Two Phase Flow, Phase Change and Numerical Modeling

The heat transfer and analysis on laser beam, evaporator coils, shell-and-tube condenser, two phase flow, nanofluids, complex fluids, and on phase change are significant issues in a design of wide range of industrial processes and devices. This book includes 25 advanced and revised contributions, and it covers mainly (1) numerical modeling of heat transfer, (2) two phase flow, (3) nanofluids, and (4) phase change. The first section introduces numerical modeling of heat transfer on particles in binary gas-solid fluidization bed, solidification phenomena, thermal approaches to laser damage, and temperature and velocity distribution. The second section covers density wave instability phenomena, gas and spray-water quenching, spray cooling, wettability effect, liquid film thickness, and thermosyphon loop. The third section includes nanofluids for heat transfer, nanofluids in minichannels, potential and engineering strategies on nanofluids, and heat transfer at nanoscale. The forth section presents time-dependent melting and deformation processes of phase change material (PCM), thermal energy storage tanks using PCM, phase change in deep CO2 injector, and thermal storage device of solar hot water system. The advanced idea and information described here will be fruitful for the readers to find a sustainable solution in an industrialized society

Analyse du transfert de chaleur par ébullition et condensation à l’Intérieur d’un caloduc horizontal

L'utilisation de différents types de caloducs dans les systèmes de climatisation, de ventilation et d'évacuation de la chaleur a considérablement augmenté. Les caloducs sont des dispositifs de transfert de chaleur à deux phases générant un flux de chaleur élevé avec un faible gradient de température et une perte de charge minimale. La réduction des coûts de maintenance est l'un des avantages de l'utilisation de caloducs, due à l'absence de pièces mécaniques, à la réduction de l'espace occupé et à la surveillance, la fabrication et la maintenance simplifiées. Les caloducs sont des conducteurs thermiques très efficaces en raison des flux de chaleur élevés obtenus lors de l’évaporation et de la condensation du fluide de travail. Pour optimiser les performances d'un caloduc, il est nécessaire d'étudier précisément ce qui se passe à l'intérieur des sections de l'évaporateur et du condenseur. Ce projet consiste à développer un modèle numérique simulant un écoulement diphasique à l'intérieur d'un caloduc à l'aide de codes CFD développés dans OpenFOAM. Le modèle est capable de prédire les principales variables représentant le comportement des deux phases telles que la vitesse, la température, la pression et la fraction volumique de chaque phase dans l’évaporateur ou le condenseur. Une attention particulière est consacrée à la simulation de la condensation car il n'existe pas de modèle numérique de ce type dans la littérature pour l'analyse du transfert de chaleur par condensation. En outre, une combinaison d'ébullition et de condensation dans un caloduc est une autre contribution de ce travail. Dans ce projet, le modèle de fractionnement du flux thermique de la paroi dans le cadre de l’approche eulérienne en deux phases a été appliqué. L'effet du transfert de quantité de mouvement et du transfert d'énergie entre les deux phases est également pris en compte. La capacité du modèle numérique a été validée par des données expérimentales obtenues à partir d’essais réalisés sur un prototype construit à l’Université de Sherbrooke. Ensuite, un modèle validé est utilisé pour évaluer les performances de ce caloduc. Enfin, deux types de structures de gorge ont été suggérés et des tests expérimentaux ont été effectués pour étudier toute amélioration des performances du caloduc.Abstract: The usage of different types of heat pipe in air conditioning, ventilation and heat removal systems has tremendously increased. Heat pipes are two-phase heat transfer devices and they are generating high heat flux with minimum temperature gradient and pressure drop. Lower service costs, due to the absence of mechanical parts, less occupied space and easier monitoring, manufacturing and maintenance are some of the advantages of using heat pipes. Heat pipes are highly effective thermal conductors due to the high heat fluxes obtained during boiling and condensation. For the optimization of heat pipe performances, it is necessary to study what happens inside the evaporator and condenser sections, precisely. This project deals with the development of a numerical model to simulate the two-phase flow inside of the heat pipe using CFD codes developed in OpenFOAM. The model is able to predict the main two-phase variables such as velocity, temperature, pressure and volume fractions of each phase in the evaporator or condenser. Special attention is devoted to the simulation of condensation since there is no such numerical model in the open literature for the analysis of condensation heat transfer. In addition, the combination of boiling and condensation inside the heat pipe is another contribution of this work. In this project, a wall heat flux partition model in the framework of two-phase Eulerian approach has been applied. The effect of the interfacial momentum and energy transfer between the two phases is also taken account. The capability of numerical model was validated by comparing numerical prediction with experimental data obtained from tests which have been conducted using a built-up prototype designed at Université de Sherbrooke. Then, the validated numerical model is used to assess the performances of this heat pipe. Finally, two types of grooves have been suggested and some experimental tests were performed to investigate any improvement in the heat pipe performance

