Research ArticleHeat Transfer of Nanofluid in a Double Pipe Heat Exchanger
Reza Aghayari,1 Heydar Maddah,2 Malihe Zarei,1
Mehdi Dehghani,3 and Sahar Ghanbari Kaskari Mahalle4
1 Department of Chemical Engineering, Shahrood Branch, Islamic Azad University, Shahrood 36199-43189, Iran2Department of Chemistry, Sciences Faculty, Arak Branch, Islamic Azad University, Arak, Iran3Department of Chemical Engineering, Damghan Branch, Islamic Azad University, Damghan 36716-39998, Iran4Department of Chemistry, Saveh Branch, Islamic Azad University, Saveh 39197-15179, Iran
Correspondence should be addressed to Reza Aghayari; [email protected]
Received 7 February 2014; Revised 10 June 2014; Accepted 6 July 2014; Published 10 November 2014
This paper investigates the enhancement of heat transfer coefficient and Nusselt number of a nanofluid containing nanoparticles(𝛾-AL
2O3) with a particle size of 20 nm and volume fraction of 0.1%–0.3% (V/V). Effects of temperature and concentration of
nanoparticles on Nusselt number changes and heat transfer coefficient in a double pipe heat exchanger with counter turbulentflow are investigated. Comparison of experimental results with valid theoretical data based on semiempirical equations shows anacceptable agreement. Experimental results show a considerable increase in heat transfer coefficient and Nusselt number up to19%–24%, respectively. Also, it has been observed that the heat transfer coefficient increases with the operating temperature andconcentration of nanoparticles.
1. Introduction
The addition of solid particles into heat transfer media haslong been known as one of the useful techniques for enhanc-ing heat transfer, although a major consideration when usingsuspended millimeter- or micrometer-sized particles is thatthey have the potential to cause some severe problems, suchas abrasion, clogging, high pressure drop, and sedimentationof particles. Compared to heat transfer enhancement throughthe use of suspended large particles, the use of nanoparticlesin the fluids exhibited better properties relating to the heattransfer of fluid. This is because nanoparticles are usuallyused at very low concentrations and nanometer sizes. Theseproperties prevent the sedimentation in the flow that mayclog the channel. From these points of view, there havebeen some previous studies conducted on the heat transferof nanoparticles in suspension. Since Choi wrote the firstreview article on nanofluids [1], Nguyen et al. [2] investigatedthe heat transfer coefficient and fluid flow characteristic ofAl2O3nanoparticles dispersed in water flowing through a
liquid cooling system of microprocessors under turbulentflow condition. The results revealed that the nanofluid gave a
higher heat transfer coefficient than the base liquid and thenanofluid with a 36 nm particle diameter gave higher heattransfer coefficient compared to the nanofluid with a 47 nmparticle diameter. He et al. [3] reported an experimental studythat investigated the heat transfer performance and flow char-acteristic of TiO
2-distilled water nanofluids flowing through
a vertical pipe in an upward direction under a constant heatflux boundary condition in both a laminar and a turbulentflow regime. Their results showed that at a given Reynoldsnumber and particle size, the heat transfer coefficient is raisedwith increasing nanoparticle concentration in both laminarand turbulent flow regimes. Similarly, heat transfer coefficientwas not sensitive to nanoparticle size at a given Reynoldsnumber and particle size. Moreover, the results indicated thatthe pressure drop of the nanofluids was very close to that ofthe base fluid.
