Experimental Investigations on Augmentation of

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(ex: tube inserts) do not require any direct input of external power. Hence many researchers preferred passive heat transfer enhancement techniques for their ...
International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 8, August 2013)

Experimental Investigations on Augmentation of Turbulent Flow Heat Transfer in A Horizontal Tube Using Square Leaf Inserts S. Naga Sarada1, P. Ram Reddy2, Gugulothu Ravi3 1

Professor, 3Lecturer, Department of Mechanical Engineering, JNTUH College of Engineering, Hyderabad, A.P, INDIA 2 Director, Malla Reddy Institute of Engineering and Technology, Hyderabad, A.P, INDIA Smith and Promvonge [1] conducted experiments on turbulent flow heat transfer enhancement for air in a horizontal tube using diamond shaped inserts. Dean and Peter [2] used stainless steel pall rings for insertion into a copper tube of 20 mm inside diameter. Sibel et al. [3] conducted experiments using equilateral triangle crosssectioned coiled wire inserts inside a horizontal tube. The use of coiled wire inserts led to a considerable increase in heat transfer and pressure drop over the smooth tube. Hsieh et al. [4, 5] conducted experimental studies on heat transfer and flow characteristics for turbulent flow of air in a horizontal circular tube with strip - type inserts. Eiamsa et al. [6] conducted experimental investigations on heat transfer enhancement in a tube fitted with combined devices i.e. twisted tape and wire coil. Sivashanmugam and Suresh [7] performed experimental investigations of heat transfer and friction factor characteristics of circular tube fitted with full - length helical screw elements of different twist ratios under uniform heat flux condition. The experimental investigations conducted by previous researchers revealed that, Nusselt number can be increased by employing inserts inside a horizontal tube at the expense of a reasonable pressure drop. Experimental results were obtained for copper inserts namely: 300 BW, 600 BW, 900, 600 FW and 300 FW squareleaf inserts. The square leaves are welded to the copper core rod of 2 mm diameter as shown in fig. 1. The length of all inserts is same as that of the test section with the distance between two adjacent leaves equal to 50 mm.

Abstract— The present work deals with the results of the experimental investigations carried out on augmentation of turbulent flow heat transfer in a horizontal tube by the means of tube inserts, with air as working fluid. Experiments were carried out initially for the plain tube. Nusselt number and friction factor obtained experimentally were validated against those obtained from theoretical correlations. Secondly experimental investigations using five kinds (900, 600FW, 600 BW, 300 FW, 300 BW) of louvered square leaf inserts were carried out to estimate the enhancement of heat transfer rate for air in the presence of insert. Nusselt number and pressure drop increased, overall enhancement ratio is calculated to determine the optimum geometry of tube insert. Keywords—Heat transfer, enhancement, pressure drop, louvered square leaf inserts.

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I. INTRODUCTION Conventional resources of energy are depleting at an alarming rate, which makes future sustainable development of energy use very difficult. As a result, considerable emphasis has been placed on the development of various augmented heat transfer surfaces and devices. Heat transfer augmentation techniques are generally classified into three categories namely: active techniques, passive techniques and compound techniques. Passive heat transfer techniques (ex: tube inserts) do not require any direct input of external power. Hence many researchers preferred passive heat transfer enhancement techniques for their simplicity and applicability for many applications. Tube inserts present some advantages over other enhancement techniques, such as they can be installed in existing smooth tube that exchanger, and they maintain the mechanical strength of the smooth tube. Their installation is easy and cost is low. It relatively easy to take out for cleaning operations too.

