1,720,997 research outputs found
Boccardi (G.). — Etude d’un groupe de 150 enfants hémiplégiques. Rev. Neur. Inf. 1960, n° 3-4, pp. 188-193
No full textBoccardi (G.). — Etude d’un groupe de 150 enfants hémiplégiques. Rev. Neur. Inf. 1960, n° 3-4, pp. 188-193. In: Bulletin de psychologie, tome 17 n°228, 1964. p. 669
Experimental investigation on the performance of a transcritical CO2 heat pump with multi-ejector expansion system [Étude expérimentale sur la performance d'une pompe à chaleur au CO2 transcritique équipée d'un système d'expansion multi-éjecteur]
No full textambient temperature in the range of −15–12 °C according to the standard UNI EN 14511/2011. The results show the possibility to reach an optimum of the COP by varying the ejector area, once the other parameters are fixed, as a consequence of the ejector regulation on the pressures at the inlet and outlet of the compressor. © 2017 Elsevier Ltd and IIRcompressor frequency in the range of 30–60 HzCarbon dioxide is becoming an interesting option also for HVAC heating appliances due to its eco-friendly characteristics. The incurrence of transcritical cycles makes the use of ejectors attractive to improve performance. Part load conditions and varied ambient temperatures imply the use of variable geometry systems of appropriate design to keep their efficiency high. This paper investigates experimentally a multi-ejector air-to-water CO2 heat pump for heating needs, measuring the performance of the system and of each component under partial and full load conditions. A sensitivity analysis was run varying, separately or in a combined way, the following parameters or operating conditions: ejector area ratio in a range of 33.1–86.6
Thermodynamic Analysis of a Multi-Ejector, CO2, Air-To-Water Heat Pump System
No full textNowadays, air conditioning systems for residential and office buildings, contribute largely to the energy consumptions and to the direct and indirect emissions of greenhouse gases. Carbon dioxide (CO2) could be an interesting option to replace traditional HFCs in space heating applications, due to its environmentally friendly characteristics: zero ODP and extremely low GWP, but, in order to spread its use, improvements in performances are needed. In fact, CO2 requires transcritical cycles with high expansion losses. The use of an ejector can reduce these losses and improve the performances up to 30% (depending on the performances of the ejector itself and on the operating conditions). In the a/c applications, characterized by variable operating conditions, multi-ejector systems could be used, where some ejectors work in parallel, in different combination, varying the operating conditions. Currently, a project of DTE-PCU-SPCT Department of ENEA and Industrial Engineering Department of Federico II University of Naples, is in progress, in order to evaluate experimentally the effect of several ejectors geometries on the global performance of a CO2 heat pump working with a transcritical cycle. As a part of this project, a complete heat pump system for production of hot water for sanitary use and for space heating is tested to investigate the effect of the ejector size on the balancing of the global performance of the whole system. © 2016 The Authors
Experimental performance evaluation for a carbon dioxide light commercial cooling application under transcritical and subcritical conditions
No full textCO2 is one of the most interesting non toxic, nor flammable, low GWP natural fluid to be used in applications with a large direct contribution to global warming, such as vending machines. In this paper the results of an experimental investigation on a small capacity, air-forced refrigerating plant working with CO2 are presented. The air inlet temperatures at gas cooler and evaporator were varied between 16 °C and 31 °C and between -25 °C and 25 °C, respectively, to cover the range of temperatures under conditions typical of commercial refrigeration, during quasi-steady operation. The thermodynamic analysis of the system performance was carried on varying independently the pressure at the gas cooler, by overfeeding the liquid receiver to point out the effect of refrigerant charge. The results allow to describe the performance of the system in a map varying the boundary conditions; at the same time the corresponding variations of the thermodynamic cycle, the COP and the mass flow rate are reported. © 2013 Elsevier Ltd. All rights reserved
Comparative analysis of calculation methods of pressure drops for two-phase flow through pipelines
No full textDespite the importance of an adequate and correct sizing of emergency relief systems, a high level of uncertainty is still present when two-phase liquid-vapor flow can establish. In such a case, no single correlation is presently generally applicable to calculate the pressure drop through a relief pipeline in a wide range of relief conditions. In the present paper a large number of new experimental data have been produced and compared with two of the most widely known and adopted calculation methods: the Friedel model and the classical Lockhart-Martinelli model. They belong to either of the two main groups of prediction models, the homogeneous and non-homogeneous models. The results provide useful insights in their accuracy and range of applicability, and are expected to be of help in the correct sizing of these fundamental safety systems. © 2015 Taylor & Francis Group
Motive flow calculation through ejectors for transcritical CO2 heat pumps. Comparison between new experimental data and predictive methods
No full textThe revival of CO2 as refrigerant is due to new restrictions in the use of current refrigerants in developed countries, as consequence of environmental policy agreements. An optimal design of each part is necessary to overcome the possible penalty in performance, and the use of ejectors instead of throttling valves can improve the performance. Especially for applications as CO2 HPs for space heating, the use of ejectors has been little investigated. The data collected in a cooperation project between ENEA (C.R. Casaccia) and Federico II University of Naples have been used to experimentally characterize several ejectors in terms of motive mass flow rate, both in transcritical CO2 conditions and not. A statistical comparison is presented in order to assess the reliability of predictive methods available in the open literature for choked flow conditions. © Published under licence by IOP Publishing Ltd
