The existence of flammable aerosols presents a high potential for fire hazards in the process industry. Various industrial fluids, most of which operate at elevated temperatures and pressures, can be atomized when released under high pressure through a small orifice. Because of the complexity in the process of aerosol formation and combustion, the availability of data on aerosol flammability and flame propagation behaviors is still quite limited, making it difficult to evaluate the potential fire and explosion risks from released aerosols in the process industry and develop safety measures for preventing and/or mitigating aerosol hazards. A study is needed to investigate the relationship between aerosol combustion behaviors and the properties of the aerosols. This dissertation presents research on the combustion behaviors of flammable aerosols. Monodisperse aerosols created by industrial heat transfer fluids were generated using electrospray. The characteristics of flame propagations in aerosols and the influence of the presence of fuel droplets in the system are studied in the aerosol ignition tests. Flames in aerosols are characterized by non-uniform shapes and discrete flame fronts. Flames were observed in different burning modes. Droplet evaporation was found to play an important role in aerosol burning modes. Droplet evaporation behaviors and fuel vapor distributions are further related to aerosol droplet size, droplet spacing, movement velocity, and liquid volatility. The burning mode of a global flame with rapid size expansion is considered the most hazardous aerosol combustion scenario. This burning mode requires a smaller droplet size and smaller space between droplets. Larger droplet sizes and spacing may hinder the appearance of global flames. But when the liquid fuel has a certain level of volatility, there is an uneven distribution of fuel vapor in the system and this may cause the unique phenomenon of burning mode variations combined with enhanced flame propagation speed. Using an integrated model, the minimum ignition energy values of aerosols were predicted. The aerosol minimum ignition energy is influenced by the fuel-air equivalence ratio and the droplet size. Higher equivalence ratios, up to 1.0, significantly reduce the minimum ignition energy, while larger droplet sizes result in a higher minimum ignition energy.

The present research work aims to develop a new heat transfer fluid by combining multiwalled carbon nanotubes (MWCNT) and microencapsulated phase change materials (MPCMs). Stable nanofluids have been prepared using different sizes of multiwalled carbon nanotubes and their properties like thermal conductivity and viscosity have been measured. Microencapsulated phase change material slurries containing microcapsules of octadecane have been purchased from Thies Technology Inc. Tests have been conducted to determine the durability and viscosity of the MPCM slurries. Heat transfer experiments have been conducted to determine the heat transfer coefficients and pressure drop of the MWCNT nanofluids and MPCM slurries under turbulent flow and constant heat flux conditions. The MPCM slurry and the MWCNT nanofluid have been combined to form a new heat transfer fluid. Heat transfer tests have been conducted to determine the heat transfer coefficient and the pressure drop of the new fluid under turbulent flow and constant heat flux conditions. The potential use of this fluid in convective heat transfer applications has also been discussed. The heat transfer results of the MPCM slurry containing octadecane microcapsules was in good agreement with the published literature. The thermal conductivity enhancement obtained for MWCNTs with diameter (60-100 nm) and length (0.5-40?m) was 8.11%. The maximum percentage enhancement (compared to water) obtained in the heat transfer coefficient of the MWCNT nanofluid was in the range of 20-25%. The blend of MPCMs and MWCNTs was highly viscous and displayed a shear thinning behavior. Due to its high viscosity, the flow became laminar and the heat transfer performance was lowered. It was interesting to observe that the value of the maximum local heat transfer coefficient achieved in the case of the blend (laminar flow), was comparable to that obtained in the case of the MPCM slurry (turbulent flow). The pressure drop of the blend was lower than that of the MWCNT nanofluid.

