• Refine Query
  • Source
  • Publication year
  • to
  • Language
  • 7
  • 1
  • 1
  • Tagged with
  • 10
  • 10
  • 4
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 2
  • 1
  • 1
  • 1
  • 1
  • About
  • The Global ETD Search service is a free service for researchers to find electronic theses and dissertations. This service is provided by the Networked Digital Library of Theses and Dissertations.
    Our metadata is collected from universities around the world. If you manage a university/consortium/country archive and want to be added, details can be found on the NDLTD website.

Evaluation of the maturity method to estimate concrete strength

Wade, Samuel Allen, Schindler, Anton K. January 2005 (has links) (PDF)
Thesis(M.S.)--Auburn University, 2005. / Abstract. Vita. Includes bibliographic references (p.200-203).

Analysis of Pump Oil and Alkanes Evaporation

Waldstein, Nathaniel A 19 November 2008 (has links)
There are many products, including hard drives, which require trace amounts, on the order of several mg, of lubricants for proper operation. The following study investigated the evaporation rates of pump oil and several alkanes, which have a wide range of applications, using a thermogravimetric machine. Both static and dynamic temperature tests were conducted. The rate of evaporation of the test specimen was determined as the percentage of mass loss per unit time. Using the Arrhenius Equation, the activation energy of the evaporation process, Ea, can be calculated as the slope of the best fit line for a plot of the ln(k) vs. 1/T (where k represents the rate of the evaporation). These values were shown to have good agreement with the enthalpy of vaporization calculated from the Clausius Clapeyron Equation and with the activation energy calculated using the Freeman and Carroll Method. The alkanes were compared using the rate of evaporation and the amount of activation energy required for evaporation as model systems. Further investigations were conducted to determine the relationship of surface area of the evaporating liquid and the rate of evaporation. It is suggested that the surface area is a function that depends on the activation, bonding, and interfacial energies of the liquid. However, the wetting angle, which aids in the description of the surface area, depends on the surface energy. Subsequent modeling was conducted in an attempt to predict the evaporation characteristics of other lubricants for the purpose of comparison.

The Study of Lifetime Prediction and Reliability Test of Co-Chromaticity Glass and Silicone Phosphor

Liou, Jyun-Sian 04 August 2011 (has links)
A Ce:YAG-doped glass phosphor layer instead of conventional Ce:YAG-doped silicone phosphor layer as phosphor-converted white-light emitting diodes (PC-WLEDs) is demonstrated. The advantage of employing doped glass encapsulation in high power PC-WLEDs could be explained the material property of glass transition temperature of 750¢J was higher than silicone of 150¢J. The lumen degradation, chromaticity shift, color temperature change, transmittance, and fluorescence spectrum in glass and silicone based high-power PC-WLEDs under thermal aging at 150¢J, 200¢J, and 250¢J is compared and presented. Under highest temperature of 250¢J, the glass and silicone encapsulation base d PC-WLEDs exhibited 8.15% and 38.85% in lumen loss, 1.07 and 7.32 in chromaticity shift, 856 K and 3666 K in color temperature change, 4.21% and 28.1% in transmittance loss, respectively. However, the excitation spectrum altered as slight as emission spectrum before and after experiments. After aging test, the mean-time-to-failure (MTTF) evaluation of glass and silicone encapsulation materials for PC-WLEDs in accelerated thermal tests is also compared and presented by the using of Weibull distribution and Arrhenius equation. The MTTF of PC-WLEDs is defined the lumen decayed to 90%. The results showed that the glass as encapsulation material of PC-WLEDs exhibited higher MTTF than the silicone encapsulation by about 4.81, 5.92, and 7.53 times in lumen loss at 150¢J, 200¢J, and 250¢J, respectively. The results of the lumen loss, chromaticity shift, and MTTF investigations demonstrated that the thermal-stability performance of the glass based PC-WLEDs were better than silicone based PC-WLEDs at 150¢J, 200¢J, and 250¢J. A better thermal stability phosphor layer of glass as encapsulation material may be beneficial to the many applications where the LED modules with high power and high reliability are demanded.

