Improved droplet breakup models for spray applications

The current study examines the performance of two zero-dimensional (0D) aerodynamically-induced breakup models, utilized for the prediction of droplet deformation during the breakup process in the bag, multi-mode and sheet-thinning regimes. The first model investigated is an improved version of the widely used Taylor analogy breakup (TAB) model, which compared to other models has the advantage of having an analytic solution. Following, a model based on the modified Navier–Stokes (M-NS) is examined. The parameters of both models are estimated based upon published experimental data for the bag breakup regime and CFD simulations with Diesel droplets performed as part of this work for the multi-mode and sheet-thinning regimes, for which there is a scarcity of experimental data. Both models show good accuracy in the prediction of the temporal evolution of droplet deformation in the three breakup regimes, compared to the experimental data and the CFD simulations. It is found that the best performance of the two is achieved with the M-NS model. Finally, a unified secondary breakup model is presented, which incorporates various models found in the literature, i.e. TAB, non-linear TAB (NLTAB), droplet deformation and breakup (DDB) and M-NS, into one equation using adjustable coefficients, allowing to switch among the different models

Numerical investigation of the aerodynamic breakup of a parallel moving droplet cluster

The present work examines numerically the aerodynamic breakup of a cluster of Diesel droplets moving in parallel with respect to the gas flow. Two- and three-dimensional simulations of the incompressible Navier-Stokes equations together with the VOF method are performed for Weber (We) numbers in the range of 5 up to 60 and non-dimensional distance between the droplets (H/D0) ranging from 1.25 to 20. The numerical results indicate that the proximity of droplets affects their breakup for distances H/D0≤5. For low droplet proximity distances (H/D0≤2.5), the droplets experience the so-called shuttlecock breakup mode, which has been also identified for droplets in tandem formations in a previous authors’ work and is characterized by an oblique peripheral stretching of the droplet. With decreasing H/D0 the breakup initiation time decreases, while the drag coefficient increases relative to that of isolated droplets. When the distance between the droplets is low enough (H/D0<1.5), this can result in critical We number, i.e. minimum We number leading to breakup, lower than that of an isolated droplet at the same conditions

Numerical Investigation of the Aerodynamic Droplet Breakup at Mach Numbers Greater Than 1

The present work examines numerically the breakup of water droplets exposed to gas flows at Mach numbers Ma>1, which resemble the ambient conditions encountered in the injection systems of scramjet (supersonic combustion ramjet) engines. A CFD model is utilized which solves the compressible Navier-Stokes equations, the energy equation, the mass conservation in volume fraction form (volume of fluid method) along with two equations of state to model the density variations of the two phases. In addition, a coupled VOF/Lagrange model is employed to capture the appearance of micro-droplets, which are smaller than the smallest grid cell. As a first step, a 2-dimensional planar simulation (water column) is performed at conditions of Ma=1.47 in order to validate the numerical model; its results are compared against published experimental and numerical data. Good agreement is observed for the temporal evolution of droplet shape, the streamwise deformation, the leading-edge displacement as well as the shock wave reflection. Subsequently, the validated model is utilized to perform a 3-D simulation at Ma=1.23, which corresponds to the conditions of previous experimental studies, and its results are compared against the experimental data as well as the results from previous numerical studies, showing good agreement. Furthermore, surface instabilities are observed at the droplet surface initiated by interfacial instabilities due to the shearing effect and the interaction with the shock-wave, pertaining to Kelvin-Helmholtz and Rayleigh-Taylor instabilities, despite the stabilising contribution of surface tension; viscosity effects are found to play an insignificant role

Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures

[EN] Abstract The present study investigates numerically the aerodynamic breakup of Diesel droplets for a wide range of ambient pressures encountered in engineering applications relevant to oil burners and internal combustion engines. The numerical model solves the Navier-Stokes equations coupled with the Volume of Fluid (VOF) methodology utilized for capturing the interface between the liquid and the surrounding gas. An adaptive local grid refinement technique is used to increase the accuracy of the numerical results around the interface. The Weber (We) numbers examined are in the range of 14 to 279 which correspond to bag, multimode and sheet-thinning breakup regimes. Model results are initially compared against published experimental data and show a good agreement in predicting the drop deformation and the different breakup modes. The predicted breakup initiation times for all cases lie within the theoretical limits given by empirical correlations based on the We number. Following the model validation, the effect of density ratio on the breakup process is examined by varying the gas density (or equivalently the ambient pressure), while the We number is kept almost constant equal to 270; ambient gas pressure varies from 1 up to 146bar and the corresponding density ratios (ε) range from 700 down to 5. Results indicate that the predicted breakup mode of sheet-thinning remains unchanged for changing the density ratio. Useful information about the instantaneous drag coefficient (Cd) and surface area as functions of the selected non-dimensional time is given. It is shown that the density ratio is affecting the drag coefficient, in agreement with previous numerical studies.Financial support from the MSCA-ITN-ETN of the European Union’s H2020 programme, under REA grant agreement n. 675676 is acknowledged.Stefanitsis, D.; Malgarinos, I.; Strotos, G.; Nikolopoulos, N.; Kakaras, E.; Gavaises, M. (2017). Numerical investigation of the aerodynamic breakup of diesel droplets under various gas pressures. En Ilass Europe. 28th european conference on Liquid Atomization and Spray Systems. Editorial Universitat Politècnica de València. 1052-1059. https://doi.org/10.4995/ILASS2017.2017.4690OCS1052105

Numerical investigation of the aerodynamic breakup of droplets in tandem

The present work examines the aerodynamic breakup of four liquid droplets in tandem formation at Diesel engine conditions using the Volume of Fluid (VOF) method. The examined Weber (We) numbers range from 15 up to 64 and the non-dimensional distances between the droplet centres (L/D0) vary from 1.25 up to 20. Focus is given on the breakup process of the third droplet of the row, which is regarded as a “representative chain droplet”; its development is compared against that of an isolated droplet at the same flow conditions. It is found that for small distances and depending on the We number, the obtained shapes and breakup modes between the droplets are different, with the representative chain droplet experiencing a new breakup mode in the multi-mode regime, termed as “shuttlecock”. This is characterized by an oblique peripheral stretching of the droplet caused by the acting of pressure forces at an off-centre region. Moreover, the drag coefficient and liquid surface area of the representative chain droplet are lower than the corresponding ones of the isolated droplet, while the breakup initiation time is longer and the minimum We number required for breakup (critical We) is higher; correlations are provided to quantify the effect of droplet distance on the aforementioned quantities. Generally, the droplet proximity becomes important for L/D0< 9. Finally, the predicted drag coefficient is utilised in a simplified 0-D model that is capable of estimating the temporal evolution of droplet velocity of the representative chain droplet

Simulation of a CFB Boiler Integrated With a Thermal Energy Storage System During Transient Operation

In the current work, a transient/dynamic 1-dimensional model has been developed in the commercial software APROS for the pilot 1 MWth CFB boiler of the Technical University of Darmstadt. Experiments have been performed with the same unit, the data of which are utilized for the model validation. The examined conditions correspond to the steady-state operation of the boiler at 100, 80, and 60% heat loads, as well as for transient conditions for the load changes from 80 to 60% and back to 80%. Fair agreement is observed between the simulations and the experiments regarding the temperature profiles in the riser, the heat extracted by the cooling lances, as well as the concentration of the main species in the flue gases; a small deviation is observed for the pressure drop, which, however, is close to the results of a CFD simulation run. The validated model is extended with the use of a thermal energy storage (TES) system, which utilizes a bubbling fluidized bed to store/return the particles during ramp up/down operation. Simulations are performed both with and without the use of TES for the load path 100–80–60–80–100%, and the results showed that the TES concept proved to be superior in terms of changing load flexibility, since the ramp up and down times proved to be much faster, and lower temperature drops between the loads are observed in this case

A simple model for breakup time prediction of Water-Heavy Fuel Oil emulsion droplets

