A numerical study of fundamental aspects of turbulent mixing has been performed,with emphasis on the behavior of passive scalars of low molecular diffusivity (high Schmidt number Sc). Direct Numerical Simulation is used to simulate incompressible, stationary and isotropic turbulence carried out at high grid resolution. Data analyses are carried out by separate parallel codes using up to 1024^3 grid points for Taylor-scale Reynolds number (R_lambda) up to 390 and Sc up to 1024.Schmidt number of order 1000 is simulated using a double-precision parallel code in a turbulent flow at a low Reynolds number of R_lambda 8 to reduce computational cost to achievable level. The results on the scalar spectrum at high Schmidt numbers appear to have a k^{-1} scaling range. In the presence of a uniform mean scalar gradient, statistics of scalar gradients are observed to deviate substantially from Kolmogorov's hypothesis of local isotropy, with a skewness factor remaining at order unity as the Reynolds number increases. However, this skewness decreases with Schmidt number suggesting that local isotropy for scalars at high Schmidt number is a better approximation. Intermittency exponents manifested by three types of two-point statistics of energy and scalar dissipation, i.e., the two-point correlator (chi(x)chi(x+r)), the second-order moment of local scalar dissipation (chi_r^2) and the variance of the logarithmic local scalar dissipation sigma^2_{lnchi_r} are discussed. Several basic issues in differential diffusion between two scalars of different molecular diffusivities transported by the same turbule nt flow, the physical process of scalar spectral transfer and subgrid-scale transfer are also briefly addressed.

High explosive charges when detonated ensue in a flow field characterized by several physical phenomena that include blast wave propagation, hydrodynamic instabilities, real gas effects, fluid mixing and afterburn effects. Solid metal particles are often added to explosives to augment the total impulsive loading, either through direct bombardment if inert, or through afterburn energy release if reactive. These multiphase explosive charges, termed as heterogeneous explosives, are of interest from a scientific perspective as they involve the confluence and interplay of various additional physical phenomena such as shock-particle interaction, particle dispersion, ignition, and inter-phase mass, momentum and energy transfer. In the current research effort, chemical explosions in multiphase environments are investigated using a robust, state-of-the-art Eulerian-gas, Lagrangian-solid methodology that can handle both the dense and dilute particle regimes. Explosions into ambient air as well as into aluminum particle clouds are investigated, and hydrodynamic instabilities such as Rayleigh- Taylor and Richtmyer-Meshkov result in a mixing layer where the detonation products mix with the air and afterburn. The particles in the ambient cloud, when present, are observed to pick up significant amounts of momentum and heat from the gas, and thereafter disperse, ignite and burn. The amount of mixing and afterburn are observed to be independent of particle size, but dependent on the particle mass loading and cloud dimensions. Due to fast response times, small particles are observed to cluster as they interact with the vortex rings in the mixing layer, which leads to their preferential ignition/ combustion. The total deliverable impulsive loading from heterogeneous explosive charges containing inert steel particles is estimated for a suite of operating parameters and compared, and it is demonstrated that heterogeneous explosive charges deliver a higher near-field impulse than homogeneous explosive charges containing the same mass of the high explosive. Furthermore, particles are observed to introduce significant amounts of hydrodynamic instabilities in the mixing layer, resulting in augmented fluctuation intensities and fireball size, and different growth rates for heterogeneous explosions compared to homogeneous explosions. For aluminized explosions, the particles are observed to burn in two regimes, and the average particle velocities at late times are observed to be independent of the initial solid volume fraction in the explosive charge. Overall, this thesis provides useful insights on the role played by solid particles in chemical explosions.

