Numerical and ExperimentalModelling of Gas Flow and Heat Transfer in the Air Gap of An Electric Machine

This work deals with the cooling of high-speed electric machines, such as motors and generators, through an air gap. It consists of numerical and experimental modelling of gas flow and heat transfer in an annular channel. Velocity and temperature profiles are modelled in the air gap of a high-speed testmachine. Local and mean heat transfer coefficients and total friction coefficients are attained for a smooth rotor-stator combination at a large velocity range. The aim is to solve the heat transfer numerically and experimentally. The FINFLO software, developed at Helsinki University of Technology, has been used in the flow solution, and the commercial IGG and Field view programs for the grid generation and post processing. The annular channel is discretized as a sector mesh. Calculation is performed with constant mass flow rate on six rotational speeds. The effect of turbulence is calculated using three turbulence models. The friction coefficient and velocity factor are attained via total friction power. The first part of experimental section consists of finding the proper sensors and calibrating them in a straight pipe. After preliminary tests, a RdF-sensor is glued on the walls of stator and rotor surfaces. Telemetry is needed to be able to measure the heat transfer coefficients at the rotor. The mean heat transfer coefficients are measured in a test machine on four cooling air mass flow rates at a wide Couette Reynolds number range. The calculated values concerning the friction and heat transfer coefficients are compared with measured and semi-empirical data. Heat is transferred from the hotter stator and rotor surfaces to the coolerair flow in the air gap, not from the rotor to the stator via the air gap, althought the stator temperature is lower than the rotor temperature. The calculatedfriction coefficients fits well with the semi-empirical equations and precedingmeasurements. On constant mass flow rate the rotor heat transfer coefficient attains a saturation point at a higher rotational speed, while the heat transfer coefficient of the stator grows uniformly. The magnitudes of the heat transfer coefficients are almost constant with different turbulence models. The calibrationof sensors in a straight pipe is only an advisory step in the selection process. Telemetry is tested in the pipe conditions and compared to the same measurements with a plain sensor. The magnitudes of the measured data and the data from the semi-empirical equation are higher for the heat transfer coefficients than thenumerical data considered on the velocity range. Friction and heat transfer coefficients are presented in a large velocity range in the report. The goals are reached acceptably using numerical and experimental research. The next challenge is to achieve results for grooved stator-rotor combinations. The work contains also results for an air gap with a grooved stator with 36 slots. The velocity field by the numerical method does not match in every respect the estimated flow mode. The absence of secondary Taylor vortices is evident when using time averagednumerical simulation.

Kuristettujen isotermisen vesivirtausten kolmidimensionaalinen mallintaminen

Työssä on tutkittu CFX ja Fluent virtauslaskentaohjelmien soveltavuutta kuristet-tujen isotermisten vesivirtausten kolmidimensionaaliseen mallintamiseen. Teoriaosassa on esitelty virtausta hallitsevat perusyhtälöt sekä eri kavitaatioteori-oita kavitaatiokuplan syntymisestä tuhoutumiseen. Laskennallisessa osassa esitellään käytetyt virtauslaskentaohjelmat ja laskentatapaukset sekä verrataan saatuja tuloksia aiemmin suoritettuihin mittauksiin. Työn pääpaino oli tutkia käytettyjen virtauslaskentaohjelmien soveltuvuutta kuris-tettujen virtauksien mallinnukseen.

Virtauslaskentaa ja energia-analyysiä hyödyntävä huoneilman lämpöolosuhteiden simulointimalli

