- Email: [email protected]
Turbulent heat transfer to separation nanofluid flow in annular concentric pipe
International Journal of Thermal Sciences 117 (2017) 14e25
Contents lists available at ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Turbulent heat transfer to separation nanofluid flow in annular concentric pipe Hussein Togun a, *, S.N. Kazi b, A. Badarudin b a b
Head of Biomedical Engineering Department, University of Thi-Qar, 64001 Nassiriya, Iraq Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 16 February 2017 Accepted 12 March 2017
Turbulent heat transfer to separation nanofluid flow in annular concentric pipe were studied numerically and experimentally. In the numerical study, finite volume method with standard k-ε turbulence model in three dimensional domains was selected. Three different types of water based (Al2O3, CuO, TiO2) nanofluids were employed in this simulation. The adopted boundary conditions were, expansion ratio (ER ¼ 1.25, 1.67, and 2), Reynolds number ranging from 20,000 to 50,000, water based nanofluids used Al2O3, CuO, TiO2 with volume fractions varied between 0 and 2% at different heat fluxes, varied from 4000 W/m2 to 16,000 W/m2. For experimental study, Al2O3 water based nanofluid was used to validate the numerical results. The results show that the volume fraction of nanofluid and Reynolds number significantly affect the surface heat transfer coefficient; an increase in surface heat transfer coefficient was noted when both volume fraction of nanofluids and Reynolds number were increased for all the cases. The improvement of heat transfer was about 36.6% for pure water at the expansion ratio of 2 compared to heat transfer obtained in a straight pipe. Augmentation of heat transfer could be achieved by using nanofluid at expansion ratio 2 where the total improvements were about 45.2% (TiO2), 47.3%(CuO), and 49%(Al2O3). Also the increment in the pressure drop was about 42% for pure water at expansion ratio 2 compared with straight pipe whereas by using nanofluid they were 62.6% (TiO2), 65.4% (CuO) and 57.6% (Al2O3). Good agreements were observed between numerical and experimental results all the way. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Turbulent heat transfer Separation flow Nanofluids Sudden expansion Thermal performance
1. Introduction Heat transfer and fluid flow through the annular concentric pipe are commonly used in power plants, chemical plants, nuclear reactors, evaporators, condensers, heat exchangers etc. Therefore, there are many experimental and numerical studies adopted analysis of temperature distributions, pressure drop, and velocity profile in annular passages. For high performance of cooling system the researcher studied effect of reconfiguration of geometry on efficiency of heat transfer devices such as addition of ribs or swirl generators in channel, expansion or contraction in passage, forward and backward-facing step passages etc. The separation of fluid flow in annular channel occurred due to the change in pressure gradient that caused by increase or decrease of cross sectional area of the annular channel. Thus the sudden expansion with one side or both
* Corresponding author. E-mail address: [email protected] (H. Togun). http://dx.doi.org/10.1016/j.ijthermalsci.2017.03.014 1290-0729/© 2017 Elsevier Masson SAS. All rights reserved.
sides of the annular pipes are representing some of the applications where the separation flow is seen. The separation region of the flow is accompanied by eddy that affects the heat transfer performance, as observed in several experimental and numerical studies by researchers [1e11]. Many methods are applied to improve the situation, among them, use of efficient materials, adjusting process parameters, modifications of design etc. are notable. Now researchers are more involved in exploration of better heat exchanging liquid where nanofluids are getting importance as heat exchanging liquid against conventional liquid. The numerical study of laminar nanofluid flow in sudden expansion is very limited and appears in the publications of Santosh Christopher et al., [12] where they performed numerical study on laminar Al2O3, Ag, Cu, SiO2, and CuO nanofluid flow in sudden expansion. They used same method [13] for analysis of sudden expansion flow and backward facing flow with Reynolds number range from 30 to 150 and nanoparticles volume fractions of suspensions 0.1, 0.2, 0.5. They observed the decrease in reattachment length about1.3% as compared with others [14].
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Nomenclature Dh h I K L Qc Qt Re
Hydraulic diameter (m) Heat transfer coefficient (W/m2 K) Heater current (AMP) Thermal conductivity (W/m K) Length of the test section (mm) Convection heat transfer (W) Total heat supplied (W) h Reynolds number Re ¼ UD n
(ro) pipe (ro)fiber Tsx Tb V
Outer radius of the outer pipe (m) Outer radius of the insulation (m) Local surface temperature (K) Bulk temperature (K) Voltage (volt)
Abu-Nada, [14] can be considering as a pioneer in numerical study of heat transfer to nanofluid over backward facing step where Cu, Ag, Al2O3,CuO, and TiO2 with volume frictions between 0.05 and 0.2 and range of Reynolds number from 200 to 600 were considered. The increase in Nusselt number at the top and at the bottom of the backward facing step was observed. Also the investigations found high thermal conductivity of nanoparticles as outside of the recirculation zones. Later, Kherbeet et al., [15] presented a numerical investigation of heat transfer and laminar nanofluid flow over a micro-scale backward-facing step. An increasing Reynolds number and volume fraction seemed to lead an increasing Nusselt number; the highest Nusselt number value was obtained with SiO2 in comparison with other types. Additional investigations concerning nanofluid flow over a backward-facing step for the laminar range [16e23]. Kherbeet et al. [24] conducted experimental study of laminar nanofluid flow over the microscale backward-facing step (MBFS) and forward-facing step (MFFS). The results showed that the highest Nusselt number noted with use MFFS in compared to the MBFS, which is approximately twice that of MBFS. Also the influences of magnetic field on nanofluid and nano boundary-layer flows over stretching surfaces are solved analytically by applying a newly developed method [25e27]. Sano Masatoshi et al. [28] presented experimental results of the turbulent channel flow over a backward-facing step by using suction through a slit at the bottom corner of the step and the direction of the suction was perpendicular and horizontal to the main flow. They measured local heat transfer coefficient and wall static pressure behind the backward-facing step and the results indicated, the enhancement of the heat transfer coefficient in the recirculating region by suction and reduction of the pressure drop. They also observed the improvement of the heat transfer coefficient with the increase in turbulent energy. Duangthongsuk & Wongwises [29] had conducted the experimental studies on performance of heat transfer and pressure drop of TiO2 nanofluid flow in horizontal double pipe under turbulent flow. Their results showed that the heat transfer coefficient was about 26% at vol. con. 1% while less than 14% at vol. con. 2%. due to their opinion that at increase of volume fraction of nanoparticle will leads combined tighter and then created bigger size which caused decrease the performance of heat transfer. While Farajollahi et al. [30] conducted experimental study on heat transfer to turbulent ɣAl2O3/water and TiO2/water flow through the shell and heat exchangers. They studied the effect of volume concentration, Peclet number and particle type on heat transfer, where the performance
15
of heat transfer for two nanofluids shows that at a certain Peclet number the heat transfer characteristics of TiO2/water nanofluid at its optimum nanoparticle concentration were higher in comparison to that of ɣ-Al2O3/water nanofluid. From the literature survey, the study of turbulent nanofluid flow in annular pipe with sudden expansion has not been investigated as yet experimentally and numerically. Therefore in the present paper flow and heat transfer of Al2O3, CuO and TiO2 water based nanofluids in an annular pipe with sudden expansion under turbulent flow regime were investigated. 2. Experimental step up 2.1. Design and construction Schematic diagram of the experimental set up is shown in Fig. 1 while the photograph of the set-up is presented in Fig. 2. The facility mainly includes a jacketed liquid tank which has the capacity of 150 L, stirrer motor (Branco Model BA 6324) with speed controller to make a homogenously dispersed solution, the chiller (Model-NWS-2AC) which is connected with the tank for adjusting the temperature of the liquid, a bypass pipe having diameter of 3 cm and the length of piping 100 cm is connected to the top part of the tank, stainless steel centrifugal liquid pump (Three Phase, 3000 RPM, 415V 3HP TO 20 HP, LIQUID Temperature 15C to þ90 C) which delivers the liquid to the test section, invertor to regulate the motor speed and run the centrifugal liquid pump and maintain different flow rates as used in this experiment, magnetic flow meter (Burkert S030) with digital display (Burkert types 8202-8222) mounted at the entrance pipe to measure the fully developed liquid velocity at 15D from the inlet region, entrance pipe section has length of 150 cm and has variable inner diameter (d ¼ 10, 8, 6, 5 cm) to create the sudden expansion in the passage which is connected to the test pipe by Teflon flange to avoid the conduction losses, the test section has inner diameter 10 cm and length of 100 cm and the solid inner pipe has outer diameter of 25 mm and length of 270 cm which extend along the passage from the entrance section up to the end of the test section mounted at the center of the passage to develop the annular passage, a bend pipe of inner diameter 10 cm and length 30 cm is connected to the test section to channelize the discharge to the storage tank. Two thermocouples of PT100Tc located at the inlet and outlet of the passage to measure the inlet and outlet temperatures. 32 Groves on straight line are made on the surface of the test section for 32 thermowalls to hold 32 thermocouples type K on the surface of the test pipe to measure the surface temperature. Two thermocouples have been installed at the inlet and outlet of the test section to measure the inlet and outlet temperatures of the nanofluids. Flexible ceramic pad heater has been used to heat the pipe and obtain uniform heat flux. A differential pressure transmitter Model (Model EJA110E-DMS4J-912DB/D3) has been used to measure the pressure drop of the flowing fluid. Two signal tubes from the inlet and the outlet of the test section were connected to the pressure transducer which provides digital display and transmits signal to the data logger (Mounting Bracket: 304 SST2-INCH PIPE). To transfer and save the experimental data, all the thermocouples and other devices connected to the data logger (YOKOGAWA, MODEL MW100-E-1Q) and then continuous monitoring and recording of the data are conducted by a personal computer. The specifications and the accuracy of the measuring equipment used in the present experimental setup are presented in Table 1: 2.2. Nanofluid preparation and its properties In the present work, a two-step method is used to produce Al2O3ewater nanofluids with volume fractions from 0.5% to 2%.
