Two-phase heat transfer inside minichannels: fundamentals and applications in refrigeration and solar technology

This thesis reports the results of many experimental tests conducted to gain a deeper insight on the two-phase heat transfer inside minichannels and to characterize the thermal performance of two refrigerants with low environmental impact: propane (R290) and R1234ze(E). Furthermore, some considerations on the application of the minichannel technology in refrigeration applications and solar concentrators are presented. As pressure drops greatly affect the heat transfer in two-phase flow, the experimental investigation on frictional pressure gradient during adiabatic flow of R134a, R1234ze(E) and propane (R290) at different mass velocities and at saturation temperatures between 30°C and 50°C has been conducted in two single copper minichannels with a circular cross section and hydraulic diameters of 0.96 mm and 2 mm. The experimental points are compared with several models available in the open literature. Heat transfer coefficients have been experimentally measured during the condensation at 40°C and during the vaporization at 31°C of R1234ze(E) and propane at different mass velocities inside a single circular cross section minichannel with an internal diameter of 0.96 mm. During the test runs, the refrigerant exchanges heat with a secondary fluid, that is distilled water, so the local heat flux is not constant along the measuring section and its accurate calculation becomes the main issue. An assessment of several predicting correlations has been presented for predicting the heat transfer coefficient both in condensation and in vaporization. The condensation process inside minichannels depends on the relative importance of shear stress, gravity and surface tension, especially in presence of corners in the cross section shape. Nevertheless, few studies concern the effect of inclination. In this work, the effect of the channel orientation has been experimentally analyzed and discussed during the condensation of R134a and R32 at 40°C saturation temperature inside a single square cross section minichannel with a hydraulic diameter equal to 1.23 mm. Several configurations of the test section from vertical upward flow to vertical downward flow have been examined. When considering the application of the minichannel technology in refrigeration, a general methodology to evaluate the potential heat transfer performance of refrigerants during in-tube condensation is a powerful tool to optimize the performance and the design of heat exchangers. The Performance Evaluation Criteria (PEC) named Penalty Factor for condensation (PF) and Total Temperature Penalization on the refrigerant side (TTP) are applied to rank several refrigerants starting from an experimental database collected in a single circular minichannel with internal diameter of 0.96 mm at the Two-Phase Heat Transfer Lab at the University of Padova. In electronics, the minichannel technology has proved to be reliable and effective in removing high heat fluxes through small heat transfer areas. This feature has suggested to use minichannel-based receivers for solar concentration systems. In this work, a parabolic trough linear solar concentrator is described and tested using two different minichannel-based receivers: a concentrating hybrid photovoltaic thermal (CPVT) receiver for the cogeneration of electrical energy and heat and a thermal receiver with a selective coating for the generation of heat in the medium temperature range. An optical modeling has been developed for the two cases in order to assess the optical efficiency and the flux distribution on the receiver. Tests with both the receivers have been performed using water in single-phase flow as working fluid in order to get a preliminary characterization of the whole system. The performance of the thermal receiver at medium temperature (up to 150°C) when two-phase heat transfer is realized inside the channels has been evaluated through a numerical model

experimental and numerical study of a parabolic trough linear cpvt system

Abstract The electric and thermal performance of a parabolic trough linear concentrating photovoltaic-thermal (CPVT) system operating in Padova (northern Italy) is experimentally investigated. The system moves about two axes and exhibits a geometrical concentration ratio around 130. The receiving module placed on the focus line displays a secondary optics made of two flat mirrors to gather some reflected radiation and to contribute to the concentrated flux on two lines of triple junction photovoltaic cells soldered on a ceramic substrate. The substrate is in thermal contact with a aluminium heat exchanger with water flow channels to cool the PV cells. During the test runs, the inlet water temperature ranges from 20 °C to 80 °C and the heat yield is obtained from mass flow rate and temperature measurements while a rheostat and a power analyzer are connected to the electric terminals of the module to assess the electrical production. The direct normal irradiation (DNI) is measured by a pyrherliometer mounted on a solar tracker. Experimental results are used to assess a numerical model of the solar receiver and the whole concentrator

Two-phase heat transfer inside minichannels: fundamentals and applications in refrigeration and solar technology

