1,721,245 research outputs found
Characteristics of R513A evaporation heat transfer inside small-diameter smooth and microfin tubes
No full textThis paper investigates the characteristics of R513A during flow boiling. R513A is an azeotropic mixture made of R1234yf and R134a (0.56/0.44 by mass), and it has been proposed as a direct drop in of the common R134a. Experimental tests were run in a wide range of operative conditions: mass velocity in the range 200-800 kg m−2 s−1, heat flux in the range 12-60 kW m−2, for saturation temperatures of 15, 20 and 25°C. Two different mini tubes were tested: a smooth tube with an inner diameter of 2.5 mm, and a microfin tube with an inner diameter at the fin tip of 2.4 mm. Heat transfer coefficients and frictional pressure drops were evaluated from the experimental measurements. The approximately same diameter of the two tubes permitted to highlight the effect of the microfins on the thermal and hydraulic behavior during R513A flow boiling. Finally, experimental heat transfer coefficients and frictional pressure drops were compared against values predicted by correlations available in the literature
A Fast On-state Voltage Measurement Circuit for Power Devices Characterization
No full textThe precise measurement of the on-state voltage of power devices working in real operating conditions is the key for their characterization as well as health monitoring. This letter proposes a new on-state voltage measurement circuit featuring very high bandwidth and accuracy. An auxiliary circuitry allows to decouple the measurement system reference potential from the noisy device-under-test environment. DC, AC, and dynamic performance have been validated from measurements on a prototype
R513A condensation heat transfer inside tubes: Microfin tube vs. smooth tube
No full textThe imminent phase-down of the common refrigerant R134a is calling for lower GWP alternatives. Real alternatives must have a lower global warming impact and they should be not flammable. In this context, R513A (azeotropic mixture made of R1234yf and R134a at 56% and 44% by mass) has been proposed as alternative to R134a due to its similar thermodynamic and transport properties and due to its lower GWP. This paper proposes a direct comparison between the thermal performances of a 3.5 mm ID smooth tube and those of a 3.4 mm ID microfin tube, during R513A condensation under the same working conditions of vapor quality (from 0.10 to 0.99), of mass velocity (from 100 to 1000 kg m−2 s−1), and of saturation temperature (30 °C and 40 °C). The comparison permits to highlight in which working conditions the microfin tube leads to a real heat transfer augmentation which is higher than the mere increase of heat transfer area. In the end, the experimental heat transfer coefficients, both for the smooth tube and for the microfin tube, are compared against values calculated with empirical correlations from the open literature
Numerical analysis of the thermal energy storage in cellular structures filled with phase-change material
Full text linkThis paper reports the results of a numerical study on the thermal performance of metal cellular structures that can be obtained by additive manufacturing (selective laser melting) when they are impregnated with phase change material (PCM) for possible applications in electronic cooling. Two body-centered cubic (BCC) periodic structures with cell sizes of 5 mm and 10 mm and a porosity of 87%, made of two solid materials (aluminum alloy and copper), and two paraffins with characteristic melting temperatures of 55 and 64 °C were considered. The numerical simulations are carried out using the commercial code ANSYS Fluent and are based on a previously validated purely conductive heat transfer model. The computational domains include just small repetitive portions of the considered composite structures, thus allowing substantial savings of computational time. Computed results show that, with both paraffins, the copper made finer BCC structure (5 mm) yields the best thermal performances, i.e, the shortest PCM melting time and the highest rate of thermal energy storage during transients
- …
