Enhancement of single- and two- phase heat transfer: inside heat generators

Heat generators are actually the most used system for producing domestic heat and hot water. These systems are most commonly fire tube heat generators, which consist in a shell-and-tube heat exchanger with the flue gases produced by a stationary combustion process flowing inside the tubes, and the secondary fluid, i.e. water, located in the shell. The first Chapter of this Thesis presents an experimental and theoretical analysis of the working conditions of a three pass fire tube heat generator, while operating both in stationary and transient regimes. In the literature, at the best of the author’s knowledge, very limited research is published on such systems. Experimental tests were performed varying heat generator working conditions as well as inserting or not turbulence generator inserts inside the tube composing the last flue gases pass. A dynamic MatLab/Simulink model for the fire tube heat generator is presented. This has been validated using the new experimental dataset. The model is characterized by a subsystem structure which makes it easily adaptable to different geometries, and it can be used to predict the behavior of heat generators working in stationary and transient conditions. Heat generators performances can be increased by enhancing heat transfer between the flue gases and the water inserting turbulence generators in the tube where the combustion products flow. In particular, inserting turbulators inside the last pass of fire tube heat generators leads to an increase of the system efficiency, due to reduced flue gases exit temperature which implies reduced thermal losses at the chimney. Thus, single phase heat transfer enhancement by means of turbulence generator inserts is an important field of research for the heat transfer industry. However, it must be kept in mind that the convective heat transfer coefficient enhancement due to the turbulators presence is associated with an augmentation of the frictional losses in the system, thus both factors have to be taken into account when evaluating inserts performances. In the second part of the present Thesis, after a review of the most common solutions presented in the literature, performance enhancement of turbulators geometries actually used by the heat generators manufacturers is evaluated by means of CFD simulations. Effects of several geometrical parameters, such as turbulator position inside the tube and pipe diameter, are analyzed with the simulations. Equations for predicting the inserts working conditions are presented. Finally, a modified geometry is presented with the aim of proposing a solution with enhanced thermal and frictional characteristics. By recovering the latent heat in the exhaust products coming out from the heat generator it is possible to achieve extremely high system efficiencies. Thus, two-phase heat transfer is a field of interest in the heat generators industry. During pure steam condensation the mayor resistance to the thermal transport is associated to the condensate layer which forms at the wall. Reducing, or eventually removing, the liquid film thickness at the wall will thus lead to an enhancement of the condensation heat transfer coefficient, increasing the heat transfer per unit area. This means the possibility to get higher performances of the system maintaining the same geometry, or to reduce the heat transfer area (thus the system cost) maintaining the same net effect. The last Section of the present Thesis is focused on the enhancement of the condensation heat transfer coefficient by means of nano-engineered surfaces. Effect of surface wetting properties on the condensation process and performance is analyzed by studying the behavior of conventional, superhydrophilic, hydrophobic and superhydrophobic surfaces during pure steam flow condensation at different mass velocities. The aim of the research is to remove (dropwise condensation mode) or eventually reduce (filmwise condensation mode) the condensate layer over the wall during the two-phase process, by acting on surface superficial characteristics, as well as to evaluate the effect of surface roughness on the filmwise condensation heat transfer coefficient

