Enhancement of saturated pool boiling using nano-patterned substrates

• Main Author: Poh, Wei Hao
• Other Authors: Leong Kai Choong
• Format: Final Year Project
• Language: English
• Published: 2014
• Subjects: DRNTU::Engineering::Mechanical engineering
• Online Access: http://hdl.handle.net/10356/60409
• Institution: Nanyang Technological University

Experiments were performed to evaluate the effect of carbon nanotubes (CNT) coatings on silicon surfaces have on pool boiling performance. Different percentages of CNT coatings were tested on 10×10mm silicon specimens. Dielectric fluids such as FC-72 and HFE-7100, were used as test fluids because of their very small contact angles which will allow deep penetration of fluid inside surface cavities. The higher the percentage of CNT coating, the higher the enhancement in both the nucleate boiling heat transfer coefficient and critical heat flux (CHF). Enhancement of heat transfer performance was recorded to be as high as 21% with CNT coating. In addition, the CHF was observed to be delayed from 17 W/cm2 to 24 W/cm2 when coated with CNTs. Surface roughness was found to be an important factor in influencing the pool boiling performance. Bare silicon samples roughness were measured and found to have an average roughness value of 30-40 nm. On the other hand, when coated with CNTs, the average roughness was recorded to be 3-4 μm. Comparing the difference in roughness value, it can be concluded that due to the smoothness of the surface, there will be lesser nucleation sites. Thus, the bare silicon sample would require higher surface superheat to initiate boiling as compared to the sample with CNT coating. An investigation on the effect of inclination angle, θ, on saturation pool boiling was also performed. The CHF was observed to decrease as the angle of inclination increased. The proposed CHF correlation is within 10-15% of the experimental data. Results also showed that CHF decreased slowly from 0° to 120° whereas, with inclination angles greater than 120° show large decrease in CHF. Using a high speed camera, still images were used for the pool boiling at different heat fluxes and at different angles of inclination. The bubble dynamics and characteristics were studied using the images to calculate the departure bubble diameter and bubble departure frequency. Through the images and calculations done, it was observed that the CNT surface was able to generate more bubbles in smaller sizes, which in turn led to a better heat transfer performance.