Huonetilojen lämpöolosuhteiden hallinta on tärkeä osa talotekniikan suunnittelua. Tavallisesti huonetilan lämpöolosuhteita mallinnetaan menetelmillä, joissa lämpödynamiikkaa lasketaan huoneilmassa yhdessä laskentapisteessä ja rakenteissa seinäkohtaisesti. Tarkastelun kohteena on yleensä vain huoneilman lämpötila. Tämän diplomityön tavoitteena oli kehittää huoneilman lämpöolosuhteiden simulointimalli, jossa rakenteiden lämpödynamiikka lasketaan epästationaarisesti energia-analyysilaskennalla ja huoneilman virtauskenttä mallinnetaan valittuna ajanhetkenä stationaarisesti virtauslaskennalla. Tällöin virtauskentälle saadaan jakaumat suunnittelun kannalta olennaisista suureista, joita tyypillisesti ovat esimerkiksi ilman lämpötila ja nopeus. Simulointimallin laskentatuloksia verrattiin testihuonetiloissa tehtyihin mittauksiin. Tulokset osoittautuivat riittävän tarkoiksi talotekniikan suunnitteluun. Mallilla simuloitiin kaksi huonetilaa, joissa tarvittiin tavallista tarkempaa mallinnusta. Vertailulaskelmia tehtiin eri turbulenssimalleilla, diskretointitarkkuuksilla ja hilatiheyksillä. Simulointitulosten havainnollistamiseksi suunniteltiin asiakastuloste, jossa on esitetty suunnittelun kannalta olennaiset asiat. Simulointimallilla saatiin lisätietoa varsinkin lämpötilakerrostumista, joita tyypillisesti on arvioitu kokemukseen perustuen. Simulointimallin kehityksen taustana käsiteltiin rakennusten sisäilmastoa, lämpöolosuhteita ja laskentamenetelmiä sekä mallinnukseen soveltuvia kaupallisia ohjelmia. Simulointimallilla saadaan entistä tarkempaa ja yksityiskohtaisempaa tietoa lämpöolosuhteiden hallinnan suunnitteluun. Mallin käytön ongelmia ovat vielä virtauslaskennan suuri laskenta-aika, turbulenssin mallinnus, tuloilmalaitteiden reunaehtojen tarkka määritys ja laskennan konvergointi. Kehitetty simulointimalli tarjoaa hyvän perustan virtauslaskenta- ja energia-analyysiohjelmien kehittämiseksi ja yhdistämiseksi käyttäjäystävälliseksi talotekniikan suunnittelutyökaluksi.

Radiaalikompressorin spiraalivirtauksen numeerinen mallinnus

Diplomityössä mallinnetaan numeerisesti radiaalikompressorin spiraalin virtaus. Spiraalin tarkoituksena radiaalikompressorissa on kerätä tasaisesti virtaus diffuusorin kehältä. Spiraaliin on viime aikoina kiinnitetty enemmän huomiota, koska on havaittu, että kompressorin hyötysuhdetta voidaan parantaa spiraalia optimoimalla. Spiraalin toimintaa tarkastellaan kolmella eri massavirralla. Työn alussa käsitellään spiraalin toimintaperiaatteita. Numeerisena ratkaisijana käytetään Teknillisessä korkeakoulussa kehitettyä FIN-FLO -koodia. FINFLO -laskentaohjelmassa ratkaistaan Navier-Stokes yhtälöt kolmeulotteiselle laskenta-alueelle. Diskretointi perustuu kontrollitilavuus menetelmään. Työssä käsitellään laskentakoodin toimintaperiaatteitta. Turbulenssia mallinnetaan algebrallisella Baldwin-Lomaxin ja kahden yhtälön Chienin k-e turbulenssimalleilla. Laskentatuloksia verrataan Lappeenrannan teknillisessä korkeakoulussa tehtyihin mittauksiin kyseessä olevasta kompressorin spiraalista. Myös eri turbulenssimalleilla ja hilatasoilla saatuja tuloksia verrataan keskenään. Laskentatuloksien jälkikäsittelyä varten ohjelmoitiin neljä eri tietokoneohjelmaa. Laskennalla pyritään saamaan lisäselvyyttä virtauksen käyttäytymiseen spiraalissa ja erityisesti ns. kielen alueella. Myös kahden eri turbulenssimallin toimivuutta kompressorin numeerisessa mallinnuksessa tutkitaan.

Turbulenssimallien soveltuminen syklonien numeeriseen virtauslaskentaan

Tässä työssä tutkittiin miten totuudenmukaisia tuloksia syklonierottimen virtauskentästä saadaan numeerisella laskennalla, kun käytetään eri turbulenssimalleja. Tarkoitus oli myös selvittää yleisesti syklonin toimintaperiaatteita, haasteita sen käytössä sekä syklonin numeerisen virtauslaskennan perusteita. Numeerisen virtauslaskennan teoria selitetään pääpiirteittäin, samoin turbulenssin mallinnus. Työn laskentaosiossa simuloitiin Fluent-ohjelmalla syklonin virtauskenttää kuumalla ilmalla sekä kahdella eri turbulenssimallilla ja verrattiin tuloksia kirjallisuudesta löytyviin mittaustuloksiin. Simuloinnit suoritettiin sekä ajasta riippuvana että ajasta riippumattomana ja kahdella eri laskentahilalla. Simulointien tulokset osoittivat, että RNG k-ε turbulenssimalli ei kykene tuottamaan totuu-denmukaista virtauskenttää. Toisen käytetyn turbulenssimallin, Reynolds-jännitysmallin tulokset vastasivat enemmän mittaustuloksia. Reynolds-jännitysmallia voidaan pitää käyttökelpoisena syklonin simuloinnissa tämän työn ja kirjallisuuden perusteella. Mallissa oli yksinkertaistuksia, esimerkiksi kiinteää ainetta ei otettu huomioon lainkaan.

