Las confusiones irresueltas de la enseñanza de la historia (2017)

The experiments were conducted in a laboratory environment using water as the working fluid. The experimental setup is shown in Fig. 1. The description of the main system components is provided below:

2.3.1 Solar Collector: A single-tube flat-plate solar collector made of commercial copper was used in this study. The collector area was about 915×155 mm2. The collector was formed by making a groove in the middle of the copper plate along its length and by attaching the tube to that groove via soldering. The thickness of the plate was around 1.1 mm. The outer and inner

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diameters of the tube were 16 mm and 13.38 mm, respectively. Thurmalox solar coating was used to increase the absorptivity of the collector.

2.3.2 The thermocouple rake assembly: The fluid temperature inside the tube was measured at nine locations in a cross-sectional plane using a rake of thermocouples. The complete rake assembly is shown in Fig. 2. It comprised of two rakes made by carefully bending stainless steel wire and attaching legs made of the same stainless steel wire. One rake has four legs and the other has five legs (see Fig. 2). All pieces were attached using epoxy adhesive maintaining an equal distance of 3.3 mm between the legs. The configuration of the rake with respect to the tube diameter is shown in Fig. 3. The rake was attached to the inner side of a thin cylinder whose outer diameter was slightly smaller than the inner diameter of the collector tube so that it could easily slide inside the collector tube. The other end of the cylinder was attached to a long thin steel tube using two metallic legs (see Fig. 2). The outer diameter of this tube was about 4.8 mm and its thickness was 0.5 mm. Nine thermocouples were passed through the tube from the free end and were taken out at the other end and attached to the legs of the rake. The thermocouples were bent carefully so that their tips became collinear with the respective leg of the rake.

2.3.3 Panel heater: An Omega QF 063610 radiant panel heater (2160 W at 240 V) was used as the radiant heat source. The radiating area of the heater was the same as the absorbing area of the collector. In order to control the temperature of the heater, a Zesta ZEL-9100 PID controller was used. A K-type thermocouple was used to provide feedback from the heater surface to the controller. The accuracy of the controller is ± 0.1 oC.

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2.3.4 The support frame: A wooden frame was build to support the collector and the heater (see Fig. 4). The function of the frame was to hold the heater and the collector at various inclination angles. It had the provision for adjusting the inclination angle with the help of two hinge joints. The system could be locked after selecting a particular angle. The structure was designed in such a way that at any given inclination, the collector and the heater remained parallel and separated by a distance of 25 mm.

2.3.5 Temperature sensors: The thermocouples used in the rake were K-type thermocouples: Omega 5TC-GG-K-36-72. Two thermistors (TJ 72–44033) with an accuracy of ±0.1˚C were also used to measure the bulk water temperatures at the inlet and outlet of the collector due to their better sensitivity. The accuracy of the thermocouples was ±0.4 oC and the response time in still air was 1 sec (for 63.2% of the change).

2.3.6 Measurement locations inside and on the collector: The locations of temperature measurements inside the collector are shown in Fig. 3. As discussed earlier, 9 thermocouples were attached to the rake to measure the fluid temperature in a given tube cross-section horizontally and vertically. A reference coordinate system was considered to ensure consistency of the rake orientation for different experimental runs (see Fig. 3). The rake assembly has the capability to move axially along the collector tube. This allowed to measure the fluid temperature at various cross-sections along the tube length. The tube surface temperature was also measured at three axial locations, x/L =0.3, 0.5 and 0.8, where L is the length of the collector and x is measured from the upstream end of the tube. These locations were selected in such a way to cover the tube wall temperature variations over the extent of the collector tube. Four thermocouples were employed at each location on the tube surface 90o apart. Note that due to

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some experimental difficulties, the surface temperatures were measured only for the horizontal orientation of the collector.

2.3.7 Calibration: All thermocouples were calibrated by placing them in an insulated hot water bath along with a high accuracy thermometer (Kessler-2150, 76 mm immersion, range -36 to 54oC) which was used as the reference. The water in the bath was allowed to cool down and data was recorded at different bath temperatures. Calibration equations were obtained for all the thermocouples and were incorporated in the LabView program to obtain corrected temperature data. All the calibration curves provided almost linear relationships with very small bias errors.

2.3.8 Insulation: Reflectix R-4 and glass wool insulations were used to insulate the collector and heater to minimize heat loss to the surrounding. The insulation has three layers i.e. the layer of glass wool was sandwiched between two layers of Reflectix. The three-layered insulation was wrapped around the collector and the heater. The system was insulated from all the sides including inlet and outlet. Pipe insulation was used to insulate pipe fittings at the ends of the collector.

2.3.9 Data Acquisition System and Labview: A National Instruments data acquisition system (SCXI-1000) was used for acquiring data from the sensors. It has a data acquisition card (PCI- 6036E), a signal conditioner (SCXI-1102) and a terminal block (SXCI-1303). The data was acquired via LabView software.

2.3.10 Experimental conditions and procedure: In order to maintain the accuracy of the flow, two flow meters were used before the inlet. Water from the water supply first passed through the

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high range rotameter (Omega FL 4205 controlled with an external valve) and then through low range rotameter (Omega FL-1448-G). The purpose of using the high range flow meter was to minimize pressure fluctuations at the low range flow meter to improve its accuracy.

The experimental procedure is described as follows: The flow rate was set first and then the heater was turned on and the PID controller was set at the desired temperature. The system was allowed to reach a steady state. The data acquisition system was turned on and the temperature measurements were begun when the rake was at position 1 (see Fig. 3). The angular position of the rake was also corrected using the reference scale to ensure that the thermocouples in the rake are positioned horizontally and vertically. Measurements were made at 11 axial locations along the collector tube length (see Fig. 3), by moving the rake outwards. The flow rate was changed and the procedure was repeated. Experiments were conducted at three collector inclination angles of 0˚, 30˚ and 45˚. At each inclination angle, the experiments were conducted at three heater temperatures of 250˚C, 275˚C and 300˚C. At each inclination angle and heat temperature, data was recorded at six different flow rates, which were 0.027, 0.037, 0.047, 0.056, 0.067 and 0.078 gpm. For reference, the corresponding Reynolds number based on no heating condition ranged from 250 to 620. Thus, 54 experimental runs were conducted at different conditions.