Copyright (c) 2013 Loyola Marymount University and Loyola Law School All rights reserved. http://digitalcommons.lmu.edu/mech_fac Recent documents in Mechanical Engineering Faculty Works en-us Mon, 28 Jan 2013 23:25:11 PST 3600 http://digitalcommons.lmu.edu/mech_fac/22 http://digitalcommons.lmu.edu/mech_fac/22 Thu, 09 Feb 2012 12:05:40 PST Concentrated solar energy has become the input for an increasing number of experimental and commercial thermal systems over the past 10-15 years [M. Thirugnanasambandam et al., Renewable Sustainable Energy Rev. 14 (2010)]. Recent papers have indicated that the addition of nanoparticles to conventional working fluids (i.e., nanofluids) can improve heat transfer and solar collection [H. Tyagi et al., J. Sol. Energy Eng. 131, 4 (2009); P. E. Phelan et al., Annu. Rev. Heat Transfer 14 (2005)]. This work indicates that power tower solar collectors could benefit from the potential efficiency improvements that arise from using a nanofluid working fluid. A notional design of this type of nanofluid receiver is presented. Using this design, we show a theoretical nanofluid enhancement in efficiency of up to 10% as compared to surface-based collectors when solar concentration ratios are in the range of 100-1000. Furthermore, our analysis shows that graphite nanofluids with volume fractions on the order of 0.001% or less are suitable for 10-100 MW(e) power plants. Experiments on a laboratory-scale nanofluid dish receiver suggest that up to 10% increase in efficiency is possible (relative to a conventional fluid)-if operating conditions are chosen carefully. Lastly, we use these findings to compare the energy and revenue generated in a conventional solar thermal plant to a nanofluid-based one. It is found that a 100 MW(e) capacity solar thermal power tower operating in a solar resource similar to Tucson, AZ, could generate similar to$ 3.5 million more per year by incorporating a nanofluid receiver. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3571565]

]]> R. A. Taylor et al. http://digitalcommons.lmu.edu/mech_fac/21 http://digitalcommons.lmu.edu/mech_fac/21 Thu, 09 Feb 2012 12:05:39 PST Solar energy is one of the best sources of renewable energy with minimal environmental impact. Direct absorption solar collectors have been proposed for a variety of applications such as water heating; however the efficiency of these collectors is limited by the absorption properties of the working fluid, which is very poor for typical fluids used in solar collectors. It has been shown that mixing nanoparticles in a liquid (nanofluid) has a dramatic effect on the liquid thermophysical properties such as thermal conductivity. Nanoparticles also offer the potential of improving the radiative properties of liquids, leading to an increase in the efficiency of direct absorption solar collectors. Here we report on the experimental results on solar collectors based on nanofluids made from a variety of nanoparticles (carbon nanotubes, graphite, and silver). We demonstrate efficiency improvements of up to 5% in solar thermal collectors by utilizing nanofluids as the absorption mechanism. In addition the experimental data were compared with a numerical model of a solar collector with direct absorption nanofluids. The experimental and numerical results demonstrate an initial rapid increase in efficiency with volume fraction, followed by a leveling off in efficiency as volume fraction continues to increase.

]]> Todd Otanicar et al. http://digitalcommons.lmu.edu/mech_fac/20 http://digitalcommons.lmu.edu/mech_fac/20 Thu, 09 Feb 2012 12:05:38 PST Suspensions of nanoparticles (i.e., particles with diameters < 100 nm) in liquids, termed nanofluids, show remarkable thermal and optical property changes from the base liquid at low particle loadings. Recent studies also indicate that selected nanofluids may improve the efficiency of direct absorption solar thermal collectors. To determine the effectiveness of nanofluids in solar applications, their ability to convert light energy to thermal energy must be known. That is, their absorption of the solar spectrum must be established. Accordingly, this study compares model predictions to spectroscopic measurements of extinction coefficients over wavelengths that are important for solar energy (0.25 to 2.5 mu m). A simple addition of the base fluid and nanoparticle extinction coefficients is applied as an approximation of the effective nanofluid extinction coefficient. Comparisons with measured extinction coefficients reveal that the approximation works well with water-based nanofluids containing graphite nanoparticles but less well with metallic nanoparticles and/or oil-based fluids. For the materials used in this study, over 95% of incoming sunlight can be absorbed (in a nanofluid thickness >= 10 cm) with extremely low nanoparticle volume fractions - less than 1 x 10(-5), or 10 parts per million. Thus, nanofluids could be used to absorb sunlight with a negligible amount of viscosity and/or density (read: pumping power) increase.