2. Experimental
2.1. Experimental Setup. Experimental apparatus used in thisstudy is depicted in Figure 1. The apparatus consists of atest section (heat exchanger), two tanks, two magnetic gear
Hindawi Publishing CorporationInternational Scholarly Research NoticesVolume 2014, Article ID 736424, 7 pageshttp://dx.doi.org/10.1155/2014/736424
2 International Scholarly Research Notices
Test tube
PC computer
Data logger with thermocouple type K
Coldwatertank
Nanofluidtank PumpCheck valve
Ballvalve
Heater
Double pipe heat exchanger
U tube manometer
Nanofluid inlet
Rotameter
Control valve
Ballvalve
Pump
Checkvalve
Rota
met
er
Control valve
Ballvalve
RTD 100 RTD 100
RTD 100
Nanofluidoutlet
(a)
(b) (c)
(d) (e)
Figure 1: Experimental setup.
pumps, and a pump for transporting nanofluid as the hot fluidand the other for the cold water. The test section is a countercurrent double pipe heat exchanger with the length of 120 cm.In this exchanger, the nanofluid flows into the pipe and coldwater in the annular space of the pipe. The inside pipe ismade of a soft steel tube with the inner diameter of 6mm,outer diameter of 8mm, and thickness of 2mm while theoutside pipe is of steel tube with the inner diameter of 14mm,outer diameter of 16mm, and thickness of 2mm. To reducethe heat loss along the axis, the top and bottom of the testsection are insulated with the plastic tubes. To measure theinlet and outlet temperature of the nanofluid and cold water
at the inlet and outlet of the test section, 4 RTD thermometersare used. It is necessary to measure the temperature at sixstations altogether at the outer surface of the test section forfinding out the average Nusselt number. All six evaluatedtemperature probes are connected to the data logger sets.The pressure drops across the test section are measured byusing inclined U-tube manometers. The 15-liter tanks madeof stainless steel are used for the storage of nanofluid and coldwater. Tomaintain the temperature of the fluid, a cooling tankand a thermostat are used. An electric heater and a thermostatinstalled on it are used to maintain the temperature of thenanofluid. Measured Nusselt number error depends on the
International Scholarly Research Notices 3
Table 1: Physical properties of the nanoparticles.
Types of nanoparticles Nanosized particles Special surface Percent purity Appearance The apparent density𝛾-Al2O3 20 nm >160m2/g +99 White powder 0.9 g/m3
Certificate of analysis (content of elements)Co M N Cl V Ca Al2O3
measurement of the temperature and the flows of the coldwater and nanofluid. During the test, the wall temperature ofthe test section, the mass flow rate, and the inlet and outlettemperatures of the nanofluid and cold water are measured.
2.2. Nanofluid Preparation. The nanofluid used in the exper-iment was 99.0+% pure aluminum oxide predispersed inwater, with an average particle size of 20 nm. The nanofluidwas mixed with deionized water. To prepare experimentalconcentrations, nanofluids with less than 4% nanoparticleswere found to be stable and the stability lasted over a week;no intermediate mixing was considered necessary (Table 1).
2.3. Data Processing. The experimental data were used tocalculate overall heat transfer coefficient, convective heattransfer coefficient, and Nusselt number of nanofluids withvarious particle volume concentrations and Peclet numbers.For fluid flows in a concentric tube heat exchanger, the heattransfer rate of the hot fluid (nanofluid AL
where 𝑚∘ is the mass flow rate of the water (cold fluid) and
𝑇in and 𝑇out are the inlet and outlet temperatures of the water(cold fluid), respectively.
The effective density of nanofluid is
𝜌nf = (1 − 𝜑𝑉
) 𝜌𝑓
+ 𝜑𝑉
𝜌𝑝. (3)
Subscripts 𝑓, 𝑝, and nf refer to the base fluid, the nanopar-ticles, and the nanofluid, respectively. 𝜑
𝑉is the nanoparticle
volume concentration. 𝐶𝑝nf
is the effective specific heat of thenanofluid which can be calculated from Xuan and Roetzelrelation [4]:
(𝜌𝐶𝑝)nf
= (1 − 𝜑𝑉
) (𝜌𝐶𝑝)𝑓
+ 𝜑𝑉
(𝜌𝐶)𝑝. (4)
The heat transfer coefficient of the test fluid, ℎ𝑖, can be
calculated as follows [5]:
1
𝑈𝑖
=1
ℎ𝑖
+𝐷𝑖Ln (𝐷
𝑜/𝐷𝑖)
2𝑘𝑤
+𝐷𝑖
𝐷𝑜
+1
ℎ𝑜
, (5)
where 𝐷𝑖and 𝐷
𝑜are the inner and outer diameters of tubes,
respectively,𝑈𝑖is the overall heat transfer coefficient based on
the inside tube area, ℎ𝑖and ℎ
𝑜are the individual convective
heat transfer coefficients of the fluids inside and outside thetubes, respectively, and 𝑘
𝑤is the thermal conductivity of the
tube wall. 𝑈𝑖is given by
𝑄 = 𝑈𝑖𝐴𝑖Δ𝑇lm, (6)
where 𝐴𝑖
= 𝜋𝐷𝑖𝐿 and Δ𝑇lm is the logarithmic mean
temperature difference. The outside heat transfer coefficientcan be computed by Bell’s procedure [6]. Nusselt number ofnanofluids is defined as follows.
