Design of NH3-H2O absorption chiller for low grade waste heat recovery

Absorption refrigeration technology is a well known subject that has been studied for a long time. The Absorption Refrigeration Cycle uses as primary energy source the heat which drives the cooling process, instead of electricity as in vapor compression cycle. Despite different technologies can be used to design an absorption chiller, the most common available on market are based on ammonia-water (NH3 – H2O) and water - lithium bromide (NH3 - LiBr) mixtures. The absorption cycle heat requirement is a characteristic that increase overall efficiency of the system when applied to industrial processes where wasted heat is available. The aim of this project is to investigate the NH3 – H2O technology and suitability for heat recovery applications. Moreover, this project is focused on the development of an analytical feasibility study method to evaluate the application of this technology to industrial processes, with a particular focus on below 0 \degree C cooling applications. This method is based on a numerical model of the system that analyses the cycle temperature influence on the overall cycle performance. The heat source is a flow of hot water (90°C – 120°C ). The analysis shows that a Coefficient Of Performance (COP) of 0.521 has been obtained for a configuration with a pre-condenser heat exchanger rectifier. This method is focused on reducing the desorber temperature required to operate the chiller for a fixed evaporation temperature. The parametric study at constant condensation and evaporation temperature shows high gradient COP drop for low values of generation temperatures. The optimal minimum generator temperature is a result of balance of many parameters, such as heat exchangers dimensions and solution flow. Differential ammonia concentration (between weak and strong solution) is also considered as a further design criteria to evaluate heat recovery application feasibility. This parameter affects directly the system operative design in terms of heat exchanger dimensions, installed pump size and the cycle performance. The operating design is obtained considering also local robustness, to reduce system instability if fluctuation of functional parameters occurs. Charts for easy first sizing of a new chiller has been developed. Furthermore, a 75 kW ammonia-water absorption chiller operated by waste heat has been designed and tested. The prototype is operated with a glycol-water solution at approximately 105°C, evaporation is fixed to -3.5°C to cool glycol-water solution down to 0.5°C and condensation temperature for summer working period is assumed as 35°C. The prototype has been built to test different technical solutions. The pump is a critical component in absorption chillers. In order to reduce cost and complexity of the system a centrifugal water pump is installed and tested, instead of more complex and costly membrane pumps. Moreover, Plate Heat Exchangers are used to obtain a compact system design, instead of Tube Heat exchangers , commonly used in the industrial refrigeration sector. The results obtained from the apparatus test confirm that the design process and all the assumptions are correct. The COP value obtained during the tests is 0.413, in line with the expected theoretical figures. All the problems encountered during the design process are reported in the Conclusions section. This section includes also some suggestions on features that can be implemented to enhance the overall chiller design and performances.Co-supervisore: Mauro ReiniopenLa refrigerazione ad assorbimento è una tecnologia ben nota e studiata da lungo tempo. Il ciclo di refrigerazione ad assorbimento, a differenza del ciclo a compressione del vapore che usa l’elettricità, permette di utilizzare come fonte primaria di energia il calore come motore per il processo di raffreddamento. Esistono diverse tecnologie che possono essere impiegate per la costruzione di un frigo ad assorbimento, ma quelle basate su miscele di ammoniaca-acqua e acqua-bromuro di litio sono le più comuni e disponibili sul mercato. La richiesta di calore del ciclo ad assorbimento è una caratteristica che può rendere questo sistema molto efficiente se applicato a processi industriali laddove il calore di scarto è disponibile. L’obiettivo di questo progetto di ricerca è studiare la tecnologia basata sulla coppia NH3 – H2O e la sua compatibilità per applicazioni di recupero di calore. Questo progetto è inoltre incentrato sullo sviluppo di un metodo analitico di valutazione di fattibilità per stimare l’impiego della tecnologia ad assorbimento a NH3 – H2O in differenti processi industriali, in particolare considerando applicazioni di raffreddamento con temperature inferiori allo 0°C. Questo metodo è basato su un modello numerico del sistema che analizza l’influenza delle temperature nel ciclo sulla complessiva prestazione del sistema. Come fonte di calore è stato considerato un flusso di acqua calda di 90-120°C. L’analisi mostra che quando si ha una configurazione con uno scambiatore di condensazione parziale il coefficiente di prestazione (COP) è pari a 0.471. Inoltre, con questo metodo si vuole ridurre la temperatura di generazione richiesta per azionare il frigo a temperature di evaporazione fisse. Lo studio parametrico fatto a temperature di condensazione ed evaporazione costanti, mostra un marcato gradiente nel calo del COP per bassi valori di temperature di generazione. La temperatura di generazione minima ottimale è il risultato dell’equilibrio di diversi parametri che comprendono le dimensione degli scambiatori di calore e la portata della soluzione. Come ulteriore criterio di progettazione sono state considerate concentrazioni diverse di ammoniaca (tra le soluzioni ricca e povera) per stimare la fattibilità di applicazione nal caso di recupero del calore. Questo parametro influenza direttamente la progettazione del sistema in termini di dimensioni degli scambiatori di calore, della potenza della pompa installata e della prestazione del ciclo. Il punto di funzionamento è stato ottenuto considerando anche la robustezza locale per ridurre l’instabilità del sistema nel caso in cui siano presenti fluttuazioni dei parametri funzionali. Grafici per un iniziale dimensionamento del nuovo frigo sono stati sviluppati. Inoltre, un frigo ad assorbimento ammoniaca-acqua della potenza di 75kW azionato tramite calore di scarto è stato progettato e testato. Il prototipo è attivato con una soluzione di acqua e glicole ad una temperatura di circa 105°C, l’evaporazione è fissata a -3.5°C per raffreddare la soluzione acqua e glicole a 0.5°C e la temperatura di condensazione è fissata a 35°C per il periodo lavorativo estivo. Il prototipo è stato costruito per testare diverse soluzioni tecniche. Considerando che la pompa è una componente critica nei frighi ad assorbimento, per ridurre i costi e la complessità del sistema è stata installata e successivamente testata una pompa centrifuga per acqua invece di pompe a membrana che sono più complesse e costose. Scambiatori di calore a piastre sono stati utilizzati per ottenere un design compatto del sistema, invece di scambiatori di calore a fascio tubiero, che sono comunemente usati nel settore della refrigerazione industriale. I risultati riportati dal test dell’apparato confermano che il processo di progettazione e tutti le ipotesi di lavoro sono corretti. Il valore di COP ottenuto durante il test è pari a 0.421 che è in linea con i valori teorici previsti. Tutte le difficoltà riscontrare durante il processo di progettazione si possono trovare nella sezione delle conclusioni, dove sono anche enunciati suggerimenti su caratteristiche che possono essere implementate per incrementare la progettazione complessiva del frigo e le sue prestazioni.Dottorato di ricerca in Tecnologie chimiche ed energeticheembargoed_20161114Cefarin, Marc

