SUBJECT: Ph.D. Dissertation Defense
BY: Zhimin Wan
TIME: Tuesday, March 29, 2016, 12:00 p.m.
PLACE: Love Building, 109
TITLE: Transport Characteristics of Pin Fin Enhanced Microgaps under Single and Two Phase Cooling
COMMITTEE: Dr. Yogendra Joshi, Chair (ME)
Dr. Andrei Fedorov (ME)
Dr. Satish Kumar (ME)
Dr. Saibal Mukhopadhyay (ECE)
Dr. Sudhakar Yalamanchili (ECE)


Microfluidic convection cooling is a promising technique for future high power microprocessors, radio-frequency (RF) transceivers, solid-state lasers, and light emitting diodes (LED). Three-dimensional (3D) stacking of chips is a configuration that allows many performance benefits. A microgap with circulating fluid is a promising cooling arrangement that can be incorporated within a 3D chip stack. Although studies have examined the thermal characteristics of microgaps under both single-phase and two-phase convection, the characteristics and benefits of microgaps with surface enhancement features have not been fully explored. In this work, firstly, the single phase thermal/fluid characteristics of microgaps with staggered pin fin arrays are studied. The effects of the pin fin dimensions including diameter, transversal and longitudinal spacing, and height are investigated computationally and experimentally over a range of Reynolds number (Re) 22-357. Micropin fin arrays investigated have pin diameter of 100 μm, pitch/ diameter ratios of 1.5 ~ 2.25, and height/ diameter ratios of 1.5 ~ 2.25. Correlations of friction factor (f) and Colburn j factor for these dense arrays of micro pins have been developed. Subsequently, microfluidic cooling with staggered pin fin arrays is employed in functional 3D integrated circuit (ICs). Thermal and electrical performance of a CMOS chip in terms of temperature and leakage power under realistic operating conditions are studied. Both experimental and modeling results show that microfluidic cooling could significantly decrease the chip temperature and leakage power, thus increasing the chip performance. Lastly, two-phase cooling is studied with dielectric fluid HFE-7200 as a baseline with mass flux from 354.5 kg/m2-s to 576.3 kg/m2-s. Critical heat flux (CHF) increases with increasing mass flux but decreases with decreasing gap height. Nonuniform heating will cause nonuniform flow with a decrease of mass flux in high power area, which decreases the thermal performance. The effects of fluid mixture (HFE-7200/Methanol) on thermal performance are studied with mass fraction of Methanol from 8.5% to 35.8%. A very small amount of addition of Methanol (8.5% mass fraction) can significantly increase the thermal performance due to the sharp decrease of saturation temperature and increase of effective thermal conductivity and latent heat. However, the Marangoni effect caused by the concentration gradient deteriorates the CHF.