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Performance evaluation of a novel solar air heater with arched absorber plate
Accepted Manuscript Performance evaluation of a novel solar air heater with arched absorber plate
Simarpreet Singh PII:
S0960-1481(17)30737-1
DOI:
10.1016/j.renene.2017.07.109
Reference:
RENE 9084
To appear in:
Renewable Energy
Received Date:
08 April 2017
Revised Date:
23 July 2017
Accepted Date:
25 July 2017
Please cite this article as: Simarpreet Singh, Performance evaluation of a novel solar air heater with arched absorber plate, Renewable Energy (2017), doi: 10.1016/j.renene.2017.07.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
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Performance evaluation of a novel solar air heater with arched
2
absorber plate
3
Simarpreet Singh*
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Department of Mechanical Engineering, BITS-Pilani, Rajasthan, India.
5
*Corresponding Author: +91-8094041444; email: [email protected]
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Abstract
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The study presents a performance evaluation of a novel solar air heating system with arched
9
absorber plate using turbulators. Simulation work is carried out in ANSYS FLUENT (v16.2) platform
10
with RNG k-ε turbulence model at constant heat flux (500 W m-2), in order to compare the thermal-
11
hydraulic performance of the proposed design for a range of Reynolds number (3800 to 14000).
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Performance of the solar air heater is projected in the terms of Nusselt number (Nu), frictional factor
13
(Fr) and thermal-hydraulic performance parameter (THPP). It is observed that the arched absorber
14
plate design increases the air turbulence and vortex generation, which results in reducing laminar sub-
15
layer generation near the surface of the absorber plate. A significant improvement is observed in
16
Nusselt number at high Reynolds Number (above 10000). However, a marginal enhancement is also
17
observed in frictional factor due to providing an extra obstacle along the duct length with arched
18
shaped design in the flow field. It is observed that arched shape design of absorber plate can
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significantly improve overall performance of the solar air heating system using various turbulators.
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This study will likewise provide a new direction to the work trend in this area.
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Keywords: Solar air heater, turbulator, THPP, dimply, equilateral triangle.
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1. Introduction
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Prior to 1970, solar energy was viewed as a prominent alternative to fulfill the continuous
24
increasing demand of energy. An extensive research work was carried out to extract the maximum
25
benefit of this renewable source of energy, due to the high rise in oil prices in 1970s (Yadav and
26
Bhagoria, 2014). Among various alternatives, solar energy stands out as the best resource option to
27
fulfill the increasing energy demand. Instead of direct use of solar energy it is more useful when
28
converted into thermal energy (Bhushan and Singh, 2010). From literature, it has been revealed that
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the solar air heater is considered as one of the most effective system among various options available
2
to capture solar energy for heating applications in various sectors. In order to improve the thermal
3
efficiency of the solar air heater, obstruction in the flow field was proposed by artificial roughness on
4
the absorber plate also known as turbulators. Turbulators of various shapes and sizes are reported to
5
increase the heat transfer coefficient in the solar air duct for the last three decades (Bhushan and Singh,
6
2011). Many efforts have been made to produce accurate prediction of the behavior of turbulator and
7
to set an optimum geometry design which makes the maximum heat transfer performance for a given
8
current.
9
In order to enhance the thermal efficiency of solar air heater, turbulators are employed on the
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inner wall surface of the absorber plate to break the laminar sub-layer and create local wall turbulence
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and as a result thermal resistance reduces. The important phenomena that increase the heat transfer
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rate are, reattachment of flow, formation of secondary flow and formation of vortices. One wall of the
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solar air heater duct with turbulators is exposed to uniform heat flux. However, it has been revealed
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that creating roughness on absorber plate is a difficult task, therefore a simple & suitable geometry
15
needs to be selected which should be easy to fabricate and repair (Bhushan and Singh, 2012). Heat
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transfer is achieved at the cost of increase in pressure drop across the heated surface (Holman, 1986).
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Considerable amount of research has been reported specifically to understand the influence of these
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factors on the overall performance of the system. Various geometries are explored and tested for
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artificial roughness or turbulators on the absorber plate along with their range of design and operating
20
parameters, are tabulated in Table 1.