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 8, August 2013) An orifice plate in the pipe connects to a U-tube water manometer to measure the airflow rate. Pressure tapings at each end of the test section connect to another manometer to measure the pressure drop across the test section. Two pressure gauges (which read in mm of water column) are also attached to verify the pressure drop noted by manometer. The test section discharges into atmosphere. III. DATA COLLECTION AND HEAT TRANSFER CALCULATIONS A total of 20 test runs were performed with mass flow rate of air varying from 0.0033 to 0.0059 kg/sec including plain flow and the flow in the presence of inserts. The blower was started and the airflow rate was adjusted by operating the control valve, so that the desired difference in manometer level (50 mm, 75 mm, 100 mm, 125 mm and 150 mm) was obtained. Fig 1: Louvered square leaf inserts 300BW, 600BW and 900, 600 FW, 300 FW (left to right)

II. EXPERIMENTAL DETAILS Experimental setup The schematic diagram of the open loop experimental setup is shown in fig.2. The loop consisted of a blower unit fitted with a tube in horizontal orientation. The blower fan runs at a constant speed and draws air through a control valve. The air then moves into the U -shaped pipe. It is connected to a smaller diameter, insulated and electrically heated copper test section of length 610 mm and 27.5 mm inner diameter. Nichrome bend heater encloses the test section to cause electric heating. The control valve in the U-shaped pipe controls the airflow rate into the test section. Power input to the test tube heater is varied using a variable transformer, which is used to vary the voltage of the AC current passing through the heater and by keeping the current less than 2A. Two thermocouples are placed one at the entrance and the other at the exit of the test section to measure air inlet and outlet temperatures (T1 and T6) respectively. Other thermocouples measure the temperatures at various points along the test tube wall. J - Type thermocouples were used to measure the temperatures. Thermocouples were installed in holes drilled into the backside of the tube walls to within 2 mm of the surface. A digital indicator on the control panel displays thermocouple temperature readings. The outer surface of the test section was well insulated to minimize convective heat loss to the surroundings.

Fig 2: Schematic diagram of Experimental setup

The test section was heated to 63 W by adjusting the dimmer stat. Steady state temperatures from T 1 to T6 are tabulated. All thermo physical properties of air are determined at the bulk temperature of air.

T1  T6 2 T T T T Ts  2 3 4 5 4 Tb 

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(1) (2)

International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 8, August 2013)

d

 w hw a

Cd Ap A0 2 ghair Ap2  A02

d Ap UD Re   C p Pr  K Q  mC p T6  T1 

U



The results of present work reasonably agree within + 11% and +7.5 % with the correlation values of Dittus Boelter and Blasius for Nusselt number and friction factor respectively.

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Qr  A c Ts4  Tb4 Q  Qr h ATs  Tb  hD Nu  K pexp f  LQaU 2 2D Nu DiccusBoelter  0.023 Re 0.8 Pr 0.4

f Blasius  0.079 Re 0.25

Nu i   Nu 1  fi  3    f 

(8) (9) (10) Fig 3: Verification of Nusselt number of plain tube

(11)

0.4 Variation of friction factor with mass flow rate for plain tube 0.35

(12)

0.3

(13) (14)

friction factor

hair 

(15)

0.25 fthe

0.2

fexp

0.15 0.1 0.05

IV. VALIDATION TEST

0 0.002

The Nusselt number and friction factor determined from experimental data are compared with the values obtained from the correlations of Dittus- Boelter for the Nusselt number and Blasius correlation for the friction factor. Comparison between present experimental work and standard correlations for for Nusselt number and friction factor turbulent internal flow is presented in fig. 3 and 4 respectively.

0.003 0.004 0.005 Mass flow rate(Kg/sec)

0.006

Fig 4: Verification of friction factor of plain tube

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 8, August 2013) V. RESULTS AND DISCUSSION

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Nusselt number

30

Friction factor

60º BW

0.4 0.3 0.2 0.1 0 0.002

0.003 0.004 0.005 Mass flow rate (Kg/sec)

