Dissertations / Theses on the topic 'Heat and mass transfer (incl. computational fluid dynamics)'

Thesis (M.S.)--West Virginia University, 2001.
Title from document title page. Document formatted into pages; contains x, 76 p. : ill. (some col.). Includes abstract. Includes bibliographical references (p. 71-72).

El entendimiento de los fenómenos de transferencia de calor y de masa en medios porosos implica el estudio de modelos de transporte de fluidos en la fracción vacía del medio; este hecho es de fundamental importancia en muchos sistemas de Ingeniería Química, tal como en procesos de extracción o en reactores catalíticos. Los estudios de flujo realizados hasta ahora (teóricos y experimentales) usualmente tratan al medio poroso como un medio efectivo y homogéneo, y toman como válidas las propiedades medias del fluido. Este tipo de aproximación no tiene en cuenta la complejidad del flujo a través del espacio vacío del medio poroso, reduciendo la descripción del problema a promedios macroscópicos y propiedades efectivas. Sin embargo, estos detalles de los procesos locales de flujo pueden llegar a ser factores importantes que influencien el comportamiento de un proceso físico determinado que ocurre dentro del sistema, y son cruciales para entender el mecanismo detallado de, por ejemplo, fenómenos como la dispersión de calor, la dispersión de masa o el transporte entre interfaces.

La Dinámica de Fluidos Computacional (CFD) como herramienta de modelado numérico permite obtener una visión mas aproximada y realista de los fenómenos de flujo de fluidos y los mecanismos de transferencia de calor y masa en lechos empacados, a través de la resolución de las ecuaciones de Navier - Stokes acopladas con los balances de materia y energía y con un modelo de turbulencia si es necesario. De esta forma, esta herramienta permite obtener los valores medios y/o fluctuantes de variables como la velocidad del fluido, la temperatura o la concentración de una especie en cualquier punto de la geometría del lecho empacado.

El objetivo de este proyecto es el de utilizar programas comerciales de simulación CFD para resolver el flujo de fluidos y la transferencia de calor y de masa en modelos bi/tri dimensionales de lechos empacados, desarrollando una estrategia de modelado aplicable al diseño de equipos para procesos de extracción o de reacción catalítica. Como referencia se tomaran procesos de tecnología supercrítica debido a la complejidad de los fenómenos de transporte involucrados en estas condiciones, así como a la disponibilidad de datos experimentales obtenidos previamente en nuestro grupo de investigación. Estos datos experimentales se utilizan como herramienta de validación de los modelos numéricos generados, y de las estrategias de simulación adoptadas y realizadas durante el desarrollo de este proyecto.
An understanding of the heat and mass transfer phenomena in a porous media implies the study of the fluid transport model within the void space; this fact is of fundamental importance to many chemical engineering systems such as packed bed extraction or catalytic reaction equipment. Experimental and theoretical studies of flow through such systems often treat the porous medium as an effectively homogeneous system and concentrate on the bulk properties of the flow. Such an approach neglects completely the complexities of the flow within the void space of the porous medium, reducing the description of the problem to macroscopic average or effective quantities. The details of this local flow process may, however, be the most important factor influencing the behavior of a given physical process occurring within the system, and are crucial to understanding the detailed mechanisms of, for example, heat and mass dispersion and interface transport.

Computational Fluid Dynamics as a simulation tool allows obtaining a more approached view of the fluid flow and heat and mass transfer mechanisms in fixed bed equipment, through the resolution of 3D Reynolds averaged transport equations, together with a turbulence model when needed. In this way, this tool permit to obtain mean and fluctuating flow and temperature values in any point of the bed.

The goal of this project is to use commercial available CFD codes for solving fluid flow and heat and mass transfer phenomena in two and three dimensional models of packed beds, developing a modeling strategy applicable to the design of packed bed chemical reaction and extraction equipment. Supercritical extraction and supercritical catalytic reaction processes will be taken as reference processes due to the complexity of the transport phenomena involved within this processes, and to the availability of experimental data in this field, obtained in the supercritical fluids research group of this university. The experimental data priory obtained by our research group will be used as validation data for the numerical models and strategies dopted and followed during the developing of the project.