16
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 1. Schematic of experimental set up.
Preparing Deionized water is prepared using water distillation W4L FAVORIT (Model WCS4L/W4L favorit Distillate output: 4 L per hour single distillation Power Supply: 220/240 V, 1.5 Watt, single phase) unit before using it in the experiments. Al2O3 spherical nanoparticles with the mean diameter of 13 nm purchased from SigmaAldrich Co., Selangor, Malaysia was selected as the source of material. The first step is to mix Al2O3 nanoparticles in the deionized water as a base fluid. pH is one of the most important parameters have effect on colloidal stability of oxide nanoparticles by controlling the electrostatic charge found on surfaces of the particles. To get good dispersed nanofluids with the nanoparticles refrain from agglomerating and settling the pH of deionized water has adjusted at about 4.5 by adding HCl based chemical components [31,32]. Four different volume concentrations of Al2O3
nanoparticles were selected as sample then dispersed in 100 ml of DI water in each case and prepared in four straight sided clear glass jars. The second step was mixing Al2O3 nanoparticles in the DI water by an ultrasonic disrupter (120 W, 50 kHz) for 60 min. The sample preparation for different volume fractions are presented in Fig. 3. To prevent the clustering of nanoparticles and obtain the enhanced thermal conductive nanofluids the time for ultrasonic vibration was maintained at 60 min. During the preparation process of nanofluids there is no surfactant added to avoid effects on the thermal conductivity and viscosity of the nanofluids. 2.2.1. Thermal conductivity KD2 Pro thermal analyzer (KD2 Pro, Decagon Devices, Inc., Pullman, WA, USA) has been used to measure the thermal
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
17
bath (WiseCircu, Witeg Labortechnik GmbH, Wertheim, Germany) with 0.1 C accuracy was used. Nanofluids were employed into the jacketed beaker, and the temperature was kept constant for each test. The water bath contain inlet and outlet tube for flowing and circulating water at a specific temperature for maintaining temperature stability. In every 30 min the measurements for each sample were recorded where the thermal conductivity of the sample was calculated as the mean of ten readings at the same temperature. Fig. 5 shows the effective thermal conductivity of the Al2O3 nanofluids as a function of the temperature at different volume fractions. The data showed that the thermal conductivity of the nanofluids increases with increase of temperature for all concentrations where it is seen linear dependence of the thermal conductivity enhancement with the rise of temperature. The enhancement of thermal conductivity after adding Al2O3 nanoparticles were in between 5% and 20%.
Fig. 2. Photograph of experimental set up.
conductivity of the nanofluids as shown in Fig. 4. The main working principle of KD2 Pro thermal analyzer is based on a transient hot wire method with a 2e4% accuracy. A single needle sensor (1.3-mm diameter 60-mm long) was used for measuring thermal conductivity and was installed in a jacketed beaker installed in a water bath. In order to get high accurate measurements the experimental set-up ensures temperature stability. To keep the temperatures at 20 C, 25 C, 30 C, 35 C, 40 C, and 45 C while measuring the thermal conductivity of the nanofluid samples, a precision water
2.2.2. The viscosity The viscosity of the Al2O3 nanofluids at a different volume fraction was measured by using an Anton Paar rheometer (Physica MCR 301, Anton Paar GmbH, Graz, Austria) at different temperatures with the 1% error rate. The instrument contains two parallel cylindrical surfaces with a gap of 0.500 mm; Among the rotating and the stationary cylinders, the mobile cylinder has a diameter of 50 mm. The viscosity of the nanofluids were measured by varying temperatures and volume fractions of Al2O3 nanofluids. Fig. 6 shows the viscosity at different temperatures and volume fractions of Al2O3 nanofluids at a high shear rate of 100 S1. The results show that the viscosity decreases with the increase of temperature for all volume fraction of Al2O3nanofluids. 2.3. TEM image Transmission electron microscopy (TEM) is the main method to
Table 1 Specifications and errors for the measuring devices utilized in the present experiment. Measured parameter Surface temperature Bulk Temperature Fluid flow rate Fluid pressure drop Cooling unit
Type
Range
Type K thermocouple RTD (PT-100) sensor Burkert S030, Electromagnetic Flow Meter Model EJA110E-DMS4J-912DB/D3 Dong Guan Naser- NWS-2AC
Fig. 3. The sample preparation with different volume fractions.
Error C
0e300 0e200 C 0.1 m/s ~ 10 m/s 0-25 kPa 5 kW
±0.1 C ±0.1 C ±0.5% ±0.075% ±0.1 C
18
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
with a non-homogeneous alumina particle distribution due to agglomeration. 2.4. Data reduction method The heat transfer from the test section can be calculated from the input power
Power ¼ V I
(1)
The total heat supplied (Qt) ¼ power. Convection heat transfer (Qc) to flowing liquid is represented by equation (2).
Q t ¼ Q C Q loss
(2)
where, Qloss is the heat losses from the outer pipe and the insulation to the surroundings. The total losses due to conduction from the outer pipe and the insulation can be calculated by equation (3).
Q loss ¼ Q loss ðinsulationÞ þ Q loss ðouter pipeÞ 0 Q loss ¼ @
DT
1 2pKL ln
Fig. 4. KD2 Pro thermal analyzer and water bath.
ro=r
i
1
0
A
þ@ Insulation
1
DT
A
1 2pKL ln
ro=r
i
(3) pipe
where, validate single particle sizes and to find agglomerations of particles. Fig. 7 represents a transmission electron microscopy (TEM) image of the Al2O3 as received by using TEM LIBRA 120; Carl Zeiss, Oberkochen, Germany). Here the samples were dried out from solution and placed in the vacuum chamber of the TEM for inspection and measurement. As it is seen, some clusters are present
(ro) pipe is the outer radius of the outer pipe and (ro)fiber is the outer radius of the insulation. (ri) pipe is the inner radius of the outer pipe, and outer (ri) is fiber inner radius of the insulation The convection heat flux is evaluated by equation (4), [33].
Fig. 5. Variations of thermal conductivity with different temperature and volume fraction of Al2O3 nanofluids.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
19
Fig. 6. Variations of viscosity with different temperature and volume fraction of Al2O3 nanofluids. o Tb ; is the bulk temperature and is calculated as, Tb ¼ Ti þT 2 [33].
The local Nusselt number Nux based on the length of the test pipe is evaluated by equation (6), Holman [32].
Nux ¼
hx $L Kf
(6)
The Reynolds number based on the hydraulic diameter is presented by equation (8) Holman [33]. The dimensionless pumping power is calculated by the nanofluid flow rate and the pressure drop across the passage, equation (7).