Application and development of numerical methods for the modelling of innovative gas cooled fission reactors

Innovative gas cooled reactors, such as the pebble bed reactor (PBR) and the gas cooled fast reactor (GFR) offer higher efficiency and new application areas for nuclear energy. Numerical methods were applied and developed to analyse the specific features of these reactor types with fully three dimensional calculation models. In the first part of this thesis, discrete element method (DEM) was used for a physically realistic modelling of the packing of fuel pebbles in PBR geometries and methods were developed for utilising the DEM results in subsequent reactor physics and thermal-hydraulics calculations. In the second part, the flow and heat transfer for a single gas cooled fuel rod of a GFR were investigated with computational fluid dynamics (CFD) methods. An in-house DEM implementation was validated and used for packing simulations, in which the effect of several parameters on the resulting average packing density was investigated. The restitution coefficient was found out to have the most significant effect. The results can be utilised in further work to obtain a pebble bed with a specific packing density. The packing structures of selected pebble beds were also analysed in detail and local variations in the packing density were observed, which should be taken into account especially in the reactor core thermal-hydraulic analyses. Two open source DEM codes were used to produce stochastic pebble bed configurations to add realism and improve the accuracy of criticality calculations performed with the Monte Carlo reactor physics code Serpent. Russian ASTRA criticality experiments were calculated. Pebble beds corresponding to the experimental specifications within measurement uncertainties were produced in DEM simulations and successfully exported into the subsequent reactor physics analysis. With the developed approach, two typical issues in Monte Carlo reactor physics calculations of pebble bed geometries were avoided. A novel method was developed and implemented as a MATLAB code to calculate porosities in the cells of a CFD calculation mesh constructed over a pebble bed obtained from DEM simulations. The code was further developed to distribute power and temperature data accurately between discrete based reactor physics and continuum based thermal-hydraulics models to enable coupled reactor core calculations. The developed method was also found useful for analysing sphere packings in general. CFD calculations were performed to investigate the pressure losses and heat transfer in three dimensional air cooled smooth and rib roughened rod geometries, housed inside a hexagonal flow channel representing a sub-channel of a single fuel rod of a GFR. The CFD geometry represented the test section of the L-STAR experimental facility at Karlsruhe Institute of Technology and the calculation results were compared to the corresponding experimental results. Knowledge was gained of the adequacy of various turbulence models and of the modelling requirements and issues related to the specific application. The obtained pressure loss results were in a relatively good agreement with the experimental data. Heat transfer in the smooth rod geometry was somewhat under predicted, which can partly be explained by unaccounted heat losses and uncertainties. In the rib roughened geometry heat transfer was severely under predicted by the used realisable k − epsilon turbulence model. An additional calculation with a v2 − f turbulence model showed significant improvement in the heat transfer results, which is most likely due to the better performance of the model in separated flow problems. Further investigations are suggested before using CFD to make conclusions of the heat transfer performance of rib roughened GFR fuel rod geometries. It is suggested that the viewpoints of numerical modelling are included in the planning of experiments to ease the challenging model construction and simulations and to avoid introducing additional sources of uncertainties. To facilitate the use of advanced calculation approaches, multi-physical aspects in experiments should also be considered and documented in a reasonable detail.

Dynamic stall for a Vertical Axis Wind Turbine in a two-dimensional study

The last few years have proved that Vertical Axis Wind Turbines (VAWTs) are more suitable for urban areas than Horizontal Axis Wind Turbines (HAWTs). To date, very little has been published in this area to assess good performance and lifetime of VAWTs either in open or urban areas. At low tip speed ratios (TSRs<5), VAWTs are subjected to a phenomenon called 'dynamic stall'. This can really affect the fatigue life of a VAWT if it is not well understood. The purpose of this paper is to investigate how CFD is able to simulate the dynamic stall for 2-D flow around VAWT blades. During the numerical simulations different turbulence models were used and compared with the data available on the subject. In this numerical analysis the Shear Stress Transport (SST) turbulence model seems to predict the dynamic stall better than the other turbulence models available. The limitations of the study are that the simulations are based on a 2-D case with constant wind and rotational speeds instead of considering a 3-D case with variable wind speeds. This approach was necessary for having a numerical analysis at low computational cost and time. Consequently, in the future it is strongly suggested to develop a more sophisticated model that is a more realistic simulation of a dynamic stall in a three-dimensional VAWT.