Damage mechanisms for near-infrared radiation induced cataract

Yu, Zhaohua January 2017 (has links)
Purpose: 1) To estimate the threshold dose and the time evolution for cataract induction by near infrared radiation (IRR) in seconds exposure time domain; 2) to determine the ocular temperature development during the threshold exposure; 3) to investigate if near IRR induces cumulative lens damage considering irradiance exposure time reciprocity; 4) to experimentally estimate the temperature in the lens indirectly from the measurement of temperature-induced light scattering increase. Methods: Before exposure, 6-weeks-old albino rats were anesthetized and the pupils of both eyes were dilated. Then the animals were unilaterally exposed to 1090 nm IRR within the pupil area. Temperature was recorded with thermocouples placed in the selected positions of the eye. At the planned post-exposure time, the animal was sacrificed and the lenses were extracted for measurements of forward light scattering and macroscopic imaging (Paper I-III). In Paper IV, the lens was extracted from six-weeks-old albino Sprague-Dawley female rats and put into a temperature-controlled cuvette filled with balanced salt solution. Altogether, 80 lenses were equally divided on four temperature groups, 37, 40, 43 and 46 ºC. Each lens was exposed for 5 minutes to temperature depending on group belonging while the intensity of forward light scattering was recorded. Results: The in vivo exposure to 197 W/cm2 1090 nm IRR required a minimum 8 s for cataract induction. There was approximately 16 h delay between exposure and light scattering development in the lens. The same radiant exposure was found to cause a temperature increase of 10 °C at the limbus and 26 °C close to the retina. The in vivo exposure to 96 W/cm2 1090 nm IRR with exposure time up to 1 h resulted in an average temperature elevation of 7 °C at the limbus with the cornea humidified and no significant light scattering was induced one week after exposure. Arrhenius equation implies that the natural logarithm of the inclination coefficient for light scattering increase is linearly dependent on the inverse of the temperature. The proportionality constant and the intercept, estimated as CI(0.95)s, were 9.6±2.4 x103 K and 22.8±7.7. Further, it implies that if averaging 20 measurements of inclination coefficients in a new experiment at constant heat load, the confidence limits for prediction of temperature correspond to ±1.9 °C. Conclusions: It is indicated that IRR at 1090 nm produces thermal but not cumulatively photochemical cataract, probably by indirect heat conduction from absorption in tissues surrounding the lens. Applying the Arrhenius equation the in vivo temperature in the lens can be determined retrospectively with sufficient resolution.

Numerical Analysis Of Ablation Process On A Two Dimensional External Surface

Aykan, Serap Fatma 01 September 2005 (has links) (PDF)
The thermal response analysis of an ablative material on a two dimensional external surface is performed. The method is applied to both rectangular and cylindrical coordinate systems, where rectangular coordinate system is used for comparison with results available in literature. The current study solves the decomposition of the material at high temperatures by using the nth order Arrhenius equation but excludes the removal of char from the surface due to mechanical erosion or phase change and considers that the ablation process takes place in a finite zone. The method considers the whole domain as one computational domain, eliminating the necessity to check the positions of the start and end of decomposition zone. The decomposition of pyrolysis gases and/or char that may occur at high temperatures and the chemical reaction between pyrolysis gases and char is neglected while pyrolysis gases are assumed to behave as ideal gas. The pressure is taken as a constant value on a whole physical domain. The formulation for one-dimensional case is validated by experimental results obtained from literature. The two-dimensional case in a Cartesian geometry is formulated and an algebraic transformation is used to normalize the region in both directions and transformed at same time into a square computational domain in order to get a solution for the variable thickness domains. The formulation for two-dimensional case is revised for the cylindrical coordinates with finite length in the axial direction. To solve geometries where the outer surface deviates from cylindrical, the formulation is scaled and transformed into a non-dimensional square computational domain. The method is also applied to a two layer material problem in axisymmetric geometry. In all problems, the radiation and constant heat flux boundary conditions exist on the outer surface while whole domain is initially at a constant temperature. Case studies are performed to demonstrate the application of the solution method in optimizing the insulation material thickness.

