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Volume-6, Issue-5, September-October 2016
International Journal of Engineering and Management Research
Page Number: 5-12
Experimental Study of Effect of Different Size of Roughened Elements
(Rhombus Shape) on Thermal Performance of Roughened Flat-Plate
Solar Air Heater
Pankaj Kumar1, Dr. S.C. Roy2, Dr. M.K. Paswan3
Assistant Professor, Department of Mechanical Engineering, B.I.T. Sindri, INDIA
2
Professor, Department of Mechanical Engineering, B.I.T. Sindri, INDIA
3
Professor, Department of Mechanical Engineering, N.I.T. Jamshedpur, INDIA
1
ABSTRACT
An experimental investigation has been carried
out for a range of system an operating parameters in order
to analyze the effect of different size of roughened elements
(rhombus shape)on thermal performance of roughened
flat-plate solar air heater. Duct has an aspect ratio W/H of
7, of different size 1”, 1.25”, 1.5” and 2”; mass flow rate
range from 0.02 – 0.045 kg/s and Reynolds Number (R e )
range from to 5100 to 28000. A considerable increase in
thermal efficiency has been observed.
Keywords--- Thermal, Solar, Air passage
I.
INTRODUCTION
Since industrial revolution and coming of
machine era there have always been needs of energy.
With every passing by day the number of machine is
increasing and so is need of power. This need of power
had propelled mankind to find every possible source of
energy and exploit it to maximum.
There is an urgent need for renewable energy
sources. The renewable energy industry has experienced
dramatic changes over the few years. Deregulation of the
electricity market failed to solve the industries problems.
Also unanticipated increase in localized electricity
demands and slower than expected growth in generating
capacity have resulted in an urgent need or alternative
energy
sources,
particularly
those
that
are
environmentally sound.
Renewable energy products suddenly faced
requests to accelerate production of new renewable
energy capacities and restore facilities that had been
closed due to poor economics. In the long term
renewable energy generating system will require
development to benefit the existing system.
Innovations in renewable energy are essential
for a sustainable energy future. In India, the national
5
solar mission spearheaded by “Mission for Enhanced
Energy.
Efficiency” aims to establish the nation as a
global leader in solar energy by creating the policy
conditions for its diffusion across the country. The
mission has a goal to achieve grid parity cost for solar
power and has targeted 20000 MW for solar power
capacity from grid and off-grid based solar technologies
both thermal and photovoltaic by 2022.
Solar air heater have low efficiency because of low
convective heat transfer coefficient between the air and
absorber plate resulting in high thermal losses to
environment.
Flat-plate solar collectors are being for thermal
conversion to raise the temperature of fluid flowing
through the collector. Conversion of solar radiations to
thermal energy is mainly due to hear transfer coefficient
between absorber plate and the fluid flowing the
collector. Several designs of solar air heater have been
developed over the years in order to improve their
performance. However, the efficiency of solar air
collectors is low because of the low value of the
convective heat transfer coefficient between the absorber
plate and the air, leading to high absorber plate
temperature and greater heat losses to surroundings.
Close (1963) discussed solar air heaters for low and
moderate temperature applications. It has been found
that the main thermal resistance to the heat transfer is
due to the formation of a laminar sub-layer on the heat
transferring surface, which can be broken by providing
artificial roughness on the heat transferring surface. The
artificial roughness has been used extensively for the
enhancement of forced convective heat transfer, which
further requires flow at heat transferring surface to be
turbulent.
The roughness was first used in solar air heater
and resulted in better heat transfer in comparison to that
in conventional solar air heater by Prasad and Mullic
(1985). Prasad and Saini (1988) studied the effect of
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roughness and flow parameters on heat transfer for
transverse ribs. It was observed that maximum heat
transfer occurred near to the reattachment points. The
maximum enhancement in Nusselt number was reported
to be 2.38 times over smooth duct. Verma and Prasad
(2000) has been carried out experimental study for
thermo hydraulic optimization of the roughness and flow
parameters for Reynolds number (R e ) range of 500020,000, relative roughness pitch (P/e) range of 10-40 and
relative roughness height (e/D h ) range of 0.01-0.03. The
optimal thermohydraulic performance was reported to be
71%. Karwa et al. (1999) has been experimentally
investigated the effect of repeated rectangular crosssection ribs on heat transfer for duct aspect ratio (W/H)
range of 7.19-7.75, P/e value of 10, e/D h range of
0.0467-0.050 and R e range of 2800-15,000. The
enhancement in the Stanton number was reported to be
1.1
65-90% Gupta et al. (1997) experimentally investigated
the effect of e/D h , inclination of rib with respect to flow
direction and Reynolds number (R e ) on the
thermohydraulic performance of roughened solar air
heater. The detailed studies on roughness geometries
used in solar air heater ducts are also available in Varun
et al. (2007), Hans et al. (2009) and Bhushan and Singh
(2010).