Enhancement of single- and two- phase heat transfer: inside heat generators

Heat generators are actually the most used system for producing domestic heat and hot water. These systems are most commonly fire tube heat generators, which consist in a shell-and-tube heat exchanger with the flue gases produced by a stationary combustion process flowing inside the tubes, and the secondary fluid, i.e. water, located in the shell. The first Chapter of this Thesis presents an experimental and theoretical analysis of the working conditions of a three pass fire tube heat generator, while operating both in stationary and transient regimes. In the literature, at the best of the author’s knowledge, very limited research is published on such systems. Experimental tests were performed varying heat generator working conditions as well as inserting or not turbulence generator inserts inside the tube composing the last flue gases pass. A dynamic MatLab/Simulink model for the fire tube heat generator is presented. This has been validated using the new experimental dataset. The model is characterized by a subsystem structure which makes it easily adaptable to different geometries, and it can be used to predict the behavior of heat generators working in stationary and transient conditions. Heat generators performances can be increased by enhancing heat transfer between the flue gases and the water inserting turbulence generators in the tube where the combustion products flow. In particular, inserting turbulators inside the last pass of fire tube heat generators leads to an increase of the system efficiency, due to reduced flue gases exit temperature which implies reduced thermal losses at the chimney. Thus, single phase heat transfer enhancement by means of turbulence generator inserts is an important field of research for the heat transfer industry. However, it must be kept in mind that the convective heat transfer coefficient enhancement due to the turbulators presence is associated with an augmentation of the frictional losses in the system, thus both factors have to be taken into account when evaluating inserts performances. In the second part of the present Thesis, after a review of the most common solutions presented in the literature, performance enhancement of turbulators geometries actually used by the heat generators manufacturers is evaluated by means of CFD simulations. Effects of several geometrical parameters, such as turbulator position inside the tube and pipe diameter, are analyzed with the simulations. Equations for predicting the inserts working conditions are presented. Finally, a modified geometry is presented with the aim of proposing a solution with enhanced thermal and frictional characteristics. By recovering the latent heat in the exhaust products coming out from the heat generator it is possible to achieve extremely high system efficiencies. Thus, two-phase heat transfer is a field of interest in the heat generators industry. During pure steam condensation the mayor resistance to the thermal transport is associated to the condensate layer which forms at the wall. Reducing, or eventually removing, the liquid film thickness at the wall will thus lead to an enhancement of the condensation heat transfer coefficient, increasing the heat transfer per unit area. This means the possibility to get higher performances of the system maintaining the same geometry, or to reduce the heat transfer area (thus the system cost) maintaining the same net effect. The last Section of the present Thesis is focused on the enhancement of the condensation heat transfer coefficient by means of nano-engineered surfaces. Effect of surface wetting properties on the condensation process and performance is analyzed by studying the behavior of conventional, superhydrophilic, hydrophobic and superhydrophobic surfaces during pure steam flow condensation at different mass velocities. The aim of the research is to remove (dropwise condensation mode) or eventually reduce (filmwise condensation mode) the condensate layer over the wall during the two-phase process, by acting on surface superficial characteristics, as well as to evaluate the effect of surface roughness on the filmwise condensation heat transfer coefficient.I generatori di calore sono attualmente i sistemi più utilizzati nelle applicazioni di riscaldamento domestico. Questi sono di norma generatori di calore a tubi di fumo, i quali consistono in uno scambiatore a fascio tubiero in cui i fumi prodotti da un processo di combustione stazionaria fluiscono all’interno dei tubi, mentre il fluido secondario, comunemente acqua, si trova nel mantello. Nel primo Capitolo di questa Tesi viene presentata un’analisi sperimentale e teorica del funzionamento di un generatore di calore a tre giri di fumo, operante sia in condizioni stazionarie che in condizioni dinamiche. In letteratura è estremamente difficile trovare dati teorici e sperimentali riguardanti questi sistemi. Le prove sperimentali sono state svolte variando le condizioni di lavoro del generatore di calore e lavorando sia con che senza generatori di turbulenza all’interno dei tubi che compongono l’ultimo passaggio del sistema. Un modello dinamico, sviluppato in ambiente MatLab/Simulink, del generatore di calore a tre giri di fumo è quindi presentato in questo elaborato. Il modello è caratterizzato da una struttura a sotto-sistemi che lo rende facilmente adattabile a geometrie diverse, e può essere utilizzato per predirre il comportamento dei generatori di calore durante funzionamento in regime stazionario o dinamico. Le prestazioni dei generatori di calore possono essere incrementate aumentando lo scambio di calore tra i gas combusti e l’acqua attraverso l’inserimento di generatori di turbulenza nei tubi dove fluiscono i gas. In particolare, l’inserimento di turbulatori all’interno dell’ultimo passaggio dei fumi nei generatori di calore comporta un aumento dell’efficienza globale del sistema, per via della minore temperatura di uscita dei gas che si riflette in minori perdite al camino. Per questo motivo, l’aumento dello scambio termico monofase attraverso l’inserimento di generatori di turbulenza è un importante ambito di ricerca per l’industria dei generatori di calore. Ad ogni modo, è importante considerare che ad un aumento del coefficiente di scambio termico è associato un incremento delle perdite di carico per attrito nel sistema, ed entrambi gli elementi devono essere considerati nel valutare le prestazione dei turbulatori. Nella seconda parte di questo elaborato, dopo aver presentato una review delle soluzioni più comuni in letteratura, sono analizzate attraverso simulazioni CFD le prestazioni delle geometrie attualmente utilizzate nei generatori di calore. Gli effetti di diversi parametri geometrici, come la posizione del turbulatore all’interno del tubo e il diametro dello stesso, sono stati analizzati. Inoltre, attraverso le simulazioni si sono ricavate delle equazioni predittive del comportamento degli inserti. In ultimo è proposta una modifica alle geometrie attuali al fine di proporre una soluzione più performante. L’efficienza dei generatori di calore può essere incrementata attraverso la condensazione del vapore presente nei gas combusti allo scarico. Per questo, lo studio dello scambio termico bifase è di interesse per l’industria dei generatori di calore. Durante il processo di condensazione del vapore la maggior parte della resistenza termica è localizzata nel condensato che si forma a contatto con la superficie fredda. Riducendo, o eventualmente annullando, lo spessore del film di liquido alla parete si possono quindi ottenere coefficienti di scambio termico bifase estremamente elevati, incrementando quindi il flusso termico specifico scambiato. Questo permetterebbe di incrementare le performance del sistema a parità di geometria, o di ridurre l’area di scambio (e quindi i costi del generatore di calore) a parità di effetto utile. Per questo, nell’ultimo Paragrafo di questa Tesi si analizza l’incremento del coefficiente di scambio termico durante condensazione di vapore su superfici nano-ingegnerizzate. L’effetto delle proprietà di bagnabilità delle superfici sulla modalità e sulle prestazioni del processo di condensazione è studiato analizzando il comportamento di superfici convenzionali, superidrofiliche, idrofobiche e superidrofobiche durante condensazione di vapore puro fluente a diverse velocità. Lo scopo della ricerca è di rimuovere (condensazione a gocce) o ridurre (condensazione a film con scivolamento del condensato) il film di liquido che si forma durante il processo bifase, agendo sulle proprietà della superficie, e di valutare l’effetto della rugosità superficiale sul coefficiente di scambio durante condensazione a film

