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Numerical Modelling of Grate Combustion
Juřena, Tomáš ; Klemeš,, Jiří (referee) ; Žitný, Rudolf (referee) ; Hájek, Jiří (advisor)
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.
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Computational analysis of fluid flow in a tumble dryer
Dohnal, Miloslav ; Juřena, Tomáš (referee) ; Hájek, Jiří (advisor)
The aim of this work is to create a computational model of radial industrial tumble dryers, to calculate and identify the amount of air that flows through the inside of the drum itself. The calculation will be performed via computational fluid dynamics (CFD). Furthermore, compile transient balance model of mentioned dryers. Perform simulation balance model and compare the results of simulations with experimental measurements. By comparing the experimental data and simulation to determine the degrees of freedom of balance model and evaluate their impact on the assembled transient model. For a better understanding of the drying process, there is constructed a system of differential equations describing heat and moisture within the material being dried on a simple model. In the section devoted to the computational analysis of fluid flow is analyzed existing geometry of the drum, which has a major impact on the flow of air inside the drum itself. Following describes how to simplify its complex geometry entering the computational fluid dynamics. Then, there is carried out a simulation of fluid flow inside the tumble dryers using MRF and Sliding Mesh models. Finally, there is an analysis of the data obtained and determined the average amount of air flowing through the drum itself. On the contrary, the aim of the work is not to create another text tool for students engaged in CFD theme.
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Computations of fluid flow in stirred reactor with floating particles
Földváry, István ; Broukal, Jakub (referee) ; Hájek, Jiří (advisor)
The main goal of this master’s thesis is to investigate the drawn down of floating particles in mechanically stirred vessels. The first part of the thesis summarizes necessary theory to cope with defined objectives of the thesis. The literature review of mixing reactors with floating particles can be found in this part of the work as well. The second part of the master’s thesis includes the preparation of the stirred tanks CFD model, simulation and comparing of experimental data from literature, as well.
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Influence of Inert Gases on Characteristic Parameters of Combustion Process
Hudák, Igor ; Jegla, Zdeněk (referee) ; Odstrčil, Miloslav (referee) ; Hájek, Jiří (advisor)
The dissertation thesis is focused on the investigation of the influence of inert gases on characteristic parameters of the combustion process. Inert gases are usually standard components of alternative gaseous fuels such as the producer gas, coal gas or biogas. For a long period some of the fuels were considered as waste gases and their potential was not sufficiently utilised. Compared with noble fuels such as natural gas, alternative fuels have different physical-chemical properties. For example, their lower heating value (LHV) can be lower even than 10 MJ/mN3. The composition of the alternative gaseous fuels can be various during their production process. Lower LHV is a result of the occurrence of inert gases that does not take a part in the combustion reactions. Inert gases have the ability to accumulate the heat. Under certain conditions it is possible to combust alternative fuels on conventional burners, but due to their different properties, problems can occur during the combustion. E.g. to achieve the same heat output as with the noble fuel, it is necessary to burn bigger volume of the alternative fuel. However, the limiting factor could be the burner's head geometry. Theoretical introduction of the thesis summarizes research results in the field of low calorific gaseous fuels combustion and the addition of inert gases into the noble fuels. The thesis also describes the mechanisms of the NOx formation and summarizes techniques which can be used to reduce NOx formation. Furthermore, fuels from alternative sources are categorized and described, including their composition and characteristics. Their physical-chemical properties were obtained by means of the combustion simulation carried out in the simulation software. Two devices had to be designed and manufactured to fulfil goals of the dissertation thesis. The first is a gas mixing station capable of mixing 4 different components. The second device is a burner utilized for the combustion of low calorific fuels. The key section of this thesis is a chapter describing the experimental plan, the performance and the evaluation of the experiments. The goal of the experiments was to dilute the natural gas by two inert gases, namely carbon dioxide and nitrogen. The lowest LHV value achieved during the experiment was 10,7 MJ/mN3. The influence of the inert gases on the NOx emissions, the flame stability and characteristics, the flue gas temperature, the in-flame temperatures, the heat flux and the thermal efficiency was investigated and evaluated. Each parameter was measured and evaluated for three different burners: the burner with the staged gas, the burner with the staged air and the burner for low calorific fuels. Generally, after the addition of the inert gas into the noble fuel, the in-flame temperatures decreased. As a consequence, the NOx emissions decreased as well. The effects of carbon dioxide on the investigated combustion parameters were more substantial than the effects of nitrogen. Each measured parameter is strongly dependent on the burner geometry. The experiments revealed that in most cases the addition of the inert gas into the fuel influenced the flame length (flame shortened) and also more heat was transfered into the chamber's walls closer to the burner tile.
