Pool boiling characteristics of nano-fluids

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Abstract

Common fluids with particles of the order of nanometers in size are termed as ‘nano-fluids’ which have created considerable interest in recent times for their improved heat transfer capabilities. With very small volume fraction of such particles the thermal conductivity and convective heat transfer capability of these suspensions are significantly enhanced without the problems encountered in common slurries such as clogging, erosion, sedimentation and increase in pressure drop. This naturally brings out the question whether such fluids can be used for two phase applications or in other words phase change in such suspensions will be assistant or detrimental to the process of heat transfer. The present paper investigates into this question through experimental study of pool boiling in water–Al2O3 nano-fluids. The results indicate that the nano-particles have pronounced and significant influence on the boiling process deteriorating the boiling characteristics of the fluid. It has been observed that with increasing particle concentration, the degradation in boiling performance takes place which increases the heating surface temperature. This indicates that the role of transient conduction in pool boiling is overshadowed by some other effect. Since the particles under consideration are one to two orders of magnitude smaller than the surface roughness it was concluded that the change of surface characteristics during boiling due to trapped particles on the surface is the cause for the shift of the boiling characteristics in the negative direction. The results serve as a guidance for the design of cooling systems with nano-fluids where an overheating may occur if saturation temperature is attained. It also indicates the possibility of such engineered fluids to be used in material processing or heat treatment applications where a higher pre-assigned surface temperature is required to be maintained without changing the fluid temperature.

Introduction

Heat transfer technology stands at cross roads today with ever increasing demand of cooling ultra high heat flux equipment on one hand and unprecedented pace of miniaturisation on the other. In the present day technology the different ranges of LASER applications, super conducting magnets, high power X-ray and above all super fast computing chips performing trillions of operations per second are becoming quite common. These devices are not only to operate in their respective applications with high precision but also to do so occupying minimum space. This puts a challenge not only to the core device design but also to their thermal management. While air based cooling systems are more common and reliable, they fail miserably with increasing heat flux. Therefore in almost all the high heat flux applications liquid cooling is preferred. The cooling liquids usually used are water/chilled water, common refrigerants and liquid nitrogen or similar cryogens depending on the specific application. While water is a convenient and safer medium, its relatively poor heat transfer characteristic is a major disadvantage. Usual refrigerants are hazardous to environment and cryogens are costly not only due to their energy intensive production process but also due to whole range of costly equipment to be deployed to use them.

Under the circumstances it makes sense to look at alternatives such as fluids with suspended solid particles in them. Though the enhancement of thermal conductivity of slurries is known for more than a century, they have not been considered as a candidate for heat transfer applications in the past due to problems associated with them such as sedimentation, erosion, fouling and increased pressure drop. The recent advancement of materials technology [1] has been able to produce particles of nanometer size which when suspended in usual fluid can overcome most of the problems encountered by common slurries. Choi [2] was the first one to call such suspension ‘nano-fluids’ which is now widely accepted. The stability of such fluids against sedimentation is remarkably improved when very small amount of stabilising agent such as laurate salt [3] is added. The erosion and pressure drop problems are also greatly reduced due to small particles and the small volume fraction (usually <5%) required for enhancement of thermal behaviour of the base fluid. A substantial enhancement of thermal conductivity of water and ethylene glycol based nano-fluids with Al2O3 or CuO nano-particles ranging from 7% to 30% with only 1–5% particle volume fraction was reported by Lee et al. [4] at room temperature. A recent study by the present authors [5] shows that the enhancement of thermal conductivity of nano-fluids increases dramatically with temperature making it more attractive for cooling at high temperature and heat flux. Further enhancement of thermal conductivity of nano-fluids with pure metallic particles was reported by Xuan and Li [3] who found enhancement comparable to Lee et al. [4] using much bigger (100 nm) particles size. Finally, the enhancement of thermal property received a quantum jump when Eastman et al. [6] reported an increase of thermal conductivity by an outstanding 40% with only 0.3 vol.% of nano-particles of copper having average size <10 nm. All the above works [3], [4], [5], [6] indicate that usual theory of suspensions and slurries such as the classical Maxwell [7] model or the extended Hamilton–Crosser [8] or Wasp [9] model fail miserably with nano-fluids. An evident theory is still missing.

However for heat transfer engineer, this enhancement of thermal conductivity is only a necessary condition for using such fluids in cooling application and not a sufficient condition. The real worth of such fluids can only be tested under convective conditions. The study of Ahuja [10], Liu et al. [11] and Sohn and Chen [12] conclusively indicate that performance of slurries under convective conditions are encouraging. Eastman et al. [13] also mention an increase of 40% in heat transfer capability for nano-fluids with 2% particle concentration under convective conditions even though no systematic study is available in this regard. The theoretical observations and proposition of dispersion model by Xuan and Roetzel [14] goes a long way in theoretical modelling of nano-fluids under convective condition the experimental validation of which is underway by the present authors.

While using nano-fluids for convective cooling, one must be aware of its boiling characteristics. This is because even if nano-fluids are unattractive with respect to two (or rather three) phase applications, during convective heat transfer with high heat flux locally boiling limit may be reached. It is important that the behaviour of nano-fluid under such conditions is accurately known to avoid unwanted effects such as local hot spot which can cause significant deterioration of reliability of components to be cooled.

The present paper is aimed at an experimental study of pool boiling characteristics of water–Al2O3 nano-fluid under atmospheric conditions. The thrust of the experiment is to compare the pool boiling parameters with that of pure water and thus bring out the applications and limitations of nano-fluids under the condition of phase change.

Section snippets

Production and characterisation of nano-fluid

Depending on base fluid and particle combination a number of nano-fluids can be produced. However in the present investigation, water based nano-fluids of Al2O3 particles have been used. The reason for this is the fact that the boiling characteristics of the base fluid water is most widely known and the thermal conductivity of water–Al2O3 nano-fluids for different particle concentration and the effect of temperature on it has already been studied [5]. Even though CuO–water nano-fluids have

Experimental set-up

The experimental set-up was designed keeping in mind the parameters the effects for which are required to be observed. For this reason no effort has been made to fabricate a so-called standard boiling apparatus but watch has been kept so that the experiments for different nano-fluids and water are performed under identical conditions. The test section is shown in Fig. 8. It consists of 120mm×100mm×200 mm rectangular stainless steel vessel (1) with thick insulation (2) outside. The vessel has

Results and discussion

Prior to the experiment the nano-fluids were separately boiled to determine the boiling point. It was observed that for nano-particle concentration from 0.1% to 4% a very nominal decrease of 0.4 °C (which is of the order of error in temperature measurement) in the boiling point occurs at atmospheric pressure.

Through the observation window it was found that for pure water at low heat flux, the flow transforms from natural convective to nucleate boiling and distinct bubbles start sliding as

Conclusion

The modern electronics, computing and optical technology has brought about a stream of equipment dealing with extremely high heat flux needing more and more cooling efficiency. In recent times nano-fluids have been claimed to be a new possibility in meeting these demands due to their enhanced thermal conductivity and capability of further enhancing convective process through particle dispersion. They have been found to be much improved with respect to sedimentation, clogging and pressure drop

Acknowledgements

This research work has been carried out during a Humboldt research stay of the first author at the Institut für Thermodynamik (Institute of Thermodynamics) of the Universität der Bundeswehr Hamburg (University of the Federal Armed Forces, Hamburg).

The grant of the Humboldt research Fellowship to Prof. Sarit Kumar Das is greatly appreciated. Thanks are due to the Deutscher Akademischer Austauschdienst (German Academic Exchange Service) who finances the present research stay of Mr. Nandy Putra in

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