Experimental and numerical study of nanofluid flow and heat transfer over microscale backward-facing step

International Journal of Heat and Mass Transfer 79 (2014) 858–867

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Experimental and numerical study of nanofluid flow and heat transfer over microscale backward-facing step A.Sh. Kherbeet a,⇑, H.A. Mohammed b, B.H. Salman c, Hamdi E. Ahmed d, Omer A. Alawi b a

Department of Mechanical Engineering, KBU International College, 47800 Petaling Jaya, Selangor, Malaysia Department of Thermofluids, Faculty of Mechanical Engineering, Universiti Teknologi Malaysia, 81310 UTM Skudai, Johor Bahru, Malaysia c FABE, Limkokwing University of Creative Technology, Jalan Teknokrat 1/1, 63000 Cyberjaya, Selangor, Malaysia d Department of Mechanical Engineering, University of Anbar, 31001 Anbar, Iraq b

a r t i c l e

i n f o

Article history: Received 19 April 2014 Received in revised form 26 August 2014 Accepted 27 August 2014 Available online 19 September 2014 Keywords: Forced convection Microscale backward-facing step Heat transfer Nanofluids

a b s t r a c t Experimental and numerical studies were presented to reveal the flow and heat transfer characteristics of nanofluid laminar flow over the microscale backward-facing step (MBFS). The duct inlet and the step height were 400 lm and 600 lm respectively. All the walls considered adiabatic except the downstream wall is heated by uniform heat flux. The experiment is conducted at the Reynolds number range from 280 to 470. The distilled water is considered as a base fluid with two types of nanoparticles SiO2 and Al2O3 immersed in the base fluid. The particle diameter is 30 nm and the range of nanoparticles volume fraction in the base fluid varied from 0 to 0.01. The measurement results revealed that the water–SiO2 nanofluid has the highest Nusselt number. It is found also that the Nusselt number increase with increases volume fraction. The water–SiO2 nanofluid with higher volume fraction has the highest Nusselt number. The friction factor of water–Al2O3 was higher than of water–SiO2 mixture. The numerical results were in good agreement with the measurement results. Ó 2014 Elsevier Ltd. All rights reserved.

1. Introduction The existence of flow separation and following reattachment which occurs due to the sudden expansion in flow geometry, such as a backward-facing step (BFS), play an important role in many engineering applications where cooling or heating is required [1]. These applications of heat transfer appear in, such as combustors, as well as in external flows such as aircraft, gas turbine engines, buildings, chemical processes and many other devices of heat transfer. The separation and the reattachment of the flow represent the key of determining the flow structure and significantly affect the heat transfer mechanism. A significant amount of mixing low and high fluid energy occurs in the reattachment region of these devices [2]. Thus, there were many studies focused on the flow separation and reattachment in the past decades, and the BFS geometry received much attention [3,4]. Flow over a BFS with heat transfer was conducted by other researchers [5,6]. The majority of the published researches discussed the isothermal flow in two dimensional geometry, and little studies discussed the heat transfer and the three dimensional flow cases. Abu-Nada [7] presented a

⇑ Corresponding author. Tel.: +60 173895660; fax: +60 7 55 66159. E-mail address: [email protected] (A.Sh. Kherbeet). http://dx.doi.org/10.1016/j.ijheatmasstransfer.2014.08.074 0017-9310/Ó 2014 Elsevier Ltd. All rights reserved.

numerical study of entropy generation over a 2D backward facing step with various expansion ratios. The expansion ratios (ER = S/H) were chosen as: 1/4, 1/3, 1/2, 2/3, and 3/4. The results showed that as the Reynolds number increases, the value of total entropy generation number (Ns) increases. For lower values of Reynolds number, the value of Ns decreases as the expansion ratio (ER) increases. Nie and Armaly [8] present a numerical study of three-dimensional laminar forced flow adjacent to backward-facing step placed in rectangular duct. The results demonstrated that the maximum reattachment length occurs at the sidewall and not at the center of the duct and as the step height increases the maximum Nusselt number increases. Biswas et al. [9] study the laminar fluid flow behavior over a three dimensional backward-facing step with various expansion ratios. The study revealed that the formation of wall jets at the side wall within the separating shear layer, formed by the spanwise of the velocity moves towards the symmetry channel plane. Armaly and Nie [10] presented an experimental study of measuring the velocity in three-dimensional laminar separated airflow adjacent to a backward-facing step by using two-component laser Doppler velocimeter. Saldana and Anand [11] studied numerically the forced convective flow over a 3-D backward-facing step. The results revealed that the spanwise average Nusselt number distributions present higher values at higher Reynolds numbers. The velocity profiles revealed that for Reynolds