]]> R. A. Taylor et al. http://digitalcommons.lmu.edu/mech_fac/19 http://digitalcommons.lmu.edu/mech_fac/19 Thu, 09 Feb 2012 11:56:21 PST The effects of curing temperature, based on new, low-temperature powder coating methods and traditional high-temperature powder coating methods, were studied. Heat-sensitive aluminum alloys (2024-T3, 6061-T6, and 7075-T6) were subjected to two different heat-treatment cycles, which were based on temperatures of 121 and 204 degrees C. Findings indicate that although both cure temperatures achieved powder coatings adhesion and thickness appropriate for industrial uses, the high-temperature cure treatment negatively affected the mechanical properties.

]]> C. Maldonado et al. http://digitalcommons.lmu.edu/mech_fac/18 http://digitalcommons.lmu.edu/mech_fac/18 Thu, 09 Feb 2012 11:56:11 PST Various thermo-mechanical processes were performed on a standard and a low oxygen content Ti-6Al-4V alloy. Testing was performed to determine whether it was possible to achieve a combination of tensile properties comparable to those of Ti-3Al-2.5V by means of cold working and annealing Ti-6Al-4V from a thickness of 0.671 cm (0.264 in.) to that between 0.081 and 0.094 cm (0.032-0.037 in.), which had never been carried out before. The resulting mechanical properties of this study were compared to the mechanical properties of Ti-3Al-2.5V to determine whether Ti-6Al-4V could be used as a suitable replacement for hydraulic tubing applications. The optimum results were achieved with 10-15% cold work and annealing at 750 A degrees C (1382 A degrees F) for 2 h between cold work reductions in thickness. It was concluded that Ti-6Al-4V was a suitable replacement for Ti-3Al-2.5V for hydraulic tubing with an increase in ultimate and yield strengths, but with a slight sacrifice of 5-10% elongation.

]]> W. Miranda et al. http://digitalcommons.lmu.edu/mech_fac/17 http://digitalcommons.lmu.edu/mech_fac/17 Thu, 09 Feb 2012 11:55:56 PST Aluminum and copper cylindrical shells were plastically buckled under quasi-static and dynamic loading conditions with an Absorption Compression-Torsion Plasticity (ACTP: Patent No. WO 2005090822) combined mechanical testing device. Optical microscopy and transmission electron microscopy (TEM) analysis were used to study the microscopic evolutions in the mechanically buckled aluminum and copper alloy samples. Optical microscopy showed evidence of the presence of second-phase particles in both the aluminum and copper alloys samples. Under dynamic loading aluminum samples showed more energy absorption as compared to copper samples. Material flow lines were more pronounced in the copper samples when observed by optical microscopy. The evidence that supports the increased energy absorption in the aluminum cylindrical shells can be supported by the TEM analysis more than the optical microscopy analysis. The TEM results showed highly oriented textured morphology with the presence of few dislocation cells structures and sub-structures.

]]> N. Drusina et al. http://digitalcommons.lmu.edu/mech_fac/16 http://digitalcommons.lmu.edu/mech_fac/16 Thu, 09 Feb 2012 11:55:32 PST The objective of this study is to determine the effect of solution treatment temperature, quenching media, and various machining sequences on the warpage behavior of aluminum 7249 alloy aged to T6 and T7' tempers. Large extrusions of 7249 aluminum alloy with fins were cut into 108 "T" sections. The samples were solution-treated, aged, and machined. Three solution temperatures (445, 474, and 505 A degrees C), two quenching media (water and 20% polyalkylene glycol), two aging treatments (T6 and T7'), and three machine sequences were used. The flatness of the samples was measured on the surfaces orthogonal to the z-axis. Three points were on top of both shoulders (six total), six were at the bottom of the sample, and six were on the top of the fin, in the cases where the fin was not milled off. They were then averaged together by surface to represent the overall warpage of each sample.