In this thesis. a three dimensional heat transfer model of heated airflow through the upper human respiratory tract consisting of nasal, oral, trachea, and the first two generations of bronchi is developed using computational fluid dynamics simulation software. Various studies have been carried out in the literature investigating the heat and mass transfer characteristics in the upper human respiratory tract, and the study focuses on assessing the injury taking place in the upper human respiratory tract and identifying acute tissue damage based on level of exposure. The model considered is for the simultaneous oronasal breathing during the inspiration phase with high volumetric flow rate of 90/liters minute and a surrounding air temperature of 100 degrees centigrade. The study of the heat and mass transfer, aerosol deposition and flow characteristics in the upper human respiratory tract using computational fluid mechanics simulation requires access to a two dimensional or three dimensional model for the human respiratory tract. Depicting an exact model is a complex task since it involves the prolonged use of imaging devices on the human body. Hence a three dimensional geometric representation of the human upper respiratory tract is developed consisting of nasal cavity, oral cavity, nasopharynx, pharynx, oropharynx, trachea and first two generations of the bronchi. The respiratory tract is modeled circular in cross-section and varying diameter for various portions as identified in this study. The dimensions are referenced from the literature herein. Based on the dimensions, a simplified model representing the human upper respiratory tract is generated.This model will be useful in studying the flow characteristics and could assist in treatment of injuries to the human respiratory tract as well as help optimize drug delivery mechanism and dosages. Also a methodology is proposed to measure the characteristic dimension of the human nasal and oral cavity at the inlet/outlet points which are classified as internal measurements.

Thesis (Ph. D.)--State University of New York at Binghamton, Thomas J. Watson School of Engineeering and Applied Science, Department of Mechanical Engineering, 2009.
Includes bibliographical references.

Periciliary liquid (PCL) is a critical component of the respiratory system for maintaining mucus clearance. As PCL homeostasis is affected by evaporation and mechanical forces, which are in turn affected by various breathing conditions, lung morphology and ventilation distribution, the complex process of PCL depth regulation in vivo is not fully understood. We propose an integrative approach to couple a thermo-fluid computational fluid dynamics (CFD) model with an epithelial cell model to study the dynamics of PCL depth using subject-specific human airway models based on multi-detector row computed-tomography (MDCT) volumetric lung images. The thermo-fluid CFD model solves three-dimensional (3D) incompressible Navier-Stokes and transport equations for temperature and water vapor concentration with a realistic energy flux based boundary condition imposed at airway wall. A corresponding one-dimensional (1D) thermo-fluid CFD model is also developed to provide necessary information to the 3D model. Both 1D and 3D models are validated with experimental measurements, and the temperature and humidity distributions in the airways are investigated. Correlations for the dimensionless parameters of Nusselt number and Sherwood number are proposed for characterizing heat and mass transfer in the airways. As one of the key applications of the thermo-fluid CFD model, the water loss rates in the both 1D and 3D airway models are studied. It is found that the secondary flows formed at the bifurcations elevate the regional heat and mass transfer during inspiration and hence the water loss rate, which can only be observed in the 3D models. Among the three human airway models studied in both 1D and 3D, little inter-subject variability is observed for the distributions of temperature and humidity. However, the inter-subject variability could be dramatic for the distribution of water loss rate, as it is greatly affected by airway diameter and regional ventilation. A method is proposed to construct an ion-channel conductance model for both normal and cystic fibrosis (CF) epithelial cells, which couples an existing fluid secretion model with an existing nucleotide and nucleoside metabolism model (collectively named epithelial cell model). The epithelial cell models for both normal and CF are capable of predicting PCL depth based on mechanical stresses and evaporation, and are validated with a wide range of experimental data. With these two models separately validated and tested, the integrated model of the thermo-fluid CFD model and epithelial cell model is applied to MDCT-based human airway models of three CF subjects and three normal subjects to study and compare PCL depth regulation under regular breathing conditions. It is found that evaporative water loss is the dominant factor in PCL homeostasis. Between three types of mechanical forces, cyclic shear stress is the primary factor that triggers ATP release and increases PCL depth. In addition, it is found that that greater diameters of the airways in the 4th-7th generations in CF subjects decrease evaporative water loss, resulting in similar PCL depth as normal subjects. Under regular breathing conditions, the average PCL depths of normal and CF is around 6 to 7 µm, with mechanical forces play a greater role in regulating CF PCL depth. Comparing to 7.68 µm normal base level (considered as optimum PCL depth), this average PCL depth is about 8 to 21% lower. This might suggest that mechanical forces alone cannot entirely balance evaporative water loss, and other mechanisms might be involved.