Mist or aerosol explosions present a serious hazard to process industries. Heat transfer fluids are widely used in the chemical process industry, are flammable above their flash points, and can cause aerosol explosions. Though the possibility of aerosol explosions has been widely documented, knowledge about their explosive potential is limited. Studying the formation of such aerosols by emulating leaks in process equipment will help define a source term for aerosol dispersions and aid in characterizing their explosion hazards. Analysis of the problem of aerosol explosions reveals three major steps: source term calculations, dispersion modeling, and explosion analysis. The explosion analysis, consisting of ignition and combustion, is largely affected by the droplet size distribution of the dispersed aerosol. The droplet size distribution of the dispersed aerosol is a function of the droplet size distribution of the aerosol formed from the leak. Existing methods of dealing with the problem of aerosol explosions are limited to enhancing the dispersion to prevent flammable concentrations and use of explosion suppression mechanisms. Insufficient data and theory on the flammability limits of aerosols renders such method speculative at best. Preventing the formation of aerosol upon leaking will provide an inherently safer solution to the problem. The research involves the non-intrusive measurement of heat transfer fluid aerosol sprays using a Malvern Diffraction Particle Analyzer. The aerosol is generated by plain orifice atomization to simulate the formation and dispersion of heat transfer fluid aerosols through leaks in process equipment. Predictive correlations relating aerosol droplet sizes to bulk liquid pressures, temperatures, thermal and fluid properties, leak sizes, and ambient conditions are presented. These correlations will be used to predict the conditions under which leaks will result in the formation of aerosols and will ultimately help in estimating the explosion hazards of heat transfer fluid aerosols. Heat transfer fluid selection can be based on liquids that are less likely to form aerosols. Design criteria also can incorporate the data to arrive at operating conditions that are less likely to produce aerosols. The goal is to provide information that will reduce the hazards of aerosol explosions thereby improving safety in process industries.

This thesis work presents a numerical study of the effects of fins and thermal fluid velocities on the storage characteristics of a cylindrical latent heat energy storage system (LHESS). The work consists of two main components: 1. The development of a numerical method to study and solve the phase change heat transfer problems encountered in a LHESS during charging of the system, which results in melting of the phase change material (PCM). The numerical model is based on the finite element method. The commercial software COMSOL Multiphysics was used to implement it. The effective heat capacity method was applied in order to account for the large amount of latent energy stored during melting of a PCM, and the moving interface between the solid and liquid phases. The fluid flow, heat transfer and phase change processes were all validated using known analytical solutions or correlations. 2. Due to the low thermal conductivity of PCMs, the heat transfer characteristics of an enhanced LHESS was studied numerically. The effects of fins and the thermal fluid velocity on the melting rate of the PCM in the LHESS were analyzed. Results obtained for configurations having between 0 and 27 fins show that the heat transfer rate increases with addition of fins and thermal fluid velocity. The effect of the HTF velocity was observed to be small with few fin configurations since the thermal resistance offered by the LHESS system, mostly PCM, is vastly more important under these conditions; while its effect becomes more pronounced with addition of fins, since the overall thermal resistance decreases greatly with the addition of fins. The total energy stored after 12 hours for 0 and 27 fins configurations range between 3.6 MJ and 39.7 MJ for a thermal fluid velocity of 0.05 m/s and between 3.7 MJ and 57 MJ for a thermal fluid velocity of 0.5 m/s. The highest system efficiencies for the 0.05 m/s and 0.5 m/s, obtained with 27 fins configuration are 68.9% and 97.9% respectively.

