Thermographers often use comparative methods to estimate surfaceemissivity. Among the most used is the tape method. In this method a knownemissivity tape in the LWIR (Long Wavelength Infrared) spectrum is placedon the surface to be inspected. After thermal equilibrium, the temperature ofthe tape and the surface under inspection must be the same. In this case, thetemperature observed on the tape is the reference temperature. The emissivityof the surface must then be changed until the reference temperature isreached. It is common practice to admit the value of the emissivity of theadhesive tape as 0.95, there are few studies that present these data withmetrological rigor, which leads to doubts about the emissivity of thecommercial tapes. In this work, experiments were performed on Tekbond,Double A, 3M 101, Rapix, Altape, adhesive tapes for temperatures of 50, 55,60, 65 and 70ºC. An experimental apparatus was developed through which itwas possible to estimate surface reflection, transmission and atmosphericemission for one and two layers of tapes, in order to make emissivitymeasurements possible. Through the data it was possible to statisticallyestimate the LWIR average total hemispheric emissivity as well as theacceptance range to 95% certainty, being therefore equal to ε=0.94±0.03. Itis possible to arm, therefore, that the value of 0.95, usually used as emissivityof the adhesive tape, is extremely reasonable because it is 0.01 of the averagevalue of the Gaussian distribution calculated by this work.
The refrigerant fluid R1234yf is a hydrofluorefine with zero potential for degradation of the ozone layer and low potential for global warming. It is one of the potential substitutes for the currently used R134a in automotive systems. In this work, the pressure drop suffered by the fluids R134a and R1234yf when flowing in a test section through a pipe with a 4.8 mm internal diameter was measured. The pressure drop was plotted as a function of the void fraction at the exit of the test section and the values were compared concerning the change in mass flux, change in saturation temperature, and comparatively between R1234yf and R134a. A significant increase in pressure drop was observed by the increases of the mass flux, showing an increment of 155.46% of the pressure loss from 200 to 300 kg·m-2·s-1 for R1234yf at 35ºC and 161.07% for R134a in the same conditions. Despite being high, those values are expected since increasing mass flux also increases the friction between both phases. On the other hand, by increasing the saturation temperature, the pressure drop is slightly lower once the differences between the densities of the liquid and vapor phases are reduced. Compared with R134a, the R1234yf ecofluid presents less pressure drop, showing a reduction of 24% for 300 kg·m-2·s-1.
Due to global warming considerations, the European Union has banned the use of refrigerants with a GWP greater than 150 in new passenger cars (air-conditioning systems) and 750 for fluids used in residential heat exchangers starting on January 1, 2017 (E. UNION, 2006). In this sense, the R1234yf was developed which consists of a hydrofluorolefin derived from alkenes and commercialized with the name of Opteon YF. Given the need for research related to the use of this fluid, this work has the objective of comparing the data of the local heat transfer coefficient in condensation extracted from the work of Del Col et al. (2010) for flow in a mini channel of 0.96 mm internal diameter, with mass flux of 200, 300, 400, 600, 800 and 1000 kg·(m²·s)-1 at saturation temperature of 40ºC with ten different correlations from literature as well as one neural network. It is verified that among the correlations analyzed the one which best suited the experimental data was presented by Cavallini and Zecchin (1974), with MRD, MARD, and Accuracy values equal to 5.42%, 7.81%, and 96.96%, respectively. The neural network used as a prediction model presents values of MRD, MARD, and Accuracy equals to 2.53%, 3.66%, and 100%, respectively
In Brazil's households, the use of electricity to heat water prevails, but its replacement by solar collectors can be justified due to the country's high level of solar radiation, average temperature and number of sunny days. A drawback preventing the broad spread of this technology its elevated inicial cost that, in spite of the short return on investment time, hinders its insertions on the population lower classes. Thereby, this study's goal is to search a cheaper alternative to one of the collector's most expensive component, by proposing the replacement of the aluminium sheets, used as absorber, by iron ore. This material was chosen due to its abundance in Minas Gerais, low cost and for being dark colored, ensuring high levels of absortance (0,4 < λ< 0,7 µm). A finely grained hematitic () ore was used to achieve an uniform compaction throughout its area, reducing this way, the thermal contact resistance between the absorber and the copper tubes. The collector was built and tested in similar conditions it would have faced during its lifespan, and the result shows that the maximum thermal efficiency was 58%.
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