PPump ¼
s
(4)
where, As is the surface area. The local heat transfer coefficient is calculated by equation (5), [33].
hx ¼
qc ðTsx Tb Þ
where, Tsx represents the local surface temperature.
(7)
where, m and r are representing the viscosity and density of the nanofluids respectively, Q is the flow rate and DP refers to the pressure drop across the passage, dh is the hydraulic diameter.
Fig. 7. TEM photograph of Al2O3 nanoparticle.
qc ¼ Qc=A
Q $DP
m3 r2 D1 h
(5)
Re ¼
UDh
n
(8)
2.5. Experimental procedure The experimental procedure was as follows: 1. A suitable entrance pipe diameter was selected to obtain the required expansion ratio. 2. All the instruments were calibrated for reasonably accurate data acquisition during experimental investigation.
20
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
3. The liquid pump -discharged liquid at different velocities was obtained by turning a regulator connected to the pump. 4. Power regulator and temperature controller were used to supply the required heat. 5. The steady state condition was achieved after 2 h in each experimental observation. However, at every half an hour increment from the beginning, the readings were taken to observe the rate of increase of temperature. The subsequent runs for other Reynolds numbers and for higher ranges the experiments were conducted by following the same procedure. 6. During the experiment, two runs at the same condition were performed to provide the data of reproducibility. It was observed that the acquired data were correct within ±2% accuracy and 95% confidence level. 7. For each of the test run, the following readings were recorded:The current in amperes, the voltage in volts, readings of all the thermocouples in ( C), and the step height in (mm). 3. Numerical simulation In this paper, Geometry, Meshing process, Boundary conditions, Governing equations, Thermo physical properties of the nanofluid, Grid-independent test, and Code validation, are performed by the same procedure which is explained elsewhere [34]. 4. Results and discussion
temperature are same for both the numerical and the experimental data. Generally, the surface temperature on the test section decreases at the inlet region and then increases towards the exit. The reduction of the surface temperature on the outer pipe represents the separation region, and the location of the separation point move far from the step with the increase of step height. The effect of the expansion ratio on the distribution of the surface temperature appears clearly at the inlet region of the passage, where the minimum of the surface temperature represents the separation point. The separation regions, which causes an enhancement of the heat transfer due to turbulence enhancement. The increase of the separation region results from the increase of the expansion ratio, which leads to an increase of the augmentation of the heat transfer. Fig. 9 shows the effect of the Reynolds number on the surface temperature of the test section, where the decrease of the Reynolds number leads to an increase of the surface temperature at the heat flux q ¼ 16,000 W/m2 and the expansion ratio ER ¼ 2. In all the cases, the same trends were seen where the surface temperature deceases at the inlet region of sudden expansion and then increase gradually up to the exit. Effect of volume fraction of Al2O3 nanofluids on distributions of surface temperature of the test section at the Reynolds number of 50,000 and expansion ratio of 2 are presented in Fig. 10. The results showed a decrease in the surface temperature with the increase of volume fraction of Al2O3 nanofluids due to the increase in heat transport by nanoparticles in the base liquid. They obtained a good cooling on the surface of test section.
4.1. Distribution of surface temperatures 4.2. Heat transfer enhancement The distribution of the surface temperature on the test section at a heat flux of q ¼ 16,000 W/m2 and Re ¼ 50,000 for different expansion ratios are shown in Fig. 8. The trends of the surface
The variations in the average convective heat transfer coefficient at different Reynolds numbers and expansion ratios of 2 for Al2O3,
Fig. 8. Distribution of the surface temperature at heat flux of q ¼ 16,000 W/m2 and Re ¼ 50,000 for different expansion ratios.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 9. Effect of the Reynolds number on the surface temperature at the heat flux q ¼ 16,000 W/m2 and the expansion ratio ER ¼ 2.
Fig. 10. Effect of volume fraction of Al2O3 nanofluids on distributions of surface temperature for Reynolds number of 50,000 and expansion ratio ER ¼ 2.
21
22
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 11. Average heat transfer coefficient with different Reynolds numbers and expansion ratio of 2 at Al2O3, CuO, TiO2 volume fractions of 2% and pure water.
Fig. 12. Effect of expansion ratio on average heat transfer coefficient at Reynolds number of 50,000 for Al2O3, CuO, TiO2 volume fractions of 2% and pure water.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
23
Fig. 13. Effect of volume faction of nanofluids on average heat transfer coefficient at expansion ratio of 2 and Reynolds number of 50,000.
Table 2 Thermal performance for using nanofluids with ER ¼ 2 and Re ¼ 50,000. Vol. %
0.5 1 1.5 2
Thermal performance TiO2
CuO
Al2O3
5.3% 6.5% 7.7% 8.6%
6.4% 7.8% 9.3% 10.7%
7% 8.9% 10.5% 12.3%
CuO, TiO2 volume fractions of 2% and pure water are presented in Fig. 11. Increases in average heat transfer coefficient with increases of Reynolds number are observed for all types of nanofluids. The maximum average heat transfer coefficient was observed with Al2O3volume fractions of 2% in comparison to others and good agreement between numerical and experimental results were observed for pure water and Al2O3 nanofluids. Fig. 12 illustrated the effect of expansion ratio on average heat transfer coefficient at Reynolds number of 50,000 for Al2O3, CuO, TiO2 volume fractions of 2% and pure water. The numerical and experimental results showed the increase of average heat transfer coefficient with increased expansion ratio where the highest value of average heat transfer coefficient was obtained for Al2O3 volume fractions of 2% and expansion ratio of 2 compared to others. Effect of volume faction of nanofluids on average heat transfer coefficient at expansion ratio of 2 and Reynolds number of 50,000 are presented in Fig. 13. It can be seen that the increase of volume faction of nanofluids leads to increase in average heat transfer coefficient due to increased thermal conductivity of the base liquid. The maximum improvement of heat transfer was observed at Al2O3 volume fractions of 2% compared with other types of nanofluids.
Generally, the thermal performance at the maximum expansion ratio (ER ¼ 2) is about 35% with using water while thermal performance on using different volume fractions of nanofluids with ER ¼ 2 and Re ¼ 50,000 is presented in Table 2. 4.3. Pressure drop Figs. 14 and 15 have shown the effect of Reynolds number and volume fractions of nanofluids on the average pressure drop at expansion ratio of 2. Generally, the average pressure drop increases linearly with the increase of both the Reynolds number and the volume fractions of nanofluids. The increment of pressure drop at expansion ratio of 2 and Reynolds number of 50,000 for different volume fractions of nanofluids are presented in Table 3 where, the highest increment of pressure drop is obtained at CuO volume fractions of 2% compared with other types of nanofluids. 5. Conclusions The experimental and numerical (using ANASYS FLUENT 14 software package) works were conducted in the turbulent heat transfer and separation nanofluid flow through an annular pipe with sudden expansion. Three dimensional study, based on finite volume method with standard k-ε turbulence model was performed. It is noted that the surface heat transfer coefficient increases with the increase of nanoparticle volume fraction or the Reynolds number. The results have shown that the separation regions formed after the sudden expansions are considerably affected by the surface heat transfer coefficient and pressure drop where the lowest of the pressure drop and the highest thermal performance are detected at the Reynolds number 50,000, 2% Al2O3 water based
24
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 14. Effect of Reynolds number on average pressure drop at expansion ratio of 2.