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Table 1. Geometry and operating parameters of various turbulators. Investigator
Geometry
Parameters Value/Range
(Prasad and Saini, 1988)
e/D:0.02-0.033 p/e:10-20 Re:5,000-50,000
(Karwa et al., 2001)
e/D:0.0197-0.0441 p/e:4.58-7.09 Re:3750-16,350 W/H:6.88-9.38
(Kumar et al., 2013)
e/D:0.022-0.043 g/e:0.5-1.5 Gd/Le:0.24-0.80 W/w:1-10 p/e:6-12, d/w:0.2-0.8
2
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e/D:0.047, e:3.2 mm p/e:10.63, p:34 mm Re:2750-11,150 W/H:7.8, W:6.58 mm a:60° e/D:0.012-0.039 L/e:25-71.87 Re:1900-13,000 S/e:15.62-46.87 e/D:0.02-0.034 p/e:10 Re:2500-18,000 W/H:10.15 a:-30°-90°
(Karwa and Chitoshiya, 2013)
(Saini and Saini, 1997)
(Ebrahim Momin et al., 2002)
(Bhagoria et al., 2002)
e/D:0.015-0.033 p/e:60.17x Re:3,000-18,000 W/H:5
(Sahu and Bhagoria, 2005)
e/D:0.0338 e:1.5 p:10, 20, 30 Re:3,000-12,000 W/H: 8
(Karwa, 1933)
e/D:0.0212-0.0390 P/e:10 Re: 2800-15,000 a:60o-90o
(Aharwal et al., 2008)
d/W:0.167-0.5 e/D:0.0377 P/e:10 Re:3000-18,000 a:60o
(Sethi et al., 2012)
e/D:0.018-0.0396 e:1.6 mm P/e:3-8 Re:2000-14,000 W/H=10
(Tanda, 2011)
e/D:0.09 e:3 mm P/e:6.66-20 Re:5000-40,000 W/H=5 a:45o & 60o
3
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(Jaurker et al., 2006)
e/D:0.0181-0.0363 g/p:0.3-0.7 p/e:4.5-10 Re:3,000-21,000
(Kumar et al., 2013)
e/D:0.0213-0.0422 W/H:12 p/e:10 Re:2,000-17,000
(Saini and Verma, 2008)
e/D:0.0189-0.038 p/e:8-12 Re:2,000-12,000 e/D:0.0168-0.0338 W/H:8:1 e:0.75-1.5 mm p/e:10 Re:3,000-15,000 e/D:0.0189-0.038 p/e:8-12 Re:2,000-12,000 e/D:0.018-0.0338 e:0.8-1.5 mm W/H:8, p/e: 10 Re:2300-14,000
(Kumar et al., 2009) (Yadav and Bhagoria, 2014) (Lanjewar et al., 2011)
e/D:0.021-0.036 e/d:0.5, p/e:10-20 Re:3600-18,000 W/H:11
(Sethi et al., 2012)
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After conducting a comprehensive literature survey, it has been observed that, no study yet has
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been reported using arched absorber plate design in a long way length with turbulators as an artificial
4
roughness. The velocity of air, is one of the main operating parameters which effects the air motion
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and influences the thermal efficiency of the system (Ebrahim Momin et al., 2002). So, there is a need
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to control air motion or air turbulence to improve the overall performance of the solar air heater
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without increasing the overall pumping loss. Main aim of this study is to evaluate the thermal-
8
hydraulic performance of a novel solar air heater with arched absorber plate. Performance is compared
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using two differnet designs; dimply and equilateral triangle turbulators on the under-side of the arched
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absorber plate, in the terms of heat transfer rate (h), Nusselt number (Nu), frictional factor (Fr) and 4
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thermo-hydraulic performance parameter (THPP).
2
2. Performance evaluation of a solar air heater
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Two-dimensional turbulent flow through the solar air heater with equilateral triangular and
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dimply turbulators on the underside of the absorber plate is modeled and simulated. The solar air duct
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considered as the computational domain is simply the physical region over which the simulation has
6
been performed. The computational domain is shown in Fig. 1.
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Fig. 1. Solar air heater duct.
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The simulation work is conducted over the relevant range of Reynolds number, i.e. (3800 to
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14000) (Gupta et al., 1997), with a constant heat flux 500 W m-2, set on the absorber plate of the
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computational domain. The air flow is considered as turbulent two-dimensional, incompressible and
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steady. The thermo-physical properties of air have been assumed to remain constant and evaluated at
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300 K temperature. One wall of the rectangular air passage duct of solar air heater is subjected to
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uniform heat flux designed with arched turbulator while the remaining three walls are insulated.
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Various geometrical and operating parameters (both fixed and variable), used to evaluate the thermal
16
performance of a solar air heater are tabulated in Table 2. In the present analysis, limited geometrical
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parameters of the absorber plates are used.
