MUNICIPIO DE MONTERIA – VEREDAS DE SANTA FE, LA ESPERANZA Y TRES BINDES -

The most important component of the flow loop was the microchannel test section on which the experiments were performed. Three test sections were manufactured and tested, each made from an Oxygen-free copper block with dimensions of 25 mm x 12 mm x 72 mm. A single rectangular microchannel was cut in the top surface of each block between the 2 mm diameter inlet and outlet plenums using a Kern HSPC 2216 high-precision micromilling machine with a speed rotation of 10000 RPM, feed rates of 100 mm/min and cut of depth of 0.24 mm (first cut) to 0.08 mm (finishing). The

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microchannel length between the plenums was 62 mm. All three microchannels had the same length and depth but were different in width, as given in Table 3.1. The width of the microchannels was measured using a field effect scanning electron microscope (SEM) (Zeiss Supra 35 VP) at the Experimental Technique Centre of Brunel University, see Fig. 3.5. The SEM has a resolution down to 1 nm and an accuracy of ±1 μm. The channel depth was measured using a microscope (TSER Technology V-200) with an accuracy of ±1 μm. The length of the test section was measured using a digital vernier calliper with a resolution of 10 μm.

Figure 3.5 Width measurements using a Zeiss Supra 35 VP electron microscope: (a) Test section 1, (b) Test section 2, (c) Test section 3.

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Table 3.1 Details of test section dimensions. Test section Width, W

(mm) Height, H (mm) Hydraulic diameter, Dh(mm) Aspect ratio, β Length, L (mm) Test section 1 0.50 0.39 0.438 0.78 62.0 Test section 2 1.00 0.39 0.561 0.39 62.0 Test section 3 1.71 0.39 0.635 0.23 62.0

Preliminary design studies and engineering drawings of the test sections are included in Appendix C.

The construction of the microchannel test section assembly can be seen in exploded view in Fig. 3.6. Six holes, 0.6 mm diameter and 6 mm deep, were drilled in the side of each copper test section, 1.5 mm below the top surface, for inserting thermocouples to measure the wall temperature distribution along the channel. The thermocouple holes were located 1.11 mm below the microchannel base and equi-spaced 12.4 mm apart in the flow direction. A 7 mm diameter hole was drilled through the copper body, parallel to the microchannel, to accommodate the cartridge heater used as a heat source. A transparent polycarbonate cover was clamped to the top of the copper test section and sealed with an O-ring. The cover formed the upper surface of the microchannel and incorporated the flow connections and associated 2 mm diameter plenums leading to and from the microchannel. Static pressure tapping holes were drilled in the cover of six positions: inlet plenum, outlet plenum and four equi-spaced locations along the microchannel.

For this research, three new copper test sections were designed and manufactured to overcome the following problems encountered in a previous study using an earlier version of the test rig:

1. Leakages due to holes in the inlet and outlet plenums and the poor seal between the top surface of the copper block and the microchannel cover.

2. The resistance heater wire was easily broken because it was very thin and a high pressure was exerted on it because the rear surface of the channel was not flat.

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1. Cover plate (polycarbonate); 2. Channel cover (polycarbonate); 3. O-ring seal; 4. Cartridge heater; 5. Copper block; 6. Nitrile foam rubber insulation; 7. Bottom plate (polycarbonate).

3. Heat losses to the ambient were high due to the difficulty of insulating the test section.

4. The previous test section was difficult to dismantle and reassemble.

Figure 3.6 Test section construction showing the main parts (all dimensions in mm).

The inlet and outlet plenums were properly sealed in the new test sections and a cartridge heater was used as the heat source which was more robust and powerful than the resistance wire heater used previously. Furthermore, the test section was insulated using nitrile foam rubber. The new test sections were easy to dismantle and reassemble for maintenance, such as changing a part or a thermocouple.

Photographs of the top views of the three new test sections are presented in Fig. 3.7. Around each microchannel there was a slot for an O-ring seal (Viton rubber cord) with a depth of 0.8 mm and a width of 1 mm to prevent leakage. This seal also provided an easy way to assemble the test section with the cover.

