Date of Award

Fall 2014

Degree Type


Degree Name

Doctor of Philosophy (PhD)


Mechanical Engineering

First Advisor

Suresh V. Garimella

Committee Chair

Suresh V. Garimella

Committee Member 1

Jayathi Y. Murthy

Committee Member 2

Jong H. Choi

Committee Member 3

Shripad T. Revankar


Direct integration of compact microchannel heat sinks is an attractive thermal management solution for the dissipation of high heat fluxes, specifically under boiling conditions that provide high rates of heat transfer at a uniform heat sink temperature. Under two-phase flow conditions, the heat transfer and pressure drop are a function of the local flow regime. Development of sensors that detect local void fraction and flow regimes may enable better understanding of the fundamental flow phenomena. ^ The void fraction in air-water two-phase adiabatic flow in a microchannel is measured in this work using a custom-designed impedance-based sensor with electrodes on opposing walls of a single microchannel, a 'crosswise' geometry. The impedance response of the sensor is calibrated against the time-averaged void fraction determined via high-speed flow visualizations. The temporal signal is depicted as a probability density function that is used for quantitative determination of two-phase flow regimes using a Kohonen Self-Organizing Map. ^ To characterize the sensor impedance response, numerical simulations are implemented in two- and three-dimensions. Electrical simulations of the crosswise electrode geometry are performed to acquire both instantaneous and time-averaged responses. For arbitrarily defined voids, the shape and distribution has no effect on the simulated impedance; the relationship between the void fraction and impedance is found to be non-linear. Time-averaged three-dimensional impedance simulations are in good agreement with the experimental data. ^ A second set of experiments are performed using multiple electrodes placed along the flow direction of a single microchannel wall, a 'streamwise' geometry. Multiple water electrical conductivities are tested, and an optimal range between 100 and 175 μS/cm is found to provide maximum instrument sensitivity. The dependency of the impedance output on water conductivity is characterized to fit all of the data to a single calibration curve, independent of water conductivity. ^ One application where the determination of the local void fraction is important is in the case of non-uniform heating in microchannels. An experimental investigation is performed to explore flow boiling phenomena in a microchannel heat sink with hotspots, as well as non-uniform streamwise and transverse heating conditions across the entire heat sink. Local heat transfer coefficients and wall temperatures are measured while the location of boiling incipience is observed via high-speed visualizations of the flow. It is found that even though the substrate thickness beneath the microchannels is very small (200 um), significant lateral conduction occurs and must be accounted for in the calculation of the local heat flux imposed. For non-uniform heat input profiles, with peak heat fluxes along the central streamwise and transverse directions, it is found that the local flow regimes, heat transfer coefficients, and wall temperatures deviate significantly from a uniformly heated case. ^ A simple computational model is developed to predict the thermal performance of a microchannel heat sink with an imposed non-uniform heating profile. While the model underpredicts the base temperatures and overpredicts the heat transfer coefficients, the trends agree with experimental data. For the cases investigated with the model, flow non-uniformities between the channels are estimated using image analysis of high-speed videos taken during the experiments. It is observed that flow maldistribution must be taken into account in the model for heating profiles that are prone to flow maldistribution in order to improve the match to experimental data. ^ Another experimental investigation is performed to measure the critical heat flux (CHF) in a microchannel heat sink with uniform heating and various hotspot heating locations. It is found that a hotspot spanning the entire length of the heat sink in the flow direction produces the lowest CHF of all the cases investigated due to the flow maldistribution induced by boiling. A single hotspot spanning the heat sink perpendicular to the flow direction produces different CHF values based on its streamwise location. The visualizations reveal that CHF occurs when there is a sudden and unalleviated upstream expansion of vapor in one or more channels above the hotspot, causing the local wall temperature to rapidly increase. The proximity of the hotspot to the inlet manifold, which communicates between all channels and can relieve upstream vapor expansion, appears to determine the resiliency of the heat sink to CHF. ^ Non-uniform heating profiles often found in actual applications greatly affect the thermal performance of microchannel heat sinks. Measuring the void fraction and understanding how the location of hotspots affects local heat transfer allows for the creation of a computational model to aid future heat sink designs.