Experimental and numerical study of wall layer development in a tribocharged fluidized bed

The effects of triboelectricity in a small-scale fluidized bed of polyethylene particles were investigated by imaging the particle layer in the vicinity of the column wall and by measuring the pressure drop across the bed. The average charge on the particles was altered by changing the relative humidity of the gas. A triboelectric charging model coupled with a computational fluid dynamics-discrete element method (CFD-DEM) model was utilized to simulate gas-particle flow in the bed. The electrostatic forces were evaluated based on a particle-particle particle-mesh method, accounting for the surface charge on the insulating walls. It was found that simulations with fixed and uniform charge distribution among the particles capture remarkably well both the agglomeration of the particles on the wall and the associated decrease in the pressure drop across the bed. With a dynamic tribocharging model, the charging rate had to be accelerated to render the computations affordable. Such simulations with an artificial acceleration significantly over-predict charge segregation and the wall becomes rapidly sheeted with a single layer of strongly charged particles.publishedVersionPeer reviewe

Unsteady laminar boundary layers of rainbow trout swimming in turbulent flows

The boundary layers of rainbow trout, Oncorhynchus mykiss [mean±s.d., 0.231±0.016 m total-body-length (L); N=6], swimming at 1.6±0.09 L s(-1) (N=6) in an experimental flow channel (the Reynolds number, Re=4×10(5)) with medium turbulence (5.6%-intensity) were examined using the particle image velocimetry technique. The tangential-flow-velocity distributions in the pectoral (arc-length from the rostrum, lx=71±8 mm, N=3) and pelvic surface regions (lx=110±13 mm, N=4) were approximated by a laminar-boundary-layer model, the Falkner-and-Skan equation. The flow regime over the pectoral and pelvic surfaces was regarded as a laminar flow, which could create less skin-friction drag than would be the case with turbulent flow. Flow separation was postponed until vortex shedding occurred over the posterior surface (lx=163±22 mm, N=3). The ratio of the body-wave velocity to the swimming speed was in the order of 1.2. This was consistent with the condition of the boundary-layer laminarisation that had been confirmed earlier using a mechanical model. These findings suggest an energy-efficient swimming strategy for rainbow trout in a turbulent environment.publishedVersionPeer reviewe

Examiners:

and Process Engineering. It has been accepted as a Master of Science thesis and it is also an extensive manual for measuring multiphase flows using PIV technique. This study is a part of the research project ProMoni funded by Tekes. The project will continue in future and further improvements of this measurement technique are expected. I am very grateful to my examiner, Prof. Reijo Karvinen for his interest during the study. Big thanks go to my supervisor and examiner, Pentti Saarenrinne, who has helped me a lot during this study. I would also like to thank Hannu Eloranta for guidance and encouragement in the use of PIV systems and applications. I want to thank you, my workmates: Hannu Mäkelä, Tero Pärssinen, Marjo Lahti and Heimo Ihalainen for your precious help and for being co-operative and creating a great work environment within thi