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Comprehensive study on solar air heater with circular and V-type turbulators attached on absorber plate
Energy xxx (2015) 1e11
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Comprehensive study on solar air heater with circular and V-type turbulators attached on absorber plate T. Rajaseenivasan, S. Srinivasan, K. Srithar* Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai 625015, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2015 Received in revised form 11 June 2015 Accepted 4 July 2015 Available online xxx
Performance enrichment in a single pass SAH (solar air heater) with circular and V-shape inserts are considered in this paper. Two identical SAH: conventional and modified SAH are tested with different Reynolds number ranging from 6000 to 12,000. Circular and V-shape turbulators are fixed in the absorber plate of modified SAH at six different configurations; inline arrangements with 4 4 (type-a), 5 4 (type-b), 6 4 (type-c) and 6 4 zigzag arrangement of circular inserts (type-d). Experiments are extended by introducing V-type inserts in convex (type-e) and concave shape (type-f) to create additional turbulence motion. Experiment results revealed that the system efficiency increases with Reynolds number and number of turbulators in absorber plate. Air temperature reaches an upper value of 66 C in type-f with the mass flow rate of 57.7 kg/hr. Nusselt number increases with the Reynolds number and reaches the maximum of 210 for type-f turbulators at Reynolds number of 11615. Thermal enhancement factor decreases with increase in Reynolds number for all modifications. First law, thermohydraulic and second law efficiency increases up to 85%, 63% and 45% respectively for type-f at Reynolds number of 11615. Theoretical analysis also carried out and agrees well with experimental results. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar air heater Circular turbulator V-shape turbulator Exergy analysis Thermal enhancement factor Performance study
1. Introduction Energy is a crucial driving factor in today's world and plays major role in economic growth and industrialization. Population growth and its material needs increase the demand of energy every year. On the other side, consumption of fossil fuel reduces the available resource and cause to environmental degradation and it creates the awareness towards renewable energy sources. Considering other alternatives, solar energy stands a promising future of renewable energy. Solar energy is free and provides an infinite and eco-friendly reservoir of energy. The easiest way to utilize solar energy is by converting it into thermal energy using solar collectors. Air heating is one of the major solar thermal applications, used for space heating and process heating like laundry, desalination, crop drying and other drying processes. Utilizing of conventional electrical energy for this process will increase the process cost as well as pollute the environment. Using solar energy for air heating will reduce the operational cost of the system, environmental free and reduce the consumption of conventional energy.
* Corresponding author. E-mail address: [email protected] (K. Srithar).
Conventional air heaters are typically low efficient due to its high thermal resistance and low heat transfer rate. Heat transfer rate can be improved by creating turbulence in the flow field. Mohammadi and Sabzphoosanim [1] investigated the influence of fins and baffles attached with the absorber plate of single pass SAH and the results revealed that attaching fins and baffles effectively increases the outlet air temperature and efficiency, in comparison to a simple conventional device. Karsli et al. [2] fabricated a single pass flat plate collector for drying applications and concluded that efficiency depends on solar radiation and the surface geometry of solar air collectors. Krishnananth and Murugavel [3] investigated the ability of a double pass SAH to store the heat with paraffin wax and found that the efficiency of SAH integrated with thermal storage medium is higher than the conventional one. Gupta and Kaushik [4] conducted an exergetic performance and parametric studies of a solar air heater. The exergy evaluation criterion routed an optimal value of aspect ratio and duct depth, which depends on mass flow rate. Sabzpooshani et al. [5] studied the exergetic analysis for single pass air heater with baffles and established that, increasing the baffle width, reducing the distance between baffles and increasing the number of fins are effective at low mass flow rates and the inverse trend in higher mass flow rate.
