International Journal of Mechanical and Production Engineering Research and Development (IJMPERD) ISSN 2249-6890 Vol. 3, Issue 2, Jun 2013, 169-180 © TJPRC Pvt. Ltd.

RECENT STUDIES ON SINGLE PHASE FLUID FLOWS AND HEAT TRANSFER IN MICROCHANNEL - A REVIEW DATTATRAYA G. SUBHEDAR1 & BHARAT RAMANI2 1

Assistant Professor, Department of Mechanical Engineering, C.S.P.I.T., Changa, India

2

Professor, Department of Mechanical Engineering Faculty of Engineering, Rajkot, India

ABSTRACT Micro-channel heat exchangers are widely used in various industries for saving energy and resources. The heat transfer rate of convective heat transfer can be enhanced by changing geometry, boundary conditions or by enhancing thermal conductivity of the fluid. Development in micromachining and micro-miniaturization techniques has made possible the fabrication of microchannel. Nano technology provided new challenge by developing Nanofluids which contains small volume fraction of suspended nano-particles in a colloidal solution. Nanofluid has excellent potential to improve the heat transfer of base fluid due to improved thermal transport properties. This paper reviews and summarizes recent research on single phase flow and heat transfer in microchannel.

KEYWORDS: Microchannel, Nanofluids INTRODUCTION Heat exchangers are a central component of many applications in industry sector including Transportation (Engine cooling/vehicle thermal

management), Electronics cooling, Defense, Space, Nuclear systems cooling, Heat

exchanger, Other applications (heat pipes, fuel cell, Solar water heating, chillers, domestic refrigerator, Diesel combustion, Drilling, Lubrications, Thermal storage,…) forces researcher to develop an efficient heat exchanger. The rate of the transport process depends on the surface area, which varies with the hydraulic diameter D of a tube where as the flow rate depends on the cross-sectional area, which varies linearly with D2. Thus the tube surface area to volume ratio varies as 1/D [19]. Thus the hydraulic diameter of channel decreases surface area to volume ratio increases. Based on hydraulic diameter channel can be classified as [31] below: Conventional channels

:

Dh > 3mm

Minichannels

:

3mm Dh > 200 μm

Microchannels

:

200 μm Dh > 10 μm

Transitional Channels

:

10μm Dh > 0.1 μm

Transitional Microchannels

:

10 μm Dh >1 μm

Transitional Nanochannels

:

1 μm Dh > 0.1 μm

Molecular Nanochannels

:

0.1 μm Dh

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Figure 1: Enhanced Thermal Conductivity of Oxide Nanofluids Systems as Measured by Lee et al. K/Ko Denotes the Ratio of Thermal Conductivity of Nanofluids to that of the Base Fluid Nano technology (Nanopowder synthesis techniques) provides good opportunity to process and produce materials with average crystallite sizes below 50 nm. The word Nanofluids was invented at Argonne National Laboratory of USA by Choi in 1995. As the heat transfer by convection can be enhanced also by using various fluid of higher conductivity. Nanofluids are also conventional fluids used for the heat transfer like water, engine oil, glycol mixtures in which a very small volume fraction of nanoparticles, nanofibers, whose conductivity is higher than the base fluid are suspended in a colloidal solution. Figure 1 shows the enhancement of conductivity of some Nanofluids with respect to the conventional fluid [33, 38, 39 and 43].

INFLUENCE OF MICROCHANNEL ON THE HYDRAULIC AND THERMAL PERFORMANCE The cross section and shape of Microchannel may have considerable effect on the thermal and hydraulic performance of a heat exchanger. In the fully developed laminar regime in microchannel, the local heat transfer coefficient h varies with the channel size [31, 32] it is given by:

h Nu

k Eq.1 Dh

Figure 2: Variation of the Heat Transfer Coefficient with Channel Size for Fully Developed Laminar Flow of Air and Water [32]

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Where k is the thermal conductivity of the fluid and Dh is the hydraulic diameter of the channel. On the other hand, the friction factor f varies inversely with Re, since the product f · Re remains constant during fully developed laminar flow [31,32]. In the turbulent regime, both the classical correlations of Gnielinski, Dittus and Boelter, seem to be quite reliable for dh

300- 500 μm [25]; The frictional pressure drop per unit length for the flow of an incompressible fluid is given by:

p f L

2 fG 2 Eq.2 D

Where pf /L is the frictional pressure gradient, f is the Fanning friction factor, G is the mass flux, and ρ is the fluid density. For fully developed laminar flow Figure 3, we can write:

f . Re C Eq.3 Where Re is the Reynolds number, Re=G.Dh/µ, and C is a constant. An experimental investigation concludes that the pressure drop and friction factor values agreed with the values of classical channel ﬂow theory [6, 13, 17, 18, 23, and 28].

