Three-dimensional model of bubbling fluidized bed combustion

 Diplomityön tarkoituksena on kehittää kolmiulotteinen malli kerrosleijupoltolle. Työn kirjallisuusosa sisältää seuraavat perusteet kerrosleijupolton tekniikasta: yleistiedot, leijutus- ja palamisilmiöt, kiinteän aineen ja kaasun sekoittuminen, päästöt ja lämmönsiirto. Lisäksi palamissysteemin mallinnuksen perusteet ja ratkaisumenetelmät ovat esitelty. Työn mallinnusosassa kehitetty koodi on ohjelmoitu Fortran-ohjelmointikielellä. Kehitetty malli perustuu olemassa olevaan malliin kiertoleijupoltosta. Yhtälö kiintoainekonsentraatioprofiilille on vaihdettu ja kiertovirta on poistettu koodista. Mallilla on tehty herkkyystarkasteluja polttoaineen ja kaasun sekoittumisen sekä reaktiokertoimen vaikutukselle. Visualisointi on tehty ohjelmassa Tecplot 360 ja mallinnustuloksia on vertailtu mitattuihin tuloksiin. Mallin laskemattulokset vastaavat hyvin mittaustuloksia ja kokemusperäisiä tietoja; monissa tapauksissa malli pystyy kvantitatiivisesti kuvaamaan parametrien variointia ja kaikissa tapauksissa malli antaa ainakin kvalitatiivisesti oikeita tuloksia. Työhön liittyvän kehityksen ja mallinnuskokemuksen perusteella on tehty ehdotukset mallin tulevaa kehitystä ja mittauksia varten.

Implementation of population balances in simulation of gas-liquid reactors using a CFD-PBM coupled approach

In bubbly flow simulations, bubble size distribution is an important factor in determination of hydrodynamics. Beside hydrodynamics, it is crucial in the prediction of interfacial area available for mass transfer and in the prediction of reaction rate in gas-liquid reactors such as bubble columns. Solution of population balance equations is a method which can help to model the size distribution by considering continuous bubble coalescence and breakage. Therefore, in Computational Fluid Dynamic simulations it is necessary to couple CFD and Population Balance Model (CFD-PBM) to get reliable distribution. In the current work a CFD-PBM coupled model is implemented as FORTRAN subroutines in ANSYS CFX 10 and it has been tested for bubbly flow. This model uses the idea of Multi Phase Multi Size Group approach which was previously presented by Sha et al. (2006) [18]. The current CFD-PBM coupled method considers inhomogeneous flow field for different bubble size groups in the Eulerian multi-dispersed phase systems. Considering different velocity field for bubbles can give the advantageof more accurate solution of hydrodynamics. It is also an improved method for prediction of bubble size distribution in multiphase flow compared to available commercial packages.

Computational studies of deformation in stainless steel rings due to torch heating

A support ring of AISI 304L stainless steel that holds vertical, parallel wires arranged in a circle forming a cylinder is studied. The wires are attached to the ring with heat-induced shrinkage. When the ring is heated with a torch the heat affected zone tries to expand while the adjacent cool structure obstructs the expansion causing upsetting. During cooling, the ring shrinks smaller than its original size clamping the wires. The most important requirement for the ring is that it should be as round as possible and the deformations should occur as overall shrinkage in the ring diameter. A three-dimensional nonlinear transient sequential thermo-structural Abaqus model is used together with a Fortran code that enters the heat flux to each affected element. The local and overall deformations in one ring inflicted by the heating are studied with a small amount of inspection on residual stresses. A variety of different cases are chosen to be studied with the model constructed to provide directional knowledge; torch flux with the means of speed, location of the wires, heating location and structural factors. The decrease of heating speed increases heat flux that rises the temperature increasing shrinkage. In a single progressive heating uneven distribution of shrinkage appears to the start/end region that can be partially fixed with using speeded heating’s to strengthen the heating of that region. Location of the wires affect greatly to the caused shrinkage unlike heating location. The ring structure affects also greatly to the shrinkage; smaller diameter, bigger ring height, thinner thickness and greater number of wires increase shrinkage.