This thesis reports the results of many experimental tests conducted to gain a deeper insight on the two-phase heat transfer inside minichannels and to characterize the thermal performance of two refrigerants with low environmental impact: propane (R290) and R1234ze(E). Furthermore, some considerations on the application of the minichannel technology in refrigeration applications and solar concentrators are presented. As pressure drops greatly affect the heat transfer in two-phase flow, the experimental investigation on frictional pressure gradient during adiabatic flow of R134a, R1234ze(E) and propane (R290) at different mass velocities and at saturation temperatures between 30°C and 50°C has been conducted in two single copper minichannels with a circular cross section and hydraulic diameters of 0.96 mm and 2 mm. The experimental points are compared with several models available in the open literature. Heat transfer coefficients have been experimentally measured during the condensation at 40°C and during the vaporization at 31°C of R1234ze(E) and propane at different mass velocities inside a single circular cross section minichannel with an internal diameter of 0.96 mm. During the test runs, the refrigerant exchanges heat with a secondary fluid, that is distilled water, so the local heat flux is not constant along the measuring section and its accurate calculation becomes the main issue. An assessment of several predicting correlations has been presented for predicting the heat transfer coefficient both in condensation and in vaporization. The condensation process inside minichannels depends on the relative importance of shear stress, gravity and surface tension, especially in presence of corners in the cross section shape. Nevertheless, few studies concern the effect of inclination. In this work, the effect of the channel orientation has been experimentally analyzed and discussed during the condensation of R134a and R32 at 40°C saturation temperature inside a single square cross section minichannel with a hydraulic diameter equal to 1.23 mm. Several configurations of the test section from vertical upward flow to vertical downward flow have been examined. When considering the application of the minichannel technology in refrigeration, a general methodology to evaluate the potential heat transfer performance of refrigerants during in-tube condensation is a powerful tool to optimize the performance and the design of heat exchangers. The Performance Evaluation Criteria (PEC) named Penalty Factor for condensation (PF) and Total Temperature Penalization on the refrigerant side (TTP) are applied to rank several refrigerants starting from an experimental database collected in a single circular minichannel with internal diameter of 0.96 mm at the Two-Phase Heat Transfer Lab at the University of Padova. In electronics, the minichannel technology has proved to be reliable and effective in removing high heat fluxes through small heat transfer areas. This feature has suggested to use minichannel-based receivers for solar concentration systems. In this work, a parabolic trough linear solar concentrator is described and tested using two different minichannel-based receivers: a concentrating hybrid photovoltaic thermal (CPVT) receiver for the cogeneration of electrical energy and heat and a thermal receiver with a selective coating for the generation of heat in the medium temperature range. An optical modeling has been developed for the two cases in order to assess the optical efficiency and the flux distribution on the receiver. Tests with both the receivers have been performed using water in single-phase flow as working fluid in order to get a preliminary characterization of the whole system. The performance of the thermal receiver at medium temperature (up to 150°C) when two-phase heat transfer is realized inside the channels has been evaluated through a numerical model.In questa tesi sono presentati i risultati di numerose prove sperimentali che mirano a migliorare la conoscenza dello scambio termico bifase all’interno di minicanali e a caratterizzare le prestazioni di due fluidi a basso impatto ambientale come il propano e il refrigerante R1234ze(E). Inoltre, sono contenute alcune considerazioni relative all’applicazione della tecnologia dei minicanali nella refrigerazione e nei concentratori solari. Dal momento che le perdite di carico influenzano notevolmente lo scambio termico in regime bifase, è stata condotta un’analisi sperimentale sul gradiente di pressione per attrito in condizioni adiabatiche di deflusso con R134a, R1234ze(E) e propano all’interno di due minicanali non lisci in rame, a sezione circolare e con diametri rispettivamente di 0.96 mm e 2.0 mm a diverse portate specifiche di massa e a in un intervallo di temperature di saturazione tra 30°C e 50°C. I punti sperimentali sono stati confrontati con i valori calcolati mediante alcuni modelli disponibili in letteratura. Sono stati misurati i coefficienti di scambio termico in condensazione a 40°C e in vaporizzazione a 31°C, utilizzando in test successivi R1234ze(E) e propano all’interno di un singolo minicanale non liscio a sezione circolare e con diametro interno di 0.96 mm. Durante le prove sperimentali, il refrigerante in esame scambia calore con un fluido secondario, che nella fattispecie è acqua distillata, pertanto il flusso termico locale non è costante e il suo calcolo accurato rappresenta l’aspetto principale della tecnica sperimentale. È stata valutata la precisione predittiva di alcuni modelli disponibili in letteratura per il calcolo dei coefficienti di scambio termico in condensazione e vaporizzazione in base ai dati sperimentali raccolti. Le forze che entrano in gioco durante un processo di condensazione all’interno dei minicanali sono dovute allo sforzo tangenziale all’interfaccia delle due fasi, all’accelerazione di gravità e alla tensione superficiale, specie se la sezione del canale presenta degli angoli. Pochissimi studi in letteratura riguardano l’effetto dell’inclinazione. In questo lavoro, è stato analizzato l’effetto dell’orientazione del canale durante la condensazione di R134a ed R32 all’interno di un minicanale a sezione quadrata con un diametro idraulico di 1.23 mm e ad una temperatura di saturazione di 40°C. Sono state esaminate diverse configurazioni della sezione di prova, dal deflusso verticale ascendente al deflusso verticale discendente. Quando si esamina l’applicazione della tecnologia dei minicanali nell’ambito della refrigerazione, avere a disposizione una metodologia per valutare le prestazioni potenziali di scambio termico di un refrigerante durante la condensazione all’interno di un tubo diventa uno strumento molto utile per ottimizzare le prestazioni dell’intero sistema e la progettazione degli scambiatori di calore. I Criteri di Valutazione delle Prestazioni (PEC) indicati come Fattore di Penalizzazione per la condensazione (PF) e Penalizzazione Totale in termini di Temperatura nel lato refrigerante (TTP) vengono applicati in questa tesi per classificare i refrigeranti che sono stati testati in un minicanale circolare con diametro interno di 0.96 mm nel Laboratorio di Scambio Termico Bifase presso l’Università degli Studi di Padova. Nell’industria elettronica, la tecnologia dei minicanali ha dimostrato di essere efficiente ed affidabile nell’asportare elevati flussi termici attraverso aree di scambio molto ridotte. Questa caratteristica ha suggerito la realizzazione di ricevitori a minicanali per concentratori solari. In questo lavoro, un concentratore parabolico a fuoco lineare è descritto e testato utilizzando due ricevitori: un ricevitore fotovoltaico termico per la cogenerazione di energia elettrica e calore ed un ricevitore termico con vernice selettiva per la produzione di energia termica a media temperatura. Per ognuno dei due dispositivi, è stato sviluppato un modello ottico per valutare l’efficienza ottica di concentrazione e la distribuzione del flusso concentrato sul ricevitore. Le prove sperimentali per entrambi i ricevitori sono state condotte utilizzando come fluido operativo acqua in deflusso bifase per avere una caratterizzazione preliminare dell’intero sidtema. Le prestazioni a media temperatura del ricevitore termico considerando uno scambio termico bifase in vaporizzazione all’interno dei minicanali sono state valutate in modo attraverso un modello numerico