The application of artificial roughness in the
form of rhombus shape on absorber plate is attractive
roughness geometry for solar air heater due to its less
complicated manufacturing process. In this paper
experimental data has been collected by performing
experiment to see the effect of different size of
roughened elements (rhombus shape)on thermal
performance of roughened flat-plate solar air heater
Roughness parameters
(a) Diagram of the absorber plate
(b) Pictorial view of the absorber plate
Fig. 1. (a) Diagram of the absorber plate.
6
(b) Pictorial view of the absorber plate
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Fig. 1.2 Isometric view of experimental set-up
Rhombus shape (sheet metal) roughness
elements of different sizes have been generated on the
absorber plate to create roughness in the duct. A
schematic and pictorial view of roughness geometry is
shown in Fig. 1(a) and (b). The rhombus shape of
roughness was produced on the underside of the absorber
plate. Fig.1.2 shows the isometric view of experimental
set-up. The range of roughness parameters and operating
parameters is given in Table 1.For studying the effect of
different size of roughened elements (rhombus shape)on
thermal performance were experimentally investigated at
various mass flow rates.
S. No.
1
2
2
3
Table 1
Values of flow and roughness parameter
Parameter
Range
Aspect ratio (W/H)
7
Sizes
1”, 1.25”, 1.5” & 2”
mass flow rates
0.02 – 0.045 kg/s
Reynolds number
5100-27000
Nomenclature
A
D
e
e+
e/D
fr
fs
h
H
I
m
����
𝑁𝑁𝑁𝑁
Nu s
Pr
p
p/e
Re
tf
ti
to
tp
W
area of absorber plate (m2)
equivalent diameter of the air passage (m)
height of roughness element
roughness Reynolds number
relative roughness height
friction factor for roughened duct
friction factor for smooth duct
convective heat transfer coefficient (W/m2 k)
height of duct (m)
intensity of solar radiation (W/m2)
mass flow rate of air (kg/s)
Nusselt number for roughened absorber plate
Nusselt number for smooth absorber plate
prandtl number
roughness pitch (m)
relative pitch roughness
Reynolds number
average fluid temperature (K)
air inlet temperature (K)
air outlet temperature (K)
average plate temperature (K)
width of duct (m)
7
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II.
EXPERIMENTAL SETUP AND PROCEDURE
Fig. 2.1 shows the schematic diagram of the experimental set-up used. Major components of this set-up are:
1.
2.
3.
4.
5.
6.
7.
8.
flow straighteners
plane solar air heater
roughened solar air heater
outlet headers & pipe fittings
manometers
selector switches
centrifugal blower
auto transformer
Two- dimensional, fully developed flow was
obtained by sucking atmospheric air through flow
straighteners by means of blower. The air thus passes
through the test section i.e. solar air heaters; one with
rhombus shape roughened and the other plane (FlatPlate) collector, and flow meters before exhausting into
the atmosphere. Thermocouples were used to measure
absorber and air temperature at different location in the
solar air heaters as flow progresses. The output of the
thermocouple fed to digital micro voltmeter displays
directly the temperature values. Mass flow rate of air
through these collector ducts were measured by two
orifice meters provided with U-tube manometers. For
each experimental run, initially all the instruments, viz.
manometer, milli-voltmeter, U-tube manometer, Blower
and electric circuit were checked for their correctness
and all joints were carefully checked to avoid any air
leakage. Data was recorded under quasi-study state
(when there is no appreciable change in temperature for
10-15 min) conditions for the air temperature at different
points on the duct and temperature of absorber plate at
Discharge of air (Q)
=
Mass flow rate of air(ṁ𝑎𝑎 ) = 𝜌𝜌 × 𝑄𝑄
Efficiency of plate (ƞ)
8
=
𝐶𝐶𝑑𝑑 ×𝑎𝑎₁×𝑎𝑎₀�2×𝑔𝑔×ℎ
�(𝑎𝑎₁)²−(𝑎𝑎₀)²
ṁa ×C p (Τ₀−Τᵢ)
𝐼𝐼×𝐴𝐴𝑐𝑐
05 different locations. Data were taken at the regular
interval of 1hour and accordingly the pressure drop
across orifice meter has been measured with the help of
U-tube manometer.
III.
RESULTS AND DISCUSSION
The total 30 numbers of test runs, raw
experimental data were collected for 3 set of roughened
solar air heater as well as smooth one. For a particular
test run, mass flow rate in the roughened and smooth
collector remained the same. Table 1 represents the
roughness and flow parameters investigated. The raw
experimental data were reduced to work out for the
values of the results with respect to heat transfer.
Fig. 3(a) and Fig. 3(b) shows the data of Solar
Intensity and Time, and Temperature and Time
respectively.