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859

Nomenclature Cp Dh dp g Gr H h h k Nu P Pr q Re s T T1 Tw u ui u1 U v V

specific heat, J/kg K hydraulic diameter, 2h, m nanoparticles diameter, nm gravitational acceleration, m/s2 Grashof number, gbqws4/(kv2) total channel height, m convective heat transfer coefficient, W/m2 K inlet channel height, m thermal conductivity, W/m K Nusselt number, h.Dh/k dimensionless pressure, P = (p + qgx)/qu21 Prandtl number, vf/af heat flux, W/m2 Reynolds number, qu1Dh/lf step height, m fluid temperature, K temperature at the inlet or top wall, K temperature of the heated wall, K velocity component in x-direction, m/s local inlet velocity, m/s average velocity for inlet flow, m/s dimensionless streamwise velocity component, u/u1 velocity component in y-direction, m/s dimensionless transverse velocity component, v/u1

number greater than 343 the flow does not reach fully developed conditions at the exit of the channel. However, for all cases considered in their study the flow never reached fully thermally flow development condition. The flow in 3-D microscale backward-facing step was investigated by Hsieh et al. [12]. In this study, the Direct Simulation Monte Carlo method (DSMC) utilized. The comparison results of the 3-D with those of the 2-D simplification showed that the side walls in the 3-D structure significantly affect the flow characteristics and heat transfer. Moreover, the stability of the vortex behind the step could be affected by the side walls of a 3-D backward-facing step channel. It is found that the flow separation, recirculation, and reattachment will disappear when the cross-section aspect ratio is less than 1. Bao and Lin [13] used the DSMC method to study the transition regime in the microscale backward-facing step. They found that at Knudsen number = 0.136, the streamwise velocity is always positive which indicated that there is no reversed flow existing after the step. The adverse pressure gradient behind the step was too small to stagnate the flow. Furthermore, the mass flow rate increases with the increase of pressure ratio and the relation is not linear as in traditional flow. However, it was found that the gradient increases with the pressure ratio. One of the techniques utilized to improve the heat transfer rate is by utilizing nanofluids. Nanofluids are conventional fluids in which particles of nanometer-size are suspended [14]. The recent researches showed that the solid nanoparticle which has high thermal conductivity when it suspended in the conventional fluids could intensify the effective thermal conductivity and convective heat transfer coefficient of these fluids [15–18]. These solid nanoparticles can be metallic or nonmetallic such as SiO2, Al2O3, TiO2, CuO, Cu and ZnO [19]. Several researchers have investigated the enhancement of the thermal conductivity by utilizing the nanofluids, and they found that the using of nanofluids could enhance the heat transfer [20–32]. The first investigation of the thermal behavior and nanofluid flow characteristics over backward-facing step was demonstrated by Abu-Nada [33]. In this study, five types of nanoparticles were utilized which are CuO, Al2O3, Ag, Cu and TiO2. He reported that the Nusselt number can be enhanced by

X Xi Xe Xr

dimensionless streamwise coordinate, x/s upstream length, lm streamwise coordinate as measured from the step, lm reattachment length, lm

Greek symbols u nanoparticles concentration af thermal diffusion of fluid, N s/m2 b thermal expansion coefficient, 1/K h dimensionless temperature qf density of fluid, kg/m3 qs density of solid, kg/m3 mf kinematic viscosity of fluid, m2/s l dynamic viscosity, N s/m2 Subscripts o outlet eff effective f fluid s solid nf nanofluid w wall 1 inlet condition

