Heat transfer enhancement of Al2O3-H2O nanofluids flowing through a micro heat sink with complex structure

doi:10.1016/j.icheatmasstransfer.2015.05.025

International Communications in Heat and Mass Transfer

Volume 66, August 2015, Pages 158-166

https://doi.org/10.1016/j.icheatmasstransfer.2015.05.025 Get rights and content

Abstract

The characteristic of flow and heat transfer of Al₂O₃-H₂O nanofluids flowing through a micro heat sink with complex structure under constant heat flux is investigated experimentally. The volume fraction and Reynolds number of Al₂O₃-H₂O nanofluids ranged from 0.1 vol.% to 1.0 vol.% and 100 to 700, respectively. Moreover, the effects of them on friction factor, Nusselt number, and thermal resistance are also considered. The results show that single phase model underestimates the friction factor and Nusselt number of nanofluids from the experiments. With increasing volume fraction and Reynolds number, both Nusselt number Nu and friction factor f of nanofluids increase while the average temperature at the bottom and thermal resistance decrease, as compared to deionized water. Moreover, new correlations of f and Nu are presented based on experimental data with the maximum deviation less than ± 5%. The performance evaluation plot shows that the higher slope of values, i.e., higher volume fraction of nanofluids, gives better comprehensive performance of heat transfer. The compound heat transfer enhancement techniques, i.e., cavities in the micro heat sink and nanofluids can obviously enhance heat transfer.

Introduction

With the development of power in micro applications, i.e., microelectronic devices, automotive and aerospace industries etc., the challenge of keeping micro heat exchangers at their best performance is inevitable [1]. Due to the high surface area to volume ratio, compactness and high heat transfer coefficient, micro heat sinks have been considered as an effective tool to provide increasing heat dissipation rates and to reduce temperature gradient across microelectronic devices. Thus, the characteristic of flow and heat transfer in the micro heat sink has become a very significant topic to attract more and more researchers' attention.

The characteristic of deionized water flowing through micro heat sinks with complex structure has been reviewed by Xu et al. [2], Zhai et al. [3], Foong et al. [4] and other authors. Xia et al. [5] numerically investigated the characteristic of flow and heat transfer in microchannels with cavities and ribs. The results showed that Nusselt number is 1.3–3 times more than that in the smooth microchannel for Reynolds number ranging from 150–600. Stacked micro heat sink was numerically studied by Cheng [6], which was shown to have better performance than that in smooth microchannels. From those reviews, heat transfer coefficient of micro heat sinks with complex structure is much higher than that of smooth micro heat sinks. However, the thermal conductivity of common coolant such as water, ethylene glycol or engine oil is relatively low, which limits the heat transfer capabilities. With increasing heat transfer capacity, nanofluids make them more attractive as a coolant in heat exchangers.

So far, many researchers have focused mainly on nanofluids flowing through smooth micro heat sinks. In these reports, working fluid as nanofluids was compared with water. Ghazvini and Shokouhmand [7], Hung et al. [8], Seyf and Nikaaein [9], Jang and Choi [10] and many others numerically or experimentally investigated various nanofluids in rectangular or circular microchannels. Few researchers have based their results on the combined effect of nanofluids and micro heat sinks with complex structure. Zhou et al. [11] and Seyf and Feizbakhshi [12] experimentally studied silver nanofluids and Al₂O₃ nanofluids through the micro-pin fin heat sink, respectively. Mohammed et al. [13] conducted the combined effect of rib–groove and nanofluids on thermal and hydrodynamic characteristics in the channel. Shalchi-Tabrizi and Seyf [14] investigated the effect of volume fractions and particle diameters on entropy generation and thermal performance in a tangential micro heat sink. Zirakzadeh et al. [15] experimentally investigated the characteristics of Al₂O₃ nanofluids through a novel heat sink. The results showed that the heat transfer coefficient was up to 20% in comparison with the conventional plate pin heat sink.

In addition to experimental investigations, numerical studies are also widely accepted by researchers to study the characteristic of flow and heat transfer in micro heat sinks. Single phase model is one of them. Single phase model assumes that liquid phase and solid phase are in thermal equilibrium [16]. Moreover, the solid particles (nanoparticles) are small enough that can be easily fluidized and approximately considered to behave as a fluid. Seyfe and Mohammadian [17] used single phase approach for the nanofluids modeling in a counterflow microchannel heat exchanger. More description can be seen in this literature. Hashemi et al. [18] used porous medium approach to simulate the SiO₂-water nanofluids flowing through miniature heat sink. They showed that the increase of aspect ratio and porosity could enhance heat transfer.

However, the relative velocity between liquid phase and nanoparticles in nanofluids might not be zero due to Brownian forces, Brownian diffusion, micro convection, gravity, sedimentation and dispersion, etc. [19]. Four individual models (single phase, VOF, mixture, Eulerian) were numerically conducted by Moraveji and Ardehali [20]. They reported that two phase models represented better approximation of experimental data comparing to single phase model, thus single phase model might not always remain true for nanofluids. Kamyar et al. [21] proposed the accurate results of numerical simulation for nanofluids only if the conditions were close to actual situations. The effect of Brownian motion and micro convection between liquid and nanoparticles always existed. Hence, the problem of modeling nanofluids through micro heat sinks as single phase model is debatable.

