Sounding Out Paper Pulp : Ultrasound Spectroscopy of Dilute Viscoelastic Fibre Suspensions Acoustics and Ultrasonics

A model of attenuation of ultrasound in fibre suspensions is compared to a model of backscattering pressure from submersed cylinders subjected to a sound wave. This analysis is carried out in the region where the wavelength is of the same order as that of the diameter of the fibre. In addition we assume the cylinder scatterer to have no intrinsic attenuation and the longitudinal axis of the scatterer is assumed to be perpendicular to the direction of propagation of the incident wave. Peaks in the frequency response of both the backscattering pressure, expressed in the form of a form function, and the attenuation are shown to correspond. Similarities between the models are discussed. Since the peaks in the form function are due to resonance of the cylinder, we infer that the peaks in the attenuation are also due to resonance. The exact nature of the waves causing the resonance are still unclear however the first resonance peaks are related to the shear wave and hence the shear modulus of the material. The aim is to use the attenuation model for solving the inverse problem of calculating paper pulp material properties from attenuation measurements. The implications of these findings for paper pulp property estimation is that the supporting fluid could, if possible, be matched to density of that of pulp fibres and that the estimation of material properties should be improved by selecting a frequency range that in the region of the first resonance peaks.Godkänd; 2006; 20150414 (aitomaki

Ultrasonic measurements and modelling of attenuation and phase velocity in pulp suspensions

In the manufacturing process of paper the mass fraction and material properties of the fibres in the pulp suspension are important for the quality of the finished product. This study presents two different methods of pulp characterisation. The first is based on phase velocity, which we use to investigate the composition of the pulp. Here a method is presented where the optimal number of circular shifts within the sampling window of the signal is determined which gives, in a weakly dispersive medium, a continuous phase spectrum and minimizes the likelihood of discontinuities within the bandwidth. Hence, the ambiguity in phase unwrapping is avoided. The results from phase velocity measurements show that the phase velocity weakly increases with increasing amount of fines in the suspension. The dispersion is caused by the fibres and it correlates with fibre mass fraction. The second method is based on attenuation and is used to characterise the wood fibres. The results of the attenuation experiments show that it is possible to inversely calculate wood fibre properties by fitting the model to the experimental data, if the fibre diameter distribution is known. However, the accuracy of these calculation is difficult to determined and more work in this area is required.Validerad; 2005; 20060913 (ysko

Ultrasonic methods in determining elastic material properties of fibres in suspension

The presented study concerns the application of the pulse-echo ultrasound measurement technique in determining the elastic properties of fibres suspended in water. The two kinds of fibre materials are investigated, nylon 6/6 fibres and softwood pulp fibres. The fibre mass fraction was 0.5% for nylon and ranges from 0% up to 1% for softwood pulp. The ultrasonic measurements are performed in the frequency range of 2-11 MHz. It is shown that the velocity dispersion of the ultrasound is small for each suspension sample. In obtaining the fibres longitudinal Young's modulus two methods are used, one based on phase velocity and one based on acoustic attenuation. It is found that both methods gives reasonable estimates of the longitudinal Young's modulus for nylon 6/6. For pulp fibres the determined Young's modulus is overestimated in comparison with earlier findings.Validerad; 2008; 20071002 (tlt

Analytical one-dimensional model for laser-induced ultrasound in planar optically absorbing layer