Quantitative Analysis of Phase-Transition Process of Light-Activatable Theranostic Agents by Pulsed Laser

Zhang, Zhe January 2018 (has links)
No description available.

Investigation into the correlation between paper insulation thermal ageing estimation using the arrhenius equation and other methods for generator transformers

Metebe, Michael Tebogo January 2015 (has links)
A dissertation submitted to the Faculty of Engineering and the Built Environment, University of the Witwatersrand, in fulfilment of the requirements for the degree of Master of Science in Engineering Date submitted: 28 August 2015 / Many generator transformers were installed many years ago during the initial commissioning of Eskom’s power stations. Many of these transformers have started showing signs of significant ageing of the paper insulation and hence require regular monitoring. There are two methods that are currently being employed to assess the degree of ageing of the paper insulation in a generator transformer, which are paper sampling and furan level measurement. This dissertation investigates an alternative method of predicting the degree of ageing of the paper insulation instead of what is used currently. This method uses the Arrhenius equation that relates time and temperature to determine the degree of degradation of organic materials. The reliability of the Arrhenius estimation method is assessed by comparing the predicted DP (Degree of polymerisation) values with the measured DP values of the same transformer paper insulation. The results obtained showed that there is reasonable correlation between the DP values estimated from the Arrhenius equation and the DP values estimated from the measured furan levels. The accuracy of the prediction method is reduced when the oil temperature greatly differs from the paper insulation temperature. The application of the Arrhenius equation to estimate the ageing of paper insulation is a great milestone in the quest to predict the remaining life of a transformer. It is the only method available to do this prediction and using online temperature measurement on transformers makes the method more reliable. / MT 2017

Superkondensatorer istället för batterier som energireserv i lågvoltssystem / Supercapacitors as energy source replacement for batteries

Eliasson, Fadi, Lundmark, Liv January 2020 (has links)
Denna rapport undersöker om superkondensatorer kan ersätta en energireserv bestående av batterier i en befintlig produkt. Bakomliggande teori av superkondensatorn redovisas samt dess användning som energireserv i lågvoltssystem. En krets konstrueras och testas för att verifiera funktionen. Simuleringar utförs för att verifiera valen till kretsen. Matlab och LTSpice används för simuleringarna. Kretsen skapas i OrCad och Altium Designer. Teorin, simuleringarna och testerna pekar på att superkondensatorer kan ersätta batterierna samt att de ställda kraven går att uppfylla. Alla tester gav dock inte resultat och därför kunde inte alla krav verifieras. Framtida tester behövs för att kunna garantera att lösningen kan uppfylla livslängds- och temperaturkraven eller om det behövs bytas till superkondensatorer med större kapacitet. / This report examines if supercapacitors could replace the existing batteries used in a product as a power source. The theory of supercapacitors and the use of these as a power source in low volt systems is presented. A circuit was created and tested to verify the function. Simulations were performed to verify the choices for the circuit. Matlab and LTSpice was used for the simulations. The circuit was created using OrCad and Altium Designer. The theory, simulations and tests all pointed towards that indeed supercapacitors can replace the existing batteries and the requirements can be met. Although not all tests resulted in results, therefore not all of the requirements could be verified. Future tests are needed to be able to guarantee that the solution can meet the lifetime and temperature requirements or if it will be necessary to replace the supercapacitors with ones that have higher capacity.

Functionalizing Ceramic Matrix Composites by the Integration of a Metallic Substructure with Comparable Feature Size

Heckman, Elizabeth Pierce 20 May 2021 (has links)
No description available.

