Cfd Investigation Of Flow And Heat Transfer Enhancement In

Journal of Mechanical Engineering Research and Developments

ISSN: 1024-1752

CODEN: JERDFO

Vol. 44, No. 5, pp. 358-370 Published Year 2021

358

Cfd Investigation Of Flow And Heat Transfer Enhancement In

Cross Flow Microchannel Heat Exchanger Using Nanofluid

Sahib Shihab Ahmed, Nabeel Abdulhadi Ghyadh, Maher A.R. Sadiq Al-Baghdadi

Mechanical Engineering Department, Engineering Faculty, University of Kufa, Najaf, Iraq.

*Corresponding Author Email: [email protected]; [email protected]

ABSTRACT

In several engineering fields, microchannel heat exchangers are commonly used, for example in microreactors

of the hydrogen micro-fuel cells and in lab-on-chip devices in biotechnology. The micro-channels have a more

excellent surface-to-volume ratio and a higher heat transfer rate with more miniaturized size and lower weight

compared to regular scale channels. Advanced channel designs and working fluids can be added to enhance the

heat transfer performance of microchannel heat exchangers. In this study, the numerical conjugate heat transfer

and hydraulic performance of working fluid with and without nanofluids flow in microchannel heat exchangers

with and without ribs on sidewalls of channels were investigated at different Reynolds numbers (0.7–22). CFD

model of the microchannel heat exchangers with and without ribs on sidewalls and working with nanofluids as

working fluids have been developed and analyzed. The findings indicate that adding ribs in microchannels could

achieve heat transfer improvement compared to straight channels, but pressure drop increases simultaneously. It

also shows the detailed effects of the design as well as the working fluids on the heat transfer and the pressure

drop.

KEYWORDS

Nanofluids; Microchannels heat exchangers; CFD; Coefficient of heat transfer, Forebody and afterbody, Next

keyword, Projectile, Supersonic speed.

INTRODUCTION

Thermal energy flow is given by heat exchangers, and a heat transfer can occur for two fluids with two different

temperatures, separated by a wall. Entropy is created by irreversible friction flow-through losses of dissipated

flow friction and by heat transfer over a finite temperature difference [1]. For several of the above processes,

heat exchangers are a central component. Over the decades, significant gains have been seen in the significance,

and diverse application of heat exchangers across multiple industries can be as power generation, transport,

HVAC/R, manufacturing, and electronics. However, in the energy and thermo-hydrodynamic areas [2], the

dominant use of heat exchangers is found. In the field of energy conservation, conversion and recovery, heat

exchangers play an essential part. So many studies have focused on a heat exchanger type of direct transfer

(recovery) where the heat transfer between fluids occurs in a transient way through separating wall or into/and

out of a wall [3]. The thermal efficiency of heat exchangers is of high importance in the efficient, economical

operation of industrial machinery. Several passive and active techniques are available, such as the adjustment of

the fluid passage, the inclusion of swirl generators, the mixing of nanofluid to increase the thermal efficiency of

heat exchangers tube convection heat transfer [4].

Electronics have become smaller, quicker, and more efficient with the recent growth of computer technology

over the past few decades, contributing to an ever-increasing rate of heat generation from electronic devices.

The chips are cooled using forced airflow [5] to preserve the temperature of the electronic components in the

safety region. For the advancement of integrated circuits, the massive number of circuits have been made into

one miniaturised chip over the last few decades. However, to sustain the operation of chips, the increase in

induced heat flux needs a highly efficient cooling approach. Microchannel heat exchangers (MCHE) have a

higher heat transfer (HT) rate relative to traditional heat exchangers; in addition, the volume and weight are

lower. MCHE are commonly applied in various industrial fields for these advantages [6]. The cooling of the

micro heat exchangers is the big challenge. The micro heat exchanger heats rapidly because of its small scale.

Therefore, the removal of heat from them is necessary for their efficient operation at optimal temperatures. The

rate of heat dissipation in them is poor when using traditional fluids because of the low specific heat and thermal

conductivity. Nanofluids are thus used to achieve a high HT rate. The mixtures of normal traditional fluids and

nanoparticles are actually nanofluids. The working fluid thermal properties [7] are enhanced by the

nanoparticle’s thermal properties of these on the base fluid.

