Pool boiling of nanoparticle-modified surface with interlaced wettability
© Hsu et al.; licensee Springer. 2012
Received: 31 January 2012
Accepted: 28 March 2012
Published: 18 May 2012
This study investigated the pool boiling heat transfer under heating surfaces with various interlaced wettability. Nano-silica particles were used as the coating element to vary the interlaced wettability of the surface. The experimental results revealed that when the wettability of a surface is uniform, the critical heat flux increases with the more wettable surface; however, when the wettability of a surface is modified interlacedly, regardless of whether the modified region becomes more hydrophilic or hydrophobic, the critical heat flux is consistently higher than that of the isotropic surface. In addition, this study observed that critical heat flux was higher when the contact angle difference between the plain surface and the modified region was smaller.
KeywordsHydrophilic Hydrophobic Critical heat flux Boiling heat transfer
Regarding the Nukiyama curve  in the heat and mass transfer textbook, the nucleate boiling region causes a higher rate of heat transfer in the boiling system. However, the nucleate boiling system is limited by the critical heat flux (CHF); therefore, CHF might be set as an index for nucleate boiling research. Several factors affect this situation, and a number of early studies reported that surface wettability is a crucial factor that affects the boiling heat transfer [2, 3]. The contact angle (CA) has a substantial influence on transition boiling, and the hydrophilic surface increases the CHF and heat transfer coefficient (HTC) in boiling heat transfer . Dhir and Liaw [5, 6] reported that the theoretical prediction of CHF efficiently compares with the experimental data in the hydrophilic region. Thin-film coatings with alumina [7, 8], zirconia , or silica [8, 9] nanoparticles applied to modify surface wettability demonstrated considerable enhancement in the pool boiling CHF. An approximate enhancement of the CHF was observed for pool boiling in a surface modified with micro/nanoscale of zircaloy-4 . Chen et al.  reported that a superhydrophilic surface that is created by the nanowire arrays on Si and Cu substrates can be utilized to increase the CHF by more than 100%. Kim et al.  reported that CHF enhancement in nanofluids is due to surface deposition of nanoparticles during boiling, and the resulting surface had significantly increased wettability. Therefore, the surface wettability is a crucial subject of boiling heat transfer. The interlaced wettability surface effect on pool boiling heat transfer was studied by Betz et al. , in which the interlaced wettability of a silicon surface formed with hydrophilic (CA = 7°) and hydrophobic (CA = 110°) regions. The ratio of the hydrophilic patterned area was 77.4%, and the measured CHF increased by 65% higher than that of a plain silicon surface treated with hydrofluoric acid (CA = 7°).
The effect of CA and details of coating methods
Regions of interlaced surface
Step 1: precursor + nanoparticle coatings (g)
Step 2: hydrophobic material treatment
93 to 111
A type of high-density anti-coating tape was used to achieve this. The sample with a plain surface was first covered with the striped tape. All stripes were aligned with the locations of thermal couples to ensure that the readouts were accurate. The details of the procedure are followed by a series of abovementioned coating steps. The desired pattern of the surface can be modified with other wettability with nanoparticles coated. SEM image of interlaced surface is shown in Figure 1c. There is an interface between nanoparticle-coated and uncoated regions. SEM image of nanoparticle-coated region is shown in Figure 1c downside, (CA = 55°), and that of uncoated (plain) areas is shown in Figure 1c upside, (CA = 105°) at a scale bar of 200 μm. Nano-silica particles coated on copper surface can be observed in Figure 1d.
All CAs are measured before the tests, using Sindatek Model 100SB (Taipei, Taiwan) to produce schematic drawings of the CA meter. A drop volume of 2 μl was chosen for the measurements, and each region was examined using more than five values. The CAs of the plain copper surfaces after baking were approximately 93° to 111°. Interlaced wettability surfaces show two CAs in different regions. The first value is the contact angle in plain regions, and the second value is the contact angle in coated regions. CAs of samples with plain and hydrophilic regions are 105° and 55°, 105° and 33°, or 97° and 10°. Besides, CAs of samples with plain and hydrophobic regions are 111° and 142° or 93° and 123°.
The heat system setup is displayed in Figure 1b. A copper block with dimensions of 1.5 cm × 1.5 cm was prepared. Holes with a depth of 0.7 mm were drilled on the sides of the copper block. The accuracy of the T-type thermocouples used in the present experiments was ±0.1°C. A thermocouple was placed in each of these holes with sink grease to reduce the contact resistance, the positions of which were at the top, side, and bottom. Each hole was separated by a space of 7 mm. The top of the copper block was the heated surface with varied surface wettability after modification. The side of the heated block was full of glass fiber for heat insulation. A glass tank with dimensions of 10 cm × 10 cm × 25 cm was placed on the top of the heated block. The opening on the top of the tank maintained at atmospheric pressure, and the pool temperature was naturally adjusted to the saturated temperature (100°C). The camera was placed on the top side of the tank to record the photos of growth bubbles. The heat source was placed under the heated block. The heating source was supplied by four 200-W electric heating rods. The control heating source passed through the electricity supplier and outputs. After heating the surface, the thermometer recorder (MX-100, Yokogawa, Musashino-shi, Tokyo, Japan) read the data that were measured by the thermocouples and transferred it to the computer for analysis. At the top position of the temperature data is the wall temperature; the temperature difference was obtained from the bottom position and the top position of the temperature data. Once the distance difference (14 mm) and copper thermal conductivity were determined, the Fourier law (Equation ) was used to calculate the heat flux into the boiling liquid. Through energy balance equation, the heat loss can be calculated. The percentage of heat loss through insulation was 0.77%. Uncertainties of the experimental measurement of heat flux and wall superheat were estimated using Equations  and . The parameter Uq in these equations represents the uncertainty of the heat flux q and the parameter UTw represents the uncertainty of the wall temperature Tw. A homogeneous temperature profile on the surface was obtained in this study. The accuracy of the T-type thermocouples used in the present experiments was ±0.1°C. All the thermocouples were calibrated using an OMEGA-HH41 thermistor (Stamford, CT, USA).
In conclusion, this study has demonstrated that surfaces with interlaced wettability significantly enhance the CHF and the HTC due to the bubble motion during pool boiling. Interlaced surface of Δθ is an important index to affect the pool boiling. A good enhancement arises in the case of hydrophilic and near-superhydrophilic regions, in which Δθ is 40° in the hydrophilic region. The best enhancement of CHF might have existed in smaller Δθ.
Prof. PHC received his master's and Ph.D. degree in mechanical engineering from the University of Minnesota in 1984 and 1988, respectively. He was awarded as a distinguished professor by the National Taiwan University in 2008, the distinguished research award by NSC in 2008 and 2011, and ASME fellow in 2009. His major research areas are in MEMS, biomedical devices, nanotechnology, and energy-harvesting chips, highly efficient energy systems, and sensors. He has published more than 127 journal papers, 2 books, and 14 patents.
The financial support of this work was provided by the KAUST Award with the project number of KUK-C1-014-12. We would also like to thank Professor Stephan Kabelac for his guidance in this research.
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