Fig. 15. Effect of volume fractions of nanofluids on average pressure drop at expansion ratio of 2.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25 Table 3 Increment of pressure drop at Reynolds number of 50,000 and expansion ratio. ER ¼ 2. Vol. %
0.5 1 1.5 2
Increment of pressure drop TiO2
CuO
Al2O3
3.5% 5.5% 8% 12%
4% 7.5% 10.5% 15.5%
3% 5% 6.5% 8%
nanofluid at expansion ratio 2 in comparison to others. The enhancement of heat transfer was about 36.6% for pure water at expansion ratio of 2 compared to the heat transfer obtained in straight pipe. Augmentation of heat transfer could be achieved by using nanofluid at the expansion ratio 2 where the total improvements were about 45.2% (TiO2), 47.3% (CuO), and 49% (Al2O3). Moreover, the increase in the pressure drop was about 42% for the pure water at expansion ratio 2 compared with the data of the straight pipe whereas by using nanofluid they were 65.4% (CuO), 62% (TiO2), and 57.6% (Al2O3). Good agreements are obtained between the numerical and the experimental data obtained in the current investigations. Acknowledgements The authors gratefully acknowledge High-Impact Research Grant UM.C/HIR/MOHE/ENG/46, UMRG RP012D-13AET, and the University of Malaya, Malaysia for support in conducting this research. References [1] Togun H, Salman YK, Hakim S, Aljibori Sultan, Kazi SN. An experimental study of heat transfer to turbulent separation fluid flow in an annular passage. Int J Heat Mass Transf 2011;54(4):766e73. [2] Tuqa A, Hussein T, Ariffin MKA, Kazi SN, Badarudin A, Adam NM, et al. Heat transfer and turbulent fluid flow over vertical double forward-facing step. World Acad Sci Eng Technol Int J Mech Aerosp Ind Mechatron Eng 2014;8: 368e72. [3] Togun H, Kazi SN, Badarudin A. A review of experimental study of turbulent heat transfer in separated flow. Aust J Basic Appl Sci 2011;5(10):489e505. [4] Togun H, Tuqa A, Kazi SN, Badarudin A, Ariffin MKA, Togun H, et al. Heat transfer to laminar flow over a double backward-facing step. Int J Mech Aerosp Manuf Ind Sci Eng World Acad Sci Eng Technol 2013;80:117e39. [5] Oon CS, Togun H, Kazi SN, Badarudin A, Zubir MNM, Sadeghinezhad E. Numerical simulation of heat transfer to separation air flow in an annular pipe. Int Commun Heat Mass Transf 2012;39(8):1176e80. [6] Togun H. Effect of laminar separation flow and nanofluids on heat transfer augmentation with passive techniques: a review. Int Commun Heat Mass Transf 2016;77:9e14. [7] Oon CS, Togun Hussein, Kazi SN, Badarudin A, Zubir MNM, Sadeghinezhad E. Computational simulation of heat transfer to separation fluid flow in an annular passage. Int Commun Heat Mass Transf 2013;46:92e6. [8] Togun H, Jassim Shkarah Ahmed, Kazi SN, Badarudin A. CFD simulation of heat transfer and turbulent fluid flow over a double forward-facing step. Math Probl Eng 2013;2013:10. [9] Togun H, Abdulrazzaq Tuqa, Kazi SN, Badarudin A, Kadhum AAH, Sadeghinezhad E. A Review of studies on forced, natural and mixed heat transfer to fluid and nanofluid flow in an annular passage. Renew Sustain Energy Rev 2014;39. [10] Safaei MR, Vafai K, Kazi SN, Badarudin A. Investigation of heat transfer enhancement in a forward-facing contracting channel using FMWCNT nanofluids. Numer Heat Transf Part A 2014:1e20.
25
[11] Togun H, Abdulrazzaq Tuqa, Kazi ABSN, MKA A. Numerical study of turbulent heat transfer in annular pipe with sudden contraction. Appl Mech Mater 2014;465e466:461e6. [12] Santosh Christopher D, Kanna PR, Madhusudhana GR, Venkumar P, Mohammed HA. Numerical investigation of heat transfer from a twodimensional sudden expansion flow using nanofluids. Numer Heat Transf Part A Appl 2012;61(7):527e46. [13] Kanna PR, Das MK. Heat transfer study of two-dimensional laminar incompressible wall jet over backward-facing step. Numer Heat Transf Part A Appl 2006;50(2):165e87. [14] Abu-Nada E. Application of nanofluids for heat transfer enhancement of separated flows encountered in a backward facing step. Int J Heat Fluid Flow 2008;29(1):242e9. [15] Kherbeet AS, Mohammed HA, Salman BH. The effect of nanofluids flow on mixed convection heat transfer over microscale backward-facing step. Int J Heat Mass Transf 2012;55(21e22):5870e81. [16] Al-aswadi AA, Mohammed HA, Shuaib NH, Campo A. Laminar forced convection flow over a backward facing step using nanofluids. Int Commun Heat Mass Transf 2010;37(8):950e7. [17] Amirhossein Heshmati HAM, Parsazadeh Mohammad, Fathinia Farshid, Wahid Mazlan A, Sies Mohsin M, Saat Aminuddin. Effect of vertical baffle installation on forced convective heat transfer in channel having a backward facing step. Appl Mech Mater 2013;388:169e75. [18] Mohammed Hussein A, Munisamy MGKM, Wahid Mazlan A. Combined convection heat transfer of nanofluids flow over forward facing step in a channel having a blockage. Appl Mech Mater 2013:185e91. [19] Kherbeet AS, Mohammed HA, Munisamy KM, Salman BH. The effect of step height of microscale backward-facing step on mixed convection nanofluid flow and heat transfer characteristics. Int J Heat Mass Transf 2014;68(0): 554e66. [20] Mohammed HA, Al-Aswadi AA, Abu-Mulaweh HI, Hussein AK, Kanna PR. Mixed convection over a backward-facing step in a vertical duct using nanofluids ;Buoyancy opposing case. J Comput Theor Nanosci 2014;11(3): 860e72. [21] Mohammed HA, Al-aswadi AA, Abu-Mulaweh HI, Shuaib NH. Influence of nanofluids on mixed convective heat transfer over a horizontal backwardfacing step. Heat Transf. Asian Res 2011;40(4):287e307. [22] Mohammed HA, Al-aswadi AA, Shuaib NH, Saidur R. Convective heat transfer and fluid flow study over a step using nanofluids: a review. Renew Sustain Energy Rev 2011;15(6):2921e39. http://dx.doi.org/10.1016/j.rser.2011.02. 019. [23] Mohammed HA, Al-aswadi AA, Yusoff MZ, Saidur R. Buoyancy-assisted mixed convective flow over backward-facing step in a vertical duct using nanofluids. Thermophys Aeromech 2012;19(1):33e52. [24] Kherbeet ASh, Mohammed HA, Munisamy KM, Saidur R, Salman BH, Mahbubul IM. Experimental and numerical study of nanofluid flow and heat transfer over microscale forward-facing step. Int Commun Heat Mass Transf 2015;57:319e29. [25] Rashidi MM, Erfani E. The modified differential transform method for investigating Nano boundary- layer over stretching surfaces. Int J Numer methods heat fluid flow 2011;21(7):864e83. [26] Rashidi MM, Fridoonimehr N, Hosseini A, Anwar Beg O, Hung T-K. Homotopy simulation of nanofluids dynamics from a Nono-linearly stretching isothermal permeable sheet with transpiration. Meccanica 2014;49(2):469e82. [27] Rashidi MM, Vishnu Ganesh N, Abdul Hakeem AK, Ganga B. Buoyancy effect on MHD flow of nanofluids over a stretching sheet in the presence of thermal radiation. J Mol Liq 2014;198:234e8. [28] Sano Masatoshi SI, Sakuraba Kenichiro. Control of turbulent channel flow over a backward-facing step by suction. J Fluid Sci Technol 2009;4(1):188e99. [29] Duangthongsuk W, Wongwises S. An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime. Int J Heat Mass Transf 2010;53(1e3):334e44. [30] Farajollahi B, Etemad SG, Hojjat M. Heat transfer of nanofluids in a shell and tube heat exchanger. Int J Heat Mass Transf 2010;53(1e3):12e7. [31] Peyghambarzadeh SM, Hashemabadi SH, Naraki M, Vermahmoudi Y. Experimental study of overall heat transfer coefficient in the application of dilute nanofluids in the car radiator. Appl Therm Eng 2013;52(1):8e16. [32] Saidur R, Leong KY, Mohammad HA. A review on applications and challenges of nanofluids. Renew Sustain Energy Rev 2011;15(3):1646e68. [33] Holman JP. Heat transfer. ninth ed. New York: McGraw Hill; 2002. [34] Togun H, Abu-Mulaweh HI, Kazi SN, Badarudin A. Numerical simulation of heat transfer and separation Al2O3/nanofluid flow in concentric annular pipe. Int Commun Heat Mass Transf 2016;71:108e17.
