Heat transfer evaluation of a micro heat exchanger cooling with spherical carbon-acetone nanofluid

Highlights

• Convection of a micro-channel heat sink cooling with carbon-acetone nanofluid studied experimentally.

• Heat transfer coefficient, friction factor, pressure drop value, and the thermo-hydraulic performance was quantified.

• The structure and morphology of the nanoparticles (NPs) were characterized.

• The value of PD was augmented by 18.3% at Reynolds number of 1400 at wt.% = 0.1.

• The THP value was increased by 76% despite an increase in the PD of the system.

Abstract

In this article, we experimentally measured the convective heat transfer of a micro-channel heat sink cooling with carbon-acetone nanofluid (NF), which potentially is a cost-effective nano-suspension with plausible heat transfer characteristics. The heat transfer and fluid flow characteristics including heat transfer coefficient (HTC), friction factor (FF), pressure drop value (PD), and the thermo-hydraulic performance (THP) was quantified. The structure and morphology of the nanoparticles (NPs) were characterised. It was found that carbon-acetone NF can enhance the HTC value by ~73%. Also, a small increase in the FF and PD values were reported due to the augmentation of particle-fluid friction forces and viscosity. The value of PD was augmented by 18.3% at Re~1400 at wt.% = 0.1. The THP value was increased by 69% despite an increase in the PD of the system. It was identified that the promotion of heat transfer was due to the micro-scale phenomena such as Brownian motion and thermophoresis. Also, reduction in the thermal boundary layer inside the micro-channel further contributed to the promotion of heat transfer within the micro-channel.

Introduction

Nowadays, efficient cooling and designing a robust heat exchanger (HEX) for high heat flux removal is by far one of the key research topics in advanced thermal engineering science [1], [2], [3]. The cooling system is the heart of a process heat, power and nuclear plants, micro-electronics systems and automotive industries [4], [5], [6]. By advancing the technology development in areas of microelectronics, voluminous and rapid calculations occur in nano- and micro-scale processors, which in turn increases the rate of heat dissipation and power consumption (heat production) from the integrated circuits (ICs) and chipsets [7, 8]. Hence, an advanced cooling system is required to remove the generated heat and increase the performance of the system over a long operation of the system [9], [10], [11]. The prevalent methods of enhancing the heat transfer include increasing the heat transfer surface area, vibrating the heat transfer's surface or fluid, applying the electric and magnetic field, or adding solid particles to the base fluid to improve the physical properties of the coolant [12], [13], [14]. In most cases, the heat transfer optimization is taking into account by enlarging the surface such as using fin and micro-channel [15], [16], [17]. The microchannel HEX is a temperature-controlling device, which can remove large amount of heat with a confined space. Microchannel cooling systems are cheap, cost-effective and easy to fabricate and can be scaled up depending on the cooling capacity and demands [18], [19], [20].

In HEXs, the thermal conductivity of the working fluid is a crucial parameter, which directly affects the techno-economic and performance index of the system [21, 22]. Since the introduction of nanofluids (NF, a mixture of solid metal oxides in a conventional coolant such as water), it has been shown that by applying NF in different configurations of HEXs, the thermal performance (THP) of the system is improved [23, 24]. For example, Abdollahi-Moghaddam et al. [25] experimented the effect of CuO/water NF on the heat transfer coefficient (HTC) of the double pipe HEX. They noticed that by adding 0.7% of nanoparticles, the HTC of the system increased by 240%. Farajollahi et al. [26] tested Al₂O₃/water and TiO₂/water NFs in a shell and tube HEX. The results indicated both NFs increased the HTC in comparison with water. Also, TiO₂/water had better THP, despite the fact that high concentration Al₂O₃/water had better heat transfer properties. In another study, the heat transfer enhancement of polyaniline/water NF in a helically tube HEX was demonstrated by Bhanvase et al. [27]. They observed that at vol.% = 0.1 and 0.5 of polyaniline, the HTC was promoted by 10.5% and 69.6%, respectively. Heat transfer of NF in a plate-fin HEX was investigated by Khoshvaght-Aliabadi and Salami [28]. The experiments were conducted with Al₂O₃/water NF at concentrations 1 to 4% (by volume) and Reynolds number between 6000 and 22,000. The results showed that the HTC can be increased by 18.6% at maximum concentration.