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Table 2. Geometrical parameters, air properties and operating parameters used for simulation. Description Geometrical parameters Entrance section Test section Exit section Air duct width, W 5
Units
Value/Range
mm mm mm mm
245 280 115 100
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Air duct height, H Hydraulic diameter, D Roughness pitch, p Roughness height, e Curve depth, d Air properties Air thermal conductivity, k Specific heat of air, cp Air density, ρ Viscosity of air, μ Operating parameters Heat flux, q Convective heat transfer coefficient, h Reynolds number, Re Prandtl number, Pr Air inlet velocity, u Air inlet temperature, T
mm
mm
20 33.33 25 1.6 4
W m-1 K-1 J kg-1 K-1 kgm-3 Nsm-2
0.0262 1007 1.117 1.857e-05
W m-2 W m-2 K-1
500 10 to 90 3800 to18000 (four steps) 0.71 1.67 to 7.9 (four steps) 300
m s-1 K
1 2
Two types of turbulators are chosen to fix on the inner surface of the absorber plate, equilateral
3
triangular turbulator and dimply turbulator, because they yield high efficiency in solar air heating
4
system with artificial roughness (Sharma and Kalamkar, 2015), as shown in Fig. 2.
5
Fig. 2. Existing and proposed turbulator design.
6 7 8
2.1.
Solution method
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For the solution method, RNG k-ε turbulence model is selected with enhanced wall treatment
10
to predict the flow and forced convection characteristics of a fully developed turbulent flow through
11
the solar sir heater. SIMPLE (semi-implicit method for pressure linked equations) algorithm is adopted 6
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for pressure and velocity coupling. First order upward discretization scheme is selected for all transport
2
equations. Computational method and procedure discussed in the present analysis is adopted from the
3
method reported (Yadav and Bhagoria, 2014). Turbulator provided on the inside surface of the
4
absorber plate increase the turbulence in air flow motion which further results in reducing laminar sub-
5
layer propagation with the heated wall surface. It is required to analyze both thermal and hydraulic
6
performance of a solar air heater for making an effective design. Therefore, the thermo-hydraulic
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performance evaluation is carried out for four non-dimensional performance parameters: Reynolds
8
number (Re), Nusselt number (Nu), frictional factor (Fr) and thermal-hydraulic performance
9
parameter (THPP), and the same are computed using Equations (1−6). Where, Nur and Fr in Equations
10
(5 & 6) represents Nusselt number obtained from Dittus–Boelter equation and frictional factor
11
obtained from Blasius equation for smooth duct of a solar air heater respectively, also reported by
12
(Yadav and Bhagoria, 2014).
13
𝑅𝑒 =
𝜌𝑢𝐷 𝜇
(1)
14
𝑁𝑢 =
ℎ𝐷 𝑘
(2)
15
𝐹=
16
𝑇𝐻𝑃𝑃 =
17
where,
18
𝑁𝑢𝑟 = 0.023𝑅𝑒0.8𝑃𝑟0.4
(5)
19
𝐹𝑟 = 0.0791𝑅𝑒 ‒ 0.25
(6)
(∆𝑝/𝑙)𝐷
(3)
2𝜌𝑢2 (𝑁𝑢/𝑁𝑢𝑟)
(4)
(𝐹/𝐹𝑟)1/3
20 21
3. Results and discussion
22
Simulation work carried out using an arched absorber plate with equilateral triangle turbulator
23
and dimply turbulator on the inner side of the absorber plate wall is presented in this section. Thermal
24
performance of the proposed design is compared with the reported design of both turbulators for the
25
similar operating conditions.
26
Fig. 3 shows velocity contour for plane, equilateral triangle and dimply turbulators (both for
27
smooth and arched absorber plate) at constant air velocity i.e. 5.2 m s-1. It can be observed that the 7
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arched design of the absorber plate provides additional velocity enhancement along the duct length in
2
all casses. Varying cross sectional area provides increasing turbulence and vortex generation which
3
further reduces laminar sub layer generation near the absorber plate surface.
8
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Fig. 3. Velocity contour for plane, equilateral triangular and dimply turbulators at constant 5.2 m s-1
2
air velocity.
3
Nusselt Number
140 120 100
Plane (Dittus-Boetler equation) Curved Plane Equilateral Triangle (Yadav and Bhagoria, 2014) Curved Equilateral Triangle Dimple (Bhushan and Singh, 2011)
80 60 40 20 0 3000
6000
9000
12000
15000
Reynolds Number
4 5
Fig. 4. Variation of Nusselt number with respect to Reynolds number for plane, equilateral triangle
6
and dimply turbulator (for both smooth and arched absorber plate).
7
Fig. 4 shows the variation of Nusselt number with respect to Reynolds number for plane,
8
equilateral triangle and dimply turbulators (for both smooth and arched absorber plate). A significant
9
improvement is observed in the thermal performance of the solar air heater with an arched absorber
10
plate design. Increment in Nusselt number is observed at high Reynolds Number (above 10000); ~80%
11
for plane, ~16% for equilateral triangular and ~55.7% for dimply turbulators. However, a marginal
12
improvement is observed at a low Reynolds number (below 7000) ) upto; ~13% for equilateral
13
triangular and ~1% for dimply turbulators. This is attributed to the fact that, at low air velocity,
14
turbulent kinetic energy and vortex generation in the flow filed is very less, along the surface of the
15
absorber plate.
