Mechanical Engineering
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Item A particle erosion model of monocrystalline silicon for high heat flux microchannel heat exchangers(2017) Squiller, David; McCluskey, Patrick; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)As package-level heat generation pushes past 1 kW/cm3 in various military, aerospace, and commercial applications, new thermal management technologies are needed to maximize efficiency and permit advanced power electronic devices to operate closer to their inherent electrical limit. In an effort to align with the size, weight and performance optimization of high temperature electronics, cooling channels embedded directly into the backside of the chip or substrate significantly reduce thermal resistances by minimizing the number of thermal interfaces and distance the heat must travel. One implementation of embedded cooling considers microfluidic jets that directly cool the backside of the substrate. However, as fluid velocities exceed 20 m/s the potential for particle erosion becomes a significant reliability threat. While numerous particle erosion models exist, seldom are the velocities, particle sizes, materials and testing times in alignment with those present in embedded cooling systems. This research fills the above-stated gaps and culminates in a calibrated particle-based erosion model for single crystal silicon. In this type of model the mass of material removed due to a single impacting particle of known velocity and impact angle is calculated. Including this model in commercial computational fluid dynamics (CFD) codes, such as ANSYS FLUENT, can enable erosion predictions in a variety of different microfluidic geometries. First, a CFD model was constructed of a quarter-symmetry impinging jet. Lagrangian particle tracking was used to identify localized particle impact characteristics such as impact velocity, impact angle and the percentage of entrained particle that reach the surface. Next, a slurry erosion jet-impingement test apparatus was constructed to gain insight into the primary material removal mechanisms of silicon under slurry flow conditions. A series of 14 different experiments were performed to identify the effect of jet velocity, particle size, particulate concentration, fluid viscosity and time on maximum erosion depth and volume of material removed. Combining the experimental erosion efforts with the localized particle impact characteristics from the CFD model enabled the previously developed Huang et al. cutting erosion model to be extended to new parameter and application ranges. The model was validated by performing CFD erosion simulations that matched with the experimental test cases in order to compare one-dimensional erosion rates. An impact dampening coefficient was additionally proposed to account for slight deviations between the CFD erosion predictions and experimental erosion rates. The product of this research will ultimately enable high fidelity erosion predictions specifically in mission-critical military, commercial and aerospace applications.Item Enhanced Gas-Liquid Absorption Utilizing Micro-Structured Surfaces and Fluid Delivery Systems(2014) Ganapathy, Harish; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Despite intensive research and development efforts in renewable energy in recent years, more than 80% of the energy supply in the year 2040 is expected to come from fossil fuel-based sources. Increasing anthropogenic greenhouse gas emissions led the United States to legislatively limit domestic CO2 emissions to between 1000-1100 lb/MWh for new fossil fuel-fired power plants, thus creating an urgent need for efficient gas separation (capture) processes. Meanwhile, the gradual replacement of coal with cleaner burning natural gas will introduce additional challenges of its own since nearly 40% of the world's gas reserves are sour due to high concentrations of corrosive and toxic H2S and CO2 gases, both of which are to be separated. Next-generation micro-structured reactors for industrial mass and heat transfer processes are a disruptive technology that could yield substantial process intensification, size reduction, increased process control and safety. This dissertation proposes a transformative gas separation solution utilizing advanced micro-structured surfaces and gas delivery manifolds that serves to enhance gas separation processes. Experimental and numerical approaches have been used to achieve aggressive enhancements for a solvent-based CO2 absorption process. A laboratory-scale microreactor was investigated to fundamentally understand the physics of multiphase fluid flow with chemical reactions at the length scales under consideration. Reactor design parameters that promote rapid gas separation were studied. Computational fluid dynamics was used to develop