The goal of the work described in this report was the experimental investigation of the mixing of coolant with different quality (temperature, boron concentration) in nuclear reactors on the way from the cold leg through the downcomer and lower plenum to the core inlet in a systematic way. The obtained data were used for the clarification of the mixing mechanisms and form a data basis for the validation of computational fluid dynamics (CFD) codes. For these purposes, experiments on slug mixing have been performed at two test facilities, modelling different reactor types in scale 1:5, the Rossendorf and Vattenfall test facilities. The corresponding accident scenario is the start-up of first main coolant pump (MCP) after formation of a slug of lower borated water during the reflux-condenser mode phase of a small break loss of coolant accident (LOCA). The matrices for the experiments were elaborated on the basis of the key phenomena, being responsible for the coolant mixing during pump start-up. Slug mixing tests have also been performed at the VVER-1000 facility of EDO Gidropress to meet the specifics of this reactor type. The mixing of slugs of water of different quality is also very important for pre-stressed thermal shock (PTS) situations. In emergency core cooling (ECC) situations after a LOCA, cold ECC water is injected into the hot water in the cold leg and downcomer. Due to the large temperature differences, thermal shocks are induced at the reactor pressure vessel (RPV) wall. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses. One of the important phenomena in connection with PTS is thermal stratification, a flow condition with a vertical temperature profile in a horizontal pipe. Due to the fluctuating character of the flow, this may cause thermal fatigue in the pipe. Besides of thermal fatigue, a single thermal shock can also be relevant for structural integrity, if it is large enough, especially in the case, that the brittle fracture temperature of the RPV material is reduced due to radiation embrittlement. Therefore, additional to the investigations of slug mixing during re-start of coolant circulation, the mixing of slugs or streams of water with higher density with the ambient fluid in the RPV was investigated. The aim of these investigations was to study the process of turbulent mixing under the influence of buoyancy forces caused by the temperature differences. Heat transfer to the wall and thermal conductivity in the wall material have not been considered. Experiments on density driven mixing were carried out at the Rossendorf and the Fortum PTS facilities.

Οι ροές φλεβών άνωσης παρουσιάζουν μεγάλο ενδιαφέρον στην περιβαλλοντική υδραυλική και στη μηχανική των ρευστών, επειδή εμφανίζονται σε αρκετά φαινόμενα που σχετίζονται με τη διάθεση υγρών αποβλήτων ή θερμικών απορρίψεων σε υδάτινους αποδέκτες καθώς επίσης και την εκπομπή αερίων ενώσεων από καμινάδες στην ατμόσφαιρα. Στην παρούσα Διδακτορική Διατριβή μελετήθηκε η ροή κεκλιμένης δισδιάστατης ή κυκλικής ανωστικής φλέβας εντός ακίνητου ή κινούμενου αποδέκτη και η αλληλεπίδραση μεταξύ ανωστικών φλεβών. Στα υπάρχοντα μαθηματικά μοντέλα, ο υπολογισμός του πεδίου ροής και διάχυσης μιας κεκλιμένης ανωστικής φλέβας πραγματοποιείται επιλύοντας το σύστημα των διαφορικών εξισώσεων της συνεχείας, της ορμής και της διατήρησης της μάζας του ιχνηθέτη, για τη μέση ροή, σε ένα καρτεσιανό ή κυλινδρικό σύστημα συντεταγμένων. Με αυτό τον τρόπο όμως, παραλείπονται όροι από τις εξισώσεις που προσδίδουν μεγαλύτερη ακρίβεια. Ακόμη, το φαινόμενο της αποκόλλησης μαζών από την ανωστική φλέβα έχει μεν παρατηρηθεί σε πειραματικές εργασίες διαφόρων ερευνητών αλλά δεν έχει προσδιοριστεί ποσοτικά. Μία ακόμη αδυναμία που χαρακτηρίζει τα υπάρχοντα μαθηματικά μοντέλα, είναι ότι θεωρούν τον αποδέκτη απεριορίστων διαστάσεων με συνέπεια τα αποτελέσματά τους να μην ακολουθούν τα αντίστοιχα πειραματικά αποτελέσματα, τα οποία φέρουν την επιρροή των ορίων, αφού πρακτικά δεν γίνεται να πραγματοποιηθούν σε αποδέκτες απεριορίστων διαστάσεων. Η παρούσα