STUDY OF NANOPARTICLE DISPERSED PHASE CHANGE MATERIALS AND THE IMPACT OF TEMPERATURE GRADIENT ON THE POTENTIAL FOR PARTICLE MIGRATION

Supercooling in phase change materials (PCMs) and the associated challenges in enhancing thermal conductivity through nanoparticle dispersion prompted this investigation. Existing literature exhibits inconsistencies in thermal conductivity improvements, suggesting a potential correlation with nanoparticle migration induced by thermophoresis. To address this, a novel temperature-dependent scaling parameter, $\xi$, was introduced to predict particle migration propensity. A strong association was observed between higher $\xi$ values and diminished thermal conductivity enhancements, indicating a significant influence of nanoparticle movement on heat transfer. To further elucidate this relationship, a Nanoparticle Interaction Parameter $N_\text{{pl}}$ was developed, incorporating critical fluid properties and interfacial effects. The derived critical Nanoparticle Interaction Parameter $N_\text{{pl}}^*$ provides a temperature-independent metric for predicting migration potential based solely on nanoparticle characteristics. This parameter offers a valuable tool for researchers to optimize experimental conditions for enhanced thermal conductivity. A computational fluid dynamics (CFD) model was employed to validate the proposed migration prediction framework. Simulations demonstrated the correlation between $\xi$ and observed particle distribution patterns in both single-phase and two-phase systems. Moreover, the impact of phase change cycling on particle dispersion was investigated, revealing the influence of thermal loading conditions on particle migration behavior. By substituting nanoparticles with nucleating clusters, the study further explored the connection between nucleation site migration and supercooling. This research offers a comprehensive understanding of nanoparticle migration in nanofluids and PCMs, providing valuable insights into the factors affecting thermal conductivity enhancement. The findings contribute to the development of effective strategies for mitigating supercooling and optimizing the performance of phase change energy storage systems

Conference Proceedings: 1st International Conference on Nanofluids (ICNf2019), 2nd European Symposium on Nanofluids (ESNf2019)

Conference proceedings of the 1st International Conference on Nanofluids (ICNf2019) and 2nd European Symposium on Nanofluids (ESNf2019), 26-28 June 2019 in Castelló (Spain), organized by Nanouptake Action (CA15119) and Universitat Jaume

Effective Thermal Conductivity of Carbon Nanotube-Based Cryogenic Nanofluids

Nanofluids consist of nanometer-sized particles or fibers in colloidal suspension within a host fluid. They have been studied extensively since their creation due to their often times anomalous and unique thermal transport characteristics. They have also proven to be quite valuable in terms of the scientific knowledge gained from their study and their nearly unlimited industrial and commercial applications. This research has expanded the science of nanofluids into a previously unexplored field, that of cryogenic nanofluids. Cryogenic nanofluids are similar to traditional nanofluids in that they utilize nanometer-sized inclusion particles; however, they use cryogenic fluids as their host liquids. Cryogenic nanofluids are of great interest due to the fact that they combine the extreme temperatures inherent to cryogenics with the customizable thermal transport properties of nanofluids, thus creating the potential for next generation cryogenic fluids with enhanced thermophysical properties. This research demonstrates that by combining liquid oxygen (LOX) with Multi-Walled Carbon Nanotube (MWCNT) inclusion particles, effective thermal conductivity enhancements of greater than 30% are possible with nanoparticle volume fractions below 0.1%. Three distinct cryogenic nanofluids were created for the purposes of this research, each of which varied by inclusion particle type. The MWCNT\u27s used in this research varied in a number of physical characteristics, the most obvious of which are length and diameter. Lengths vary from 0.5 to 90 microns and diameters from 8 to 40 nanometers. The effective thermal conductivity of the various cryogenic nanofluids created for this research were experimentally determined by a custom made Transient Hot Wire (THW) system, and compared to each other and to more traditional nanofluids as they vary by type and particle volume fraction. This work also details the extensive theoretical, experimental, and numerical aspects of this research, including a rather detailed literature review of many of the salient sciences involved in the study of cryogenic nanofluids. Finally, a selection of the leading theories, models, and predictive equations is presented along with a review of some of the potential future work in the newly budding field of cryogenic nanofluids