inexpensive stationary (fixed) interface models for incorporation with optimization engines, as well as high fidelity unsteady (deforming) interface models featuring universal flow regime predictive capabilities. Scalability was investigated by developing a multiport microreactor and a stacked multiport microreactor that represented one and two orders magnitude increase in throughput, respectively. The present reactors achieved mass transfer coefficients as high as 400 1/s, which is between 2-4 orders of magnitude higher than conventional gas separation technologies and can be attributed to the impressive interfacial contact areas as high as 15,000 m2/m3 realized in this study through innovative design of the system. The substantial enhancement in performance achieved is indicative of the high level of process intensification that can be attained using the proposed micro-structured reactors for gas separation processes for diverse energy engineering applications. This dissertation is the first comprehensive work on the application of micro-structured surfaces and fluid delivery systems for gas separation and gas sweetening applications. More than ten refereed technical publications have resulted from this work, part of which has already been widely received by the community.Item LIMITS OF THIN FILM COOLING IN MICROGAP CHANNELS(2011) Rahim, Emil; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)The forced flow of dielectric liquids, undergoing phase change while flowing in a narrow channel, is a promising candidate for the thermal management of advanced semiconductor devices. Such channels may be created by the spacing between silicon ribs in a microchannel cooler, between stacked silicon chips in a three-dimensional logic, RF, or heterogeneous microsystem, narrowly-spaced organic or ceramic substrates, or between a chip and a non-silicon polymer cover in a microgap cooler. These microgap configurations provide direct contact - and hence cooling - between a chemically-inert, dielectric liquid and the back surface of an active electronic component, thus eliminating the significant thermal resistance associated with a Thermal Interface Material (TIM) or the solid-solid contact resulting from the attachment of a microchannel cold plate to the chip. This dissertation explores the physics underpinning two-phase flow in miniature channels, through an extensive literature survey, and employs analytical, numerical, and experimental techniques to determine the thermal transport phenomena in microgap channels, with emphasis on the thermal limits of thin film heat transfer in annular flow. The applicability of several flow regime mapping methodologies has been examined. The predictions of these mapping methodologies have been compared to the visual observations of two-phase flow in microtubes and microchannels. The axial variation of two-phase heat transfer coefficients with local vapor qualities is reported, and the association of this variation with the dominant flow regime is discussed. The measured two-phase flow heat transfer coefficients are then sorted according to the dominant flow regime, and compared to the predictions of classical heat transfer correlations. Two-phase flow experiments were performed in a microgap cooler with the flow of HFE7100 and FC-87. The microgap cooler is 125 mm long, 14 mm wide, and was operated with three distinct gap sizes: 100, 200, and 500 micron. An instrumented Intel thermal test vehicle (TTV) flip-chip mounted via a BGA on an organic substrate, and equipped with 9 pre-calibrated temperature sensors, was used as the heated section of the microgap channel. Pressure drop across the channel, fluid inlet and exit temperature, and wall temperature were measured. Using commercial software, an "inverse" numerical technique was developed to identify the local heat flux and heat transfer coefficient. Local Annular heat transfer coefficients, for FC-87 flowing in the 100 micron channel, were found to display elements of the M-shaped variation with flow quality and reached a maximum value of 15 kW/m^2-K.Item Characterization and Modeling of Two-Phase Heat Transfer in Chip-Scale Non-Uniformly Heated Microgap Channels(2010) Ali, Ihab A.; Bar-Cohen, Avram; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)A chip-scale, non-uniformly heated microgap channel, 100 micron to 500 micron in height with dielectric fluid HFE-7100 providing direct single- and two-phase liquid cooling for a thermal test chip with localized heat flux reaching 100 W/cm2, is experimentally characterized and numerically modeled. Single-phase heat transfer and hydraulic characterization is performed to establish the single-phase baseline performance of the microgap channel and to validate the mesh-intensive CFD numerical model developed for the test channel. Convective heat transfer coefficients for