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Cfd Investigation Of Flow And Heat Transfer Enhancement In Cross Flow Microchannel Heat Exchanger Using Nanofluid

359

In conventional HT fluids, such as oil, water, and ethylene glycol, nanofluids are applied to engineered fluids

containing nanoparticles with an average size less than 100 nm. Compared to conventional heat transfer fluids,

nanofluids exhibit far superior heat transfer characteristics. Not a new concept is the principle of using tiny

metallic particles to improve the thermal conductivity of fluids. In 1873, Maxwell was the one who first

suggested the concept of using metallic particles to improve the thermal properties of fluids [3], recognizing that

metals in solid form have much more excellent thermal conductivity than fluids. These studies showed that

structured roughness components in micro-channels could interrupt the flow, and compared to straight ones, the

heat transfer ability was improved. However, at the cost of a higher-pressure drop, this increase was

accomplished. Therefore, to increase HT with a lower penalty on pressure drop [6], it is essential to propose

novel structures. The literature-based on micro-channel heat exchangers without and with Nano-fluid for

hydrodynamic and thermal behaviour would be presented here. Pan et al. [8] modelled a micro-channel heat

exchangers series with cavities of fan-shaped. Their performance compared with straight experimentally. They

suggested that with the penalty of a lower pressure drop micro-channels of a fan-shaped cavity might achieve

better HT performance.

They also found those heat exchangers with symmetrically arranged cavities performed better in HT than those

with asymmetrically arranged cavities. In order to determine the impact of the structure, contraction and

expansion cross-sectional cavities on the thermal and flow characteristics, Binghuan et al. [6] examined

microchannel heat exchanger systems with cavities in two different design and were conducted experimentally.

The results show that in micro-channels inserting cavities could achieve HT improvement and reduction in

pressure drop simultaneously compared with the straight channels. The effects of the design on the HT and the

pressure drop are also disclosed, as are the contraction and expansion cross-sectional lengths of cavities. The

estimated performance of the micro-heat exchangers for plastics, ceramics and aluminium by chad et al. [9], as

well as the latest advanced automotive radiator is being compared to each other. The micro cross-flow heat

exchanger transfers more mass or volume/ heat than the other existing heat exchangers within the design

constraints. This may be critical for a broad range of applications (home heating, automotive, and aerospace).

They manufactured heat exchanger by aligning and by welding two identical plastic pieces, which are moulded

using LIGA process.

After heat exchanger assembled, the liquid pumped through it and minimal leakage was observed. Abdollahi et

al. [10] examined the numerically laminar flow of Nano-fluids in a square, longitudinally internal microchannel.

The outer sections of the micro-channel are heated utilizing constant heat flux, and the inner walls are attached

with four rectangular fins. As operating fluids, Al2O3, SiO2, ZnO, and CuO are used in water-based fluids in four

different forms. There will be various 1–2 percent volume fractions and various 30–60 nm nanoparticles with

diameters. The results indicate that SiO2 has highest HT rate between all the Nano-fluids used. The rise of the

volume fraction (VF) of Nano-parts and the fall in the diameter of the nanoparticles will increase the value of

the Nusselt number. Nano-fluid with different volume fractions and different diameters of nanoparticles does

not change the friction factor significantly. In addition, the results show that Nano-fluid can enhance

longitudinal fins in microchannel efficiency. In the shape of microchannel devices (of various hydraulic

diameters) and microcolumn array devices (of different microcolumn layouts), Brandner et al. [11] introduced

micro HEs and created comparisons. With accurate mechanical micromachining processes and materials, may

produce surface roughness of less than 0.1 percent of the channel height. In most cases, microchannels devices

work within a range of mass flux where the flow is laminar since only negligible disturbance inflow profiles are

formed by surfaces of such smoothness.

