טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
M.Sc Thesis
M.Sc StudentRodes Lior
SubjectInvestigation of Two-Phase Flow and Heat Transfer in Micro-
Channels for Cooling Electronic Devices
DepartmentDepartment of Mechanical Engineering
Supervisor Professor Emeritus Gad Hetsroni (Deceased)
Full Thesis text - in Hebrew Full thesis text - Hebrew Version


Abstract

The semiconductors industry has seen a trend of minimization and increased density of transistors. This trend has created new challenges in the field of cooling technologies. One of the promising cooling solutions is the usage of micro-channels heat sinks, which can increase the rate of heat removal and be an integral part of the chip.

This study deals with the characterization of the flow processes and heat exchange in rectangular aluminum micro channels, with regards to single-and two-phase heat sinks.

The friction coefficient, which was concluded from the measured pressure gradient in the channel, was found to be lower than the theoretical value. A better fit to trapezoidal model rather than rectangular model was conducted alone with an accurate error calculation.

An examination of the micro channels' surface roughness, led to the conclusion, that the channels can be considered as smooth.

In the single-phase flow, examination of the dependency of the non-dimensional pressure gradient upon the non-dimensional temperature was compatible with theoretical predictions. When boiling starts and the flow comprises two phases the pressure gradient increases rapidly as heat flux and wall temperature increases.

The critical nucleation radius for bubbles creation was determined.

With regards to two-phase flow in micro-channels, the pressure gradient in the channel was estimated according to a homogenous model. The pressure gradient in the two-phase flow and its dependency on the electrical power was found to be highly compatible with the theory.

The thermal performance of the single-phase heat sink was characterized. The heat transfer coefficient and the non-dimensional heat transfer coefficient number were found for single-phase flow and were compared to known correlations for conventional theory in the literature. The experimental coefficients were found to be lower than the theoretical coefficients and also, in contrast to the theory, an increase in Nu number as dependent on the Re number has been observed in the developed flow regime. The suggested explanation for these differences was the axial conduction mechanism.

 Two-phase heat sink thermal performances were concluded from the heat transfer coefficient dependency upon the void fraction. This study identified different boiling regimes in the two-phase flow, and characterized the bubbles formation point. In addition, the rate of bubble growth, expansion, and its progress along the channel were measured. The boiling phenomenon was measured in the frequency domain and a dependency of the boiling frequencies on the mass flow and on the applied electrical power was found.