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An emerging trend in the building services industry is the installation of passive deaerators on the flow line of domestic wet central heating systems. To date, no data and theoretical models predicting the two-phase flow characteristics in domestic wet central heating systems are available in the open literature. This gap in literature has prevented essential design improvements to passive deaerators thus impeding the efficiency enhancement of such devices. Hence, the current study is aimed at assisting designers of deaeration devices by providing fundamental data and model correlations with respect to the two-phase flow characteristics typical in a wet domestic central heating system. For this purpose an experimental research project was adopted and several studies were carried out, including; (1) a comprehensive review to understand the background of the phenomena, (2) the design and construction of an experimental test rig to conduct the necessary investigations into the phenomenon of two-phase flow in domestic wet central heating systems, (3) the development of a reliable image capture and analysis technique, (4) the completion of a number of experiments to investigate typical bubble sizes, volumetric void fractions, bubble distributions and nucleation and dissolution rates and (5) the correlation of the data gathered as part of the present study with existing bubble size, nucleation and dissolution prediction models. This research has, for the first time, provided an in depth analysis into two-phase flow characteristics in wet domestic central heating systems through the use of a high speed camera and image analysis techniques. The two-phase phenomenon finds its origins in high dissolved gas concentrations present in the water flowing through the closed loop system, thus resulting in super saturation conditions at the primary heat exchange wall conditions. Bubble sizes at the boiler flow line were found to be dependent on the bulk fluid velocity, heat flux and pressure, with a measured mean diameter in the range of 0.13 mm to 0.39 mm. The Winterton (1972a) force balance model for bubble size prediction was in reasonable agreement with the experimental results. This model was further improved through the correlation of our data with the inclusion of dimensionless groups. Bubble nucleation rates have been calculated in the range of 0.3 to 4 bubbles / cm2 s with total system bubble production rates measured in the range of 784 to 6920 bubbles per second. Bubble nucleation rates have been calculated through the consideration of the heat exchanger surface under super saturation conditions. A correlation for the model by Hepworth et al. (2003) for nonclassical heterogeneous nucleation is proposed based on the experimental data gathered during the present study. Experimental results have shown dissolution rates for the bubble size ratio in the range of 0.4 to 12 % per second with system conditions. A modification of the model developed by Epstein and Plesset (1950) for stationary bubble dissolution is proposed with the inclusion of the Sherwood number to capture the effects of turbulent diffusion. The volumetric void fraction distribution in vertical pipes was found to be quasi-homogenous across the pipe section while being strongly dependent on gravitational and turbulence effects in horizontal pipe bubbly flow. A CFD simulation predicted the volumetric void fraction distribution with reasonable accuracy.

Este trabalho disserta sobre os métodos usados para determinar a aceitabilidade de defeitos tipo trinca em um bocal de caldeira de geração de vapor. Inicia com uma avaliação sobre as tensões atuantes em bocais e introduz os conceitos de mecânica da fratura para aceitação de defeitos em estruturas. Propõe um método analítico para estimativa de tensões longitudinais em bocais e o compara ao cálculo de tensões por elementos finitos. Explora um caso de um defeito circunferencial e não aflorado em um bocal de uma caldeira real a qual fornece corpos de prova que são submetidos a ensaios de impacto a partir do quais é traçada sua curva de transição ductil-frágil. Analisa os resultados obtidos a partir destes ensaios e os confronta com os valores nominais e de referência. Esta análise embasa discussão sobre probabilidades de falha e fatores de segurança. / This work discusses the methods used to determine the acceptability of crack-like flaws in a boiler nozzle. It begins with an evaluation of the stresses acting on nozzles and introduces the concepts of fracture mechanics for the acceptance of defects in structures. An analytical method for estimating longitudinal stresses in nozzles is proposed and compared with numerical results obtained from the finite element method. It explores a case of a circumferential embedded flaw in a nozzle of a real boiler that provides specimens that are subjected to impact tests from which its ductile-brittle transition curve is drawn. It analyzes the results obtained from these tests and confront them with the nominal and reference values. This analysis bases discussion on probabilities of failure and safety factors.

