Experimental Investigation on Heat Transfer By Natural Convection Over A Cylinder for Different...

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International Journal of Research and Innovation (IJRI)


Experimental Investigation on Heat Transfer By Natural Convection Over A Cylinder for Different Orientations

S. Madhava rao1, D. Santha rao2, Dr.S. Rajesh3

1 P.G student, Department Of Mechanical Engineering, B.V.C. Engineering College, Odalarevu-533210, India.2 Associate professor , Department Of Mechanical Engineering, B.V.C. Engineering College, Odalarevu-533210, India.3 Assistant professor , Department Of Mechanical Engineering, S.R.K.R. Engineering College, Bhimavaram-534204, India

*Corresponding Author:

S. Madhava rao , P.G Student, Department Of Mechanical Engineering, B.V.C. Engineering College, Odalarevu, India.

Published: December 16, 2014Review Type: peer reviewedVolume: I, Issue : I

Citation: S. Madhava rao, P.G student (2014) Experimen-tal Investigation on Heat Transfer By Natural Convection Over A Cylinder for Different Orientations

INTRODUCTION

The problem of natural convection heat transfer across a channel of various cross section (rectan-gular , circular , concentric annulus and parallel plates) has received considerable attention in view of its fundamental importance germane to numer-ous engineering application such as electronic sys-tems , chemical process equipments , combustion chambers , environmental control system chemi-cal catalytic reactors, fiber and granular insulation ,packed beds ,petroleum reservoirs ,nuclear waste repositories ,boiler design ,air cooling systems in air conditioners and so on [1-2] .Atayilmaz and Teke [3] studied natural convection heat transfer from a horizontal cylinder experimentally and numerically and concluded that Nusselt numbers increases with increasing Rayliegh numbers. Akeel et al. [4] car-ried out experiments to investigate natural convec-tion heat transfer in an inclined uniformly heated circular cylinder and deduced an empirical equa-tion of average nusselt number as a function of ray-liegh number. Akeel [5] carried out experiments to study the local and average heat transfer by natural convection in a vertical concentric cylinder annu-lus and deduced an empirical equation of average nusselt number as a function of rayliegh number.

Reymond et al. [6] investigated natural convection heat transfer from a single horizontal cylinder and a pair of vertically aligned horizontal cylinders and concluded that spectral analysis of surface heat transfer signals has established the influence of the plume oscillations on the heat transfer H.S.Takhar et al. [7] studied the laminar natural convection boundary layer flow on an isothermal vertical thin cylinder

embedded in a thermally stratified high porosity medium. It is observed that for certain values of the ambient stratification parameters, the skin fric-tion vanishes and the direction of the heat transfer changes. R.Roslan et al. [8] studied the problem of unsteady natural convection induced by a tempera-ture difference between a cold outer square enclo-sure and a hot inner circular cylinder and obtained that the maximum heat transfer augmentation for frequency between 25π and 30π for a high ampli-tude and moderate source radius. Bae and Hun [9] carried out a study on air cooling in an unsteady laminar natural convection in a vertical rectangu-lar channel with three flush mounted heat sources on one vertical wall .The results show the effects of the thermal conditions of the lowest source on the downstream sources . The study emphasizes that the transient temperatures may exceed average values in time This is important for designing elec-tronic equipment projects. Madhavan and Sastri [10] developed a parametric study of natural con-vection in a set of boards inside an enclosure. Each board has heat sources. This layout has direct ap-plication on electronic equipment cooling. It’s noted that the Rayliegh and the Prandtl numbers as well as the boundary conditions strongly affect the fluid flow and heat transfer features. M.M.Molla et al. [11] investigated the effect of radiation on natural convection flow from an isothermal circular cylinder numerically and concluded that the effect of the ra-

Abstract

Experiments were carried out to investigate natural convection heat transfer over uniformly heated hollow cylinder mod-els made of aluminium alloy and pure copper. The effect of surface temperature, heat transfer coefficient and Nusselt’s number with respect to different heat fluxes and different orientations of two hollow cylinders. In the current study the heat fluxes range covers from 124w/m2 to 621 w/m2 . Whereas, the different orientations consider for the present in-vestigation are 00(vertical), 300, 450, 600, 900(horizontal) respectively for conducting experiments on both hollow cylin-ders. Based on the experimental result (surface temperature) the following parameters such as theoretical heat transfer coefficient, experimental heat transfer coefficient and Nusselt number are evaluated and depicted graphically for both hollow cylinders made of aluminium alloy and pure copper.

