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 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

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

POwer production through the utilization of the pressure retarded osmosis concept - Computational Fluid Dynamics (CFD) simulations in PRO systems

166 σ.Η ενέργεια που δημιουργείται απο τη μίξη γλυκού και αλμυρού νερού αποτελεί μια πηγή ανανεώσιμης ενέργειας η οποία μπορεί να αξιοποιηθεί μέσω της καθυστεριμένης ώσμωσης (pressure retarded osmosis - PRO). Στο PRO, νερό από ένα διάλυμα με χαμηλή συγκέντρωση αλατιού περνάει μέσα από μία μεμβράνη σε ένα συμπιεσμένα διάλυμα με υψηλή περιεκτικότητα σε αλάτι. Η ισχύς προκύπτει αποσυμπιέζοντας το νέο διάλυμα αλατόνερου, που έχει αυξημένη παροχή, με μία τουρμπίνα. Ο συνδυασμός του αξημένου ενδιαφέροντος στις εναλλακτικές πηγές ενέργειας και στη σύγχρονη εξέλιξη στην τεχνολογία των μεμβρανών έχει οδηγήσει σε ένα αυξημένο ενδιαφέρον για την τεχνολογία PRO την τελευταί δεκαετία. Αυτό το ενδιαφέρον οδήγησαι στο πρώτο πρότυπο εργοστάσιο παραγωγής ενέργειας με τη χρήση του PRO, το οποίο λειτουργεί στη Νορβηγία από το 2009. Αν και πολλοί ερευνητές έχουν μελετήσει την παραγωγή ενέργειας μέσω του PRO υπάρχει ακόμα έλλειψη θεωριτικού και τεχνολογικού υποβάθρου που θα εξασφαλίσουν την επιτυχία του PRO. Η παρούσα εργασία παρουσιάζει ένα μοντέλο CFD (υπολογιστική ρευστομηχανική) για να μελετήσει τη διεργασία του PRO σε μιά δοκιμαστική γεωμετρία. Τα αποτελέσματα συγκ΄ρινονται με τα πειραματικά μιας δημοσιευμένης εργασίας και γίνονται προτάσεις για βελτίωση ώστε να επιτευχθεί μεγαλύτερη ακρίβεια. Ο ρόλος του πλέγματος και της μεθόδου επίλυσης έχουν μελετηθεί επίσης, Επιπλέον, έχει μελετηθεί η επιροή βασικών παραμέτρων (παροχή εισόδου, συγκέντρωση αλατιού, συντελεστής περατότητας του νερού για τη μεμβράνη) στο PRO και βάσει αυτόν έχουν εξαχθεί συμπεράσματα για τον ρόλο τους στο PRO. Τέλος, 2 καινούριες γεωμετρίες βασισμένες στην αρχική δοκιμάζονται για τις επιδόσεις τουσ στο PRO και εξάγονται συμπεράσματα για το πώς συγκεκριμένες αλλάγες στη γεωμετρία επηρεάζουν την απόδοση, και με βάση αυτά γίνονται προτάσεις για μελλοντική έρευνα.The energy released from the mixing of freshwater with saltwater is a source of renewable energy that can be harvested using pressure retarded osmosis (PRO). In PRO, water from a low salinity solution permeates through a membrane into a pressurized, high salinity solution; power is obtained by depressurizing the permeate through a hydro turbine. The combination of increased interest in renewable and sustainable sources of power production and recent progress in membrane science has led to a high increase in PRO interest in the last decade. This interest led in the first prototype installation of PRO which opened in Norway in late 2009. Although many researchers have studied the power production through PRO there is still lack of theoretical and experimental investigations to ensure the success of PRO. The present work introduces a Computational Fluid Dynamics (CFD) model to study the PRO process in a test cell, examines its accuracy by comparing these results with those experimental and of an existing model, and makes suggestions for improvements in order to provide more precision. The role, also, of the grid and of the solution method have been addressed. Moreover, the effects of basic variables (inlet volumetric flow, salt concentration difference, water permeability of the membrane) in the PRO process have been investigated and conclusions were made about their role in PRO. Finally, by testing two new designs, conclusions have arisen about how certain modifications in the design affect the process and also suggestions for further investigation of the design have been made.Διονύσιος Γ. Στεφανίτση

Development and numerical investigation of a DDPM-KTGF model for modelling flow hydrodynamics and heat transfer phenomena in a bubbling calciner reactor

In this work, a DDPM-CFD model is developed in ANSYS® Fluent for the simulation of the indirectly heated, bubbling calciner of the 300kWth dual fluidized bed pilot plant located at Technische Universität Darmstadt. The calciner is heated by 72 heat pipes that carry the heat from an external combustor. Regarding the heat transfer, both convection and radiation are considered in the model. Regarding the modelling of the drag forces, flow heterogeneity aspects are considered by applying the Energy Minimization Multi-Scale (EMMS) scheme. However, the application of DDPM in such dense, bubbling flows considered here proved to be challenging, demanding several advancements and customizations. To this end, this study proposes mainly three advancements; i) The inter-particle forces are modelled using custom user defined functions incorporating both normal and tangential components. In particular, KTGF-based correlations are applied at dilute regions, while at dense regions the solid pressure is modelled according to Harris and Crighton, and the shear and bulk viscosities are modelled using correlations based on the plastic theory. ii) It is shown that, in order to correctly predict the overall pressure drop, the Lagrangian particle momentum equation should be reformulated according to Model A formulation to be consistent with the solved gas-phase momentum equation. iii) In order to capture the correct heat flux levels, the heat flux on the heat pipe heat exchanger walls is modelled in the Eulerian reference frame scaling the temperature gradient on the wall to take into account the thin thermal boundary layer. Τhe DDPM results are compared against those of an already validated Eulerian TFM model, in terms of calculated flow patterns, volume fractions, pressure profiles and heat fluxes. In addition, both models are assessed for their computational cost. The developed DDPM model predicts practically the same overall pressure drop with the TFM model. However, it overpredicts the bed length by 12% when using the default grid. This reduces to 6% when using a finer grid comprising double computational cells. As for the heat fluxes and the calcination reaction rate, both models predict similar levels and their differences are attributed to the differences in hydrodynamics.Peer reviewe