As stated above, experimental studies on convective heat transfer of nanofluids in micro heat sinks with complex structure are limited. This is the main purpose to recover some research gap on it. Based on Chai et al.'s [22] work, Al₂O₃-H₂O nanofluids flowing through the micro heat sink with fan-shaped reentrant cavities are studied experimentally in the present work.

This study can be structured in the following manner. Firstly, the characteristic of flow and heat transfer of Al₂O₃-H₂O nanofluids flowing through the micro heat sink with fan-shaped reentrant cavities is investigated experimentally. Secondly, numerical data of friction factor and Nusselt number are compared with those of experimental data to validate whether single phase model is valid for nanofluids or not. Thirdly, performance evaluation plot is also used to estimate the overall heat transfer performance.

Section snippets

Thermophysical property measurements of nanofluids

The thermal conductivity of nanofluids is measured experimentally (Hot Disk 2500 type) by the transient plane source (TPS) method [23]. On the other side, vibrating string viscometer is used to measure dynamic viscosity at the room temperature. The ultrasonic vibrator (KH-100DB) is used to vibrate the nanofluids. We try several times to get the best stability of nanofluids, 2 h, 4 h, 6 h, 8 h, then the vibration with 8 h can effectively avoid nanopowders sediment. Base fluid (water) is used to

Numerical simulation of single phase model

As stated in the Introduction section, single phase model is widely accepted by many researchers. However, nanofluids are solid–liquid mixture in nature. Therefore, the numerical data are compared with experimental data to validate whether single phase model is suitable for nanofluids or not.

Single phase model of the continuity, momentum and energy equations can be extended to nanofluids by directly replacing the thermophysical properties appearing into nanofluids' effective properties,

Data for flow and heat transfer

The friction factor is calculated by means of an equation given below, $f = \frac{2 Δ p D_{h}}{ρ_{n f} L_{c h} u_{m}^{2}}$ where, Δp is the pressure drop of the micro heat sink, which is the difference between total pressure drop and local pressure drop, Pa. u_m is the mean velocity, m/s.

The local pressure drop is written as follows, $Δ p_{loss} = \sum ξ_{i} \frac{ρ {u_{i}}^{2}}{2} i = 1, 2, \dots, n .$

Due to the rectangular cross section of single microchannel, the hydraulic diameter is written as, $D_{h} = \frac{2 W_{c h} H_{c h}}{W_{c h} + H_{c h}} .$

The average heat transfer coefficient is calculated as

Validation of experiment and simulation

Initially, to validate the experimental and numerical methods, comparisons with the available correlations of pressure drop Δp presented by Kandlikar et al. [40] and the outlet temperature of fluid derived from simple energy conservation have been done, respectively. The experiments have been performed by deionized water, i.e., φ = 0%.

The correlation of pressure drop presented by Kandlikar et al. [40] was shown as follows, $Δ p = \frac{2 (f R e) μ u_{m} L_{c h}}{D_{h}} + K (\infty) \frac{ρ u_{m}^{2}}{2}$ $f R e = 24 (1 - 1.3553 α_{c} + 1.9467 α_{c}^{2} - 1.7012 α_{c}^{3} + 0.9564 α_{c}^{4} - 0.2537)$

Conclusions

In this paper, Al₂O₃-H₂O nanofluids as a coolant in the micro heat sinks with complex structure subjected to a constant heat flux have been experimentally investigated. The volume fraction and Reynolds number vary from 0 to 1 vol.% and 100 to 700, respectively. Therefore, the following remarks can be concluded as follows:

1.
Single phase model underestimates the friction factor and Nusselt number of the nanofluids from the experiments. Many factors, such as Brownian diffusion, gravity, slip and

Acknowledgments

Our research program is supported by the National Natural Science Foundation of China (No. 51176002), the National Basic Research Program of China (2011CB710704), and the Beijing Natural Science Foundation (3142004).

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^☆: Communicated by W.J. Minkowycz.

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Heat transfer enhancement of Al2O3-H2O nanofluids flowing through a micro heat sink with complex structure☆

Abstract

Introduction

Section snippets

Thermophysical property measurements of nanofluids

Numerical simulation of single phase model

Data for flow and heat transfer

Validation of experiment and simulation

Conclusions

Acknowledgments

Superlattice Microst

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Int. J. Therm. Sci.

Appl. Therm. Eng.

Int. Commun. Heat Mass Transfer

Energy Convers. Manag.

Int. J. Heat Mass Transfer

Int. J. Therm. Sci.

Appl. Therm. Eng.

Int. J. Therm. Sci.

Int. Commun. Heat Mass Transfer

Int. J. Heat Mass Transfer

Int. J. Therm. Sci.

Int. Commun. Heat Mass Transfer

Int. J. Heat Fluid Flow

Int. J. Therm. Sci.

Int. J. Heat Mass Transfer

Int. J. Heat Mass Transfer

Appl. Therm. Eng.

Int. J. Therm. Sci.

Heat transfer enhancement of Al₂O₃-H₂O nanofluids flowing through a micro heat sink with complex structure☆