Chapter 6. Pool Boiling CHF Enhancement of Nanofluids
6.2. Experimental Setup
6.2.1. Preparation of silver, copper oxide and aluminum oxide nanofluids by EEWL
The silver, copper oxide and aluminum oxide nanofluids for pool boiling experiment were prepared by electrical explosion of the wire in liquid method. The preparation of nanofluids was progressed the identical process. Detailed experimental conditions are summarized in Table 6-1. The systemic conditions were based on the design of experiment (DOE) methodology of a previous study in chapter 5. And, two copper oxide nanofluids were more prepared with adding 10ml of NH3∙H2O under same experimental conditions.
79 Table 6-1. Summary of the EEWL process conditions
Capacitance Charging voltage
Material / type Wire diameter Wire length
Liquid Liquid volume
Concentration
30 µF 3 kV
Aluminum, copper, or silver / wire 0.1 mm
3.8 mm Deionized water
500 mL 0.001 vol%
6.2.2. Preparation of xGnPs and xGnPs oxide nanofluids
xGnPs (M-5) particles were purchased from XG Sciences, Inc. The xGnPs combines a lower price and layered structure with superior thermal and electrical properties. The xGnPs used in this study displayed an average thickness of ~6-8nm and were synthesized by alkali metal intercalation.
The material properties of the xGnPs powders are summarized in Table 3-2.
xGnPs oxide was synthesized from natural xGnPs (M-5) by the method of Hummers and Offeman[15]. The xGnPs (M-5) (5g) was put into and 80℃ solution of concentrated H2SO4 (30mL), K2S2O8 (2.5g), and P2O5 (2.5g). The mixture was thermally isolated and allowed to cool to room temperature over a period of 6h. The mixture was then carefully diluted with distilled water, filtered, and washed on the filter until the rinse water pH became neutral. The product was dried in air. The oxidized xGnPs (M-5) powder(5g) was put into cold (0℃) concentrated H2SO4(115mL). KMnO4 (15g) was added gradually with stirring and cooling, so that the temperature of the mixture was not allowed to reach 20℃. The mixture was then stirred at 35℃ for 2h, and distilled water (230mL) was added.
The reaction was terminated by the addition of a large amount of distilled water (700mL) and 30%
H2O2 solution (12.5mL), after which the color of the mixture changed to bright brown. The mixture was filtered and washed with 1:10 HCl solution (1.25L) in order to remove metal ions. Then, the GO product was suspended in distilled water, which was subjected to dialysis to completely remove metal ions and acids.
The high-resolution-transmission electron microscopy (HR-TEM) and field emission-scanning electron microscopy (FE-SEM) images of xGnPs and xGnPs oxide particles are shown in Figure 6-1.
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The thickness of xGnPs (M-5) and xGnPs oxide particles are about 15.3nm and 11.5nm, respectively.
Table 6-2. Material properties of xGnPs
Physical structure Interaction Thermal conductivity Density
Platelet π-π 3000W/mK ll, 6W/mK ⊥ ~2.2 g/㎤
Figure 6-1. TEM images of (a) xGnPs, (b) xGnPs oxide powders dispersed in water, and (c) xGnPs(left)/xGnPs oxide(right) nanofluids with concentration of 0.001vol%
The two different nanofluids, including xGnPs and xGnPs oxide, were prepared by a two-step method:
xGnPs and xGnPs oxide powders were added to distilled water with no surfactant. To stably disperse the particles, sonication was used. The xGnPs particles consist of a higher number of graphene sheets that form a platelet. The xGnPs and xGnPs oxide particles dispersed in deionized water are shown in
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Figure 6-1 (c). The zeta potential of the nanofluids (Nano ZS, Malvern, UK) were -53.5 mV for xGnPs nanofluids and -62.5 mV for xGnPs oxide nanofluids at a particle concentration of 0.001 vol%.
Figure 6-1 (d) shows the Raman spectrum of the xGnPs and xGnPs oxide particles. The spectrum of the xGnPs oxide shows a broadened and blue shifted G-band at 1597 cm-1 and the D-band at 1353 cm-1. The G-band arises from the E2g vibrational mode of sp2-carbon systems. The spectrum of the xGnPs shows a strong G-band at 1585 cm-1 with a small shoulder, identified as the D’-band at 1626 cm-1, and a D-band at 1353 cm-1. The D-band is caused by disordered structure into sp2-carbon systems. The intensity ratio of the D-band to the G-band is determined the quality of xGnPs structures.
D’-band is also proportional to the concentration of defects. If there are some disordered impurities or surface charges, the G-band is split into two bands, G-band (1585 cm-1) and D’-band (1626 cm-1). All kinds of sp2-carbon materials have a strong 2D-band in the range of 2500 - 2800 cm-1. 2D-band is a second-order two-phonon assisted double resonant processes. 2D-band can be also used to determine the number of layer of graphene. The single-layer graphene exhibits a sharper 2D-peak located below 2700 cm-1 while bilayer have a broader peak. Multi-layer has a milder and broader peak located above 2700 cm-1. 2D-band of xGnPs (M-5) particles shows around 2720 cm-1, it can be explained that xGnPs (M-5) is probably consisted with five or more layers.
6.2.3. Pool boiling experiment
A schematic illustration of the experimental apparatus is shown in Figure 6-2. The apparatus consisted of a rectangle vessel, a Teflon cover, a reflux condenser to maintain the volume concentration of the working fluid during boiling, two electrodes, and a power supply. A horizontally suspended NiCr wire was located between two copper electrodes. The voltage, current, and temperature signals were collected by a data acquisition system (DAQ). The experiments were conducted in a stabilized atmosphere at a saturated temperature of about 100°C. Power was supplied to the wire heater until the CHF was reached. The power was increased in small steps as the heat flux approached the CHF.
The CHF was calculated using the following equation:
"
CHF
q VI
= A
(6.1)where I and V are the current and the voltage of the heater, respectively, and A is the heat-transfer area of the heater.
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Figure 6-2. Schematic illustration of the pool boiling experimental apparatus