83 Test section 1, W = 0.5 mm, H = 0.39 mm, Dh = 0.438 mm. Test section 2, W = 1 mm, H = 0.39 mm, Dh = 0.561 mm. Test section 3, W = 1.71 mm, H = 0.39 mm, Dh = 0.635 mm.

Figure 3.7 Photographs showing the top view of the three microchannel test sections.

Because the roughness of the wall surfaces may influence the flow behaviour for in small channels, a surface analysis was performed. This investigation involved scanning electron microscope (SEM) and surface roughness measurements on the base surfaces of the microchannel test sections. The SEM measurements were conducted at the Experimental Techniques Centre, Brunel University using a Zeiss Supra 35 VP electron microscope, using setting of 20 kV, 26 mm lens distance, 30 μm aperture and 100X magnification. The surface roughness measurements were performed using a Zygo NewView 5000 surface profiler in the Advanced Manufacturing Technology laboratory, Brunel University. The SEM images obtained presented in Fig. 3.8 show that all test sections appear to have similar surface characteristics. The surface of Test section 1 looks dirty and there are many white black marks. Test section 2 has many scratches as a result of machining, but there is no debris. Test section 3 seems rougher than Test section 1 or Test section 2 and there are many scratches and possibly some debris or dust (circled in yellow).

Surface roughness measurements made on the channel base surface at mid-length along each microchannel are shown in Fig 3.9 for each test section. The roughness was measured at three different axial locations and an average absolute surface roughness was calculated. Local and average surface roughness values for each test section are given in Table 3.2. Relative surface roughness values for the three test sections were

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estimated based on the data in Tables 3.1 and 3.2 to be 0.0023, 0.00226 and 0.00176 for test sections 1, 2 and 3 respectively. These values may be compared with those of Kandlikar et al. (2003), who employed microchannels with relative surface roughnesses ranging from 0.00178 to 0.00355. They concluded from their experiments that the relative roughness of 0.00355 only affected the friction factors and heat transfer coefficients for microchannels with a hydraulic diameter of 0.62 mm or less. Since the relative roughnesses of the channel tested in this work are smaller than those of Kandlikar et al. (2003), the acceptance of their conclusions means that surface roughness should not affect the measured values of friction factor and heat transfer coefficient in this study.

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Figure 3.8 SEM images of microchannel base surfaces: (a) Test section 1, (b) Test section 2 and (c) Test section 3.

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Figure 3.9 Surface roughness measurements; (a) Test section 1, (b) Test section 2 and (c) Test section 3. The measurements shown were taken at a location near the middle of each microchannel for a sample test area of 0.265 x 0.199 mm2.

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Table 3.2 Local surface roughness (Ra) measurements and averaged values for each microchannel.

Test section Near inlet (μm) Near middle (μm) Near outlet (μm) Average (μm) 1 0.979 1.167 0.89 1.01 2 0.954 1.474 1.382 1.27 3 0.961 0.947 1.449 1.12

The local Ra values shown in Table 3.2 evaluated over sample areas (0.265 x 0.199 mm²) on the base of each microchannel test section near the inlet, middle and outlet. The average Ra value for each microchannel is the arithmetic mean of the local values.

3.4 Visualization system

To observe flow boiling patterns, a high-speed camera Phantom V6.0 camera was mounted on a movable camera support above the test section. The camera support could be easily traversed vertically and horizontally, so that the images of bubble movement and growth inside the test section could be captured at different locations along the entire microchannel length. The CMOS camera head sensor had an image resolution of 512 x 512 pixels. For the experiments, the camera was set for a sample rate of 1000 pictures per second, allowing full sensor resolution and post-trigger recording set at 10000 pictures.

In preparation for each experiment, the camera position, focus, speed and resolution were adjusted. Image capture was started once an experiment was running steadily at the desired mass flow rate, inlet temperature and heat flux with flow boiling established. Readings of 1 second duration (1000 pictures) were saved to a computer, in CIN image file format by the Phantom software at each operating condition for subsequent flow patterns analysis. In the visualisation system, illumination should be provided. Phillips halogen lights, 24V, 250 W, were employed for illumination as shown in Fig. 3.10.

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Figure 3.10 Visualisation system.