Numerical Modelling of Grate Combustion

Předkládaná práce je zaměřena na numerické modelování spalování tuhých paliv na roštu metodami výpočtové dynamiky tekutin (CFD). Jelikož výsledky CFD simulací roštového spalování závisí na kvalitě vstupních dat, která zahrnují i údaje o teplotě, hmotnostním toku a chemickém složení spalin vystupujících z lože, pozornost je věnována především procesům, probíhajícím v loži během spalování na roštu. Velká část práce je věnována vývoji spolehlivého modelu spalování v sypaných ložích, jelikož může napomoci zkvalitnit výsledky simulací i rozšířit znalosti principů spalování tuhých paliv v sypaných ložích. V rámci práce byl vyvinut jednorozměrný nestacionární model spalování v experimentálním reaktoru a implementován do počítačového programu GRATECAL 1.3 včetně grafického uživatelského rozhraní. Zvláštní důraz byl kladen na konzervativnost modelu. Proto byla vyvinuta metoda pro kontrolu hmotnostní a energetické bilance systému a následně aplikována v řadě studií, v rámci nichž byly odhaleny některé chyby týkající se definic zdrojových členů, které byly převzaty z literatury a opraveny. Pomocí modelu byla provedena analýza šíření čela sušení a reakce hoření koksu po výšce lože pšeničné slámy. Na základě výsledků těchto analýz bylo doporučeno zahrnout i modelování změny porozity částic paliva, aby šířka reakční zóny byla predikována korektně v případě, že je uvažována změna porozity celého lože. Rovněž vyvinutá bilanční metoda byla použita k analýze vlivu kritérií konvergence na hmotnostní a energetickou nerovnováhu simulovaného systému. Bylo zjištěno, že škálovaná rezidua rovnic všech veličin by měla poklesnout aspoň na hodnotu $10^{-6}$, aby bylo dosaženo nízké hmotnostní a energetické nerovnováhy a tudíž uspokojivě přesných výsledků ze simulací v loži. Druhá část práce je věnována vývoji a implementaci knihovny uživatelem definovaných funkcí pro komerční CFD nástroj ANSYS FLUENT, které slouží k propojení modelu lože s modelem komory reálné spalovací jednotky, aby byla umožněna dynamická změna okrajových podmínek na vstupu do komory v závislosti na výstupech ze simulací v loži. Vytvořené rozhraní pro propojení těchto dvou modelů je dostatečně obecné pro aplikaci na širokou škálu modelů roštových kotlů. Popsané výsledky přispívají k lepšímu porozumění numerickému modelování spalování na roštu, a to zejména ve fázi sestavování numerického modelu a nastavení parametrů řešiče pro kontrolu konvergence.The present work is focused on numerical modelling of grate combustion of solid fuels by means of computational fluid dynamics (CFD) methods. Since CFD results from simulations of grate combustion depend on the quality of input data including information on temperature, mass flux and chemical composition of flue gas leaving a fuel bed, the attention is turned to modelling of processes, that take place within the fuel bed on a grate. A great part of the work is devoted to development of a reliable numerical model of packed-bed combustion as it may help improve both results from simulations and knowledge of principles of solid fuel combustion in fixed or moving beds. A one-dimensional transient numerical model of combustion in an experimental reactor is developed and implemented into a computer program called GRATECAL 1.3 with a grapical user interface. A special emphasis is put on the conservativeness property of the model. Therefore, a method for control of mass and energy balance over the system is developed and applied to a series of case studies, which have revealed certain errors in definitions of mass source terms, so that data adopted from literature have been reconciled. The model is used for analysis of propagation of drying and char combustion reaction fronts in a bed of wheat straw particles. It is suggested to include modelling of particle internal porosity change in order to obtain correct reaction zone thickness, if porosity of the bed is allowed to change during combustion. The balance-based method is also used to analyse effects of convergence criteria on mass and energy imbalance of the modelled system. It is found that all the scaled residuals must drop to as low as $10^{-6}$ or lower in order to obtain sufficiently accurate results from in-bed simulations in terms of mass and energy conservation within the packed bed. The second part of the work is devoted to development of a library of user-defined functions for the commercial CFD software ANSYS FLUENT for coupling the bed model with a freeboard model of a real combustion unit in order to specify the boundary conditions indirectly using results from in-bed simulations. The created interface is general enough to be used for a wide range of models of grate furnaces. The presented results contribute to better understanding of numerical modelling of grate combustion, especially in the setup of a numerical model and parameters of solver for the control of the convergence.