Electrostatic Precipitators (ESPs) are widely used to capture particulate matter from flue gas. In coal-based power stations, they are used for capturing fly ash before the flue gas is released to the environment. Coal-based power plants are still one of the major suppliers of energy because they are more reliable and have lower unit cost of power generation. Under the current environmental protection regulation controlled by the Environment Protection Agency (EPA), only the finer particles can be released to the environment. However, this is likely to change and coal-based power plants will then have to face stricter rules about permissible size limits for particulate matter (PM) discharged in flue gas, namely the particle size of PM 2.5 (micron) or less. It is therefore required that the capabilities of ESPs are enhanced so that they will be able to capture these finer particles. The main aim of the research is to investigate the micro-size particulate matter capture ability of existing ESPs and determine the operational parameter relationships to improve the collection efficiency of ESPs. In particular, this research focuses specifically on the flow phenomena of the finer fly ash particles inside the ESP model and how they are impacted by the changed geometries and varied electric fields. This involves studying the flow velocity and forces associated with the flow and the electric field and the relevant parameters affecting the dust collection and thus establishing and validating a relationship between the interaction of two phase flow and electric field to reveal the underlying physics for collecting finer particles. To achieve the aim, a laboratory scale ESP was constructed for undertaking various tests and measurements using a novel method. This method involved flow measurement in the ESP chamber using a pitot tube and a cobra tube, whilst employing different shaped baffles in the chamber, varying production of electrostatic field in the ESP model and testing its capturing capacity. This research investigated the influence of internal geometry of the ESP on the flow in the ESP chamber. Two different shaped baffles – semicircular and arrow shaped - were designed, fabricated and inserted in the ESP chamber to effect changes to the flue gas pathway to enhance collection efficiency and collection capability of submicron particles. The flow measurements and experimental results were compared and validated with the 2D ii simulation results. Results using baffles indicate that internal geometry of the ESP has an influence on collection efficiency and changing the internal shape produces swirling flow inside ESP, which, in turn, improves collection efficiency. In addition, baffles increase residence time, which allows capture of sub-micron particles. A high voltage transformer and associated electrode plates and rods were designed, constructed and fitted into the model ESP for measuring and investigating particle collection efficiency under various velocities and electric/voltage characteristics. Production of electric field in a lab model ESP of this type and its testing constitutes a novel approach as such work is not found in the public domain. The experimental results show that ESP collection efficiency is higher at high voltages and at low fly ash velocity and the collection efficiency rapidly decreases when voltage reduces. A mathematical model was developed and validated with the experimental measurements to confirm the collection efficiency. By analysing the various conditions and scenarios, an optimum operational condition within an operational range were developed and recommended for future ESP operation. By implementing a TR (Transformer–Rectifier) in different collection chambers, power consumption of the ESP can be reduced. The research also revealed new information on the particulate matter size distribution and the collection of submicron particles from flue gas of coal-fired power plants. Particle size distribution analysis was conducted using a Mastersizer and the morphology of the particles was analysed using a Scanning Electron Microscope (SEM). Size distribution analysis suggested that higher voltage and lower flue gas velocity will be more suitable to capture submicron particles. The morphology study indicated that smaller particles have a tendency to agglomerate with bigger particles. Overall, this thesis provides new knowledge about Electrostatic Precipitator operation with new geometries and under various electric field conditions at a laboratory scale, whilst achieving operational efficiency improvement and improving the capture of sub-micron particulate matter. The knowledge obtained from this research would be a good basis to operate industrial ESPs for future sustainable coal-fired power generation.

Particulate matter emission is one of the major air pollution problems of coal fired power plants. Fine particulates constitute a smaller fraction by weight of the total suspended particle matter in a typical particulate emission, but they are considered potentially hazardous to health because of the high probability of deposition in deeper parts of the respiratory tract. Electrostatic precipitators (ESP) are the most widely used devices that are capable of controlling particulate emission effectively from power plants and other process industries. Although the dust collection efficiency of the industrial precipitator is reported as about 99.5%, an anticipation of future stricter environmental protection agency (EPA) regulations have led the local power station seeking new technologies to achieve the new requirements at minimum cost and thus control their fine particulate emissions to a much greater degree than ever before.