Contents lists available at ScienceDirect
International Journal of Thermal Sciences journal homepage: www.elsevier.com/locate/ijts
Turbulent heat transfer to separation nanofluid flow in annular concentric pipe Hussein Togun a, *, S.N. Kazi b, A. Badarudin b a b
Head of Biomedical Engineering Department, University of Thi-Qar, 64001 Nassiriya, Iraq Department of Mechanical Engineering, University of Malaya, 50603 Kuala Lumpur, Malaysia
a r t i c l e i n f o
a b s t r a c t
Article history: Received 19 October 2016 Received in revised form 16 February 2017 Accepted 12 March 2017
Turbulent heat transfer to separation nanofluid flow in annular concentric pipe were studied numerically and experimentally. In the numerical study, finite volume method with standard k-ε turbulence model in three dimensional domains was selected. Three different types of water based (Al2O3, CuO, TiO2) nanofluids were employed in this simulation. The adopted boundary conditions were, expansion ratio (ER ¼ 1.25, 1.67, and 2), Reynolds number ranging from 20,000 to 50,000, water based nanofluids used Al2O3, CuO, TiO2 with volume fractions varied between 0 and 2% at different heat fluxes, varied from 4000 W/m2 to 16,000 W/m2. For experimental study, Al2O3 water based nanofluid was used to validate the numerical results. The results show that the volume fraction of nanofluid and Reynolds number significantly affect the surface heat transfer coefficient; an increase in surface heat transfer coefficient was noted when both volume fraction of nanofluids and Reynolds number were increased for all the cases. The improvement of heat transfer was about 36.6% for pure water at the expansion ratio of 2 compared to heat transfer obtained in a straight pipe. Augmentation of heat transfer could be achieved by using nanofluid at expansion ratio 2 where the total improvements were about 45.2% (TiO2), 47.3%(CuO), and 49%(Al2O3). Also the increment in the pressure drop was about 42% for pure water at expansion ratio 2 compared with straight pipe whereas by using nanofluid they were 62.6% (TiO2), 65.4% (CuO) and 57.6% (Al2O3). Good agreements were observed between numerical and experimental results all the way. © 2017 Elsevier Masson SAS. All rights reserved.
Keywords: Turbulent heat transfer Separation flow Nanofluids Sudden expansion Thermal performance
1. Introduction Heat transfer and fluid flow through the annular concentric pipe are commonly used in power plants, chemical plants, nuclear reactors, evaporators, condensers, heat exchangers etc. Therefore, there are many experimental and numerical studies adopted analysis of temperature distributions, pressure drop, and velocity profile in annular passages. For high performance of cooling system the researcher studied effect of reconfiguration of geometry on efficiency of heat transfer devices such as addition of ribs or swirl generators in channel, expansion or contraction in passage, forward and backward-facing step passages etc. The separation of fluid flow in annular channel occurred due to the change in pressure gradient that caused by increase or decrease of cross sectional area of the annular channel. Thus the sudden expansion with one side or both
* Corresponding author. E-mail address: [email protected] (H. Togun). http://dx.doi.org/10.1016/j.ijthermalsci.2017.03.014 1290-0729/© 2017 Elsevier Masson SAS. All rights reserved.
sides of the annular pipes are representing some of the applications where the separation flow is seen. The separation region of the flow is accompanied by eddy that affects the heat transfer performance, as observed in several experimental and numerical studies by researchers [1e11]. Many methods are applied to improve the situation, among them, use of efficient materials, adjusting process parameters, modifications of design etc. are notable. Now researchers are more involved in exploration of better heat exchanging liquid where nanofluids are getting importance as heat exchanging liquid against conventional liquid. The numerical study of laminar nanofluid flow in sudden expansion is very limited and appears in the publications of Santosh Christopher et al., [12] where they performed numerical study on laminar Al2O3, Ag, Cu, SiO2, and CuO nanofluid flow in sudden expansion. They used same method [13] for analysis of sudden expansion flow and backward facing flow with Reynolds number range from 30 to 150 and nanoparticles volume fractions of suspensions 0.1, 0.2, 0.5. They observed the decrease in reattachment length about1.3% as compared with others [14].
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Nomenclature Dh h I K L Qc Qt Re
Hydraulic diameter (m) Heat transfer coefficient (W/m2 K) Heater current (AMP) Thermal conductivity (W/m K) Length of the test section (mm) Convection heat transfer (W) Total heat supplied (W) h Reynolds number Re ¼ UD n
(ro) pipe (ro)fiber Tsx Tb V
Outer radius of the outer pipe (m) Outer radius of the insulation (m) Local surface temperature (K) Bulk temperature (K) Voltage (volt)
Abu-Nada, [14] can be considering as a pioneer in numerical study of heat transfer to nanofluid over backward facing step where Cu, Ag, Al2O3,CuO, and TiO2 with volume frictions between 0.05 and 0.2 and range of Reynolds number from 200 to 600 were considered. The increase in Nusselt number at the top and at the bottom of the backward facing step was observed. Also the investigations found high thermal conductivity of nanoparticles as outside of the recirculation zones. Later, Kherbeet et al., [15] presented a numerical investigation of heat transfer and laminar nanofluid flow over a micro-scale backward-facing step. An increasing Reynolds number and volume fraction seemed to lead an increasing Nusselt number; the highest Nusselt number value was obtained with SiO2 in comparison with other types. Additional investigations concerning nanofluid flow over a backward-facing step for the laminar range [16e23]. Kherbeet et al. [24] conducted experimental study of laminar nanofluid flow over the microscale backward-facing step (MBFS) and forward-facing step (MFFS). The results showed that the highest Nusselt number noted with use MFFS in compared to the MBFS, which is approximately twice that of MBFS. Also the influences of magnetic field on nanofluid and nano boundary-layer flows over stretching surfaces are solved analytically by applying a newly developed method [25e27]. Sano Masatoshi et al. [28] presented experimental results of the turbulent channel flow over a backward-facing step by using suction through a slit at the bottom corner of the step and the direction of the suction was perpendicular and horizontal to the main flow. They measured local heat transfer coefficient and wall static pressure behind the backward-facing step and the results indicated, the enhancement of the heat transfer coefficient in the recirculating region by suction and reduction of the pressure drop. They also observed the improvement of the heat transfer coefficient with the increase in turbulent energy. Duangthongsuk & Wongwises [29] had conducted the experimental studies on performance of heat transfer and pressure drop of TiO2 nanofluid flow in horizontal double pipe under turbulent flow. Their results showed that the heat transfer coefficient was about 26% at vol. con. 1% while less than 14% at vol. con. 2%. due to their opinion that at increase of volume fraction of nanoparticle will leads combined tighter and then created bigger size which caused decrease the performance of heat transfer. While Farajollahi et al. [30] conducted experimental study on heat transfer to turbulent ɣAl2O3/water and TiO2/water flow through the shell and heat exchangers. They studied the effect of volume concentration, Peclet number and particle type on heat transfer, where the performance
15
of heat transfer for two nanofluids shows that at a certain Peclet number the heat transfer characteristics of TiO2/water nanofluid at its optimum nanoparticle concentration were higher in comparison to that of ɣ-Al2O3/water nanofluid. From the literature survey, the study of turbulent nanofluid flow in annular pipe with sudden expansion has not been investigated as yet experimentally and numerically. Therefore in the present paper flow and heat transfer of Al2O3, CuO and TiO2 water based nanofluids in an annular pipe with sudden expansion under turbulent flow regime were investigated. 2. Experimental step up 2.1. Design and construction Schematic diagram of the experimental set up is shown in Fig. 1 while the photograph of the set-up is presented in Fig. 2. The facility mainly includes a jacketed liquid tank which has the capacity of 150 L, stirrer motor (Branco Model BA 6324) with speed controller to make a homogenously dispersed solution, the chiller (Model-NWS-2AC) which is connected with the tank for adjusting the temperature of the liquid, a bypass pipe having diameter of 3 cm and the length of piping 100 cm is connected to the top part of the tank, stainless steel centrifugal liquid pump (Three Phase, 3000 RPM, 415V 3HP TO 20 HP, LIQUID Temperature 15C to þ90 C) which delivers the liquid to the test section, invertor to regulate the motor speed and run the centrifugal liquid pump and maintain different flow rates as used in this experiment, magnetic flow meter (Burkert S030) with digital display (Burkert types 8202-8222) mounted at the entrance pipe to measure the fully developed liquid velocity at 15D from the inlet region, entrance pipe section has length of 150 cm and has variable inner diameter (d ¼ 10, 8, 6, 5 cm) to create the sudden expansion in the passage which is connected to the test pipe by Teflon flange to avoid the conduction losses, the test section has inner diameter 10 cm and length of 100 cm and the solid inner pipe has outer diameter of 25 mm and length of 270 cm which extend along the passage from the entrance section up to the end of the test section mounted at the center of the passage to develop the annular passage, a bend pipe of inner diameter 10 cm and length 30 cm is connected to the test section to channelize the discharge to the storage tank. Two thermocouples of PT100Tc located at the inlet and outlet of the passage to measure the inlet and outlet temperatures. 32 Groves on straight line are made on the surface of the test section for 32 thermowalls to hold 32 thermocouples type K on the surface of the test pipe to measure the surface temperature. Two thermocouples have been installed at the inlet and outlet of the test section to measure the inlet and outlet temperatures of the nanofluids. Flexible ceramic pad heater has been used to heat the pipe and obtain uniform heat flux. A differential pressure transmitter Model (Model EJA110E-DMS4J-912DB/D3) has been used to measure the pressure drop of the flowing fluid. Two signal tubes from the inlet and the outlet of the test section were connected to the pressure transducer which provides digital display and transmits signal to the data logger (Mounting Bracket: 304 SST2-INCH PIPE). To transfer and save the experimental data, all the thermocouples and other devices connected to the data logger (YOKOGAWA, MODEL MW100-E-1Q) and then continuous monitoring and recording of the data are conducted by a personal computer. The specifications and the accuracy of the measuring equipment used in the present experimental setup are presented in Table 1: 2.2. Nanofluid preparation and its properties In the present work, a two-step method is used to produce Al2O3ewater nanofluids with volume fractions from 0.5% to 2%.