At a given mass flow with regard to HT capabilities, they compared two different layouts of microcolumn

arrays. In comparison with the aligned array, it was concluded that the staggering array of microcolumns

maximized the HT and pressure drop minimized across the unit. Mapa and Mazhar [12] tested the effect of

nanofluids in a mini heat exchanger. They tested the efficiency of HT in the heat exchanger utilizing water-

water as working fluids and with a concentration of 0.01 percent and 0.02 percent volume using water and

Nano-fluid. They concluded that nanofluids increase the rate of HT and noted that the existence of nanoparticles

decreased the thickness of the thermal boundary layer. Park [13] has performed another analysis on

microchannel heat exchanger model. In order to find the highest cooling performance when combined with

pumping output, a liquid microchannel heat exchanger has been developed and investigated with different

channel widths. Another analysis contrasted the experimental findings with the newly established association of

the heat transfer rate in terms of the Nusselt number and the Brinkman number. To estimate the cooling

performance of the heat exchangers, traditional HT theories and numerical codes were also used. Compared

with numerical and traditional theories, the thermal resistance effects were compared. The analysis of the new

associations was found to agree well with the micro heat exchanger is calculated minimum thermal resistance

relative to commercial numerical codes and traditional HT theories.


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The channel geometry effect on the efficiency of a heat exchanger of a counter-flow microchannel was studied

by Hasan et al.[14]. To determine the influence of the shape and size of channels on the output, a simulation was

performed to deal with 3D development flow and conjugate 3D HT. Using forms such as circle, rectangle,

rectangular, iso-triangular, and trapezoidal, the influence of form was analyzed. It was noted that: increasing the

number of channels or reducing the volume of the channels gives an increase in HT and increased pumping

capacity and pressure drop required; circular shape offers best overall hydrodynamic and thermal characteristics

efficiency. Their analysis has also developed associations for productivity prediction as well as the output index,

which has been shown to be right across a wide variety of operational parameters. Yang et al.[15], a

computational 3D simulation of incompressible, continuous fluid flow from a trapezoidal microchannel using

(CuO-water) nanofluid is carried out. They observed that the nanofluid's thermal resistance is lower than that of

water and that the pressure drop increases significantly with respect to pure water using the nanofluid.

Karamallah et al. [16] studied the flow and the enhancement of heat transfer characteristics of four fluid types in

a double-pipe heat exchanger. The results show that the mixture of ethylene glycol and water has Nusselt

number lower than that for the ethylene glycol alone.

Aljabair et al. [17] introduced a modern method to design and manufacture a solar thermal collector using

parabolic dish collector with a cavity receiver-type by using a thermal heat exchanger radiator. The test proved

this system could be used for many applications such as a boiler in a small steam power plant. Twisted tapes are

one of the most important methods that can be used to increase the heat transfer in a heat exchanger.

Researchers have studied the using on twisted tape as a passive technology to increase the heat transfer using

different shapes and types of twisted tapes [18-21]. Moreover, both nanofluid and twisted tape was used to rise

the effectiveness of heat exchangers. It was found that the addition of nanofluid and twisted type were highly

effective as well as using other passive technologies such as fins [22-26]. Buonomo et al. [27] numerically

studied the forced convection flow of laminar water – the nanofluid Al2O3 is developed in a rectangular

microchannel. They utilized a particle size of 38 nm and steady-state regime. To solve the 3D model, a CFD

Fluent code was employed. A rectangular duct consists of the geometrical shape is considered. Laminar steady

flow and various volume fractions of nanoparticle have been utilized. The fluid was water, and alumina is

composed of nanoparticles (Al2O3). The height and the length of the edge of the conduit are respectively1.1 x10-

7 m and 0.030 m, 1.7 x10-7.

The findings are defined in terms of velocity and temperature distributions, Nusselt number, convective heat

transfer coefficient and surface shear stress, and the necessary pumping power profiles. A comparison with fluid

mechanics and thermal activities is carried out to determine the change attributed to the existence of

nanoparticles with regards to volumetric concentration. Computational Fluid Dynamics (CFD) has been widely

used in recent years to evaluate and improve heat exchanger efficiency for particular applications. CFD research

of micro-channel heat exchangers using multi-physical COMSOL geometric simulation, meshing and solution

was performed in this research. The procedure of analysis been developed by assuming suitable boundary

conditions (BCs) and properties to nanofluids. This research, which involves straight, smooth channels, side-

walled ribs, both water and Al2O3-water, has been included in this study. They show the increased convective

coefficient of heat transfer and thermal conductivity relative to their base fluid. Most of the literature focused on

a particular parameter of the heat transfer study; the present study focuses on the comparative function of two

different fluids on different geometric channels.

