A one-fluid,two-dimensional flow simulation model for a kettle reboiler

A one-fluid, or algebraic slip, model has been developed to simulate two-dimensional, two-phase flow in a kettle reboiler. The model uses boundary conditions that allow for a change in flow pattern from bubbly to intermittent flow at a critical superficial gas velocity, as has been observed experimentally. The model is based on established correlations for void fraction and for the force on the fluid by the tubes. It is validated against pressure drop measurements taken over a range of heat fluxes from a kettle reboiler boiling R113 and n-pentane at atmospheric pressure.The model predicts that the flow pattern transition causes a reduction in vertical mass flux, and that the reduction is larger when the transition occurs at a lower level. Before transition, the frequently-used, one-dimensional model and the one-fluid model are shown to predict similar heat-transfer rates because similar magnitudes of mass flux are predicted. After transition, the one-dimensional model significantly over-predicts the mass fluxes. The average heat-transfer coefficient predicted by the one-fluid model is consequently about 10% lower. The one-fluid model shows that tube dryout can be expected at much lower heat fluxes than previously thought and that the fluid kinetic energy available to induce tube vibrations is significantly smaller.     

Unobstructed electron transfer on porous polyelectrolyte nanostructures and its characterization by electrochemical surface plasmon resonance

Thin organic films with desirable redox properties have long been sought in biosensor research. We report here the development of a polymer thin film interface with well-defined hierarchical nanostructure and electrochemical behavior, and its characterization by electrochemical surface plasmon resonance (ESPR) spectroscopy. The nano-architecture build-up is monitored in real time with SPR, while the redox response is characterized by cyclic voltammetry in the same flow cell. The multilayer assembly is built on a self-assembled monolayer (SAM) of 1:1 (molar ratio) 11-ferrocenyl-1-undecanethiolate (FUT) and mercaptoundecanoic acid (MUA), and constructed using a layer-by-layer deposition of cationic poly(allylamine hydrochloride) (PAH) and anionic poly(sodium 4-styrenesulfonate) (PSS). Electron transfer (ET) on the mixed surface and the effect of the layer structures on ET are systematically studied. Under careful control, multiple layers can be deposited onto the 1:1 FUT/MUA SAM that presents unobstructed redox chemistry, indicating a highly ordered, extensively porous structure obtained under this condition. The use of SPR to trace the minute change during the electrochemical process offers neat characterization of local environment at the interface, in particular double layer region, allowing for better control over the redox functionality of the multilayers. The 1:1 SAM has a surface coverage of 4.1 ± 0.3 × 10⁻¹⁰ mol cm⁻² for ferrocene molecules and demonstrates unperturbed electrochemistry activity even in the presence of a 13 nm polymer film adhered to the electrode surface. This thin layer possesses some desirable properties similar to those on a SAM while presenting ∼15 nm exceedingly porous structure for high loading capacity. The high porosity allows perchlorate to freely partition into the film, leading to high current density that is useful for sensitive electrochemical measurements.     

Fiber-optic infrared reflection absorption spectroscopy for trace analysis on surfaces of varying roughness: Sodium dodecyl sulfate on stainless steel

The effect of surface roughness on the analytical performance of a fiber-optic grazing-angle infrared reflection absorption spectroscopy (IRRAS) system has been investigated. The instrument was used to develop calibrations to quantify trace surface loadings for sodium dodecyl sulfate (SDS) on 316 stainless steel with three different surface finishes. Partial least squares (PLS) calibrations were developed for both individual finishes and combinations of finishes. For SDS on individual surface finishes, the root mean square (rms) error of cross-validation (RMSECV) varied between 0.08 and 0.12 microg cm(-2) with values of R2 between 0.89 and 0.96. A combined model for SDS on all surfaces gave an RMSECV of 0.18 microg cm(-2) with an R2 of 0.85.     

Mercury accumulation and attenuation at a rapidly forming delta with a point source of mining waste

The Walker Creek intertidal delta of Tomales Bay, California is impacted by a former mercury mine within the watershed. Eleven short sediment cores (10 cm length) collected from the delta found monomethylmercury (MMHg) concentrations ranging from 0.3 to 11.4 ng/g (dry wt.), with lower concentrations occurring at the vegetated marsh and upstream channel locations. Algal mats common to the delta's sediment surface had MMHg concentrations ranging from 7.5 to 31.5 ng/g, and the top 1 cm of sediment directly under the mats had two times greater MMHg concentrations compared to adjacent locations without algal covering. Spatial trends in resident biota reflect enhanced MMHg uptake at the delta compared to other bay locations. Eighteen sediment cores, 1 to 2 m deep, collected from the 1.2 km² delta provide an estimate of a total mercury (Hg) inventory of 2500 ± 500 kg. Sediment Hg concentrations ranged from pre-mining background conditions of approximately 0.1 μg/g to a post-mining maximum of 5 μg/g. Sediment accumulation rates were determined from three sediment cores using measured differences of ¹³⁷Cs activity. We estimate a pre-mining Hg accumulation of less than 20 kg/yr, and a period of maximum Hg accumulation in the 1970s and 1980s with loading rates greater than 50 kg/yr, corresponding to the failure of a tailings dam at the mine site. At the time of sampling (2003) over 40 kg/yr of Hg was still accumulating at the delta, indicating limited recovery. We attribute observed spatial evolution of elevated Hg levels to ongoing inputs and sediment re-working, and estimate the inventory of the anthropogenic fraction of total Hg to be at least 1500 ± 300 kg. We suggest ongoing sediment inputs and methylation at the deltaic surface support enhanced mercury levels for resident biota and transfer to higher trophic levels throughout the Bay.     

