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133 subhash presentation 1

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Page 1: 133 subhash presentation 1
Page 2: 133 subhash presentation 1

An Analytical Investigation on Thermal and

Thermohydraulic Performance of Finned

Absorber Solar Air HeaterPresented by

Dr. Prabha ChandAssociate Professor

Department of Mechanical Engineering

National Institute of Technology, Jamshedpur – 831014 Jharkhand, India

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INTRODUCTIONThis paper deals with theoretical parametric analysis of finned absorber

solar air heater. Two models of solar air heater one with rectangular fins

and other with triangular fins

has been developed. The fluid channel is formed by two transversely

positioned fins

attached on the absorber plate, bottom side thermally insulated and top

surface of

absorber subjected to uniform heat flux. The expression for collector

efficiency factor

and collector heat removal factor of such collector has been developed.

Effects of mass flow rates on thermal performance have been presented

and results are compared with flat plate air heaters.Further, the

thermohydraulic performance parameter called

“effective efficiency” has been employed and presented to express the net

useful thermal energy gain, taking into account the equivalent thermal

energy required to produce

the work energy necessary to overcome the additional friction or hydraulic

losses as

a result of extended surfaces on the absorber plate.

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Mathematical Analysis

Solar air heater with extended surface absorber

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Cont.Considering a slice of average width W and thickness dx at a distance x from inlet then the energy balance equations for the absorber plate, the bottom plate and the air flowing in between respectively can be written as

S. W. dx = Ul W dx (Tp-Ta)+hfpWdx (Tp - Tf)+ 2Df dxfhff (Tp-Tf)+hrWdx(Tp-Tb) (1)

hr W dx (Tp-Tb) = hfb W dx (Tb-Tf)+UbW dx (Tb-Ta) (2)

fp2

dTC mL

W =hfp W dx (Tp - Tf)+ 2Df dx fhff (Tp- Tf)+ hfb W dx (Tb - Tf) (3)

he is the effective heat transfer co-efficient and can be given as

he=

fb

fb

fp

fff

h

h

h

h 21

r

rffp h

h

W

Dh

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F' is the collector efficiency factor and expressed as 1

' 1

e

l

h

UF

For the heat transfer coefficient, the characteristic dimension used in the definitions of Nu and Re is the equivalent diameter de given by

de = )(2

)(4

channelfin a ofperimeter

channelfin a of area sec.4

fLWfLfWL

Wetted

tionCross

for

rectangular fin

)2)(2)5.0((2

)5.0(4

fLfW

fLfWL

= for triangular fin

Cont.

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Cont.

The temperature distribution along the flow direction and collector heat removal factor can be expressed as

Lp

Cm

F

aUS

aT

fiT

lUS

aT

foT

c

A l

U'exp

FR= pCmFlUcAlUcApCm

/'exp1

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Cont.

The collector efficiency can be expressed as

I

TCm

I

TTCm pfifop where I = S ()e

Here, the solar air heater works on an open cycle so the inlet temperature coincides with the ambient temperature and the above equation becomes 

I

TTCm afop

or = G CpT/I

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THERMOHYDRAULIC PERFORMANCEThermohydraulic performance is the performance of the system that includes the consideration of thermal as well as hydraulic characteristics. The pumping performance of the collector in terms of the effective efficiency that taken into account the useful thermal gain and equivalent thermal energy that will be required to provide corresponding mechanical energy for overcoming friction power losses. Effective efficiency, ηe, of a solar air heater is given by,ηe =

The useful energy gain is written as

Qu = ṁCp(Tfo - Tfi)

The net energy gain, Qn of the collector can be expressed as the different between the useful thermal energy gain, Qu, and the equivalent thermal energy required for producing the work energy necessary to overcome the pressure energy losses. This net energy can be written as

Qn = Qu – Pm/Cf

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Cont.

Cf is the conversion factor representing conversion from thermal energy to compression energy of the fan/blower imparted to air and is given as

Cf = ηf .ηm .ηt .ηth

where,ηf= Efficiency of fan.ηm = motor efficiencyηt= Efficiency of electrical transmission.ηth = thermal conversion efficiency of power plant.where Cm is the equivalent temperature drop due to friction.