0.006

Fig 6: Variation of friction factor with mass flow rate for all inserts

The overall enhancement ratio gradually decreased with the rise of mass flow rate as observed in fig. 7. This is used to determine the quality of enhancement technique. Overall enhancement ratio is observed highest for 600 BW insert. The maximum overall enhancement ratios are 1.30, 1.32, 1.34, 1.02 and 0.79 for 900, 600 FW, 600 BW, 300 FW, 300 BW inserts respectively. Although Nusselt number increases w. r. to plain tube is less for 60 0 BW insert compared to that of 900 insert, as the obstruction to air flow is less in this case, it might have caused the increase of overall enhancement ratio. Also for 300 FW & 300 BW inserts, both Nusselt numbers and friction factors are less. Hence overall enhancement ratios are also less for 30 0 inserts. Hence, among all inserts in the present work, 600 BW inserts are found to be optimum and they are suggested as a viable enhancement device as its overall enhancement ratio is greater than that of other inserts in the present work.

90º 60º FW 60º BW

20 10 0.002

60º FW

0.5

60

40

90º

0.6

Variation of Nusselt number in the presence of inserts

50

Variation of Friction factor in the presence of inserts

0.7

In the present work, experimental investigations on turbulent flow heat transfer enhancement for air inside the horizontal tube in the presence of five types of inserts (900, 600 forward flow, 600 backward flow, 300 forward flow, 300 backward flow) are carried out. Figure 5 shows the variation of Nusselt number with mass flow rate for all inserts in comparison to plain tube. Nusselt number increased with the rise of mass flow rate as shown in the figure. It is observed that 900 insert yielded the highest value of Nusselt number. This may be due to better turbulence created on air side in the presence of 900 insert which increased the heat carrying capacity of air that led to increase of Nusselt number. Due to more resistance offered to air flow, Nusselt number could not be obtained for higher flow rates for the case of 900 insert.

0.004 0.006 Mass flow rate (Kg/sec)

Fig 5: Variation of Nusselt number with mass flow rate for all inserts

Figure 6 shows the variation of friction factor with mass flow rate for all inserts in comparison to plain tube. As shown in the figure, friction factor decreased with the rise of mass flow rate. Friction factor is also observed to be highest for 900 insert and lowest for 300 BW insert. This may be due to highest obstruction caused to air flow in the presence of 900 insert.

1.4

Variation of overall enhancement ratio with mass flow rate

1.3 Overall enhancement ratio

1.2 90º

1.1 1

60º FW

0.9

60º BW

0.8 0.7 0.6 0.5 0.002

0.003 0.004 0.005 mass flow rate(Kg/sec)

0.006

Fig 7: Variation of overall enhancement ratio with mass flow rate

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International Journal of Emerging Technology and Advanced Engineering Website: www.ijetae.com (ISSN 2250-2459, ISO 9001:2008 Certified Journal, Volume 3, Issue 8, August 2013) (hDh/K) 𝜟pexp Experimental Pressure drop for plain tube Pr Prandtl number Q total heat transferred to air (Qc + Qr), (W) Qc heat transferred to air by convection, (W) Qr heat transferred to air by radiation, (W) Re Reynolds number, (UD/ν) T1 air temperature at inlet, (°C) T2, T3, T4, T5 - tube wall temperatures, (°C) T6 air temperature at outlet, (°C) Tb bulk temperature, (°C) Ts Average of tube wall temperatures, (°C) U air velocity through test section, (m/sec) ρa Density of air, (kg/ m3) ρw Density of water, (kg/m3) εC emissivity of copper σ Stefan-Boltzmann’s constant

VI. CONCLUSIONS Experimental investigations were performed to investigate the friction factor and heat transfer characteristics of air in an externally heated horizontal tube fitted with louvered square leaf inserts in comparison to plain tube. With the increase in mass flow rate, Nusselt number increased and friction factor decreased. Nusselt number increased by a maximum of 128.39%, 121%, 81.31%, 30.03% and 32.72 % in the presence of 900, 600 forward, 600 backward, 300 forward, 300 backward square leaf inserts respectively. Friction factor increased by a maximum of 441.31%, 369.17%, 143.43%, 116.48% and 80.39%. in the presence of 900, 600 forward, 600 backward ,300 forward, 300 backward square leaf inserts respectively. Maximum overall enhancement ratios were 1.30, 1.32, 1.34, 1.02 and 0.79 for 900, 600 FW, 600 BW, 300 FW, 300 BW inserts respectively. Based on our experimental investigations it is observed that overall enhancement ratio is highest for louvered square insert - 600BW. This may be due to better turbulence given to air by this insert, which increased the contact of air with tube wall that led to enhancement of heat transfer rates. Moreover, friction factor w. r. to plain tube is also not very high. Also the inclined angle (600) of square strip with core rod could effectively drive the air flow towards the externally heated tube wall. Hence, the overall enhancement ratio is highest for 600 BW insert and it is a viable alternative that can be recommended.