The convection heat transfer from the test section can bewritten by
where the effective thermal conductivity (𝑘nf) of the nanoflu-ids can be evaluated by Maxwell’s model that is given asfollows [7]:
𝑘nf = 𝑘𝑓
𝑘𝑝+2𝑘𝑓
− 2𝜑𝑉
(𝑘𝑓
− 𝑘𝑝)
𝑘𝑝+2𝑘𝑓
+ 𝜑𝑉
(𝑘𝑓
− 𝑘𝑝)
. (9)
Maxwell’s formula shows that the effective thermal conduc-tivity of nanofluids (𝑘nf) relies on the thermal conductivity ofspherical particles (𝑘
𝑝), the thermal conductivity of base fluid
(𝑘𝑓), and volume concentration of the solid particles (𝜑
𝑉).
4 International Scholarly Research Notices
1000
1500
2000
2500
3000
3500
4000
8000 14000 20000 26000 32000Reynolds number
Experimentaleory
Ui
(ove
rall
heat
tran
sfer
coe
ffici
ent)
T = 50∘C
Figure 2: Comparison between the measured overall heat transfercoefficient and predicted values for distilled water.
3. Results and Discussion
To evaluate the accuracy of the measurements, experimentalsystem was tested with distilled water before measuringthe convective heat transfer of nanofluids. Figure 2 showsthe comparison between the measured overall heat transfercoefficient and prediction of (5) in which ℎ
𝑖is evaluated by
Gnielinski correlation for turbulent flow through a tube [8]:
Nu = 0.012 (Re0.87 − 280)Pr0.4. (10)
As shown in Figure 2, the good agreement exists between theexperimental data and predicted values.
3.1. The Convective Heat Transfer of the Nanofluid. Figure 3shows the overall heat transfer coefficient of aluminumoxide nanofluid and water in terms of the Reynolds numberat different volume concentrations show. The results showthe increase of the overall heat transfer coefficient withthe Reynolds number and temperature of the nanofluid.Compared to the base fluid, the heat transfer coefficient ofaluminum oxide nanofluid increases with the increase ofconcentration in a fixed Reynolds number. The overall heattransfer coefficient is found to be the highest for aluminumoxide nanofluid at the concentration of 0.3 and a Reynoldsnumber of about 27000, increasing up to 5 and 9.2% at thetemperatures of 35 and 40∘C compared to the base fluid. Forwater, this value is 4.6 and 6.82 percent for the temperatures of35 to 40∘C (Reynolds number of 27000 and concentration of0.1).This increase in the convective heat transfer coefficient isalso observed in Figure 4. For example, this value increases to24.12 and 32.20% for the temperatures of 35–40∘C comparedto the base fluid (the concentration of 0.3 and Reynoldsnumber of 27000). For water, this amount is 21.3 and 24.35percent at the same Reynolds number and the concentrationof 0.1. As seen in Figure 3, the overall heat transfer coefficient
increases with the increase of Reynolds number.The possiblereasons for this increase may be as follows:
(1) a nanofluid with suspended nanoparticles whichincreases the thermal conductivity of the mixture,