Διατριβή επιχειρεί να συμβάλλει στη βελτίωση των παραπάνω αδυναμιών, προτείνοντας τη μαθηματική περιγραφή της ελεύθερης κεκλιμένης τυρβώδους ανωστικής φλέβας σε ένα καμπυλόγραμμο ορθογώνιο ή κυλινδρικό σύστημα συντεταγμένων, ώστε να επιτευχθεί καλύτερη ακρίβεια στον υπολογισμό των μέσων χαρακτηριστικών της ροής. Επίσης, αναπτύσσεται ένα μαθηματικό μοντέλο, το οποίο προσομοιώνει τη διαφυγή των μαζών που αποκολλώνται από το σώμα της ανωστικής φλέβας, και όπως φαίνεται, τα δύο αυτά στοιχεία επηρεάζουν τόσο την τροχιά της φλέβας όσο και την αραίωση. Στο σημείο αυτό, η παρούσα Διατριβή χρησιμοποιεί τη μέθοδο της συμμετρικής εικονικής πηγής για την απόκτηση λύσης του προβλήματος σε ημίχωρο. Θεωρείται, λοιπόν, κατοπτρικά ως προς το όριο όμοια εικονική πηγή με την πραγματική, ανωστική φλέβα, η οποία αλληλεπιδρά δυναμικά με την ανωστική φλέβα της πραγματικής πηγής. Τέλος, τα ανωτέρω συνδυάζονται στην ανάπτυξη ενός μοντέλου, το οποίο υπολογίζει τα χαρακτηριστικά του πεδίου μέσης ροής και διάχυσης από την αλληλεπίδραση N κατακορύφων ανωστικών φλεβών σε διάταξη τύπου ροζέτας εντός ήρεμου αποδέκτη. Το μοντέλο αυτό παρέχει ακρίβεια 2ης τάξης. Η παρούσα διατριβή αναπτύσσεται σε έξι κεφάλαια. Στο πρώτο Κεφάλαιο, πραγματοποιείται μια εισαγωγή στο αντικείμενο της Διατριβής. Αναφέρονται τα πεδία στα οποία συναντώνται φαινόμενα ροών από ανωστικές φλέβες και η πρακτική σημασία τους. Επίσης, αναφέρονται οι στόχοι οι οποίοι επιδιώκονται μέσω της Διατριβής. Το δεύτερο Κεφάλαιο, περιέχει βιβλιογραφική ανασκόπηση, στην οποία αναφέρονται τα κυριότερα μαθηματικά μοντέλα για τις περιπτώσεις των ελευθέρων και των πολλαπλών κυκλικών ή δισδιάστατων ανωστικών φλεβών. Επίσης, παρουσιάζονται οι σημαντικότερες διαθέσιμες ερευνητικές εργασίες διεξαχθέντων συναφών πειραμάτων, των οποίων οι μετρήσεις χρησιμοποιούνται στον έλεγχο των αποτελεσμάτων, τα οποία προκύπτουν από την εφαρμογή των μαθηματικών μοντέλων που αναπτύσσονται στην παρούσα Διατριβή. Στο τρίτο Κεφάλαιο, αναπτύσσονται τα υποστηρικτικά μοντέλα, τα οποία ενσωματώνονται στο γενικότερο μαθηματικό μοντέλο υπολογισμού των κεκλιμένων ανωστικών φλεβών και στο μοντέλο της αλληλεπίδρασής τους. Αρχικώς, κατασκευάζεται το σύστημα το καμπυλογράμμων ορθογωνίων και κυλινδρικών συντεταγμένων και διατυπώνονται οι εξισώσεις της συνεχείας, της ορμής και της διατήρησης της μάζας του ιχνηθέτη για τη μέση ροή στο αντίστοιχο σύστημα. Στην συνέχεια, περιγράφεται η ανάπτυξη του μοντέλου για τον πυρήνα της Ζώνης Εγκατάστασης της Ροής, το οποίο υπολογίζει τις πραγματικές εγκάρσιες κατανομές των ταχυτήτων και των συγκεντρώσεων ξεκινώντας από την έξοδο του ακροφυσίου έως το πέρας του πυρήνα. Τέλος, αναπτύσσεται το μαθηματικό μοντέλο, το οποίο περιγράφει τη διαφυγή των μαζών που αποκολλώνται από το πεδίο ροής μιας κεκλιμένης δισδιάστατης ή κυκλικής τυρβώδους ανωστικής φλέβας. Στο τέταρτο Κεφάλαιο, παρουσιάζεται ένα ολοκληρωμένο μαθηματικό μοντέλο για μια κεκλιμένη δισδιάστατη ή κυκλική τυρβώδη ανωστική φλέβα σε ήρεμο ή κινούμενο αποδέκτη, το οποίο ενσωματώνει τα επί μέρους μοντέλα του τρίτου Κεφαλαίου. Το μοντέλο, συγκρίνεται με τα διαθέσιμα πειραματικά δεδομένα της διεθνούς βιβλιογραφίας και ταυτοχρόνως ρυθμίζεται η προσομοίωση της διαφυγής των μαζών που αποκολλώνται. Στο πέμπτο Κεφάλαιο εξετάζεται η αλληλεπίδραση μεταξύ τυρβωδών ανωστικών φλεβών. Στο πρώτο μέρος του Κεφαλαίου προτείνεται ένα μοντέλο 2ης τάξης, το οποίο υπολογίζει τα χαρακτηριστικά του μέσου πεδίου ροής και διάχυσης από την αλληλεπίδραση Ν κατακορύφων κυκλικών ανωστικών φλεβών από διαχύτη τύπου ροζέτας. Στο δεύτερο μέρος, μελετάται η δυναμική αλληλεπίδραση μεταξύ ανωστικών φλεβών. Αναπτύσσεται το σύστημα των εξισώσεων, το οποίο ενσωματώνει τη δυναμική αλληλεπίδραση, και αντιμετωπίζεται η ύπαρξη στερεών ορίων στο πεδίο ροής ανωστικής φλέβας μέσω της αλληλεπίδρασης των ανωστικών φλεβών από την πραγματική και τη συμμετρική εικονική της πηγή. Στο έκτο Κεφάλαιο, παρουσιάζονται τα συμπεράσματα, τα οποία προκύπτουν από την εφαρμογή των διαφόρων μαθηματικών μοντέλων της Διατριβής, τα οποία εν συντομία, είναι τα εξής: • Αναπτύσσεται ένα μαθηματικό μοντέλο, το οποίο υπολογίζει με πολύ μεγάλη ακρίβεια την κεκλιμένη δισδιάστατη ή κυκλική τυρβώδη ανωστική φλέβα, για αρχικές γωνίες κλίσης -75° ≤ θ0 ≤ 90°. • Η προσομοίωση της διαφυγής των μαζών, που αποκολλώνται από την ανωστική φλέβα, προσεγγίζει ακριβέστερα την τροχιά των κεκλιμένων φλεβών, όπως αυτή προσδιορίζεται από τις πειραματικές μετρήσεις. • Λόγω έλλειψης πειραματικών δεδομένων για την περίπτωση της κεκλιμένης δισδιάστατης τυρβώδους ανωστικής φλέβας, ο συντελεστής Λ, που χαρακτηρίζει τις αποκολλήσεις, λαμβάνεται ίσος με 0,06, για την περίπτωση όπου θ0 = 0°. • Για την κεκλιμένη κυκλική τυρβώδη ανωστική φλέβα, προτείνεται Λ = 0,34 για -75° ≤ θ0 ≤ -15° και Λ = 0,00 για -15° < θ0 ≤ 90°, δηλώνοντας ότι οι διαφυγές σε αυτές τις γωνίες είναι αμελητέες. • Προβλέπεται με χαρακτηριστική ακρίβεια το εξωτερικό όριο των κεκλιμένων κυκλικών τυρβωδών ανωστικών φλεβών και με ικανοποιητική ακρίβεια η τροχιά, συγκριτικά με τα διαθέσιμα αποτελέσματα των αντιστοίχων πειραμάτων. Η σύγκριση του εσωτερικού ορίου εξαιτίας της ύπαρξης των διαφυγών είναι δυσδιάκριτη και πιθανόν να εμπεριέχει σφάλματα. • Ο υπολογισμός των χαρακτηριστικών της ροής, από την αλληλεπίδραση N κατακορύφων κυκλικών τυρβωδών ανωστικών φλεβών από διαχύτη τύπου ροζέτας εντός ήρεμου αποδέκτη, δίνουν ικανοποιητικά αποτελέσματα συγκρινόμενα με τα υπάρχοντα πειραματικά και θεωρητικά δεδομένα • Η αλληλεπίδραση Ν ομοίων ή διαφορετικών δισδιάστατων ή κυκλικών τυρβωδών ανωστικών φλεβών αντιμετωπίζεται χρησιμοποιώντας τη θεωρία του δυναμικού πεδίου. • Η αντιμετώπιση των εξωτερικών ορίων, χρησιμοποιώντας τη δυναμική αλληλεπίδραση μεταξύ ανωστικών φλεβών δίνει ενθαρρυντικά αποτελέσματα. Τέλος, στα Παραρτήματα περιγράφεται αναλυτικώς η μαθηματική ανάπτυξη του κάθε μοντέλου και δίνονται σε μορφή διαγραμμάτων τα θεωρητικά αποτελέσματα με τα αντίστοιχα πειραματικά δεδομένα για τα διάφορα χαρακτηριστικά των κεκλιμένων κυκλικών ανωστικών φλεβών. / In this Doctoral Thesis a mathematical model that predicts the mean flow and mixing parameters of inclined plane and round turbulent jets in a stationary or moving uniform fluid environment is developed. Also, the interaction of multiple buoyant jets is mathematically examined. The existing mathematical models predict the mean flow and mixing properties of an inclined plane and round turbulent buoyant jets in a uniform stationery or moving environment solving the system of partial differential equations of continuity momentum and tracer conservation of mass written in cartesian or cylindrical coordinates. By this way, the terms that give a better precision are omitted. Also, the escaping masses from the main buoyant jet flow that are experimentally observed are not quantified. Furthermore, these models assume that the receiver is infinite so the predicted properties do not coincide to experimental data. These experimental data are affected by the boundaries as the experiments cannot be conducted in boundless environment. The present Thesis, attempts to improve the aforementioned weaknesses. In this Thesis, a mathematical description of an inclined turbulent plane or round buoyant jet is proposed, where the partial differential equations for continuity, momentum and tracer conservation are written in orthogonal and cylindrical curvilinear coordinates in order to achieve better accuracy of the mean flow and mixing parameters. The escaping masses from the main buoyant jet flow are simulated, and the model can be successfully applied to initial discharge inclinations θ0 from 90° to -75° with respect to the horizontal plane. This is based on the idea that masses may escape from the buoyant jet zones where considerable intermittency occurs and entrainment shows large variations having a very weak, zero or negative mean value and at the same time some buoyant chunks lose their inertia due to reversal motions imposed by the large eddies. Thus, the only governing force on these masses is buoyancy. This complementary approach introduces a concentration coefficient, called Λ, which