REFERENCES Smith Eiamsa-ard, Pongjet Promvonge, 2010, “Thermal characterization of turbulent tube flows over diamond-shaped elements in tandem”, International Journal of Thermal Sciences 49 (6), pp. 1051-1062. [2] Dean Burfoot and Peter Rice, 1982, “Turbulent forced convection heat transfer enhancement using pall rings in a circular duct”, Ind. Eng. Chem. Process Des. Dev., 21 (4), pp. 646–650. [3] Sibel Gunes, Veysel Ozceyhan, Orhan Buyukalaca, 2010, “Heat transfer enhancement in a tube with equilateral triangle cross sectioned coiled wire insert”, Experimental Thermal Fluid Science, 34(6), pp.684-691. [4] Hsieh, I.W.Huang, 2000, “Heat transfer and pressure drop of laminar flow in horizontal tubes with/without longitudinal inserts”, Journal of Heat Transfer, 122, pp.465-475. [5] Hsieh, Ming-Ho Liu, Huang-Hsiu Tsai, 2003, “Turbulent Heat transfer and flow characteristic in a horizontal circular tube with strip-type inserts”, Part-II (Heat transfer), International Journal of Heat and Mass Transfer, 46, pp. 837-849. [6] Eiamsa-ard, Nivesrangsan, Chokphoemphun. Promvonge, 2010, “Influence of combined non-uniform wire coil and twisted tape inserts on thermal performance characteristics”, International Communications in Heat and Mass Transfer, 37, pp. 850–856. [7] Sivashanmugam, Suresh, 2006, “Experimental studies on heat transfer and friction factor characteristics of laminar flow through a circular tube fitted with helical screw- tape inserts”, Applied Thermal Engineering, 26, pp.1990-1997. [8] Smith Eiamsa-ard, Somsak Pethkool, Chinaruk Thianpong and Pongjet Promvonge, 2008, “Turbulent flow heat transfer and pressure loss in a double pipe heat exchanger with louvered strip inserts”, International Communications in Heat and Mass Transfer, 35, pp. 120-129. [9] Necati Ozisik, 2007. Heat transfer, A basic Approach, Mc.Graw-Hill Book Company. [10] Naga Sarada.S., Raju A.V.S, Kalyani. K. Radha, 2009, “Experimental investigations in a circular tube to enhance turbulent heat transfer using mesh inserts”, ARPN Journal of Engineering and Applied Sciences, 4(5), pp. 53-60. [1]

Nomenclature A convective heat transfer area (πDL), (m2) 2 A0 Area of orifice, (m ) Ap test section inner tube area (ΠD2 /4), (m2) BW Backward flow direction Cd coefficient of discharge Cp Specific heat of air, (J/kg K) d air flow rate through test section, (m3/sec) D Inner diameter of test section, (m) Dh Hydraulic diameter (4A/P) f friction factor for plain tube (experimental) fi friction factor obtained in the presence of tube inserts FW forward flow direction h convective heat transfer coefficient, (W/m2K) hair Equivalent height of air column, (m) hw manometer level difference, (m) K thermal conductivity, (W/mK) L length of test section, (m) m. mass flow rate of air, (kg/sec) Nu Nusselt number for plain tube, (hD/K) Nui Nusselt number obtained using inserts, 424