10
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0.06
Plane (Modified Blasius equation
Frictional Factor
Curved Plane
0.05
Equilateral Triangle (Yadav and Bhagoria, 2014) Curved Equilateral Triangle
0.04 0.03 0.02 0.01 0 3000
6000
9000
12000
15000
Reynolds Number
1 2
Fig. 5. Variation of frictional factor with respect to Reynolds number of plane, equilateral triangle
3
and dimply turbulators (for both smooth and arched absorber plate).
4
Fig. 5 shows the variation of frictional factor with respect to Reynolds number for plane,
5
equilateral triangle and dimply turbulators (for both smooth and arched absorber plate). It is observed
6
that the frictional factor increases with arched absorber plate design, providing extra obstruction in the
7
air flow field along the duct length. However, a marginal enhancement is observed in frictional factor
8
for all cases and the difference reduces further as the air velocity increases. It is observed that, at low
9
Reynolds number (below 7000), enhancement in frictional factor is by ~15% for plane, ~25% for
10
equilateral triangular and ~11% for dimply turbulators. However, it reduced upto ~9% for plane, ~8%
11
for equilateral triangular and ~2% for dimply turbulators at high Reynolds number (above 10000).
12
Along the duct length the vortex generation increases and this reduces the insulation provided by
13
laminar sub-layer generation.
11
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THPP
3.2
Plane Curved Plane
2.8
Equilateral Triangle (Yadav and Bhagoria, 2014)
2.4
Dimple (Bhushan and Singh, 2011)
Curved Equilateral Triangle
2 1.6 1.2 0.8 0.4 3000
6000
9000
12000
15000
Reynolds Number
1 2
Fig. 6. Variation of THPP with respect to Reynolds number for smooth, equilateral triangle and
3
dimply turbulators (for both smooth and arched absorber plate).
4
Fig. 6 shows the variation of THPP with respect to Reynolds number for plane, equilateral
5
triangle and dimply turbulators (for both smooth and arched absorber plate). Improvement is observed
6
in THPP for all the cases. Thermal-hydraulic performance of the solar air heater tends to increase with
7
increase in Reynolds number for all the cases and then decreases with further rise as expected for all
8
cases.
9 10
4. Conclusion
11
Applications in solar energy, one of the prominent renewable resource available, is expected
12
to increase in near future. Solar air heater is considered to be the most efficient system in order to
13
fulfill the increasing energy demand among the various options available.
14
In this study, an attempt is made to improve the thermal-hydraulic performance of a solar air
15
heater using arched shape absorber plate. A comparative study is carried out using two dimensional
16
CFD model with plane, equilateral triangle and dimply turbulators (both for smooth and arched
17
absorber plate design). Performance evaluation is carried out in the terms of heat transfer coefficient
18
(h), Nusselt number (Nu), Reynolds number (Rn) and Thermo-hydraulic performance parameter
19
(THPP). A significant improvement is observed in the thermal-hydraulic performance for solar air
12
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heater with an arched absorber plate design. Significant enhancement is observed in Nusselt number
2
at high Reynolds Number (above 10000) upto; ~80% for plane, ~16% for equilateral triangular and
3
~55.7% for dimply turbulators. However, a marginal improvement is observed at low Reynolds
4
number (below 7000) ) upto; ~13% for equilateral triangular and ~1% for dimply turbulators. With
5
respect to hydraulic performance, a marginal enhancement is observed in frictional factor by providing
6
an extra obstractle due to arched shaped absorber plate design, for all cases and the difference reduces
7
further as the air velocity increases. At low Reynolds number (below 7000), frictional factor is
8
enhanced by ~15% for plane, ~25% for equilateral triangular and ~11% for dimply turbulators. But, it
9
reduced upto ~9% for plane, ~8% for equilateral triangular and ~2% for dimply turbulators at high
10
Reynolds number (above 10000). The results are restricted to the design and operating parameters
11
adopted in this study. It is concluded that the arched absorber plate design increases the turbulence
12
and vortex generation in the air flow field along the duct length and which results in reducing laminar
13
sub-layer generation near the absorber surface and therefore, improves both thermal and hydraulic
14
performance of the solar air heater.
15
5. Future scope
16
Various turbulators of different shapes and sizes reported in the literature, can be re-tested with arched
17
shape design of the absorber plate and extract the additional improvement in the thermal and hydraulic
18
performance of the solar air heater for various operating parameters. The same trend is expected to
19
follow by the various turbulator designs.
20
References
21
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ACCEPTED MANUSCRIPT
Highlights A novel solar air heater with arched absorber plate is proposed and a comparative study is made using plane, equilateral triangle and dimple turbulators (for both smooth and arched absorber plate design). Significant improvement is observed in Nusselt number at high Reynolds number (above 10000); ~80% for plane, ~16% for equilateral triangular and ~55.7% for dimple turbulators. A marginal enhancement in the frictional factor is also observed and which reduces further at high Reynolds number (above 10000) upto; ~9% for plane, ~8% for equilateral triangular and ~2% for dimple turbulators.