HFE-7100 flowing in a 100-micron microgap channel reached 9 kW/m2K at 6.5 m/s fluid velocity. Despite the highly non-uniform boundary conditions imposed on the microgap channel, CFD model simulation gave excellent agreement with the experimental data (to within 5%), while the discrepancy with the predictions of the classical, "ideal" channel correlations in the literature reached 20%. A detailed investigation of two-phase heat transfer in non-ideal micro gap channels, with developing flow and significant non-uniformities in heat generation, was performed. Significant temperature non-uniformities were observed with non-uniform heating, where the wall temperature gradient exceeded 30°C with a heat flux gradient of 3-30 W/cm2, for the quadrant-die heating pattern compared to a 20°C gradient and 7-14 W/cm2 heat flux gradient for the uniform heating pattern, at 25W heat and 1500 kg/m2s mass flux. Using an inverse computation technique for determining the heat flow into the wetted microgap channel, average wall heat transfer coefficients were found to vary in a complex fashion with channel height, flow rate, heat flux, and heating pattern and to typically display an inverse parabolic segment of a previously observed M-shaped variation with quality, for two-phase thermal transport. Examination of heat transfer coefficients sorted by flow regimes yielded an overall agreement of 31% between predictions of the Chen correlation and the 24 data points classified as being in Annular flow, using a recently proposed Intermittent/Annular transition criterion. A semi-numerical first-order technique, using the Chen correlation, was found to yield acceptable prediction accuracy (17%) for the wall temperature distribution and hot spots in non-uniformly heated "real world" microgap channels cooled by two-phase flow. Heat transfer coefficients in the 100-micron channel were found to reach an Annular flow peak of ~8 kW/m2K at G=1500 kg/m2s and vapor quality of x=10%. In a 500-micron channel, the Annular heat transfer coefficient was found to reach 9 kW/m2K at 270 kg/m2s mass flux and 14% vapor quality level. The peak two-phase HFE-7100 heat transfer coefficient values were nearly 2.5-4 times higher (at similar mass fluxes) than the single-phase HFE-7100 values and sometimes exceeded the cooling capability associated with water under forced convection. An alternative classification of heat transfer coefficients, based on the variable slope of the observed heat transfer coefficient curve), was found to yield good agreement with the Chen correlation predictions in the pseudo-annular flow regime (22%) but to fall to 38% when compared to the Shah correlation for data in the pseudo-intermittent flow regime.Item STUDY OF CONDENSATION OF REFRIGERANTS IN MICRO-CHANNELS FOR DEVELOPMENT OF FUTURE COMPACT MICRO-CHANNEL CONDENSERS(2008) Chowdhury, Sourav; Ohadi, Michael; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Mini- and micro- channel technology has gained considerable ground in the recent years in industry and is favored due to its several advantages stemming from its high surface to volume ratio and high values of proof pressure it can withstand. Micro-channel technology has paved the way to development of highly compact heat exchangers with low cost and mass penalties. In the present work, the issues related to the sizing of compact micro-channel condensers have been explored. The considered designs encompass both the conventional and MEMS fabrication techniques. In case of MEMS-fabricated micro-channel condenser, wet etching of the micro-channel structures, followed by bonding of two such wafers with silicon nitride layers at the interface was attempted. It was concluded that the silicon nitride bonding requires great care in terms of high degree of surface flatness and absence of roughness and also high degree of surface purity and thus cannot be recommended for mass fabrication. Following this investigation, a carefully prepared experimental setup and test micro-channel with hydraulic diameter 700 microns and aspect ratio 7:1 was fabricated and overall heat transfer and pressure drop aspects of two condensing refrigerants, R134a and R245fa were studied at a variety of test conditions. To the best of author's knowledge, so far no data has been reported in the literature on condensation in such high aspect ratio micro-channels. Most of the published experimental works on condensation of refrigerants are concerning conventional hydraulic diameter channels (> 3mm) and only recently some experimental data has been reported in the sub-millimeter scale channels for which the surface tension and viscosity effects play a dominant role and the effect of gravity is diminished. It is found that both experimental data and empirically-derived correlations tend to under-predict the present data by an average of 25%. The