increasing the nanoparticles volume fraction. However, the high value of the Nusselt number inside the recirculation zone is independent of Reynolds number value, while it strongly depends on the thermophysical properties of the nanoparticles. Mohammed et al. [34,35] studied the effect of nanofluids on mixed convective heat transfer over a vertical and horizontal backward-facing step. In this investigation, eight types of nanoparticles were utilized with 5% of the nanoparticles volume fraction. They illustrated that the nanofluids with secondary recirculation regions found to have a lower Nusselt number. Furthermore, the diamond nanofluid has the highest Nusselt number in the primary recirculation region, while downstream the primary recirculation region the SiO2 nanofluid has the highest. Al-Aswadi et al. [36] investigated numerically the laminar forced convection flow over a BFS in a duct using different nanofluids. They reported that the recirculation size and reattachment length increase as the Reynolds number increases. Nanofluids with low dense nanoparticles such as SiO2 have a higher velocity than those with high dense nanoparticles such as Au. Very recently Kherbeet et al. [37] presented a numerical investigation of the nanofluid effect of laminar flow on a mixed convection heat transfer over 2D microscale backward-facing step. The nanoparticle size was in the range of 25 nm 6 dp 6 70 nm. Four types of nanoparticles were utilized which are Al2O3, CuO, SiO2 and ZnO, with a volume fraction of the range 1–4%. The results revealed that there is no recirculation region observed behind the step for all the mentioned nanofluids. The fluids with SiO2 nanoparticles showed to have the highest Nusselt number. In addition, the results showed Nusselt number increases with the increment of the volume fraction of the nanoparticles in the base fluid. Heshmati et al. [38] examined numerically a forced convective heat transfer in channel over a backward facing step having a baffle on the top wall. In this study four different geometries with different expansion ratios and different type of baffles were investigated. The study clearly showed that the geometry with expansion ratio 2 and solid baffle has the highest Nusselt number compared to other geometries. Kherbeet et al. [39] investigated a numerically the laminar mixed convection flow of nanofluids over a 3-D horizontal microscale forward-facing step (MFFS) using a finite volume

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method. Various nanoparticle materials, such as SiO2, Al2O3, CuO, and ZnO, were dispersed in ethylene glycol as a base fluid with volume fractions in the range of 0 and 0.04. The duct has a step height of 650 lm. The results revealed that the SiO2 nanofluid had the highest Nusselt number, which increased with decreasing nanoparticle material density, increasing volume fraction and decreasing nanoparticles diameter. Kherbeet et al. [40] presented a numerical study of mixed convective flow over the 3-D horizontal microscale backward-facing step (MBFS). In this study EG-SiO2 nanofluid was considered with 25 nm nanoparticle diameter, 0.04 volume fraction. The results revealed that the Nusselt number and skin friction coefficient increase with the increase of the step height. The Reynolds number and pressure drop found to decrease with the increase of the step height. It is obvious from the above literature review that the experimental study of fluid flow and heat transfer over MBFS utilizing nanofluids seems not to have been investigated in the past and this has encouraged the present study. In addition, most of the previous researches on backward-facing step involved conventional fluids and there is a very little work reported in the open literature that involved nanofluids in their studies. The present study deals experimental and numerical investigation of laminar forced convective flow over a MBFS placed in a horizontal duct with utilizing nanofluid. The deionized water is considered as a base fluid with two types of nanoparticles SiO2 and Al2O3 immersed in the base fluid. The particle diameter is 30 nm and the range nanoparticles volume fraction in the base fluid from 0 to 0.01.

Table 1 Thermophysical properties for distilled water and various types of nanoparticles.

q (kg/m3)

l (N s/m2)

k (W/m K)

Cp (J/kg K)

b (1/K)

Pure water Al2O3 SiO2

998.203 3970 2200

1.01E03 – –

6.13E01 40 1.2

4182.2 765 703

2.06E04 5.80E06 5.50E06

2.2. Governing equations The continuity, momentum and energy governing equations in non-dimensional form, in Cartesian coordinates are given as [33]:

@U @V @W þ þ ¼0 @X @Y @Z U

ð1Þ

@U @U @U 1 @P þV þW ¼ @X @Y @Z ð1  uÞ þ u qqs @X f

1 1    þ Re ð1  uÞ2:5 ð1  uÞ þ u qs qf ! @2U @2U @2U Grx þ 2h þ þ  @X 2 @Y 2 @Z 2 Re U

ð2Þ

@V @V @V 1 @P þV þW ¼ @X @Y @Z ð1  uÞ þ u qqs @Y f

1 1    þ Re ð1  uÞ2:5 ð1  uÞ þ u qs qf ! @2V @2V @2V Gry þ þ þ 2h  @X 2 @Y 2 @Z 2 Re

2. Mathematical modeling 2.1. Physical model and assumptions The schematic diagram of the considered geometry and the flow configuration used in this study is presented in Fig. 1. In order to ensure the fully developed flow in the channel inlet and outlet, the downstream wall length is considered 0.15 m and the upstream wall length is 0.1 m. The channel inlet and the step height are 400 lm and 600 lm respectively. The downstream wall maintained at uniform heat flux, while the other walls considered adiabatic surfaces. The flow at the duct entrance is assumed to be hydrodynamically steady and fully developed. Streamwise gradients of all quantities at the duct exit are set to be zero. The base fluid (Distilled Water) and the nanoparticles are assumed to have a thermal equilibrium and no slip condition occurs. The fluid flow is considered to be Newtonian and incompressible. The thermophysical properties of the nanofluids are given in Table 1.

