Numerical analyses of concrete thermal energy storage systems: effect of the modules' arrangement

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Modelli di scoring per il rischio paese

Country risk and sovereign risk are two of the most important topics in risk management. The first part of this work introduces these concepts and shows the differences between them. The following chapters fit linear and ordinal regression models to a data-set with more than 100 countries, where the response variable is an appropriate measure of their creditworthiness. The main purposes are to identify the most relevant explanatory variables and to make predictions for those countries whose response variable is not available. For the second aim it is important to verify that records with missing values are not systematically different from the complete ones: a Little test for the MCAR hypothesis is implemented. About model selection, ad hoc algorithms are used and the theory of reduction, proposed by David Hendry, is also briefly described

Valutazione dell'influenza di parametri clinico-strumentali sui livelli di mesotelina sierica in una popolazione di lavoratori ex esposti ad amianto

L’esposizione lavorativa all’amianto può provocare varie patologie respiratorie come l'asbestosi, il versamento pleurico, le placche pleuriche, fino a patologie tumorali come il tumore del polmone ed il mesotelioma pleurico maligno, patologia per la quale ad oggi non esistono trattamenti radicali in grado di incidere sulla mortalità. Diversi sono i protocolli medici di sorveglianza sanitaria dei lavoratori ex esposti ad amianto ad oggi proposti. Su una coorte di più di 600 ex esposti seguiti presso l'ambulatorio di Medicina del Lavoro dell'Azienda Ospedaliero Universitaria Pisana, è stato adottato un protocollo che oltre alla visita medica con particolare attenzione all'anamnesi lavorativa, ad alcuni accertamenti radiologici (Rx ed eventuale TAC torace) e alle prove di funzionalità respiratoria ha visto anche l'innovativa introduzione di alcuni markers biologici, tra i quali la mesotelina sierica. Questo marker, ancora in fase di studio, è da molti suggerito come un utile strumento nella individuazione di sub-popolazioni a maggior rischio di patologie neoplastiche amianto correlate. Non del tutto conosciute sono le funzioni ed interazioni fisiologiche e fisiopatologiche di questa molecola. Conseguentemente, con la presente tesi si è voluto valutare la possibile influenza di alcuni parametri clinico strumentali sui livelli di mesotelina sierica, che potrebbero infine agire come fattori di confondimento inficiando così l’utilizzo concreto della mesotelina

Modelli di scoring per il rischio paese

Country risk and sovereign risk are two of the most important topics in risk management. The first part of this work introduces these concepts and shows the differences between them. The following chapters fit linear and ordinal regression models to a data-set with more than 100 countries, where the response variable is an appropriate measure of their creditworthiness. The main purposes are to identify the most relevant explanatory variables and to make predictions for those countries whose response variable is not available. For the second aim it is important to verify that records with missing values are not systematically different from the complete ones: a Little test for the MCAR hypothesis is implemented. About model selection, ad hoc algorithms are used and the theory of reduction, proposed by David Hendry, is also briefly described

3D CFD Simulation of a New Ventilated Roof

In the last decades, energy management and saving have become challenging issues. Considering the building sector (residential or industrial), different technologies have been developed in order to realize tangible energy savings, such as: ventilated roof, double facades, glazed surfaces, etc. Nonetheless, it is important for these new technologies to contemporary assure the human thermal comfort. Passive cooling (or heating) technologies are of actual interest. Low or near-zero energy buildings can only be realized as a result of the good design of all their components; specifically, the roofs call for particular attention as they take large parts of a building’s total surface area. This paper presents a comparison between an innovative ventilated roof, based on an original design of the support and a traditional one. A 3D numerical model is developed to analyze the air flow and to compute the achievable benefits in terms of reduction of the summer heating gains. The simulations were performed by varying the solar irradiance from 600 to 1000 W m2. The investigation is conducted comparing a ventilated roof assembly to the same traditional structure, assuming buoyancy-driven airflow. Two roof types are studied: an insulated roof and a non-insulated one. The results reveal that the ventilated roof leads to a great reduction of the total amount of solar heat gains for all the simulated scenarios

