1,721,015 research outputs found
Measurement of local boiling heat transfer coefficient (HTC) within a brazed plate heat exchanger (BPHE) and heat transfer assessment of low-GWP R134a replacements
No full textThis article shows the local boiling HTCs of R134a and three low-GWP replacements, R1234yf, R1234ze(E) and R152a within a (BPHE). The experimentation was performed in a test-section specially set up for the measurement of the local two-phase HTC at a saturation temperature of 10 °C, with a refrigerant mass velocity from 9.5 to 35.2 kg m−2 s−1, incoming vapour quality between 0.2 and 0.3 and almost complete vaporisation. Both nucleate boiling and forced-convection boiling play an important role in these tests, depending, mainly, on the location along the plate. The comparison with theoretical models shows that both Longo et al. (2015) and Amalfi et al. (2016) models reproduce very well the experimental HTCs. The measurement of local temperature profiles within the BPHE allows to couple the evaluation of thermal and hydraulic performances of refrigerants into a single parameter named Total Temperature Penalty (TTP). The TTP analysis highlights similar boiling performance for R134a, R1234yf and R152a refrigerants, while R1234ze(E) displays worse performance
Measurement of local condensation heat transfer coefficient (HTC) within a brazed plate heat exchanger (BPHE) and heat transfer assessment of low-GWP R134a replacements
Full text linkThis paper presents local condensation Heat Transfer Coefficients (HTCs) and temperature profiles of R134a and three low-GWP replacements, R1234yf, R1234ze(E), and R152a, within Brazed Plate Heat Exchangers (BPHEs). The measurement of local temperature profiles allows to couple the evaluation of thermal and hydraulic performances of refrigerants into a single parameter named Total Temperature Penalty (TTP). The experimentation was carried out in a prototype BPHE instrumented with thermocouples inserted into the plate wall to measure the local heat flux and wall temperature. The transition from gravity-controlled condensation to mixed condensation was observed in the refrigerant mass velocity interval 15−20 kg m−2s−1. The experimental HTCs were compared with existing theoretical models finding a Mean Absolute Percentage Error (MAPE) within 10–15 %. The TTP analysis highlights similar condensation performance for R134a, R1234yf and R1234ze(E) refrigerants, while R152a performs better than R134a
Programma di Calcolo MVP-CARD: Acquisizione ed Elaborazione di Immagini. Manuale d'Uso e Manuale del Programmatore
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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
Heat-transfer assessment of the low GWP substitutes for R245fa in ORC
No full textThis paper presents the heat-transfer assessment of different low GWP substitutes for traditional R245fa as a working fluid for ORC. The assessment is carried out by applying a theoretical analysis based on specific Performance Evaluation Criteria (PEC), the Penalty Factor (PF) and the Total Temperature Penalisation (TTP), in the specific case of boiling and condensation inside smooth and microfin tubes. The results of the assessment confirm that the refrigerants R1233zd(E), R1234ze(Z) and R1224yd(Z) have comparable heat transfer characteristics to R245fa. The use of theoretical assessments tuned on experimental measurements provides a sound procedure for the performance comparison and optimization of the low GWP alternatives for traditional HFC working fluids in ORC
Saturated vapour condensation of R134a inside a 4 mm ID horizontal smooth tube: Comparison with the low GWP substitutes R152a, R1234yf and R1234ze(E)
No full textThis study performs the comparative analysis of R134a, R152a, R1234yf and R1234ze(E) condensation inside a 4 mm ID smooth tube. The experimental tests were carried out at three different saturation temperatures, 30, 35, and 40 °C, at different vapour qualities and mass velocities to evaluate the specific contribution of refrigerant mass flux, mean vapour quality, and condensation temperature (pressure). The frictional pressure drops exhibit great sensitivity to all the operating variables considered, while the condensation heat transfer coefficients show great sensitivity only to refrigerant mass flux and mean vapour quality. The transition between gravity dominated and forced convection condensation occurred in the range of the equivalent Reynolds number 10,000–20,000. The Friedel (1979) correlation was able to predict properly the whole set of frictional pressure drops data, while the Akers et al. (1959) model gave a very good estimation of the forced convection condensation heat transfer coefficients. R152a, R1234yf and R1234ze(E) show condensation heat transfer and pressure drop performances similar to those of R134a, therefore they seem valuable long-term low GWP substitutes for R134a
Experimental study on the role of metal foams in a vertical PCM-based thermal energy storage tube
No full textIn this work an aluminum foam is proposed and studied to increase the thermal conductivity of a commercial paraffin wax having a melting temperature of around 40 °C and to decrease the charging and discharging times. Two samples were experimentally tested. Both are tube-in-tube heat exchangers where water flows in the tube and there is Phase Change Material (PCM) in the annulus. In one sample there is a 40PPI aluminum foam inside the PCM, while the reference sample contains only PCM. Experimental tests were carried out by running charging/discharging cycles feeding the tube with hot and cold water, respectively. The influence of water flow rate and inlet water temperature were investigated. The use of the aluminum foam was demonstrated to speed up both the charging and the discharging processes of the latent thermal storage, and to make the temperature field more unifor
Comparative analysis of microfin vs smooth tubes in R32 and R410A condensation
No full textThis study performs the comparative analysis of R32 and R410A condensation inside a 4.2 mm ID microfin tube: the effects of saturation temperature, vapour quality and mass velocities are evaluated. The heat transfer coefficients show a great sensitivity to vapour quality, a non-negligible sensitivity to saturation temperature/pressure, and a non-monotonic trend versus mass velocity. The microfin tube exhibits a maximum heat transfer enhancement with respect to the smooth tube around 3.5−3.6 at a mass velocity of 200 kg m−2s−1 and a vapour quality of 0.8. The optimum mass velocity for R32 and R410A condensation inside the microfin tube is in the range 150−300 kg m−2s−1. R32 exhibits condensation heat transfer coefficients 20 − 30% higher and frictional pressure drops 10 − 20% higher than those of R410A and it seems a valuable low GWP substitute for R410A
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