Ultrasound generated by means of laser-based photoacoustic principles are in common use today and applications can be found both in biomedical diagnostics, non-destructive testing and materials characterisation. For certain measurement applications it could be beneficial to shape the generated ultrasound regarding spectral properties and temporal profile. To address this, we studied the generation and propagation of laser-induced ultrasound in a planar, layered structure. We derived an analytical expression for the induced pressure wave, including different physical and optical properties of each layer. A Laplace transform approach was employed in analytically solving the resulting set of photoacoustic wave equations. The results correspond to simulations and were compared to experimental results. To enable the comparison between recorded voltage from the experiments and the calculated pressure we employed a system identification procedure based on physical properties of the ultrasonic transducer to convert the calculated acoustic pressure to voltages. We found reasonable agreement between experimentally obtained voltages and the voltages determined from the calculated acoustic pressure, for the samples studied. The system identification procedure was found to be unstable, however, possibly from violations of material isotropy assumptions by film adhesives and coatings in the experiment. The presented analytical model can serve as a basis when addressing the inverse problem of shaping an acoustic pulse from absorption of a laser pulse in a planar layered structure of elastic materials.Validerad; 2014; 20131029 (erisva

Speed of sound measurements in gas-mixtures at varying composition using an ultrasonic gas flow meter with silicon based transducers

This paper concerns speed of sound measurements performed in three different gas mixtures at constant temperature and pressure while the concentration of the gases was varied. The performed experiments used an ultra sonic, sing-around, gas flow meter equipped with silicon based transducers. The center frequency of the transducers was 800 kHz. Speed of sound was measured in mono-, di- and triatomic gases: argon (Ar), oxygen (O2) and carbon dioxide (CO2), in either air or nitrogen (N2) as a background gas. The gas under investigation was mixed with the background gas in a test chamber and the concentration of the gas under examination was varied between 0% and 100%. A gas chromatograph was used in order to accurately determine the composition of the gas mixture. The experiments show that measured speed of sound, as a function of gas composition, agrees with the speed of sound obtained from theory. The achieved data also show that the speed of sound measurements was performed with low standard deviation. Thus, one can conclude that this type of ultrasonic gas flow meter is well suited in determining gas concentration in a binary gas mixture as well as flow velocity. The technique could be of value in both industrial and medical applications.Godkänd; 2003; Bibliografisk uppgift: Undertitel på proceedings: International Conference on Flow Measurement Sider: 036-; 20060913 (ysko

Speed of sound measurements in humid air using an ultrasonic flow meter

This paper presents results from experiments using an ultrasonic gas flow meter measuring the speed of sound in air at varying air velocities, humidities and temperatures. The meter utilises the sing-around technique. The transducers in the meter are silicon-based ultrasonic transducers with a centre frequency of 800 kHz. In order to investigate the performance of the flow meter it was tested in a novel gas flow facility connected to a calibration facility for flow meters used for liquids. The Reynolds’ numbers for the investigated flow velocities ranged from 0 to 3,2·104, the relative humidity varied from 40% to 80% RH and the temperature varied from 20°C to 46°C. It was found that the experimentally measured speed of sound corresponded well with the speed of sound obtained from theory. It is also concluded that the flow meter could potentially be used in determining the relative humidity in flowing air at atmospheric pressures using speed of sound and temperature measurements.Godkänd; 2003; 20121227 (jerker

On a new sensing strategy using a combination of ultrasonic and photoacoustic techniques