Molecular Dynamics Study Of Thermal Conductivity Enhancement Of Water Based Nanofluids

A systematic investigation using molecular dynamics (MD) simulation involving particle volume fraction, size, wettability and system temperature is performed and the effect of these parameters on the thermal conductivity of water based nanofluids is discussed. Nanofluids are a colloidal suspension of 10 -100 nm particles in base fluid. In the last decade, significant research has been done in nanofluids, and thermal conductivity increases in double digits were reported in the literature. This anomalous increase in thermal conductivity cannot be explained by classical theories like Maxwell\u27s model and Hamilton-Crosser model for nanoparticle suspensions. Various mechanisms responsible for thermal conductivity enhancement in nanofluids have been proposed and later refuted. MD simulation allows one to predict the static and dynamic properties of solids and liquids, and observe the interactions between solid and liquid atoms. In this work MD simulation is used to calculate the thermal conductivity of water based nanofluid and explore possible mechanisms causing the enhancement. While most recent MD simulations have considered Lennard Jones (LJ) potential to model water molecule interactions, this work uses a flexible bipolar water molecule using the Flexible 3 Center (F3C) model. This model maintains the tetrahedral structure of the water molecule and allows the bond bending and bond stretching modes, thereby tracking the motion and interactions between real water molecules. The choice of the potential for solid nanoparticle reflects the need for economic but insightful analyses and reasonable accuracy. A simple two body LJ potential is used to model the solid nanoparticle. The cross interaction between the solid and liquid atoms is also modeled by LJ potential and the Lorentz-Berthelot mixing rule is used to calculate the potential parameters. The various atomic interactions show that there exist two regimes of thermal conductivity enhancement. It is also found that increasing particle size and decreasing particle wettability cause lower thermal conductivity enhancement. In contrast to the previous studies, it is observed that increasing system temperature does not enhance thermal conductivity significantly. Such enhancement with temperature is proportional to the conductivity enhancement of base fluid with temperature. This study demonstrates that the major cause of thermal conductivity enhancement is the formation of ordered liquid layer at the solid-liquid interface. The enhanced motion of the liquid molecules in the presence of solid particles is captured by comparing the mean square displacement (MSD) of liquid molecules in the nanofluid to that of the base fluid molecules. The thermal conductivity is decomposed into three modes that make up the microscopic heat flux vector, namely kinetic, potential and collision modes. It was observed by this decomposition analyses that most of the thermal conductivity enhancement is obtained from the collision mode and not from either the kinetic or potential mode. This finding also supports the observation made by comparing the MSD of liquid molecules with the base fluid that the interaction between solid and liquid molecules is important for the enhancement in thermal transport properties in nanofluids. These findings are important for the future research in nanofluids, because they suggest that if smaller, functional nanoparticles which have higher wettability compared to the base fluid can be produced, they will provide higher thermal conductivity compared to the regular nanoparticles

