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Experimental studies of Al2O3/water nanofluid in double pipe heat exchanger with serrated twisted tape insertM.Raja1Assistant Professor, Department of Mechanical Engineering

Government College of Engineering

Salem, Tamilnadu, India

raaj.nml@gmail.com.M.Venkatesan2 PG Scholar,

Department of Mechanical Engineering

Government College of Engineering

Salem, Tamilnadu, India

venkatesanm1972@gmail.comJ.Vinoth kumar3 PG Scholar, Department of Mechanical EngineeringGovernment College of Engineering

Salem, Tamilnadu, India

Vinothmech1809@gmail.com

Abstract Augmentation of heat transfer in heat exchanging devices plays vital role in present day heat exchanger design. Among the passive heat transfer method one of the efficient technique is the insertation of turbulator. In this study, thermal performance of double pipe heat exchanger with and without inserting serrated twisted tape (STT) using water and Al2O3 / water nanofluid for the Reynolds number range of 5000 to 15000 is analysed. STT pitch and width ratio is 0.2.The results showed that the maximum heat transfer enhancement is about 49%,on the other hand friction factor is increased by 90% for 1% addition of nanoparticles with STT compared to water in plain tube.Keywords Nanofluids; Serrated twisted tape; double pipe heat exchanger; thermal performance.I. Introduction Application of twisted tape in heat exchanger increases the heat transfer rate by creating turbulence in flow region which tends to increase the pressure drop. In order to reduce the pressure drop without affecting the increase in heat transfer rate, we use some modification in twisted tape. Among them we select serrated twisted tape.

Addition of small amount of nano sized particle in conventional base fluids gives much enhanced thermo physical properties. In our experiment 0.25%,0.5% and 1% of Al2O3 / water nanofluids are employed.

II. EXPERIMENTAL SETUP AND PROCEDURE The test section of double pipe heat exchanger is 1.5m. The inner tube is made up of copper material with inner and outer diameter of 33 mm and 38 mm. The outer tube is made up of mild steel with inner and outer diameter of 71 mm and 76 mm. The outer pipe is insulated by wounding the asbestos rope to minimize heat loss.Four digital thermometer with an accuracy of Now the experimental setup is ready, the storage tank is filled with water and nanofluid at desired volume concentration and the reading is taken with and without STT at different flow rates(60ml/sec to 180ml/sec).Three different volume concentration of nanofluid 0.25%,0.5% and 1% are used for conducting experiment. The experimental layout of the system is given in the figure 1.

Figure 1.Experimental layout

Initially readings are taken using water and nanofluid at different concentration. Later the same procedure is carried out by insertion of turbulator in the inner tube with same concentration fluid.Nomenclature

A Area, m2C Specific heat, J/kg K

D Inner diameter of the tube, m

f Friction factor

h Heat transfer coefficient, W/m2K

k Thermal conductivity, W/mK

L Length of the tube, m

m Mass flow rate, kg/s

Nu Nusselt number, hD/k

Pr Prandtl number, C/k

Q Heat flow, WattsRe Reynolds number

T Temperature, C

v Velocity, m/s

Greek symbols

T Temperature difference

p Pressure drop

Volume concentration of nanoparticles, %

Dynamic viscosity, kg/m2s

Density, kg/m3Subscripts

bf Base fluid(water)

c cold

Exp Experimental

h hotnf Nanofluid

o Outlet

Figure 2.Photograph of an experimental setup

Figure 3.Serrated twisted tape

Specification of serrated twisted tape turbulatorWidth of cut w=6mm

Depth of the cut d=6mm

Width ratio /=0.2

Depth ratio /=0.2

Length of tape L = 1500mm

Twist ratio (y/ W) = 4

Width of tape (W) = 30mm

Length of angle of twist(y) = 120mm

Angle of Twist = 180oIII. NANOFLUID PREPARATION AND ITS PROPERTIESNanofluid is prepared by dispersing Al2O3 nanoparticles with an average diameter of supplied (Alfa acer Ltd). in distilled water. The dispersion process was done with ultrasonic vibrator (Toshiba, India).The quantity of nanoparticles required for different volume concentration is calculated by using the following equation given below, (1)

Density of nanofluid

(2)Thermal conductivity (3)Specific heat capacity (4)Viscosity (5)

Sonication is the act of applying sound (usually ultrasound about 30kHz) energy to agitate particles in a sample, for various purposes. In the laboratory, it is usually applied using an ultrasonic bath or an ultrasonic probe, colloquially known as a sonicator. Sonication is the process of converting an electrical signal into a physical vibration that can be directed toward a substance. The vibration has a very powerful effect on solutions, causing their molecules to break apart. The primary part of a sonication device is the ultrasonic electric generator. 3.1 Measurement of heat rate transfer coefficient

Rate of heat flow from hot fluid to the cold fluid is calculated depending on the measurements of inlet and outlet temperatures along with the mass flow rates of hot and cold fluids using the following set of equations:

(6)

(7)

(8)

The heat loss to the surrounding from the hot fluid is of the order of 3%.Experimental heat transfer coefficient for nanofluid with and without insert is calculated based on the Newtons law of cooling and the expression is given below:

(9)

(10)

(11)Dittus - Boelter Equation is given is used to determine the value of Nusselt number,

(12)3.2 Measurement of friction factor

Experimental friction factor is evaluated from the pressure drop measurement along the length of test section and using the following equation

(13)Blasius Equation given below is used to determine the theoretical value of friction factor

(14)IV. EXPERIMENTAL SETUP VALIDATION

Figure 4 shows the variation of Nusselt number with Reynolds number in the plain tube. The data obtained from the experiment has discrepancy of 11% when compared to the values obtained from Dittus- Boelter Equation.

