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[en] Heat transfer from a heated plate to circular air jets issuing from a square array of nozzles is measured by means of an Infrared Scanning Radiometer (IRSR). Experimental tests are performed for different values of jet Reynolds number, nozzle-to-plate distance, nozzle-to-nozzle spacing and nozzle diameter. The two-dimensional character of IRSR and the possibility of processing digital thermal images on a computer allow to obtain surface temperature maps as well as to compute local heat transfer coefficient profiles. (author)
[en] The reed-type valves employed in refrigeration compressors must provide adequate sealing when closed to avoid leakage of gas between the compression chamber and the suction and discharge chambers. Recent studies show that valve leakage can considerably affect the performance of the small reciprocating compressors used for domestic refrigeration. The present paper reports an investigation on the adequacy of simplifying the hypothesis adopted in the simulation models of valve leakage. The results indicate that the transient effects related to both the valve deflection and fluid flow are negligible. Also, the ideal gas formulation was found suitable in some operating conditions found in domestic refrigeration. On the other hand, leakage was found to be overpredicted by almost 20% when the reed valve geometry was simplified to a circular plate in order to reduce the computational processing cost. (paper)
[en] A characterization of differently sized aluminum powders, by using BET (specific surface measurements), Scanning Electron Microscopy (SEM), X-Ray Diffraction (XRD), and X-ray Photoelectron Spectroscopy (XPS), was performed in order to evaluate their performance in solid propellant. These aluminum powders were used in manufacturing composite rocket propellants, that are based on Ammonium Perchlorate (AP) as oxidizer and Hydroxyl-Terminated-PolyButadiene (HTPB) as binder. The reference formulation was AP/HTPB/Al with 68/17/15 mass fractions, respectively. The ballistic characterization of studied propellants, made in terms of steady burning rates, showed how better is the performance of nano-aluminized compared to micro-aluminized propellants. Measurements of Al powder ignition time and temperature were also carried out
[en] Titanium oxide/carbon nanotubes-based nanocomposites (TiO2/CNTs, prepared by sol-gel method, and 2%Pt/TiO2/CNTs, obtained by wetness impregnation of the TiO2/CNTs base material with a solution of platinum acetylacetonate) have been recently used as active layer in hydrogen sensing devices at near room temperature, obtaining quite different responsiveness. The microstructure of these hybrid materials is here systematically investigated by micro-Raman spectroscopy at 2.41 eV. The results show that regardless of the nominal C/Ti molar ratio (3.6 or 17.0) only the anatase phase of titania is formed. Theoretical calculations demonstrate that phonon confinement is fully responsible for the large blue-shift (∼10 cm-1) and broadening (∼20 cm-1) of the lowest-frequency Raman mode with respect to bulk anatase. The average size (4.3-5.0 nm) of TiO2 crystallites, resulting from Raman spectra fitting, is in excellent agreement with those inferred from transmission electron microscopy and X-ray diffraction measurements. - Graphical Abstract: Micro-Raman analysis of TiO2/CNTs and Pt/TiO2/CNTs hybrids for H2 sensing applications evidences that regardless of C/Ti molar ratio titania crystallizes in the anatase phase. The very small size of TiO2 crystallites (4.3-5.0 nm) is responsible for the observed phonon confinement effects.
[en] The characterization of several differently sized aluminium powders, by BET (specific surface), EM (electron microscopy), XRD (x-ray diffraction), and XPS (x-ray photoelectron spectroscopy), was performed in order to evaluate their application in solid rocket propellant compositions. These aluminium powders were used in manufacturing several laboratory composite solid rocket propellants, based on ammonium perchlorate (AP) as oxidizer and hydroxil-terminated polybutadiene (HTPB) as binder. The reference formulation was an AP/HTPB/Al composition with 68/17/15% mass fractions respectively. The ballistic characterization of the propellants, in terms of steady burning rates, shows better performance for propellant compositions employing nano-aluminium when compared to micro-aluminium. Results obtained in the pressure range 1-70 bar show that by increasing the nano-Al mass fraction or decreasing the nano-Al size, larger steady burning rates are measured with essentially the same pressure sensitivity