Viability of server module thermal management using enhanced heat sinks and low global warming potential dielectric fluids

Abstract

Immersion cooling is making a resurgence for use in server applications driven by an increase in chip densities and the need for reduction in overall data center power usage. This thesis focuses on pool boiling characteristics from an array of heaters which simulate electronic chips on a vertically-oriented printed circuit board. A pool boiling study was conducted on an array of four bare die using two different dielectric fluids; namely Novec-649 and HFE-7100, both with low global warming potential (GWP). Tests were conducted at two die spacings; 25mm and 10mm under two different pool conditions: saturated and 15°C subcooled representing a start-up transient. Data were collected for increasing and decreasing heat flux cycles. The flux dissipated was found to be 14.6 W/cm2 and 14.2 W/cm2 with Novec-649 and HFE-7100, respectively for the test board with 10mm die spacing under subcooled conditions. These values are recorded at self-imposed maximum surface temperatures that ensured operation well below the critical heat flux. These flux values are quite high especially since they are attained without the need to modify the surface or add heat sinks. In an effort to increase thermal performance, tests were also conducted on die spaced 25mm apart augmented with two different enhanced heat sinks featuring microporous and microfinned surfaces. Enhanced heat sinks performed better than bare die and the flux dissipated was found to be 18.9 W/cm2 and 18.3 W/cm2 using Novec-649 and HFE-7100, respectively under subcooled conditions. Additionally, these values were achieved at surface temperatures 15°C lower than the surface temperatures recorded by the bare die for the same heat flux. Additionally, a batch of Novec-649 was intentionally contaminated using high levels of dioctyl phthalates to address the effect of contamination. The superheat required to dissipate 10 W/cm2 with contaminated fluid was found to be 10°C greater than with clean fluid. High speed images were obtained to provide a better understanding of nucleation characteristics.