Simarpreet Singh PII:
S0960-1481(17)30737-1
DOI:
10.1016/j.renene.2017.07.109
Reference:
RENE 9084
To appear in:
Renewable Energy
Received Date:
08 April 2017
Revised Date:
23 July 2017
Accepted Date:
25 July 2017
Please cite this article as: Simarpreet Singh, Performance evaluation of a novel solar air heater with arched absorber plate, Renewable Energy (2017), doi: 10.1016/j.renene.2017.07.109
This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.
ACCEPTED MANUSCRIPT
1
Performance evaluation of a novel solar air heater with arched
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absorber plate
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Simarpreet Singh*
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Department of Mechanical Engineering, BITS-Pilani, Rajasthan, India.
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*Corresponding Author: +91-8094041444; email: [email protected]
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Abstract
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The study presents a performance evaluation of a novel solar air heating system with arched
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absorber plate using turbulators. Simulation work is carried out in ANSYS FLUENT (v16.2) platform
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with RNG k-ε turbulence model at constant heat flux (500 W m-2), in order to compare the thermal-
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hydraulic performance of the proposed design for a range of Reynolds number (3800 to 14000).
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Performance of the solar air heater is projected in the terms of Nusselt number (Nu), frictional factor
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(Fr) and thermal-hydraulic performance parameter (THPP). It is observed that the arched absorber
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plate design increases the air turbulence and vortex generation, which results in reducing laminar sub-
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layer generation near the surface of the absorber plate. A significant improvement is observed in
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Nusselt number at high Reynolds Number (above 10000). However, a marginal enhancement is also
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observed in frictional factor due to providing an extra obstacle along the duct length with arched
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shaped design in the flow field. It is observed that arched shape design of absorber plate can
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significantly improve overall performance of the solar air heating system using various turbulators.
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This study will likewise provide a new direction to the work trend in this area.
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Keywords: Solar air heater, turbulator, THPP, dimply, equilateral triangle.
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1. Introduction
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Prior to 1970, solar energy was viewed as a prominent alternative to fulfill the continuous
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increasing demand of energy. An extensive research work was carried out to extract the maximum
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benefit of this renewable source of energy, due to the high rise in oil prices in 1970s (Yadav and
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Bhagoria, 2014). Among various alternatives, solar energy stands out as the best resource option to
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fulfill the increasing energy demand. Instead of direct use of solar energy it is more useful when
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converted into thermal energy (Bhushan and Singh, 2010). From literature, it has been revealed that
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the solar air heater is considered as one of the most effective system among various options available
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to capture solar energy for heating applications in various sectors. In order to improve the thermal
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efficiency of the solar air heater, obstruction in the flow field was proposed by artificial roughness on
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the absorber plate also known as turbulators. Turbulators of various shapes and sizes are reported to
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increase the heat transfer coefficient in the solar air duct for the last three decades (Bhushan and Singh,
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2011). Many efforts have been made to produce accurate prediction of the behavior of turbulator and
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to set an optimum geometry design which makes the maximum heat transfer performance for a given
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current.
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In order to enhance the thermal efficiency of solar air heater, turbulators are employed on the
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inner wall surface of the absorber plate to break the laminar sub-layer and create local wall turbulence
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and as a result thermal resistance reduces. The important phenomena that increase the heat transfer
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rate are, reattachment of flow, formation of secondary flow and formation of vortices. One wall of the
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solar air heater duct with turbulators is exposed to uniform heat flux. However, it has been revealed
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that creating roughness on absorber plate is a difficult task, therefore a simple & suitable geometry
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needs to be selected which should be easy to fabricate and repair (Bhushan and Singh, 2012). Heat
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transfer is achieved at the cost of increase in pressure drop across the heated surface (Holman, 1986).
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Considerable amount of research has been reported specifically to understand the influence of these
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factors on the overall performance of the system. Various geometries are explored and tested for
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artificial roughness or turbulators on the absorber plate along with their range of design and operating
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parameters, are tabulated in Table 1.