Figure 3: Variation of Pressure Gradient with Channel Size for Fully Developed Laminar Flow of Air and Water [32] The behavior of the flow in micro-channels, at least down to 50 μm diameters, shows no differences with macroscale flow. For smooth and rough micro-channels with relative roughness 0.32% to 7% the transition from laminar to turbulent flow founds for Reynolds number 1800 to 2200, in full agreement with flow visualization and flow resistance data. For single-phase fluid flow in smooth micro-channels of hydraulic diameter from 15 to 4010 μm, in the range of Reynolds number Re < Recritical the Poiseuille number, Po is independent of the Reynolds number, Re [5]. The streamwise mean velocity proﬁles and turbu-lent intensities by micro-PIV show that the transition from laminar ﬂow to turbulent ﬂow occurred for at Re = 1700–1900.The ﬂow become fully developed turbulent in the microtubes when the Reynolds number exceeds 2500[26].

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Figure 4: Variation of Pressure Drop for One Channel of Different Shapes [20] Various cross sections such as square, rectangular, circular, trapezoidal etc also affects on the performance of microchannel heat exchanger [20]. Moreover circular channels give the best overall performance (thermal and hydraulic) among others various channel shapes for microchannel heat exchanger as shown in Figure 4 and 5. Microchannel performance for channel shape like zigzag, wavy, curvy, and step microchannel and compare with straight channels[12,41,11] concludes that for the same cross section of a microchannel the temperature and the heat transfer coefficient of the zigzag or wavy channel is least and greatest respectively shown in Figure 6, but at the penalty of the pressure drop. Friction factor for all the shape is highest than conventional straight Microchannel Figure 7. In case of curved rectangular microchannel the geometrical aspect ratio has significant influence on the pressure drop [15].

Figure 5: Variation of Heat Transfer Rate Over Pumping Power Heat Transfer Rate Over Pumping Power Required for Different Channel Shapes (Re = 50) [20]

Figure 6: Dimensionless Average Heat Transfer Coefficient with Number of Channels for Different Shapes (Re=60) [12]

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Figure 7: Variation of Friction Factor versus Reynolds Number for Different Channel Shapes [12] The shape of the bends and branching has a significant influence on both, pressure loss and heat transfer. Redirecting and splitting the fluid in microchannel leads to an enhanced heat transfer, but at the same time the pressure loss is increased [3] as shown in Figure 8.

Figure 8: Simulated Elements with Normalized Pressure Distribution with Re = 200 at Inlet: (a) L-Bend (Heat Transfer Optimized); (b) T-Joint (Heat Transfer Optimized); and (c) Fork-Shaped Element (Wedge/Radius, Pressure Optimized). [3] While calculating the Nusselt number always entrance effect has to be considered. The Nusselt number for laminar flows in a channel is constant only for fully developed flows, i.e. when both the velocity and the temperature profile remain unchanged. The Graetz number is defined as in Eq. (4) and is used as a criterion for neglecting the entrance effects. [25].

Figure 9: Scaling Effects in Microchannels [25]

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Gz

Re . Pr d h Eq.4 L

If GZ<10 entrance effect on average Nusselt number can be neglected [8]. Figure 9 shows, for a water flow (Pr = 5.5), the channel length L above which entrance effects may be neglected, as a function of the Reynolds number. As Compactness is one of the most important characteristics of microchannel heat sinks and their length L can be small. Therefore for the most typical microchannel dimensions, according to Eq.4 we have to consider the entrance effect while calculating the Nusselt number. Experimental study by Chein-Yuh [1] has concluded that frictional coefficient of gas flows in microtubes and conventional larger tubes with consideration of compressibility will remain same. There is no significant size effect for air flows air flows in tube.

