Numerical and experimental investigation of a vertical LiBr falling film absorber considering wave regimes and in presence of mist flow

The absorber represents the most critical component in absorption systems and one of the key issues. In this component complex heat and mass transfer phenomena during the absorption process takes place simultaneously. For this reason the development of mathematical models validated against experimental data always constitutes useful tools for the design and improvement of falling film absorbers. A testing device has been designed and built to reproduce absorption phenomena in vertical LiBr-H2O falling film absorbers with the primary objective to obtain experimental data. On the other hand, a mathematical model of falling film absorption of H2O vapour in LiBr aqueous solutions has been implemented. Wave regime is considered by including and solving the Free Surface Deflection Equation. The numerical results are validated using the experimental data. During the development of this work, the authors have paid careful attention to the verification of experimental data. Such verification consists of performing energy and mass balances in the fluid film side. Important discrepancies were found in our experimental data. Therefore, an extensive study was carried out in order to find the source of such errors. The conclusion is that there is a drag of LiBr solution in the water vapour which increases with the Re number. This mist flow cannot be measured experimentally, but can be evaluated in an indirect way. The mathematical models have been adapted in order to consider the influence of mist flow. On the other hand, in the literature there are not many experimental works related to falling film absorbers which expose enough information to verify the reliability of their experimental data.Peer ReviewedPostprint (author's final draft

Analysis of Binary Fluid Heat and Mass Transfer in Ammonia-Water Absorption

An investigation of binary fluid heat and mass transfer in ammonia-water absorption was conducted. Experiments were conducted on a horizontal-tube falling-film absorber consisting of four columns of six 9.5 mm (3/8 in) nominal OD, 0.292 m (11.5 in) long tubes, installed in an absorption heat pump. Measurements were recorded at both system and local levels within the absorber for a wide range of operating conditions (nominally, desorber solution outlet concentrations of 5 - 40% for three nominal absorber pressures of 150, 345 and 500 kPa, for solution flow rates of 0.019 - 0.034 kg/s.). Local measurements were supplemented by high-speed, high-resolution visualization of the flow over the tube banks. Using the measurements and observations from videos, heat and mass transfer rates, heat and vapor mass transfer coefficients for each test condition were determined at the component and local levels. For the range of experiments conducted, the overall film heat transfer coefficient varied from 923 to 2857 W/m2-K while the vapor and liquid mass transfer coefficients varied from 0.0026 to 0.25 m/s and from 5.51×10-6 to 3.31×10-5 m/s, respectively. Local measurements and insights from the video frames were used to obtain the contributions of falling-film and droplet modes to the total absorption rates. The local heat transfer coefficients varied from 78 to 6116 W/m2-K, while the local vapor and liquid mass transfer coefficients varied from -0.04 to 2.8 m/s and from -3.59×10-5 (indicating local desorption in some cases) to 8.96×10-5 m/s, respectively. The heat transfer coefficient was found to increase with solution Reynolds number, while the mass transfer coefficient was found to be primarily determined by the vapor and solution properties. Based on the observed trends, correlations were developed to predict heat and mass transfer coefficients valid for the range of experimental conditions tested. These correlations can be used to design horizontal tube falling-film absorbers for ammonia-water absorption systems.Ph.D.Committee Chair: Garimella, Srinivas; Committee Member: Bergin, Michael; Committee Member: Frederick, James; Committee Member: Graham, Samuel; Committee Member: Wepfer, Willia

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

Film condensation heat transfer of low integral-fin tube.

PhDFor condensation on horizontal low-finned tubes, the dependence of heat-transfer performance on fin spacing has been investigated experimentally for condensation of refrigerant 113 and ethylene glycol. Fourteen tubes have been used with inside diameter 9.78 mm and working length exposed to vapour 102 mm. The tube had rectangular section fins having the same width and height (0.5 mm and 1.59 mm) and with the spacing between fins varying from 0.25 mm to 20 mm. The diameter of the tube at the fin root was 12.7 mm. Tests were also made using a plain tube having the same inside diameter and an outside diameter equal to that at the root of the fins for the finned tubes. All tests were made at near atmospheric pressure with vapour flowing vertically downward with velocities of 0.24 m/s and 0.36 m/s for refrigerant 113 and ethylene glycol respectively. Optimum fin spacings were found at 0.5 mm and 1.0 mm for refrigerant 113 and ethylene glycol respectively. In earlier experiments for steam using the same tubes, the optimum fin spacing was found to be 1.5 mm. Maximum enhancement ratios of vapour-side heat-transfer coefficient (vapour-side coefficient for a finned tube / vapour-side coefficient for a plain tube. for the same vapour-side temperature difference) were 7.5, 5.2 and 3.0 for refrigerant 113, ethylene glycol and steam respectively. Enhancement phenomena have also been studied theoretically. Consideration has been given to a role of surface tension forces on the motion and configuration of condensate film. On the basis of this study, several semi-empirical equations, to predict heat-transfer performance, have been obtained. These are considered to represent recent reliable data (present and other recent works) satisfactorily

Review of Dehumidifier with Association to Solar Circular Collector for Close Water Open Air System (CWOA) Humidification Dehumidification Process

Coastal area where clean drinking water availability is measured problem, which insist to innovate cheep, decentralized small-scale water production.The geographical conditions of vadodara (22.00N, 72.10E ),and kachh,Gujarat are best suitable for humidification-dehumidification (HDH) technique based on closed-water, open-air cycle where air heated system is used. There are different types of heat exchangers available as dehumidifiers for HDH applications vary but they have required strength to withstand corrosive nature of seawater, there for frames, collecting plates, fins are made of aluminum. In addition, special attention was exercised to avoid leakage of distillate water. Dehumidifiers is heat exchanger in which heat exchange is takes places between two fluids i.e. hot and cold that are at different temperatures. The heat exchange in the heat exchanger may be in the form of latent heat or sensible heat or combination of both. The HDH concept are also reviewed and compared. Further, novel proposals for improvement of the HDH cycle are outlined. It is notice that HDH technology has great promise to produced fresh water using circular solar collector, although additional research and development is needed for improving system efficiency and reducing capital cost

The Effects of Vapour Shear and Inundation on Roped Tubes

This investigation is concerned with the effects of both vapour shear and inundation on the performance of the overall heat transfer coefficient of plain and roped tubes. A series of experiments were conducted using a specially designed test condenser and three types of tubes to initially determine the effect of steam velocity and tube geometry on the overall heat transfer coefficient. Subsequent tests were performed to investigate the combined effects of vapour shear and inundation on bundles of condenser tubes. The main experimental variables in the tests were steam velocity, cooling water velocity and the number of equivalent tubes in a vertical array. The results of the vapour shear tests conclusively showed that as the velocity of steam increased so did the amount of heat transferred by all of the three types of tubes. These tests also determined that the geometry of a tube bundle (i.e. staggered or in-line configurations) made little or no difference to the overall heat transfer coefficient. The multi-tube experiments confirmed that inundation reduced the heat transfer in condensers and showed that the effect of inundation was moderated through the use of a higher steam velocity. This result was convincingly shown for bundles of 2-start roped tubes but was less apparent for similar bundles of both plain and 6-start roped tubes. In all of the above experiments the roped tubes were found to consistently outperform the equivalent plain tubes with respect to the amount of heat transferred. In addition to the experimental work, two computer programs were used to predict the effects of vapour shear and inundation on both single tubes and tube bundles. The theoretical predictions were compared with the test results and very good agreement was found for the plain tube simulations. The predictions of the roped tube simulations were judged to be less accurate than those of the plain tube, but still good considering the complex hydrodynamic and thermal processes associated with roped tubes