Innovative solar thermal concentrating roof-integrated collector: technological and formal issues

The design and development of solar thermal concentrating collectors integrated into buildings is becoming an interesting strategy for the promotion of green energy production. This paper reports an innovative system of roof envelope in which a cylindrical solar linear concentrating collector with circular cross-section for the production of thermal energy at medium temperature (80\ub0C - 250\ub0C) is integrated. The optical performance of the solar concentrating collector has been evaluated using a Monte- Carlo ray tracing tool and a thermal model of the system has been developed. The results highlighted that the proposed solar device can be suitable for many industrial and commercial applications such as solar cooling with double effect absorption chillers and process heat. The envelope integrated system meets the requirements of the upper horizontal enclosures. It consists of plane, inclined and curves parts, the last constituting the fixed primary optics which concentrates the direct solar irradiance on an evacuated tubular receiver moved by a domotic system of mechanized arms. The formal and technological issues of the architectural integration are studied by case studies, both on existing and new buildings. The analysis has led to the development of different solutions depending on the different climate zones considered

Modelling of a direct absorption solar receiver using carbon based nanofluids under concentrated solar radiation

The addition of nanoparticles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus, nanofluids can be successfully used in solar collectors to absorb the solar radiation in their volume and avoid using an absorber plate. This paper investigates the application of aqueous suspensions as volumetric absorber in a concentrating direct absorption solar collector: a suspension of single wall carbon nanohorns (SWCNHs) in water is chosen as the nanofluid. A model of a solar receiver with a planar geometry to be installed in a parabolic trough concentrator is developed: the radiative transfer equation in participating medium and the energy equation are numerically solved to predict the thermal performance of the receiver. The developed model is capable to predict the temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. The simulated performance of the direct absorption receiver has been compared with calculations and experimental data of two surface absorption conventional receivers under the same operating conditions

Coupled radiative and fluid flow modelling for a direct absorption solar receiver

The application of nanofluids has the potential to solve technical issues in many solar thermal engineering systems. Recent literature indicates that nanofluids offer unique advantages over conventional fluids. Nanofluids are solid nanoparticles suspended in a liquid: the average dimensions of these particles may vary from 1 to 100 nm. The addition of particles in a base fluid can enhance its optical properties, in particular its absorption properties. Thus nanofluids can be successfully used in direct absorption solar collectors to directly absorb the solar radiation in their volume. In this kind of devices, it is possible to surpass the constraints of conventional collectors due to the absence of the absorber plate. An important advantage of direct absorption of solar radiation is to avoid the thermal resistance between the absorber surface and the heat transfer fluid. This paper investigates the application of water based nanofluids as volumetric absorber in a direct absorption solar collector: a suspension of single wall carbon nanohorns in water is chosen as nanofluid. A new model of a solar receiver with a planar geometry is developed: radiative transfer equation in participating medium and energy equation are numerically solved to predict the performance of the receiver; the optical and thermal behaviors of the nanofluid are modelled according to the properties available in the current scientific literature. Monte Carlo ray tracing is used to determine the directional and spatial distribution of the concentrated solar radiation coming from a parabolic trough concentrator. This distribution is then applied to the receiver geometry using a commercial computational fluid dynamic software to simulate the incoming solar flux. The developed model is capable to predict temperature distribution, heat transfer rate and penetration distance of the concentrated solar radiation inside the nanofluid volume. Two different configurations of the bottom surface are considered: transparent wall and reflecting wall. The effects of inlet temperature, flow rate and nanoparticle concentration on the energy efficiency of the receiver are studied. Finally, the model is applied to compare the performance of a direct absorption receiver to a conventional surface receiver under the same operating conditions