Particle type

U

ð3Þ

@W @W @W 1 @P þV þW ¼ @X @Y @Z ð1  uÞ þ u qqs @Z f

1 1    þ Re ð1  uÞ2:5 ð1  uÞ þ u qs qf ! @2W @2W @2W Grz þ 2h þ þ  @X 2 @Y 2 @Z 2 Re

ð4Þ

0 1 ! knf @h @h @h 1 @ @2h @2h @2h kf A U þV þW ¼ þ þ @X @Y @Z RePr ð1  uÞ þ u ðqCpÞs @X 2 @Y 2 @Z 2 ðqCpÞ

Fig. 1. Schematic diagram of a horizontal 3D backward-facing step (MBFS).

f

ð5Þ

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where

Table 2 b Value for SiO2 particles and it is boundary conditions.

qf um Dh vf gbqw s ; Pr ¼ ; Gry ¼ 2 lf af kv

4

Re ¼

u v w ; V¼ ; W¼ ; u1 u1 u1 ðp þ qgxÞ T  T1 P¼ ; h¼ qu21 ðqw s=kÞ

U¼

X¼

Type of particles

x ; Dh

Y¼

y ; Dh

Z¼

z ; Dh

(i) Upstream conditions at X ¼  Dxi , Ds 6 Y 6 DH : Ui ¼ uu1i ; V ¼ 0, h h h and  Dw 6 W 6 Dw ; W ¼ 0; h ¼ 0. h h The flow at the duct inlet is considered to be fully developed with an average velocity u1. Thus, the inlet velocity distribution is parabolic. Fully developed flow and thermal conditions are imposed at the exit section of the calculation domain by equating the streamwise gradients of all quantities at that exit section to zero. (ii) Downstream exit conditions at X ¼ X e : 0 6 Y 6 DH and h  Dw 6 W 6 Dw h

@ U

2

@X

2

¼ 0;

@ V @X

2

(iii) Straight wall  Dw 6 W 6 Dw h

¼ 0; at

@2W @X

2

¼ 0;

@2h

¼0

@X 2

X ¼  Dxi 6 X 6 Dxe h

h

V ¼ 0;

W ¼ 0;

and

Y ¼ DH

h

and

h¼0

(iv) Stepped wall conditions: Upstream of the step at  Dxi 6 X 6 0, and Y ¼ Ds : h h @h U ¼ 0; V ¼ 0; W ¼ 0; @Y ¼ 0. At the step X ¼ 0 and 0 6 Y 6 Ds : U ¼ 0; V ¼ 0; W ¼ 0; h @h ¼ 0. @X Downstream of the step @h U ¼ 0; V ¼ 0; W ¼ 0; @Y ¼ 1.

at

Y ¼ 0;

leff 1 ¼ lf 1  34:87ðdp =df Þ0:3 u1:03 "

6M df ¼ Npqfo

298 K 6 T 6 363 K 298 K 6 T 6 363 K

and

0 6 X 6 Dxe : h

2.4. Nanofluids thermophysical properties The effective thermal conductivity equation of nanofluid is presented by Vajjha et al. [41] as:

ð8Þ

#1=3

where leff and lf are the viscosity of nanofluid and base fluid, respectively, dp is the nanoparticle diameter, df is the base fluid equivalent diameter and u is the nanoparticle volume fraction. M is the molecular weight of the base fluid and N is the Avogadro number, and qfo is the mass density of the base fluid calculated at temperature T = 293 K. The effective density of nanofluid is given as [43]:

qeff ¼ ð1  uÞqf þ uqs

h

U ¼ 0;

8.4407(100/) [41] 1.9526(100/)1.4594 [44]

In the above Eqs. (1)–(5), the viscosity of the nanofluid is approximately considered as viscosity of a base fluid if containing dilute suspension of fine spherical particles and is given by Corcione [43]:

The boundary conditions for the above set governing equations are:

h

1.07304

Al2O3 SiO2

2.3. Boundary conditions

2

Temperature (K)

b

ð9Þ

where qeff and qf are the nanofluid and base fluid densities respectively, and qs is the density of nanoparticle. The effective heat capacity is given as [43]:

ðCpÞeff ¼

ð1  uÞðqCpÞf þ uðqCpÞs ð1  uÞqf þ uqs

ð10Þ

where Cps is the heat capacity of the solid particles, and Cpf is the heat capacity of the base fluid. The effective thermal expansion is expressed as [43]:

beff ¼

ð1  uÞðqbÞf þ uðqbÞs ð1  uÞqf þ uqs

ð11Þ

where bs and bf are the thermal expansion of the solid particles and base fluid respectively. The b equations for SiO2 and Al2O3 particles are expressed in Table 2 as it is given by Vijjha [41,44].

keff ¼ kstatic þ kbrownian 3. Experimental setup

where the kstatic is given by Ghasemi et al. [42] as:

kstatic ¼ kf



ðks þ 2kf Þ  2uðkf  ks Þ ðks þ 2kf Þ þ uðkf  ks Þ

 ð6Þ

where ks and kf are the thermal conductivities of the solid particles and the base fluid respectively. While the thermal conductivity due to the Brownian motion is given by Vajjha [41] as:

kbrownian

sffiffiffiffiffiffiffiffiffi KT f ðT; uÞ ¼ 5  10 buqf Cpf qs ds 4

ð7Þ

where

  T  f ðT; uÞ ¼ 2:8217  102 u þ 3:917  103 T0   2 þ 3:0669  10 u  3:91123  103 where K is the Boltzmann constant, T is the fluid temperature, T0 is the reference temperature.

3.1. Experimental apparatus The experimental facility explained in Fig. 2. The working fluid was contained in the tank driven by high pressure water pump type BOSCH AQUATAK ECO-100. At the two ends of the tested micro backward-facing step, two sumps was fabricated to connect the channel. Between the high-pressure pump and the pump, a micro screen filter was placed to prevent big particles from entering the test section. Between the high pressure pump and the inlet sump a distribution valves were placed to control the inlet flow rate to the test section and to reduce the so high pressure on the system by the bypass pipe from the valves to the tank. At the two ends of the test section there were two digital pressure gauges placed type MD-S910W to measure the pressure drop through the test section. All the test section walls (except the downstream wall) fabricated by using high purity Acrylic material with 1 cm thickness

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Fig. 2. Schematic diagram of experimental setup.

for all the walls to ensure a good isolation with ambient. The downstream wall made from high purity Aluminum. Seven of micro holes were made for thermocouples along the downstream wall and close to internal surface the surface by 200 lm. The heating system of the present experimental work is including providing a uniform heat flux along the downstream wall of the test section (Aluminum platform). Therefore, a special design of cartridge heater is fabricated. In this heater cartridge there was 7 through holes with a diameter of 1 mm along the length of the heater and in a position exactly same as they in the Aluminum platform. These holes are made to allow the thermocouple wires to pass through the cartridge heater to be connected to Aluminum platform and close the internal surface. The working temperature range of this heater is stated from 0 °C to maximum 150 °C. The heater connected to the power supply controller, which has a voltage controller of the input voltage. There were seven K-type thermocouples with the diameter of 500 lm installed on the Aluminum platform wall. Two thermocouples placed in the inlet and outlet of the test section to measure the inlet and outlet temperature in the whole experimental process, and the other seven were placed on the outside of the downstream wall surface along the flow direction to measure the wall temperature in the heat transfer experiment process. The isolation used to cover the outside of the heater cartridge in order to reduce the losses of the heat flux with the ambient. At the outlet of the test section the microflow meter placed (Omega FTB332D) which is considered to measure the mass flow rate of the fluid flow. After the flow meter there is a water heat exchanger used to reduce the temperature of the working fluid and return back its temperate to the ambient temperature in order to re-use it by sending it back to the main tank.

3.2. Nanofluid preparation The two-step method which is a process by dispersing nanoparticles into base liquids is considered to prepare the nanofluids. Distilled water is adopted as base fluids to prepare the nanofluids. Al2O3 and SiO2 are adopted as the nanoparticles materials with circular shapes and diameter of 30 nm. A very sensitive balance (compact analytical balance HR-250 AZ) with 0.1 mg resolution is used to weight the nanoparticles very accurately. The nanofluid is prepared by adding the calculated amount of nanoparticles in a known amount of distilled water and sonicated for 30 min. The ultrasonic pulses of 750 W and 20 kHz generated by an ultrasonic cell disrupter were used to improve the dispersion of nanoparticles into the base fluid.

