Condensation in Horizontal Smooth Tubes: A New Heat Transfer Model for Heat Exchanger Design

This paper proposes a new method to determine the condensation heat transfer coefficient of fluids flowing into horizontal smooth tubes with internal diameters D > 3 mm. The method has been drawn up as simply as possible and is ready to use in heat exchanger modeling and design applications. It is also suitable to work very well with old and new fluids used in the refrigeration, air conditioning, and heat pump industries. Particular attention is given to accuracy: it has been tested over a wide updated experimental database and comes from many different independent researchers with reduced experimental uncertainties. In order to obtain an easy structure, only two equations are employed, related respectively to & Delta; T-independent and to & Delta; T-dependent fluid flows. All the parameters that influence the condensation heat transfer have been included.

A comparison has been conducted against HCFCs, HFCs, HCs, carbon dioxide, ammonia, and water data. Zeotropic mixtures with two and three components are also considered in the comparison by applying the Bell and Ghaly [1 Bell, K. J. and Ghaly, M. A. 1973. An Approximate Generalized Design Method for Multicomponent/Partial Condenser. AIChE Symp. Ser., vol. 69: 72–79. [CSA] [Google Scholar]] correction to calculate the relative heat transfer penalization. A model has been developed with the idea of getting high accuracy through an easy structure, and the results show a very satisfactory agreement with experimental data: average deviation e_R = +2%, absolute mean deviation e_AB = 14%, and standard deviation σ_N = 19% for the total number of 5478 data points.     

Mechanical alloying and subsequent heat treatment of Ag–Zn powders

Microstructural evolution during mechanical alloying of Ag and Zn, and subsequent heat treatments were investigated. The mechanical alloying was carried out in a SPEX 8000D miller. The microstructural characterization was obtained by X-ray diffraction (XRD), transmission electron microscopy (TEM) and scanning electron microscopy (SEM). The thermal behavior was studied using differential scanning calorimetry (DSC). Based on the results obtained, it can be concluded that at the early stages of milling was possible to detect the ?, β, β′, α solid solutions and remaining Zn. Later, the ?, β, β′ and Zn phases disappeared while the Zn concentration of the α solid solution was strongly increased. After 7.2 ks of milling, the mechanical alloying process reached a steady state. During this period, both the composition and crystallite size of the α solid solution remained practically unchanged. On the other hand, subsequent heat treatments of milled powders showed that the α solid solution could also be obtained by the combination of mechanical alloying and heat treatment. Finally, the evolution of the microstructure during milling and annealing was combined to propose an optimal processing route in order to obtain a α solid solution.     

Acid and alcohol tolerance of Escherichia coli O157:H7 in pulque, a typical Mexican beverage

Pulque is a traditional Mexican fermented alcoholic beverage produced from the nectar of maguey agave plants. No data exist on the behavior of Escherichia coli O157:H7 in agave nectar and pulque. An initial trial was done of the behavior of E. coli O157:H7 during fermentation of nectar from a single producer, a nectar mixture from different producers and "seed" pulque. A second trial simulating artisanal pulque production was done by contaminating fresh nectar with a cocktail of three E. coli O157:H7 strains, storing at 16 ° and 22 °C for 14 h, adding seed pulque and fermenting until pulque was formed. A third trial used pulque from the second trial stored at 22 °C as seed to ferment fresh nectar at 22 °C for 48 h (fermentation cycle). This procedure was repeated for an additional two fermentation cycles. During incubation at 16 ° or 22 °C in the first trial, the E. coli O157:H7 strains multiplied in both the single producer nectar and nectar mixture, reaching maximum concentration at 12h. E. coli O157:H7 cell concentration then decreased slowly, although it survived at least 72 h in both fermented nectars. E. coli O157:H7 did not multiply in the seed pulque but did survive at least 72 h. In the second trial, the numbers of E. coli O157:H7 increased approximately 1.5 log CFU/ml at 22 °C and 1.2 log CFU/ml at 16 °C after 14 h. After seed pulque was added, E. coli O157:H7 concentration decreased to approximately 2 log CFU/ml, and then remained constant until pulque was produced. In the third trial, the E. coli O157:H7 cells multiplied and survived during at least three nectar fermentation cycles. The results suggest that E. coli O157:H7 can develop acid and alcohol tolerance in pulque, and constitutes a public health risk for pulque consumers.     

Enhancing the accuracy of subcutaneous glucose sensors: a real-time deconvolution-based approach

Minimally invasive continuous glucose monitoring (CGM) sensors can greatly help diabetes management. Most of these sensors consist of a needle electrode, placed in the subcutaneous tissue, which measures an electrical current exploiting the glucose-oxidase principle. This current is then transformed to glucose levels after calibrating the sensor on the basis of one, or more, self-monitoring blood glucose (SMBG) samples. In this study, we design and test a real-time signal-enhancement module that, cascaded to the CGM device, improves the quality of its output by a proper postprocessing of the CGM signal. In fact, CGM sensors measure glucose in the interstitium rather than in the blood compartment. We show that this distortion can be compensated by means of a regularized deconvolution procedure relying on a linear regression model that can be updated whenever a pair of suitably sampled SMBG references is collected. Tests performed both on simulated and real data demonstrate a significant accuracy improvement of the CGM signal. Simulation studies also demonstrate the robustness of the method against departures from nominal conditions, such as temporal misplacement of the SMBG samples and uncertainty in the blood-to-interstitium glucose kinetic model. Thanks to its online capabilities, the proposed signal-enhancement algorithm can be used to improve the performance of CGM-based real-time systems such as the hypo/hyper glycemic alert generators or the artificial pancreas.     