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Table 1. Geometry and operating parameters of various turbulators. Investigator
Geometry
Parameters Value/Range
(Prasad and Saini, 1988)
e/D:0.02-0.033 p/e:10-20 Re:5,000-50,000
(Karwa et al., 2001)
e/D:0.0197-0.0441 p/e:4.58-7.09 Re:3750-16,350 W/H:6.88-9.38
(Kumar et al., 2013)
e/D:0.022-0.043 g/e:0.5-1.5 Gd/Le:0.24-0.80 W/w:1-10 p/e:6-12, d/w:0.2-0.8
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e/D:0.047, e:3.2 mm p/e:10.63, p:34 mm Re:2750-11,150 W/H:7.8, W:6.58 mm a:60° e/D:0.012-0.039 L/e:25-71.87 Re:1900-13,000 S/e:15.62-46.87 e/D:0.02-0.034 p/e:10 Re:2500-18,000 W/H:10.15 a:-30°-90°
(Karwa and Chitoshiya, 2013)
(Saini and Saini, 1997)
(Ebrahim Momin et al., 2002)
(Bhagoria et al., 2002)
e/D:0.015-0.033 p/e:60.17x Re:3,000-18,000 W/H:5
(Sahu and Bhagoria, 2005)
e/D:0.0338 e:1.5 p:10, 20, 30 Re:3,000-12,000 W/H: 8
(Karwa, 1933)
e/D:0.0212-0.0390 P/e:10 Re: 2800-15,000 a:60o-90o
(Aharwal et al., 2008)
d/W:0.167-0.5 e/D:0.0377 P/e:10 Re:3000-18,000 a:60o
(Sethi et al., 2012)
e/D:0.018-0.0396 e:1.6 mm P/e:3-8 Re:2000-14,000 W/H=10
(Tanda, 2011)
e/D:0.09 e:3 mm P/e:6.66-20 Re:5000-40,000 W/H=5 a:45o & 60o
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(Jaurker et al., 2006)
e/D:0.0181-0.0363 g/p:0.3-0.7 p/e:4.5-10 Re:3,000-21,000
(Kumar et al., 2013)
e/D:0.0213-0.0422 W/H:12 p/e:10 Re:2,000-17,000
(Saini and Verma, 2008)
e/D:0.0189-0.038 p/e:8-12 Re:2,000-12,000 e/D:0.0168-0.0338 W/H:8:1 e:0.75-1.5 mm p/e:10 Re:3,000-15,000 e/D:0.0189-0.038 p/e:8-12 Re:2,000-12,000 e/D:0.018-0.0338 e:0.8-1.5 mm W/H:8, p/e: 10 Re:2300-14,000
(Kumar et al., 2009) (Yadav and Bhagoria, 2014) (Lanjewar et al., 2011)
e/D:0.021-0.036 e/d:0.5, p/e:10-20 Re:3600-18,000 W/H:11
(Sethi et al., 2012)
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After conducting a comprehensive literature survey, it has been observed that, no study yet has
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been reported using arched absorber plate design in a long way length with turbulators as an artificial
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roughness. The velocity of air, is one of the main operating parameters which effects the air motion
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and influences the thermal efficiency of the system (Ebrahim Momin et al., 2002). So, there is a need
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to control air motion or air turbulence to improve the overall performance of the solar air heater
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without increasing the overall pumping loss. Main aim of this study is to evaluate the thermal-
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hydraulic performance of a novel solar air heater with arched absorber plate. Performance is compared
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using two differnet designs; dimply and equilateral triangle turbulators on the under-side of the arched
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absorber plate, in the terms of heat transfer rate (h), Nusselt number (Nu), frictional factor (Fr) and 4
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thermo-hydraulic performance parameter (THPP).
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2. Performance evaluation of a solar air heater
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Two-dimensional turbulent flow through the solar air heater with equilateral triangular and
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dimply turbulators on the underside of the absorber plate is modeled and simulated. The solar air duct
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considered as the computational domain is simply the physical region over which the simulation has
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been performed. The computational domain is shown in Fig. 1.
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Fig. 1. Solar air heater duct.
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The simulation work is conducted over the relevant range of Reynolds number, i.e. (3800 to
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14000) (Gupta et al., 1997), with a constant heat flux 500 W m-2, set on the absorber plate of the
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computational domain. The air flow is considered as turbulent two-dimensional, incompressible and
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steady. The thermo-physical properties of air have been assumed to remain constant and evaluated at
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300 K temperature. One wall of the rectangular air passage duct of solar air heater is subjected to
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uniform heat flux designed with arched turbulator while the remaining three walls are insulated.
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Various geometrical and operating parameters (both fixed and variable), used to evaluate the thermal
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performance of a solar air heater are tabulated in Table 2. In the present analysis, limited geometrical
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parameters of the absorber plates are used.
