Luis Silva-Llanca

Rapid prediction of exergy destruction in data centers due to airflow mixing

ABSTRACT A novel lumped parameter approach is introduced to predict data center exergy destruction due to airflow mixing, resulting in a speedup of several orders of magnitude compared to detailed computational fluid dynamics (CFD) simulations. Both lumped parameter and detailed CFD methods agree within 8.5% for 11 test cases on an example data center. A significant time-saving design strategy is also introduced using two detailed CFD simulations to predict bulk flow parameters, and then applying lumped parameter analysis on flow-independent design parameters. The strategy shows agreement within 0.39% when computer room air conditioner (CRAC) supply temperature is varied from 12–20°C.

Replicating impinging synthetic jets as a train of consecutive viscous Lamb-Ossen vortex pairs

In small scale applications, synthetic jets have proven to be an efficient cooling technique when impinged onto heating surfaces. These jets are produced by the quick injection-ejection of fluid from an orifice, which generates a train of counter-rotating vortex pairs that sustain a fluctuating jet flow with positive momentum. Previously, in an effort to understand the fundamental mechanisms that drive this phenomenon, an idealized numerical canonical geometry was created and studied using CFD, which liberated the jet from actuator artifacts. Due to its highly vortical nature, the fluid can penetrate the thermal boundary layer better than a conventional steady jet. In the wall jet region, the passing of the main vortices gives rise to secondary vortices with opposite rotation that cause the entrainment of cold fluid towards the vicinity of the heated surface, thus broadening the effective impinging area and further enhancing the heat transfer. This study intends to advance prior fundamental studies by focusing in the fluid dynamics associated with this type of flow. Counter-rotating viscous Lamb-Ossen vortex pairs were repeatedly placed inside a domain at a given time interval (frequency) with a given intensity. The method of images was used to replicate the presence of the perpendicular static surface that acts as an inviscid wall. A numerical code written in Matlab™ language was developed to calculate the unsteady interaction between the N vortices, and the consequently induced fluid flow. This was used to compare the approach proposed with the canonical CFD data. A method is proposed to predict the vortex intensity evolution, which presented excellent agreement with the numerical data. It was found that the Lamb-Ossen vortex pair translational velocity and trajectory were comparable to the synthetic jet in the free jet region. The canonical vortex slowed when entering the stagnation region due to wall effects and the presence of the secondary vortex that induced a velocity onto the primary vortex opposite to its translation. Four effects were identified, each having different or opposite relationships with the jet parameters and the heat transfer, providing multiple options when it comes to finding optimum operating conditions.

Meteorological assessment and implementation of an air-side free-cooling system for data centers in Chile

Data center energy consumption in Latin America has increased considerably during last years. According to Datacenter dynamics, energy requirements during 2016 were expected to be around 3.85 GW. In Chile, the data center industry grew 14% between 2009 and 2010, whereas energy consumption increased 21.4% between 2012 and 2013. For this reason, many data centers in the country have started to evaluate efficient alternatives to reduce energy consumption such as the use of air containment techniques, air-side and water-side cooling systems. To date, existing free-cooling maps do not provide information about available hours during the year for implementing either air-side or water-side cooling systems in data centers in South America. This paper presents a thermo-dynamic analysis aimed to evaluate the potential use of air-side free-cooling systems in the Chilean data center industry. First, temperature and Relative Humidity (RH) variations, during three years, were obtained at 29 different stations throughout the entire country. The objective was to identify regions in Chile that meet data center thermal requirements proposed by the ASHRAE. Fiber-optic availability was also considered during the analysis. The thermodynamic model considered a white room with a thermal load of 20 kW, for which an air treatment unit was incorporated with the objective of providing cold air at 18° and 60% RH. An air treatment system was calculated at three different locations in Chile. These locations were selected since they offer high availability of fiber-optic connections (Chacalluta, Arica y Parinacota Region), strategic position for companies (Quinta Normal, Metropolitan Region), and low temperatures through the year (Carlos Ibanez, Aysen Region). Preliminary results have demonstrated that Chile is a relatively humid country. For this reason, cooling air must be dehumidified most of the time. The results also showed that even when low temperatures can be found in Carlos Ibanez, both Chacalluta and Quinta Normal offer excellent possibilities for the data centers industry. These two last locations offer more fiber-optic connections and temperature variations that lay within the range established by the ASHRAE.

Two-dimensional numerical analysis of a low-re turbulent impinging synthetic jet

Synthetic jets are produced by the symmetric cyclic ejection and injection of fluid from a cavity. Due to the particular fluid dynamics, they generate an oscillating outward flow, which can be used as a cooling technique by impinging it onto a heated stationary surface. In small scale applications, synthetic jets have been found to remove heat more efficiently than steady jets at equal Reynolds numbers. Two-dimensional numerical simulations of this phenomenon have been presented in the literature, with turbulence as the preferred numerical model. The authors previously demonstrated that laminar flow has shown good to excellent agreement with experimental data under a certain range of jet-to-surface distance, frequency and Re. It was found that the most suitable turbulent model was the SST/k-ω. The heat transfer along the heated wall was influenced by the vortex dynamics. For cases with large jet-to-surface distance and low Reynolds number, the vortices were weak and with low vorticity. It was hypothesized that to the low heat transfer coefficient is due to the convection, conduction and radiation losses.

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

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.Copyright