Computational fluid dynamic (CFD) optimization of microfluidic mixing in a MEMS steam generator

Thesis (S.B.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2008.Includes bibliographical references (p. 23-24).The challenge of achieving rapid mixing in microchannels is addressed through a computational fluid dynamics (CFD) study using the ADINA-F finite element program. The study is motivated by the need to design an adequate mixing chamber for aqueous chemical reactants in a micro steam generator. The study focuses on the geometric optimization of a static micromixer channel by considering the trade-off between mixing quality and pressure drop. Both zigzag and straight channels are evaluated, in addition to channels with differing amounts of added obstruction features. Due to computational limits, the numerical analysis is conducted in two dimensions. The results indicate that hydrodynamic focusing of the reactant at the inlet, in addition to the amount and density of added obstruction features, has the most significant impact on mixing efficiency and increased pressure drop. The study presents mixing quality and pressure drop trends that provide useful information for the micro steam generator mixing chamber design.by Kimberlee C. Collons.S.B

Experimental investigations of solid-solid thermal interface conductance

Thesis (S.M.)--Massachusetts Institute of Technology, Dept. of Mechanical Engineering, 2010.Cataloged from PDF version of thesis.Includes bibliographical references (p. 70-84).Understanding thermal interface conductance is important for nanoscale systems where interfaces can play a critical role in heat transport. In this thesis, pump and probe transient thermoreflectance methods are used to measure the thermal interface conductance between solid materials. Two experimental studies of thermal interface conductance are presented, each revealing the complexity of phonon interactions at interfaces which are inadequately captured by current models of phonon transmissivity. The first study considers interfaces of different metals with graphite, and finds that atomic-scale roughness at the interface could be appreciably influencing the heat transport due to the extreme anisotropy of graphite. The thermal interface conductance of graphite is found to be similar to that of diamond, suggesting that when estimating the thermal interface conductance between metal and multi-walled carbon nanotubes (MWCNTs), a reasonable assumption may be that the conductance with the side walls of the MWCNTs is similar to the conductance with the ends of the MWCNTs. The second study considered aluminum on diamond interfaces where the diamond samples were functionalized to have different chemical surface terminations. The surface termination of the diamond is found to significantly influence the heat flow, with oxygenated diamond, which is hydrophilic, exhibiting four times higher thermal interface conductance than hydrogen-treated diamond, which is hydrophobic. Microstructure analysis determined that the Al film formed similarly, independent of diamond surface termination, suggesting that differences in interface bonding likely caused the observed difference in thermal interface conductance, a phenomenon which is not captured in current models of solid-solid phonon transmissivity.by Kimberlee Chiyoko Collins.S.M

Studies of non-diffusive heat conduction through spatially periodic and time-harmonic thermal excitations

Thesis: Ph. D., Massachusetts Institute of Technology, Department of Mechanical Engineering, 2015.Cataloged from PDF version of thesis.Includes bibliographical references (pages 127-133).Studies of non-diffusive heat conduction provide insight into the fundamentals of heat transport in condensed matter. The mean free paths (MFPs) of phonons that are most important for conducting heat are well represented by a material's thermal conductivity accumulation function. Determining thermal conductivity accumulation functions experimentally by studying conduction in non-diffusive regimes is a recent area of study called phonon MFP spectroscopy. In this thesis, we investigate nondiffusive transport both experimentally and theoretically to advance methods for determining thermal conductivity accumulation functions in materials. We explore both spatially periodic and time-harmonic thermal excitations as a means for probing the non-diffusive transport regime, where the Fourier heat diffusion law breaks down. Boltzmann transport equation calculations of one-dimensional (1D) spatially sinusoidal thermal excitations are performed for gray-medium and fully spectral cases. We compare our calculations to simplified transport models and demonstrate that a model based on integrating gray-medium solutions can reasonably model materials with a narrow range of dominant heat-carrying phonon MFPs. We also consider the inverse problem of determining thermal conductivity accumulation functions from experimental measurements of thermal-length-scale-dependent effective thermal conductivity. Based on experimental measurements of Si membranes of varying thickness, we reproduce the thermal conductivity accumulation function for bulk Si. To investigate materials with short phonon MFPs, we developed an experimental approach based on microfabricating 1D wire grid polarizers on the surface of a material under study. This work finds that the dominant thermal length scales in polycrystalline Bi 2Te3 are smaller than 100 nm. We also determine that even small amounts of direct sample optical excitation, which occurs when light transmits through the grating and directly excites electron-hole pairs in the substrate, can appreciably influence the measured results, suggesting that an alternate approach that prevents all direct optical excitation is preferable. To study thermal length scales smaller than 100 nm without the need for microfabrication, we develop a method for extracting high frequency response information from transient optical measurements. For a periodic heat flux input, the thermal penetration depth in a semi-infinite sample depends on the excitation frequency, with higher frequencies leading to shallower thermal penetration depths. Prior work using frequencies as high as 200 MHz observed apparent non-diffusive behavior. Our method allows for frequencies of at least 1 GHz, but we do not observe any deviation from the heat diffusion equation, suggesting that prior observations attributed to non-diffusive effects were likely the result of transport phenomena in the metal transducer.by Kimberlee Chiyoko Collins.Ph. D