Well-defined Cu2O photocatalysts for solar fuels and chemicals

The shape-controlled synthesis of cuprous oxide (Cu2O) photocatalysts with both low or high index crystal planes has received increasing attention due to their unique facet-dependent properties. Since they are cheap and earth abundant, these well-defined Cu2O nanostructures are extensively used for different photocatalytic reactions, also because of their strong visible light absorption capability. However, further development will still be needed to enhance the efficiency and photostability of Cu2O to expand its industrial application. We start this review by summarizing the synthetic advancement in the facet engineering of Cu2O and other associated hybrid Cu2O-based heterostructures with a special emphasis put on their growth mechanism. We then discuss different facet-dependent properties, which are relevant to photocatalysis. In the subsequent section, we present a critical discussion on the photocatalytic performance of faceted Cu2O nanostructures during organic synthesis, hydrogen production, and carbon dioxide photoreduction. The relation between photocatalytic efficiency and product selectivity with exposed crystal facets or with different compositions of hybrid nanostructures is also discussed. Finally, important strategies are proposed to overcome the photostability issue, while outlining the course of future development to further boost the technological readiness of well-defined Cu2O-based photocatalysts

A prediction method of frictional pressure drop during two-phase flow in small diameter channels

Two-phase flow in minichannels is widely present in evaporators and condensers, such as, for example, air-cooled condensers for automotive and air-conditioning applications. It is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization because pressure drop have a strong influence on the two-phase heat transfer. In this work, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is presented: it accounts for the effects of surface roughness on the internal surface. Predictions by the present model are compared against a new two-phase pressure drop database to enlarge its validity range. The new pressure drop database is obtained from experimental tests with refrigerants R290 (propane), R1234ze(E), and two mixtures R32/R1234ze(E) (23/77% and 50/50% by mass composition) at mass flux ranging between 200 and 800 kg m-2 s-1

Frictional Pressure Drop during Two-Phase Flow of Pure Fluids and Mixtures in Small Diameter Channels

Two-phase flow is widely encountered in minichannels heat exchangers such as air-cooled condensers and evaporators for automotive, compact devices for electronic cooling and aluminum condenser for air-conditioning applications. In the present work, frictional pressure drop during adiabatic liquid-vapor flow is experimentally investigated inside a single 0.96 mm diameter minichannel. Tests have been run with three mixtures of R32/ R1234ze(E) (23/77%, 50/50% and 75/25% by mass composition) at mass flux ranging between 200 and 600 kg m 122 s 121. Since pressure drop has a strong influence on the twophase heat transfer, it is crucial to have reliable pressure drop prediction methods for two-phase heat transfer modeling and optimization. Therefore, with the aim of extending its validity range, a model to calculate the frictional pressure gradient during two-phase flow in small diameter channels is tested against the present two-phase pressure drop database. An assessment is also done with two low- GWP refrigerants: the halogenated olefin R1234ze(E) and the hydrocarbon R290. The present model accounts for the effect of internal surface roughness as a function of the liquid-only Reynolds number