Evaluating Technical Lifetime of Radio Circuit Boards : by Measuring Degradation and Aging of Hold-up Capacitors / Utvärdering av teknisk livslängd för radiokretskort : Genom att mäta degradering och åldring av hold-up kondensatorer

Tellberg, Otto January 2023 (has links)
The aim of this thesis is to improve the knowledgebase regarding the technical lifetime of radio circuit boards. The purpose is to create opportunities to facilitate the design of products that live up to the customers' requirements and to Ericsson's sustainable responsibility goals. The problem at hand was to look for the average temperature that has contribiuted to the performance degradation of circuit boards used in the field. The method was to measure the performance degradation of capacitors used in a power distribution function on the circuit board and combining those values with the time the circuit board has been in operation, to derive the average operation temperature with the help of the Arrhenius equation. The radio circuit board used in the field showed signs of performance degradation that could be interpreted as having reached the end of its technical lifetime after being used for 31 800 hours (3,6 years) at an average temperature of 78 °C to 79 °C. For it to be possible to interpret the temperature results of the unit used in the field, the three parameters: measured performance degradation, time of operation and average temperature at that time, had to be collected. All three parameters are key when evaluating technical lifetime since they need to be interpreted in relation to each other. The temperature results exists in a thermodynamic system that includes the capacitors, circuit board, the temperature of the surrounding environment, and the global climate. The temperature results and the measured performance degradation is a consequence of how the radio was operated, which is linked to the desired performance it was designed for. The radio circuit boards are also designed to comply with customers' requirements and the requirements based in the companys's sustainability responsibility goals. It is likely that the desired performance of radios in the future needs to be balanced with the company's sustainability responisibility goals, and the temperatures created as an effect of climate change. / Målet med detta examensarbete var att öka kunskapen om radiokretskorts tekniska livslängd. Syftet var att skapa möjligheter att främja utveckling av produkter som möter kunders krav samt de krav som följer av Ericssons strategi för hållbarhet och ansvarsfullt företagande. Problemformuleringen består av att söka efter medeltemperaturen som har bidragit till den prestandadegradering som lett till att kretskort i fält nått slutet på sin tekniska livslängd. Metoden som användes för att kunna härleda medeltemperaturen under drift var att mäta prestandadegraderingen hos kondensatorer som används i en specifik kraftfördelningsfunktion på radiokretskort och kombinera dessa mätvärden med tiden kretskortet varit i drift. Tiden och temperaturen härleddes med hjälp av Arrhenius ekvation. Den radiokretskortsenhet som användes i fält kan tolkas ha nått slutet på sin tekniska livslängd efter 31 800 timmar (3,6 år) och under tiden i drift haft medeltemperaturen på ungefär 78 °C till 79 °C. För att det skulle vara möjligt att tolka temperaturresultatet för radiokretskortet som användes i fält behövde följande tre parameterar samlas in: uppmätt prestandadegradering, tiden som radiokretskortet varit i drift samt medeltemperaturen under driftstiden. Samtliga tre parameterar behöver analyseras i relation till varandra när den tekniska livslängden ska utvärderas. Temperaturresultatet existerar i ett termodynamiskt system som består av kondensatorerna, krestkortet, temperaturen i den omkringliggande miljön samt det globala klimatsystemet. Temperaturresultatet och den uppmätta prestandadegraderingen är en konsekvens av hur radion har använts och är länkat till den den prestanda radion är designad för. Radiokretskorten är också designade för att följa kundernas krav och de krav som följer av Ericssons strategi för hållbarhet och ansvarsfullt företagande. Det är sannolikt att den önskade prestandan hos radioprodukter i framtiden behöver balanseras med de krav som följer av Ericssons strategi för hållbart företagande samt de temperaturer som skapas som konsekvens av klimatförändringarna.

Page generated in 0.2271 seconds