is calibrated using experimental evidence. This phenomenon is sharper in motionless environment. The present model has incorporated the second-order approach and, regarding the jet-core region, a jet-core model based on the advanced integral model for the production of more correct transverse profiles of the mean axial velocities and mean concentrations than the common Gaussian or top-hat profiles. The partial differential equations for momentum and tracer conservation are written in orthogonal and cylindrical curvilinear coordinates for inclined plane and round buoyant jets, respectively, and they are integrated under the closure assumptions of (a) quasi-linear spreading of the mean flow and mixing fields, and (b) known transverse profile distributions. The integral forms are solved by employing the Runge–Kutta algorithm. This model is applied to predict the mean flow properties (trajectory characteristics, mean axial velocities and mean concentrations) for inclined plane and round buoyant jets. The results predicted are compared with experimental data available in the literature, and the accuracy obtained is more than satisfactory. The best values of Λ were found to be in the range from 0.30 to 0.42, indicating a mean value ± standard deviation of Λ = 0.344 ± 0.053 for -75° ≤ θ0 ≤ -15°. Thus Λ = 0.34 is adopted as the suitable value for all cases of round buoyant jets with -75° ≤ θ0 ≤ -15°, while for the rest range -15° < θ0 ≤ 90° the pertinent value is Λ = 0. For the inclined plane buoyant jets, the available experimental data are rather restricted to only trajectories and concentrations of horizontal discharges, which allow the determination of a suitable value Λ = 0.08. The Entrainment Restriction Approach is employed in interacting round buoyant jets discharged vertically upwards from a rosette type diffuser into a calm environment. Incorporating the second order approach, the prediction of the mean-flow properties achieves better accuracy. The present Thesis solves the interaction of N identical or not inclined turbulent plane or round buoyant jets using the potentional theory. The occurrence of boundaries is handled via the method of symmetric virtual origin. A mirror image to the boundary, which is identical to the real buoyant jet, is assumed that dynamically interacts with the buoyant jet issued form the real source.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

The High Pressure Test Unit (HPTU) forms part of the Pebble Bed Modular Reactor (PBMR) Heat Transfer Test Facility (HTTF). One of the test sections that forms part of the HPTU is the Braiding Effect Test Section (BETS). This test section allows for the evaluation of the so–called ‘braiding effect’ that occurs in fluid flow through a packed pebble bed. The braiding effect implies an apparent enhancement of the fluid thermal conductivity due to turbulent mixing that occurs as the flow criss–crosses between the pebbles. The level of enhancement of the fluid thermal conductivity is evaluated from the thermal dispersion effect. The so–called thermal dispersion quantity r K is equivalent to an effective Peclet number eff Pe based on the inverse of the effective thermal conductivity eff k . This thesis describes the experiments carried out on three different BETS test sections with pseudo–homogeneous porosities of 0.36, 0.39 and 0.45, respectively. It also provides the values derived for the enhanced fluid thermal conductivity for the range of Reynolds numbers between 1,000 and 40,000. The study includes the following: * Compilation of a literature study and theoretical background. * An uncertainty analysis to estimate the impact of instrument uncertainties on the accuracy of the empirical data. * The use of a Computational Fluid Dynamics (CFD) model to simulate the heat transfer through the BETS packed pebble bed.