reason for this deviation could be because a high aspect ratio channel tends to collect the condensate in the corners of its cross-section leaving only a thin liquid film on the flat side surfaces for better heat transfer than in circular or low aspect ratio channels.Item ENHANCEMENT OF SPRAY COOLING HEAT TRANSFER USING EXTENDED SURFACES AND NANOFLUIDS(2007-11-05) Coursey, Johnathan Stuart; Kim, Jungho; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)Spray cooling is a powerful heat transfer technique in which an atomizing nozzle provides a flow of liquid droplets directed towards a hot surface. This dissertation explores two potentially powerful techniques capable of improving traditional spray cooling: nanofluids and extended surfaces. Nanofluids were experimentally studied in a pool boiling system to elucidate the underlying mechanisms of critical heat flux (CHF) enhancement. Dilute suspensions of nanoparticles were found to have a degrading or no effect on boiling performance. Greater concentrations (≥ 0.5 g/L) lead to modest (up to ~37%) increase in the CHF. The results were highly dependent on the working fluid/substrate combination, specifically wetting characteristics. Poorly wetting systems (e.g. water on copper) could be enhanced by nanofluids, whereas better wetting systems (e.g. ethanol on glass) showed no improvement. This conclusion was re-enforced when nanofouling caused by dryout of nanofluid was found to improve wetting as shown by a reduction in the advancing threephase contact angle. Interestingly, similar CHF enhancement was achieved without nanofluids using an oxidized surface, which is easily wetted with pure fluids. In fact, surface treatment alone resulted in similar CHF enhancement at ~20°C less wall superheat than required using nanofluids. Spray cooling was found to be adversely affected by the addition of nanoparticles due to changing thermophysical properties and/or nozzle clogging due to particle deposition. The addition of high aspect ratio open microchannels to the sprayed surface resulted in significant enhancement at all wall superheats and over 200% enhancement in the low temperature single-phase regime. The two-phase regime began at lower temperatures with microchannels, which lead to heat transfer enhancements of up to 181%. The onset of two-phase effects was found to be a strong function of channel depth. However, the onset of two-phase effects was found to occur at a temperature that was independent of nozzle pressure/mass flow rate. Therefore, nucleation and two-phase effects are likely triggered by the unique liquid distribution caused by the extended structures. Using high aspect ratio open microchannels, these mechanisms resulted in spray efficiencies approaching one, indicating almost complete utilization of the spray's ability to absorb heat.Item Heat Transfer And Mechanical Design Analysis Of Supercritical Gas Cooling Process Of CO2 In Microchannels(2006-12-15) Kuang, Guohua; Ohadi, Michael M; Mechanical Engineering; Digital Repository at the University of Maryland; University of Maryland (College Park, Md.)An extensive review of the literature indicates a lack of systematic study of supercritical CO2 gas cooling and no prior work on CO2/oil mixture in supercritical region, suggesting a lack of fundamental understanding of supercritical gas cooling process and a lack of comprehensive data that would help quantify the performance potential of CO2 microchannel heat exchangers for engineering applications. This dissertation presents a systematic and comprehensive study on gas cooling heat transfer characteristics of supercritical CO2 in microchannels. Semi-empirical correlation is developed for predicting heat transfer performance of supercritical CO2 in microchannels. The effect of oil addition on heat transfer performance has been experimentally investigated as well. It is shown that presence of lubricant oil mixed with supercritical CO2 in the heat exchangers can substantially affect heat transfer and pressure drop coefficients. Because of the outstanding performance of supercritical CO2 and its promising potential as a substitute for current refrigerants, attention has been paid to the design of CO2 microchannel heat exchangers. The extensive review of the literature also indicates no previous study in systematically developing a simulation model for structural design of microchannel heat exchangers. The dissertation extends the research to the mechanical design analysis of microchannel heat exchangers. A finite-element method (FEM) based mechanical design analysis of tube-fin heat exchangers is carried out to develop a simulation model of the heat exchangers. The solid modeling and simulation scheme can be served as a guide for mechanical design of CO2 heat exchangers. Experiments are conducted to validate the developed models as well.