Figure 4.Experimental & Theoretical Nusselt number vs Reynolds number

Flow friction characteristics in terms of friction factor from the experimental setup is compared with standard correlations as shown in Figure5.It depicts ,the experimental value deviates by 19% from the values obtained from Blasius equation.

Figure 5.Experimental & Theoretical friction factor vs Reynolds number

V.EXPERIMENTAL RESULTS AND DISCUSSION

5.1 Nusselt number of nanofluid in a tube

Experiments are further conducted to evaluate the heat transfer coefficient with nanofluid flowing in a tube .The obtained results from the experiment is plotted in the Figure 6 in which Nusselt number of nanofluid at different volume fraction is compared with water at Reynolds numbers range of 5000 to 15000 respectively. The heat transfer enhancement in terms of Nusselt number for 0.25%, 0.5% and 1% volume concentration of nanofluid is of 19%, 25% and 39% compared to that of water for the Reynolds number of 5000. Figure 6.Experimental Nusselt numbers (water, 0.25% nf, 0.5% nf & 1% nf) vs. Reynolds number

While for Reynolds number around 10000, heat transfer in terms of Nusselt number enhances by 22% ,29% and 44% for 0.25%,0.5% and 1% volume concentration nanofluid compared to that of water. At the Reynolds number around 15000, heat transfer enhancement in terms of Nusselt number increases by 25%, 31% and 46% for 0.25%,0.5% and 1% volume concentration nanofluid compared to that of water.

The enhancement in heat transfer is contributed due to the properties of nanofluid in terms of high thermal conductivity and lower specific heat compared to that of water. Due to which they take less heat to attain a particular temperature when compared to the base fluid. The heat transfer enhancement in the Al2O3 nanofluid is also contributed to particle Brownian motion and hence the heat transport capability of Al2O3 nanoparticles will increase further. Al2O3 nanoparticles have large surface area over the coarse grains and this large surface area of nanoparticles improves the heat transfer rate of nanofluid.5.2 Nusselt number of nanofluid in a tube with serrated twisted tape insert.

Further the experiments conducted to calculate heat transfer coefficient with nanofluid flowing in a tube along with STT inserts. The obtained results from the experiment are plotted in the Figure 7.

Figure 7. Experimental Nusselt numbers (water with STT, 0.25% nf with STT, 0.5% nf with STT & 1% nf with STT) vs. Reynolds number

The heat transfer enhancement in terms of Nusselt number for 0.25%, 0.5% and 1% volume concentration nanofluid is of 24%, 28% and 42% compared to that of water for the Reynolds number of 5000.While for Reynolds number around 10000, heat transfer in terms of Nusselt number enhances by 26%, 32% and 47% for 0.25%,0.5% and 1% volume concentration nanofluid compared to that of water. At the Reynolds number around 15000, heat transfer enhancement in terms of Nusselt number increases by 28%, 34% and 49% for 0.25%, 0.5% and 1% volume concentration nanofluid compared to that of water.

It is observed that the use of STT insert enhancement in convective heat transfer coefficient is achieved in both base fluid and nanofluid. The helices developed in the inner tube induce turbulence and swirl in the fluid and cause effective mixing of fluid. The nanoparticles in the vicinity of inner wall of the tube will carry heat energy and transmit the same to the adjacent fluid layers of cold fluid at a higher rate. The helices also increase the retention time of flow in the test section as the fluid has to move in a helical path, consequently increasing the flow path length. This could be another reason which is responsible for enhancement in the performance of heat transfer in nanofluid.This is due to intensification of swirl which promotes convective heat transfer in the nanofluid.

5.3 Friction factor for nanofluid flowing in a plain tube

Experiments are conducted to determine friction factor with different volume concentrations of Al2O3 nanofluid and are shown in Figure 8 along with the friction factor data of water.

It is observed that friction factor increases with increase of volume concentration and Reynolds number. While for Reynolds number around 5000, friction factor enhances by 20%, 36% and 90% for 0.25%, 0.5% and 1% volume concentration nanofluid compared to that of water.