This study aims to identify the options for controlling fine particle emission through improvement of the ESP performance efficiency. An ESP system consists of flow field, electrostatic field and particle dynamics. The performance of an ESP is significantly affected by its complex flow distribution arising as a result of its complex internal geometry, hence the aerodynamic characteristics of the flow inside an ESP always need considerable attention to improve the efficiency of an ESP. Therefore, a laboratory scale ESP model, geometrically similar to an industrial ESP, was designed and fabricated at the Thermodynamics Laboratory of CQUniversity, Australia to examine the flow behaviour inside the ESP. Particle size and shape morphology analyses were conducted to reveal the properties of the fly ash particles which were used for developing numerical models of the ESP.

Numerical simulations were carried out using Computational Fluid Dynamics (CFD) code FLUENT and comparisons were made with the experimental results. The ESP was modelled in two steps. Firstly, a novel 3D fluid (air) flow was modelled considering the detailed geometrical configuration inside the ESP. A novel boundary condition was applied at the inlet boundary of this model to overcome all previous assumptions on uniform velocity at the inlet boundary. Numerically predicted velocity profiles inside the ESP model are compared with the measured data obtained from the laboratory experiment. The model with a novel boundary condition predicted the flow distribution more accurately. In the second step, as the complete ESP system consists of an electric field and a particle phase in addition to the fluid flow field, a two dimensional ESP model was developed. The electrostatic force was applied to the flow equations using User Defined Functions (UDF). A discrete phase model was incorporated with this two dimensional model to study the effect of particle size, electric field and flue gas flow on the collection efficiency of particles inside the ESP. The simulated results revealed that the collection efficiency cannot be improved by the increased electric force only unless the flow velocity is optimized.

The CFD model was successfully applied to a prototype ESP at the power plant and used to recommend options for improving the efficiency of the ESP. The aerodynamic behaviour of the flow was improved by geometrical modifications in the existing 3D numerical model. In particular, the simulation was performed to improve and optimize the flow in order to achieve uniform flow and to increase particle collection inside the ESP. The particles injected in the improved flow condition were collected with higher efficiency after increasing the electrostatic force inside the 2D model. The approach adopted in this study to optimize flow and electrostatic field properties is a novel approach for improving the performance of an electrostatic precipitator.

The thermal efficiency of gas turbines increases with the temperature of the gas entering its turbine component. To enable high inlet temperatures, even those that far exceed the melting point of the turbine materials, the turbine must be cooled. One way is by internal cooling, where cooler air passes through U-ducts embedded inside turbine vanes and blades. Since the flow and heat transfer in these ducts are highly complicated, computational fluid dynamics (CFD) based on RANS have been used extensively to explore and assess design concepts. However, RANS have been found to be unreliable – giving accurate results for some designs but not for others. In this study, large-eddy simulations (LES) were performed for a U-duct with a trapezoidal cross section to assess four widely used RANS turbulence models: realizable k-ε (k-ε), shear-stress transport (SST), Reynolds stress model with linear pressure strain (RSM-LPS), and the seven-equation stress-omega full Reynolds stress model (RSM).

Heat transfer and solidification models were developed for use in a numerical model of a continuous caster to provide a means of predicting how the developing shell would react under variable operating conditions. Measurement data of the operating conditions leading up to a breakout occurrence were provided by an industrial collaborator and were used to define the model boundary conditions. Steady-state and transient simulations were conducted, using boundary conditions defined from time-averaged measurement data. The predicted shell profiles demonstrated good agreement with thickness measurements of a breakout shell segment – recovered from the quarter-width location. Further examination of the results with measurement data suggests pseudo-steady assumption may be inadequate for modeling shell and flow field transition period following sudden changes in casting speed. An adaptive mesh refinement procedure was established to increase refinement in areas of predicted shell growth and to remove excess refinement from regions containing only liquid. A control algorithm was developed and employed to automate the refinement procedure in a proof-of-concept simulation. The use of adaptive mesh refinement was found to decrease the total simulation time by approximately 11% from the control simulation – using a static mesh.

Despite the central importance of porous electrodes to any advanced electrochemical system, there is no clear answer to “How to make the best electrode?”. The source of ambiguity lies in the incomplete understanding of convoluted material interactions at smaller – difficult to observe length and timescales. Such mesoscopic interactions, however, abide by the fundamental physical principles such as mass conservation. The porous electrodes are investigated in such a physics-based setting to comprehend the interplay among structural arrangement and off-equilibrium processes. As a result, a synergistic approach exploiting the complementary characteristics of controlled experiments and theoretical analysis emerges to allow mechanistic insights into the associated mesoscopic phenomena. The potential of this philosophy is presented by investigating three distinct electrochemical systems with their unique peculiarities.