R134a Flow Boiling Heat transfer on an Electrically Heated Carbon/Carbon Surface

With the increase of heat flux densities following the Moore’s law, electronic cooling challenge is focused on the high heat flux to be dissipated by the operating fluid and more and more efficient heat spreaders, dissipators, and compact heat exchangers are in great demand for various applications. Considering the device efficiency, the boiling heat transfer ensures very high heat transfer coefficients, which can even be improved via specific surface treatments that have been shown to be very effective. In particular, several authors, experimentally demonstrated the interesting enhancement capabilities of microparticles coatings on the Critical Heat Flux. Furthermore, the recent work on nanoscale domain has led to new concepts for surface modification. In the last decade, nano-structured materials (i.e. nanowires coatings, nanoporous layers, Carbon Nano Tube arrays, etc.) have been proved to enhance the boiling heat transfer. Unfortunately, almost all of this kind of surface treatments fail when scaled up to industrial implementation because of the relatively high costs and complex operations involved. Furthermore, compactness and lightness of cooling systems are becoming even more challenging design constraints leading the research efforts towards new light and efficient materials. In this scenario, the Carbon/Carbon material appears to be a viable option for future thermal management devices because it exploits interesting properties having a low density and a high thermal conductivity; moreover, it is already used in many industrial applications where it is shaped in various forms even complex. This paper presents the experimental measurements carried out during flow boiling heat transfer of R134a on a Carbon/Carbon surface. The test section with the Carbon/Carbon sample, is electrically heated from the bottom and it is instrumented with 18 wall thermocouples to monitor the temperature distribution at an imposed heat fluxes of 50 kW m-2, and refrigerant mass flow rates from 50 to 200 kg m-2 s-1, at constant saturation temperature of 30 °C. The sample is tested in a new experimental facility built at the Nano Heat Transfer Lab of the Department of Management and Engineering of the University of Padova especially designed to study the flow boiling heat transfer process on innovative materials and enhanced micro- and nano-structured surfaces.

Saturated R134a flow boiling inside a 4.3 mm inner diameter microfin tube

The refrigerant charge minimization in refrigerating and air-conditioning systems represents a challenging issue due to the new environmental national and international regulations. The use of smaller smooth tubes, such as with the outer diameter around 5 mm, is becoming more and more common in many applications. More recently, the microfin tubes have also started to be reduced in size to cope with the continuously increasing demand of new, efficient, and compact heat exchangers for air-conditioning and refrigeration equipment. This work investigates the performance of R134a during saturated flow boiling inside a microfin tube with internal diameter at the fin tip of 4.3 mm. Boiling heat transfer coefficients, frictional pressure drops, and critical vapor qualities were measured at 30\ub0C of saturation temperature, by varying the refrigerant mass velocity between 100 and 800 kg m 122 s 121 and the vapor quality from 0.1 to 0.95 at four different heat fluxes: 15, 30, 60, and 90 kW m 122. Moreover, the reliability of several models for flow boiling heat transfer and pressure drop estimations was assessed by comparing the experimental results with the calculations

R134a Flow Boiling inside a 4.3 mm ID Microfin Tube

The energy and environmental performance of refrigeration and air conditioning machines are commonly described by their Total Equivalent Warming Impact, so called TEWI, which is defined as the sum of the indirect and direct emissions. The direct emissions are related to charge inventory of the system and to the type of refrigerant used, while the indirect emissions basically depend on the system energy performance. Even if there is a strong interest in the new low-GWP refrigerants, the traditional HFC fluids, with huge GWPs, are still widely used in the refrigeration and air conditioning equipment. For this reason, there is a still strong demand of innovative solutions which can be implemented with the current fluids and then applied to the new ones, when there will be the final phase-out of the HFCs. From this standpoint, looking at the TEWI index, the charge minimization and the system performance optimization represent the main targets of the innovation to cope with the environmental challenges. Since the early 1970s, traditional microfin tubes have been widely used in air and water heat exchangers for heat pump and refrigerating applications because they have been demonstrated to significantly improve the heat transfer performance during both in-tube condensation and boiling. The possible downsizing of microfin tubes could lead to more efficient and compact heat exchangers and thus to a reduction of the refrigerant charge of the systems and to an overall improving of their performance. Nowadays, large manufacturers are exploring the possible use of mini microfin tubes and there is a strong interest in understanding the heat transfer and pressure drop behaviours of this enhanced tube. This paper presents the R134a flow boiling heat transfer and pressure drop measurements inside a mini microfin tube with internal diameter at the fin tip of 4.3 mm. This study is carried out in a new experimental facility built at the Dept. of Management and Engineering of the University of Padova. The microfin tube was brazed inside a copper plate and electrically heated from the bottom. Sixteen T-type thermocouples are located in the copper plate to monitor the temperature distribution during the heat transfer process. In particular, the experimental measurements were carried out at constant mean saturation temperature of 30 °C, by varying the refrigerant mass velocity between 200 kg m-2 s-1 and 800 kg m-2 s-1, the vapour quality from 0.1 to 0.95, at four different heat fluxes: 15, 30, 60, and 90 kW m-2. The experimental results are presented in terms of two-phase heat transfer coefficient, onset dryout vapour quality, and frictional pressure drop as a function of the operative test conditions