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
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.
Mathematical Analysis
Solar air heater with extended surface absorber
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
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.
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
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
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
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]
Results And Discussions
Effect of mass flow rate on collector efficiency Effect of mass flow rate on collector efficiency factor Factor
Cont.
Effect of mass flow rate on heat removal factor. Effect of mass flow rate on heat removal factor.
Cont.
Effect of mass flow rate on ∆T/I Effect of mass flow rate on ∆T/I
Cont.
Effect of mass flow rate on instantaneous efficiency Effect of mass flow rate on instantaneous efficiency
Cont.
Variation of pressure drop with mass flow rate Variation of pressure drop with mass flow rate
Cont.
Effect of mass flow rate on thermohydraulic Effect of mass flow rate on thermohydraulic and thermal efficiency and thermal efficiency
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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