16
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 1. Schematic of experimental set up.
Preparing Deionized water is prepared using water distillation W4L FAVORIT (Model WCS4L/W4L favorit Distillate output: 4 L per hour single distillation Power Supply: 220/240 V, 1.5 Watt, single phase) unit before using it in the experiments. Al2O3 spherical nanoparticles with the mean diameter of 13 nm purchased from SigmaAldrich Co., Selangor, Malaysia was selected as the source of material. The first step is to mix Al2O3 nanoparticles in the deionized water as a base fluid. pH is one of the most important parameters have effect on colloidal stability of oxide nanoparticles by controlling the electrostatic charge found on surfaces of the particles. To get good dispersed nanofluids with the nanoparticles refrain from agglomerating and settling the pH of deionized water has adjusted at about 4.5 by adding HCl based chemical components [31,32]. Four different volume concentrations of Al2O3
nanoparticles were selected as sample then dispersed in 100 ml of DI water in each case and prepared in four straight sided clear glass jars. The second step was mixing Al2O3 nanoparticles in the DI water by an ultrasonic disrupter (120 W, 50 kHz) for 60 min. The sample preparation for different volume fractions are presented in Fig. 3. To prevent the clustering of nanoparticles and obtain the enhanced thermal conductive nanofluids the time for ultrasonic vibration was maintained at 60 min. During the preparation process of nanofluids there is no surfactant added to avoid effects on the thermal conductivity and viscosity of the nanofluids. 2.2.1. Thermal conductivity KD2 Pro thermal analyzer (KD2 Pro, Decagon Devices, Inc., Pullman, WA, USA) has been used to measure the thermal
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
17
bath (WiseCircu, Witeg Labortechnik GmbH, Wertheim, Germany) with 0.1 C accuracy was used. Nanofluids were employed into the jacketed beaker, and the temperature was kept constant for each test. The water bath contain inlet and outlet tube for flowing and circulating water at a specific temperature for maintaining temperature stability. In every 30 min the measurements for each sample were recorded where the thermal conductivity of the sample was calculated as the mean of ten readings at the same temperature. Fig. 5 shows the effective thermal conductivity of the Al2O3 nanofluids as a function of the temperature at different volume fractions. The data showed that the thermal conductivity of the nanofluids increases with increase of temperature for all concentrations where it is seen linear dependence of the thermal conductivity enhancement with the rise of temperature. The enhancement of thermal conductivity after adding Al2O3 nanoparticles were in between 5% and 20%.
Fig. 2. Photograph of experimental set up.
conductivity of the nanofluids as shown in Fig. 4. The main working principle of KD2 Pro thermal analyzer is based on a transient hot wire method with a 2e4% accuracy. A single needle sensor (1.3-mm diameter 60-mm long) was used for measuring thermal conductivity and was installed in a jacketed beaker installed in a water bath. In order to get high accurate measurements the experimental set-up ensures temperature stability. To keep the temperatures at 20 C, 25 C, 30 C, 35 C, 40 C, and 45 C while measuring the thermal conductivity of the nanofluid samples, a precision water
2.2.2. The viscosity The viscosity of the Al2O3 nanofluids at a different volume fraction was measured by using an Anton Paar rheometer (Physica MCR 301, Anton Paar GmbH, Graz, Austria) at different temperatures with the 1% error rate. The instrument contains two parallel cylindrical surfaces with a gap of 0.500 mm; Among the rotating and the stationary cylinders, the mobile cylinder has a diameter of 50 mm. The viscosity of the nanofluids were measured by varying temperatures and volume fractions of Al2O3 nanofluids. Fig. 6 shows the viscosity at different temperatures and volume fractions of Al2O3 nanofluids at a high shear rate of 100 S1. The results show that the viscosity decreases with the increase of temperature for all volume fraction of Al2O3nanofluids. 2.3. TEM image Transmission electron microscopy (TEM) is the main method to
Table 1 Specifications and errors for the measuring devices utilized in the present experiment. Measured parameter Surface temperature Bulk Temperature Fluid flow rate Fluid pressure drop Cooling unit
Type
Range
Type K thermocouple RTD (PT-100) sensor Burkert S030, Electromagnetic Flow Meter Model EJA110E-DMS4J-912DB/D3 Dong Guan Naser- NWS-2AC
Fig. 3. The sample preparation with different volume fractions.
Error C
0e300 0e200 C 0.1 m/s ~ 10 m/s 0-25 kPa 5 kW
±0.1 C ±0.1 C ±0.5% ±0.075% ±0.1 C
18
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
with a non-homogeneous alumina particle distribution due to agglomeration. 2.4. Data reduction method The heat transfer from the test section can be calculated from the input power
Power ¼ V I
(1)
The total heat supplied (Qt) ¼ power. Convection heat transfer (Qc) to flowing liquid is represented by equation (2).
Q t ¼ Q C Q loss
(2)
where, Qloss is the heat losses from the outer pipe and the insulation to the surroundings. The total losses due to conduction from the outer pipe and the insulation can be calculated by equation (3).
Q loss ¼ Q loss ðinsulationÞ þ Q loss ðouter pipeÞ 0 Q loss ¼ @
DT
1 2pKL ln
Fig. 4. KD2 Pro thermal analyzer and water bath.
ro=r
i
1
0
A
þ@ Insulation
1
DT
A
1 2pKL ln
ro=r
i
(3) pipe
where, validate single particle sizes and to find agglomerations of particles. Fig. 7 represents a transmission electron microscopy (TEM) image of the Al2O3 as received by using TEM LIBRA 120; Carl Zeiss, Oberkochen, Germany). Here the samples were dried out from solution and placed in the vacuum chamber of the TEM for inspection and measurement. As it is seen, some clusters are present
(ro) pipe is the outer radius of the outer pipe and (ro)fiber is the outer radius of the insulation. (ri) pipe is the inner radius of the outer pipe, and outer (ri) is fiber inner radius of the insulation The convection heat flux is evaluated by equation (4), [33].
Fig. 5. Variations of thermal conductivity with different temperature and volume fraction of Al2O3 nanofluids.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
19
Fig. 6. Variations of viscosity with different temperature and volume fraction of Al2O3 nanofluids. o Tb ; is the bulk temperature and is calculated as, Tb ¼ Ti þT 2 [33].
The local Nusselt number Nux based on the length of the test pipe is evaluated by equation (6), Holman [32].
Nux ¼
hx $L Kf
(6)
The Reynolds number based on the hydraulic diameter is presented by equation (8) Holman [33]. The dimensionless pumping power is calculated by the nanofluid flow rate and the pressure drop across the passage, equation (7).