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Table 2. Geometrical parameters, air properties and operating parameters used for simulation. Description Geometrical parameters Entrance section Test section Exit section Air duct width, W 5
Units
Value/Range
mm mm mm mm
245 280 115 100
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Air duct height, H Hydraulic diameter, D Roughness pitch, p Roughness height, e Curve depth, d Air properties Air thermal conductivity, k Specific heat of air, cp Air density, ρ Viscosity of air, μ Operating parameters Heat flux, q Convective heat transfer coefficient, h Reynolds number, Re Prandtl number, Pr Air inlet velocity, u Air inlet temperature, T
mm
mm
20 33.33 25 1.6 4
W m-1 K-1 J kg-1 K-1 kgm-3 Nsm-2
0.0262 1007 1.117 1.857e-05
W m-2 W m-2 K-1
500 10 to 90 3800 to18000 (four steps) 0.71 1.67 to 7.9 (four steps) 300
m s-1 K
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Two types of turbulators are chosen to fix on the inner surface of the absorber plate, equilateral
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triangular turbulator and dimply turbulator, because they yield high efficiency in solar air heating
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system with artificial roughness (Sharma and Kalamkar, 2015), as shown in Fig. 2.
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Fig. 2. Existing and proposed turbulator design.
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2.1.
Solution method
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For the solution method, RNG k-ε turbulence model is selected with enhanced wall treatment
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to predict the flow and forced convection characteristics of a fully developed turbulent flow through
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the solar sir heater. SIMPLE (semi-implicit method for pressure linked equations) algorithm is adopted 6
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for pressure and velocity coupling. First order upward discretization scheme is selected for all transport
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equations. Computational method and procedure discussed in the present analysis is adopted from the
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method reported (Yadav and Bhagoria, 2014). Turbulator provided on the inside surface of the
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absorber plate increase the turbulence in air flow motion which further results in reducing laminar sub-
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layer propagation with the heated wall surface. It is required to analyze both thermal and hydraulic
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performance of a solar air heater for making an effective design. Therefore, the thermo-hydraulic
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performance evaluation is carried out for four non-dimensional performance parameters: Reynolds
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number (Re), Nusselt number (Nu), frictional factor (Fr) and thermal-hydraulic performance
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parameter (THPP), and the same are computed using Equations (1−6). Where, Nur and Fr in Equations
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(5 & 6) represents Nusselt number obtained from Dittus–Boelter equation and frictional factor
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obtained from Blasius equation for smooth duct of a solar air heater respectively, also reported by
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(Yadav and Bhagoria, 2014).
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𝑅𝑒 =
𝜌𝑢𝐷 𝜇
(1)
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𝑁𝑢 =
ℎ𝐷 𝑘
(2)
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𝐹=
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𝑇𝐻𝑃𝑃 =
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where,
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𝑁𝑢𝑟 = 0.023𝑅𝑒0.8𝑃𝑟0.4
(5)
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𝐹𝑟 = 0.0791𝑅𝑒 ‒ 0.25
(6)
(∆𝑝/𝑙)𝐷
(3)
2𝜌𝑢2 (𝑁𝑢/𝑁𝑢𝑟)
(4)
(𝐹/𝐹𝑟)1/3
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3. Results and discussion
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Simulation work carried out using an arched absorber plate with equilateral triangle turbulator
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and dimply turbulator on the inner side of the absorber plate wall is presented in this section. Thermal
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performance of the proposed design is compared with the reported design of both turbulators for the
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similar operating conditions.
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Fig. 3 shows velocity contour for plane, equilateral triangle and dimply turbulators (both for
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smooth and arched absorber plate) at constant air velocity i.e. 5.2 m s-1. It can be observed that the 7
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arched design of the absorber plate provides additional velocity enhancement along the duct length in
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all casses. Varying cross sectional area provides increasing turbulence and vortex generation which
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further reduces laminar sub layer generation near the absorber plate surface.
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Fig. 3. Velocity contour for plane, equilateral triangular and dimply turbulators at constant 5.2 m s-1
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air velocity.
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Nusselt Number
140 120 100
Plane (Dittus-Boetler equation) Curved Plane Equilateral Triangle (Yadav and Bhagoria, 2014) Curved Equilateral Triangle Dimple (Bhushan and Singh, 2011)
80 60 40 20 0 3000
6000
9000
12000
15000
Reynolds Number
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Fig. 4. Variation of Nusselt number with respect to Reynolds number for plane, equilateral triangle
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and dimply turbulator (for both smooth and arched absorber plate).
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Fig. 4 shows the variation of Nusselt number with respect to Reynolds number for plane,
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equilateral triangle and dimply turbulators (for both smooth and arched absorber plate). A significant
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improvement is observed in the thermal performance of the solar air heater with an arched absorber
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plate design. Increment in Nusselt number is observed at high Reynolds Number (above 10000); ~80%
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for plane, ~16% for equilateral triangular and ~55.7% for dimply turbulators. However, a marginal
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improvement is observed at a low Reynolds number (below 7000) ) upto; ~13% for equilateral
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triangular and ~1% for dimply turbulators. This is attributed to the fact that, at low air velocity,
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turbulent kinetic energy and vortex generation in the flow filed is very less, along the surface of the
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absorber plate.
