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Pool Boiling on silicon Nanofins |
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Pool Boiling Studies on Nano-finned surfaces
Objective: In this study, we create silicon nano-fins on double-side-polished, 4 inch silicon wafers through Step and Flash Imprint Lithography (SFIL - developed at University of Texas at Austin) and test them on a pool boiling heat transfer set-up. The heat transfer properties of the nano-finned surface are compared to that of a bare surface to arrive at the degree of improvement.
Problem Definition: In the electronic cooling industry today, devices are so dense and powerful that a large amount of heat is produced that has to be dissipated safely, and the power and density are only going higher and higher. Pool boiling along with surface modification has shown to improve the heat transfer rate in the past. In this study, we explore the effect of silicon nano-fins on pool boiling heat transfer.
General Procedure: Through the SFIL process (Figure 1), the test surfaces are covered with an array of 200nm diameter pillars with a pitch of 1?m in one direction and 0.8?m in the other (Figure 3, Figure 4). The average height of the pillars is 100nm (Figure 2). The nano-fin containing test pieces are tested on a cubical pool boiling experimental set-up. The test surface in the chamber is mounted on a copper block with embedded cartridge heaters which are heated by a DC source. The chamber is filled with the test fluid which is maintained as a constant temperature bath with the help of a heat exchanger coil. The test fluid used in these studies is PF 5060 (Manufacturer: 3M Co.), a fluoroinert with a boiling point of 56°C. A number of thermocouples help in recording temperatures at different points in the set-up as well as calculate the heat flux across the test surface. The results are reported for the nucleate boiling regime and the respective boiling curves are studied. The apparatus is of the constant heat flux type.
Conclusion and Future Direction: Depending on the controlled temperature of the fluid in the test chamber, tests were run in the saturation, 10 °C sub-cooling and 20 °C sub-cooling conditions. The results are compared with those obtained with the bare, untreated surfaces. The nano-finned surface clearly shows more nucleation close to CHF (Figures 5 and 6). It is clearly seen that, for all the test conditions, there is an increase in the performance (heat transfer rate) when compared to the bare surface. The plots also clearly indicate that with increasing sub-cooling, the CHF increases as would be expected (Figure 7). All the results are reported at the maximum heat flux point (CHF point). The saturation condition showed a 41% enhancement in heat flux. The 10°C sub-cooling case showed 19% enhancement and the 20°C sub-cooling case showed 27% enhancement. Future studies include varying the height, pitch and diameter of the nano-fins and observing their effect on the pool boiling process.
Graduate Students: Sharan Sriraman, Jiwon Yu
Undergraduate Students: Rudolfo De Leone, Randy Williams
Project funded by: NSF
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"This material is based upon work supported by the National Science Foundation under Grant No. 0630703." "Any opinions, findings, and conclusions or recommendations expressed in this material are those of the author(s) and do not necessarily reflect the views of the National Science Foundation."