The main accomplishments of this research are

(1) developed a high fidelity computational methodology based on large eddy simulation to capture lean blowout (LBO) behaviors of different fuels;

(2) developed fundamental insights into the combustion processes leading to the flame blowout and fuel composition effects on the lean blowout limits;

(3) developed artificial intelligence-based models for early detection of the onset of the lean blowout in a realistic complex combustor.

The methodologies are demonstrated by performing the lean blowout (LBO) calculations and statistical analysis for a conventional (A-2) and an alternative bio-jet fuel (C-1).

High-performance computing methodology is developed based on the large eddy simulation (LES) turbulence models, detailed chemistry and flamelet based combustion models. This methodology is employed for predicting the combustion characteristics of the conventional fuels and bio-derived alternative jet fuels in a realistic gas turbine engine. The uniqueness of this methodology is the inclusion of as-it-is combustor hardware details such as complex hybrid-airblast fuel injector, thousands of tiny effusion holes, primary and secondary dilution holes on the liners, and the use of highly automated on the fly meshing with adaptive mesh refinement. The flow split and mesh sensitivity study are performed under non-reacting conditions. The reacting LES simulations are performed with two combustion models (finite rate chemistry and flamelet generated manifold models) and four different chemical kinetic mechanisms. The reacting spray characteristics and flame shape are compared with the experiment at the near lean blowout stable condition for both the combustion models. The LES simulations are performed by a gradual reduction in the fuel flow rate in a stepwise manner until a lean blowout is reached. The computational methodology has predicted the fuel sensitivity to lean blowout accurately with correct trends between the conventional and alternative bio-jet fuels. The flamelet generated manifold (FGM) model showed 60% reduction in the computational time compared to the finite rate chemistry model.

The statistical analyses of the results from the high fidelity LES simulations are performed to gain fundamental insights into the LBO process and identify the key markers to predict the incipient LBO condition in swirl-stabilized spray combustion. The bio-jet fuel (C-1) exhibits significantly larger CH₂O concentrations in the fuel-rich regions compared to the conventional petroleum fuel (A-2) at the same equivalence ratio. It is observed from the analysis that the concentration of formaldehyde increases significantly in the primary zone indicating partial oxidation as we approach the LBO limit. The analysis also showed that the temperature of the recirculating hot gases is also an important parameter for maintaining a stable flame. If this temperature falls below a certain threshold value for a given fuel, the evaporation rates and heat release rated decreases significantly and consequently leading to the global extinction phenomena called lean blowout. The present study established the minimum recirculating gas temperature needed to maintain a stable flame for the A-2 and C-1 fuels.

The artificial intelligence (AI) models are developed based on high fidelity LES data for early identification of the incipient LBO condition in a realistic gas turbine combustor under engine relevant conditions. The first approach is based on the sensor-based monitoring at the optimal probe locations within a realistic gas turbine engine combustor for quantities of interest using the Support Vector Machine (SVM). Optimal sensor locations are found to be in the flame root region and were effective in detecting the onset of LBO ~20ms ahead of the event. The second approach is based on the spatiotemporal features in the primary zone of the combustor. A convolutional autoencoder is trained for feature extraction from the mass fraction of the OH ( data for all time-steps resulting in significant dimensionality reduction. The extracted features along with the ground truth labels are used to train the support vector machine (SVM) model for binary classification. The LBO indicator is defined as the output of the SVM model, 1 for unstable and 0 for stable. The LBO indicator stabilized to the value of 1 approximately 30 ms before complete blowout.