The combination of NF and micro-channel has been demonstrated to potentially increase the THP of the cooling system. For example, Azizi et al. [29] evaluated the THP of a cylindrical microchannel in the presence of Cu/water NF. They observed that adding 0.3 wt.% of Cu nanoparticles (NPs) to the base fluid can enhance the Nusselt number by 23%. Vinoth et al. [30] experimentally studied various cross sections of a microchannel while Al₂O₃/water NF was used as a coolant in the system. The results indicated that trapezoidal cross-section augments the NF heat transfer by 5.88% more than square cross section. Duangthongsuk and Wongwises [31] investigated SiO₂/water NF flowing into a zigzag microchannel. They found that 0.3 to 0.8 vol.% NF lead to 3 to 15% more THP compared to pure water. In an interesting study, Anoop and Sadr [32] measured the near-wall velocity of NFs. They prepared 0.1–1 wt.% concentration of SiO₂/water NF and utilized nanoparticle image velocimetry for NF flowing in rectangular cross-section microchannel. They found that the near-wall velocity of water and NFs at given flow rate are similar to each other. Experimentally results of Putra et al. [33] about demonstrated that using 1 vol.% SnO₂/water and 5 vol.% Al₂O₃/water NFs in microchannel instead of pure water can enhance the heat transfer coefficient almost 14% and 13%. THP of CNT/water NF flowing in a solar microchannel collector was investigated by Ahlatli et al. [34]. The experiments were conducted in laminar flow for Reynolds numbers between 50 and 1500 and weight fractions between 0.01 and 0.5. The observations illustrated that heat transfer and PD increase with concentration. A serpentine microchannel was studied by Sivakumar et al. [35] under CuO/water and Al₂O₃/water NFs laminar flows. Overall, NFs increased heat transfer coefficient compared to the base fluid and it is more obvious for higher concentrations. Furthermore, CuO/water NF had better thermal efficiency than to the Al₂O₃/water NF.

Sohel et al. [36] analytically investigated the heat transfer properties of copper, and alumina dispersed in water and ethylene glycol as a coolant inside two microchannel and minichannel HEX. The results showed that Cu/water NF at vol.% = 6 had the maximum THP in comparison with other NFs. Abdollahi-Moghaddam et al. [37] carried out a statistical optimization of microchannel operating conditions in which Al₂O₃/water NF flowing at concentrations of vol.% = 0.1 to 1. A NSGA-II algorithm was utilised to obtain the concentrations and Reynolds numbers, in which the friction factor (FF) and Nusselt number are optimised. They proposed an equation to predict the lowest FF versus optimum Nusselt number. The effect of magnetohydrodynamic flow of NF was analysed by Ganguly et al. [38]. The NF was flowing in a parallel plate microchannel while magnetic field, pressure-driven and electroosmotic transport affecting the fluid simultaneously. The results indicated adding nanoparticle causes heat transfer reduction. Total entropy generation decreased by increasing NF's concentration, which in turn decreased the thermodynamic irreversibility. Halelfadl et al. [39] experimented a rectangular microchannel when CNT/water was utilised as a working fluid at wt.% = 0 to 0.01. They optimized the THP of the NF by means of NSGA-II algorithm. They found that the NF can improve the THP of the microchannel as it decreases the average value of thermal resistance in comparison with water. Malvandi et al. [40] studied a circular microchannel by considering the migration of Al₂O₃ nanoparticle and Brownian motion. They assumed that the fluid flow is laminar and fully developed. Optimization results showed that the smaller diameter of nanoparticle is more plausible for increasing the THP of the system. In Table 1 some studies on the THP characteristics of microchannel operating with NFs are summarized.

In light of above, it can be stated that the performance of different aqueous and non-aqueous NFs such as Al₂O₃, CNT, CuO, TiO₂, and Ag in microchannel heat exchanging systems has widely been studied. A comparison between the outcome of the research shows that carbon nanotubes NPs have plausible THP in comparison with the metal oxides, which is largely due to the unique thermal features of carbon structure. However, production of carbon nanotube is expensive and requires complex chemical process of production. Hence, in the present work, a new NF is prepared using spherical carbon particles with plausible thermal conductivity dispersed in acetone. The heat transfer characteristics of the NF in a microchannel heat exchanging system are experimentally investigated and the mechanism for the enhancement of heat transfer is discussed.