4. Experimental data reduction and measurement uncertainty analysis 4.1. Friction factor The fully developed pressure drop across the micro channels, Dp is obtained [45] by

Dp ¼ Dpin;out  pd

ð12Þ

pd ¼ 1:18qu2av e mo uav e ¼ q Ac uav e :Dh Re ¼

ð13Þ

m

Dh 2 f ¼ Dp: : 2 L quav e

ð14Þ ð15Þ ð16Þ

4.2. Heat transfer The power supply added to the downstream channel wall can be obtained from Eqs. 17, 18, 20 [45]:

U ¼ U:I o

Q ¼ m cp ðT out  T in Þ Q q¼ Aw

ð17Þ ð18Þ ð19Þ

The local heat transfer coefficient hx and Nusselt number Nux are calculated by the following equations:

hx ¼

q:Lx =L DT x

hx :Dh q:Lx =L:Dh mo cp ðT out  TinÞDh :Lx =L ¼ ¼ Aw :DT x :kl kl DT x :kl DT x ¼ T wx  T b;x Nux ¼

T b;x

Q :Lx =L ¼ T in þ o m cp

ð20Þ

ð21Þ ð22Þ ð23Þ

The experimental results of heat transfer and flow characteristics in micro channel measurement accuracy are of decisive importance to ensure the validity of the test results. So that, uncertainty analysis is performed to give some quantitative description of the test data validity, even though the analysis results are something uncertain. The uncertainty in determining the friction factor consists of the uncertainties from Dp, Dh, L, mo, Ac of Eq. (5). The uncertainty analysis of Nusselt number consists of the uncertainties from mo cp, Tb,

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0.05

Table 3 Measurement uncertainties. Parameters

Maximum uncertainty (%)

q Dp Tw Dh

2.2 0.5 5 3 0.1 3.03 6 1.3 4.8 5 2 3 2.925 6.4 10.4

Ac mo L Aw Tb k

l uave Nusselt number Friction factor

0.045

0.04

Friction factor

q

1 % SiO2 1% Al2O3 Pure water 1 % SiO2 1% Al2O3 Pure water

CFD CFD CFD Exp. Exp. Exp.

0.035

0.03

0.025

0.02

Tin, Tout, L, x, kl of Eq. (22). The uncertainty analysis of the measurements is conducted along the line depicted in [46]. The maximum uncertainties in friction factor and Nusselt number are estimated about 10.4% and 6.4%, respectively. The details of the related parameters uncertainties and the resulted uncertainties in friction factor and Nusselt number are listed in Table 3.

0.015

300

350

400

450

Re Fig. 4. The experimental and numerical comparison of effect nanofluid type on friction factor.

5. Results and discussion 5.1. Flow characteristics According to Eqs. (12)–(16), the friction factors in the laminar flow experimentally were calculated. As explained previously, the fluid temperatures at the inlet and the outlet were measured in all experimental tests and the average temperature is considered as the reference temperature for calculating the thermal properties [47]. For the MBFS, the experimental friction factors and the deviations between the experimental and the numerical results are evaluated. The effects of using different particle material on the friction factor trend are presented in Figs. 3 and 4. The numerical results revealed that there is no noticeable difference in the friction factor of the water–Al2O3 and pure water. However, a slight difference of friction factor can be seen clearly by using water–SiO2

nanofluid which is a little higher than the case of using water– Al2O3 nanofluid and pure water. The experimental results of effects nanofluid type on the friction factor presented in Fig. 3. From this figure, the experimental results revealed that the friction factor of water–Al2O3 was the lowest and the water–SiO2 has the highest friction factor. This is due to the high variance in the fluid densities and the low variance in the pressure drop [48,49]. However, in terms of the pressure drop the results illustrated that water– Al2O3 nanofluid has the highest pressure drop and then water– SiO2, and pure water respectively. The comparison between the experimental and numerical results presented in Fig. 4. From this figure the results showed that the experimented results are less than of numerical. It is found also that the deviation between the experimental and the numerical results start from its maximum value at low Reynolds number and decreases with the increases 0.04

0.04

1 % SiO2 Exp. 0.5 % SiO2 Exp. Pure water Exp.

1 % SiO2 Exp. 1% Al2O3 Exp. Pure water Exp.

0.035

Friction factor

Friction factor

0.035

0.03

0.025

0.025

0.02

0.02

0.015

0.03

300

350

400

450

Re Fig. 3. The experimental results of nanofluid type effect on friction factor.

0.015

300

350

400

450

Re Fig. 5. The experimental results of effect volume fraction on the friction factor.

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12

0.05 1 % SiO2 0.5% SiO2 Pure water 1 % SiO2 0.5 % SiO2 Pure water

0.045

0.035

EXP. 1% SiO2 EXP. 0.5% SiO2 EXP. Pure water

10

8

Nu

Friction factor

0.04

CFD CFD CFD Exp. Exp. Exp.