EFFECT OF NANOFLUIDS ON THE HYDRAULIC AND THERMAL PERFORMANCE OF MICROCHANNEL Many researchers concluded after their painful experiments the thermo physical properties like thermal conductivity, viscosity, thermal diffusivity, convective heat transfer coefficient enhanced compared to the base fluid according to particle size, percentage volume fraction as shown in Table 1 and 2[35]. Conductivity of fluid (eq.1) is also one of the important parameter to enhance the heat transfer rate in microchannel heat exchanger. It is investigated that by using the nano- particles having higher conductivity (Table 2.1) than the basic fluid we can used to prepare a fluid called Nano-fluid which can be used to enhance the heat transfer due to their high conductivity [35]. Table 1: Thermo-Physical Properties of Nano-Particle, Base Fluid [35] Materials Nanoparticle Base Fluid

Al2O3 TiO2 Cu Water

ρ (kgm-3) 3970 4157 8933 997

Cp (Jkg-1c-1) 765 710 385 4187

μ (Pas) 0.000855

k (Wm-1C-1) 36 84 401 0.613

Table 2: Thermo-Physical Properties Associated with Various Nanoparticles, Particle Volume Fractions, and Pumping Powers for Particle Size D=38 Nm. [35] Nanofluid Ф = 0.5 %

Ф=1%

Al2O3- Water TiO2 -Water Cu- Water Al2O3- Water TiO2 -Water Cu- Water

ρ (kgm-3) 1012 1013 1034 1027 1022 1071

Cp (Jkg-1c-1) 4111 4107 4033 4046 4038 3889

k (Wm-1C-1) 0.8942 0.8794 0.8643 1.1567 1.1167 1.0745

μ (Pas) 0.000858 0.000945 0.001098 0.000919 0.001060 0.001312

The conductivity of the suspended nano-particles in a conventional fluids is found greater than the conventional fluids motivates many researchers of their application in Heat exchanger. The large surface area of nanoparticles allows for more heat transfer. Another advantage is the mobility of the particles, attribute to the tiny size, which may bring about micro-convection of fluid and increased heat transfer. Because of the small particles

they weigh less, and the chances of

sedimentation are also less. This will make the nanofluids more stable. Large enhancement of conductivity was achieved

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with a very small concentration of particles that completely maintained the Newtonian behavior of the ﬂuid. The rise in viscosity was nominal; hence, pressure drop as increased only marginally. Unlike the situation with micro- slurries, the enhancement of conductivity was found to depend not only on particle concentration but also on particle size. In general, with decreasing particle size, an increase in enhancement was observed. [30, 29, 43, 21, 35] The particle size has an insignificant effect on cooling performance above certain value because Brownian motion may be inhibited for larger particles [27]. There are many applications of Nanofluids [37] like Transportation (Engine cooling/vehicle thermal management), Electronics cooling, Defense, Space, Nuclear systems cooling, Nuclear systems cooling, Heat exchanger, Biomedicine,

Other applications (heat pipes, fuel cell, Solar water heating, chillers, domestic refrigerator, Diesel

combustion, Drilling, Lubrications, Thermal storage,…) forces researcher to study the Nanofluids behavior in depth. There are also challenges to use the Nanofluids due to some disadvantages [30] like:

The particles settle rapidly, forming a layer on the surface and reducing the heat transfer capacity of the ﬂuid.

If the circulation rate of the ﬂuid is increased, sedimentation is reduced, but the erosion of the heat transfer devices, pipelines, etc., increases rapidly.

The large size of the particles tends to clog the ﬂow channels, particularly if the cooling channels are narrow.

The pressure drop in the ﬂuid increases considerably.

Finally, conductivity enhancement based on particle concentration is achieved (i.e., the greater the particle volume fraction is, the greater the enhancement—and greater the problems, as indicated in 1–4 above).

EFFECT OF FLOW ARRANGEMENT ON THE HYDRAULIC AND THERMAL PERFORMANCE OF MICROCHANNEL The heat flux obtained from the counter-flow arrangement is always higher than that obtained from the parallelflow one: the value obtained from the counter-flow is 1.1 to 1.2 times of that obtained from the parallel-flow. This means that the heat transfer behaviors of the microchannel heat exchanger with counter-flow are better than those with parallelflow [34]. Swirl microchannel Figure 10 improves the heat transfer performance over straight Microchannels by 50% on average [44].

Figure 10: Swirl Microchannels Layout [44]

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The effect of Nanofluid (Al2O3) in radial flow cooling system Figure. 11 [16, 45] on heat transfer. The study conclude that heat transfer coefficient increases with the increase of the Reynolds number and the nanoparticles volume fraction, through increase in Pressure drop is more significantly associated with the increase in particle concentration.