Thesis (M.S.)--Virginia Polytechnic Institute, 1951. / Abstract. Vita. Includes bibliographical references (leaves 30-31). Also available via the Internet.

Magnesium oxide-based additives have been used in kraft recovery boilers to minimize deposit accumulation on superheater and generating bank tubes. Due to inconclusive results observed in boilers and claims made for additive effectiveness backed by little scientific evidence, only a few mills still use the additives for fouling control. A systematic study was performed to examine the effect of MgO-based additives on deposit accumulation and removal under controlled laboratory conditions, and to evaluate the additive effect in a kraft recovery boiler. The results show that while the additive coating has no effect on deposition, it could ease deposit removal if a coating greater than 5 µm can be applied on the tube surface effectively. Separate injection of additive and black liquor had insignificant effect on deposition and removal. Results of the field study show that the amount of additive collected on the tube is too small to have an impact.

Magnesium oxide-based additives have been used in kraft recovery boilers to minimize deposit accumulation on superheater and generating bank tubes. Due to inconclusive results observed in boilers and claims made for additive effectiveness backed by little scientific evidence, only a few mills still use the additives for fouling control. A systematic study was performed to examine the effect of MgO-based additives on deposit accumulation and removal under controlled laboratory conditions, and to evaluate the additive effect in a kraft recovery boiler. The results show that while the additive coating has no effect on deposition, it could ease deposit removal if a coating greater than 5 µm can be applied on the tube surface effectively. Separate injection of additive and black liquor had insignificant effect on deposition and removal. Results of the field study show that the amount of additive collected on the tube is too small to have an impact.

The research focuses on the fate of heavy metals and their emissions from pulp mill recovery boilers. Heavy metals are extremely toxic and cause adverse effects on human health. There is limited information about factors affecting heavy metal vaporization from recovery boilers, in which spent pulping liquors, (also called black liquor) is burnt. The heavy metals are present in trace quantities in black liquor. They are emitted in the air as submicron particles, and their amount of emission depends on the input of metals into the recovery boiler, volatilization of metals in the boiler and the efficiency of the electrostatic precipitator (ESP) to remove the metals. The heavy metals vaporization from different recovery boilers vary widely and the reason is not known. My thesis will focus on determining what conditions affect the volatility of the heavy metals, and whether differences in heavy metal vaporization are due to differences in boiler operating conditions or the composition of the liquor. I will examine black liquor pyrolysis, combustion and gasification in well-controlled laboratory conditions. I will analyze three different black liquors. The liquors will be spray-dried and burnt or gasified in a laminar entrained-flow reactor at high temperatures of 1000-1100C and residence times ranging from 0.5 to 1.5 s. Equilibrium calculations will be performed to determine the distribution of heavy metals and species under set conditions from previous devolatilization experiments. The equilibrium calculations will serve as a useful tool for prior estimation of the distribution of heavy metals. The impact of different liquors on volatilization of heavy metals in the experiments will be determined. The char residue and condensation aerosol (fume) particles from each experiment will be analyzed using ICP-AES. The amount of heavy metals volatilization during combustion and gasification will be determined based on the amount of the metals in the char residues and fume particles. Based on the results of the experimental study combined with data from the equilibrium analysis program the main cause of variability in heavy metal vaporization can be found. Also the boiler operating conditions can be studied to determine its affects on the volatility of the heavy metals.

Thesis (M.S.)--Ohio University, June, 1986. / Title from PDF t.p.

Thesis (Ph.D.)--Mechanical Engineering, Georgia Institute of Technology, 2008. / Committee Co-Chair: Preet M. Singh; Committee Co-Chair: Richard W. Neu; Committee Member: Hamid Garmestani; Committee Member: Timothy Patterson; Committee Member: W. Steven Johnson.