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Method Article
* Wspomniani autorzy wnieśli do projektu równy wkład.
Pool-boiling heat-transfer experiments were carried out to observe the effects of hybrid wettable patterns on the heat-transfer coefficient (HTC). The parameters of investigation are the number of interlines and the pattern orientation of the modified wettable surface.
In this study, pool-boiling heat-transfer experiments were performed to investigate the effect of the number of interlines and the orientation of the hybrid wettable pattern. Hybrid wettable patterns were produced by coating superhydrophilic SiO2 on a masked, hydrophobic, cylindrical copper surface. Using de-ionized (DI) water as the working fluid, pool-boiling heat-transfer studies were conducted on the different surface-treated copper cylinders of a 25-mm diameter and a 40-mm length. The experimental results showed that the number of interlines and the orientation of the hybrid wettable pattern influenced the wall superheat and the HTC. By increasing the number of interlines, the HTC was enhanced when compared to the plain surface. Images obtained from the charge-coupled device (CCD) camera indicated that more bubbles formed on the interlines as compared to other parts. The hybrid wettable pattern with the lowermost section being hydrophobic gave the best heat-transfer coefficient (HTC). The experimental results indicated that the bubble dynamics of the surface is an important factor that determines the nucleate boiling.
A high heat flux-sustaining system providing cooling in the range of 10-105 W/cm2 is required in the emerging fields of electronics, defense, avionics, and nuclear device development. Conventional cooling with air is insufficient for these applications due to the low heat-transfer coefficient (HTC) for both free- and forced-convection conditions. The phase change-based cooling techniques, such as pool boiling and flow boiling, are good enough to remove high heat fluxes on the order of 10 - 1,000 W/cm2 1. Since the two-phase heat-transfer process is isothermal, the cooled device temperature is almost constant over its surface. Due to the negligible variation of the temperature along the surface, the thermal shock of the device can be eliminated. However, the major limiting parameter in boiling heat-transfer is the critical heat flux (CHF), which causes an abnormal rise in temperature2.
In the last few decades, extensive research has been carried out to improve the CHF by using surface modification, nanofluids, and surface coatings3,4,5,6,7,8,9,10,11. Among the various methods, surface coatings are found to be the best method to improve the CHF due to the substantial increase in the surface area. Surface coatings generally increase the heat transfer by fin action, porosity effects, and surface wettability12. Surface wettability plays a significant role in boiling heat-transfer. Previous studies show that at lower heat-flux conditions, the hydrophobic surface shows better HTC due to the early nucleation. However, at higher heat flux, the detachment of the formed bubbles is slow due to the low affinity of water towards the surface. This leads to bubble coalescence and results in a lower CHF3. On the other hand, a hydrophilic surface produces a higher CHF, because of the fast detachment of the formed bubbles, but it gives a lower HTC at low heat fluxes, due to the delay in bubble nucleation13.
The hybrid structures show a remarkable enhancement in boiling heat-transfer for all heat fluxes due to the combined effect of hydrophobicity and hydrophilicity14,15,16. Hsu et al. produced heterogeneous wettable surface by coating superhydrophilic Si nanoparticles on a masked copper surface. They achieved different wettability ratios by varying the coating time. The onset of boiling occurred earlier on the heterogeneous surfaces compared to the homogeneous surface, which substantially reduced the wall superheat17. Jo et al. conducted nucleate boiling heat-transfer studies on hydrophilic, hydrophobic, and heterogeneous wetting surfaces. The heterogeneous wetting surface was composed of hydrophobic patterned dots on the hydrophilic surface. They got higher HTCs and the same CHF for the heterogeneous surface as compared to the hydrophilic surface. An improvement in boiling heat-transfer directly depends upon the number of dots on the surface and upon the boiling conditions18.
In this study, axial hybrid wettable patterns were produced on a cylindrical copper surface using the dip coating technique. Pool-boiling heat-transfer studies were conducted to determine the effects of the number of interlines and of the orientation of the hybrid wettable pattern. Boiling heat flux, HTC, and bubble dynamics were analyzed for the all coated substrates and were compared with the copper substrate.