abstract: Fluids such as steam, oils, and molten salts are commonly used to store and transfer heat in a concentrating solar power (CSP) system. Metal oxide materials have received increasing attention for their reversible reduction-oxidation (redox) reaction that permits receiving, storing, and releasing energy through sensible and chemical potential. This study investigates the performance of a 111.7 MWe CSP system coupled with a thermochemical energy storage system (TCES) that uses a redox active metal oxide acting as the heat transfer fluid. A one-dimensional thermodynamic model is introduced for the novel CSP system design, with detailed designs of the underlying nine components developed from first principles and empirical data of the heat transfer media. The model is used to (a) size components, (b) examine intraday operational behaviors of the system against varying solar insolation, (c) calculate annual productivity and performance characteristics over a simulated year, and (d) evaluate factors that affect system performance using sensitivity analysis. Time series simulations use hourly direct normal irradiance (DNI) data for Barstow, California, USA. The nominal system design uses a solar multiple of 1.8 with a storage capacity of six hours for off-sun power generation. The mass of particles to achieve six hours of storage weighs 5,140 metric tonnes. Capacity factor increases by 3.55% for an increase in storage capacity to eight hours which requires an increase in storage volume by 33% or 737 m3, or plant design can be improved by decreasing solar multiple to 1.6 to increase the ratio of annual capacity factor to solar multiple. The solar reduction receiver is the focal point for the concentrated solar energy for inducing an endothermic reaction in the particles under low partial pressure of oxygen, and the reoxidation reactor induces the opposite exothermic reaction by mixing the particles with air to power an air Brayton engine. Stream flow data indicate the solar receiver experiences the largest thermal loss of any component, excluding the solar field. Design and sensitivity analysis of thermal insulation layers for the solar receiver show that additional RSLE-57 insulation material achieves the greatest increase in energetic efficiency of the five materials investigated. / Dissertation/Thesis / Masters Thesis Civil and Environmental Engineering 2017

Le marché du gaz naturel liquéfié (GNL) est en plein essor et présente l'avantage de diversifier les approvisionnements. Le GNL est acheminé à une température de -162 °C et à pression atmosphérique. L'objet principal de la thèse vise à valoriser l'exergie contenue dans le GNL lors de sa gazéification et de son réchauffage avant d'être distribué sur le réseau. La thèse étudie un nouveau concept de gazéification qui se différentie des développements en cours par l'usage de l'air ambiant comme seule source de chaleur et par la valorisation d'une partie de l'exergie du GNL en énergie mécanique.Une analyse énergétique et exergétique a permis de définir des architectures de cycles moteurs utilisant l'énergie calorifique de l'air comme source chaude (récupérée par un fluide intermédiaire qui est le propane) et le GNL comme puits froid.L'usage de l'air ambiant conduit à gérer les cycles de givrage et dégivrage sur les évaporateurs du fluide intermédiaire récupérant les calories de l'air. Ceci nécessite la compréhension des phénomènes couplés de transferts de chaleur et de masse. Pour cela un banc d'essai a été construit afin de caractériser des échangeurs à ailettes rondes discontinues en condition de givrage et de valider un modèle numérique simulant le comportement de ces échangeurs en présence de givre. Cette modélisation a permis de proposer une logique de dimensionnement et d'exploitation des évaporateurs à air en prenant en compte l'effet du givre. Une étude saisonnière a montré que le procédé proposé est énergétiquement excédentaire tout au long de l'année. / The market of liquefied natural gas (LNG) is growing and presents the advantage of diversifying supplies. The LNG is fed at a temperature of -162°C and at atmospheric pressure. The main objective of the thesis aims at the valorization of the LNG exergy during its gasification and heating before being delivered through the network. The thesis investigates a new concept of gasification that differs from the current developments by the use of ambient air as the only heat source and by partial valorization of the LNG exergy in mechanical energy.An energy and exergy analysis helped to the definition of engine-cycle architectures using the air heating capacity as a heat source (recovered by an intermediate fluid, here propane) and LNG as a cold sink.The use of ambient air led to manage frosting and defrosting cycles on the evaporators of intermediate fluid recovering heat from the air. This requires understanding the coupled heat and mass-transfer phenomena. A test bench was built to characterize Circular Finned-Tube Heat Exchangers in frosting conditions and to validate a numerical model simulating the behavior of such heat exchangers in presence of frost. As a result, a logic for the design and operation of air heat exchangers is proposed taking into account the frosting effect. A seasonal study showed that the output power generated by the new concept of gasification is superior to its energy consumption throughout the year.