PPump ¼
s
(4)
where, As is the surface area. The local heat transfer coefficient is calculated by equation (5), [33].
hx ¼
qc ðTsx Tb Þ
where, Tsx represents the local surface temperature.
(7)
where, m and r are representing the viscosity and density of the nanofluids respectively, Q is the flow rate and DP refers to the pressure drop across the passage, dh is the hydraulic diameter.
Fig. 7. TEM photograph of Al2O3 nanoparticle.
qc ¼ Qc=A
Q $DP
m3 r2 D1 h
(5)
Re ¼
UDh
n
(8)
2.5. Experimental procedure The experimental procedure was as follows: 1. A suitable entrance pipe diameter was selected to obtain the required expansion ratio. 2. All the instruments were calibrated for reasonably accurate data acquisition during experimental investigation.
20
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
3. The liquid pump -discharged liquid at different velocities was obtained by turning a regulator connected to the pump. 4. Power regulator and temperature controller were used to supply the required heat. 5. The steady state condition was achieved after 2 h in each experimental observation. However, at every half an hour increment from the beginning, the readings were taken to observe the rate of increase of temperature. The subsequent runs for other Reynolds numbers and for higher ranges the experiments were conducted by following the same procedure. 6. During the experiment, two runs at the same condition were performed to provide the data of reproducibility. It was observed that the acquired data were correct within ±2% accuracy and 95% confidence level. 7. For each of the test run, the following readings were recorded:The current in amperes, the voltage in volts, readings of all the thermocouples in ( C), and the step height in (mm). 3. Numerical simulation In this paper, Geometry, Meshing process, Boundary conditions, Governing equations, Thermo physical properties of the nanofluid, Grid-independent test, and Code validation, are performed by the same procedure which is explained elsewhere [34]. 4. Results and discussion
temperature are same for both the numerical and the experimental data. Generally, the surface temperature on the test section decreases at the inlet region and then increases towards the exit. The reduction of the surface temperature on the outer pipe represents the separation region, and the location of the separation point move far from the step with the increase of step height. The effect of the expansion ratio on the distribution of the surface temperature appears clearly at the inlet region of the passage, where the minimum of the surface temperature represents the separation point. The separation regions, which causes an enhancement of the heat transfer due to turbulence enhancement. The increase of the separation region results from the increase of the expansion ratio, which leads to an increase of the augmentation of the heat transfer. Fig. 9 shows the effect of the Reynolds number on the surface temperature of the test section, where the decrease of the Reynolds number leads to an increase of the surface temperature at the heat flux q ¼ 16,000 W/m2 and the expansion ratio ER ¼ 2. In all the cases, the same trends were seen where the surface temperature deceases at the inlet region of sudden expansion and then increase gradually up to the exit. Effect of volume fraction of Al2O3 nanofluids on distributions of surface temperature of the test section at the Reynolds number of 50,000 and expansion ratio of 2 are presented in Fig. 10. The results showed a decrease in the surface temperature with the increase of volume fraction of Al2O3 nanofluids due to the increase in heat transport by nanoparticles in the base liquid. They obtained a good cooling on the surface of test section.
4.1. Distribution of surface temperatures 4.2. Heat transfer enhancement The distribution of the surface temperature on the test section at a heat flux of q ¼ 16,000 W/m2 and Re ¼ 50,000 for different expansion ratios are shown in Fig. 8. The trends of the surface
The variations in the average convective heat transfer coefficient at different Reynolds numbers and expansion ratios of 2 for Al2O3,
Fig. 8. Distribution of the surface temperature at heat flux of q ¼ 16,000 W/m2 and Re ¼ 50,000 for different expansion ratios.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 9. Effect of the Reynolds number on the surface temperature at the heat flux q ¼ 16,000 W/m2 and the expansion ratio ER ¼ 2.
Fig. 10. Effect of volume fraction of Al2O3 nanofluids on distributions of surface temperature for Reynolds number of 50,000 and expansion ratio ER ¼ 2.
21
22
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 11. Average heat transfer coefficient with different Reynolds numbers and expansion ratio of 2 at Al2O3, CuO, TiO2 volume fractions of 2% and pure water.
Fig. 12. Effect of expansion ratio on average heat transfer coefficient at Reynolds number of 50,000 for Al2O3, CuO, TiO2 volume fractions of 2% and pure water.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
23
Fig. 13. Effect of volume faction of nanofluids on average heat transfer coefficient at expansion ratio of 2 and Reynolds number of 50,000.
Table 2 Thermal performance for using nanofluids with ER ¼ 2 and Re ¼ 50,000. Vol. %
0.5 1 1.5 2
Thermal performance TiO2
CuO
Al2O3
5.3% 6.5% 7.7% 8.6%
6.4% 7.8% 9.3% 10.7%
7% 8.9% 10.5% 12.3%
CuO, TiO2 volume fractions of 2% and pure water are presented in Fig. 11. Increases in average heat transfer coefficient with increases of Reynolds number are observed for all types of nanofluids. The maximum average heat transfer coefficient was observed with Al2O3volume fractions of 2% in comparison to others and good agreement between numerical and experimental results were observed for pure water and Al2O3 nanofluids. Fig. 12 illustrated the effect of expansion ratio on average heat transfer coefficient at Reynolds number of 50,000 for Al2O3, CuO, TiO2 volume fractions of 2% and pure water. The numerical and experimental results showed the increase of average heat transfer coefficient with increased expansion ratio where the highest value of average heat transfer coefficient was obtained for Al2O3 volume fractions of 2% and expansion ratio of 2 compared to others. Effect of volume faction of nanofluids on average heat transfer coefficient at expansion ratio of 2 and Reynolds number of 50,000 are presented in Fig. 13. It can be seen that the increase of volume faction of nanofluids leads to increase in average heat transfer coefficient due to increased thermal conductivity of the base liquid. The maximum improvement of heat transfer was observed at Al2O3 volume fractions of 2% compared with other types of nanofluids.
Generally, the thermal performance at the maximum expansion ratio (ER ¼ 2) is about 35% with using water while thermal performance on using different volume fractions of nanofluids with ER ¼ 2 and Re ¼ 50,000 is presented in Table 2. 4.3. Pressure drop Figs. 14 and 15 have shown the effect of Reynolds number and volume fractions of nanofluids on the average pressure drop at expansion ratio of 2. Generally, the average pressure drop increases linearly with the increase of both the Reynolds number and the volume fractions of nanofluids. The increment of pressure drop at expansion ratio of 2 and Reynolds number of 50,000 for different volume fractions of nanofluids are presented in Table 3 where, the highest increment of pressure drop is obtained at CuO volume fractions of 2% compared with other types of nanofluids. 5. Conclusions The experimental and numerical (using ANASYS FLUENT 14 software package) works were conducted in the turbulent heat transfer and separation nanofluid flow through an annular pipe with sudden expansion. Three dimensional study, based on finite volume method with standard k-ε turbulence model was performed. It is noted that the surface heat transfer coefficient increases with the increase of nanoparticle volume fraction or the Reynolds number. The results have shown that the separation regions formed after the sudden expansions are considerably affected by the surface heat transfer coefficient and pressure drop where the lowest of the pressure drop and the highest thermal performance are detected at the Reynolds number 50,000, 2% Al2O3 water based
24
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25
Fig. 14. Effect of Reynolds number on average pressure drop at expansion ratio of 2.
Fig. 15. Effect of volume fractions of nanofluids on average pressure drop at expansion ratio of 2.