Numerical modelling and simulation in sheet metal forming

The application of numerical modelling and simulation in manufacturing technologies is looking back over about a 20–30 years history. In recent years, the role of modelling and simulation in engineering and in manufacturing industry has been continuously increasing. It is well known that during manufacturing processes simultaneous the effect of many different parameters can be observed. This is the reason why in former years, detailed analysis of manufacturing processes could have been done only by time-consuming and expensive trial-and-error methods. Due to the recent developments in the methods of modelling and simulation, as well as in computational facilities, modelling and simulation has become an everyday tool in engineering practice. Besides the aforementioned facts, the emerging role of modelling and simulation can also be explained by the growing globalisation and competition of the world market requiring shorter lead times and more cost effective solutions. In spite the enormous development of hardware and software facilities, the exclusive use of numerical modelling still seems to be very time- and cost consuming, and there is still often a high scepticism about the results among industrialists. Therefore, the purpose of this paper is to overview the present situation of numerical modelling and simulation in sheet metal forming, mainly from the viewpoint of scientific research and industrial applications

Modelling Primordial Gas in Numerical Cosmology

We have reviewed the chemistry and cooling behaviour of low-density (n<10^4 cm^-3) primordial gas and devised a cooling model wich involves 19 collisional and 9 radiative processes and is applicable for temperatures in the range (1 K < T < 10^8 K). We derived new fits of rate coefficients for the photo-attachment of neutral hydrogen, the formation of molecular hydrogen via H-, charge exchange between H2 and H+, electron detachment of H- by neutral hydrogen, dissociative recombination of H2 with slow electrons, photodissociation of H2+, and photodissociation of H2. Further it was found that the molecular hydrogen produced through the gas-phase processes, H2+ + H -> H2 + H+, and H- + H -> H2 + e-, is likely to be converted into its para configuration on a faster time scale than the formation time scale. We have tested the model extensively and shown it to agree well with former studies. We further studied the chemical kinetics in great detail and devised a minimal model which is substantially simpler than the full reaction network but predicts correct abundances. This minimal model shows convincingly that 12 collisional processes are sufficient to model the H, He, H+, H-, He+, He++, and H2 abundances in low density primordial gas for applications with no radiation fields.Comment: 26 pages of text, 4 tables, and 6 eps figures. The paper is also available at http://zeus.ncsa.uiuc.edu:8080/~abel/PGas/bib.html Submitted to New Astronomy. Note that some of the hyperlinks given in the paper are still under constructio

Numerical modelling of in-plane behaviour of adobe walls

Some tests for material characterization of adobe blocks and adobe masonry have been carried out in universities and laboratories around the world. However, the number of tests is quite limited in comparison with those carried out with other structural materials, such as masonry or reinforced concrete, and even those tests just refers to elastic properties. The results of adobe tests (i.e. compression strength, elasticity modulus, shear strength, etc.), as well as the results of cyclic and dynamic tests on adobe masonry components and small buildings show that the mechanical properties of adobe masonry and the seismic performance of adobe constructions highly depend on the type of soil used for the production of units and mortar. Basic properties, such as elasticity modulus, can have significant variation from one soil type to another. The state-of-the-art for the numerical modelling of unreinforced masonry point to three main approaches: macro-modelling, simplified micro-modelling and detailed micro-modelling. In all three approaches, the use of elastic and inelastic parameters is required. For adobe masonry, the lack of knowledge concerning some of the material properties makes numerical modelling more difficult. In the proposed work, the mechanical properties of the typical adobe masonry in Peru have been calibrated based on a cyclic in-plane test carried out on an adobe wall at the Catholic University of Peru (PUCP). The mechanical parameters calibration and the modelling results of the in-plane behaviour of the adobe wall are presented. Macro-modelling and simplified micro-modelling strategies are used in finite element software with an implicit solution strategy. The results of this work represent the first step for the numerical modelling of the seismic behaviour of adobe constructions