http://dx.doi.org/10.1016/j.energy.2015.07.020 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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Singh et al. [6] made an exergy analysis on air heater having vdown rib roughness on absorber plate and resulted in better performance compared with conventional air heater under same climatic conditions. Lanjewar et al. [7] experimentally studied the heat transfer and friction factor in duct with w-shaped rib roughness on absorber plate and concluded that thermo-hydraulic performance improves with angle of attack of the flow with maximum at 60 and relative roughness height. Sun et al. [8] studied the influence of duct depth on the performance of solar air heaters and discovered an optimum height of 10 cm for single pass and for double pass, the ratio of upper to down channel height should not be less than 11 cm. Sebaii et al. [9] considered the double pass flat and v-shaped corrugated plate solar air heaters and concluded that v-shaped air heater displays 11e14% more efficiency and better thermo hydraulic performance than the flat plate double pass air heater. Tanda [10] tested the SAH with different types of ribs on absorber plate and concluded that repeated ribs were considered as effective way to improve convective heat transfer and also that all other rib configurations are better than smooth channel air heater. Karwa and Chitoshiya [11] examined the SAH having v-down ribs on absorber plate and found that 12.5e20% more thermal efficiency than the normal smooth duct. Aldabagg et al. [12] reported the performance of single and double pass air heaters with wire mesh as packing bed instead of flat absorber plate and found that efficiency of double pass is 34e45% greater than single pass and also displays that packed bed collector indicates a substantial enhancement in thermal efficiency over the conventional collector. Ramadan et al. [13] stated that double pass SAH with packed bed materials like limestone and gravel exhibits increased thermo hydraulic performance. Chaube et al. [14] studied the different geometries square, rectangular, chamfered, semicircular, circular rib for a Reynolds number range from 3000 to 20,000 and the best thermal performance was found with chamfered ribs. Nowzari et al. [15] considered the single and double pass SAH with partially perforated covers and packed mesh and concluded that efficiency of double pass is always 5e22.7% greater than single pass SAH. Sara et al. [16] investigated the performance of flat plate collector fitted with rectangular and perforated blocks and confirmed that energy gain of about 20% larger than the channel without perforated blocks. Gentry and Jacobi [17] achieved an average of 50e60% heat enhancement in a flat plate collector using delta-wing vortex generators. Zhou and Ye [18] stated that curved trapezoidal winglet has best thermo-hydraulic behavior at fully turbulent flow region. Alvarez et al. [19] reported that SAH with aluminium cans on absorber plate has maximum efficiency of 74%. Ozg et al. [20] experimentally investigated the double flow air heater with aluminium cans and it also leads to improved thermal efficiency than the single flow SAH with aluminum cans fitted on absorber plate. Aforementioned literatures explained that the ability of solar air heater is considerably improved by creating the turbulence effect in flow field. In previous works, fins with baffles, different types of rib (v shape, w shape, square, rectangular, circular, chamfered and semi circular), corrugated plate, wire mesh, packed bed materials, perforated duct, rectangular and solid block, delta wing vortex generator, trapezoidal winglet and aluminium cans were used as insert in SAH. Presence of these turbulators enhances the heat transfer rate in the solar air heater. Thus the present objective of this work is to impact the recasting of a solar air heater with different new configurations namely circular and V - shape turbulators in the flow field to augment its efficiency. Initially, experiments are carried out with circular turbulator in the flow passage as inline arrangement with number of insert ranging as 4 4, 5 4,
6 4 and then number of insert with 6 4 zigzag arrangement for optimized result. To create the additional turbulence in the flow field, two V-type inserts are placed inside the 6 4 zigzag circular turbulators at convex and concave shape. A conventional SAH is simultaneously experimented with modified SAH for comparison purpose. 2. Experimental setup and procedure A graphic view of the conventional single pass SAH is displayed in Fig. 1. Both solar air heaters consist of a sheet metal enclosure (1 mm thickness) with the size of 0.75 0.95 0.1 m3 which makes the test section of SAH. This enclosure is extended as duct in both sides with the reduction in breath from 0.75 m to 0.2 m in 0.3 m length. Window glass of 6 mm thickness used as transparent cover. Black coated steel plate with the thickness of 1.6 mm and area of 0.7 0.9 m2 is used as an absorber plate in both SAH. Thermocol of thickness 5 cm is used as insulation material to reduce the heat loss through the side and bottom. SAH is placed at an inclination of 10 equal to latitude of Madurai to receive possible utmost radiation (Fig. 2). Modified SAH is designed with same features of conventional SAH except providing turbulators (Circular and V-type turbulators) in the absorber plate to create turbulence effect. Circular turbulators are made by hollow steel pipes of 2 inch diameter with 10 cm length. V-type turbulators are made by L-angles with the size of 5 2 0.03 cm. The modifications and number of turbulators are given in Table 1 and in Fig. 3. Air blower (1 hp) is connected with the flow control valve to provide appropriate quantity of air and flow rate is measured by adopting an orifice meter setup. Pressure drop across the test section is measured with the help of U-tube manometer. The temperature of the systems is measured in absorber plate (4 points), glass cover (2 points), inlet and outlet air by using K-type thermocouple. Thermocouples are linked with digital temperature indicator and selector switch arrangement. Solar radiation is measured with the help of solarimeter. All the experiments were carried out from 9 am to 4 pm local time at Thiagarajar College of Engineering, Madurai, India in the month of June to September 2014. Due to varying climatic condition, the experiments are operated at different days and discussion is made for average solar radiation and ambient temperature condition within the deviation of 10% for comparison purpose. 3. Theoretical analysis One dimensional steady state energy balance is formulated for the single pass solar air heater considered in this work. Some assumptions are made to simplify the analysis: (i) There is no air leakage from the heater and negligible edge los; (ii) Air inside the heater does not absorb the solar radiation; (iii) Air has uniform velocity inside the heater; (iv) The air temperature varies in the flow direction only. On the basis of above assumptions, the following energy balance equations are formulated as follows for conventional solar air heater. Energy balance at absorber plate [21]
Iap tg Ap ¼ mp Cp;p
dTp þ qc;pa þ qr;pa þ qloss dt
(1)
Energy balance for fluid medium [21]
qc;pa þ qr;pa þ qc;ga ¼ ma Cp;a
dTa dt
(2)
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Fig. 1. Schematic diagram of conventional solar air heater (Dimensions are in cm e Not to scale).