Αντικείμενο της διατριβής είναι η υπολογιστική και πειραματική διερεύνηση των φαι-νομένων μεταφοράς θερμότητας και μάζας σε πρότυπη εργαστηριακή μονάδα μηχανικής ξήρανσης δια συναγωγής, η οποία σχεδιάσθηκε, κατασκευάσθηκε και εξοπλίσθηκε με μετρητικό εξοπλισμό και ολοκληρωμένο σύστημα ελέγχου, συλλογής και επεξεργασίας των πειραματικών μετρήσεων. Στην εργαστηριακή μονάδα ξήρανσης παρέχεται η δυνατότητα μεταβολής και ελέγχου των βασικών παραμέτρων από τις οποίες επηρεάζεται η θερμική διεργασία της ξήρανσης, όπως η ταχύτητα, η θερμοκρασία και η υγρασία του αέρα ξήρανσης. Η μέτρηση της αποβολής της περιεχομένης υγρασίας στο υπό ξήρανση προϊόν πραγματοποιείται μέσω δυναμοκυψελών υψηλής ακρίβειας, ενώ η χωρική κατανομή της ταχύτητας ροής στην είσοδο του θαλάμου ξήρανσης κατά τη διεξαγωγή κάθε κύκλου πειραμάτων συνεχώς μετρείται με συστοιχία σωλήνων pitot και ενός συστήματος συγκροτούμενου από ηλεκτροβαλβίδες και μορφομετατροπέα πίεσης. Η χωρική κατανομή της θερμοκρασίας και της ταχύτητας στον θάλαμο ξήρανσης είναι δυνατή μέσω αισθητηρίων που προσαρμόζονται σε καρτεσιανό σύστημα μετακίνησης ελεγχόμενου από υπολογιστή το οποίο σχεδιάσθηκε, κατασκευάσθηκε και τοποθετήθηκε στην έξοδο του κατακόρυφου θαλάμου ξήρανσης αποτελώντας πλέον αναπόσπαστο τμήμα της εργαστηριακής μονάδας. Όλες οι μετρήσεις πραγματοποιήθηκαν στον κατακόρυφο θάλαμο ξήρανσης, ενώ παρέχεται η δυνατότητα διεξαγωγής μετρήσεων και στο θάλαμο ξήρανσης οριζόντιας διάταξης. Στα πλαίσια της διατριβής, έγινε συστηματική πειραματική διερεύνηση της ξήρανσης δια συναγωγής σε φέτες κυδωνιών και μελετήθηκε η επίδραση διαφόρων παραμέτρων που επηρεάζουν τη θερμική διεργασία της ξήρανσης σε αυτό το αγροτικό προϊόν, για θερμοκρασίες αέρα 40, 50 και 60οC και ταχύτητες αέρα 1, 2 και 3 m/s. Σκοπός των με-τρήσεων ήταν ο προσδιορισμός: (i) της επίδρασης της θερμοκρασίας και της ταχύτητας του αέρα στις καμπύλες ξήρανσης κυλινδρικών φετών κυδωνιού, (ii) της επίδρασης του πάχους των κυλινδρικών φετών του κυδωνιού στις καμπύλες ξήρανσης, (iii) της επίδρασης του προσανατολισμού των κυλινδρικών φετών κυδωνιού, ως προς τη διεύθυνση της προσπίπτουσας ροής, στις καμπύλες ξήρανσης, (iv) της προσαρμογής των καμπύλων ξήρανσης σε διάφορα απλά μοντέλα ξήρανσης λεπτού στρώματος, v) των ενεργών συντελεστών διάχυσης υγρασίας για την κάθε περίπτωση με την μέθοδο της κλίσης (slope method) και οι οποίοι συσχετίστηκαν με τη θερμοκρασία του αέρα ξήρανσης έτσι ώστε ο συντελεστής διάχυσης της υγρασίας να εκφρασθεί με την εξίσωση μορφής τύπου Arrhenius και vi) των διεπιφανειακών συντελεστών μεταφοράς θερμότητας και μάζας οι οποίοι στη εκφράζονται ως συνάρτηση των αδιάστατων αριθμών Nu, Re και Pr με τη μορφή Nu=aRebPr1/3. Η προσομοίωση του ρευστοθερμικού πεδίου στο θάλαμο ξήρανσης και ο υπολογισμός των διεπιφανειακών συντελεστών μεταφοράς θερμότητας και μάζας γύρω από την επι-φάνειας του προϊόντος πραγματοποιήθηκε με χρήση της εργαλείων της υπολογιστικής ρευστοδυναμικής (CFD). Έγιναν προσομοιώσεις CFD μόνιμης κατάστασης (steady-state), θεωρώντας τυρβώδη ροή ενώ ο θάλαμος ξήρανσης και η κυλινδρική φέτα του κυδωνιού εξιδανικεύθηκε ως μια δισδιάστατη αξονοσυμμετρική διαμόρφωση. Ως μοντέλο τύρβης χρησιμοποιήθηκε το μοντέλο SST (Shear Stress Transport) k-ω, ενώ για την προσέγγιση του οριακού στρώματος στα τοιχώματα του προϊόντος επιλέχθηκε το μοντέλο LRNM (Low Reynolds Number Model). Από την επίλυση των πεδίων ροής και θερμοκρασίας προσδιορίσθηκαν οι κατανομές των διεπιφανειακών συντελεστών στην προσήνεμη και στην υπήνεμη πλευρά της κυλινδρικής φέτας του κυδωνιού για όλες τις πειραματικές συνθήκες. Από τον υπολογισμό του μέσων σταθμισμένων τιμών του διεπιφανειακού συντελεστή μεταφοράς θερμότητας συνάγεται μια συσχέτιση των αδιάστατων αριθμών Nu, Re και Pr, στη μορφή Nu=aRebPr1/3, που ως εύρημα εμπλουτίζει την υφιστάμενη βιβλιογραφία. Στο τελικό στάδιο της διατριβής, αναπτύχθηκε και αποτιμήθηκε σε