Hybrid-Cooled Data Center Server Layout Optimization for Air-Side Heat Recovery

The rapid increase in energy demand for data center requires improved cooling techniques. This study investigates, numerically and experimentally, the energy efficiency optimization based on server level air flows and also identifies the potential for waste heat recovery from the air stream for a hybrid air/liquid cooled server. Multi-objective genetic algorithm and entropy generation minimization are chosen as tools to address the multiple objectives involved in the problem and examine the cooling performance and waste heat recovery potential. Selected experimental measurements have successfully validated the CFD simulation model

Multi-objective Layout Optimization of a Generic Hybrid-cooled Data Centre Blade Server

The rapid global increase in energy consumption by data centres requires new improved cooling solutions and techniques to be developed and implemented. In a typical data centre, approximately a third of the total power consumption is needed for the cooling infrastructure, resulting in high power usage effectiveness (PUE) values. The main culprits of raised PUE are legacy air-cooled data centres, exhausting only low grade waste heat for which capture and re-use is challenging. This study investigates numerically the potential for energy recuperation by a server-level internal layout optimization for a hybrid air/liquid-cooled server. The approach combines multi-objective genetic algorithm (MOGA) and entropy generation minimization (EGM) techniques to incorporate the multiple objectives involved in solving this problem, and examines the cooling performance and waste heat recovery potential. In order to evaluate the potential for waste heat recovery, an extra entropy generation term ṠΔT,ext role= presentation style= box-sizing: border-box; margin: 0px; padding: 0px; display: inline-block; line-height: normal; font-size: 16.2px; word-spacing: normal; overflow-wrap: normal; white-space: nowrap; float: none; direction: ltr; max-width: none; max-height: none; min-width: 0px; min-height: 0px; border: 0px; position: relative; \u3eṠΔT,extis introduced, representing an air/liquid heat exchanger at the rear of the server. The effect of modifying the internal component layout on pressure drop and the outlet temperature profile are of primary interest, due to their direct impact on fan power usage and energy recuperation potential. The CFD model of the baseline configuration is validated using experimental pressure measurements conducted on a real blade server. The research demonstrates that a basic server layout optimization such as changing the memory module angles and spacing could enhance both the cooling effectiveness but also improve the potential for waste heat recovery from the air stream. The maximum reduction in entropy generation rate due to server layout optimization is 15%, while the outlet temperature uniformity can be improved by up to 42%

Laminar flow and heat transfer characteristics of interrupted microchannel heat sink with ribs in the transverse microchambers

publisher: Elsevier articletitle: Laminar flow and heat transfer characteristics of interrupted microchannel heat sink with ribs in the transverse microchambers journaltitle: International Journal of Thermal Sciences articlelink: http://dx.doi.org/10.1016/j.ijthermalsci.2016.06.029 content_type: article copyright: © 2016 Elsevier Masson SAS. All rights reserved.The work was supported by the Engineering and Physical Sciences Research Council (EPSRC) of the UK through research grant (EP/L001233/1) and the National Natural Science Foundation of China (51576005)