Realizzazione di un prototipo di macchina frigorifera caldo/freddo dedicata al settore alimentare
Full text linkI costi energetici legati ai trattamenti termici ad alta e bassa temperatura degli alimenti costituiscono una importante porzione dei costi energetici globali sostenuti dal sistema industriale: nel mondo essi sono stimati in circa due miliardi di dollari l’anno. Tali costi potrebbero essere considerevolmente ridotti realizzando i trattamenti con sistemi tecnologicamente avanzati e integrati. In particolare, si ritiene di poter contribuire a tali obiettivi attraverso la realizzazione di macchine reversibili in grado di produrre sia “caldo”
che “freddo” a seconda delle necessità, con ridotto impatto economico. In generale, i processi termici prevedono una fase di riscaldamento fino a temperature comprese tra i 65‐80°C ed una successiva fase di raffreddamento per la conservazione a temperatura controllata (generalmente dell’ordine di 4°C) dell’alimento trattato. In questa linea di attività è stata valutata l’opportunità di realizzare tali trattamenti avvalendosi di macchine
reversibili a CO2 (R744) per una serie di motivi, legati sia ad aspetti prestazionali, che di ridotto impatto ambientale del sistema complessivo. La scelta della CO2 come fluido di lavoro è legata a caratteristiche peculiari della stessa quali l’atossicità e la non infiammabilità. In più tale fluido è caratterizzato da un ridottissimo impatto ambientale, comprovato da valori del potenziale di riduzione dell’ozono (ODP) pari a 0 e del potenziale di riscaldamento globale (GWP) pari a 1. Sotto il profilo delle prestazioni di scambio termico è stata spesso evidenziata l’elevata attitudine di un ciclo transcritico a CO2 a fornire calore con rendimento elevato, attraverso il gas‐cooler, a un fluido che debba subire un trattamento termico di riscaldamento, per l’ottimale corrispondenza dei profili di temperatura dei due fluidi avviati allo scambiatore. D’altra parte, è noto che è possibile realizzare cicli frigoriferi utilizzanti CO2, che possono garantire livelli di
prestazione (COP frigorifero) soddisfacenti a diversi livelli di temperature delle sorgenti calda e fredda. Per i livelli di temperatura minima previsti per la maggior parte dei trattamenti termici (dell’ordine dei 5°C) richiesti dai prodotti alimentari, si possono peraltro prevedere valori di COP frigoriferi soddisfacenti anche disponendo di sorgenti calde (aria o acqua) al gas cooler a temperature relativamente elevate. Per questi motivi, si è ritenuto possibile proporre in questa attività di ricerca una macchina a CO2 acqua-acqua di tipo reversibile in grado di realizzare trattamenti termici di raffreddamento e/o di riscaldamento. Si è proceduto quindi a verificare i range di temperatura ottimali di utilizzo di una siffatta macchina e sono stati proposti vari schemi di impianto. Sono stati selezionati i principali componenti adatti a realizzare il ciclo termico completo per una situazione di riferimento caratterizzata da condizioni rmodinamiche che includessero il maggior numero di trattamenti termici possibili. E’ stato inoltre affrontato il problema dell’inversione del ciclo di funzionamento della macchina, che rappresenta il principale nodo tecnico da risolvere, dato che gli scambiatori, lavorando da evaporatori o da condensatori/gas‐cooler secondo le
necessità (produzione di caldo o freddo) devono presentare delle caratteristiche specifiche. La scelta dello schema d’impianto più appropriato all’applicazione in parola e il definitivo dimensionamento degli scambiatori di calore è infine subordinato alla completa conoscenza delle prestazioni garantite dal compressore (bistadio con inverter) e dal sistema di espansione elettronico selezionati per l’applicazione
nelle condizioni termodinamiche di riferimento. Per ottenere informazioni più dettagliate sulle prestazioni ottenibili con tali componenti ci si è infine dedicati allo studio delle problematiche legate alla realizzazione degli idonei sistemi di gestione e controllo del compressore e della valvola elettronica di laminazione
Water two-phase flow through pressure safety valve with variable backpressure: check of calculation methods to estimate mass flow-rate and critical flow conditions
No full textA careful design of pressure safety valves (PSV) is an essential requirement for safeguarding of industrial plants; reliable correlations are available for PSV design in the cases of liquid or gas discharge. Instead, if two-phase flow is possible, especially for low vapour quality (less than 10%), PSV design becomes very difficult owing to complex thermal hydraulic phenomena that happen between the two phases; moreover, in this situation, the prediction of the critical flow onset (which occurs when the fluid velocity becomes equal to the sound velocity in the fluid) is very important for a correct design because it involves a choked flow situation.
Currently there are some calculation methods, based on different simplifying hypotheses, that try to predict the two-phase flow-rate through a PSV knowing the inlet fluid conditions (pressure, quality or temperature) and the outlet pressure; however, none of them is acknowledged as being reliable for every situation and, therefore, there are not standards for PSV design under two-phase conditions.
This paper shows the results of an experimental research carried out through a PSV with steam-water two-phase flow. The experimental data are compared with the results of a calculation method based on the homogeneous model with non equilibrium hypotheses. The performance of the model is evaluated as a function of inlet and outlet parameters; an analysis of model critical flow prediction capability is presented too, with some uncertainties caused by the test procedures (here the mass flow-rate is imposed
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
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