Flow boiling heat transfer and pressure drop of R-134a in a mini tube: an experimental investigation

This paper presents the results of an experimental study carried out with R-134a during flow boiling in a horizontal tube of 2.6 mm ID. The experimental tests included (i) heat fluxes in the range from 10 to 100 kW/m², (ii) the refrigerant mass velocities set to the discrete values in the range of 240-930 kg/(m² s) and (iii) saturation temperature of 12 and 22 °C. The study analyzed the heat transfer, through the local heat transfer coefficient along of flow, and pressure drop, under the variation of these different parameters. It was possible to observe the significant influence of heat flux in the heat transfer coefficient and mass velocity in the pressure drop, besides the effects of saturation temperature. In the low quality region, it was possible to observe a significant influence of heat flux on the heat transfer coefficient. In the high vapor quality region, for high mass velocities, this influence tended to vanish, and the coefficient decreased. The influence of mass velocity in the heat transfer coefficient was detected in most tests for a threshold value of vapor quality, which was higher as the heat flux increased. For higher heat flux the heat transfer coefficient was nearly independent of mass velocity. The frictional pressure drop increased with the increase in vapor quality and mass velocity. Predictive models for heat transfer coefficient in mini channels were evaluated and the calculated coefficient agreed well with measured data within a range 35% for saturation temperature of 22 °C. These results extend the ranges of heat fluxes and mass velocities beyond values available in literature, and add a substantial contribution to the comprehension of boiling heat transfer phenomena inside mini channels.     

Electron transfer rates in DNA films as a function of tether length

A homologous series of DNA-modified electrodes has been investigated in which the molecular tether length varies. Using intercalated, covalently bound daunomycin as a redox probe, an exponential dependence of electron transfer rates on the number of intervening methylene groups in the sigma-bonded tether is observed. In contrast, variation in DM position within DNA yields no detectable change in rate. These data confirm that overall electron transfer rates in DNA films are limited by the tether, not the DNA.     

Das Atomgewicht des Heliums

Ohne Zusammenfassung     

Copper-catalyzed hydroamination of propargyl imidates

Propargyl imidates derived from aromatic and aliphatic nitriles cyclize at room temperature in high yields when treated with a catalytic amount of copper (I) iodide. This 5-exo-dig process affords dihydrooxazoles which do not aromatize under the reaction conditions, and which are isolated without chromatography. Investigations of the reaction scope, subsequent functionalization of the reaction products, and preliminary mechanistic data are presented.     

Microwave-assisted immobilization of the REDOX indicator 2,6-dichloroindophenol on PEGylated Merrifield resins

The blue organic dye, 2,6-dichloroindophenol (DCIP), was immobilized on PEGylated Merrifield resin after a chlorination step followed by a nucleophilic substitution reaction, using microwave-assisted methodology. The final product was a blue pigmented resin that can be reversibly reduced by ascorbic acid and changes color from blue to light yellow. Structural elucidation of this immobilized indicator was confirmed through FTIR and elemental analyses.     

Preparation and characterization of magnetic polymeric composite particles by miniemulsion polymerization

A water‐based magnetite ferrofluid, with an average size of about 10 nm, was prepared in a first step by the chemical coprecipitation of ferrous and ferric salts. Oil‐based styrene (St) magnetite ferrofluid was obtained by the acidification of the water‐based magnetite ferrofluid and the dispersion of the acidified magnetite in St. Magnetic polymeric composite particles (MPCPs) were prepared by miniemulsion polymerization in the presence of the oil‐based St magnetite ferrofluid with hexadecane as a hydrophobe, 2,2′‐azobisisobutyronitrile as an initiator, and sodium dodecyl sulfate as an emulsifier. Methacrylic acid was used as a comonomer, and hydroxyethyl cellulose and polyvinylpyrrolidone were used as aid stabilizers subsequently. With the aim of improving the encapsulation degree of magnetite, avoiding pure polymer particles and exposed magnetite particles, and obtaining the narrowest particle size distributions, the encapsulation conditions of magnetite were investigated in detail. The results show that miniemulsion polymerization is an effective method for encapsulating magnetite into a hydrophobic polymer successfully. Exposed magnetite particles and pure polymer particles can be avoided completely by the selection of the appropriate preparation conditions. All the resulting MPCPs exhibited superparamagnetism and possessed some magnetic response. © 2006 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 44: 4187–4203, 2006