Immiscible heavy fuel-water (W/HFO) emulsion droplets inside combustion chambers are subjected to explosive boiling and fragmentation due to the different boiling point between the water and the surrounding host fuel. These processes, termed as either puffing or micro-explosion, are investigated with the aid of a CFD model that solves the Navier-Stokes and energy conservation equations alongside with three sets of VoF transport equations resolving the formed interfaces. The model is applied in 2-D axisymmetric configuration and it is valid up to the time instant of HFO droplet initiation of disintegration, referred to as breakup time. Model predictions are obtained for a wide range of pressure, temperature, water droplet surface depth and Weber number; these are then used to calibrate the parameters of a fitting model estimating the initiation breakup time of the W/HFO droplet emulsion with a single embedded water droplet. The model assumes that the breakup time can be split in two distinct temporal stages. The first one is defined by the time needed for the embedded water droplet to heat up and reach a predefined superheat temperature and a vapor bubble to form; while the succeeding stage accounts for the time period of vapor bubble growth, leading eventually to emulsion droplet break up. It is found that the fitting parameters are ±10% accurate in the examined range o

Financial knowledge and financial behaviour in households and small and medium sized firms

Recent crisis in the financial markets worldwide and the complexity of financial products offered, bring into the forefront, the businesses’ potentials for sustaining past performance, and the issue of their owners’ and managers’ financial literacy. Business ability to comprehend and successfully use and exploit financial information seems today more crucial than ever before since access to financial resources is restricted and delivery channels for financial services are under transformation. Similarly, individuals are in charge of their own financial matters and are confronted with similar complex financial decisions. However, there is suspicion that many households are not well equipped to make sound financial decisions. The purpose of this research is twofold. First, an attempt is made to shed some light on the links between businesses’ and households’ financial knowledge and financial behaviour and second, determine the patterns under which small and medium sized enterprises (SMEs) and households make financial decisions. Special focus is given to the Greek case, by considering, in detail, a range of core variables and relationships which form the financial knowledge and relevant practices of SMEs and households. In this context, the following questions are inevitably raised: do entrepreneurs and households make “poor” financial decisions due to financial ignorance, in other words, because they simply do not know how to take the “right” ones? Further, is it well realized by businesses and households that if financial knowledge is associated with appropriate financial behaviour, there is a chance of reporting less financial difficulties in the forthcoming years? Finally, what is the role of financial knowledge in SMEs’ and households’ financial performance? Despite their importance, there is little research in academia that directly investigates the links between financial knowledge and financial behaviour. Data were collected through a questionnaire survey. Results so far, reveal a positive relationship between specific variables of financial knowledge and specific variables of financial behaviour. Despite Greek SMEs’ and households’ adequate financial knowledge in specific thematic areas, the aggregate results illustrate that the majority of the respondents fail to capture gains in overall performance. These results indicate the complexity of the relationship between knowledge and behaviour. Further research in the field seems however necessary.