Effects of Al2O3/R-123 nanofluids containing C19H40 core–shell phase change materials on critical heat flux

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Abstract

In this paper, the curiosity is coming from how to bring out the fluidic capability of nanofluids (fluid itself) for critical heat flux (CHF) enhancement away from surface deposition effects such as improved wettability. The pool boiling characteristics of dilute dispersions of alumina and the microencapsulated C19H40 phase change material (MPCM) in R-123 were studied. Whereas other nanofluid studies only reported that a significant enhancement of CHF was achieved by buildup of a porous layer of nanoparticles on the heater surface during nucleate boiling, it was found that the additional CHF enhancement of 24% occurred with the MPCM compared to alumina nanomaterials. With solid–liquid phase changes, PCMs in suspension delay the occurrence of CHF by absorbing heat around from the heater, nucleate bubbles and merged bubbles while PCM shells prevent leakage of molten cores and allows the return to solid with exchanges of heat at some distances. The present study found that PCMs could make fluidic effects of nanofluid not relying on the surface depositions.

Introduction

Boiling is efficient heat transfer mode using the vaporization of a liquid. Critical heat flux (CHF) is an undesirable phenomenon causing an excessive increase of the temperature in the boiling phenomenon. It is important to enhance the CHF in order to improve the safety margin and economic performance in a thermal system. Recently, nanofluids have been attractive to the researchers because of the unique characteristics on the CHF [1], [2], [3], [4], [5], [6]. Nanofluids are colloidal dispersions of nanoparticles in a base fluid which are water, oil, bio-fluid and ethylene. Such nanofluids have the capability to enhance the critical heat flux (CHF) significantly about three times [7]. The enhancement of the CHF is caused by build-up of nanoparticles deposition layer on the heater. This nanoparticle layer is formed through vaporization of microlayer underneath bubbles during the boiling of nanofluids. And a characteristic of this layer depends on the kind of nanoparticles dispersed in the base fluid.

Therefore, some studies have been performed to find the optimized coating geometry or more beneficial nanoparticles. A variety of materials are used as nanoparticles [8], [9], [10]. The materials include metals (e.g., silver, copper, and gold), metal oxides (e.g., titania, alumina, silica, and zirconia), carbon allotrope (e.g., graphite, carbon nanotube, graphene, and fullerene). As the results, most of researches report observations focused on characterization of nanoparticles deposition layer formed on the heated surface. However, the surface effects are not key ideas to apply nanofluids to the CHF applications. Nanofluids originally attracted great interest because of their enhanced thermal conductivity. In other words, the fundamental properties of fluid itself such as thermal conductivity and viscosity of nanofluids have been emphasized by many researchers (in here, we call the properties of fluids into “fluidic effects” compared to surface effects). It is very different from the experimental findings in boiling to change the surface characteristics by deposition of nanoparticles on the heater surface. For practical applications, nanofluids should have their own improved capability focused on fluid property, itself. In this paper, the curiosity is coming from how to bring out the fluidic capability of nanofluids for CHF enhancement away from surface deposition effects such as improved wettability.

We got the inspiration on the point that some particles which are well absorbing and storing the heat could make a positive effect on the CHF. The microencapsulated phase change material (MPCM) is the suitable material to apply our idea in the boiling CHF test [11], [12], [13]. MPCM is a kind of particles (with dimensions on the order of 0.1–1000 μm) in which the core material is enveloped by a thin layer of shell. The core materials should have the following requirements; suitable temperature range, low reactivity, large latent heat, low volume change and reasonable thermal conductivity. In the same manner, the shell materials should have the following requirements; good sealing tightness, endurance, good elastic strength, water and fire resistance. The most interesting feature of the MPCM is that latent heat of the solid–liquid phase change is beneficially related to heat transfer in addition to the liquid-vapor phase change of the base fluid. When the MPCM is dispersed into the carrier fluid, a kind of suspension named as microencapsulated phase change slurry (MPCS) is formed. The MPCM has been applied to carrier fluids which can be used as both the heat transfer and energy storage media in the single phase flow although the phase of the core is changed. The heat transfer coefficients of the fluid containing the MPCM increased due to the increase in apparent specific heat caused by phase change process [14], [15]. It is well known that the latent heat of fusion, melting and freezing has a large effect on the heat transfer. Therefore, the key point of the current work is newly to apply the MPCM to the original concept of the nanofluid like a “heat boat” to enhance the CHF more than ever before.

Section snippets

Preparation of test fluids characterization

In this study, C19H40 phase change material was used as the MPCM. Average dimension of the MPCM is 20 μm (10–30 μm). A differential scanning calorimeter (DSC) was used to check the latent heats and phase-change temperatures of melting and freezing. Thermal denaturation of the MPCM is shown in Fig. 1. The material of core is paraffin (n-nonadecane C19H40). The solid paraffin is the best candidate as the core material due to its stable chemical and thermal properties. The thermal property of the

Results and discussion

The CHF data obtained from the present work at various conditions were shown in Table 2. As the MPCM was added in R-123 at 0.01 vol%, it is found that the CHF was enhanced up to about 44% compared to pure R-123. However, the enhancement ratio was relatively lower than the value of alumina nanofluid (∼70%). The reason why the enhancement occurred is due to the change of heater surface characteristics caused by the MPCM/nanoparticles deposition during the boiling. Fig. 4 shows the build-up of

Conclusions

We can conclude that the MPCM is attractive material to enhance the CHF by changing the state of the core in the case of that the MPCM is used independently and together with nanoparticles because the additional heat removal occurs on both the wetted heater surface and bubble interface. With solid–liquid phase changes, PCMs in suspension delay the occurrence of CHF by absorbing heat around from the heater, nucleate bubbles and merged bubbles while PCM shells prevent leakage of molten cores and

Acknowledgments

This work was supported by the Human Resources Development of the Korea Institute of Energy Technology Evaluation and Planning (KETEP) grant funded by the Korea government Ministry of Knowledge Economy (No. 20114030200010) and by the Nuclear Energy Research Program through the National Research Foundation of Korea (NRF) funded by the Ministry of Education, Science and Technology.

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