6

0.03 4

0.025 2

0.02 0

0.015

300

350

Re

400

0

0.025

0.05

450

0.075

0.1

0.125

0.15

x (m)

Fig. 6. The experimental and numerical comparison of volume fraction effect on friction factor.

of the Reynolds number. This may due to the disability of the pressure gauges to measure accurately too low values of pressure at low Reynolds numbers. The average deviation between the numerical end the experimental work was about 8.5%. The effects of the increasing volume fraction on the friction factor behavior are investigated numerically and experimentally. In this investigation the water–SiO2 nanofluid considered and the volume fraction varied between 0% 6 u 6 1%. The numerical results revealed that the friction factor increases with the increases of the nanoparticle volume fraction to 1%. Whereas, a very slight difference in friction factor between the pure water and 0.5% water–SiO2 nanofluid. However, the friction factor of water–SiO2 nanofluid with 0.5% volume fraction was higher than pure water. This is due to the increases of the pressure drop value with the increases of the volume fraction. The experimental results of volume fraction effects are parents in Fig. 5. The results clearly

Fig. 8. The experimental results of effects nanoparticle volume fraction on the Nusselt number.

showed the decrease of the friction factor with the increases of SiO2 nanoparticles volume fraction. The comparison of the numerical with the experimental results showed a good agreement between them as shown in Fig. 6. The results illustrated that there is high deviation with numerical results for low Reynolds number which is decreased with the increases of the Reynolds number. This is due to the decreases of the pressure gauge accuracy with low values of the Reynolds number. The average deviation of the total experimental results in comparison to numerical is 8.64%. 5.2. Heat transfer characteristics The experimental results and numerical simulation predictions of Nusselt number are presented in this section. The experimental results of using different nanofluids type, nanoparticles volume fraction and Reynolds number on the Nusselt number are presented in the Figs. 7–9. In order to study the nanofluid type effects 14

12 EXP. 1% SiO2 EXP. 1% Al2O3 EXP. Pure water

10

EXP. Re = 470 EXP. Re = 375 EXP. Re = 280

12 10

8

Nu

Nu

8 6

6 4

4

2

0

2

0

0.025

0.05

0.075

0.1

0.125

0.15

x (m) Fig. 7. The experimental results of effects nanofluid type on the Nusselt number.

0

0

0.025

0.05

0.075

0.1

0.125

0.15

x (m) Fig. 9. The experimental results of effects Reynolds number on the Nusselt number.

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12

12

(a)

(b)

CFD 1% SiO2 CFD 1% Al2O3 CFD Pure water EXP. 1% SiO2 EXP. 1% Al2O3 EXP. Pure water

10

10

8

Nu

Nu

8

6

6

4

4

2

2

0

CFD 1% SiO2 CFD 0.5% SiO2 CFD Pure water EXP. 1% SiO2 EXP. 0.5% SiO2 EXP. Pure water

0

0.025

0.05

0.075

0.1

0.125

0

0.15

0

0.025

0.05

0.075

x (m)

0.1

0.125

0.15

x (m) 14

(c)

CFD Re = 470 CFD Re = 375 CFD Re = 280 EXP. Re = 470 EXP. Re = 375 EXP. Re = 280

12

10

Nu

8

6

4

2

0

0

0.025

0.05

0.075

0.1

0.125

0.15

x (m) Fig. 10. The experimental and numerical comparison of effects different factors on the Nusselt number (a) nanofluid type (b) nanoparticle volume fraction (c) Reynolds number.

on the Nusselt number, the Reynolds number considered to be 280 and the volume fraction of 1%. Two types nanoparticles materials considered SiO2 and Al2O3. The experimental results revealed that the nanofluids can significantly enhance the Nusselt number due to the enhancement in the thermal conductivity of these fluids. The results showed that the water–SiO2 nanofluid has the highest Nusselt number in comparing with water–Al2O3 nanofluid and pure water. However, the Nusselt number of using water–Al2O3 nanofluid is higher than the pure water Nusselt number. This may attributed to tat the nanofluids with a high Prandtl number have a higher Nusselt number [34]. The effects of the increases volume fraction on the Nusselt number are investigated numerically and experimentally as shown in Fig. 8. The numerical and experimental results were performed for water–SiO2 nanofluid at Reynolds number of 280 and volume fraction range of 0 6 u 6 1%. The results exhibited that the Nusselt number increases with the increase of the nanoparticles volume