The demand for clean and environmentally benign energy resources has been a great concern in the last two decades. To alleviate the associated environmental problems, reduction of the use of fossil fuels by developing more cost-effective renewable energy technologies becomes more and more significant. Among various types of renewable energy sources, solar energy and bioenergy take a great proportion. This dissertation focuses on the heat transfer and flow in solar energy and bioenergy systems, specifically for Thermal Energy Storage (TES) systems in Concentrated Solar Power (CSP) plants and open-channel algal culture raceways for biofuel production. The first part of this dissertation is the discussion about mathematical modeling, numerical simulation and experimental investigation of solar TES system. First of all, in order to accurately and efficiently simulate the conjugate heat transfer between Heat Transfer Fluid (HTF) and filler material in four different solid-fluid TES configurations, formulas of an effective heat transfer coefficient were theoretically developed and presented by extending the validity of Lumped Capacitance Method (LCM) to large Biot number, as well as verifications/validations to this simplified model. Secondly, to provide design guidelines for TES system in CSP plant using Phase Change Materials (PCM), a general storage tank volume sizing strategy and an energy storage startup strategy were proposed using the enthalpy-based 1D transient model. Then experimental investigations were conducted to explore a novel thermal storage material. The thermal storage performances were also compared between this novel storage material and concrete at a temperature range from 400 °C to 500 °C. It is recommended to apply this novel thermal storage material to replace concrete at high operating temperatures in sensible heat TES systems. The second part of this dissertation mainly focuses on the numerical and experimental study of an open-channel algae culture raceway for biofuel production. According to the proposed flow field design of ARID-HV algal raceway, experiments and numerical simulation have been conducted to understand the enhancement of flow mixing in the flow field of ARID-HV raceway by cutting slots on top of the dam near the dead zones. A new method was proposed to quantitatively evaluate the flow mixing by using the statistics of temporal and spatial distribution of the massless fluid particles (centered in each cell at the inlet surface) in the raceway collecting the data of path-lines of fluid particles from CFD results. It is hoped that this method can be applied to assist the algal raceway flow field design as well as other engineering applications. The third part introduces the details about the construction work of a high temperature molten salt test loop. Because of the limited operating temperature of conventional synthetic oils, in order to obtain higher energy conversion efficiency, higher operating temperature is always desirable in a CSP plant which leads to the requirement of new generation of HTF. Currently, a halide salt eutectic mixture (NaCl-KCl-ZnCl₂) as a potential HTF for future CSP applications has been proposed by a multi-institute research team, led by University of Arizona. The thermophysical properties of the halide eutectic salt have been measured. However, this new developed halide eutectic salt has not been tested in a circulating loop at a high operating temperature for the measurement of heat transfer coefficient. It is a significant effort to build such a test system due to extremely high operating temperature. As a consequence, in the third part of this dissertation, details about the design of the lab-scale test system and all the equipment items will be introduced. The investigations included in this dissertation for the heat transfer and flow in solar energy and bioenergy systems are of particular interest to the renewable energy engineering community. It is expected that the proposed methods can provide useful information for engineers and researchers.