qr;pg ¼ Ac hr;pg Tp Tg
(4)
The radiative heat transfer coefficient between the plate to glass is given by Ref. [1].
hr;pg
s Tp2 þ Tg2 T p þTg ¼ 1 1 εp þ εg 1
(5)
The heat loss from absorber plate to the ambient is given by Ref. [1].
qloss ¼ Ac Uloss Tp Tamb
(6)
where the bottom heat loss coefficient can be calculate by Ref. [1].
Uloss ¼
Lins kins
(7)
The convective and radiative heat transfer from glass cover to ambient air is given by Ref. [1]. Fig. 2. Photographic view of modified (left) and conventional (right) solar air heater.
Energy balance at glass cover [21]
Iag Ag þ qr;pg ¼ mg Cp;g
dTg þ qc;gamb þ qr;gsky þ qc;ga dt
(3)
The radiative heat transfer rate between the plate to glass can be computed by Ref. [1].
qc;gamb ¼ Ac hc;gamb Tg Tamb
(8)
qr;gsky ¼ Ac hr;gsky Tg Tsky
(9)
where the convective and radiative heat transfer coefficient from the cover to the atmosphere is as follows [1].
Table 1 Turbulators arrangement and notation in modified solar air heater. S. No
Notation
Arrangement
No. of turbulators (Circular þ V-shape)
1 2 3 4 5 6 7
Conventional Type (a) Type (b) Type (c) Type (d) Type (e) Type (f)
Conventional 4 4 inline 5 4 inline 6 4 inline 6 4 inline with zigzag 6 4 inline with zigzag and convex pattern inside the cylinder 6 4 inline with zigzag and concave pattern inside the cylinder
e 16 20 24 39 39 39
þ þ þ þ þ þ
0 0 0 0 78 78
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Fig. 3. Arrangement of turbulators in absorber plate.
hc;g;amb ¼ 2:8 þ 3V hr;gsky ¼ εg s
Tg2
þ
2 Tsky
(10)
Tsky ¼ Tamb 6
Tg þ Tsky
(11) (12)
The Convection heat transfer rate from absorber plate to air is given by Ref. [1].
qc;pa ¼ Ac hc;pa Tp Ta
(13)
ka Dh
(14)
hc;pa ¼ Nu
The Nusselt number is estimated by following equation for fully developed turbulent flow [1,5].
Nu ¼ 0:0158ðReÞ0:8
Tout ¼ Tout þ dTout
(18)
The experimentally measured values of solar radiation and ambient temperature of the corresponding day are used for evaluating these parameters. The parameters are used in this modeling given in Table 2. The above same procedure is followed for the modified solar air heater expect the convective heat transfer coefficient relation due to the presence of turbulators in absorber plate. Nusselt number of the solar air heater is depends on the surface geometry of collector which affects by the hydraulic diameter of the system. The plate fin heat exchanger offset strip fin array correlations are used in this theoretical analysis due to lack of correlations available for circular pipe turbulators and the turbulators arrangement is almost same as fin heat exchanger offset strip fin [22,23]. There are three correlations used to find out the Nusselt Number and it is compared with the experimental values to validate the result. Wieting correlation [24]
!0:322
(15)
For the first time step, initial temperatures of absorber plate, glass and air are assumed equal to the atmosphere temperature. Different heat transfer coefficients are computed as given in Eqns. (4)e(15) and used to determine the temperatures. The change in absorber plate, glass cover and air is calculated by solving the Eqns. (1)e(3), respectively for the interval of 10 s. For the next time step, the parameter is redefined as
Tp ¼ Tp þ dTp
Tg ¼ Tg þ dTg
(16) (17)
Nu ¼ 0:242
where
l DH;turb
DH;turb ¼
!0:089
t
1
Re0:632 Pr3
DH;turb
2sh sþh
(19)
(20)
Dubrovsky and Vasiliev correlation [25]
Nu ¼ 0:000437
t Dh;turb
!2:6
l Dh;turb
!0:15 Rex
(21)
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Table 2 Parameters used for theoretical study [26,27].
