1.
An Analytical Investigation on
Thermal and Thermohydraulic
Performance of Finned
Absorber Solar Air Heater
Presented by
Dr. Prabha Chand
Associate Professor
Department of Mechanical Engineering
National Institute of Technology, Jamshedpur – 831014
Jharkhand, India
2.
INTRODUCTION
This 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
3.
Mathematical Analysis
Solar air heater with extended surface absorber
4.
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 dxηfhff (Tp-Tf)+hrWdx(Tp-Tb)
(1)
hr W dx (Tp-Tb) = hfb W dx (Tb-Tf)+UbW dx (Tb-Ta)
(2)
W
m C p dTf =hfp W dx (Tp - Tf)+ 2Df dx ηfhff (Tp- Tf)+ hfb W dx (Tb - Tf)
L2
he is the effective heat transfer co-efficient and can be given as
2 D f ηf h ff
he= h fp 1 +
W h fp
hr h fb
+
h +h
r
fb
(3)
5.
Cont.
F' is the collector efficiency factor and expressed as
Ul
F = 1 +
he
−1
'
For the heat transfer coefficient, the characteristic dimension used in the definitions of Nu and Re is the equivalent
diameter de given by
4 (WL − ∂ f L f )
4.Cross −sec tion area of a fin channel
for
de = Wetted perimeter of a fin channel = 2 (W + L ) rectangular fin
f
=
4 (WL −0.5 ∂ f L f )
2 (W + ( 0.5 ∂ f ) 2 +( L f ) 2 )
for triangular fin
6.
Cont.
The temperature distribution along the flow direction and collector heat removal factor can be
expressed as
S
T
−T −
F' U A
fo
a U
l =exp
l
c L
−
S
mC
T −T −
p
fi
a U
a
F R=
mC p
AcU l
[ (
1− exp − AcU l F ' / mC p
)]
7.
Cont.
The collector efficiency can be expressed as
η=
mCp ( Tfo − Tfi )
I
mCp ∆T
=
I
where I = S (τα)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
η=
mC p ( Tfo − Ta )
I
or
η =
G Cp∆T/I
8.
THERMOHYDRAULIC
PERFORMANCE
Thermohydraulic 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 = ṁ Cp(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
9.
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 efficiency
ηt= 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]
10.
Results And Discussions
Effect of mass flow rate on collector efficiency
factor
Effect of mass flow rate on collector efficiency
Factor
11.
Cont.
Effect of mass flow rate on heat removal factor.
Effect of mass flow rate on heat removal factor .
12.
Cont.
Effect of mass flow rate on ∆T/I
Effect of mass flow rate on ∆T/I
13.
Cont.
Effect of mass flow rate on instantaneous efficiency
efficiency
Effect of mass flow rate on instantaneous
14.
Cont.
Variation of pressure drop with mass flow rate
Variation of pressure drop with mass flow rate
15.
Cont.
Effect of mass flow rate on thermohydraulic
and thermal efficiency
Effect of mass flow rate on thermohydraulic
and thermal efficiency
16.
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 .
17.
REFERENCE
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18.
Cont.
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19.
Cont.
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absorber
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Mass Transfer Conference, January 4-6, IIT Mumbai, India.
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