1401-1402

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diation the skin–friction coefficients as well as the rate of heat transfer increased. Vande Sande and Hamer [12] and Aitsaada et.al.[13]] have obtained empirical correlations for natural convection heat transfer in concentric and eccentric annuli of con-stant heat flux inner cylinder while the outer cylin-der was subjected to the ambient temperature. An empirical equation of average Nusselt number as a function of Rayliegh number was deduced. P.K. Sarma et.al.[14] and M.A.Hossain et.al.[15] have in-vestigated the heat transfer rates from horizontal cylinder surface of an internally heated tube under constant heat flux conditions and the effect of con-duction–radiation on natural convection flow of an optically dense viscous incompressible fluid along an isothermal cylinder of ellipitical cross section. it is found that the rate of heat transfer from the slen-der body is higher than from the blunt body. There are no available literatures concerning the heat transfer by natural convection over a circular cylin-der for different orientations. The present study cov-ers this lack and gives a clear view to actual physi-cal behavior in the heat transfer process by natural convection.

EXPERIMENTAL APPARATUS

The apparatus consist of wooden box with alu-minum alloy and copper hollow cylinders as a test section mounted on a heating coil, analog ammeter (0-2A), analog voltmeter (0-300v), digital tempera-ture indicator (0-4000c), thermocouples, AC con-troller (220/240v) & rotary switch. Aluminum al-loy and copper hollow cylinder pipe with finite wall thickness is exposed to a ambient medium Of air at a constant wall temperature. The thermal con-ditions at a inner wall corresponds to the case of constant heat flux. The test section consist of an aluminium hallow cylinder with a wall thickness of10mm ,inner diameter 40mm,outer diameter 50mm and length of cylinder is 450mm.The cylin-der was heated electrically using an electrical heater which consist of 250kw .It is used to heat external surface with a constant heat. The cylinder surface temperature was measured by 8 thermocouples arranged along the cylinder. Thermocouples were fixed by drilling 8 holes of 0.5mm thickness along the cylinder. The excess material was cleaned care-fully by fine grain paper. The insulation material glass wool was placed in between the holder and cylinder.

All thermocouples are fixed with the help of studs. The distance between these thermocouples are var-ied constantly from bottom to top for both the alu-minum alloy and copper hollow cylinders. The ex-perimental set up developed for the current work for various orientations of cylinder was depicted in the Fig.1 to Fig 10.

EXPERIMENTAL PROCEDURETo carry out the experiments the following proce-dure was followed:

a) The inclination angle of the cylinder was ad-justed as required.b) The electrical heater was switched on and the heater input power then adjusted to give the re-quired heat flux at particular anglec) The apparatus was left at least two hours to establish steady state condition .the thermocou-ple readings were measured every half an hour by means of the digital electronic thermometer until the reading became constant ,a final reading of tem-peratured) Now whole rectangular box is tilted to re-quired angle and wait for half an hour to establish steady state condition and the note down readings .then again change the angles with respective verti-cal and note down the readings,e) The input power to the heater could be in-creased to cover another run in a shorter period of time and to obtain steady state condition s for next heat flux .subsequent runs for other ranges of cyl-inder inclination angles were performed in the same previous procedure.f) during each test run ,the following readings were recorded:> The angle of inclination of the cylinder in de-grees > The readings of thermocouples in degrees centigrade> The heater current in amperes.