]]> N. Keleshian et al. http://digitalcommons.lmu.edu/mech_fac/15 http://digitalcommons.lmu.edu/mech_fac/15 Thu, 09 Feb 2012 11:55:03 PST The room temperature tensile properties of Ti-6Al-4V alloy prepared under two different processing routes were evaluated and compared. One group of samples was prepared by conventional casting-forging-rolling into flat plates. The other group was prepared by using Triton's Laser Free-Form Fabrication (LF3)(TM) processes, i.e., a laser was used to melt pre-alloyed powders of the required metallic composition as they were dropped onto a moveable substrate programmed to move in such a manner as to form a solid alloy plate. Five populations of Ti-6Al-4V were evaluated: a standard wrought form, an as-deposited form, a machined as-deposited form, a heat-treated as-deposited form, and a machined as-deposited and heat-treated form. The poorest mechanical properties occurred with the rough surfaces, likely due to existing microcracks and stress concentrations. The LF3 (TM) as-deposited material had mechanical properties comparable to, if not higher than, the mechanical properties of the wrought material. Further evaluations of the laser-formed material for complex spacecraft piece parts were warranted, specifically in regards to improving the surface finish of the materials.

]]> J. Alcisto et al. http://digitalcommons.lmu.edu/mech_fac/14 http://digitalcommons.lmu.edu/mech_fac/14 Thu, 09 Feb 2012 11:54:17 PST Rapid Prototyping (RP) is a method used everywhere from the entertainment industry to healthcare. Layer orientation is an important aspect of the final product. The objective of this research was to evaluate the effect of layer orientation on the mechanical strength and toughness of a polymer. The polymer used was a combination of two materials, ZP 130 and ZB 58, fused together in the Z Corporation Spectrum Z510 Rapid Prototyping Machine. ZP 130 is a powder composed of vinyl polymer (2-20%), sulfate salt (0-5%), and plaster that contains < 1% crystalline silica (50-95%). ZB 58 is a liquid composed of glycerol (1-10%), preservative (sorbic acid salt) (0-2%), surfactant (< 1%), pigment (< 1%), and water (85-95%). After removal from the machine the samples were sealed with Z bond 101 which is Beta-methoxyethyl cyanoacrylate (60-100%). The layer orientations studied were the crack arrestor, crack divider, and short transverse with various combinations of the three, for a total of seven orientations. The mechanical strength was evaluated using tensile testing and three-point bend testing. The toughness was evaluated by Izod impact testing. Five samples for tensile testing and three-point bend testing as well as 15 samples for the Izod impact test for each of the seven orientations were made. The total number of samples was 175. The crack arrestor orientation was the strongest main orientation for the tensile and three-point bend test. Weibull analysis was done on the Izod impact testing due to high variation in the results for the crack arrestor and short transverse directions. It was found that the layer orientation and surface roughness played a significant role in the penetration of the Z bond 101 coating and in the overall strength of the samples.

]]> V. Vega et al. http://digitalcommons.lmu.edu/mech_fac/13 http://digitalcommons.lmu.edu/mech_fac/13 Thu, 09 Feb 2012 11:53:35 PST The objective of this study was to investigate the feasibility of performing retrogression and reaging (RRA) heat treatments on 2099 aluminum-lithium alloy. The retrogression temperatures were 200-250 A degrees C and retrogression times were 5-60 min. Half of the samples were exposed to a salt fog environment. Interestingly, the samples exposed to salt spray had consistently higher mechanical tensile properties than those which were not exposed.

