Acoustic Attenuation in Fans and Ducts by Vaporization of Liquid Droplets

A cloud of small water droplets in saturated air attenuates acoustic disturbances by viscous drag, heat transfer, and vapor exchange with the ambient gas. The viscous and heat transfer phenomena attenuate at frequencies above 104 Hz for I-J.l droplets. The processes associated with phase exchange attenuate at a much lower frequency that may he controlled by choice of the liquid mass fraction. The strength of this attenuation is proportional to the mass of water vapor in the air, a factor controlled by air temperature. For plane waves, the attenuation magnitude e~ceeds 5 db!m ~t a temperature of 25°C with a cloud of 0.7 J.l radius droplets constituting 1 % of the gas mass. ThiS attenuation mcreases to more than 7 dbjm at frequencies above 1000 Hz where viscous and heat transfer mechanisms contribute significantly. The attenuation of higher order duct modes is strongly increased over the above values, similarly to the attenuation by duct lining. When the droplet cloud occupies only a fraction of the duct height close to the walls, the droplet clond may be up to twice as elfective as the uniform cloud, and a significant saving is possible in the water required to saturate the air and furnish the water droplets

Shear and longitudinal viscosity of non-ionic C12E8 aqueous solutions

We present measurements of the steady shear viscosity, the longitudinal elastic modulus and the ultrasonic absorption in the one-phase isotropic liquid region of the nonionic surfactant C12E8 aqueous solutions. The overall results support the presence of two separated intervals of concentration corresponding to different structural properties. In the surfactant-rich region the temperature dependence of the steady shear viscosity follows an equation characteristic of glass-like systems. The ultrasonic absorption spectra show unambiguous evidence of viscoelastic behaviour described by a Cole-Cole relaxation formula. In the water-rich region the behaviour of the measured quantities are more complex and reflect the presence of dispersed aggregates whose size increases with temperature and concentration. An additional low frequency contribution is also observed, which is ascribed to the exchange of water molecules and/or surfactant monomers between the aggregates and the bulk solvent region.Comment: 23 Pages, 7 Figures, 1 Table, submitted to J. Phys. Chem B, accepted for publicatio

Preparation and properties of potassium-vermiculite films

Flexible films of vermiculite have been prepared from aqueous suspensions after swelling by cation exchange and mechanical delamination. Two different swelling cations, lithium and butylammonium, have been investigated. The degree of swelling and delamination during the suspension preparation was characterized by the percentage of water reabsorbed by dried clays and the adsorption of methylene blue. The vermiculite saturated with lithium ions is more easily delaminated but contains more water than those saturated with butylammonium. Good quality coherent flexible films could be prepared from both the lithium and butylammonium exchanged vermiculites but the high percentage of water found in the films has a detrimental effect on their dielectric properties. To reduce the amount of water in the exchanged vermiculites a second ion exchange with potassium, a less hydratable cation, was investigated. Films prepared after exchange with potassium showed significant improvements in their dielectric properties, with a dielectric constant ∈ around 10 and a dissipation factor tan δ around 0.06 at 25 °C and a frequency of 1 kH

Microwave and tunable far-infrared laser spectroscopy of the ammonia–water dimer

Microwave and far-infrared spectra of the H3N–HOH dimer have been recorded from 36 to 86 GHz and 520 to 800 GHz with a planar supersonic jet/tunable laser sideband spectrometer. The a-type pure rotational microwave data extend the previous m=0, K=0 A symmetry manifold measurements of Herbine and Dyke [J. Chem. Phys. 83, 3768 (1980)] to higher frequency and also provide an additional set of microwave transitions in the mK=+1 E symmetry manifold. Two sets of five b-type rotation–tunneling bands, one set shifted from the other by an approximately constant 113 MHz, have been observed in the far infrared. The splitting into two sets arises from water tunneling, while the overall band structure is due to internal rotation of the ammonia top. Nonlinear least-squares fits to an internal rotor Hamiltonian provided rotational constants, and an estimation of V3=10.5±5.0 cm–1 for the barrier height to internal rotation for the NH3 monomer. A nonlinear equilibrium hydrogen bond is most consistent with the vibrationally averaged rotational constants; with the angle co^s–1[] determined from , the projection of the ammonia's angular momentum onto the framework; and with the nitrogen quadrupole coupling constants of Herbine and Dyke. The water tunneling splitting and observed selection rules place constraints on the barrier height for proton exchange of the water as well as the most feasible water tunneling path along the intermolecular potential energy surface. An estimated barrier of ~700 cm^–1 is derived for the water tunneling motion about its c axis

Ultrasound attenuation dependence on air compression or expansion processes

In this work variation of ultrasonic attenuation coefficient is analyzed in terms of air compression or expansion processes. In\ud closed spaces changing air volume, the ultrasound attenuation coefficient depends on thermodynamic processes which occur\ud during the air volume change. Two limiting cases are possible: 1) if the change of air volume is very slow or intensive heat\ud exchange occurs between the system and surrounding environments, so the system stays in a thermodynamic equilibrium;\ud therefore an isothermal process occurs; 2) if the change of air volume is very fast or the working environment has a good thermal\ud insulation, so the heat exchange between the system and the surrounding environment does not occur. In this case an adiabatic\ud process is presented. The attenuation coefficient of ultrasound varies very differently depending on the process (isothermal or\ud adiabatic) that occurs during the change of air volume. In particular, these differences occur when measurements are carried out in\ud a frequency range above 500 kHz during air compression. Initial relative air humidity has high influence on the ultrasonic signals\ud attenuation. Carrying out ultrasonic measurements in such systems, due to reliability of the measurements it is necessary to\ud evaluate thermodynamic process and ultrasound attenuation variation during the process. Oversaturated water vapour may occur\ud during the measurement process, therefore the measurement conditions become more complicated.\ud Keywords: attenuation of acoustic signals in air, relative humidity, isothermal process, adiabatic proces

Demand for imports in Venezuela : a structural time series approach

Using structural time series models, Cuevas estimates common stochastic trends of real GDP and imports in Venezuela from 1974-2000. The real imports trend drifts upward at almost twice the rate of growth of GDP. This highlights the powerful structural tendency toward increasing imports in Venezuela. The author also explicitly estimates common stochastic cycles, which he finds to have 5 and 17 year periods. In addition, he finds that a 1 percent real exchange rate appreciation leads to a 0.4 percent increase in imports. And in the long-run, 1 percent real GDP growth is associated with 1.7 percent real imports growth. The author also shows that the GDP elasticity of imports uniformly falls with cycle period, with the elasticity reaching 4.55 at the frequency associated with the 5-year cycle. A powerful imports responsiveness at the higher cycle frequency is associated with the recurrence of external imbalances in Venezuela.Environmental Economics&Policies,Climate Change,Consumption,Economic Theory&Research,Water Conservation,Economic Theory&Research,Environmental Economics&Policies,Macroeconomic Management,Economic Stabilization,Consumption