Cm = ∆P/Cfρ Cp

Cm = equivalent temperature drop due to friction The effective temperature rise is given as;

∆Te = [(To - Ti) - Cm]

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Results And Discussions

Effect of mass flow rate on collector efficiency Effect of mass flow rate on collector efficiency factor Factor

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Cont.

Effect of mass flow rate on heat removal factor. Effect of mass flow rate on heat removal factor.

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Cont.

Effect of mass flow rate on ∆T/I Effect of mass flow rate on ∆T/I

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Cont.

Effect of mass flow rate on instantaneous efficiency Effect of mass flow rate on instantaneous efficiency 

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Cont.

Variation of pressure drop with mass flow rate Variation of pressure drop with mass flow rate

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Cont.

Effect of mass flow rate on thermohydraulic Effect of mass flow rate on thermohydraulic and thermal efficiency and thermal efficiency 

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CONCLUSIONS

Considerable improvement in air temperature rise parameter (T/I) and efficiency of flat-plate solar air heater is obtained by providing rectangular and triangular fins on the absorber plate of solar air heaters. An enhancement of thermal efficiency in triangular finned and rectangular finned absorber is 32% and 34% reported respectively compared with flat-plate absorber solar air heater and this enhancement is a strong function of operating parameter and system parameter.

However, any attempt to increase the heat transfer rate hence performance will, by the Reynolds Analogy, also result in an increase in pressure drop leading to an increase in the pumping power and hence there is a need to optimize the system.

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REFERENCES

 [1] BAA Yousef, NM Adam,( 2008)“Performance analysis for flat plate collector with and without porous media” Journal of Energy in Southern Africa , Vol 19 No 4.

[2] Hottel, HC., and Woertz, B.B., (1955) "Evaluation of flat plate solar collector performance", Trans. of the Conference on use of Solar Energy, Part I, Arizona, 74-104.

[3] Hüseyin Benli, (2012) “Experimentally derived efficiency and exergy analysis of a new solar air heater having different surface shapes” Renewable Energy 50 ,58-67.

[4] Duffice, J.A., and Backman, W.A., (1974)"Solar energy thermal processes", Jhon Wiley and Sons, New York.

[5] Sukhatme, S.P. (1997), “Solar Energy: Principles of Thermal Collection and Storage “Tata McGraw Hill .

[6] Blaine, F. Parker, (1981) "Derivation of efficiency and loss factor for solar air heaters", Solar Energy, 26, 27-32.

[7] Ye-Di. Liu, Diaz, L.A., and Suryanarayana, N.V. , (1984) "Heat transfer enhancement in air heating flat-plate solar collector", ASME Trans. 106, 358-363.

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[8] Kays, W.M. and London, A.L., "Compact heat exchangers", 2nd Ed., McGraw Hill, New York.

[9]Sharma, S.P., Saini. J.S. and Verma, H.K., (1991) "Thermal performance of packed-bed solar air

heaters", Solar Energy, Vol. 47, 2, 59-67.

[10] Piao, Y., Hauptmann, E.G., and Iqbal, M., (1994) "Forced convective heat transfer in cross-corrugated solar air heaters", J. of Solar Energy Engineering, 116, 212-214.

[11] Ho-Ming Yeh, Chii-Dong Ho and Chi-Yen Lin, (1998) "The influence of collector aspect ratio on the collector efficiency of baffled solar air heaters”, Energy, 23, 11-16.

[12] Kumari, P., and Sharma, S.P., (2000) "Enhancement of thermal performance of solar air heater with

extended surfaces absorber", XVI National Convection of Mechanical Engineers, Sept. 29-30, 2000 Roorkee, pp. 621-627.

[13] Kumari, P., and Sharma, S.P., (2000) "An analytical investigation on thermal performance of finned

absorber solar air heater", National Renewable Energy Conference 2000, Nov. 30-Dec. 2, 2000, IIT, Bombay, pp. 60-64.

Cont.

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[14] Kumari, P., and Sharma, S.P., (2001) "Thermohydraulic performance of extended surfaces absorber

solar air heaters", Third National Conference on Thermal Systems, Organised by Mechanical Engg. Deptt. I.T. B.H.U., Feb. 17-19.

[15] Chand,P.and Sharma, S.P., (2010) ”Thermal performance prediction of extended absorber solar air

heater” Proc. of the 20thNational and 9thInternational ISHMT-ASME Heat and Mass Transfer Conference, January 4-6, IIT Mumbai, India.

Cont.

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