The current trend of electronics miniaturization presents thermal challenges which limit the performance of processors. The high heat fluxes in CPUs are affecting the reliability and processing ability of the servers. Due to these thermal limitations, large amount of energy is required to cool the servers in data centers. The advanced two-phase boiling heat transfer systems are significantly more efficient than currently used single phase coolers. In the current work, a novel approach is adopted by using a dual tapered microgap over the heated boiling surface for enhanced heat transfer. The theoretical work has identified the role of two-phase pressure recovery effect induced by the expanding bubbles in the tapered microgap configuration that leads to a self-sustained flow over the heater surface. This effectively transforms the pool boiling into a pumpless flow boiling system. Additionally, the tapered microgap introduces a bubble squeezing effect that pushes the liquid along the expanding taper direction. High fluid velocities are generated through this mechanism thus creating the pumpless flow boiling process in a conventional pool boiling system. Using water as the working fluid, a critical heat flux (CHF) of 288 W/cm2 was achieved at a wall superheat of 24.1°C. The baseline configuration without any tapered manifold resulted in a CHF of 124 W/cm2 at a wall superheat of 23.8°C. This represents the largest enhancement ever reported for water on a plain surface without incorporating any surface modifications during pool boiling. For dielectric liquid as the working fluid, the dual tapered micogap obtained ~2X enhancement in the heat transfer coefficient (HTC) compared to the configuration with the baseline configuration. This dual tapered microgap design is also implemented in a thermosiphon loop for CPU cooling where no pumping power is required for fluid circulation in a closed loop. The loop contains an evaporator with a dual tapered microgap and was able to dissipate heat from an actual CPU (i7-930 processor, TDP 130W) more efficiently as compared to air, or water-based coolers during thermal stress tests. This dual taper evaporator configuration will be able to mitigate the hotspots generated on the CPU surface and improve the efficiency and mechanical integrity of the processor. Such boiling heat transfer systems can significantly reduce the cooling water temperature requirement thereby reducing the operational cost of data centers.

Library of Congress Subject Headings

Data centers--Cooling; Heat sinks (Electronics); Heat exchangers; Heat--Transmission; Ebullition

Publication Date


Document Type


Student Type


Degree Name

Engineering (Ph.D.)

Department, Program, or Center

Engineering (KGCOE)


Satish Kandlikar

Advisor/Committee Member

Kathleen Lamkin-Kennard

Advisor/Committee Member

Michael Schertzer


RIT – Main Campus

Plan Codes