Fig. 1 Aluminium alloy hollow Cylinder when Ø = 00

Fig. 2 coppper hollow Cylinder when Ø = 00

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Fig.3 Aluminium alloy hollow Cylinder when Ø = 300

Fig. 4 coppper hollow cylinder when Ø = 300





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Data Analysis

Simplified steps were used to analyze the heat transfer process for the air flow in a cylinder when it surface was subjected to a uniform heat flux. The total input power supplied to the cylinder can be calculatedTotal heat transfer Q = V×I (watt ) (1)Average heat transfer coefficient can be obtained ash = Q / (A*(Ts-T∞)) ( w/m2k) (2)whereTs = average heat transfer coefficient obtained from table (0c)T∞ = ambient temperature ( 0c)A = surface area of cylinder ( m2)h value from empirical correlation taken from heat &mass transfer data bookA. For vertical cylinderNu = 0.59(GrlPr)0.25 for constant heat flux or con-stant wall temperature, When GrlPr < 109 (3)B. For inclined cylinderNuL =[0.60-0.488(sinθ)1.03](GrLcPr)Z for constant heat flux, When GrLcPr < 2 ×108 and Z=0.25+0.083(sinθ)1.75 (4)C. For horizontal cylinder

Nu = C×(Grd Pr)m for constant wall temperatureGrd Pr = 104 to 107 where C = 0.48 m = 0.25 (5)

Results and Discussion

A. Average Temperature Variation

The variations of average surface temperature over uniformly heated hollow cylinder models made of Copper and Aluminium alloy at different heat fluxes and angle of inclinations 00(vertical),300,450,600, 900(horizontal)) was studied on the corresponding graphs are plotted and depicted in figures 23&24. From the plots it was observed that the average sur-face temperatures increases with increase of heat flux for both the hollow cylinders. It was also ob-served that average surface temperatures for hollow cylinders made of copper was better than aluminum alloy cylinder for different heat fluxes and angle of inclinations(moves from vertical to horizontal)

The effect of angle of inclinations on the tem-perature distribution along the cylindrical surfaces for particular voltage (100v) is plotted. From the plots are show in figures 25&26. it was observed that average surface temperature of copper is better than the aluminum alloy

B.Average Heat Transfer Coefficient Variation

The variations of average heat transfer coefficient over uniformly heated cylinder models made of Cop-per and Aluminum alloy at different heat fluxes and angle of inclinations(00(vertical),300,450,600,900(horizontal)) was studied on the corresponding graphs are plotted and depicated in figures 11&12.

From the plots it was observed that the average heat transfer coefficient increases with increase of heat flux for both the hollow cylinders. It was also ob-served that average heat transfer coefficient for hol-low cylinders made of copper was better than alu-minum alloy cylinder for different heat fluxes and angle of inclinations (moves from vertical to hori-zontal)

The effect of angle of inclinations of the heat trans-fer coefficient along the cylindrical surfaces for par-ticular voltage (100v) is plotted. From the plots are shown in figures13&14. it was observed that heat transfer coefficient of copper is better than the alu-minum alloy

From the plots 35 and 36 it was observed that the experimental average heat transfer coefficient in-creases with increase of heat flux for both the hol-low cylinders. It was also observed that average heat transfer coefficient for hollow cylinders made of alu-minum alloy was better than copper cylinder for dif-ferent heat fluxes and angle of inclinations (moves from horizontal to vertical)

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C.Local Heat Transfer Coefficient Variation

From figures15-22 it shows the variation of local heat transfer coefficient with distance between ther-mocouples from the bottom at different voltages and again drawn for different angles respectively It tells that as the distance increases from the bot-tom, local heat transfer coefficient decreases.From the plots it was observed that local heat trans-fer coefficient of copper is better than the alumini-um alloy

D.Average Nusselt Number Variation

From figures 27-34 it shows the variation of local nusselt number with distance between thermocou-ples from the bottom at different voltages and again drawn for different angles respectively .

It tells that as the distance increases from the bot-tom ,local nusselt number also increases. Nux = (h*x)/ k From the plots it was observed that the average nusselt number of copper is better than the alu-minium alloy. Local nusselt number is directly proportional to dis-tance between thermocouples from the bottom.

CONCLUSION

Natural convection heat transfer experiments were conducted on two hollow cylindrical models made of aluminium alloy and copper in order to study the various theoretical heat transfer coefficient ,experi-mental heat transfer coefficient and nusselt number for different heat fluxes and orientations .Based on the experimental observation the following conclu-sions were observed.