σύγκριση με τις πειραματικές μετρήσεις ένα μονοδιάστατο αριθμητικό μοντέλο μη-μόνιμης μεταφοράς θερμότητας και μάζας για την προσομοίωση των καμπυλών ξήρανσης σε κυλινδρικές φέτες κυδωνιών. Στο μοντέλο, η μεταφορά θερμότητας εντός του προϊόντος γίνεται με αγωγή ενώ η μεταφορά μάζας γίνεται με υγρή διάχυση, με την εξάτμιση του περιεχόμενου νερού στις φέτες του κυδωνιών να λαμβάνει χώρα από την προσήνεμη και την υπήνεμη επιφάνεια. Στο αριθμητικό μοντέλο, λαμβάνεται υπόψη η συρρίκνωση της κυλινδρικής φέτας του κυδωνιού, θεωρώντας ότι ο όγκος της προϊόντος μειώνεται κάθε φορά κατά τον όγκο του νερού που εξατμίζεται τις δύο επιφάνειες της φέτας. Στον αριθμητικό κώδικα, οι θερμοφυσικές ιδιότητες του κυδωνιού και του αέρα προσδιορίζονται από σχέσεις που συναντώνται στη βιβλιογραφία, ο ενεργός συντελεστής διάχυσης της υγρασίας εισάγεται ως αποτέλεσμα της επεξεργασίας των πειραματικών μετρήσεων, ενώ για τους συντελεστές μεταφοράς χρησιμοποιήθηκαν οι μέσες σταθμισμένες τιμές των διεπιφανειακών συντελεστών μεταφοράς θερμότητας και μάζας, ως αποτέλεσμα των CFD προσομοιώσεων και για περίπτωση μη-συζυγούς προσέγγισης (non-conjugated approach). Στοχεύοντας στην καλύτερη προσαρμογή των πειραματικών μετρήσεων και των υπολογιστικών αποτελεσμάτων, χρησιμοποιήθηκε η ανάλυση μη-γραμμικής παλινδρόμησης, με τους αλγόριθμους SQP (Sequential Quadratic Programming) και εσωτερικού σημείου (internal point), για τον προσδιορισμό των συντελεστών της εξίσωσης διάχυσης της υγρασίας, με μικρή όμως βελτίωση των υπολογιστικών αποτελεσμάτων, σε συνδυασμό με τη σημαντική αύξηση του χρόνου υπολογισμού. Συμπερασματικά, από τη συνολική αξιολόγηση των αποτελεσμάτων του αριθμητικού κώδικα αποδείχθηκε ότι το προτεινόμενο αριθμητικό μοντέλο που βασίζεται στη διάχυση είναι ικανό να περιγράψει αποτελεσματικά τη σύζευξη της μεταφοράς της θερμότητας και της μάζας όπως και να αποτυπώσει ικανοποιητικά τη χρονική εξέλιξη της περιεχόμενης υγρασίας και θερμοκρασίας εντός του προϊόντος, με την ελάχιστη χρήση πειραματικών μεταβλητών εισόδου ενώ έχει ελάχιστες υπολογιστικές απαιτήσεις. Για αυτούς τους λόγους μπορεί να θεωρηθεί κατάλληλο για την ανάλυση της διαδικασίας της ξήρανσης δια συναγωγής σε οποιοδήποτε οργανικό ή μη-οργανικό προϊόν.
The objective subject of this thesis is the computational and experimental investigation of heat and mass transfer phenomena in a new laboratory mechanical convection drying unit, which was designed, constructed and equipped with measuring equipment and an integrated control system of collection and processing of experimental measurements. In laboratory drying unit there is an option to change and control the main parameters of which affected the thermal drying process, such as speed, temperature and humidity of the drying air. Measurement of the removal of moisture content in the dried product is carried out through high-precision load cells, and the spatial distribution of the flow velocity at the entrance of the drying chamber during of each experiment, is continuously measured by pitot tube array and a system composed of solenoids and a pressure transducer. The spatial distribution of temperature and velocity in the drying chamber is possible by means of sensors fitted to a computer controlled cartesian motion system which is designed, constructed and placed at the outlet of the vertical drying chamber, constituting an integral part of the facility. All measurements were performed on the vertical drying chamber while it is possible to conduct measurements in a horizontal layout of the drying chamber. In this thesis became systematic experimental investigation of convective drying sliced quince and studied the effect of various parameters affecting the thermal drying process in this agricultural product, for air temperatures of 40, 50 and 60°C and air velocities 1, 2 and 3 m/s. The purpose of the measurements was to determine: (i) the effect of temperature and air velocity in drying curves of cylindrical quince slice, (ii) the effect of the thickness of the cylindrical slice of quince in drying curves, (iii) the effect of the orientation of the cylindrical quince slice, in the direction of incident flow, in the drying curves (iv) the adjusting of the drying curves in several simple thin layer drying models v) the effective moisture diffusivity coefficients for each case with the slope method which correlated with the temperature of the drying air so that the diffusion coefficient of moisture be expressed by Arrhenius type equation form and vi ) the interfacial heat and mass transfer coefficients which expressed as a function of dimensionless numbers Nu, Re and Pr in the form Nu = aRebPr1/3. The simulation of the flow and temperature fields in the drying chamber and the calcu-lation of the interfacial heat and mass transfer coefficients around the surface of the product were performed using the tools of Computational Fluid Dynamics (CFD). CFD simulations were steady state, considering turbulent flow while drying chamber and cy-lindrical slice of quince specialized as an axisymmetric two-dimensional configuration. As turbulence model was used the SST k-ω model while on the approximation of the boundary layer near the walls of the product the LRNM was chosen. By solving the flow and temperature fields determined distributions of interfacial heat and mass transfer coefficients in front and rear of the cylindrical slice of quince for all experimental conditions. The calculation of the weighted average prices of the interfacial heat transfer coefficient indicates a correlation between dimensionless numbers Nu, Re and Pr, in the form Nu = aRebPr1/3, which as finding enriches the existing literature. In the final stage of the thesis, developed and evaluated in comparison with the experi-mental measurements, a one-dimensional transient numerical model of heat and mass transfer to simulate drying curves in cylindrical slices of quince. The heat transfer inside the quince is considered to be by conduction while the moisture transfer is considered to be governed solely by liquid diffusion. Evaporation is considered to take place only from the windward and leeward surface of the quince slice. The numerical model takes into account the shrinkage of the cylindrical slice of quince, assuming that the cylindrical volume decreases each time as much as the volume of water that evaporates on both surfaces of the slice. The numerical code used the thermophysical properties of quince and air from the literature, the effective diffusion coefficient of moisture experimentally determined by the method of the slopes, while the transfer coefficients used the weighted average prices of interfacial heat and mass transfer coefficients derived from the simulations with CFD (non-conjugated approach). In order to achieve higher accuracy between experimental data and predictions, a non-linear regression analysis, using an Arrhenius type effective diffusion equation, was also performed. However, preliminary result, obtained using the SQP (Sequential Quadratic Programming) and Interior Point algorithms for the minimization of the Chi-square function (χ2) showed only small improvement of the calculated results with a significant increase of the computational cost. In conclusion, the overall assessment of the results of the numeric code shown that the proposed numerical model based on diffusion is able to effectively describe the coupling of heat transfer and mass, as to capture the time evolution of moisture content and temperature within the product, with minimum use of experimental input variables and minimum computational requirements. For these reasons it may be considered appropriate to analyze the convective drying process in any organic or non-organic product.