* Application of the CFD model combined with a numerical search technique to extract the effective fluid thermal conductivity values from the measured results. * The assessment of the results of the experiments by comparing it with the results of other investigations found in the open literature. The primary outputs of the study are the effective fluid thermal conductivity values derived from the measured data on the HPTU plant. The primary variables that were measured are the temperatures at radial positions at different axial depths inside the bed and the total mass flow rate through the test section. The maximum and minimum standard uncertainties for the measured data are 10.80% and 0.06% respectively. The overall effective thermal conductivities that were calculated at the minimum and maximum Reynolds numbers were in the order of 1.166 W/mK and 38.015 W/mK respectively. A sensitivity study was conducted on the experimental data and the CFD data. A maximum uncertainty of 5.92 % was found in the calculated effective thermal conductivities. The results show that relatively high values of thermal dispersion quantities or effective Peclet numbers are obtained for the pseudo–homogeneous packed beds when compared to randomly packed beds. Therefore, the effective thermal conductivity is low and it can be concluded that the radial mixing in the structured packing is low relative to the mixing obtained in randomly packed beds. / Thesis (M.Ing. (Mechanical Engineering))--North-West University, Potchefstroom Campus, 2011.

The goal of the work described in this report was the experimental investigation of the mixing of coolant with different quality (temperature, boron concentration) in nuclear reactors on the way from the cold leg through the downcomer and lower plenum to the core inlet in a systematic way. The obtained data were used for the clarification of the mixing mechanisms and form a data basis for the validation of computational fluid dynamics (CFD) codes. For these purposes, experiments on slug mixing have been performed at two test facilities, modelling different reactor types in scale 1:5, the Rossendorf and Vattenfall test facilities. The corresponding accident scenario is the start-up of first main coolant pump (MCP) after formation of a slug of lower borated water during the reflux-condenser mode phase of a small break loss of coolant accident (LOCA). The matrices for the experiments were elaborated on the basis of the key phenomena, being responsible for the coolant mixing during pump start-up. Slug mixing tests have also been performed at the VVER-1000 facility of EDO Gidropress to meet the specifics of this reactor type. The mixing of slugs of water of different quality is also very important for pre-stressed thermal shock (PTS) situations. In emergency core cooling (ECC) situations after a LOCA, cold ECC water is injected into the hot water in the cold leg and downcomer. Due to the large temperature differences, thermal shocks are induced at the reactor pressure vessel (RPV) wall. Temperature distributions near the wall and temperature gradients in time are important to be known for the assessment of thermal stresses. One of the important phenomena in connection with PTS is thermal stratification, a flow condition with a vertical temperature profile in a horizontal pipe. Due to the fluctuating character of the flow, this may cause thermal fatigue in the pipe. Besides of thermal fatigue, a single thermal shock can also be relevant for structural integrity, if it is large enough, especially in the case, that the brittle fracture temperature of the RPV material is reduced due to radiation embrittlement. Therefore, additional to the investigations of slug mixing during re-start of coolant circulation, the mixing of slugs or streams of water with higher density with the ambient fluid in the RPV was investigated. The aim of these investigations was to study the process of turbulent mixing under the influence of buoyancy forces caused by the temperature differences. Heat transfer to the wall and thermal conductivity in the wall material have not been considered. Experiments on density driven mixing were carried out at the Rossendorf and the Fortum PTS facilities.