(2) high energy exchange process, which is resultedfrom the amorphous movement of the nanoparticles.Comparison of convective heat transfer coefficientbetween the nanofluid and the base fluid shows thatthis value is higher for the nanofluid at the sameReynolds number than the base fluid (Figure 4). Thisresults in the increase of heat transfer efficiencycaused by the increase of thermal conductivity, con-vective heat transfer, and the thinness of thermalboundary layer. Figure 5 shows the effects of tempera-ture and concentration of aluminum oxide nanofluidin terms of the Nusselt number at the temperaturesof 35 and 40∘C, respectively. As can be seen, Nusseltnumber of the nanofluid under the condition of sameReynolds number is greater than the base fluid. Forexample, this value is 19% for the nanofluid witha concentration of 0.3 at the temperature of 35∘Ccompared to the base fluid (the Reynolds numberof 26500). This amount is 25% at the temperatureof 40∘C. This increase can be attributed to the ther-mal conductivity. There are several mechanisms toincrease the thermal conductivity of the nanofluid:the formation of the liquid layer on the surface ofthe nanoparticles, Brownian motion, classification ofparticles, the transmission of the phonons projectilesin the nanoparticles, and the increase of the ther-mal conductivity of fluids with the increase of thenanoparticles in the pipe wall. The increase in thethermal conductivity can increase the heat transfercoefficient in the thermal boundary layer near thetube wall. Temperature is one of the factors increasingthe thermal conductivity of the nanofluid and therebyincreasing the heat transfer coefficient and Nusseltnumber. Experimental results indicate that the effectsof the nanoparticles on the thermal conductivityincrease with the temperature. It is assumed that themain mechanism for the thermal conductivity of thenanofluid is the random motion of the nanoparti-cles. This pseudo-Brownian motion is a function offluid temperature. Thus, the increase in the thermalconductivity is higher for smaller particles than forlarger particles at the high temperatures. Brownianmotion at low temperatures is of less importance andtherefore the difference in the increase of the thermalconductivity between the smaller and larger particlesis reduced.
3.2. Comparison between Experimental Results and AvailableCorrelations. In Figure 6 the experimental results for theNusselt number of 𝛾-Al
2O3/water nanofluid are compared
International Scholarly Research Notices 5
2000
2100
2200
2300
2400
2500
2600
2700
13000 16000 19000 22000 25000 28000Reynolds number
13000 15000 17000 19000 21000 23000 25000 27000 29000Reynolds number
Water
Ui
(ove
rall
heat
tran
sfer
coe
ffici
ent) T = 40
∘C
Nanofluid Al2O3 (0.1%)Nanofluid Al2O3 (0.3%)
(b)
Figure 3: Overall heat transfer coefficient of 𝛾-Al2O3/water nanofluid versus Reynolds number for various volume concentrations (𝑇 = 35∘C,
40∘C).
6000
7000
8000
9000
10000
11000
12000
13000
14000
15000
13000 16000 19000 22000 25000 28000Reynolds number
WaterNanofluid Al2O3 (0.1%)Nanofluid Al2O3 (0.3%)
hi
(con
vect
ive
heat
tran
sfer
coe
ffici
ent)
T = 35∘C
(a)
6000700080009000
100001100012000130001400015000
10000 13000 16000 19000 22000 25000 28000Reynolds number
WaterNanofluid Al2O3 (0.1%)Nanofluid Al2O3 (0.3%)
hi
(con
vect
ive
heat
tran
sfer
coe
ffici
ent)
T = 40∘C
(b)
Figure 4: Convective heat transfer coefficient of 𝛾-Al2O3/water nanofluid versus Reynolds number for different volume concentrations (𝑇 =
35∘C, 40∘C).
