G. Huelsz

Particle image velocimetry of the unstable capillary flow of a micellar solution

The unstable capillary flow of the micellar system formed by cetylpyridinium chloride 100 mM/sodium salicylate 60 mM (CPyCl/NaSal) in tridistilled water was studied in this work using a combination of particle image velocimetry (PIV) and rheometrical measurements. The experiments were run in a pressure controlled capillary rheometer at a temperature of 26 °C and covered all the different flow regimes occurring in the nonmonotonic flow curve characteristic of micellar solutions. First, we show the suitability of the PIV technique to study the unstable capillary flow of this micellar system. Then, and more important, we provide evidence of the development of shear banding and the velocity profiles for the different flow regimes, including the transition to the high shear branch because of spurt. The velocity profiles at low shear rates exhibited a Newtonian behavior, followed by a still Newtonian with apparent slip up to the onset of spurt. There was a jump of one order of magnitude in the shear rate with a...

Heat-to-electricity thermoacoustic-magnetohydrodynamic conversion

A design for the conversion of heat into electricity is proposed. The conversion is based on the combined effects of a thermoacoustic prime mover coupled to a magnetohydrodynamic generator where different working fluids can be optimally chosen for each process. The results of preliminary experiments are presented.

Temperature measurements inside the oscillatory boundary layer produced by acoustic waves

In this work experimental techniques developed for acoustic temperature oscillation measurements in gases using a cold wire anemometer are reported. The techniques also proved accurate for measuring of the relative phase between temperature and pressure oscillations. Specifically, the structure of the oscillatory thermal boundary layer with isothermal conditions in the solid boundary of an acoustic standing wave in air with 130-Hz frequency and pressure oscillation amplitude up to 200 Pa is determined. Experimental results are satisfactorily compared to theoretical results of a linear theory.

Experimental observation of dramatic differences in the dynamic response of Newtonian and Maxwellian fluids.

An experimental study of the dynamic response of a Newtonian fluid and a Maxwellian fluid under an oscillating pressure gradient is presented. Laser Doppler anemometry is used in order to determine the velocity of the fluid inside a cylindrical tube. In the case of the Newtonian fluid, the dissipative nature is observed. In the dynamic response of the Maxwellian fluid an enhancement at the frequencies predicted by theory is observed.

A thermal study of optical fibres transmitting concentrated solar energy

In this paper we develop a theoretical thermal study of optical fibres transmitting concentrated solar energy. An energy equation for simultaneous conduction and radiation of heat through optical fibres is obtained. To transmit concentrated solar energy an optical fibre tip is placed in the focus of a small paraboloidal mirror. The role of aluminium and silver as the reflective surface on the mirror that allows one to concentrate the solar energy is studied. The power supply is estimated to be 26 W at the end of a 10 m long fibre with 88% transmission efficiency. The thermal study considers a wavelength-dependent absorption coefficient of the optical fibre core in order to obtain the radiative heat flux in the fibre. The time evolution of the temperature distribution is obtained by a finite-difference method. With this result we predict that the fibre can be used 6 h.

Measurements of the bulk and interfacial velocity profiles in oscillating Newtonian and Maxwellian fluids.

We present the dynamic velocity profiles of a Newtonian fluid (glycerol) and a viscoelastic Maxwell fluid (CPyCl-NaSal in water) driven by an oscillating pressure gradient in a vertical cylindrical pipe. The frequency range explored has been chosen to include the first three resonance peaks of the dynamic permeability of the viscoelastic-fluid--pipe system. Three different optical measurement techniques have been employed. Laser Doppler anemometry has been used to measure the magnitude of the velocity at the center of the liquid column. Particle image velocimetry and optical deflectometry are used to determine the velocity profiles at the bulk of the liquid column and at the liquid-air interface respectively. The velocity measurements in the bulk are in good agreement with the theoretical predictions of a linear theory. The results, however, show dramatic differences in the dynamic behavior of Newtonian and viscoelastic fluids, and demonstrate the importance of resonance phenomena in viscoelastic fluid flows, biofluids in particular, in confined geometries.

Total Energy Balance Method for Venting Electric Clothes Dryers

A total energy balance method for venting electric tumbler dryers was developed and the experimental methodology and calculation details are provided in this article. The energy to evaporate the water in the load; the energy to heat the water, textile, and dryer; energy losses by convection and radiation; and the energy not used in the process were considered to calculate the total output energy. This method was applied to a domestic venting electric tumble dryer. The results of the method suggested areas of improvement. The maximum difference between the measured total input and the calculated total output energies was 2.6%, indicating the accuracy of the method.

A theoretical and experimental thermal study of SiO2 optical fibres transmitting concentrated radiative energy

In this work a theoretical and experimental thermal behaviour study of optical fibres with a high-purity SiO2 core transporting concentrated radiative energy is carried out. A theoretical unidimensional model for the simultaneous transport of heat by conduction and radiation in optical fibres, including the heat losses by convection at the surface, is developed. This model considers a constant linear absorption coefficient and it is solved analytically. An experimental method to determine the linear coefficient of absorption is developed. The time evolution of the axial temperature distribution of two kinds of fibres is recorded and compared with the theoretical predictions. These experimental results validate the theoretical model proposed.

Effect of a magnetic field on the linear stability of a thermoacoustic oscillation

Abstract The effect of a constant magnetic field on the stability of a compressible, electrically conducting fluid contained in a duct with an axial temperature gradient is studied. A plausible physical model of the phenomenon is presented and its linear stability is analyzed. The study includes the consideration of a general state equation for the working fluid and the variation of its physical properties with temperature. An expression for the critical thermal gradient required to generate thermoacoustic standing waves is found as a function of the external magnetic field. The potential application of this work is in the context of thermoacoustic prime movers with magnetohydrodynamic transducers.

On the phase difference of the temperature and pressure waves in the thermoacoustic effect

Abstract In this comment, the single-plate, linear theory for the thermoacoustic phenomenon in ideal conditions, Prandtl Number (σ) zero and a plate with infinite heat capacity, presented by Swift [1] has been used to find expressions for the phase difference α between the temperature and pressure waves. The effect of Prandtl Number different from zero and a plate with a finite heat capacity has also been analyzed. It has been found that the behavior is governed by the relation between a non-dimensional temperature gradient Γ and σ. Attenuation occurs for Γ σ and excitation for Γ > 1 + √ σ which correspond to | α | π /2 and | α | > π /2 respectively. Explicit expressions for a as a function of the transverse coordinate are given. A physical interpretation of the results is presented in the context of concepts offered by Lord Rayleigh.