]]> N. Ward et al. http://digitalcommons.lmu.edu/mech_fac/12 http://digitalcommons.lmu.edu/mech_fac/12 Thu, 09 Feb 2012 11:51:25 PST A retrogression and reaging (RRA) treatment was performed on 2195 Al-Li Alloy. The exposure times were from 5 to 60 min, and the temperatures were from 200 to 250 A degrees C. Samples that were exposed to a salt spray test had overall similar mechanical properties as compared to those that were not exposed. The percent elongation, however, was significantly deteriorated due to the salt spray exposure. The mechanical properties of the 2195 samples were compared to those of 2099 samples exposed to similar treatments in an earlier study.

]]> N. Ward et al. http://digitalcommons.lmu.edu/mech_fac/11 http://digitalcommons.lmu.edu/mech_fac/11 Thu, 09 Feb 2012 11:48:49 PST This letter discusses experimentation with optically induced phase change in nanoparticle liquid suspensions-commonly termed nanofluids. Four different types of nanofluids at five concentrations were exposed to a similar to 120 mW, 532 nm laser beam to determine the minimum laser flux needed to create vapor. Laser irradiance was varied between 0-770 W cm(-2). While the experiments were simple, they involved many complex, interrelated physical phenomena, including: subcooled boiling, thermal driven particle/bubble motion, nanoparticle radiative absorption/scattering, and nanoparticle clumping. Such phenomena could enable novel solar collectors in which the working fluid directly absorbs energy and undergoes phase change in a single step. c 2009 American Institute of Physics. [doi: 10.1063/1.3250174]

]]> R. A. Taylor et al. http://digitalcommons.lmu.edu/mech_fac/10 http://digitalcommons.lmu.edu/mech_fac/10 Thu, 09 Feb 2012 11:45:52 PST The analysis of the combined efficiencies in a coupled photovoltaic (PV)/thermal concentrating solar collector are presented based on a coupled electrical/thermal model. The calculations take into account the drop in efficiency that accompanies the operation of PV cells at elevated temperatures along with a detailed analysis of the thermal system including losses. An iterative numerical scheme is described that involves a coupled electrothermal simulation of the solar energy conversion process. In the proposed configuration losses in the PV cell due to reduced efficiencies at elevated temperatures and the incident solar energy below the PV bandgap are both harnessed as heat. This thermal energy is then used to drive a thermodynamic power cycle. The simulations show that it is possible to optimize the overall efficiency of the system by variation in key factors such as the solar concentration factor, the band gap of the PV material, and the system thermal design configuration, leading to a maximum combined efficiency of similar to 32.3% for solar concentrations between 10-50 and a band-gap around 1.5-2.0 eV. (C) 2010 American Institute of Physics. [doi: 10.1063/1.3514590]

]]> Todd Otanicar et al. http://digitalcommons.lmu.edu/mech_fac/9 http://digitalcommons.lmu.edu/mech_fac/9 Thu, 09 Feb 2012 10:51:41 PST Accurately predicting the coefficient of thermal expansion for many aerospace components is critical to ensure proper functionality on orbit where the temperature gradient across a spacecraft can vary from +300 degrees F to -450 degrees F. Under these conditions, the linear approximations generated by theoretical equations no longer hold true, and experimental methods are needed. Although several methods exist for measuring the coefficient of thermal expansion of materials, laser interferometry yields high-resolution results, and the technique is widely accepted in the scientific community.

]]> G. Wallace et al. http://digitalcommons.lmu.edu/mech_fac/8 http://digitalcommons.lmu.edu/mech_fac/8 Thu, 09 Feb 2012 10:51:39 PST Aluminum alloys 2099-T6 die forgings, 2099-T83 extrusions, 7075-T7651 plate, 7085-T7452 die forgings, 7085-T7651 plate, and 2397-T87 plate were thermally exposed at temperatures of 180 degrees C (350 degrees F), 230 degrees C (450 degrees F), and 290 degrees C (550 degrees F) for 0.1, 0.5, 2, 10, 100, and 1000 h. The purpose of this study was to determine the effect of thermal exposure on the mechanical properties and electrical conductivity of these alloys. The data shows that higher temperatures and longer exposure times generally resulted in decreased strength and hardness and increased percent elongation and electrical conductivity.