CFD SIMULATION MODEL

Computational domain and solution methodology

The computational domains in this work consist of two designs for cross-flow micro-channel heat exchangers

from stainless steel material. The first design contains straight, smooth channels, while the second design

contains channels with ribs on sidewalls. Quadratic elements were used in these 3D models (Figure 1). The

method of the finite volume was applied to discretizing and solving the equations by using a multi-physics

COMSOL CFD code. A grid independence study was conducted to ensure that the solution independent of the

grid size (Figure 2). The inlet cold stream temperature was considered to be 25oC, while the inlet hot stream

temperature was considered to be 50oC. The velocity was assumed uniform at the inlet of the flow field, while

the pressure was assumed uniform at the outlet. The coupled equations were solved using an iterative solution.

A criterion of error less than or equal to 1.0×10-6 was assumed adequate to achieve the convergence of the

solution.


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Micro smooth channels

Microchannels with ribs on sidewalls

Figure 1. Computational domains and meshes for two geometric configurations of the cross-flow micro-channel

heat exchangers

Figure 2. Grid independency study

Mesh size

Domain elements Boundary elements Edge elements

Smooth

ch. Ribs ch.

Smooth

ch. Ribs ch.

Smooth

ch. Ribs ch.

1 Grid 1 14952 61904 4248 11406 864 2138

2 Grid 2 26462 105063 6806 16138 1150 2620

3 Grid 3 64190 255764 13166 31280 1666 3754

4 Grid 4 115819 444190 20408 45696 2090 4602

5 Grid 5 311466 1099035 38922 83654 2880 6368

0

200

400

600

800

1000

1200

4000

4500

5000

5500

6000

6500

7000

7500

8000

8500

0 1 2 3 4 5 6

P [

Pa]

h [

W/m

2.K

]

Mesh size

h - smooth channel h - ribs channel

P - smooth channel P - ribs channel


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Conservation equations

Resolving the steady-state Navier Stokes equations gives the flow field of the fluid in the microchannels of the

cross-flow heat exchanger, which is assumed to be incompressible, single-phase and laminar [28].

Conservation of mass and momentum are expressed as;

∇⃗⃗ . (𝜌𝑓 . �⃗� ) = 0 (1)

∇⃗⃗ . (𝜌𝑓 . �⃗� �⃗� ) = −∇⃗⃗ 𝑝 + ∇⃗⃗ . �⃡� (2)

Stress tensor is defined as;

�⃡� = 𝜇𝑓 [(∇⃗⃗ ∇⃗⃗ + ∇⃗⃗ ∇⃗⃗ 𝑇) −3

2∇⃗⃗ ∇⃗⃗ ] (3)

Where 𝜌𝑓, �⃗� , 𝜇𝑓 and p, are : density [kg/m3], velocity components [m/s], dynamic viscosity [N.s/m2] and

pressure [Pa], of the fluid respectively.

The energy equation is given by [29];

𝜌𝑓𝐶𝑝𝑓[∇⃗⃗ . ∇⃗⃗ 𝑇] = ∇⃗⃗ . [𝑘𝑓 ∇⃗⃗ 𝑇] (4)

where 𝑘𝑓 , is the thermal conductivity [W/m.K], 𝐶𝑝𝑓 is the specific heat capacity [J/kg.K] and T is the

temperature [K] of the fluid respectively.

For the solid region (cell components), the energy equation is expressed as [29];

𝑘𝑠. ∇2𝑇 = 0 (5)

where 𝑘𝑠 is the thermal conductivity [W/m.K] of the solid (heat exchanger).