Development of high performance heat sink for power electronics

Thermal management in the field of electronic devices has become a challenge due to the recent miniaturization trend, which results in an inevitable increase in power density requirement. Moreover conventional cooling technologies may not maintain a low junction temperature required for the chip. In order to meet such strict requirements, improvement in cooling technologies is needed. Heat sinks are one of the potential candidates capable of satisfying the junction temperature requirements in terms of both performance and reliability. Increased power rating and shrinking size of power electronics systems require advanced thermal management technologies. Introduction of micro-channel heat sink (MCHS) into power electronics cooling has significantly improved the cooling performance. In this thesis, two advanced micro-channel structures, i.e. double-layered (DL) and double-sided (sandwich) with water as coolant are proposed. Both the structures are optimized and compared using computational fluid dynamics (CFD) study. The micro-channels are embedded inside the Cu-layer of direct bond copper (DBC). The effects of inlet velocity, inlet temperature, heat flux are investigated during geometry optimization. The major scaling effects including temperature-dependent fluid properties and entrance effect are also considered. Based on the optimal geometry, the sandwich structure with counter flow shows a reduction in thermal resistance by 59%, 52% and 53% when compared to single-layer (SL), DL with unidirectional flow and DL with counter flow respectively. Furthermore, when water based Al2O3 (with concentrations of 1% and 5%) nanofluid is applied, a remarkable improvement for wide channels is observed. In this study, numerical investigation of the passive heat transfer improvement such as wavy walls in MCHS applying nanofluids as a coolant is also carried out. In wavy MCHS the coolant flows through the curved passages rather than in conventionally straight microchannels. These curved passages makes them the potential candidates for incorporation into efficient heat transfer devices. The effects of wavy amplitude, wavelength, volumetric flow rate and volume fraction of different types of nanofluids are investigated. The wavy MCHS yields better cooling performance than traditional rectangular MCHS with pure water as a coolant. High amplitude and short wavelength provide the lowest thermal resistance, while low amplitude and long wavelength do not show any significant improvement when compared with rectangular channel. The influence of nanofluid markedly deteriorates with increment in wavy amplitude and even more with wavelength. The effect of 2-5% of volume concentration of diamond nanofluids is quite close to each other and it becomes less distinguishable with increasing wavy amplitude and wavelength. Hence, higher amplitude as well as shorter wave length results in small overall improvement while a major enhancement owes to enhanced thermal conductivity which increases with volume concentration of nanofluid. The highest impact of nanofluid is observed in the case of rectangular channels than wave channel. High volume concentration of nanofluid makes the effect of wavy geometry imperceptible and it exhibits the same thermal performance as conventional MCHS with the same nanofluid. In this thesis, heat sink application in aerospace sector with different types of coolant is also investigated. The demand for cooling power in aerospace sector is progressively increasing owing to the replacement of mechanically driven engine accessories with electrically driven parts. This replacement causes an increase in electric loads compared to the traditional aircraft system. Size and weight are the main challenges in aeronautical industry. Higher number of electrically driven components will result in increase in power dissipation density. Therefore, for heat to be dissipated effectively thermal management is important. Despite these modifications, cooling system must be able to cope with the increased thermal loads as a result of system upgrades and avoid any contribution to the weight and size. Air cooled heat sink approach is the most widely used cooling system due to its simplicity and primitive nature. However, as system volumetric density increases it precludes usage of air as a coolant. Liquid cooling is the most viable method to meet the essential requirements. Engine oil and fuel are readily available as on-board coolants. Using already available vehicle coolants has variety of applications in automotive sector. There are no studies available in literature for the adaption of those coolants in avionics. This thesis investigates the weight contribution to the liquid coolant system for power electronics converter in future aircraft both experimentally and numerically. For that end, a cooling system of 2 and 6 pass cold plates is designed for aircraft power converter and cooling performance is then analyzed. It also discusses the contribution to the weight and size using available coolants on the aircraft at a flow rate ranging from 2-8LPM and considering 1-3% dissipation power loss. The addition of water cooling was examined to complete the studies. From the study, the most promising coolant is arrived at and further improvement in cooling system is highlighted. Currently, oil is an inappropriate coolant for avionics industry unless the required improvement can be done in cooling system.Doctor of Philosophy (EEE

Comparison of pin-fin and finned shape heat sink for power electronics in future aircraft

This study investigates the heat transfer performance of finned and pin-fin heat sinks for high power density converter in future aircraft. There is a lack of studies evaluating the cooling performance of pin-fin as compared to the finned heat sink configuration. The influence of the aspects such as type of fluid flow (laminar or turbulent), type of working fluid (fuel or air) and geometry configuration (pin-fin or finned shaped heat sink) on heat transfer performance is compared. Circular, cone and hydrofoil pin fin arrays are considered. It is concluded that the thermal performance of pin-fin heat sink is superior to the finned configuration by 1.6-2 times in all cases except when fuel coolant is employed at turbulent flow. As weight of heat sink is concerned, the use of pin-fin shape can reduce half the weight of heat sink. The results of this paper are the guidance which can be applied while designing the heat sink for power electronics in future aircraft.NRF (Natl Research Foundation, S’pore)Accepted versio

Performance improvements of microchannel heat sink using wavy channel and nanofluids

To improve the heat transfer performances of microchannel heat sink (MCHS), the advanced channel structures and working fluids can be applied. In this paper, the wavy channel structure and application of nanofluids are investigated. The effects of wavy amplitude, wavelength, volumetric flow rate and volume fraction of different type of nanofluids are presented. Three wave amplitudes of 25 μm, 50 μm and 75 μm with two wavelength of 250 μm and 500 μm at volumetric flow rate ranges from 0.152 L/min to 0.354 L/min are considered. Three different types of nanofluids with volume concentration ranges from 1% to 5% are applied. The effect of wavy MCHS is shown on thermal resistance, pressure drop, friction factor. It is found that in case of the pure water is applied as the coolant the heat transfer performance of the MCHS is significantly improved comparing with the traditional straight channel MCHS, while the replacement of the pure water by nanofluids makes the effect of wavy wall unnoticeable.Accepted versio

Investigation on weight consideration of liquid coolant system for power electronics converter in future aircraft

Cooling systems significantly contribute to the total mass and volume of power electronic systems. In the case of aerospace application, it will directly increase the operating cost of the aircraft. This paper experimentally and numerically investigates the weight contribution of the liquid cooling system for power electronics converter in future aircraft. In order to investigate, a cooling system of 2 and 6 pass cold plates is designed and its cooling performance is analyzed. The weight and size contribution is discussed based on available coolants in the aircraft, flow rate ranges from 2 to 8 LPM and 1% to 3% power loss dissipation. Water is added and examined for completeness of the studies. This paper concludes that oil is inappropriate coolant for this particular case. The optimum parameters (Q = 8 LPM with 9.5 kg pump weight) for most promising coolant (fuel) that give high extraction rate with low weight contribution for the highest density cooling system are indicated.NRF (Natl Research Foundation, S’pore)Accepted versio