Pool boiling heat transfer characteristics of iron oxide nano-suspension under constant magnetic field
Introduction
Boiling is one of the most efficient mechanism of heat transfer for cooling and heating applications at high heat flux conditions. This plausible feature is attributed to the formation of the bubbles which creates a two-phase heat transfer mechanism. The presence of the bubbles can renew the thermal and fluid boundary layer, which in turn promotes the heat transfer coefficient (HTC) of the system. There are extensive studies in the literature towards the lack of understanding of the governing mechanisms and exact behavior of fluids in a two-phase boiling regime. Nano-suspensions are the promising working fluids for the future of the advanced thermal engineering and have been assessed for various applications. When the boiling heat transfer comes to the nano-suspensions, understanding the mechanism of heat transfer becomes challenging as bubble-bubble, bubble-surface and bubble-liquid interactions affect the heat transfer rate. Moreover, deposition of the nanoparticle (NP) is another challenge affecting the properties of the surface, which in turn can suppress or promote the performance of the system. It is worth saying that, in boiling of nano-suspensions, the stability of the nano-suspension can be altered due to the evaporation of the base fluid, which can intensify the rate of deposition and surface change. Thus, boiling heat transfer of nano-suspensions requires further investigation.
Critical Heat Flux referred to as CHF point is a heat flux (HF), in which the HTC of the system is maximum and thereby, the rate of bubble formation is superior such that the bubbles surround the boiling surface. By increasing the population density of the bubbles over the surface, bubble coalescence occurs, which in turn creates a large-size bubbles and blanket of vapor on the surface. Such large blankets can insulate the surface and reduce the heat transfer. Overall, the HTC value decreases and performance of the system is suppressed. This can lead to the system failure. There are extensive studies conducted on heat transfer mechanisms involved in boiling heat transfer. Hence, the studies can be categorized into three different groups. The first group of study shows that the boiling heat transfer is suppressed once NPs are added to the system, while there is a second group demonstrating that the boiling heat transfer can be promoted due to the presence of the NPs. These two groups of study have one point in common that the deposition of the NPs on the surface can improve the CHF value. Therefore, a vision was created to use passive and active methods with the aim to prevent or regulate the deposition of the NPs such that the HTC remains untouched or be promoted. The utilization of magnetic field (MF) is one plausible passive approach, which can contribute to the mitigation of the fouling of NPs, while providing conditions for the enhancement of HTC. For example, Verplaetsen and Berghmans [1], conducted a set of experiments to evaluate the potential role of MF on the HTC value of a magnetic nano-suspension in a film boiling regime. They showed that the formation of the bubbles can be affected by the direction and strength of the MF. Arias [2] showed that MF can improve the HTC in liquid metals. He demonstrated that the frequency of the bubble separation decreases by increasing the strength of the MF. Fan et al. [3] evaluated the heat transfer characteristics of aqueous graphene oxide nano-sheets nano-suspensions at wt.% = 0.1% to cool down some metallic balls. They reported that the performance of the system was enhanced once nano-suspension was used in the system and the quenching mechanism was boiling heat transfer. By conducting a series of experiments, Aminfar et al. [4] demonstrated the potential of combination of MF and nano-suspension to improve the CHF in flow boiling regime. Results showed that the CHF value can be promoted once a MF is applied to a magnetic nano-suspension. They attributed the enhancement of CHF value to the change in surface properties and deposition of the NPs. Abdollahi et al. [5] conducted some experiments to further evaluate the effect of MF on the HTC value in pool boiling of iron based nano-suspensions. They experimentally proved that the MF can promote the HTC by affecting the surface deposition, however, they did not report any information on the potential thermal resistance over the surface.
In light of above, despite extensive research conducted on the influence of the MF on the boiling performance of the nano-suspensions, the influence of zeta potential as one of the key characteristics of the nano-suspension together with MF on the value of the CHF, the boiling heat transfer and fouling thermal resistance (FTR) has not been fully studied. In the previous studies reported in the literature, the dependence of the deposition rate of NPs with zeta potential and its influence on CHF has not been studied. Hence, there is a need to further investigate the effect of ferro-fluid deposition layer on the CHF, HTC and thermal fouling resistance values of the nano-suspension with the consideration of a MF and zeta potential. Zeta potential not only influences the rate of the deposition of the NPs on the surface, but also changes the stability behavior and thermal conductivity of the nano-suspension. Thus, in this work, experimental study on the thermal performance of Fe3O4/water nano-suspension stabilized with nonylphenol ethoxilates is carried out. Characteristic tests are performed to analyze the quality of the prepared nano-suspensions. The influences of various operating parameters including the concentration of NPs, HF and zeta potential and surfactant concentration on the HTC, CHF and FTR are experimentally measured and discussed. In addition, a visualization study on the bubble formation of NPs on boiling surface covered by NPs under the influence of MF is conducted to understand the role of MF on fouling formation of iron oxide particles. Also, the wettability of the surface was also investigated using contact angle measurement. The findings presented here will develop a new insight on the potential influence of the MF and zeta potential on the enhancement of boiling heat transfer and CHF for a ferro-fluid and correlates the influence of zeta potential on the value of the CHF, HTC together with the boiling surface characteristics.
Section snippets
Nano-suspension preparation and characterization
Fe3O4 NPs with size of 20 nm was dispersed in water and nonylphenol ethoxilates was used as a surfactant. The pH value of the nano-suspension was regulated with a buffer solution (a mixture of HCl and NaOH at 0.1 mM. Ultrasonic homogenizer was used to further disperse the agglomerated NPs within the base fluid. Also, zeta potential was measured at each step and for each nano-suspension to obtain the most stable nano-suspension. To investigate the particle morphology, and also the structure of
Validation of the test rig
In Fig. 7, the variation of the HTC with applied heat flux has been represented for pure water. The experimental data were compared to two well-known correlations including Rohsenow and Stephan-Abdelsalam equations to ensure that the test rig was valid and reliable. As shown in the Figure, the HTC values were within deviation of the ±8.9% and ±10.1% against Rohsenow and Stephan-Abdelsalam and ±5.1% of each other, respectively.
Surface heat flux
In Fig. 8, the variation of HTC with applied HF is depicted for
Conclusion
In the present work, by conducting a series of experiments, we reported the HTC value of the Fe3O4/water nano-suspensions at different MF magnitudes and following results were obtained:
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Zeta potential was found to be a key parameter which can promote or lower the boiling HTC. For the case in which MF was applied to the system, when zeta potential is −15 mV and +20 mV, nano-suspensions were unstable and NPs deposited on the surface resulting in the decrease in HTC. However, for zeta potential −45
Acknowledgement
Authors of this work appreciate the University of Semnan and University of Adelaide for sharing the facilities.
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