fraction and the higher volume fraction the higher Nusselt number. This due to the increases of the nanoparticles volume fraction caused increases in the nanofluid viscosity [37]. Moreover, the increase in the nanoparticles volume fraction increase the irregular and random movements of particles and the energy exchange rates in the fluid [50]. The effect of increases Reynolds number on the Nusselt number enhancement are performed numerically and experimentally for three values of Reynolds 280, 375 and 470. Water–SiO2 nanofluid with 1% volume fraction considered in this case. The experimental results revealed that the Reynolds number increases have a direct proportion with Nusselt number enhancement as shown in Fig. 9. The comparison between the measurements and the predicted results are illustrated in Fig. 10. In this figure the comparison of effects nanofluid type, volume fraction and Reynolds number are presented. The experimental results were in a good agreement with numerical with a deviation percentage of 8.39% in case of

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nanofluid type effects and 6.97% in case of volume fraction effects as shown in Fig. 10a and b. However, a high deviation found in case Reynolds number effects and the results of comparison showed an increase of the deviation percentage with the increases of Reynolds number as shown in Fig. 10c. The average deviation between the measurements and the numerical results of the Nusselt number with the increases of the Reynolds number was 18.52%. This is due to the Nusselt number is highly dependent on the bulk temperature value which is being much difficult to be accurately measured in the microscale size duct with the increasing of fluid velocity. 6. Conclusion In this paper, in order to clarify the feasibility of conventional theory to predict the flow and heat transfer characteristics of incompressible fluid flow in micro backward-facing step, both experiments and numerical simulation were conducted. The different types of nanofluids effect and the nanoparticles volume fraction effects were investigated. From the results and analysis of the experiment and numerical simulation, the results showed there is an enhancement found experimentally by using the nanofluid. The experimental results revealed that the water–Al2O3 nanofluid has the higher friction factor in comparing to the numerical which showed there is no different from the nanoparticles type of the friction factor. However, for the same nanoparticles material the higher volume fraction has the higher friction factor value. The water–SiO2 nanofluid showed a higher enhancement of the average Nusselt number in comparing to the pure water and water– Al2O3 nanofluid. However, the increases of the water–SiO2 nanofluid showed increases in the Nusselt number enhancement. In general the experimental results were in a good agreement with the numerical. Conflict of interest None declared. Acknowledgements The authors would like to acknowledge the University of Malaya, for their extremely diligent and magnificent efforts in providing whatever materials and instruments were required for the experiments during nanofluids preparation. References [1] Y. Chen, J. Nie, B. Armaly, H. Hsieh, Turbulent separated convection flow adjacent to backward-facing step-effects of step height, Int. J. Heat Mass Transfer 49 (2006) 3670–3680. [2] B. Armaly, A. Li, J. Nie, Measurements in three-dimensional laminar separated flow, Int. J. Heat Mass Transfer 46 (2003) 3573–3582. [3] J.T. Lin, B.F. Armaly, T.S. Chen, Mixed convection heat transfer in inclined backward-facing step flows, Int. J. Heat Mass Transfer 34 (1991) 1568–1571. [4] H.I. Abu-Mulaweh, B.F. Armaly, T.S. Chen, Measurements in buoyancyassisting laminar boundary layer flow over a vertical backward-facing stepuniform wall heat flux case, Exp. Therm. Fluid Sci. 7 (1993) 39–48. [5] G.C. Vradis, V. Outgen, J. Sanchez, Heat transfer over a backward-facing step: solutions to a benchmark, in: Benchmark Problems for Heat Transfer Codes ASME HTD, vol. 222, 1992, pp. 27–34. [6] G. Vradis, L. VanNostrand, Laminar coupled flow downstream an asymmetric sudden expansion, J. Thermophys. Heat Transfer 6 (2) (1992) 288–295. [7] E. Abu-Nada, Entropy generation due to heat and fluid flow in backward facing step flow with various expansion ratios, Int. J. Energy 3 (2006) 419–435. [8] J. Nie, B. Armaly, Three-dimensional convective flow adjacent to backwardfacing step-effects of step height, Int. J. Heat Mass Transfer 45 (2002) 2431– 2438. [9] G. Biswas, M. Breuer, F. Durst, Backward-facing step flows for various expansion ratios at low and moderate Reynolds numbers, J. Fluids Eng. 126 (2004) 362–374. [10] B.F. Armaly, A. Li, J.H. Nie, . Measurements in three-dimensional laminar separated flow, Int. J. Heat Mass Transfer 46 (19) (2003) 3573–3582.

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