Critical Heat Flux of Pool Boiling on Zinc Oxide Porous Nanostructures

• Main Authors: Chen, Yu-Chi, 陳郁其
• Other Authors: Lu, Ming-Chang
• Format: Others
• Language: zh-TW
• Published: 2018
• Online Access: http://ndltd.ncl.edu.tw/handle/3b332p

碩士 === 國立交通大學 === 機械工程系所 === 107 === To enhance the efficiency and performance of semiconductor devices, designers reduce the volume and increase the number of electronic chips; however, the heat flux emitted by electronic chips increases simultaneously. Therefore, keeping electronic chips away from burnout under high heat flux is a tremendous challenge for the heat management of electronic chips. Due to liquid-vapor phase-change which is accompanied with a large amount of latent heat absorption, pool boiling is an extremely effective heat transfer mechanism. The effectivity of pool boiling heat transfer can be quantified with critical heat flux and heat transfer coefficient. To enhance the critical heat flux and heat transfer coefficient of pool boiling heat transfer, this research presents the results of pool boiling experiments on zinc oxide porous nanostructures and plain silicon dioxide surfaces. The experiments were conducted in saturated deionized water and under atmospheric pressure. In terms of critical heat flux, compared to the plain silicon dioxide surface, the zinc oxide porous nanostructures had smaller equilibrium contact angles which represented higher surface wettability. The highest critical heat flux of the zinc oxide porous nanostructures was 200.19 ± 0.65 W/cm2 and achieved a 50.8 % enhancement compared to the plain silicon dioxide surface. If the critical heat fluxes of the different zinc oxide porous nanostructures are compared, it can be found that the critical heat fluxes of the different zinc oxide porous nanostructures had no significant change regardless of cavity size or structure height. A possible reason is that both cavity size difference and structure height difference are not significant. In terms of heat transfer coefficient, the nucleation site density on the zinc oxide porous nanostructures was much larger than that on the plain silicon dioxide surface at similar heat flux, so working fluid was easy to absorb latent heat and achieved phase-change phenomena on the zinc oxide porous nanostructures. The highest heat transfer coefficient of the zinc oxide porous nanostructures was 54756.5 ± 895 W/m2 ⁰C and achieved a 100.4 % enhancement compared to the plain silicon dioxide surface. If the heat transfer coefficients of the different zinc oxide porous nanostructures are compared, firstly it can be found that heat transfer coefficient increased as cavity size increased. From the nucleation theory and experimental results, it is known that the surfaces with larger cavity size are more easily excited, and it have sufficient nucleation sites to dissipate heat with pool boiling under low wall superheat. Therefore, heat transfer coefficient increased as cavity size increased; on the other hand, although average nucleation density increased as structure height increased, heat transfer coefficient had no obvious relationship with structure height. A possible reason is that the variations of bubble departure frequency and bubble departure volume with structure height are also related to heat transfer coefficient. Summarizing the above results, it can be seen that the application of the zinc oxide porous nanostructures to the boiling heat transfer of saturated deionized water under atmospheric pressure could increase the critical heat flux and heat transfer coefficient of pool boiling compared with the plain silicon dioxide surface.