Development of high performance heat sink for power electronics
Date of Issue2016-06-23
School of Electrical and Electronic Engineering
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.