1,720,967 research outputs found
Combined effects of refrigerant leakages and fouling on air-source heat pump performances in cooling mode
No full textFor electric heat pump systems (EHP), soft faults such as refrigerant leakages and heat exchangers fouling can lead to significant performance degradations, which can remain unidentified for a long period. This paper, through a digital model, wants to simulate the cooling season behavior of a case study air-source heat pump for residential air-conditioning, analyzing the performance degradation in case of both standalone and simultaneous occurrence of soft faults. The study is carried-out in different scenarios of not planned and ordinary maintenance of the machine and in typical climate conditions of Naples, Miami and Shanghai. The impact of faults on seasonal performances (SCOP) and total equivalent warming impact (TEWI) has also been analyzed. Results show that refrigerant leakages can have an impact on system performance higher than 25%, whereas heat exchangers fouling can downgrade the efficiency of approximately 15%. It is also found that, for each climate condition, a different maintenance strategy can halve the cases with SCOP penalization higher than 10%. Since faults have not the same probability of occurrence, a smart monitoring and a fault detection system can be the solution to guarantee good performances avoiding at the same time excessive maintenance costs
Flow boiling heat transfer and pressure drop data of non-azeotropic mixture R455A in a horizontal 6.0 mm stainless-steel tube
No full textThis paper carries out an experimental investigation on R455A, a low GWP (148) non azeotropic mixture, to determine its performance in terms of flow boiling heat transfer coefficient and pressure drop in a smooth circular horizontal stainless-steel tube, with an internal diameter of 6 mm. The effect of the operating parameters, such as mass flux (from 300 to 500 kg‧m−2‧s−1), bubble saturation temperature (from 10 to 40 °C), heat flux (from 5 to 40 kW‧m−2) and vapor quality (between 0 and 1) is analyzed. Results show an increase of the heat transfer coefficient with mass and heat flux, while it decreases with saturation temperature. On the other hand, pressure drop increases with mass flux, and decreases with bubble saturation temperature. An assessment of several predictive methods from literature is finally conducted. Two-phase heat transfer methods based on an exclusive convective contribution can predict quite well the experimental results, while all other methods improve their statistics once a glide-related modification on the nucleate boiling contribution is assumed. The fictional pressure gradient results are well fitted with the separated flow model of Müller Steinhagen and Heck and with the Cicchitti two-phase viscosity expression considering a homogeneous flow approach
Flow pattern, condensation and boiling inside and outside smooth and enhanced surfaces of propane (R290). State of the art review
No full textA comprehensive review of two-phase flow characteristics, boiling heat transfer and condensation outside and inside tubes of propane (R290) is presented in this paper. The first section focuses on the global heat transfer performances in heat exchangers, with a collection of 11 works dealing with boiling and condensation of propane in fin and tubes, shell and tubes, plate heat exchangers and complex geometries. The second section reviews local analyses on fluid flow (8 works) and heat transfer characteristics of propane during flow boiling (23 works), pool boiling (10 works), condensation inside (13 works) and outside (2 works) single and multi-channel geometries. The critical analysis of the collected papers has the objective to provide useful information regarding the components performance for those involved in the assembly of refrigeration plants, and also to offer a critical summary on local heat transfer and fluid flow behavior of propane, particularly useful during the processes of heat exchangers design and manufacturing
State of the art and working fluids for high temperature heat pumps
Full text linkIn the current framework of the European policy that pushes toward the ecological transition by promoting the use of low GWP refrigerants and the system electrification, high temperature heat pumps (HTHPs) can be considered as an important technology with huge applicative potential. Their capacity to boost the heat quality from lower to higher temperatures, in fact, makes these machines very interesting across numerous domains. On this regard, this paper presents a comprehensive critical review of the state of the art for applications and working fluids related to high temperature heat pumps. The focus is on the analysis of the heat pump thermodynamic cycles and performance, possible suitable refrigerants, operating source and sink temperatures, potential applications and related technical challenges. More than 25 scientific papers are reviewed, showing large application potential in the food, paper, metal and chemical industries, together with the residential sector
Thermal-hydraulic characterization of R513A during flow boiling inside a 6.0 mm horizontal tube, comparison with R134a and development of a new correlation
Full text linkThis paper presents two-phase heat transfer coefficient and pressure drop data of refrigerant R513A, a new
azeotropic mixture conceived as possible alternative to R134a for medium temperature small-size refrigeration
systems. All the experiments were performed in a commercial horizontal stainless-steel tube having an internal
diameter of 6.0 mm and an outer diameter of 8.0 mm. The channel heating was obtained through DC current and
Joule effect. The effect of the main operating parameters in terms of mass flux (from 150 to 500 kg/m2s), heat
flux (from 5.0 to 40 kW/m2), saturation temperature (from 30 to 50 ◦C) and vapor quality (from the onset of
boiling up to the dry-out occurrence) was analyzed and discussed for the heat transfer coefficient values, finding
that both convective and nucleate boiling were significant contributions. The same ranges of mass flux and
saturation temperature were also applied to frictional pressure gradient trends with vapor quality, taken in
adiabatic conditions. The thermo-hydraulic performances were then compared with those of R134a, obtaining
lower heat transfer coefficients and very similar pressure gradients. Finally, the collected experiments were
assessed with values predicted from the most quoted correlations available in literature. A new composite
method for nucleate-dominant or convective-dominant mechanisms was proposed for the evaluation of the heat
transfer coefficient, with a mean absolute error of 25.3%, whereas the frictional pressure gradient values were
well predicted with the Friedel correlation, that provides a mean absolute error of 13.8%
Performance degradation of air source heat pumps under faulty conditions
Full text linkThe ongoing European regulations towards the decarbonization of energy intensive sectors such as heating and cooling will lead to an increase of heat pump appliances. Apart from anomalies that can be detected in a preliminary phase after the start-up, soft faults such as refrigerant leakage and heat exchanger fouling may cause a performance degradation in such systems that evolves in time and that must be detected before it becomes too big. From this perspective, in a previous paper (Pelella et al. [1]) it has been demonstrated that, in cooling mode, a seasonal performance penalization up to 50% can be reached in case of a not-planned maintenance scenario, whereas wherever a timed or intelligent maintenance strategy is implemented based on data-monitoring, this impact can be significantly reduced. Therefore, in order to extend the analysis previously carried out to the heating cases, and to evaluate the consequent energetic degradation of the system in case of soft faults occurring, depending on different climate conditions and maintenance scenarios, this paper develops a physic-based model to simulate system components and to describe the fault phenomenology on a residential 2.6 kW air-to-air heat pump, operating in winter mode. The results show that in heating mode a 40% condenser fouling and a 30% refrigerant leakage cause a performance degradation of respectively 16% and 12%, whereas in case of evaporator fouling the performance penalization is only of 3.2%. Moreover, the performance degradation is enhanced by the overlapping effect of simultaneous faults. Finally, from seasonal simulations of the heat pump along an entire machine lifetime of 12 years, it is found that none of the maintenance strategies analysed is able to significantly reduce the number of scenarios penalized by faults, opening to the potential development of systems for the automatic fault detection, diagnosis, and evaluation (FDDE)
Going Beyond Counting First Authors in Author Co-citation Analysis
Full text linkThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Variations on the Author
Full text link“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Full text linkWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
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