with the prediction of Xuan and Li correlation. The corre-lation was provided by Xuan and Li for turbulent flow ofnanofluid inside a tube [9]:
Nunf = 0.0059 (1 + 7.6286𝜑0.6886
𝑉Pe0.001𝑝
)Re0.9238nf Pr0.4nf . (11)
As seen in Figure 6, there is an agreement between theexperimental and calculated values for nanofluid. In thepresent study, aluminum oxide nanoparticles mixed withwater to the volume percent of 0.1–0.3% (V/V) are used toinvestigate the effects of Reynolds number, the temperature ofthe flowing nanofluid, and the nanoparticle concentration onthe heat transfer. Nusselt number increases with the Reynolds
number. The obtained results are consistent with the resultsfrom the relationship between Xuan and Li [9]. The particlePeclet number, Reynolds number, and the Prandtl number fornanofluid are defined, respectively, as
Pe𝑝
=𝑉𝑚
𝑑𝑝
𝛼nf,
Renf =𝑉𝑚
𝐷
𝜗nf,
Prnf =𝜗nf𝛼nf
,
(12)
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50
70
90
110
130
150
170
190
210
15000 17000 19000 21000 23000 25000 27000 29000Reynolds number
T = 35∘C
Nus
selt
num
ber(
Nu i
)
WaterNanofluid Al2O3 (0.1%)Nanofluid Al2O3 (0.3%)
(a)
80
100
120
140
160
180
200
15000 17000 19000 21000 23000 25000 27000 29000Reynolds number
T = 40∘C
Nus
selt
num
ber(
Nu i
)
WaterNanofluid Al2O3 (0.1%)Nanofluid Al2O3 (0.3%)
(b)
Figure 5: Nusselt number of 𝛾-Al2O3/water nanofluid versus Reynolds number for different volume concentrations (35∘C, 40∘C).
80
90
100
110
120
130
140
150
160
170
14000 16000 18000 20000 22000 24000 26000 28000Reynolds number
Figure 6: Comparison between the experimental results and calcu-lated values from correlation (11) for 𝛾-Al
2O3/water nanofluids.
where the thermal diffusivity is given by
𝛼nf =𝑘nf
(𝜌𝐶𝑝)nf
=𝑘nf
(1 − 𝜑𝑓) (𝜌𝐶
𝑝)𝑓
+ 𝜑𝑓
(𝜌𝐶𝑝) 𝑝
. (13)
4. Conclusion
With respect to utilizing nanoparticles in many processes,attention has been focused on the improvement of heat
exchanger efficiency by adding solid particles to heat trans-fer fluids. Many researches have investigated the effect ofnanoparticles on different process parameters like hydrody-namic and thermophysical properties. However, researcheswere seldom performed to evaluate the effect of turbulentnanofluid flow on heat transfer. This study investigatedthe heat transfer enhancement of the nanofluid containingaluminumoxide nanoparticles andwater under the conditionof turbulent flow in a double pipe heat exchanger. Theheat transfer values were measured in the turbulent flow ofa nanofluid containing 20 nm aluminum oxide suspendedparticles with the volume concentration of 0.1–0.3% (V/V) inwater. Properties of nanofluid are good and there is plentyof fluid. Heat transfer coefficient and Nusselt number ofthe nanofluid increase from 15 to 20% compared to thebase fluid according to the comparison on the basis of fixedReynolds number. Experimental results showed the increaseof the average heat transfer coefficient in the turbulent flowregime with the addition of the nanoparticles to the fluid.The obtained results are in agreement with the results fromthe relationship between Xuan and Li [9]. This increase inthe heat transfer coefficient may be due to the high densityof nanoparticles on the wall pipe and the migration of theparticles. The extensive research is needed to understand theheat transfer characteristics of the nanofluid and to obtain theother relations.
Nomenclature
𝐴: Heat transfer area (m2)𝐶𝑝: Specific heat (kJ kg−1 ∘C−1)
Nu: Nusselt number (dimensionless)Pe: Peclet number (dimensionless)Pr: Prandtl number (dimensionless)𝑄: Heat transfer rate (W)Re: Reynolds number (dimensionless)𝑇: Temperature (∘C)𝑈: Overall heat transfer coefficient
(Wm−2 ∘C−1)𝑉: Velocity (m2 s−1).
Greek Symbols
Δ𝑇Im: Logarithmic mean temperaturedifference ( ∘C)
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