Natural turbulent convection studies encompass a wide range of flows occurring in nature, for example, atmospheric and oceanic flows, con-vection in the Earth’s mantle, convection in the stars and also in many engineering applications. Rayleigh-Benard convection (RBC), i.e. con-vection in a horizontal fluid layer confined between two plates with a temperature differential maintained across them, has been a proto-type problem in the studies of turbulent natural convection. Many small scale and global features of the flow in the turbulent regime of RBC are known, yet the flow dynamics is not fully understood, es-pecially at high Rayleigh numbers (Ra). Present work comprises of experimental investigations of a different type of flow, high Rayleigh number turbulent convection in a long vertical tube (abbreviated as tube convection or TC). The tube of aspect ratio (length to diameter) of about 10, open at both the ends interconnects two large tanks. The flow driven by an unstable density difference created between the two tanks, has some unique features, different from RBC. The net flow at any tube cross-section is zero and the time averages of the velocities, the Reynolds shear stress and the mean shear are also zero. Turbu-lent energy production is therefore solely due to buoyancy. The flow is axially homogeneous and axisymmetric. In the homogeneous region, the mean density gradient is linear. Rayleigh number in TC is conve-niently defined based on the mean (linear) density gradient (denoted by Rag). Two sets of experiments are carried out. In one set of experiments, the density difference is created using brine and fresh water and in another set, it is created using heat. The ranges of Rag achieved are 3 × 108 < Rag < 8 × 109 in the experiments using salt (Schmidt number, Sc ≈ 600) and 5 × 104 < Rag < 5 × 106 in the experiments using heat (Prandtl number, P r ≈ 6). From the measured salt and heat fluxes in both the sets of experiments, the non dimensional flux 1 1 scaling above a certain value of Rag is obtained as N ug ∼ Rag2 P r 2 and from the velocity measurements in the experiments using salt, the 1 Reynolds number scaling is obtained as Re ∼ Rag2 P r− 12 . Both these are as per the predicted scalings by the mixing length model proposed by Arakeri et al. (2000) for high Rag convection in the vertical tube. The flux scaling N u ∼ (RaP r)2 , also known as the ‘ultimate regime’ of convection, expected at very high Ra but not yet observed in the experiments in classical RBC, is easily achieved in TC at relatively lower values of Ra. The fluxes and Reynolds numbers in TC are orders of magnitude higher as compared to those obtained in RBC for similar values of Ra and P r. In the lower range of Rag values for P r ≈ 6, a transition to a new flux scaling, N u ∼ (RaP r)0.29 is found. Similar transitions are also found to be present in the results of Tovar (2002) for Sc ≈ 600 and in the DNS results of Schmidt et al. (2012) for P r = 1, at different values of Rag. Collecting all these data, it is shown that the transition occurs at a fixed Grashof number of 1.6 × 105, independent of P r. Velocity measurements are carried out using particle image velocime-try (PIV) in the salt experiments. Kinetic energy spectra computed from the velocity fields are presented for the locations from the tube axis to the wall, for the lowest and the highest values of Rag achieved in the experiments. The spatial energy spectrum of lateral velocity at the tube axis follows Kolmogorov-Obukhov (KO) scaling (−5/3 scaling exponent) while the spatial spectrum of longitudinal velocity shows a scaling slightly higher than −5/3 but lower than −11/5 (the Bolgiano-Obukhov (BO) scaling). The scalar spectra is computed from the concentration fields obtained from planar laser induced fluorescence (PLIF) in the experiments using salt, and also from the temperature measurements from the experiments using heat. Both the concentra-tion and temperature fluctuations spectra show some evidence of dual scaling - BO scaling (−7/5 scaling exponent) in the inertial subrange followed by Obukhov-Corrsin (OC) scaling (−5/3 scaling exponent) over a narrow range of scales. Light propagation through the buoyancy driven turbulent flow in TC has also been experimentally investigated. Light propagation through convective turbulence is encountered in many situations. In some cases e.g. in observational astronomy it is undesirable, while in some other cases it is useful, e.g. in remote sensing of meteorological parameters. In the present study, light intensity and angle of arrival fluctuations in a parallel beam of light are measured. Laser shadowgraphy is used in the intensity measurements while the angle of arrival is obtained by measuring deflections of narrow laser beams, created by passing collimated laser light through a mask having equispaced grid of holes. Background oriented schlieren (BOS) measurements have also been carried out to obtain the displacements, which are proportional to the angle of arrivals. The equations for frequency spectrum of intensity and angle of arrival from the literature, developed for isotropic, ho-mogeneous turbulent media, are modified for the flow in the present case and the asymptotic scalings for high and low frequency ranges are obtained. The scalings in the frequency spectra computed from the measurements of intensity and angle of arrival fluctuations are com-pared with the obtained asymptotic scalings. The results from the present work are also compared with results from studies in the atmo-sphere and lab experiments.

