Thermal performance of a micro heat exchanger (MHE) working with zirconia-based nanofluids for industrial cooling

International Journal of Industrial Chemistry https://doi.org/10.1007/s40090-019-0183-6 RESEARCH Thermal performance of a micro heat exchanger (MHE) working with zirconia‑based nanofluids for industrial cooling V. Nikkhah1 · SH. Nakhjavani2 Received: 10 September 2018 / Accepted: 9 April 2019 © The Author(s) 2019 Abstract An experimental investigation was performed with the view to assess the heat transfer characteristics of a water-based nanofluid in a micro heat exchanger employed to quench a high heat flux heater for industrial and microelectronic cooling applications. The experiments were conducted at heat fluxes 10–70 kW/m2 and for nanofluids at various mass concentrations of 0.1–0.3% and passing flow rates of 0.1–5 l/min. Thermo-physical properties of the nanofluid including thermal conductivity, heat capacity, density and viscosity of nanofluid were experimentally measured at 40 °C close to the temperature of the experiments. Results showed that the heat transfer coefficient and pressure drop were augmented by 40.1% and 67% at wt% = 0.3 compared to the base fluid, respectively. The enhancement in the heat transfer coefficient was associated with the improvement in the thermal conductivity of the base fluid together with the intensification of Brownian motion and thermophoresis effect. The increase in the pressure drop was also attributed to the increase in the viscosity of the working fluid which induces layer–layer frictional forces in the bulk of the coolant in micro heat exchanger. Keywords Nanofluid · Zirconia/water · Micro heat exchanger · Convective heat transfer · Heat transfer coefficient Abbreviations A The microchannel surface area, m2 Cp The heat capacity of the eutectic, kJ/kg °C d Diameter of the microchannel (hydraulic diameter), m f Friction factor H Height of microchannel, m I Digital current, A k Thermal conductivity, W/m °C L or L Length, m m Fin constant ṁ Mass flow rate, kg/m2 s N Number of microchannel Q Heat transfer, W Qʹʹ Applied heat flux, kW/m2 s Distance between thermocouple and wall of microchannel, m T Temperature, °C u Fluid mean velocity, m/s * SH. Nakhjavani 1 School of Chemical Engineering, Semnan University, Semnan, Iran 2 School of Engineering, University of Yazd, Yazd, Iran V Voltage, V W Width of microchannel, m z Axial distance, m Greek letters 𝜂 Efficiency of the fin 𝜌 Density, kg/m3 𝜈 Fluid flow rate, m/s 𝜇 Viscosity, cP Introduction Heat exchangers play a key role in cooling systems and power cycles thanks to their anomalous thermal features, contact surface area and the great heat transfer coefficient. Depending on the application, type of the coolant, the rate of cooling and amount of the heat transfer rate, different models and standards have been defined to fabricate efficient heat exchanging media [1–7]. When it comes to the surface cooling technology, liquid blocks and heat sinks are pioneer types of heat exchanging media with the capability to remove the significant amount of thermal heat dissipated from the heating surface using a very small space [4, 8–10]. For instance, to cool a central processing unit (CPU), normally a heat sink equipped with a forced convective fan is 13 Vol.:(0123456789) International Journal of Industrial Chemistry used which is not only a noiseless technology but also an efficient conductive/convective cooling system. However, air and conventional coolants used in such systems have reached their limitations [11–14]. For example, the thermal conductivity of water is around 0.59 W/m K, which is way lower than other coolants such as liquid metals, siliconebased liquid coolants and oil-based working fluids [12, 15, 16]. Thereby, seeking some alternatives to replace the conventional coolant has been targeted by the researchers. Since nanofluid was introduced in Aragon National Laboratories (ANL) [17, 18], extensive studies have been conducted to further understand the micro-mechanisms involved in the nanofluids. By a definition, a nanofluid is a colloidal mixture of a conventional coolant and some conductive particles with the nominal size of zero to 100 nano-meters [19–22]. The presence of these nanoparticles can intensify some thermo-physical properties of the nanofluids including thermal conductivity, heat capacity, and density. Together with these properties, viscosity has also been reported to be changed due to the particle–particle behaviour [23–27]. Thanks to the changes occurred in the physical properties of the coolant, an enhancement in the heat transfer coefficient has been reported in the literature [28–30], while there are some studies that vote against the enhancement of heat transfer due to the nanofluids [31–33]. Such controversial reports are the main driver for further study on nanofluids. That being said, much effort has been made to implement nanofluid for cooling purposes not only in the heat exchangers but also in cooling systems. For example, in several studies conducted by Sarafraz et al., on various groups of nanofluids [12, 34–37] and base fluids [34, 38–43] and for different thermodynamic systems [11, 37, 44–48], the heat transfer characteristics of carbon nanotube-based nanofluid inside the various heat exchangers were investigated. They also conducted the experiments on some modified surfaces with circular fins, rectangular channels and tubular cross sections with the view to enhance the heat transfer coefficient, bubble formation (in two-phase experiments) while taking advantages of nanofluid to enhance the thermo-physical properties of the water. They found out that the HTC on the smooth heating surface decreased, while for the modified one, the HTC increased by 56% and 77% for wt% = 0.1 and wt% = 0.3, respectively. They also noticed that the bubble formation might be affected due to the presence of nanofluids which created a fouling layer on the surface resulting in the change in HTC. Apart from two-phase experiments, some researchers focused on the nanofluid’s physical properties. Much effort has made to investigate the influence of nanoparticles on the enhancement of the thermal conductivity and the density of nanofluids with the view to enhance the HTC and the ability of nanofluid for thermal energy storage. It has been shown that such conductive particles have plausible influence on 13 the heat capacity, thermal conductivity and density of the base fluid. However, disadvantage of presence of nanoparticles is reflected in the increase in the value of the pressure drop and also a deterioration in the HTC in boiling and evaporation studies [49–53]. There are other groups of study focusing on the potential of nanofluids on the enhancement of the forced convective heat transfer in the cooling and heating systems. For example, in an empirical study performed by Han et al. [54], several experiments were conducted to identify and measure the effect of aluminium oxide nanofluid on the HTC of the system in a pipe–pipe heat exch (...truncated)