H. Togun et al. / International Journal of Thermal Sciences 117 (2017) 14e25 Table 3 Increment of pressure drop at Reynolds number of 50,000 and expansion ratio. ER ¼ 2. Vol. %
0.5 1 1.5 2
Increment of pressure drop TiO2
CuO
Al2O3
3.5% 5.5% 8% 12%
4% 7.5% 10.5% 15.5%
3% 5% 6.5% 8%
nanofluid at expansion ratio 2 in comparison to others. The enhancement of heat transfer was about 36.6% for pure water at expansion ratio of 2 compared to the heat transfer obtained in straight pipe. Augmentation of heat transfer could be achieved by using nanofluid at the expansion ratio 2 where the total improvements were about 45.2% (TiO2), 47.3% (CuO), and 49% (Al2O3). Moreover, the increase in the pressure drop was about 42% for the pure water at expansion ratio 2 compared with the data of the straight pipe whereas by using nanofluid they were 65.4% (CuO), 62% (TiO2), and 57.6% (Al2O3). Good agreements are obtained between the numerical and the experimental data obtained in the current investigations. Acknowledgements The authors gratefully acknowledge High-Impact Research Grant UM.C/HIR/MOHE/ENG/46, UMRG RP012D-13AET, and the University of Malaya, Malaysia for support in conducting this research. References [1] Togun H, Salman YK, Hakim S, Aljibori Sultan, Kazi SN. An experimental study of heat transfer to turbulent separation fluid flow in an annular passage. Int J Heat Mass Transf 2011;54(4):766e73. [2] Tuqa A, Hussein T, Ariffin MKA, Kazi SN, Badarudin A, Adam NM, et al. Heat transfer and turbulent fluid flow over vertical double forward-facing step. World Acad Sci Eng Technol Int J Mech Aerosp Ind Mechatron Eng 2014;8: 368e72. [3] Togun H, Kazi SN, Badarudin A. A review of experimental study of turbulent heat transfer in separated flow. Aust J Basic Appl Sci 2011;5(10):489e505. [4] Togun H, Tuqa A, Kazi SN, Badarudin A, Ariffin MKA, Togun H, et al. Heat transfer to laminar flow over a double backward-facing step. Int J Mech Aerosp Manuf Ind Sci Eng World Acad Sci Eng Technol 2013;80:117e39. [5] Oon CS, Togun H, Kazi SN, Badarudin A, Zubir MNM, Sadeghinezhad E. Numerical simulation of heat transfer to separation air flow in an annular pipe. Int Commun Heat Mass Transf 2012;39(8):1176e80. [6] Togun H. Effect of laminar separation flow and nanofluids on heat transfer augmentation with passive techniques: a review. Int Commun Heat Mass Transf 2016;77:9e14. [7] Oon CS, Togun Hussein, Kazi SN, Badarudin A, Zubir MNM, Sadeghinezhad E. Computational simulation of heat transfer to separation fluid flow in an annular passage. Int Commun Heat Mass Transf 2013;46:92e6. [8] Togun H, Jassim Shkarah Ahmed, Kazi SN, Badarudin A. CFD simulation of heat transfer and turbulent fluid flow over a double forward-facing step. Math Probl Eng 2013;2013:10. [9] Togun H, Abdulrazzaq Tuqa, Kazi SN, Badarudin A, Kadhum AAH, Sadeghinezhad E. A Review of studies on forced, natural and mixed heat transfer to fluid and nanofluid flow in an annular passage. Renew Sustain Energy Rev 2014;39. [10] Safaei MR, Vafai K, Kazi SN, Badarudin A. Investigation of heat transfer enhancement in a forward-facing contracting channel using FMWCNT nanofluids. Numer Heat Transf Part A 2014:1e20.
25
[11] Togun H, Abdulrazzaq Tuqa, Kazi ABSN, MKA A. Numerical study of turbulent heat transfer in annular pipe with sudden contraction. Appl Mech Mater 2014;465e466:461e6. [12] Santosh Christopher D, Kanna PR, Madhusudhana GR, Venkumar P, Mohammed HA. Numerical investigation of heat transfer from a twodimensional sudden expansion flow using nanofluids. Numer Heat Transf Part A Appl 2012;61(7):527e46. [13] Kanna PR, Das MK. Heat transfer study of two-dimensional laminar incompressible wall jet over backward-facing step. Numer Heat Transf Part A Appl 2006;50(2):165e87. [14] Abu-Nada E. Application of nanofluids for heat transfer enhancement of separated flows encountered in a backward facing step. Int J Heat Fluid Flow 2008;29(1):242e9. [15] Kherbeet AS, Mohammed HA, Salman BH. The effect of nanofluids flow on mixed convection heat transfer over microscale backward-facing step. Int J Heat Mass Transf 2012;55(21e22):5870e81. [16] Al-aswadi AA, Mohammed HA, Shuaib NH, Campo A. Laminar forced convection flow over a backward facing step using nanofluids. Int Commun Heat Mass Transf 2010;37(8):950e7. [17] Amirhossein Heshmati HAM, Parsazadeh Mohammad, Fathinia Farshid, Wahid Mazlan A, Sies Mohsin M, Saat Aminuddin. Effect of vertical baffle installation on forced convective heat transfer in channel having a backward facing step. Appl Mech Mater 2013;388:169e75. [18] Mohammed Hussein A, Munisamy MGKM, Wahid Mazlan A. Combined convection heat transfer of nanofluids flow over forward facing step in a channel having a blockage. Appl Mech Mater 2013:185e91. [19] Kherbeet AS, Mohammed HA, Munisamy KM, Salman BH. The effect of step height of microscale backward-facing step on mixed convection nanofluid flow and heat transfer characteristics. Int J Heat Mass Transf 2014;68(0): 554e66. [20] Mohammed HA, Al-Aswadi AA, Abu-Mulaweh HI, Hussein AK, Kanna PR. Mixed convection over a backward-facing step in a vertical duct using nanofluids ;Buoyancy opposing case. J Comput Theor Nanosci 2014;11(3): 860e72. [21] Mohammed HA, Al-aswadi AA, Abu-Mulaweh HI, Shuaib NH. Influence of nanofluids on mixed convective heat transfer over a horizontal backwardfacing step. Heat Transf. Asian Res 2011;40(4):287e307. [22] Mohammed HA, Al-aswadi AA, Shuaib NH, Saidur R. Convective heat transfer and fluid flow study over a step using nanofluids: a review. Renew Sustain Energy Rev 2011;15(6):2921e39. http://dx.doi.org/10.1016/j.rser.2011.02. 019. [23] Mohammed HA, Al-aswadi AA, Yusoff MZ, Saidur R. Buoyancy-assisted mixed convective flow over backward-facing step in a vertical duct using nanofluids. Thermophys Aeromech 2012;19(1):33e52. [24] Kherbeet ASh, Mohammed HA, Munisamy KM, Saidur R, Salman BH, Mahbubul IM. Experimental and numerical study of nanofluid flow and heat transfer over microscale forward-facing step. Int Commun Heat Mass Transf 2015;57:319e29. [25] Rashidi MM, Erfani E. The modified differential transform method for investigating Nano boundary- layer over stretching surfaces. Int J Numer methods heat fluid flow 2011;21(7):864e83. [26] Rashidi MM, Fridoonimehr N, Hosseini A, Anwar Beg O, Hung T-K. Homotopy simulation of nanofluids dynamics from a Nono-linearly stretching isothermal permeable sheet with transpiration. Meccanica 2014;49(2):469e82. [27] Rashidi MM, Vishnu Ganesh N, Abdul Hakeem AK, Ganga B. Buoyancy effect on MHD flow of nanofluids over a stretching sheet in the presence of thermal radiation. J Mol Liq 2014;198:234e8. [28] Sano Masatoshi SI, Sakuraba Kenichiro. Control of turbulent channel flow over a backward-facing step by suction. J Fluid Sci Technol 2009;4(1):188e99. [29] Duangthongsuk W, Wongwises S. An experimental study on the heat transfer performance and pressure drop of TiO2-water nanofluids flowing under a turbulent flow regime. Int J Heat Mass Transf 2010;53(1e3):334e44. [30] Farajollahi B, Etemad SG, Hojjat M. Heat transfer of nanofluids in a shell and tube heat exchanger. Int J Heat Mass Transf 2010;53(1e3):12e7. [31] Peyghambarzadeh SM, Hashemabadi SH, Naraki M, Vermahmoudi Y. Experimental study of overall heat transfer coefficient in the application of dilute nanofluids in the car radiator. Appl Therm Eng 2013;52(1):8e16. [32] Saidur R, Leong KY, Mohammad HA. A review on applications and challenges of nanofluids. Renew Sustain Energy Rev 2011;15(3):1646e68. [33] Holman JP. Heat transfer. ninth ed. New York: McGraw Hill; 2002. [34] Togun H, Abu-Mulaweh HI, Kazi SN, Badarudin A. Numerical simulation of heat transfer and separation Al2O3/nanofluid flow in concentric annular pipe. Int Commun Heat Mass Transf 2016;71:108e17.