SURFACE ROUGHNESS IN MICROCHANNEL Surface roughness may have a significant impact on microchannel performances [10], since at such a small scale it is nearly impossible to obtain an actual smooth surface. A remarkable effect of roughness on pressure drop, and a weaker one on the Nusselt number. The friction factors in stainless steel tubes (D = 119â€“300 Âľm) are much higher than the theoretical predictions for tubes of conventional size. This difference is resulted from the large relative surface roughness in the stainless steel tubes [7]. The performances are dependent on the geometrical details of the roughness elements. Large roughness of the micro-tube causes high friction factor [47, 46]. The roughness elements have a significant impact on the flow characteristics. For rarefied gases, it is found that roughness effect leads to an increase in the Poiseuille number with increasing roughness height and decreasing element spacing [42]. The surface roughness has a more significant effect on the flow with a lower inlet Kn. Compressible gas flow is also sensitive to the height of the wall roughness elements. In addition, an increase of the relative roughness height leads to a pronounced decrease in the local heat flux for both rarefied and compressible flow. The average Nusselt numbers have a much more significant reduction for a rarefied flow than a compressible flow. The influence of wall roughness on the average heat transfer rate is smaller than that on the Poiseuille number [42]. In Many works researcher found that in microchannel heat exchanger that the Nu decreases when Re increases. The conventional friction prediction is valid for water flow through micro tube with a relative surface roughness less than about 1.5% [47].

Figure 11: Radial Flow Cooling System [45] The local Nusselt number approaches the conventional theory prediction.Y.Liu.et.al studied performance of Microchannels with different surface microstructures numerically and concluded that shield shape groove microchannel [40] possesses the highest heat exchange performance. In microchannel surface roughness, which likely responsible for early transition from laminar to turbulent flow and the increased friction factor and Nusselt number [46].

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HEAT TRANSFER CHARACTERISTICS OF MICROCHANNEL The Nu in Microchannels can be predicted by classical conventional correlation without and significant error. For smooth walls the continuum mechanics laws for convection and fluid mechanics remain valid in Microchannels of hydraulic diameter greater than or equal to 100 µm[24,22]. It has been observed that increase in the heat transfer is possible using Nanofluid in comparison to conventional fluids. Convective heat transfer coefficient for nanofluids is greater than that of the base liquid. Heat transfer enhancement increases with the particle volume concentration, but it is accompanied by increasing wall shear stress values [36]. Heat transfer characteristics of gaseous flows in micro tubes with constant heat flux whose value is positive or negative are investigated on two-dimensional compressible laminar flow for no-slip regime and compared with those of the incompressible flow in a conventional sized tube. In the case of fast flow, temperature profiles normalized by heat flux have different trends whether heat flux is positive or negative [2]. Two dimensional simulations was performed For low Reynolds number flow of liquid water in a single channel subjected to localized heat flux boundary conditions. The velocity field was highly coupled with temperature distribution and distorted through the variations of viscosity and thermal conductivity. The induced cross-flow velocity had a marked contribution to the convection. The heat transfer enhancement due to viscosity-variation was pronounced, though the axial conduction introduced by thermal-conductivity-variation was insignificant unless for the cases with very low Reynolds numbers [14]. Depending on the contact angle and the wall shear rate, variations in the heat transfer rate exceeding 10% can be expected. Thus the contact angle is an important consideration in the design of micro, and even more so, nano heat exchangers [6]. Poiseuille and Nusselt numbers are found to be significant functions of aspect ratio, Knudsen number, slip model parameters, Brinkman number, and Peclet number [4].

CONCLUSIONS

In the microchannel, the local heat transfer coefficient h varies with the channel size, cross section of tube, shape of channel, fluid properties and the fluid flow arrangement.

Conductivity of Nanofluid is more than conventional fluid. Heat transfer coefficient is higher than base liquid and increased with increasing Reynolds number and particle concentration. Pressure drop increases than base fluid with increasing volume concentration.

To enhance the heat transfer in microchannel heat exchanger or microchannel heat sink it is necessary to study simultaneous effects of various parameter like size of channel, shape of channel, Nanofluid properties, Reynolds number, friction factor, pressure drop, pumping power etc.

ACKNOWLEDGEMENTS I thanks to our Principal,Dr. Niraj Shah and HOD, Mechanical Engineering Dr. Piyush Gohil for their valuable support.

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