Exploring the jamming transition over a wide range of critical densities

We numerically study the jamming transition of frictionless polydisperse spheres in three dimensions. We use an efficient thermalisation algorithm for the equilibrium hard sphere fluid and generate amorphous jammed packings over a range of critical jamming densities that is about three times broader than in previous studies. This allows us to reexamine a wide range of structural properties characterizing the jamming transition. Both isostaticity and the critical behavior of the pair correlation function hold over the entire range of jamming densities. At intermediate length scales, we find a weak, smooth increase of bond orientational order. By contrast, distorted icosahedral structures grow rapidly with increasing the volume fraction in both fluid and jammed states. Surprisingly, at large scale we observe that denser jammed states show stronger deviations from hyperuniformity, suggesting that the enhanced amorphous ordering inherited from the equilibrium fluid competes with, rather than enhances, hyperuniformity. Finally, finite size fluctuations of the critical jamming density are considerably suppressed in the denser jammed states, indicating an important change in the topography of the potential energy landscape. By considerably stretching the amplitude of the critical "J-line", our work disentangles physical properties at the contact scale that are associated with jamming criticality, from those occurring at larger length scales, which have a different nature.Comment: 19 pages, 11 figures, resubmission to SciPos

Large-Eddy Simulation of a Wankel Rotary Engine for Range Extender Applications

The Wankel rotary engine offers unrivalled power density as a consequence of having a combustion event every revolution, as well as lightness, compactness and vibrationless operation due to its perfect balance. These attributes have led to its success as a powerplant for unmanned aerial vehicles, and which should also make it an attractive proposition for range-extended electric vehicles. However, it is not currently in production for automotive applications having historically struggled with poor combustion efficiency, and high fuel consumption and hydrocarbon emissions. The purpose of this work is to study the in-chamber flow motion in order to better understand how such limitations may be overcome in future. A 225cc 30 kW Wankel rotary engine is modelled using Large-Eddy Simulation (LES) to ensure turbulent flow features are faithfully recreated, since Reynolds-averaged Navier-Stokes-based approaches are insufficient in this regard. The LES-predicted peak chamber pressure lies within 3.7% of the engine test data, demonstrating good model validation. Combustion simulation parameters are calibrated to match the measured heat release profile, for high-load engine operation at 4000 rpm. Simulation results provide insight into the generation of turbulent structures as the incoming flow interacts with the throttle, intake port, rotor and housing surfaces; how the turbulence breaks down as the combustion chamber is compressed; and how the flame propagates following ignition, leaving a pocket of reactants unburned. Indeed, the computational approach described here allows detailed understanding of the impact of design parameters on the detailed in-chamber flow phenomena, and consequently engine performance and emissions. This will enable the optimization of Wankel rotary engine geometry, port and ignition timing for maximum combustion efficiency and low emissions, reasserting its potential as an effective and efficient prime mover for hybrid and range extended electric vehicles

Shock waves in nuclear matter : proof by circumstantial evidence

In the present paper we develop the essential theoretical tools for the treatment of the dynamics of High Energy Heavy Ion Collisions. We study the influence of the nuclear equation of state and discuss the new phenomena connected with phase transitions in nuclear matter (pion condensation). Furthermore we investigate the possibility of a transition from nuclear to quark matter in High Energy Heavy Ion Collisions. In this context we discuss exotic phenomena like strongly bound pionic states, limiting temperatures, and exotic nuclei

Numerical Simulation of Compressible Flows with Interfaces

Compressible interfacial flows exist in a variety of applications: reacting fronts, droplet break up, jets and sprays in high speed, shock passage in foams, etc. These flows behave in a complex multi-scale way including interface deformation, wave interface interaction and complex transport phenomena. In the first section, the interaction of a laminar flame with a compression wave is investigated. More precisely, the evolution of the burning interface is investigated and discussion over different compression waves and their effects on the flame geometry and burning rate are made. In the second part, a numeral framework for simulation of compressible multiphase flows using adaptive wavelet collocation method is developed. This study was originally motivated by the desire for a numerical tool capable of simulating the atomization process during start-up conditions in a supersonic combustor. To model such physics, the solver needs to handle high density ratios, transport terms and capillary effects. The multi-scale behaviour of these flows requires a multi-scale approach. Parallel Adaptive Wavelet Collocation Method (PAWCM) makes use of second generation wavelets to dynamically adapt the grid to localized structures in the flow in time and space. This approach allows the solution to be approximated using a subset of the points that would normally be used with a uniform grid scheme. Thus, computation on this subset is efficient and high levels of data compression is achieved