1,721,001 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
Transient melting of paraffin waxes embedded in aluminum foams: Experimental results and modeling
No full textReticular Structures Embedded in Phase Change Material: Effects of Orientation on Thermal Performance
No full textPhase change materials (PCMs) are plagued by very low thermal conductivity. Metal foams and conductive fillers are often used to increase the overall thermal conductivity of composite PCM structures. Additive Manufacturing (AM) technology like Selective Laser Melting (SLM) can also be employed. This paper presents a numerical investigation on the influence of cell orientation on the thermal performance of PCM-impregnated metal reticular structures produced through Additive Manufacturing (AM). The study includes four cubic cell reticular structures with varying cell sizes (5 mm and 10 mm) and porosities (87% and 93%). Additionally, four distinct cell orientations are examined. Rubitherm’s RT42 paraffin wax is used as the Phase Change Material (PCM), and AlSi10Mg aluminum alloy constitutes the material for the reticular structures. Numerical simulations were carried out using a simplified purely conductive model implemented in the commercial Ansys Fluent software. The results indicate that, for a specific cell size and porosity, cell orientation significantly influences the thermal energy storage rate. Furthermore, a cell orientation has been identified that, in addition to providing good thermal performance, allows easier fabrication by SLM
Novel dimensionless predictive flow pattern map for HFOs inside microscale enhanced tubes
Full text linkModels designated to predict flow patterns in microscale geometries with enhanced surfaces such as micro-finned tubes are scarce in the literature and new low-GWP HFOs could benefit from the utilization of such geometries. Therefore, HFO1234ze(E)’s condensation flow patterns were subjected to visualization inside micro-finned tubes ranging from 4 to 7 mm outer diameters with various geometrical figures. The saturation temperature was set to 30 °C, vapor qualities ranged in the scope of 0.01 to 0.9, and mass fluxes in the magnitude of 100 to 400 kg m-2 s-1. Four distinctive flow patterns are observed, namely intermittent, annular, wavy-stratified, and transitional. Stratification only transpired at low mass fluxes (mainly below 100 kg m-2 s-1) and intermittent flow was only present at vapor qualities close to full condensation. The range in which transitional flow is observable shrinks with a progressive trend of mass flux. The impact of diameter was observed to be negligible, however, a more meticulous assessment highlights smaller ranges of vapor qualities in which transitional flow is present for the tube of 5 mm OD whose helix angle is substantially larger. Datapoints were juxtaposed to previous models of Doretti et al., Chen et al., Mandhane et al., and Jige et al. and the results attested to an inadequacy of accurate predictions made for tube of 4 mm OD. Noting the absence of surface tension force in the aforementioned maps, a novel flow pattern map based on modified Froude versus modified Weber numbers provided an accurate prediction for the three cases under study. Ultimately the model was also deemed fairly suitable for visualization datasets collected from 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
Two-phase flow patterns inside inclined microfin tubes: Experimental investigation and empirical modelling
No full textThe paper studies the effect of inclination angle on condensation flow patterns inside helical micro-fin tubes. Since the open literature is completely lacking in studies concerning such topics with particular reference to low GWP refrigerants, R515B was selected and its flow patterns were investigated utilizing a high-speed camera. Three micro-fin tubes with different outer diameters (4, 5, and 7 mm) were explored at three different upward flow inclinations: 30°, 45°, and 60° with respect to the horizontal. Tests were conducted at constant saturation temperature of 30 °C, by varying the mass flux in the range 50–400 kg m−2 s−1 and the vapor quality from 0.02 to 0.96. Among the over than 500 experimental points, five types of flow patterns were detected: annular, transitional, churn, wavy-stratified, and intermittent. At increasing inclination angles, stratification tends to phase out and be replaced by intermittent flow and churn flow. Regarding the occurrence of an annular flow pattern, which is of high importance as it infers a shear-stress controlled heat transfer mechanism, the inclination did not impose a significant discrepancy, which was surmised to stem from the turbulence-inducing effect of microfin tubes boosting the inertia forces. The flow pattern map of Irannezhad and Diani provides fairly accurate predictions regarding the transition to annular flow pattern
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
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