data centers housing hybrid cooled server racks. 

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3. Experimental Analysis of Chiller Cooling Failure in a Small Size Data Center Environment Using Wireless Instrumentation

   https://doi.org/10.1115/IPACK2018-8334  

   Tradat, Mohammad; Sammakia, Bahgat; Alissa, Husam; Nemati, Kourosh (January 2018, ASME 2018 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems)

   Given the vital rule of data center availability and since the inlet temperature of the IT equipment increase rapidly until reaching a certain threshold value after which IT starts throttling or shut down because of overheat during cooling system failure. Hence, it is especially important to understand failures and their effects. This study presented experimental investigation and analysis of a facility-level cooling system failure scenario in which chilled water interruption introduced to the data center. Quantitative instrumentation tools including wireless technology such as wireless temperature and pressure sensors were used to measure the discrete air inlet temperature and pressure differential though cold aisle enclosure, respectively. In addition, Intelligent Platform Management Interface (IPMI) and cooling system data during failure/recovery were reported. Furthermore, the IT equipment performance and response for opened and contained environments were simulated and compared. Finally, an experiment based analysis of the Ride Through Time (RTT) of servers during chilled water interruption of the cooling infrastructure presented as well. The results showed that for all three classes of servers tested during the cooling failure, CAC helped keep the server’s cooler for longer. The containment provided a barrier between the hot and cold air streams and caused slight negative pressure to build up, which allowed the servers to pull cold air from the underfloor plenum. In addition, the results show that the effect of CAC in containment solutions on the IT equipment performance and response could vary and depend on the server’s airflow, generation and hence types of servers deployed in cold aisle enclosure. Moreover, it was shown that when compared to the discrete sensors, the IPMI inlet temperature sensors underestimate the Ride Through Time (RTT) by 42% and 12% for the CAC and opened cases, respectively. 

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4. STOCHASTIC OPTIMIZATION FOR FEASIBILITY DETERMINATION: AN APPLICATION TO WATER PUMP OPERATION IN WATER DISTRIBUTION NETWORK

   https://doi.org/10.1109/WSC.2018.8632513  

   Tsai, Yi-An; Pedrielli, Giulia; Mathesen, Logan; Zabinsky, Zelda B.; Huang, Hao; Candelieri, Antonio; Perego, Riccardo (December 2018, Proceedings of the 2018 Winter Simulation Conference)

   Water Distribution Networks are a particularly critical infrastructure for the high energy costs and frequent failures. Variable Speed Pumps have been introduced to improve the regulation of water pumps, a key for the overall infrastructure performance. This paper addresses the problem of analyzing the effect of the VSPs regulation on the pressure distribution of a WDN, which is highly correlated to leakages and energy costs. Due to the fact that water network behavior can only be simulated, we formulate the problem as a black box feasibility determination, which we solve with a novel stochastic partitioning algorithm, the Feasibility Set Approximation Probabilistic Branch and Bound, that extends the algorithm previously proposed by two of the authors. We use, as black box, EPANet, a widely adopted hydraulic simulator. The preliminary results, over theoretical functions as well as a water distribution network benchmark case, show the viability and advantages of the proposed approach. 

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5. The Effectiveness of Data Center Overhead Cooling in Steady and Transient Scenarios: Comparison of Downward Flow to a Cold Aisle Versus Upward Flow From a Hot Aisle

   https://doi.org/10.1115/IPACK2015-48423  

   Silva-Llanca, Luis; del Valle, Marcelo; Ortega, Alfonso (July 2015, ASME 2015 International Technical Conference and Exhibition on Packaging and Integration of Electronic and Photonic Microsystems collocated with the ASME 2015 13th International Conference on Nanochannels, Microchannels, and Minichannels)

   The most common approach to air cooling of data centers involves the pressurization of the plenum beneath the raised floor and delivery of air flow to racks via perforated floor tiles. This cooling approach is thermodynamically inefficient due in large part to the pressure losses through the tiles. Furthermore, it is difficult to control flow at the aisle and rack level since the flow source is centralized rather than distributed. Distributed cooling systems are more closely coupled to the heat generating racks. In overhead cooling systems, one can distribute flow to distinct aisles by placing the air mover and water cooled heat exchanger directly above an aisle. Two arrangements are possible: (i.) placing the air mover and heat exchanger above the cold aisle and forcing downward flow of cooled air into the cold aisle (Overhead Downward Flow (ODF)), or (ii.) placing the air mover and heat exchanger above the hot aisle and forcing heated air upwards from the hot aisle through the water cooled heat exchanger (Overhead Upward Flow (OUF)). This study focuses on the steady and transient behavior of overhead cooling systems in both ODF and OUF configurations and compares their cooling effectiveness and energy efficiency. The flow and heat transfer inside the servers and heat exchangers are modeled using physics based approaches that result in differential equation based mathematical descriptions. These models are programmed in the MATLAB™ language and embedded within a CFD computational environment (using the commercial code FLUENT™) that computes the steady or instantaneous airflow distribution. The complete computational model is able to simulate the complete flow and thermal field in the airside, the instantaneous temperatures within and pressure drops through the servers, and the instantaneous temperatures within and pressure drops through the overhead cooling system. Instantaneous overall energy consumption (1st Law) and exergy destruction (2nd Law) were used to quantify overall energy efficiency and to identify inefficiencies within the two systems. The server cooling effectiveness, based on an effectiveness-NTU model for the servers, was used to assess the cooling effectiveness of the two overhead cooling approaches 

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