> Experimental setup was successfully estab-lished for analysing the heat transfer over a hollow cylinder for different orientations..> From the results , it is concluded that the average surface temperature of hollow cylinders made of copper is better than the aluminium alloy for different heat fluxes and orientations .Hence, the copper was obtained higher value at the horizontal position (Ø = 900) when compared with other orien-tations.> The experimental average heat transfer coef-ficient of aluminium alloy is better than copper at different orientations. Average heat transfer coeffi-cient of aluminium alloy is better in vertical position (Ø = 00) compared with other orientations .The heat flux increase with increase of average heat transfer coefficient. > The theoretical average heat transfer coeffi-cient of copper alloy is better than aluminium alloy at different orientations. Average heat transfer coef-ficient of copper alloy is better in horizontal position (Ø = 900) compared with other orientations. > The local heat transfer coefficient of cylin-ders made of aluminium alloy is better than copper

at different orientations. Local heat transfer coeffi-cient of aluminium alloy was better vertical posi-tion (Ø = 00) compared with the other orientations at distance between thermocouples top to bottom. The local heat transfer coefficient increases when hollow cylinder moves from horizontal to vertical position> The local nusselt number of hollow cylin-ders made of copper is better than aluminium alloy at different orientations. The local nusselt number of copper was better vertical position (Ø = 00) com-pared with the other orientations. The local nusselt number increases with increase of distance between thermocouples bottom to top. The nusselt number increases when hollow cylinder moves from vertical to horizontal position.> The average nusselt number of hollow cyl-inder made of copper is better than aluminium al-loy at different orientations. The heat flux increases with increase of average nusselt number. The cop-per has obtained the higher value of average nus-selt number at vertical position. Hence, the average nusselt number is better in vertical position.

REFERENCES

[1].J.Grimson “Advance Fluid dynamic and heat transfer “McGraw-Hill.,England,1971.[2].J.P.Holman"Experimental Method For Engi-neers" McGraw-Hill,'Tokyo,4th Edition 1984.[3].OAtayilmaz,Ismail Teke "Experimental and nu-merical study of the natural convection from a heat-ed horizontal cylinder”, Int.Com in Heat and mass Transfer,36,731-738,2009.[4].A.M. Akeel, A.M. Mahmood and S.A. Raad, “Nat-ural convection heat transfer in a inclined circular cylinder”, Journal of Engg., vol. 17, pp. 659-673, 2011.[5].A.M Akeel, "Natural convection heat trans-fer in a vertical concentric annulus” ,journal of Engg.,vol.13,pp1417-1423,2007.[6].Olivier Reymond, Darina B.Murray, Tadhg S.O’Donovan, “ Natural convection heat transfer from two horizontal cylinders” Experimental ther-mal and fluid science,32,1702-1709,2008.[7].H.S.Takhar, A.J.Chamkhar, G.Nath “Natural convection on a vertical cylinder embedded in a thermally stratified high-porosity medium” Int. J. Therm. Sci.41,83-93,2002.[8].R.Rosian, H.Saleh, I.Hashim, A.S.Bataineh “nat-ural convection in an enclosure containing a sinu-soidally heated cylindrical source” Int.J. Of Heat and Mass Transfer, 70,119-127, 2014.[9].J.H. Bae and J.M. Hun “Time dependent buoy-ant convection in an enclosure with discrete heat sources”, Int. J. Therm. Sci., 2003.[10].P.N. Madhavan and V.M.K. Sastri, “Conjucate natural convection colling of protruding heat sourc-es mounted on a substrate placed inside an eclo-sure: a parametric study”, Comp. Methods Appl. Mech. Engg., vol. 188, pp. 187-202, 2003.[11].M.M.Molla, S.C.Saha, M.A.I.Khan, M.A.Hossain “Radiation effects on natural convection laminar flow from a horizontal circular cylinder” Desalina-tion publications, 30,89-97,2011.