Unsteady flamelet progress variable modeling of reacting diesel jets

Accurate modeling of turbulence/chemistry interactions in turbulent reacting diesel jets is critical to the development of predictive computational tools for diesel engines. The models should be able to predict the transient physical and chemical processes in the jets such as ignition and flame lift-off. In the first part of this work, an existing unsteady flamelet progress variable (UFPV) model is employed in Reynolds-averaged Navier-Stokes (RANS) simulations and large-eddy simulations (LES) to assess its accuracy. The RANS simulations predict that ignition occurs toward the leading tip of the jet, followed by ignition front propagation toward the stoichiometric surface, and flame propagation upstream along the stoichiometric surface until the flame stabilizes at the lift-off height. The LES, on the other hand, predicts ignition at multiple points in the jet, followed by flame development from the ignition kernels, merger of the different flames and then stabilization. The UFPV model assumes that combustion occurs in thin zones known as flamelets and turbulent strain characterized by the scalar dissipation rate modifies the flame structure. Since the flamelet is thinner than the smallest grid size employed in RANS or LES, the effect of the turbulence is modeled through probability distribution functions of the independent variables. The accuracy of the assumptions of the model is assessed in this work through direct numerical simulations (DNS) which resolves the flame. The DNS is carried out in turbulent mixing layers since the combustion in a diesel jet occurs in the fuel/air mixing layer surrounding the jet. ^ The DNS results show that the flamelet model is applicable but that its implementation in the UFPV model is flawed because the effects of expansion due to heat release and increase in diffusivity due to rise in temperature are not accounted for in the formulation of the scalar dissipation rate. A new diffusivity-corrected flamelet model is proposed which leads to an improved prediction of flame development. Furthermore, it is shown that the most commonly used approach to calculate the scalar dissipation rate in LES of reacting flows leads to large errors when the LES grid size is large. The DNS results are used to determine the best model for the filtered scalar dissipation rate and its PDF under diesel engine conditions. A new model is derived for the variance of the scalar dissipation rate. The DNS results are also used to compare the performance of the UFPV model with the Perfectly Stirred Reactor (PSR) model predictions. It is shown that the UFPV model performance is superior for turbulent intensities and grid sizes encountered in diesel engine application

Nanofluids based on molten nitrates for thermal energy storage and heat transfer in concentrated solar power.

385 p.El suministro de energía es un tema de vital importancia que afecta especialmente a la sociedad debido a la emisión de Gases de Efecto Invernadero (GEI) y la necesidad de reducir el uso de combustibles fósiles. Es bien conocido que estas emisiones contribuyen al cambio climático y el calentamiento global, al mismo tiempo que conducen a una seria degradación del entorno y provocan enfermedades. Además, existen otras cuestiones serias relacionadas con el uso de fuentes de energía no renovable, como la seguridad en la cadena de suministro y su disponibilidad limitada.En este contexto, la energía solar de concentración (CSP, por sus siglas en inglés) destaca como una opción muy valiosa dentro del marco de las energías renovables. Su disponibilidad es su característica principal comparada con otras energías alternativas. La energía solar no está disponible bajo demanda cuando y donde es necesaria. Como consecuencia, la mayoría de las plantas CSP cuentan con un sistema de almacenamiento térmico. Este sistema almacena la energía térmica como calor sensible, a través de dos tanques a diferentes temperaturas llenos con una sal fundida (Sal Solar, NaNO3:KNO3 60:40 %masa). El mismo material se utiliza como fluido de transferencia térmica para transportar el calor del campo solar al bloque de potencia. La madurez de esta tecnología está más que probada después de varias décadas desde que la primera planta CSP se puso en funcionamiento. Sin embargo, existen aún muchas oportunidades para desarrollar nuevos métodos de almacenamiento térmico o mejorar los que existen actualmente.Las modestas propiedades termofísicas (calor específico y conductividad térmica) están entre las principales desventajas de la Sal Solar utilizada actualmente, lo que obliga al uso de una gran cantidad de sal para poder almacenar calor durante el tiempo necesario. Varias soluciones se han propuesto, como el uso de otras sales inorgánicas dentro de complicados sistemas de almacenamiento térmico para alcanzar una tasa de transferencia de calor adecuada. Recientemente, ha emergido una opción que considera el uso de la nanotecnología. Esta solución consiste en añadir pequeñas cantidades de nanopartículas a las sales para mejorar su comportamiento térmico. Estos innovadores materiales se han denominado como nanofluidos basados en sales fundidas o materiales de cambio de fase nanomejorados, dependiendo del método empleado para almacenar la energía térmica: calor sensible o latente, respectivamente.Esta tesis analiza detalladamente el diseño, síntesis y caracterización de estos materiales. Su reciente descubrimiento, unido a las dificultades técnicas de trabajar con sales fundidas, han ocasionado que ciertas propiedades apenas se hayan estudiado. Se ha puesto especial atención en el desarrollo de un método preciso para medir el calor específico y un proceso de síntesis adecuado y escalable. La caracterización de los materiales incluye propiedades térmicas como el calor específico, la conductividad térmica, el calor latente y la temperatura de cambio de fase. También se han estudiado otras propiedades interesantes como la estabilidad de las nanopartículas en la sal fundida durante largos periodos y su comportamiento reológico.Zabalduz Tecnali