Pool boiling of nano-fluids on horizontal narrow tubes
Introduction
In recent times heat transfer technology is confronted with increasing demand of cooling applications of miniaturised high heat flux components. Applications such as LASER diagnostics, superconducting magnets and most importantly superfast computing are posing tremendous challenge to thermal management. Usually air based cooling systems are more common and reliable but they are inadequate for high heat flux applications where liquid cooling is preferred. The cooling liquids usually used are chilled water, refrigerants or cryogens depending on the requirement of heat removal. Looking at the requirement, it makes sense to consider alternatives such as fluid suspensions of ultrafine solid particles. The fluids with suspended solid particles have not been considered as an alternative for heat transfer applications so far due to associated technological problems such as sedimentation, clogging, erosion, fouling and increase of pressure drop. However in recent times a fresh look has been cast on these fluid–solid suspensions with particles of nano-meter size which have been named as ‘nano-fluids’ by Choi (1995). The erosion, clogging and pressure drop problems are also greatly reduced due to small particles and the small volume fraction (usually 1–5%) required and the stability of such fluids against sedimentation is remarkably improved. Lee et al. (1999) reported a substantial enhancement of thermal conductivity of water and ethylene glycol based nano-fluids with Al2O3 or CuO nano-particles at room temperature. In a recent study (Das et al., 2001) the present authors have shown that the enhancement of thermal conductivity of nano-fluids increases even more at elevated temperature which makes it more attractive for cooling at high heat flux applications. This enhancement of thermal conductivity received an impressive breakthrough when Eastman et al. (2001) reported an increase of thermal conductivity by an outstanding 40% with only 0.04% of nano-particles of pure copper having average size less than 10 nm. The above works indicate that usual theories of thermal conductivity of suspensions such as the Hamilton and Crosser (1962) model fail in case of nano-fluids. A satisfactory theory is yet to evolve.
However, for heat transfer applications, the enhancement of thermal conductivity is not the only concern, the real worth of such fluids as coolants can only be examined under convective conditions. Ahuja (1975) and Liu et al. (1988) have shown that performance of suspensions even with micrometer size particles are encouraging under convective conditions. The proposition of dispersion model by Xuan and Roetzel (2000) can be a useful tool in theoretical modelling of nano-fluids under convective conditions. While using nano-fluids for convective cooling, one cannot overlook the need of proper knowledge of its boiling characteristics. This is because during convective heat transfer with high heat flux local boiling condition may be reached. It is important to know the behaviour of nano-fluids under such conditions to avoid unwanted effects if any. The present authors have carried out an experimental study (Das et al., 2003) of pool boiling characteristics of water–Al2O3 nano-fluid under atmospheric conditions on a tube of large diameter (20 mm). The study shows that the nano-particles degrade the boiling performance systematically with increasing particle concentration resulting in an increase of wall superheat for a given heat flux. The deterioration in boiling performance is observed to be more drastic at a higher surface roughness.
The present paper is aimed at understanding the pool boiling of nano-fluids in the regime of small diameter tubes (4 and 6.5 mm) which are on the one hand more important for application in miniaturised heat sources, on the other hand they are different from the usual tubes with respect to pool boiling due to the closeness of bubble and tube diameter and a consequent deviation in bubble sliding mechanism. Thus, the present study can act as a guidance for the use of these fluids in the applications where high heat flux is associated with smaller dimensions of the components.
Section snippets
Characterisation of nano-fluid
Although a number of combinations of base fluid and particle can be used for nano-fluids, in the present investigation, water–Al2O3 particles nano-fluids have been used. This is due to the fact that the boiling characteristics of the base fluid water is widely known and the enhancement of thermal conductivity of water–Al2O3 nano-fluids has already been studied (Das et al., 2001). The particles were supplied by nano-phase Technologies Corporation, IL (USA) produced by physical vapour deposition
Experimental setup
A simple experimental setup was designed keeping watch that the experiments for different nano-fluids and water were performed under identical conditions. The test section is shown in Fig. 4. It consists of 120 mm × 100 mm × 200 mm rectangular stainless steel vessel (1) with thick insulation (2) outside. The vessel has two cooling arrangements cascaded together. The first one (3) is a counter current copper condenser which on one hand connects the vessel directly to atmosphere maintaining an
Results and discussion
Previous study (Das et al., 2003) shows that for nano–particle concentration of present range a very small decrease of 0.4 K in the boiling point occurs which is of the order of error in temperature measurement and can be neglected. The visual observation for pure water shows that for tube of large diameter (20 mm) bubbles sliding bubble regimes similar to those observed by a series of studies by Cornwell and Schüller (1982), Cornwell (1990) and the bubbles coalesce forming larger bubbles
Conclusion
The use of nano-fluids for cooling of high heat flux devices in modern electronics, computing and optical technology has been claimed to be a new possibility due to their enhanced thermal conductivity and capability of further enhancing convective process through particle dispersion. They have been found to be much improved compared to common slurries with respect to sedimentation, clogging and pressure drop. However under phase change conditions this possibility has been conclusively negated
Acknowledgements
This research work has been carried out during the stay of the first author in Hamburg under the Humboldt Research Fellowship which is gratefully acknowledged.
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