Figure 8.Experimental Friction factor (water, 0.25% nf,0.5% nf & 1% nf) vs. Reynolds number.The friction factor enhancement for 0.25%,0.5% and 1% volume concentration nanofluid is of 16%,28% and 82% compared to that of water for the Reynolds number of 10000. At the Reynolds number around 15000, friction factor increases by 5%, 11% and 56% for 0.25%, 0.5% and 1% volume concentration nanofluid compared to that of water. However, it may not affect the pump performance because of very low volume concentrations considered in the present experiment.5.4. Friction factor for nanofluid flowing in a tube with Serrated twisted tape insert.

Further experiments are conducted to evaluate the friction factor for nanofluid with serrated twisted tape insert inside a tube and the experimental results are shown in figure 9 given below.

The friction factor for 0.25%, 0.5% and 1% volume concentration nanofluid is of 15%, 28% and 46% compared to that of water for the Reynolds number of 5000.While for Reynolds number around 10000, friction factor decreased by 11% ,20% and 33% for 0.25%,0.5% and 1% volume concentration nanofluid compared to that of water. At the Reynolds number around 15000 the friction factor decreased by 7%, 16% and 23% for 0.25%, 0.5% and 1% volume concentration nanofluid compared to that of water.

From the graph it depicts that the friction factor of water with STT in plain tube increases around 40% compared to that of friction factor obtained from water in plain tube. The percentage increase of friction factor in plain tube for all volume concentrations of nanofluid to water is higher than that of percentage increase of friction factor in STT inserted tube.

Figure 9. Experimental friction factor (water, 0.25% nf with STT, 0.5% nf with STT & 1% nf with STT) vs. Reynolds number.

But, the increase in magnitude of nanofluid friction factor with STT inserts is negligible. It is expected that, it may not cause severe penalty on the pumping power of nanofluid into the test section when both STT and higher concentration nanofluid is used due to the increase in pressure drop.5.5. Thermal Performance factor for nanofluid flowing in a tube with serrated twisted tape insert

The thermal performance factor( ), of the test tube fitted with serrated twisted tape is defined as the ratio of heat transfer coefficient for the inserted tube (ht) to that of the plain tube (hp) at the same level of pumping power can be expressed as follows, = ht / hp = (Nut / Nup) / (ft / fp)1/3

Thermal performance factor for the Reynolds number of 5000 at 0.25% volume concentration Al2O3 nanofluid with STT, 0.5% volume concentration Al2O3 nanofluid with STT and 1% volume concentration Al2O3 nanofluid with STT when compared to water in plain tube increases by 22%,26% and 34%.At the Reynolds number of 10000,all the above values increases by 24%,27% and 38% when compared to that of water in plain tube Increase in thermal performance of when compared to water in plain tube increases for the Reynolds number of 15000 is of 26%,30% and 41%.

.Thermal performance factor denotes the actual increase in energy transfer by taking into account of both increase in heat transfer and increase in pumping power. Increase of thermal performance at 1% volume concentration nanofluid is higher compared to that of all other volume concentration. But further increase in volume concentration tends to decrease in thermal performance as the friction factor increases drastically which in turn increases the pressure drop and pumping power.

Figure 10.Thermal performance factor (water, 0.25% nf with STT, 0.5% nf with STT & 1% nf with STT) vs. Reynolds number

VI. CONCLUSIONS

The present study is concerned about heat transfer, friction factor and thermal performance factor of Al2O3/ water nanofluid flowing in circular tube with and without serrated twisted tape insert. The observations from the experimental investigation are given below.1. In plain tube using Al2O3/ water nanofluid of 1% volume concentration, the increase of heat transfer rate in terms of Nusselt number is from 39% to 46% when Reynolds number is altered in the range from 5000 to 15000 compared to water.2. Heat transfer rate in terms of Nusselt number further enhance while using serrated twisted tape. It is observed that in the presence of serrated twisted tape,heat transfer enhancement for 1% volume concentration nanofluid is of 42% to 49% when Reynolds number varies from 5000 to 15000.It clearly shows the heat transfer enhancement due to combined effect of serrated twisted tube and addition nanoparticle.3. Friction factor in plain tube for 1% volume concentration of Al2O3/ water nanofluid increases from 90% to 56% as the Reynolds number varies from 5000 to 15000 on comparison with water .While friction factor in tube with serrated twisted tape for 1% volume concentration of Al2O3/ water nanofluid decreases from 46% to 23% when compared to water with serrated twisted tape.4. Thermal performance factor of 1% volume concentration of Al2O3/ water nanofluid with serrated twisted tape increases from 26% to 41% when Reynolds number varies from 5000 to 15000.5. The addition of Al2O3 nanoparticle increases the heat transfer and at the mean while it introduces pressure drop in the system due to increase in friction factor. So nanoparticle concentration should not be increased beyond 2%.

6. By using serrated twisted tape and nanofluid, we can reduce the size of the heat exchanger.REFERENCES[1] Zan Wu, Lei Wang, Bengt Sundn, Pressure drop and convective heat transfer of water and nanofluids in a double-pipe helical heat exchanger, Applied Thermal Engineering, Vol. 60, Issues 12, pp.266-274, 2013.

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