1. Preparation of the Modified Surfaces
Figure 1. Selection of Various Interlined Surfaces. (a) Schematic of various interlined surfaces with different orientations. The area ratio of a plain copper surface and a superhydrophilic surface is 1:1 in all conditions. (b) Orientation selection criteria. (c) Isometric view of the 2 interline 0° angle oriented surface. Orientation is selected as the angle between the baseline and coating center line of the first hydrophilic pattern from the top side and it is measured in a clockwise direction. Please click here to view a larger version of this figure.
2. Experimental Procedure
Figure 2. Schematic of the Pool-boiling Chamber. Glass tubes are connected to both sides of the hollow copper cylinder with silicon paste. This is fixed to the pool-boiling chamber with silicon paste. Please click here to view a larger version of this figure.
Figure 3. Thermocouple Positioning. 8 thermocouples are placed inside the 1 mm diameter holes circumferentially in the test piece place at a diameter of 20 mm. The depths of alternate 1mm diameter holes are fixed at 5 mm and 7 mm respectively. Please click here to view a larger version of this figure.
3. Data Reduction
Figure 4. Schematic of Wall Temperature Analysis. Wall temperature is calculated using the measured average temperature and known cylindrical thermal resistance. Please click here to view a larger version of this figure.
Pool-boiling heat-transfer experiments were conducted on a hybrid wettable cylindrical surface using the experimental setup whose schematic is shown in Figure 5. The pool-boiling experimental procedure explained in step 2 of the protocol section was successfully carried out while investigating the effect of the number of interlines and of the orientation of the hybrid wettable pattern on the pool-boiling performance. The pool-boiling performances of the different-treated ...
The main goal of this investigation was to develop a pool-boiling heat sink for high heat dissipation applications, such as nuclear reactors, boilers, and heat pipes, by introducing the hybrid wettable surface, as described in the protocol section. These surfaces can produce better pool-boiling performances than homogeneous wettable surfaces (hydrophilic and hydrophobic). The improvement in the boiling heat-transfer performance is due to an increase in active nucleation sites and the easy detachment of the formed bubbles...
The authors declare that they have no competing financial interests.
The authors gratefully acknowledge funding support from the Ministry of Science and Technology, MOST (project numbers: MOST 104-2218-E-002 -004, MOST 105-2218-E-002-019, MOST 105-2221-E-002 -107 -MY3, MOST 102-2221-E-002 -133 -MY3, and MOST 102-2221-E-002 -088 -MY3).
Name | Company | Catalog Number | Comments |
Deionized water | |||
Silica nanopowder,40 nm | UniRegion Bio-Tech | 60676860 | |
Ethanol | ECHO Chemical co. Ltd | 64175 | |
Hydrochloric acid | SHOWA Chemical co. Ltd. | 7647010 | |
Tetraethoxysilane | SHOWA Chemical co. Ltd. | 78104 | |
Acetone | UNI-ONWARD CORP. | 67641 | |
Cartridge Heater | Chung Shun Heater & Instrument Co, Ltd. | ||
Pyrex glass | Automotive Glass service , Taiwan | ||
Ordinary toughened glass | Automotive Glass service , Taiwan | ||
Thermal paste | Electrolube | EG-30 | |
Insulation Tape | Chuan Chi Trading Co. Ltd | Kapton Tape | |
Sandpaper | Chuan Chi Trading Co. Ltd | #2000 | |
Heating furnace | Chung Chuan | Hong Sen HS-101 | |
Electronic scales | A&D co. Ltd | GX400 | |
Ultrasonic cleaner | Bransonic | Bransonic 3510 | |
Magnet stirrer | Yellow line | MST D S1 | |
Data logger | Yokogawa | MX-100 | |
CCD camera | JVC | LY35862-001A | |
Silicon paste | Permatex | 599BR | |
Power supply | Gwinstek | GPR-20H50D | |
Teflon tape | Chuan Chi Trading Co. Ltd | CS170000 | |
Contact Angle Goniometer | Sindatek | Model 100SB | |
Auxiliary Heater | Chuan Chi Trading Co. Ltd | ||
T- type thermocouples | Chuan Chi Trading Co. Ltd | ||
Reflux Condenser | Chuan Chi Trading Co. Ltd | ||
Fiber glass | Professional Plastics, Taiwan |
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