Browsing by Subject "Thermal"

Now showing 1 - 4 of 4
  • Thumbnail Image
    Item
    Bridging scales in modeling and simulation of thermal transport processes
    (2014-08) Wheeler, Vincent Michael
    The vastly disparate length and time scales existing in new devices and materials born out of nanotechnology have made thermal modeling and simulation more important and more difficult. The experimental thermal characterization of such systems, e.g. modern computer processors, can be prohibitively difficult or expensive making numerical simulation the only route to effective technology design. However, obtaining solutions that account for small scales, but are still computationally feasible, requires innovative modeling approaches. The research contained herein represents three independent contributions to the understanding of the modeling of thermal transport processes in systems with nano-sized features. At their common core, all contributions in this thesis are rooted in transport theory--the solution or approximation of the Boltzmann equation (BE)--to statistically describe a system made up of a great many energy-carrying particles. The work roughly divides into the three modes of heat transfer--convection, conduction, and radiation. First, a framework for the discretization of the BE (in its many forms) based on lattices is presented. The widely-used lattice Boltzmann method for the simulation of fluid flow is shown to be a sub-case. The framework gives a new rigorous foundation to the use of lattice methods which have emerged in recent years with applications ranging from Brownian motion to astrophysical radiation. Second, we give a thorough presentation of recently proposed models of heat conduction derived from the phonon BE which provides rigor and insight into the different approaches. Most notably, the "new heat equation" is derived directly from the phonon BE for the first time along with a novel boundary condition. The result is shown to give excellent agreement with the more detailed description provided by the equation of phonon radiative transport. Last, we provide the radiative characterization of a nano-porous material using Maxwell's equations in order to recover coefficients to the linear BE governing thermal radiative transfer.
  • Thumbnail Image
    Item
    Data for: The influence of an in-stream thermal gradient on chironomid emergence during winter
    (2019-10-17) Ferrington, Leonard C Jr.; Nyquist, Corrie E; Vondracek, Bruce; ferri016@umn.edu; Ferrington, Leonard C Jr.; Chironomidae Research Group
    Chironomid surface floating pupal exuviae were collected from November 2017 through March 2018 from sites within one head water trout stream, Ike's Creek, and two comparative head water trout streams, Pine Needles Creek and Arcola Mills Stream. Pupal exuviae were identified to genus and species. Relative abundance over time at each site was used to calculate Jaccard’s coefficient of similarity among sites and to observe if there were differences in community composition along the length of a trout stream with longitudinal thermal heterogeneity. Mean daily water temperatures were also collected from November 2017 to April 2018 from each of the sites. Mean daily air temperatures were obtained from November 2017 to April 2018 from the NOAA National Weather Service. This data were collected in order to create air-water linear regressions for trout stream sites over winter. Our goal was to determine if fine-scale thermal heterogeneity exists along a first and second order trout stream and to compare the thermal regime with two other trout streams. The data, here, are released in accordance with the terms of publication.
