Investigation of Transport Mechanisms during Pool Boiling on Nano-Structured Surfaces
Objective: In this study we investigate the role various transport mechanisms on nanostructured surfaces. Pool boiling heat flux is measured on heater surfaces are coated with Multi Walled Carbon Nanotubes (CNT) and compared with heat flux on an uncoated surface.
Problem Definition: Significance of the nanoscale transport mechanisms in pool boiling is profound. Major proportion of the heat flux in boiling occurs in a thin layer, termed as the "micro-layer" (in reality it is only several molecules thick and is nm in thickness).
Standard methodologies exist to model and predict the micro-scale transport phenomena in pool boiling. For example, Hsu's criterion can be used to predict the onset of nucleation in micro-cavities. The existing theories in the literature rely on continuum models. However, there is no theory to model, describe and predict the transport phenomena on the nano-scale without relying on empirical assumptions.
The objective of this study is to experimentally measure the effect of nano-textured surfaces on the pool boiling heat flux for various wall superheats and liquid subcoolings in both nucleate and film boiling regimes. Nano-scale spatial and temporal perturbations will help to identify the dominant nano-scale mechanisms in pool boiling.
General Procedure: A schematic and photograph of the experimental apparatus is shown in Fig 1. SEM micrographs of silicon substrates coated with CNT are shown in Fig. 2. CNT is synthesized on the silicon wafers using CVD (Chemical Vapor Deposition). An array of Thin Film Thermocouples (TFT) are fabricated on the silicon wafer apriori, to measure the thermal resistance and the spatial distribution of the surface temperature fluctuations.
Conclusion and Future Direction: Based on the experimental results the following micro/nano-scale transport mechanisms are identified:
(a) Hair like protrusions of the CNT which act as "nanofins";
(b) Higher thermal conductivity of the CNT;
(c) Disruption (or enhancement) of the micro-layer and the vapor film in nucleate boiling and film boiling, respectively; and
(d) Enhancement of coupled transient hydrodynamic and thermal fluctuations (e.g., cold spots).
We are investigating the relative contribution from these mechanisms to the total heat flux using high speed photography, measurement of surface temperature transients using TFT and non-linear dynamics models (based on fractal models and chaos theory).
H.-S. Ahn, N. Sinha, M. Zhang, S. Feng, D. Banerjee, D., and R. Baughman, "Pool boiling experiments on Multi Walled Carbon Nanotube (MWCNT) Forests," ASME Journal of Heat Transfer, December, 2006.
H.-S. Ahn, N. Sinha, D. Banerjee, and S.C. Lau, "Pool Boiling Experiments Using Surface Micromachined Thermocouples," SAE Paper No. 06PSC-44, SAE Power Systems Conference, New Orleans, Nov. 7-9, 2006.
J. Guinn and D. Banerjee, "Experimental Study of Nanofluids for Droplet Cooling Applications Using Temperature Microsensors," Proceedings of the ASME-IMECE 2006, Paper No. 14101, Chicago, IL, Nov. 5-9, 2006.
H.-S. Ahn, N. Sinha, D. Banerjee, and S.C. Lau, "Boiling on experiments on Carbon Nanotube Forests and using Micromachined Thin Film Thermocouples," AIAA-ASME Joint Thermophysics Conference, San Francisco, CA, June 5-8, 2006.
H.-S. Ahn, N. Sinha, and D. Banerjee, "Micro-Machined Temperature Sensor Arrays for Studying Micro-Scale Features in Film Boiling," Proceedings of ASME-International Mechanical Engineering Congress and Exposition 2005, November 5-11, Orlando, FL.
S. Jeon, B. Jo, and D. Banerjee, "Enhancement of saturation boiling of PF-5060 on microporous surface," Paper No. AJTEC2011-44408, ASME/JSME 8th Thermal Engineering Joint Conference, Honolulu, Hawaii, March 13-17, 2011 (submitted).