Kraichnan–Leith–Batchelor similarity theory and two-dimensional inverse cascades

We study the scaling properties and Kraichnan–Leith–Batchelor (KLB) theory of forced inverse cascades in generalized two-dimensional (2D) fluids (α-turbulence models) simulated at resolution 8192x8192. We consider α=1 (surface quasigeostrophic flow), α=2 (2D Euler flow) and α=3. The forcing scale is well resolved, a direct cascade is present and there is no large-scale dissipation. Coherent vortices spanning a range of sizes, most larger than the forcing scale, are present for both α=1 and α=2. The active scalar field for α=3 contains comparatively few and small vortices. The energy spectral slopes in the inverse cascade are steeper than the KLB prediction −(7−α)/3 in all three systems. Since we stop the simulations well before the cascades have reached the domain scale, vortex formation and spectral steepening are not due to condensation effects; nor are they caused by large-scale dissipation, which is absent. One- and two-point p.d.f.s, hyperflatness factors and structure functions indicate that the inverse cascades are intermittent and non-Gaussian over much of the inertial range for α=1 and α=2, while the α=3 inverse cascade is much closer to Gaussian and non-intermittent. For α=3 the steep spectrum is close to that associated with enstrophy equipartition. Continuous wavelet analysis shows approximate KLB scaling ℰ(k)∝k−2 (α=1) and ℰ(k)∝k−5/3 (α=2) in the interstitial regions between the coherent vortices. Our results demonstrate that coherent vortex formation (α=1 and α=2) and non-realizability (α=3) cause 2D inverse cascades to deviate from the KLB predictions, but that the flow between the vortices exhibits KLB scaling and non-intermittent statistics for α=1 and α=2.

Simulação computacional do escoamento em bombas de cavidades progressivas

The use of Progressing Cavity Pumps (PCPs) in artificial lift applications in low deep wells is becoming more common in the oil industry, mainly, due to its ability to pump heavy oils, produce oil with large concentrations of sand, besides present high efficiency when compared to other artificial lift methods. Although this system has been widely used as an oil lift method, few investigations about its hydrodynamic behavior are presented, either experimental or numeric. Therefore, in order to increase the knowledge about the BCP operational behavior, this work presents a novel computational model for the 3-D transient flow in progressing cavity pumps, which includes the relative motion between rotor and stator, using an element based finite volume method. The model developed is able to accurately predict the volumetric efficiency and viscous looses as well as to provide detailed information of pressure and velocity fields inside the pump. In order to predict PCP performance for low viscosity fluids, advanced turbulence models were used to treat, accurately, the turbulent effects on the flow, which allowed for obtaining results consistent with experimental values encountered in literature. In addition to the 3D computational model, a simplified model was developed, based on mass balance within cavities and on simplification on the momentum equations for fully developed flow along the seal region between cavities. This simplified model, based on previous approaches encountered in literature, has the ability to predict flow rate for a given differential pressure, presenting exactness and low CPU requirements, becoming an engineering tool for quick calculations and providing adequate results, almost real-time time. The results presented in this work consider a rigid stator PCP and the models developed were validated against experimental results from open literature. The results for the 3-D model showed to be sensitive to the mesh size, such that a numerical mesh refinement study is also presented. Regarding to the simplified model, some improvements were introduced in the calculation of the friction factor, allowing the application fo the model for low viscosity fluids, which was unsuccessful in models using similar approaches, presented in previous works

Numerical simulation of a radial diffuser turbulent airflow

In the present work are presented results from numerical simulations performed with the ANSYS-CFX (R) code. We have studied a radial diffuser flow case, which is the main academic problem used to study the flow behavior on flat plate valves. The radial flow inside the diffuser has important behavior such as the turbulence decay downstream and recirculation regions inside the valve flow channel due to boundary layer detachment. These flow structures are present in compressor reed valve configurations, influencing to a greater extent the compressor efficiency. The main target of the present paper was finding the simulation set-up (computational domain, boundary conditions and turbulence model) that better fits with experimental data published by Tabatabai and Pollard. The local flow turbulence and velocity profiles were investigated using four different turbulence models, two different boundary conditions set-up, two different computational domains and three different flow conditions (Re-in - Reynolds number at the diffuser inlet). We used the Reynolds stress (BSL); the k-epsilon; the RNG k-epsilon; and the shear stress transport (SST) k-omega turbulence models. The performed analysis and comparison of the computational results with experimental data show that the choice of the turbulence model, as well as the choice of the other computational conditions, plays an important role in the results physical quality and accuracy. (c) 2007 Elsevier B.V. All rights reserved.