  • Thumbnail Image
    Item
    Impact of Thermal Processing on Taste Development in Food
    (2016-06) Zhang, Liyun
    Despite the increasing demand, healthier foods suffer from lower consumer acceptability due to inferior flavor quality. The flavor of food is greatly affected by the food composition and thermal processing. This study specifically investigates the thermal processing impact on the positive and negative taste attributes of foodstuffs, which enables the optimization of processing strategies to improve the palatability and ultimately the consumption of ‘healthier’ formulated food products. The overall goal of this work was to characterize the effect of thermal processing on the taste-active compounds or the generation of taste modulating compounds in foodstuffs. This work mainly focuses on the influence of three common thermal processing techniques on the resulting taste profiles in three food systems respectively: the roasting of cocoa, the extrusion of corn cereal, and the frying of potato chips. These three processing techniques are widely utilized by the food industry. Bitterness was investigated in roasted cocoa and extruded whole grain corn while umami was characterized in deep fried potato chips. The influence of roasting on the taste attributes of cocoa was studied first. Roasting processes involve the Maillard reaction, which is a ubiquitous thermally catalyzed chemical pathway that is well-known to impact aroma development as well as taste, such as bitterness. Bitterness is a challenge for consumer acceptability, which is typically masked by adding sugar. The goal of this part of the research was to discover the impact of Maillard chemistry on endogenous bitter-tasting compound, catechin, in raw cocoa, and likewise on the resulting bitter taste profile of the roasted beans. Catechin-Maillard reaction products were identified by stable isotope labeling techniques in model reactions using the simulated cocoa roasting conditions. Eight reaction products were identified and reported for the first time. One of the newly-identified compounds significantly suppressed the perceived bitter intensity of the caffeine solution, which is a novel bitter blocker. Further analysis revealed that this bitter blocking compound was present in both the raw and roasted cocoa beans; however its concentration was higher in roasted cocoa beans. A generation mechanism of the bitter blocker was proposed. The results of the first phase indicated that the bitter profile of cocoa beans was altered by thermal reactions of the endogenous bitter compounds. In the second research phase of this study, the influence of extrusion on the taste profile of puffed whole grain corn products was examined. The goal of this phase is to identify the key bitter compounds and understand how they are influenced by extrusion. Three phenolic compounds (chaenorpine, coumaryl-spermidine, terrestribisamide) and one amino acid (L-tryptophan) were identified as the main bitter compounds in the extruded whole grain corn product. Based on sensory recombination analysis, chaenorpine was found to have the highest contribution to the bitterness intensity, based on the concentration of the bitter compounds reported in the saliva during mastication. Additionally, all of the identified bitter compounds were found to be degraded during extrusion, suggesting that the further optimization of extrusion could be utilized to suppress bitterness in order to improve the flavor quality of whole grain extruded products. In the last phase of this research project, the role of the thermal process, deep frying, on the taste profile of potato chips was examined. Potato chips are a highly desired food product and the umami taste is well known to positively contribute to the taste profile. Initial analysis indicated that the umami taste attribute of potato chips increases with frying time thus the compounds that contribute to umami were characterized. A dehydration product of monosodium glutamate (MSG), monosodium L-pyroglutamate (L-MSpG) and monosodium D-pyroglutamate (D-MSpG) were identified for the first time as umami enhancing compounds that contributed to the umami flavor of potato chips. The generation of pyroglutamates was reported to be directly related to the frying time. Sensory time-intensity taste analysis of potato chips with topical added L-MSpG and D-MSpG revealed significantly higher umami intensity and the overall higher potato chip flavor intensity. In summary, the impact of three thermal processes on taste profile was studied in three different food products. This study provides a novel basis for flavor optimization by investigating the thermal impact on taste chemistry. The ultimate goal of this study is to increase the market demand for health conscious foods, thus benefiting the food industry as well as promoting a healthy lifestyle.
  • Thumbnail Image
    Item
    Synthetic jet flow and heat transfer for electronics cooling
    (2014-05) Huang, Longzhong
    The progressive increase of heat dissipation from modern electronics requires more and more powerful cooling systems. Various cooling technologies have been developed such as liquid cooling, micro-channel cooling, and active cooling. The present study focuses on applying a unique device called a synthetic jet to cool electronics. A synthetic jet is able to generate an unsteady flow with a simple structure that makes it effective in convective heat transfer. This study provides both practical and fundamental view of synthetic jets in the application of electronics cooling. A mock-up synthetic jet is fabricated to study heat transfer and fluid mechanics of synthetic jet cooling. The scaled synthetic jet is geometrically and dynamically similar to the actual jet. The heat transfer performance characteristics of a synthetic jet impinging on a fin are tested with different operating frequencies and with different orifice shapes. Flow visualizations and detail flow field measurements of the impinging synthetic jet flow are documented to support the heat transfer experiment. The optimized parameters obtained from the scaled experiment are applied to the actual synthetic jet design. The actual synthetic jet is realized using a piezoelectric stack and applied on a cooling system based on a full-sized heat sink module. The cooling performance of the whole system is documented. The noise characteristics of the actual synthetic jet is tested and analyzed. A muffler with optimized parameters is found and used for noise reduction. Numerical simulation is used to find the optimal design for the synthetic jets. The computation is realized by the commercial software ANSYS Fluent. The numerical model is verified by comparing the computational results with experimental results. A parametric study of heat transfer performance of synthetic jet cooling is documented.