Numerical modelling of rubber vibration isolators

An important cause for interior noise in vehicles is structure-borne sound from the engine. The vibrations of the source (engine) are transmitted to the receiver structure (the vehicle) causing interior noise in the vehicle. For this reason the engine is supported by rubber isolators for passive isolation in especially the high-frequency region. To make a good judgment of the characteristics of a vibration isolator in the design process, it is useful to use numerical models. In this paper a cylindrical vibration isolator is modelled numerically with the Finite Element package ABAQUS. The investigation is split in two parts: first a nonlinear analysis is performed for different pre-deformations of the mount. After that, a linear harmonic analysis is superimposed on the pre-deformed isolator. The structure-borne sound is transmitted by the isolator by six degrees of freedom, so the harmonic analysis must be performed for different excitations. With the results the behavior of the isolator can be represented by dynamic stiffness matrices as function of the frequency and predeformation. These matrices can be used to model the passive isolation components as part of numerical models of hybrid isolation systems. These isolation systems describe a combination of active and passive isolation to reduce the structure-borne sound transmission to receiver structures

Proposition of numerical modelling of BEC

We propose extension of the numerical method to model effect of Bose-Einstein correlations (BEC) observed in hadronization processes which allows for calculations not only correlation functions $C_2(Q_{inv})$ (one-dimensional) but also corresponding to them $C_2(Q_{x,y,z})$ (i.e., three-dimensional). The method is based on the bunching of identical bosonic particles in elementary emitting cells (EEC) in phase space in manner leading to proper Bose-Einstein form of distribution of energy (this was enough to calculate $C_2(Q_{inv})$). To obtain also $C_2(Q_{x,y,z})$ one has to add to it also symmetrization of the multiparticle wave function to properly correlate space-time locations of produced particles with their energy-momentum characteristics.Comment: 7 pages, 2 figures, poster presented at QM2005, to be published in Nukleonika (2006); acknowledgements adde

Numerical modelling of Bose-Einstein correlations

We propose extension of the algorithm for numerical modelling of Bose-Einstein correlations (BEC), which was presented some time ago in the literature. It is formulated on quantum statistical level for a single event and uses the fact that identical particles subjected to Bose statistics do bunch themselves, in a maximal possible way, in the same cells in phase-space. The bunching effect is in our case obtained in novel way allowing for broad applications and fast numerical calculations. First comparison with $e^+e^-$ annihilations data performed by using simple cascade hadronization model is very encouraging.Comment: LaTeX file and 5 eps file with figures, 9 pages altogethe

Integrated Numerical Modelling System for Extreme Wave Events at the Wave Hub Site

This paper examines an extreme wave event which occurred during a storm at the Wave Hub site in 2012. The extreme wave of 9.57 m height was identified from a time series of the heave data collected by an Oceanor Seawatch Mini II Buoy deployed at the site. An energy density spectrum was derived from this time series and then used to drive a physical model, which represents the extreme wave at 1:20 scale in Plymouth University’s new COAST Lab. The NewWave technique was used to define the input to the physical model. The experiment is reproduced in a numerical wave tank using the fully nonlinear CFD library OpenFOAM® and the wave generation toolbox waves2Foam. Results are evaluated, and issues regarding the predictions of a numerical model that is driven by the NewWave input signal are discussed. This study sets the basis for further research in coupling field data, physical modelling and numerical modelling in a more efficient and balanced way. This will lead to the new approach of composite modelling that will be implemented in future work