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Ejectors and it's utilization in industry
Zejda, Vojtěch ; Hájek, Jiří (referee) ; Vondál, Jiří (advisor)
This work aims at introducing ejector pumps. It describes the working principle, advantages and disadvantages and restrictions of their usage in various applications. Main disadvantage of ejectors is the low efficiency which limits the usage only in specific applications where it is not considered to be a crucial factor. Therefore, examples of usage of various ejectors in industrial processes are listed further. Last but not least, in these examples are also demonstrated difficulties and reasons why it is favourable to use an ejector rather than other types of pumps. Further off, this work deals with design methods of ejectors and compares their suitability. It appears that not all the methods are suitable for ejector design, because it depends greatly on presumed shape of a certain ejector. In the end, there is compared energy intensity of an ejectors and common centrifugal pumps and confirms energy disadvantage of ejectors.
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Quality Management System Improvement according to ČSN EN ISO 9001:2001
Hájek, Jiří ; Jaroš, David (referee) ; Videcká, Zdeňka (advisor)
This thesis deals with a detailed analysis of the systems of quality management in a particular company according to EN ISO 9001:2000. The aim of this work is to analyse the system deeply and then focus on how the system could become more effective. In this case the topic was the system of measurement of the customers´satisfaction. In the theoretical part there is the overall view of quality from the historical point of view up to the present. For better understanding of the problem the principles of ISO standards, maintenance of the systems and improvement instruments are mentioned. In the practical part there is the analysis of the particular system of quality management and the process of application of improvement procedures.
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Nonlinear diffusion equations
Polášek, Radek ; Hájek, Jiří (referee) ; Franců, Jan (advisor)
This work is focused on various types of diusion equations. The diffusion equations are derived for various assumptions of media. We describe in detail Barenblatt solution for slow diffusion equation, whose features are demonstrated.
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Design optimization of water-cooled thermocouple probe
Dohnal, Miloslav ; Vondál, Jiří (referee) ; Hájek, Jiří (advisor)
The object of this bachelor’s thesis is to create a computational model of an existing (basic) structure water-cooled thermocouple probe, to calculate and evaluate the quality of existing cooling probe and to propose possible design modifications to improve the cooling effect of the probe tip. Verify and evaluate design modifications by further calculation. The calculation will be carried out using computational fluid dynamics (CFD method). On the contrary, the aim of this work is not to create another text tool for students dealing with CFD topics. The first part describes the meaning and history of the CFD methods, then mentions the work on the principle of thermocouple probes and its application in practice. The second part of the work is devoted to problem solving. It describes the reason for the introduction of model situations when calculating the cooling effect of thermocouple probes and calculation indicates what type of flow occurs inside the thermocouple probe. It further describes the results of calculations for several approximations gradually increasing accuracy of the resulting solution of the problem and assessing the quality of cooling in the probe tip current (basic) structure. The final part deals with design changes for better cooling effect especially in the probe tip, which is verified by further calculation as well.
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Velocity profile measurement downstream of swirler
Zejda, Vojtěch ; Hájek, Jiří (referee) ; Vondál, Jiří (advisor)
A burner is very important device in process furnaces that significantly affect the production of emissions during the combustion process. One of the key things in development of the modern low-NOX burners is the evaluation of flow field downstream of an axial blade swirler inside the burner. The computational fluid dynamics (CFD) is often used to predict the attributes of the flow. Predicted values should be validated with measurement. It is the reason why the velocity fields for several choosen swirlers were measured. The hot wire anemometry was choosen and the dual-sensor probe was used during the measurement. The data can be then used for CFD validation. This thesis describes procedure of measurement set-up. The experimental facility was designed according to the anemometry method. The new probe traversing system was designed, which provides desired accuracy. Five different swirlers were measured. Large data set, need for customized post-processing and control over calculation procedures lead to new software design. For each swirler the velocity profiles were gathered and the swirl numbers calculated. That final data were transferred in to graphical format. Uncertainty of measured data was calculated. Results show counter-rotating flow in some areas closed to the swirler. Some drawbacks of current measurement set-up are discussed. Based on the thesis reader can obtain the information and knowledge for consequent measurements of swirl burners velocity profiles.
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