This thesis deals with turbulent mixing processes occuring in internal combustion engines, when applying exhaust gas recirculation (EGR). EGR is a very efficient way to reduce emissions of nitrogen oxides (NOx) in internal combustion engines. Exhaust gases are recirculated and mixed with the intake air of the engine, thus reducing the oxygen concentration of the combustion gas and the maximum combustion tempera- ture. This temperature decrease results in a reduction of NOx emissions, since NOx is produced at high temperatures.The issue of NOx reduction is of high importance for current engine development (particularly for heavy-duty engines), since NOx is the main cause for smog formation and subject to increasingly stronger emission legislation. One of the practical problems when applying EGR is the non-uniformity of the mixture among and inside the cylinders deteriorating the engine and emission performance.The aim of this work is to develop and assess methods suited for the computation of turbulent mixing processes in engine conditions. More specifically, RANS and LES computations are considered. The flow structures responsible for the mixing are analyzed for two different T-junctions and a six-cylinder Scania engine-manifold. Shortcomings and advantages of the applied mixing models are explained.The main results are, that commonly applied scalar flux models for the RANS framework do not predict correct scalar flux directions. In stationary flow, the applied k-ε-model in combination with a gradient-diffusion-model gives too small mixing rates as compared to LES and experiments. Furthermore, the LES computations of the T-junctions show, that Dean vortices occuring due to the curvature of the flow are broken up and dissipated only a few diameters downstream of the junction. The RANS computations do not predict this break-up, giving fundamentally different flow structures and mixing distributions. In pulsating flow, a resonance between the natural stabilities and the pulsation frequency is found by LES results, which could not be predicted by RANS.Computations of the flow in a Scania intake manifold with generic boundary con- ditions indicate, that inlet pulsations are important for the mixing process and that the smoothing effect of URANS is not adequate for accurate mixing computations. LES, on the other hand, is more promising, since it is able to capture the physics of pulsating flows much better. / QC 20111117

The test facility ROCOM (Rossendorf Coolant Mixing Model) has been built for the investigation of coolant mixing processes in the reactor pressure vessel of pressurised water reactors (PWR). ROCOM is a 1:5 model of the German PWR KONVOI and has been designed for a wide range of different mixing scenarios. ROCOM disposes of four loops with fully controllable coolant pumps. The test facility is operated with demineralised water. For the investigation of mixing, tracer solution (water labelled with salt) is injected into the facility. The transient distribution of the electrical conductivity is is measured at different positions of the flow path by means of wire-mesh sensor technique with high resolution in space and time. The measured conductivity is transformed into a dimensionless mixing scalar. The mixing at quasi-stationary conditions (constant loop mass flow rates) has been investigated in the presented experiments. That concerned nominal operation conditions, the operation with a reduced number of loops and the investigation of cold-water transients with running pumps and conditions of developed natural circulation. In special experimental series, the reproducibility of the results at identicla boundary conditions within the confidence intervalls has been shown. Further, the influence of various factors on the mixing has been investigated. This included the pressure losses at the core bottom plate, the global coolant flow level and the influence of the loop flow rate on the perturbed sector at the core inlet. An analysis of the measurement error of the used measurement technique completes the report.

