POOL BOILING STUDIES ON NANO-TEXTURED SURFACES: IMPACT OF HIGH SUBCOOLING

摘要

The effect of surface nano-texturing using Multi-Walled Carbon Nano-Tubes (MWCNT) on pool boiling heat transfer is studied under high subcooling (～30 °C). Three different substrates were used in this study: a bare silicon wafer (450 μm thick), and two similar silicon wafers coated with 9 μm (Type-A) and 25 μm tall (Type-B) MWCNT. The MWCNT are deposited by Chemical Vapor Deposition (CVD), have diameters ranging between 8-16 nanometers and a randomized center to center pitch ranging between 24-45 nm. PF-5060, a fluorocarbon is used as the test fluid. In the nucleate boiling regime the surface coated with Type-A MWCNT shows a significant enhancement in heat transfer in comparison to the bare silicon wafer and Type-B MWCNT coated wafer. Type-B MWCNT on the silicon substrate results in only a slightly higher heat transfer rate as compared to the bare silicon wafer. The presence of MWCNT seems to disrupt the formation of a stable vapor film thus delaying the onset of film boiling. Based on the experimental results, the following transport mechanisms have been identified for the observed enhancement of pool boiling heat flux on the MWCNT: the enhanced surface area (nano-fin effect), enhanced nucleation, disruption of the "microlayer" region in nucleate boiling, and the high thermal conductivity of MWCNT.