Η πρόσφατη κρίση στις χρηματοπιστωτικές αγορές παγκοσμίως και η πολυπλοκότητα των προσφερόμενων χρηματοοικονομικών προϊόντων, φέρνουν στην επικαιρότητα, την δυνατότητα των επιχειρήσεων να διατηρήσουν τις προηγούμενες αποδόσεις τους και το θέμα της επάρκειας χρηματοοικονομικών γνώσεων των ιδιοκτητών και των στελεχών τους. Η ικανότητα κατανόησης και επιτυχημένης εκμετάλλευσης των χρηματοοικονομικών πληροφοριών καθίσταται σήμερα περισσότερο κρίσιμη παρά ποτέ αφού η πρόσβαση στις χρηματοοικονομικές πηγές είναι περιορισμένη και τα κανάλια διανομής σε διαδικασία μετατροπής. Ομοίως, τα νοικοκυριά επιφορτίζονται με τα δικά τους χρηματοοικονομικά προβλήματα και αντιμετωπίζουν εξίσου περίπλοκα χρηματοοικονομικά διλλήματα. Υπάρχουν πάντως ενδείξεις, ότι αρκετά νοικοκυριά δεν έχουν το υπόβαθρο να πάρουν σωστές χρηματοοικονομικές αποφάσεις. Ο στόχος της παρούσας διατριβής είναι διττός. Αρχικά, γίνεται προσπάθεια να διαλευκανθούν οι συσχετίσεις μεταξύ χρηματοοικονομικής γνώσης και χρηματοοικονομικής συμπεριφοράς των επιχειρήσεων και των νοικοκυριών, καθώς επίσης, να βρεθούν οι προσδιοριστικοί παράγοντες με βάση τους οποίους οι Μικρές – Μεσαίες Επιχειρήσεις (Μ.Μ.Ε.) και τα νοικοκυριά λαμβάνουν τις χρηματοοικονομικές αποφάσεις που τους αφορούν. Ιδιαίτερη προσοχή δίνεται στην ελληνική επικράτεια, εξετάζοντας λεπτομερώς, το φάσμα των βασικών παραγόντων και σχέσεων που προσδιορίζουν τη χρηματοοικονομική γνώση και συμπεριφορά των Μικρών – Μεσαίων επιχειρήσεων και των νοικοκυριών. Σε αυτό το πλαίσιο, οι ακόλουθες ερωτήσεις αναπόφευκτα εγείρονται: παίρνουν οι επιχειρηματίες και τα νοικοκυριά «ακατάλληλες» χρηματοοικονομικές αποφάσεις εξαιτίας χρηματοοικονομικής άγνοιας, με άλλα λόγια επειδή ενδεχομένως δεν γνωρίζουν πώς να πάρουν τις «κατάλληλες»; Επιπρόσθετα, έχει γίνει κατανοητό από τους επιχειρηματίες και τα νοικοκυριά ότι αν η χρηματοοικονομική γνώση συνοδεύεται από πρέπουσα χρηματοοικονομική συμπεριφορά, υπάρχει πιθανότητα να βιώσουν λιγότερες δυσκολίες τα επόμενα χρόνια; Τελικά, ποιος ο ρόλος της χρηματοοικονομικής γνώσης στις χρηματοοικονομικές επιδόσεις των Μικρών – Μεσαίων επιχειρήσεων και των νοικοκυριών; Παρά τη σπουδαιότητα των παραπάνω, σχετικά μικρή επιστημονική έρευνα έχει πραγματοποιηθεί που να διερευνά τη σχέση μεταξύ χρηματοοικονομικής γνώσης και συμπεριφοράς. Τα στοιχεία συγκεντρώθηκαν μέσω ερωτηματολογίων. Τα αποτελέσματα μέχρι τώρα αποκαλύπτουν θετική σχέση μεταξύ ορισμένων μεταβλητών χρηματοοικονομικής γνώσης και ορισμένων μεταβλητών χρηματοοικονομικής συμπεριφοράς. Παρόλο που τόσο οι Μ.Μ.Ε. όπως και τα νοικοκυριά έχουν αξιόλογη απόδοση σε συγκεκριμένους τομείς της χρηματοοικονομικής γνώσης, δεν καταφέρνουν να επωφεληθούν της γνώσης αυτής επιδεικνύοντας αντίστοιχα υπεύθυνη συνολική χρηματοοικονομική συμπεριφορά. Τα αποτελέσματα αυτά, αναδεικνύουν την πολυπλοκότητα της σχέσης μεταξύ γνώσης και συμπεριφοράς. Περαιτέρω έρευνα στο συγκεκριμένο πεδίο πάντως, κρίνεται απαραίτητη

Asymmetric information and employment: evidence from the U.S. banking sector.

The goal of this paper is to analyze and assess the role of asymmetric information for employment performance in the case of the U.S. banking industry. To this end, the analysis performs a number of methodological approaches, such as panel cointegration and long- and short-run panel causality, spanning the period 2000–2013. The findings provide evidence that asymmetric information exerts a negative effect on employment. The results remain robust after the implementation of further checks.N/