La réduction de la consommation des bâtiments passe par : l'économie d'énergie, l'efficacité énergétique et l'utilisation des énergies renouvelables. Sur ce dernier point, l'intégration à grande échelle de composants photovoltaïques (PV) est une solution. Le rendement et la durée de vie des cellules PV en silicium cristalin diminuant avec l'augmentation de leur température de fonctionnement, il est essentiel de mettre au point des configurations d'intégration limitant leur échauffement. L'intégration en configuration de double-peau – la surface PV est séparée du bâtiment par une lame d'air – est une solution prometteuse. Sous l'effet de la chaleur, un écoulement de convection naturelle se met en place entre les deux parois, refroidissant ainsi les panneaux PV. Cet écoulement peut également servir de moteur pour la ventilation en été, et de préchauffage de l'air en hiver. Cette étude, expérimentale, fait partie d'un projet visant à comprendre le fonctionnement des double-peaux PV en analysant séparément les différents phénomènes physiques avant de prendre en compte l'ensemble des couplages. Elle porte plus particulièrement sur la convection naturelle au sein des double-façades verticales. Ici, la double-peau est modélisée par un canal vertical dont les deux parois principales sont chauffées sous des conditions de flux imposé. Un banc d'essais a été développé pour étudier la convection naturelle dans un canal vertical en eau. L'eau a été choisie pour se placer dans le cas d'un écoulement de convection pure (pas de rayonnement entre les parois). C'est un canal vertical de 65 cm de haut avec un écartement réglable placé dans une cuve de 1,5 m de haut contenant 160 L d'eau. Les parois sont chauffées à l'aide de 24 chaufferettes indépendantes délivrant un flux de chaleur uniforme, ce qui permet d'appliquer différentes configurations de chauffage. Des mesures de flux et de température sont réalisées au niveau des parois, et un système couplant de la velocimétrie laser Doppler deux composantes à un micro-thermocouple (25 μm) a été développé pour avoir accès aux vitesses verticale et horizontale et à la température dans le canal. Ce banc et son instrumentation sont décrits et les incertitudes de mesure associées ont été caractérisées. La configuration de chauffage uniforme symétrique a été étudiée ici pour différentes puissances injectées. Les profils de vitesse et de température moyennes mettent en évidence la présence d'un changement de régime d'écoulement dans le canal pour un nombre de Rayleigh indéntifié. L'étude approfondie des profils des fluctuations de vitesse et de température a permis de mettre au point une modélisation comportementale de ce changement de régime. De plus, une première approche est développée pour évaluer la pression dans le canal à partir de l'analyse et de l'estimation des différents termes de l'équation de conservation de la quantité de mouvement. / Reducing the building consumption passes through : energy saving, energy efficiency and the use of renewable energy sources for a local production of electricity. For the last point, the building integrated photovoltaic systems represent a promising solution. However, electrical yield and life time of silicon solar cells decrease with the increase of its operating temperature, that is why integrated configurations which limits cell overheating must be developed. The solution considered is the double-skin façade configuration : photovoltaic panels are separated from the building wall(or roof) by an opened air channel. This induces a natural convection flow that cools the rear surface of the photovoltaic panels. This flow can also be used for natural ventilation of buildings in summer or for air preheating in winter. This experimental study is part of a project on photovoltaic double-skin façades. In this project, each physical phenomena are analysed separately first, and the coupling between the different phenomena are studied in a second time. This thesis deals with the natural convection into the vertical double-skin façades. The system is modelled by a vertical channel with isoflux heating at the two mains walls. An experimental apparatus was developed in the laboratory for studying natural convection in water in a vertical channel. Water is used as the working fluid to avoid radiative heat transfer and obtain a pure convective flow. The channel is 65 cm high with an adjustable gap. It is placed in a 1.5 m high glass tank filled with 160 L of distilled water. The main walls are heated through 24 independant electrical heaters maintaining a constant heat flux, which allows differents boundary condition (symmetrical, asymmetrical, uniforme, alternated, etc). Heat flux and temperature measurements are made at the walls. The velocity (horizontal and vertical) and temperature of the flow are measured through a two components Laser Doppler Velocimetry system combined with a micro-thermocouple (25 μm diameter). This apparatus is described in details and the measurement uncertainties were characterized. The symmetrical uniform heating configuration was studied here with the analyse of velocity and temperature profiles in the channel for several input power (46 W to 562 W). These profiles show a change in flow regime in the channel that is analysed with turbulent quantities along the channel. Moreover, the pressure is estimated in the channel from the analysis of all the terms of the momentum conservation equations.