The following relations are used to find the
thermal performance of roughened collector:
(3.1)
(3.2)
(3.3)
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Table 3(A)
Thermal performance of roughened collector at m=0.02 kg/s
Efficiency(in %) comparison of plane and rough absorber plate at m=0.02 kg/s
Rough plate
Rough plate
Rough plate
Rough plate
Length of side of
Length of side of
Length of side of
Length of side of
rhombus = 1”
rhombus = 1.25”
rhombus = 1.5”
rhombus = 2”
12.31
14.72
18.37
19.91
13.64
14.51
19.02
20.44
13.85
15.47
19.79
21.36
20.39
23.77
23.83
25.25
20.95
22.80
23.92
28.25
21.25
21.54
20.38
23.36
18.85
19.11
19.10
16.20
Time
10.00
11.00
12.00
13.00
14.00
15.00
16.00
Table 3(B)
Time
Thermal performance of roughened collector at m=0.03 kg/s
Efficiency(in %) comparison of plane and rough absorber plate at m=0.03 kg/s
Rough plate
Length of side of
rhombus = 1”
Rough plate
Length of side of
rhombus = 1.25”
Rough plate
Length of side of
rhombus = 1.5”
Rough plate
Length of side of
rhombus = 2”
10.00
11.00
12.00
12.42
12.93
15.10
15.20
18.90
19.44
20.72
21.63
13.28
16.65
20.81
22.15
13.00
21.47
25.38
25.94
27.17
14.00
23.38
24.84
25.89
30.22
15.00
22.86
23.54
21.74
24.52
16.00
20.06
20.63
20.39
17.22
Table 3(C)
Time
Thermal performance of roughened collector at m=0.035 kg/s
Efficiency(in %) comparison of plane and rough absorber plate at m= 0.035 kg/s
10.00
11.00
12.00
13.00
14.00
15.00
16.00
Rough plate
Length of side of
rhombus = 1”
12.42
12.93
13.28
21.47
23.38
22.86
20.06
Rough plate
Length of side of
rhombus = 1.25”
15.10
15.20
16.65
25.38
24.84
23.54
20.63
Rough plate
Length of side of
rhombus = 1.5”
18.90
19.44
20.81
25.94
25.89
21.74
20.39
Rough plate
Length of side of
rhombus = 2”
20.72
21.63
22.15
27.17
30.22
24.52
17.22
Table 3(D)
Thermal performance of roughened collector at m=0.04 kg/s
Efficiency(in %) comparison of plane and rough absorber plate at m= 0.04 kg/s
Rough plate
Rough plate
Rough plate
Rough plate
Time
Length of side
Length of side of
Length of side of
Length of side of
of rhombus = 1”
rhombus = 1.25”
rhombus = 1.5”
rhombus = 2”
10.00
14.06
15.83
18.90
19.52
9
11.00
15.20
16.17
19.87
23.58
12.00
18.22
18.11
20.70
24.54
13.00
23.34
27.84
27.98
28.91
14.00
25.99
27.36
27.94
34.07
15.00
24.58
25.46
23.57
26.77
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16.00
Time
10.00
11.00
12.00
13.00
14.00
15.00
16.00
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21.04
22.15
22.26
18.17
Table 3(E)
Thermal performance of roughened collector at m=0.045 kg/s
Efficiency(in %) comparison of plane and rough absorber plate at m=0.045 kg/s
Rough plate
Rough plate
Rough plate
Rough plate
Length of side
Length of side of
Length of side of
Length of side of
of rhombus = 1”
rhombus = 1.25”
rhombus = 1.5”
rhombus = 2”
15.38
15.22
21.27
22.77
17.25
16.18
21.98
23.50
18.89
18.01
22.01
24.70
23.48
27.96
28.51
30.11
26.29
27.73
28.09
34.66
24.68
25.65
23.45
27.62
21.12
22.12
22.44
20.94
Figure 3(a)
Figure 3(b)
10
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26
size=1"
size =1.25"
size=1.5"
size=2"
24
η
22
20
18
0.020
0.025
0.030
0.035
0.040
0.045
m, kg/s
Figure 3(c)
Table 3(A), 3(B), 3(C), 3(D) & 3(E) shows the
values of thermal efficiency at various mass flow rate
like 0.02, 0.03, 0.035, 0.040 and 0.045 kg/s for different
size of roughened element (rhombus). The average
values of thermal efficiency have been found to be 21.0,
21.8, 23.9, and 26.3 for a particular mass flow rate, m,
equals to 0.045 kg/s for different size of roughened
element (rhombus), 1”, 1.25”, 1.5” and 2” respectively.
It is clear from fig.3(c) that the thermal performance of
roughened solar air heater increases with increasing size
of rhombus but it is small in amount. Hence we can say
that the size of rhombus does not affect more the thermal
efficiency of roughened solar air heateras in case of
other parameters like relative roughness pitch or relative
roughness height.
IV.
CONCLUSION
The effect of size of roughened geometry on
thermal performance leads the following conclusions:
1. The rate of enhancement of thermal performance of
roughened flat plate solar air heater strongly depends on
mass flow rate and size of roughened element (rhombus
shape).
2. It is worthy to note here that thermal performance
increases as the mass flow rate increases.
3. As the size of roughened element (rhombus shape)
increases the thermal efficiency of roughened collector
of solar air heater also increases.
4. It is worthy to note here that the rate of increase of
thermal efficiency is small as compare to increase in size
of roughened element (rhombus shape).
11
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