Passive modelocking of InGaAsP/InP laser diode over wide operating temperature range

The passive modelocked operation of an InGaAsP/InP laser diode, with a novel design of adaptable saturable absorber, up to an operating temperature of 75/spl deg/C, is reported. To the authors' best knowledge, this is the first demonstration of high-temperature operation of a passively modelocked laser diode. The effect of the operating temperature on the operating frequency and the tuning range is described.     

Hydrogels Constructed from Engineered Proteins

Due to their various potential biomedical applications, hydrogels based on engineered proteins have attracted considerable interest. Benefitting from significant progress in recombinant DNA technology and protein engineering/design techniques, the field of protein hydrogels has made amazing progress. The latest progress of hydrogels constructed from engineered recombinant proteins are presented, mainly focused on biorecognition‐driven physical hydrogels as well as chemically crosslinked hydrogels. The various bio‐recognition based physical crosslinking strategies are discussed, as well as chemical crosslinking chemistries used to engineer protein hydrogels, and protein hydrogels' various biomedical applications. The future perspectives of this fast evolving field of biomaterials are also discussed.     

Monolithic integration via a universal damage enhanced quantum-wellintermixing technique

A novel technique for quantum-well intermixing is demonstrated, which has proven a reliable means for obtaining postgrowth shifts in the band edge of a wide range of III-V material systems. The technique relies upon the generation of point defects via plasma induced damage during the deposition of sputtered SiO₂, and provides a simple and reliable process for the fabrication of both wavelength tuned lasers and monolithically integrated devices. Wavelength tuned broad area oxide stripe lasers are demonstrated in InGaAs-InAlGaAs, InGaAs-InGaAsP, and GaInP-AlGaInP quantum well systems, and it is shown that low absorption losses are obtained after intermixing. Oxide stripe lasers with integrated slab waveguides have also enabled the production of a narrow single lobed far field (3°) pattern in both InGaAs-InAlGaAs, and GaInP-AlGaInP devices. Extended cavity ridge waveguide lasers operating at 1.5 μm are demonstrated with low loss (α=4.1 cm^-1) waveguides, and it is shown that this loss is limited only by free carrier absorption in waveguide cladding layers. In addition, the operation of intermixed multimode interference couplers is demonstrated, where four GaAs-AlGaAs laser amplifiers are monolithically integrated to produce high output powers of 180 mW in a single fundamental mode. The results illustrate that the technique can routinely be used to fabricate low-loss optical interconnects and offers a very promising route toward photonic integration     

A study of impurity-free vacancy disordering in GaAs-AlGaAs forimproved modeling

A study of the parameters of the process of impurity-free vacancy disordering (IFVD) of GaAs-AlGaAs quantum-well structures is presented. The study includes photoluminescence excitation measurements which show that the as-grown barrier/well interface is better fitted by an exponential profile than a square profile. This has a significant effect on the intermixed diffusion profiles. Also, deep level transient spectroscopy measurements have been conducted on samples that were processed using IFVD. The measurements show an elevated concentration of the trap EL2 in the processed samples, which is known to be related to As antisites. The concentration of such defects agrees with the concentration calculated for IFVD to within an order of magnitude, suggesting a correlation between the point defects required for IFVD and EL2. Finally, temporally and spatially resolved photoluminescence measurements were conducted on processed samples which indicate a factor of 3 reduction in the photogenerated carrier life time after undergoing IFVD. A spatial resolution better than 3 μm has been observed     

Suppression of bandgap shifts in GaAs/AlGaAs quantum wells using strontium fluoride caps

Quantum well intermixing using dielectric caps has been studied using photoluminescence at 77 K. The structures which have been investigated, including shallow depth single quantum wells and multiquantum well waveguiding material, are highly sensitive to the presence of surface defects during annealing. Samples capped with either silicon nitride or silica have shown considerable energy shifts after processing in a rapid thermal annealer, and large energy shifts have also been found in uncapped material. Samples capped with strontium fluoride have shown negligible intermixing of the quantum wells.<>