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0.06
Plane (Modified Blasius equation
Frictional Factor
Curved Plane
0.05
Equilateral Triangle (Yadav and Bhagoria, 2014) Curved Equilateral Triangle
0.04 0.03 0.02 0.01 0 3000
6000
9000
12000
15000
Reynolds Number
1 2
Fig. 5. Variation of frictional factor with respect to Reynolds number of plane, equilateral triangle
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and dimply turbulators (for both smooth and arched absorber plate).
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Fig. 5 shows the variation of frictional factor with respect to Reynolds number for plane,
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equilateral triangle and dimply turbulators (for both smooth and arched absorber plate). It is observed
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that the frictional factor increases with arched absorber plate design, providing extra obstruction in the
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air flow field along the duct length. However, a marginal enhancement is observed in frictional factor
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for all cases and the difference reduces further as the air velocity increases. It is observed that, at low
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Reynolds number (below 7000), enhancement in frictional factor is by ~15% for plane, ~25% for
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equilateral triangular and ~11% for dimply turbulators. However, it reduced upto ~9% for plane, ~8%
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for equilateral triangular and ~2% for dimply turbulators at high Reynolds number (above 10000).
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Along the duct length the vortex generation increases and this reduces the insulation provided by
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laminar sub-layer generation.
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THPP
3.2
Plane Curved Plane
2.8
Equilateral Triangle (Yadav and Bhagoria, 2014)
2.4
Dimple (Bhushan and Singh, 2011)
Curved Equilateral Triangle
2 1.6 1.2 0.8 0.4 3000
6000
9000
12000
15000
Reynolds Number
1 2
Fig. 6. Variation of THPP with respect to Reynolds number for smooth, equilateral triangle and
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dimply turbulators (for both smooth and arched absorber plate).
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Fig. 6 shows the variation of THPP with respect to Reynolds number for plane, equilateral
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triangle and dimply turbulators (for both smooth and arched absorber plate). Improvement is observed
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in THPP for all the cases. Thermal-hydraulic performance of the solar air heater tends to increase with
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increase in Reynolds number for all the cases and then decreases with further rise as expected for all
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cases.
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4. Conclusion
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Applications in solar energy, one of the prominent renewable resource available, is expected
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to increase in near future. Solar air heater is considered to be the most efficient system in order to
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fulfill the increasing energy demand among the various options available.
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In this study, an attempt is made to improve the thermal-hydraulic performance of a solar air
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heater using arched shape absorber plate. A comparative study is carried out using two dimensional
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CFD model with plane, equilateral triangle and dimply turbulators (both for smooth and arched
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absorber plate design). Performance evaluation is carried out in the terms of heat transfer coefficient
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(h), Nusselt number (Nu), Reynolds number (Rn) and Thermo-hydraulic performance parameter
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(THPP). A significant improvement is observed in the thermal-hydraulic performance for solar air
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heater with an arched absorber plate design. Significant enhancement is observed in Nusselt number
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at high Reynolds Number (above 10000) upto; ~80% for plane, ~16% for equilateral triangular and
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~55.7% for dimply turbulators. However, a marginal improvement is observed at low Reynolds
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number (below 7000) ) upto; ~13% for equilateral triangular and ~1% for dimply turbulators. With
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respect to hydraulic performance, a marginal enhancement is observed in frictional factor by providing
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an extra obstractle due to arched shaped absorber plate design, for all cases and the difference reduces
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further as the air velocity increases. At low Reynolds number (below 7000), frictional factor is
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enhanced by ~15% for plane, ~25% for equilateral triangular and ~11% for dimply turbulators. But, it
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reduced upto ~9% for plane, ~8% for equilateral triangular and ~2% for dimply turbulators at high
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Reynolds number (above 10000). The results are restricted to the design and operating parameters
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adopted in this study. It is concluded that the arched absorber plate design increases the turbulence
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and vortex generation in the air flow field along the duct length and which results in reducing laminar
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sub-layer generation near the absorber surface and therefore, improves both thermal and hydraulic
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performance of the solar air heater.
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5. Future scope
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Various turbulators of different shapes and sizes reported in the literature, can be re-tested with arched
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shape design of the absorber plate and extract the additional improvement in the thermal and hydraulic
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performance of the solar air heater for various operating parameters. The same trend is expected to
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follow by the various turbulator designs.
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Highlights A novel solar air heater with arched absorber plate is proposed and a comparative study is made using plane, equilateral triangle and dimple turbulators (for both smooth and arched absorber plate design). Significant improvement is observed in Nusselt number at high Reynolds number (above 10000); ~80% for plane, ~16% for equilateral triangular and ~55.7% for dimple turbulators. A marginal enhancement in the frictional factor is also observed and which reduces further at high Reynolds number (above 10000) upto; ~9% for plane, ~8% for equilateral triangular and ~2% for dimple turbulators.