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- All Subjects: Mechanical Engineering
- Genre: Masters Thesis
Description
Microchannel heat sinks can possess heat transfer characteristics unavailable in conventional heat exchangers; such sinks offer compact solutions to otherwise intractable thermal management problems, notably in small-scale electronics cooling. Flow boiling in microchannels allows a very high heat transfer rate, but is bounded by the critical heat flux (CHF). This thesis presents a theoretical-numerical study of a method to improve the heat rejection capability of a microchannel heat sink via expansion of the channel cross-section along the flow direction. The thermodynamic quality of the refrigerant increases during flow boiling, decreasing the density of the bulk coolant as it flows. This may effect pressure fluctuations in the channels, leading to nonuniform heat transfer and local dryout in regions exceeding CHF. This undesirable phenomenon is counteracted by permitting the cross-section of the microchannel to increase along the direction of flow, allowing more volume for the vapor. Governing equations are derived from a control-volume analysis of a single heated rectangular microchannel; the cross-section is allowed to expand in width and height. The resulting differential equations are solved numerically for a variety of channel expansion profiles and numbers of channels. The refrigerant is R-134a and channel parameters are based on a physical test bed in a related experiment. Significant improvement in CHF is possible with moderate area expansion. Minimal additional manufacturing costs could yield major gains in the utility of microchannel heat sinks. An optimum expansion rate occurred in certain cases, and alterations in the channel width are, in general, more effective at improving CHF than alterations in the channel height. Modest expansion in height enables small width expansions to be very effective.
ContributorsMiner, Mark (Author) / Phelan, Patrick E (Thesis advisor) / Herrmann, Marcus (Committee member) / Chen, Kangping (Committee member) / Arizona State University (Publisher)
Created2011
Description
As additive manufacturing grows as a cost-effective method of manufacturing, lighter, stronger and more efficient designs emerge. Heat exchangers are one of the most critical thermal devices in the thermal industry. Additive manufacturing brings us a design freedom no other manufacturing technology offers. Advancements in 3D printing lets us reimagine and optimize the performance of the heat exchangers with an incredible design flexibility previously unexplored due to manufacturing constraints.
In this research, the additive manufacturing technology and the heat exchanger design are explored to find a unique solution to improve the efficiency of heat exchangers. This includes creating a Triply Periodic Minimal Surface (TPMS) geometry, Schwarz-D in this case, using Mathematica with a flexibility to control the cell size of the models generated. This model is then encased in a closed cubical surface with manifolds for fluid inlets and outlets before 3D printed using the polymer nylon for thermal evaluation.
In the extent of this study, the heat exchanger developed is experimentally evaluated. The data obtained are used to derive a relationship between the heat transfer effectiveness and the Number of Transfer Units (NTU).The pressure loss across a fluid channel of the Schwarz D geometry is also studied.
The data presented in this study are part of initial experimental evaluation of 3D printed TPMS heat exchangers.Among heat exchangers with similar performance, the Schwarz D geometry is 32% smaller compared to a shell-and-tube heat exchanger.
In this research, the additive manufacturing technology and the heat exchanger design are explored to find a unique solution to improve the efficiency of heat exchangers. This includes creating a Triply Periodic Minimal Surface (TPMS) geometry, Schwarz-D in this case, using Mathematica with a flexibility to control the cell size of the models generated. This model is then encased in a closed cubical surface with manifolds for fluid inlets and outlets before 3D printed using the polymer nylon for thermal evaluation.
In the extent of this study, the heat exchanger developed is experimentally evaluated. The data obtained are used to derive a relationship between the heat transfer effectiveness and the Number of Transfer Units (NTU).The pressure loss across a fluid channel of the Schwarz D geometry is also studied.
The data presented in this study are part of initial experimental evaluation of 3D printed TPMS heat exchangers.Among heat exchangers with similar performance, the Schwarz D geometry is 32% smaller compared to a shell-and-tube heat exchanger.
ContributorsChandrasekaran, Gokul (Author) / Phelan, Patrick E (Thesis advisor) / Rykaczewski, Konrad (Committee member) / Schultz, Karl U (Committee member) / Arizona State University (Publisher)
Created2018
Description
Thermoelectric devices (TED's) continue to be an area of high interest in both thermal management and energy harvesting applications. Due to their compact size, reliable performance, and their ability to accomplish sub-ambient cooling, much effort is being focused on optimized methods for characterization and integration of TED's for future applications. Predictive modeling methods can only achieve accurate results with robust input physical parameters, therefore TED characterization methods are critical for future development of the field. Often times, physical properties of TED sub-components are very well known, however the "effective" properties of a TED module can be difficult to measure with certainty. The module-level properties must be included in predictive modeling, since these include electrical and thermal contact resistances which are difficult to analytically derive. A unique characterization method is proposed, which offers the ability to directly measure all device-level physical parameters required for accurate modeling. Among many other unique features, the metrology allows the capability to perform an independent validation of empirical parameters by measuring parasitic heat losses. As support for the accuracy of the measured parameters, the metrology output from an off-the-shelf TED is used in a system-level thermal model to predict and validate observed metrology temperatures. Finally, as an extension to the benefits of this metrology, it is shown that resulting data can be used to empirically validate a device-level dimensionless relationship. The output provides a powerful performance prediction tool, since all physical behavior in a performance domain is captured using a single analytical relationship and can be plotted on a singe graph.
ContributorsLofgreen, Kelly (Author) / Phelan, Patrick E (Thesis advisor) / Posner, Jonathan (Committee member) / Devasenathipathy, Shankar (Committee member) / Arizona State University (Publisher)
Created2011
Description
Biogas’s potential as a renewable fuel source has been an area of increased research in recent years. One issue preventing wide-spread use of biogas as a fuel is the trace amounts of impurities that damage fuel-burning equipment by depositing silicon, sulfur, calcium and other elements on their surface. This study aims to analyze the effects of a high concentration of L4 linear siloxane on solid oxide fuel cell performance until failure occurs. L4 siloxane has not been extensively researched previously, and this investigation aims to provide new data to support similar, though slower, degradation compared to D4, D5 and other siloxanes in solid oxide fuel cells. The experiments were conducted inside a furnace heated to 800℃ with an Ni-YSZ-supported (Nickel-yttria-stabilized zirconia) fuel cell. A fuel source with a flow rate of 20 mL/min of hydrogen gas, 10 mL/min of nitrogen gas and 0.15 mL/min of L4 siloxane was used. Air was supplied to the cathode. The effects of siloxane deposition on cell voltage and power density degradation and resistance increase were studied by using techniques like the current-voltage method, electrochemical impedance spectroscopy, and gas chromatography. The results of the experiment after reduction show roughly constant degradation of 8.35 mV/hr, followed after approximately 8 hours by an increasing degradation until cell failure of 130.45 mV/hr. The initial degradation and stagnation match previous research in siloxane deposition on SOFCs, but the sharp decline to failure does not. A mechanism for solid oxide fuel cell failure is proposed based on the data.
ContributorsRiley, Derall M. (Author) / Milcarek, Ryan J (Thesis advisor) / Wang, Liping (Committee member) / Phelan, Patrick E (Committee member) / Arizona State University (Publisher)
Created2021