Thermophysical properties of the fluid

The base fluid thermophysical properties "bf" (i.e. water) depends on its temperature and can be considered

using [30];

𝜌𝑏𝑓 =

999.84+18.225(𝑇+273.15)−7.92×10−3(𝑇+273.15)2−

5.545×10−5(𝑇+273.15)3+1.498×10−7(𝑇+273.15)4

−3.933×10−10(𝑇+273.15)5

1+1.816−2(𝑇+273.15) [kg/m3] (6)

𝜇𝑏𝑓 = 2.414 × 10−5 × 10247.8

𝑇−140 [kg.m/s] (7)

𝐶𝑝𝑏𝑓 = 8958.9 − 40.535𝑇 + 0.11243𝑇2 − 1.014 × 10−4𝑇3 [J/kg.K] (8)

𝑘𝑏𝑓 = −0.58166 + 6.3556 × 10−3𝑇 − 7.964 × 10−6𝑇2 [W/m.K] (9)

The nanoparticles Alumina (Al2O3) is used to prepare the nanofluid with water and use it in the micro heat

exchanger due to its good chemical stability in water and its relatively low cost in comparison to other

nanoparticles. The nanofluids thermophysical properties “nf” depends on the volume fraction ∅ of the

nanoparticles and can be found using [30];

𝜌𝑛𝑓 = (1 − ∅). 𝜌𝑏𝑓 + ∅𝜌𝑛𝑝 (10)

𝜌𝑛𝑓𝐶𝑝𝑛𝑓 = (1 − ∅). 𝜌𝑏𝑓𝐶𝑝𝑏𝑓 + ∅ 𝜌𝑛𝑝𝐶𝑝𝑛𝑝 (11)

Based on the Brownian motion, the thermal conductivity of nanoparticles is:

𝑘𝑒𝑓𝑓 = 𝑘𝑠𝑡𝑎𝑡𝑖𝑐 + 𝑘𝐵𝑟𝑜𝑤𝑛𝑖𝑎𝑛 (12)

The static thermal conductivity is:

𝑘𝑠𝑡𝑎𝑡𝑖𝑐 = 𝑘𝑏𝑓 [𝑘𝑛𝑝+2𝑘𝑏𝑓−2(𝑘𝑏𝑓−𝑘𝑛𝑝)∅

𝑘𝑛𝑝+2𝑘𝑏𝑓+(𝑘𝑏𝑓−𝑘𝑛𝑝)∅] (13)


363

Brownian thermal conductivity is given as

𝑘𝐵𝑟𝑜𝑤𝑛𝑖𝑎𝑛 = 5 × 104𝛽. ∅. 𝜌𝑏𝑓 . 𝐶𝑝𝑏𝑓√𝜎𝐵.𝑇

𝜌𝑛𝑝 . 𝑑𝑛𝑝. 𝑓(𝑇, ∅) (14)

The dynamic viscosity is given as

𝜇𝑒𝑓𝑓 = 𝜇𝑠𝑡𝑎𝑡𝑖𝑐 + 𝜇𝐵𝑟𝑜𝑤𝑛𝑖𝑎𝑛 (15)

𝜇𝑠𝑡𝑎𝑡𝑖𝑐 =𝜇𝑏𝑓

(1−∅)2.5 (16)

𝜇𝐵𝑟𝑜𝑤𝑛𝑖𝑎𝑛 = 5 × 104𝛽. ∅. 𝜌𝑏𝑓 . √𝜎𝐵.𝑇

𝜌𝑛𝑝 . 𝑑𝑛𝑝. 𝑓(𝑇, ∅) (17)

𝑓(𝑇, ∅) = (2.8217 × 10−2. ∅ + 3.917 × 10−3) (𝑇

𝑇𝑜) + (−3.0699 × 10−2. ∅ − 3.91123 × 10−3) (18)

where 𝛽 is the fraction of the moving liquid volume with the nanoparticles (Table 2), 𝜎𝐵 is the Boltzmann

constant, and 𝑑𝑛𝑝 is the nanofluids particles diameter [nm]. The thermophysical properties of the Al2O3

nanoparticles evaluated at 300 K are in Table 3.

