Study of the Nanoparticle Coating Thickness On Critical Heat Flux in Pool Boiling Heat Transfer

Abstract

Since nanofluids, which are colloidal dispersions of nanoparticles in a base fluid such as water, were known as a way to significantly enhance the CHF, the pool boiling CHF test was conducted in a variety conditions depending on the material, the concentration etc. The concentration is the factor which influences the amount of the deposited thickness on the heater surface. In most papers which are reporting the results of the pool boiling CHF test for nanofluids, the exact heating time is not described according to the heat flux level. The heating time is also an important factor that determines the amount of deposited nanoparticles. Alumina nanofluid is prepared by dispersing nanoparticles into the distilled water as a base fluid. The concentration of nanofluid is 0.01Vol.%. Pre-coating process was carried out from 1 to 180 minutes at 600kW/m2. Increased heating time has a positive effect on CHF in the region of a relatively short pre-coating time. The trend of CHF change is slowly decreased as pre-coating time is increased. When more nanoparticles are coated on the heater surface, the heat resistance which is disturbing the heat flow is increasing. The static contact angle was measured with pure water (1?l) at 25? in air on the clean and nanoparticles coated surfaces boiled in nanofluids. The dramatic decrease of contact angle was observed on the coated heater surface. The results of contact angle are not uncovering the slight decrease of CHF in high pre-coating region. Hydrodynamic instability theory was studied to explain the effect of thickness on CHF. The nanoparticle-coated surface has a shorter average distance compared with a bare surface. A case of high pre-heating time has a short wavelength in this work. A short wavelength allows to the vapor to prevent the formation of bulk vapor by venting the vapor equally across the heater surface. But the results of wavelength are not similarly uncovering the slight decrease of CHF in high pre-coating region. Coating thickness was measured by analyzing the SEM images. The thickness is continually increased but the gradient is becoming gentle. Reversely, porosity is decreased as the pre-boiling time is longer. It means that the coating layer formed by nanoparticles is becoming dense. It is clear that the coating layer functions as a heat transfer resistance.