1,721,040 research outputs found
Peripheral heat transfer coefficient during flow boiling: Comparison between 2-D and 1-D data reduction and discussion about their applicability
No full textThis paper presents a critical analysis of possible data reduction procedures for the evaluation of local heat transfer coefficient during flow boiling experiments. The benchmark method using one-dimensional (1-D) heat transfer in a heated tube was compared to a new data reduction method in which both radial and circumferential contributions to the conductive heat transfer inside a metal tube are considered. Using published experimental flow boiling data, the circumferential profiles of the wall superheat, inner wall heat flux, and heat transfer coefficients were independently calculated with the two data reduction procedures. The differences between the two methods were then examined according to the different heat transfer behavior observed (symmetric or asymmetric), which in turn was related to the two-phase flow regimes occurring in a channel during evaporation. A statistical analysis using the mean absolute percentage error (MAPE) index was then performed for a database of 417 collected flow boiling data taken under different operating conditions in terms of working fluid, saturation temperature, mass velocity, vapor quality, and imposed heat flux. Results showed that the maximum deviations between the two methods could reach up to 130% in the case of asymmetric heat transfer. Finally, the possible uses of the two data reduction methods are discussed, pointing out that the two-dimensional (2-D) model is the most reliable method to be employed in the case of high-level modeling of two-phase flow or advanced design of heat exchangers and heat spreader systems
Flow boiling of carbon dioxide: Heat transfer for smooth and enhanced geometries and effect of oil. state of the art review
Full text linkThis paper presents a state-of-the-art review on flow boiling of carbon dioxide, including experimen- tal studies and correlations for smooth and enhanced tubes, with pure CO 2 and CO 2 /lubricant mixtures. Specifically, 5223 CO 2 heat transfer coefficient data in smooth tubes are collected, and the effect of the operating conditions is discussed. Additional 883 data points in microfin tubes and 1184 experimental heat transfer coefficients in smooth tubes with CO 2 /oil mixture are also collected, and the influence of the microfin structure and of the oil presence on the heat transfer mechanism is analyzed. The statistical analysis has highlighted that the CO 2 -based correlation of Fang et al. is very accurate ( MAE = 5.1%) for the smooth tube database, whereas the heat transfer coefficients in microfin tubes are satisfactorily predicted ( MAE = 30.5%) with the model of Mehendale. Among the available cor- relations for CO 2 /oil mixture in smooth tubes, the method of Gao et al. provides the highest accuracy ( MAE = 63.2%)
Un confronto tra misure di riverberazione ottenute con eccitazione impulsiva e con rumore interrotto
No full textSulla sensibilità alla temperatura di sonde anemometriche a filo nella convezione naturale ad aria
No full textUna procedura per la stima della sensibilità alla temperatura di sonde anemometriche a filo
No full textAn experimental investigation on optimal heat rejection pressure for a carbon dioxide split system
No full textFlow boiling of R452A: Heat transfer data, dry-out characteristics and a correlation
Full text linkThis paper presents an experimental investigation on two-phase heat transfer and dry-out occurrence for refrigerant R452A in a single horizontal circular stainless-steel tube having an internal diameter of 6.0 mm. The effects of mass flux (from 150 to 600 kg/m2s), saturation (bubble) temperature (from 23 to 55 °C) and heat flux (from 10 to 65 kW/m2) are investigated and discussed. Heat transfer coefficient and dry-out vapor quality data are then compared to R404A results in the same operating conditions, observing that the nucleate boiling contribution of the new blend is penalized by its very high temperature glide during evaporation. The assessment of some dry-out and flow boiling heat transfer coefficient prediction methods is finally carried-out and a correction factor on the nucleate boiling term is proposed to take into account the negative effect of the temperature glide difference on the mass diffusion in the liquid. By implementing this modification on two chosen asymptotic models, the statistical error analysis is considerably improved
Progettazione costruttiva di un prototipo di pompa di calore a CO2 (R744) invertibile del tipo aria-aria e realizzazione della facility di prova
Full text linkQuesta relazione riguarda le attività svolte nell’ambito dell’accordo di collaborazione tra ENEA e l’Università degli studi di Napoli Federico II, Dipartimento di Energetica, Termofluidodinamica Applicata e Condizionamenti Ambientali (DETEC) per una attività di ricerca dal titolo “Progettazione costruttiva di un roof‐top a pompa di calore ad R744 (CO2)”, a sua volta rientrante nell’ accordo di programma MSE‐ENEA sulla Ricerca di Sistema Elettrico, Piano Annuale di Realizzazione 2011. L’ENEA ha poi seguito tutta la parte di progettazione e realizzazione della facility di prova della macchina prototipo.
L’applicazione considerata è pensata per la climatizzazione sia estiva che invernale di grandi locali commerciali; inoltre prevede l’utilizzo di sbrinamento a gas caldo in diverse soluzioni circuitali in modo da consentire il funzionamento tramite il solo allaccio alla rete di distribuzione dell’energia elettrica. Il sistema è stato progettato nei suoi componenti e successivamente tramite la modellazione fisica dei singoli componenti è stato sviluppato un codice di calcolo operante in matlab (e riportato in appendice) che
ha consentito la previsione del funzionamento in condizioni diverse da quelle di progetto, mostrando prestazioni previsionali soddisfacenti. Infine sono riportati anche i risultati, anch’essi soddisfacenti, di alcuni test preliminari svolti nell’ultima settimana del mese di luglio 2012 sul prodotto assemblato e le foto della posa in opera presso la sede di ENEA – Casaccia in Roma avvenuta nella prima settimana di settembre
Factors affecting energy consumptions and carbon footprint in refrigerated road transport: A brief review of recent advances in technology, materials and methods
No full textIn this paper some research related to the thermal insulation of the body, the energy consumption and the environmental impact of the cooling units are presented. The use of new materials such as the aerogels (both at ambient pressure and under vacuum conditions) is considered as an option and discussed in terms of performance, reliability and costs. Concerning usual solutions for the insulating walls based on polyurethane foams, the actual thermophysical properties of the blowing agents used in insulating walls are discussed in the view of the new regulations for the use of F-GAS refrigerants. The methods to calculate the air infiltration rate and radiative properties of the walls are commented. The ageing of the material used for the insulation body is discussed with the relative issues related to the appropriate prediction of the life of in-service units. Concerning the energy consumptions and environmental impact results from works related to the classic option of vapor compression cycles is compared to new ones such as cryogenic units in terms of total environmental impact, running costs and feasibility. Finally, works discussing new alternatives for refrigerants in vapor compression systems are presented
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