Cette thèse, financée dans le cadre du projet européen CSP2, porte sur l'étude d'un nouveau type de récepteur solaire thermique à concentration utilisant comme fluide caloporteur une suspension dense de fines particules en circulation ascendante dans des tubes verticaux. Ladite suspension est obtenue par fluidisation de particules de classe A. Le principe consiste à créer un écoulement ascendant de la suspension dans un tube vertical exposé au rayonnement solaire concentré qui chauffe la paroi du tube, qui transmet ensuite cette chaleur aux particules, qui la transportent jusqu'à un cycle de conversion d'énergie pour la production d'électricité. Au contraire des fluides solaires classiques, les particules peuvent atteindre les hautes températures (> 700 °C) permettant l'utilisation de cycles à haut rendement de conversion (Brayton, cycles combinés), tout en permettant un stockage direct de la pour une production continue. Au cours de la thèse, un récepteur à un tube a été testé avec succès au grand four solaire du laboratoire PROMES-CNRS à Odeillo, les particules en sortie atteignant 750 °C, ce qui a prouvé la faisabilité du concept et permis la détermination des premières valeurs de coefficient d'échange de chaleur tube-suspension. L'hydrodynamique de l'écoulement et les mécanismes d'échange de chaleur ont été observés grâce à des simulations numériques 3D. Un récepteur de 150 kWth à 16 tubes a ensuite été testé et modélisé, validant l'utilisation du procédé à plus grande échelle. / This thesis, financed in the frame of the CSP2 European project, concerns the study of a new kind of thermal concentrating solar receiver using a dense suspension of solid particles circulating upward in vertical tubes. The suspension is obtained by fluidizing Geldart A-type particles. The principle consists in creating an upward flow of the suspension in a vertical tube exposed to the concentrated solar radiation that heats the tube wall. The heat is then transmitted to the particles circulating inside that transport it to a conversion cycle for electricity production. Contrarily to usual solar heat transfer fluids, particles can reach high temperatures (> 700 °C) that permit to power high efficiency thermodynamic cycles such as Brayton or combined cycles. Moreover they can be used as a direct heat storage medium for continuous electricity production. During this thesis, a one-tube solar receiver was successfully tested at the PROMES-CNRS solar furnace in Odeillo, with particle outlet temperatures of 750 °C reached. The first values of wall-to-suspension heat transfer coefficient were calculated and a Nusselt correlation was determined. A specific flow pattern with a particle downward flux close to the wall and upward flux in the tube center was underlined. The flow hydrodynamics and the heat transfer mechanisms were studied thanks to 3D numerical simulations. A 16-tube 150 kWth receiver was finally tested and modeled, proving the process applicability at larger scale.

Stárnutí teplonosných kapalin na organické bázi je dlouhodobým problémem, který je znám od počátku jejich používání. První část této disertační práce je tak věnována případové studii funkčního experimentálního systému, který byl jako nový naplněn teplonosnou kapalinou na bázi propan-1,2-diol a pozorován po období 7 let. Pro analýzu stárnutí kapaliny v tomto systému byly sledovány základní provozní vlastnosti kapaliny jako jsou hustota, viskozita, teplota tuhnutí, pH a obsah kovů. Skrze tyto vlastnosti tak bylo sledováno stárnutí kapaliny nepřímo. Přímé sledování stárnutí bylo posléze provedeno analýzou degradačních produktů, jako jsou organické kyseliny a změny ve složení směsi pomocí izotachoforézy a hmotnostní spektrometrie. Pro srovnání byly taktéž analyzovány vybrané vzorky z několik dalších systémů plněných identickou kapalinou s prokazatelně pokročilou formou degradace. V druhé části práce jsou představeny základní fyzikálně-chemické vlastnosti směsí propan-1,3-diolu s vodou a jejich analytické hodnocení a matematické modelování pro universální použití jakožto nového základu pro nemrznoucí teplonosné kapaliny. Na základě dostupných informací je pak hodnocena použitelnost této směsi. Výhoda propan-1,3-diolu je spatřována především ve výrobě z obnovitelných zdrojů a v některých fyzikálních a chemických vlastnostech, které dle dosavadních poznatků předčívají doposud používané glykolové směsi.