The process industry is today aiming for more advanced process control strategies. These strategies rely on quick and accurate sensing of process variables. Considering processes where particles are suspended in a fluid, e.g. paper and pulp industry, there is a need for development of a new or improved in-line sensor technique determining different properties of the particles. In this study we present a sensing strategy where a combination of acoustical and optical measurement techniques is used. For this purpose have we designed and built a measurement cell which utilises the ultrasonic signal generated from an ultrasonic transducer as well as the ultrasonic, or photoacoustic, signal generated using a pulsed laser. The photoacoustic method is using the same transducer as in the transducer based method above. In the transducer based method, the well known pulse-echo technique is used. The pulse is generated by an ultrasonic transducer, travels through the suspension and reflects at a steel reflector. In the photoacoustic method, the ultrasonic transducer receives the ultrasound that is generated when a pulse of laser light is scattered and absorbed in the suspension. The laser beam enters the cell through fused silica windows. The laser beam is crossing the cell orthogonally to the ultrasonic transducer. The photoacoustic signal contains two echoes. The first echo is the acoustic signal which is generated when photons are absorbed by the medium. The second echo is the signal that has been travelling through the suspension and reflected at the steel reflector. The two measurement techniques were tested experimentally in the cell using aqueous suspensions of Nylon 66 fibres. The samples were mixed from fibres of three different diameters 17, 51 and 55 µ m with the length of 1.2mm for the 17µm and 1.5 mm for the 51 and 55µm. The fibres were suspended in distilled and degassed water to mass fractions of 0.12% and 0.25%. Distilled and degassed water was also used as a reference medium when determining the calibration constants for the cell. The photoacoustic signal is generated using a double-pulsed laser having pulse energy of 10 mJ and the laser beam is focused at the centre of the cell. The suspension was pumped around in a closed loop using a peristaltic pump and the temperature of the suspension was monitored during the measurement cycle. The result shows that the signal generated using the transducer based method scales linearly with mass fraction for each type of fibre. The results also show that the photoacoustic signal is sensitive to the number density of scatters in the suspension, i.e. the amplitude of the signal decreases with decreasing fibre diameters. These preliminary results indicate that a combination of these two techniques might be used in resolving the proportion of different particles with different diameter in a suspension.Validerad; 2006; 20061229 (ysko

Acoustic design principles for energy efficient excitation of a high intensity cavitation zone

Energy-efficient process intensification is a key aspect for a sustainable industrial production. To improve energy conversion efficiency high intensity cavitation is a promising method, especially in cases where the material to be treated is valuable and on the micro meter scale. Transient collapsing cavitation bubbles gives powerful effects on objects immersed in fluids, like cellulose fibers, mineral particles, enzymes, etc. The cavitation process needs optimization and control, since optimal conditions is multivariate challenge. This study focuses on different design principles to achieve high intensity cavitation in a specific volume in a continuous flow. This study explores some potential design principles to obtain energy efficient process intensification. The objective is to tune several different resonance phenomena to create a powerful excitation of a flowing suspension (two-phase flow and cavitation bubbles). The reactor is excited by sonotrodes, connected to two coupled resonant tube structures, at the critical frequency. Finally cavitation bubbles are initiated by a flow through a venturi nozzle. The acoustically optimised reactor geometry is modelled in Comsol Multiphysics®, and excited by dedicated ultrasound signals at three different frequencies. The effect of the high intensity cavitation is experimentally evaluated by calorimetric method, foil tests and degree of fibrillation on cellulose fibers.ISBN för värdpublikation: 978-3-939296-15-7</p

Energy Efficient Fibrillation of Cellulose Fibers using an Ultrasound Reactor

The pulp and paper industry is in continuous need for energy-efficient production processes. Therefore, there is a focus in reducing electrical energy use in the production of paper.  The most energy demanding processes are related to fibrillation, which in some cases use up to 80% of required electrical power, with a net efficiency of 1%. The presented work focus on ultrasound controlled cavitation in concentrating the processing energy to provide an energy efficient development of cellulose fibers. The objectives are to develop a scalable cavitation reactor to obtain energy-efficient fibrillation of cellulose fibers aiming at reducing the energy use by 50%. Our goal is to develop a methodology based on multiphysic simulation for the design of an alternative refiner based on ultrasound cavitation. The reactor concept is of a flow through type where cavitation bubbles are initiated in the fiber suspension by the pressure release when the pulp flow through a venturi nozzle. The induced cavitation bubbles are collapsed by high intensity ultrasound at resonant frequencies. The collapsing bubbles and their associated shock waves modify the fiber wall properties which enables fibrillation.  Energy efficient fibrillation of cellulose fibers is therefore possible to achieve through an optimized combination of hydrodynamic and ultrasonic controlled cavitation. Initial results shows a positive effect on fiber quality. However, further optimization of process parameters like temperature and static pressure is required