Table 1. Values of 𝛽 for Al2O3 nanoparticles

Nanoparticles 𝛽 Volume fraction [%] Temperature [K]

Al2O3 8.4407(100∅)−1.07304 1 % ≤ ∅ ≤ 10 % 298 ≤ 𝑇 ≤ 363 𝐾

Table 2. The thermophysical properties of Al2O3 nanoparticles at T=300K [30]

Thermophysical properties Al2O3

Density 𝜌 [kg/m3] 3970

Thermal conductivity k [W/m.K] 40

Specific heat Cp [J/kg.K] 765

Data acquisition

The Bulk temperature (mixing-cup) of the fluid that leaves the micro heat exchanger is calculated as;

𝑇𝑏𝑢𝑙𝑘 =∫ 𝜌𝐶𝑝𝑇𝑉𝑑𝑠𝑜𝑢𝑡

∫ 𝜌𝐶𝑝𝑉𝑑𝑠𝑜𝑢𝑡

(19)

The Reynolds number is calculated by;

𝑅𝑒 =𝜌𝑓𝑉𝑚𝐷ℎ

𝜇𝑓 (20)

where 𝑉𝑚 is the average fluid velocity at the inlet of the microchannel [m/s], 𝐷ℎ is the hydraulic diameter of the

micro channel [m].

The hydraulic diameter of the microchannels is calculated by the following equation;

𝐷ℎ =2𝑊𝐻

𝑊+𝐻 (21)

where H is the height, and W is the width of the microchannel, respectively [m].

The heat transfer performance of the cross-flow micro-channel heat exchanger can be measure by calculating

the overall heat transfer coefficient [31];

ℎ̅ =�̇�

𝐴.(�̅�ℎ𝑜𝑡−�̅�𝑐𝑜𝑙𝑑) (22)

Where �̇� is the total exchanged power [W], 𝐴 is the surface area [m2] through which the total exchanged power

flows, and �̅�ℎ𝑜𝑡 and �̅�𝑐𝑜𝑙𝑑 are the average temperatures of the hot and cold fluids flow in the microchannels [K].

The following equation defines the average Nusselt number [31];


364

𝑁𝑢̅̅ ̅̅ =ℎ̅𝐷ℎ

𝑘𝑓

RESULTS AND DISCUSSION

In microchannel heat exchangers, the CFD study was carried out by considering the following two different

fluid flow cases: (1) water and (2) Al2O3-water pairs nanofluid as a working fluid, and two prototypes for cross-

flow microchannel heat exchangers made of stainless steel material. The first design contains straight, smooth

channels, while the second design contains channels with ribs on sidewalls. A relative analysis of water and

water-based nanofluids using Al2O3 on various volume fractions of nanoparticles was conducted. In each study,

the variance of the pressure difference, temperature differential, heat transfer rate and heat transfer coefficient

were obtained for the same inlet range of the Reynolds number conditions. In order to ensure that the flow has to

be preserved as laminar, the Reynolds number is considered in the range of 0.7 to 22. Figures (3, 4) demonstrate

the temperature distribution profiles in the water and Al2O3-water micro-channel heat exchangers, with various

Reynolds number values (Re = 0.7 - 22) for channels with and without ribs. It has been understood from

temperature distribution that an upsurge in the number of Reynolds induces a decrease in the difference in

temperature. The reduction in the difference in temperature takes the rate of HT down. From this, it can obtain a

lower Reynolds number with a higher rate of HT. Related behaviour has been found in both water with and

without nanofluids with the same volume fraction. Figures (5, 6) provides a contour plot indicating the contour

of the pressure distribution in the micro heat exchangers for all studied cases at 0.7 and 22 Reynolds number,

respectively. The figure indicates that the pressure reduces over the microchannel length of the micro heat

exchangers for all kinds of the operating fluid. It is clear that in the case of water-Al2O3, the highest pressure

drop becomes. Of course, this is due to the dynamic viscosity-Al2O3 and higher water velocity.