]]> J. Jabra et al. http://digitalcommons.lmu.edu/mech_fac/7 http://digitalcommons.lmu.edu/mech_fac/7 Thu, 09 Feb 2012 10:51:38 PST Navy AM2 mats are used as portable aircraft landing platforms for the Short Take-off/Vertical Landing (STOVL) aircraft operations. This investigation presents the study performed to determine whether the surface discoloration is a precursor to degradation in the mechanical property of the AM2 mat material. The red discoloration on the mat surfaces had a clear correlation with the decrease in yield strength, ultimate strength, and hardness properties.

]]> V. Vega et al. http://digitalcommons.lmu.edu/mech_fac/6 http://digitalcommons.lmu.edu/mech_fac/6 Thu, 09 Feb 2012 10:50:57 PST Extruded I sections of 7075-T6 aluminum were machined into four different sections shapes: L, short depth L, T, and short depth 7. The furnace was preheated to 416 degrees C (780 degrees F) and the samples were placed inside. The temperature was raised to 471 degrees C (880 degrees F) and then the samples were quenched in either a 30% polyalkylene Glycol solution or water, both at 15 degrees C (59 degrees F). Points on the distorted samples were recorded before and after the solution treatment; the difference between the measurements indicated the extent of warpage.

]]> Omar S. Es-Said et al. http://digitalcommons.lmu.edu/mech_fac/5 http://digitalcommons.lmu.edu/mech_fac/5 Thu, 09 Feb 2012 10:50:56 PST Sheets of 2195 aluminum-lithium alloy were solution-treated at 507 A degrees C for 30 min. One set was stretched to 3-5% in the 0A degrees, 45A degrees, and 90A degrees angle with respect to the original rolling direction. Two other sets were rolled 6% reduction in thickness and 24% reduction in thickness in the 0A degrees, 45A degrees, and 90A degrees angle with respect to the original rolling direction. All specimens were aged at 143 A degrees C for 36 h. A second group of samples was rolled at 24 and 50% reduction in thickness after a solution treatment of 507 A degrees C for 1 h prior to aging at 190 A degrees C for 24 h. Tensile specimens were machined from each sheet at 0A degrees, 45A degrees, and 90A degrees angles to the original grain orientation. Tensile testing was used to determine the mechanical properties and anisotropic behavior of each condition. Rolling 6% reduction in thickness in the 45A degrees orientation yielded anisotropy of 7.6% in the yield strength.

]]> Omar S. Es-Said et al. http://digitalcommons.lmu.edu/mech_fac/4 http://digitalcommons.lmu.edu/mech_fac/4 Thu, 09 Feb 2012 10:50:54 PST Polymer adhesives are finding increased use in panel joining applications in aircraft and aerospace structures where the applied stresses permit their use and where a uniform stress distribution is needed. One such adhesive, Hysol EA-9394 (TM), was compared to three other formulations in this study. The new formulations were Hysol EA-9396, Hysol EA-9396 filled with nickel nanofibers and mixed by machine (Jamesbury Blender), and Hysol EA-9396 filled with nickel nanofibers and hand mixed in the laboratory. The comparison consisted of measuring shear lap strengths of aluminum test pieces bonded together with the candidate adhesives. The mechanical tests were supplemented by a Weibull analysis of the strength data and by a visual inspection of the failure mode (adhesive/cohesive). The lap shear strengths (fracture stress values) of all three Hysol EA-9396 adhesives were greater than that of the baseline Hysol EA-9394 polymer.

]]> P. Stoyanov et al. http://digitalcommons.lmu.edu/mech_fac/3 http://digitalcommons.lmu.edu/mech_fac/3 Thu, 09 Feb 2012 10:11:29 PST Aluminum alloys 6061, 2024, and 7075 were heat treated to various tempers and then subjected to a range of plastic strain (stretching) in order to determine their strain limits. Tensile properties, conductivity, hardness, and grain size measurements were evaluated. The effects of the plastic strain on these properties are discussed and strain limits are suggested.

]]> D. Ortiz et al.