Development of a low-Reynolds-number k-ω model for FENE-P fluids

A low-Reynolds-number k-ω model for Newtonian fluids has been developed to predict drag reduction of viscoelastic fluids described by the FENE-P model. The model is an extension to viscoelastic fluids of the model for Newtonian fluids developed by Bredberg et al. (Int J Heat Fluid Flow 23:731-743, 2002). The performance of the model was assessed using results from direct numerical simulations for fully developed turbulent channel flow of FENE-P fluids. It should only be used for drag reductions of up to 50 % (low and intermediate drag reductions), because of the limiting assumption of turbulence isotropy leading to an under-prediction of k, but compares favourably with results from k-ε models in the literature based on turbulence isotropy. © 2012 Springer Science+Business Media Dordrecht.

Análise experimental e numérica de convecção forçada em arranjo de obstáculos dentro de canal

Pós-graduação em Engenharia Mecânica - FEIS

Um modelo de turbulência baseado no conceito de vórtice

O fenômeno da turbulência está presente na maioria dos escoamentos observados na indústria e na natureza. Muitas são as considerações a respeito das dificuldades relacionadas à caracterização dos escoamentos turbulentos. Uma das muitas questões trata do procedimento de análise do problema através da descrição estatística dos campos por grandezas “médias”, o que leva ao problema de fechamento e à modelagem do tensor de Reynolds, normalmente com modelos baseados no conceito de viscosidade turbulenta. Os modelos de turbulência já existentes apresentam algumas deficiências na previsão do escoamento, além de outras limitações, o que justifica a busca por novas abordagens para o tratamento da turbulência. Neste trabalho, o problema de fechamento é tratado segundo a modelagem turbulenta baseada no conceito de viscosidade turbulenta. Um novo modelo de turbulência é proposto, que admite a existência de vórtices imersos no escoamento e aplica conceitos e definições relacionados à identificação de vórtices, com o uso do critério de identificação Q , que caracteriza a região do escoamento ocupada pelo vórtice. Propõe-se a investigação da aplicabilidade do critério Q em conjunto com o modelo k − ε , para o desenvolvimento de um novo modelo de turbulência chamado k − ε −Q . Validou-se a aplicabilidade do modelo através de um código numérico computacional para tratamento de escoamentos turbulentos. A solução numérica foi obtida através da discretização do domínio fluido, utilizando o método de volumes finitos e o método multigrid foi utilizado para resolver o sistema linear resultante. Como verificação, foi utilizado este modelo de turbulência para simular o escoamento em uma cavidade quadrada com tampa deslizante e o escoamento turbulento sobre um degrau. Os resultados obtidos foram confrontados com dados experimentais e demonstraram que o modelo aqui proposto se apresenta mais eficiente que o clássico modelo k − ε , no tratamento da turbulência nesses dois problemas clássicos.

Análise do escoamento turbulento por um queimador industrial a gás utilizando dinâmica dos fluidos computacional

Pós-graduação em Engenharia Mecânica - FEG

Numerical modeling of flow through an industrial burner orifice

This paper presents numerical modeling of a turbulent natural gas flow through a non-premixed industrial burner of a slab reheating furnace. The furnace is equipped with diffusion side swirl burners capable of utilizing natural gas or coke oven gas alternatively through the same nozzles. The study is focused on one of the burners of the preheating zone. Computational Fluid Dynamics simulation has been used to predict the burner orifice turbulent flow. Flow rate and pressure at burner upstream were validated by experimental measurements. The outcomes of the numerical modeling are analyzed for the different turbulence models in terms of pressure drop, velocity profiles, and orifice discharge coefficient. The standard, RNG, and Realizable k-epsilon models and Reynolds Stress Model (RSM) have been used. The main purpose of the numerical investigation is to determine the turbulence model that more consistently reproduces the experimental results of the flow through an industrial non-premixed burner orifice. The comparisons between simulations indicate that all the models tested satisfactorily and represent the experimental conditions. However, the Realizable k-epsilon model seems to be the most appropriate turbulence model, since it provides results that are quite similar to the RSM and RNG k-epsilon models, requiring only slightly more computational power than the standard k-epsilon model. (C) 2014 Elsevier Ltd. All rights reserved.