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It is critical to have an efficient energy budget in all the industrial process applications involving multiphase flow system where a significant amount of power is invested to achieve a desired outcome such as valuable particle collection and recovery in mineral flotation circuits. In order to achieve this aim there needs to be an ability to properly characterise the energy dissipation in the system; and from this knowledge to develop methodologies so that the supplied energy is distributed suitably among the eddies of different sizes which are responsible for enhancing different transport events such heat/mass transfer, mixing etc. The aim of the study was to obtain the 2D instantaneous velocity field in a homogeneous near isotropic turbulence field using particle image velocimetry (PIV) and then compute the space and time averaged specific energy dissipation rate from velocity field using four different methods, namely: (1) dimensional analysis, (2) velocity gradient, (3) structure function, and (4) energy spectrum. The system was studied in the Taylor Reynolds number range of 24–60, where it was found that the difference between the computed specific energy dissipation rates could be as much as 100 percent. Whilst it was found that there were uncertainties in all four methodologies, it is argued that the energy spectrum method is likely to give the most realistic quantification of the specific energy dissipation rate value since it was shown to satisfy the system energy balance which was not possible to do so for the other three methods. The energy spectrum method also had the added benefit of incorporating integral scale, Taylor microscale and Kolmogorov length scales in the quantification of the specific energy dissipation rate; whereas the other three methods are limited to either integral scale or Taylor microscale only. The limitation of the energy spectrum method, however, is the resolution of the energy spectrum down to the Kolmogorov length scale due to the noise in the measurement; and to resolve this problem a filter was applied to denoise in the dissipation range. [ABSTRACT FROM AUTHOR]

Transport phenomena occur frequently in industrial problems. Most of the turbulent transport properties can be directly associated with the turbulent energy dissipation rate; hence it is a very significant parameter in the design of chemical processing equipment. To develop a better chemical processing equipment design, a thorough knowledge of the effect flow structure on local turbulence parameters like turbulent kinetic energy, eddy diffusivity and the energy dissipation rate are required. Turbulence is heterogeneous in most of the process equipment. Hence, the use of spatial average energy dissipation rate causes error in modelling of turbulent transport processes. In this present work, particle image velocimetry (PIV) is used to obtain the energy spectrum from grid generated homogeneous turbulence velocity data. The model of energy spectrum given by Kang et al. (2003) has been fitted to this energy spectrum using energy dissipation rate. A different approach, based on a third order structure function and velocity gradient technique has been used to compute the energy dissipation rate. The model predictions have been verified by experimental PIV velocity data from oscillating grid apparatus. [ABSTRACT FROM AUTHOR]

Abstract: Particle image velocimetry (PIV) is a promising technique to measure dispersed phase size, dispersed phase hold-up and velocity of both the phases. The current work reports measurement of the shape, size, velocity and acceleration of bubbles using shadowgraphy, and liquid velocity measurement obtained using PIV/LIF with fluorescent tracer particles. Measurements were performed in a narrow rectangular column at moderate gas hold-up (∼5%) with wide variation of bubble sizes (0.1–15mm). The liquid velocity field was subjected to 2D discrete wavelet transform (DWT) to visualize the flow structures in the bubbly flow. Further, the slip velocity of individual bubbles was obtained from the DWT filtered liquid velocity field. The results are compared with the slip velocity correlations reported in literature for single bubbles rising in quiescent water. The comparison shows the difference in slip velocity of single bubbles and bubbles rising in swarm. The scale wise decomposition obtained from DWT was also used to quantify the liquid velocity field in terms of wavenumber spectrum. The velocity and acceleration measurements are demonstrated on a single spherical cap bubble rising in quiescent water. The measurements show the potential of the 2D acceleration measurement to facilitate the estimation of unsteady drag on bubbles. [Copyright &y& Elsevier]

Turbulence modulation of a nearly isotropic flow field due to the presence of single glass particles, with diameters in the range of 1−8 mm (~10–77 times the Kolmogorov scale), was studied experimentally in an oscillating grid apparatus. Particle image velocimetry (PIV) was used to obtain the instantaneous, two-dimensional velocity field for grid Reynolds number, Re g , varying from 1080 to 10,800. Fluctuating velocity components, flow field length scales, energy dissipation rates, turbulence intensity modulation and energy spectra were determined. An apparent increase of ~2–25% in the turbulence fluctuating velocity in the inertial subrange was noted compared with the fluid-only system. Presence of the particle led to enhancement in the flow field isotropy ratio and this ratio was found to be more dependent on the particle size compared with grid Reynolds number. The integral length scale for both fluid-only and fluid–particle systems exhibited a decreasing power law dependency on the grid Reynolds number. A critical ratio (0.41) of particle size to integral length scale was obtained which demarcated the regime of attenuation and enhancement of turbulence intensity and found to be valid for general grid turbulence in multiparticle systems as well. Energy dissipation rate was observed to increase with increase in particle size. Both longitudinal and transverse energy spectrum exhibited less steep slope than −5/3 in the presence of particle which was reasoned to the production of turbulence in the inertial subrange region. Energy enhancements from large scale to smaller scale were explained by the dissipative spectrum which showed maximum energy enhancement in the inertial subrange followed by a decaying trend. In general, the role of a particle in modulating turbulence was explained through possible wake oscillation and vortex shedding due to boundary layer separation on the particle surface and interaction with bulk eddies. [ABSTRACT FROM AUTHOR]