Poly(ester)s and poly(amide)s with fluorene and diphenyl-silane units in the main chain: Effects of iodine doping on the structure and electrical conductivity

Intramolecular charge transfer interaction between the electron donor and electro acceptor units within the polymeric structure and its optoelectronic properties were studied. The monomer, 9H-fluorene-2,7-dicarboxylic acid, was prepared from 9H-fluorene-2,7-dicarbonitrile using CuCN/N,N-dimethylformamide followed by the decomposition of the complex with FeCl₃x6H₂O in HCl and KOH/H₂O. The formation of two new classes of polymers was reported at different reaction times. The poly(ester) (PEF) was synthesized by the reaction of the diacid monomer with bis(4-hydroxiphenyl)diphenylsilane using tosyl chloride/pyridine/dimethylformamide system as condensing agent. Alternatively, the poly(amide) (PAF) was synthesized by the direct polycondensation of the diacid monomer and bis(4-aminophenyl)diphenylsilane in N-methyl-2-pyrrolidine solution containing dissolved calcium chloride. The resulting new polymers were obtained in good yields and were characterized by FTIR, NMR (¹H, ¹³C, and ²⁹Si), ESI, Raman, UV (optical gap) and fluorescence spectroscopy. The thermal properties were characterized by DSC and TGA. The electrical conductivity of the polymers was measured before and after exposure to iodine vapor, utilizing films of different thickness. Ellipsometric studies were used for the determination of the film thickness. Morphological differentiation was carried out by SEM-EDX analysis. Oxidation of the polymer films of low thickness decreased their conductivities, mainly due to the small structural changes. For a polymeric sample with a higher thickness, the doping process slightly increased the conductivity. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2012     

Si-SiC-ZrB2 ceramics by silicon reactive infiltration

Silicon carbide ceramics obtained by silicon reactive infiltration are nowadays employed within industry in several high temperature applications. Although these ceramics show good thermo-mechanical properties and oxidation resistance, they suffer temperature limitations (1400 °C). At higher temperatures another type of ceramics, commonly known as ultra high temperature ceramics (UHTCs), is under study. These include the transition metal diborides of group IV; one in particular, zirconium diboride, is interesting in certain applications (e.g. aerospace) because of its low relative density. ZrB₂-SiC ceramics show good thermo-mechanical properties and maintain the “protective” passive oxidation regime of their scales over a wide range of temperatures.This paper presents a feasibility study on a manufacturing methodology to produce Si-SiC-ZrB₂ bulk ceramics taking advantage of the reactive infiltration technique. This technique allows lower processing temperatures and near to net shape capability due to low shrinking of the green compacted bodies. C-SiC-ZrB₂ preforms were successfully infiltrated with molten silicon. The resulting Si-SiC-ZrB₂ composites showed promising oxidation behavior, similarly to that reported in other works. Bulk material optimization was performed with a view to manufacturing Si-SiC-ZrB₂ ceramic matrix composites by silicon reactive infiltration in future.     

Reactivity of 3-sulfanyl-1-hexanol and catechol-containing phenolics in vitro

Volatile species with thiol functions are important contributors to the flavour of a wide variety of wine types. However, in spite of their importance, their fate during winemaking has not been fully elucidated. In this work, the iron-catalysed reaction between 3-sulfanyl-1-hexanol, catechol-containing phenolics, and sulfurous acid, under in vitro aerobic conditions was evaluated by means of electrospray ionisation mass spectrometry (ESI-MS). The results indicate that a direct addition reaction between the thiol and some of the phenolics tested, and between sulfite and some of the phenolics may occur, thus contributing evidence of a possible route of thiol losses in wines.     

Correlation of animal diet and fatty acid content in young goat meat by gas chromatography and chemometrics

The meat fatty acids (FA) profiles of caprines submitted to different dietary treatments were determined by gas chromatography. The data were treated by Chemometrics to consider all variables together. The contents of saturated FA (SFA), monounsaturated FA (MUFA), polyunsaturated FA (PUFA), omega-3 (n-3) FA, and omega-6 (n-6) FA in 32 samples were analyzed. PUFA:SFA and n-6:n-3 ratios were also considered. The multivariate methods of hierarchical cluster analysis (HCA) and principal component analysis (PCA) were used to analyze the experimental results. HCA can group samples according to their basic composition, and PCA can explain the relationship among the dietary treatments according to the meat fatty acid composition. Treatment 1 presented the highest n-6 FA concentration, PUFA:SFA, and n-6:n-3 ratios, and the lowest MUFA and n-3 concentrations. Opposite results were observed for treatment 4. Treatments 2 and 3 were highly similar with differences mainly in SFA and MUFA concentrations.