Water

Water-Al2O3

Water

Water-Al2O3

Figure 3. Temperature distribution in the micro heat exchangers at 0.7 Reynolds number


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Water

Water-Al2O3

Water

Water-Al2O3

Figure 4. Temperature distribution in the micro heat exchangers at 22 Reynolds number

Figure 5. Pressure distribution in the micro heat exchangers at 0.7 Reynolds number


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Figure 6. Pressure distribution in the micro heat exchangers at 22 Reynolds number

The Bulk temperature (mixing-cup) of the fluid leaving the micro heat exchanger from upper and lower channels

are shown in Figures (7, 8) for the different working fluid with and without ribs at different Re. At Re = 22, The

Bulk temperature of the fluid leaving from upper ribs channels in case of using water-Al2O3 is the highest (~41.3 ͦ

C), also gives the lowest temperature of the fluid leaving lower ribs channels. However, the differences in the Bulk

temperature identical to smaller Re, and they become discovered when Re further increases. The same shape can

be seen in Figure (8), the Bulk temperature leaving lower channels for all cases. The temperatures rate change at

low Re in both figures is higher than that at high Re.

Figure 7. Bulk temperature (mixing-cup) of the fluid leaving the micro heat exchanger from upper channels

34

35

36

37

38

39

40

41

42

0 2 4 6 8 10 12 14 16 18 20 22

Bu

lk t

emp

erat

ure

, up

per

ch

ann

els

[C]

Re

Tu - smooth channel - water

Tu - smooth channel - Al2O3

Tu - ribs channel - water

Tu - ribs channel - Al2O3


367

Figure 8. Bulk temperature (mixing-cup) of the fluid leaving the micro heat exchanger from lower channels

Figure (9, 10) show the heat transfer coefficient and Nusselt number inside the microchannel for water, Al2O3-

water for channels with and without ribs at different Reynolds number. The result shows that the expansion of

the Reynolds number also increases the number of Nusselt and it depends on the heat transfer coefficient and the

comparative technique of the heat transfer coefficient has been connected with various operating fluids and also

the design of the inside channels. It demonstrates that the best coefficient of heat transfer is in the case of water-

Al2O3 and ribs channels due to its high thermal conductivity for working fluid. Figure (11) indicates the drop in

pressure along with the microchannel heat exchanger. The figure depicts that nanofluids have a higher-pressure

drop than the water and that they rise with the increase in the number of Reynolds. In comparison, the pressure

drop in the ribs channel is clearly rising relative to the smooth channel. This indicates that nanofluids can be

useful at a low Reynold number since the nanofluid heat transfer coefficient rises at higher rates at a low

Reynold number.

Figure 9. Variation of the HT coefficient of the cross-flow micro-channel heat exchanger with Reynolds

numbers for different cases

33

34

35

36

37

38

39

40

41

0 2 4 6 8 10 12 14 16 18 20 22

Bulk

tem

pera

ture

, low

er c

hann

els

[C]

Re

Tl - smooth channel - water

Tl - smooth channel - Al2O3

Tl - ribs channel - water

Tl - ribs channel - Al2O3

1000

2000

3000

4000

5000

6000

7000

8000

9000

0 2 4 6 8 10 12 14 16 18 20 22

h [

W/m

2.K

]

Re

h - smooth channel - water

h - smooth channel - Al2O3

h - ribs channel - water

h - ribs channel - Al2O3


368

Figure 10. Variation of the Nusselt numbers of the cross-flow micro-channel heat exchanger with Reynolds


Figure 11. Variation of the pressure drop inside the cross-flow micro-channel heat exchanger with Reynolds


CONCLUSION

In the current work, fluid flow and heat transfer were simulated in a microchannel heat exchanger with two

working fluid water and water - Al2O3 pairs, using the finite volume approach for two cases with different

Reynolds numbers varying from 0.7 to 22. The findings show that Al2O3 nanofluid presents the maximum heat

transfer coefficient and Nusselt number value among tested nanofluid compared pure water also it best values for

channels with ribs from smooth channels. Improvement in the performance of the heat exchanger in the use of the

ribs inside the channels as well as the use of water-Al2O3 as a working fluid for the micro heat exchanger is evident

in the temperature of the liquid leaving the upper rib channels in the case of using water - Al2O3 is the highest (~

41.3°C), and also gives the lowest temperature heat of the fluid leaving the lower rib channels.

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0.2

0.3

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0 2 4 6 8 10 12 14 16 18 20 22

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Cfd Investigation Of Flow And Heat Transfer Enhancement In