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[12].E. Vande Sarde and B.J.G. Hamer, “Study and transient natural convection in enclosure between horizontal circular cylinders (constant heat flux)”, Int. J. Heat mass Transfer, vol. 22, pp. 361-370, 1979.[13].M. Ait Saada, S. Chikh, A. Campo “Natural con-vection around a horizontal solid cylinder wrapped with a layer of fibrous or porous material” Inter. J. of Heat and Fluid Flow 28, 483–495,2007.[14].P.K.Sarma , P.V.Sunitha “Interaction of ther-mal radition with laminar natural convection from a horizontal cylinder in air” Warme And Stoffubertra-gung,26, 654-69,1991.[15].M.A.Hossain, M.A.Alim, D.A.S.Rees “Effect of thermal radiation on natural convection over cyl-inders of elliptic cross section” Acta mechanica, 129,177-186, 1998.

Fig. 11 variation of average heat transfer coefficient w.r.t voltage for different orientations of aluminium alloy hollow cylinder

Fig. 12 variation of average heat transfer coefficient w.r.t voltage for different orientations of copper hol-low cylinder

Fig 13 variation of average heat transfer coefficient w.r.t particular voltage (80 V) for different orienta-tions of aluminiumalloy hollow cylinder

Fig 14 variation of average heat transfer coefficient w.r.t particular voltage (80V) for different orienta-tions of copper hollow cylinder

Fig 15 variation of local heat transfer coefficient w.r.t distance from the bottom at = 00 at different voltages for aluminium alloy hollow cylinder

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Fig 16 variation of local heat transfer coefficient w.r.t distance from the bottom at = 00 at different voltages for copper hollow cylinder

Fig. 17 variation of local heat transfer coefficient w.r.t distance from the bottom at =300 at different voltages for aluminium alloy hollow cylinder

Fig. 18 variation of local heat transfer coefficient w.r.t distance from the bottom at =300 at different voltages for copper hollow cylinder

Fig.19 variation of local heat transfer coefficient w.r.t distance from the bottom at =450 at different voltages for aluminium alloy hollow cylinder

Fig.20 variation of local heat transfer coefficient w.r.t distance from the bottom at =450 at different voltages for copper hollow cylinder

Fig.21 variation of local heat transfer coefficient w.r.t distance from the bottom at =600 at different voltages for aluminium alloy hollow cylinder

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Fig.22 variation of local heat transfer coefficient w.r.t distance from the bottom at =600 at different voltages for copper hollow cylinder

Fig 23 variation of average surface temperature w.r.t voltage for different orientations of aluminium alloy hollow cylinder

Fig 24 variation of average surface temperature w.r.t voltage for different orientations of copper hollow cylinder

Fig 25 variation of average surface temperature w.r.t voltage for different orientations of aluminium alloy hollow cylinder

Fig 26 variation of average surface temperature w.r.t voltage for different orientations of copper hollow cylinder

Fig 27 variation of local Nusselt number w.r.t dis-tance from the bottom at =00 at different voltages for aluminium alloyhollow cylinder

Fig 28 variation of local Nusselt number w.r.t distance from the bottom at =00 at different voltages for copper hollow cylinder

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Fig 29 variation of local Nusselt number w.r.to distance from the bottom at =300 at different voltages for aluminium alloy hollow cylinder


Fig 31 variation of local Nusselt number w.r.t distance from the bottom at=450 at different voltages for aluminium alloy hollow cylinder


Fig.33 variation of local Nusselt number w.r.t dis-tance from the bottom at =600 at different voltages for aluminiumalloy hollow cylinder


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Fig 35 variation of therritical and experimental heat transfer coefficient w.r.t voltage for different orientations of aluminium alloy hollow cylinder

Fig.36 variation of therritical and experimental heat transfer coefficient w.r.t voltage for different orientations of copper hollow cylinder

AUTHOR

S. Madhava rao,P.G Student,Department Of Mechanical Engineering, B.V.C. Engineering College, Odalarevu, [email protected]

D. Santha rao Associate professor Mechanical dept.Experience 14 YEARS.B.V.C. Engineering CollegeOdalarevu-533210, [email protected]

Dr.S. RajeshMechanical EngineeringAssistant professor(9years)S.R.K.R. Engineering CollegeBhimavaram-534204, [email protected]

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Experimental Investigation on Heat Transfer By Natural Convection Over A Cylinder for Different...

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