x ¼ 2:2
S. No
Thermo physical values used
1 2 3 4 5 6 7 8 9 10 11 12
Area of absorber plate (Ap) Absorptivity of absorber plate (ab) Transmitivity of glass (tg) Specific heat of absorber plate (Cp,p) Specific heat of air (Cp,a) Absorptivity of glass (ag) Specific heat of glass (Cp,g) Density heat of air ðra Þ Thermal conductivity of air (ka) Dynamic viscosity of air ðma Þ Mass of absorber plate (mp) Mass of glass (mg)
t Dh;turb
!0:55
l
!0:02
Dh;turb
(22)
Manglik and Bergles [22]
Nu ¼ 0:6522Re0:4597 a0:1541 d0:1499 g0:0678 i0:1 h 1 þ 5:269 105 a0:504 d0:456 g1:055 Pr1=3
(23)
Pr ¼ 0.7, a ¼ s=h, d ¼ t=l, g ¼ t=s: Where the hydraulic diameter with turbulator is given as [22,23].
Dh;turb ¼
4shl 2ðsl þ hl þ thÞ þ ts
(24)
This theoretically calculated Nusselt number for conventional and modified solar air heater is compared with the experimental values discussed in Section 4.1.1.
0.63 m2 0.95 0.9 473 J/kg C 1.005 þ 0.000066 (Tf 27) 0.0475 800 J/kg C 1.1774e0.00359 (Tf 27) 0.02624 þ 0.0000758 (Tf 27) ½ 1:983 þ 0:00184 ðTf 27Þ 105 4.5 kg 2.7 kg
4.1.2. Friction factor Friction factor ðf Þ is the parameter used to calculate the frictional resistance of the fluid passing through the channel. It is related with the pressure drop ðDPÞ across the channel [7]. Friction factor is inversely proportional to the heat transfer rate.
f ¼
DPD 2rLV 2
(28)
4.1.3. Nusselt number and friction factor ratio Nusselt number ratio ðNur Þ and friction factor ratio ðfr Þ indicates the Nusselt number and friction factor of modified SAH increased than conventional SAH in number of times respectively.
Nur ¼
fr ¼
Num Nusselt number of improved SAH ¼ Nusselt number of conventional SAH Nuc
fm Friction factor of improved SAH ¼ Friction factor of conventional SAH fc
(29)
(30)
4. Experimental data reduction Performance of solar air heater should evaluate to know the effect of different modifications in absorber plate. For this purpose, experimentally measured values are used and the results are discussed in terms of Nusselt number, friction factor, TEF (thermal enhancement factor), energy efficiency, thermohydraulic and exergy efficiency. Method to estimate the above parameters are discussed below.
4.1.4. Thermal enhancement factor Nusselt number ratio Nur reveals the performance improvement in modified SAH without considering the effect of friction factor. Thermal enhancement factor (TEF) is used to evaluate the capacity of the modified system over conventional by considering constant pumping power [29,30]. This factor is calculated by considering the effect of Nusselt number and friction factor and given as [29,30].
4.1. Heat transfer analysis
4.1.1. Nusselt number The heat transfer rate, Qu to the air from absorber plate can be equated as [7,28].
Qu ¼ mCp ðTout Tin Þ ¼ hc Ap Tp Tf Tf ¼
Tin þ Tout 2
hc D k
c
¼ f Re3
m
(31)
TEF is the ratio between the convective heat transfer coefficient of the modified collector and conventional collector at constant pumping power [30,31].
(25) TEF ¼
ðNur Þ 1
ðfr Þ3
(32)
(26)
From Eqn. (25), experimental heat transfer coefficient can be determined and based on this experimental Nu (Nusselt number) can be calculated from following equation [7].
Nu ¼
f Re3
(27)
4.2. Energy efficiency Energy (First law) efficiency measures the potential of solar air heater in energy conversion process. It is the ratio between useful heat gain to the incident solar radiation on collector area [11]. Better thermal efficiency is an indication of more heat absorption of air.
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hI ¼
mCp ðTout Tin Þ Useful heat gain Qu ¼ ¼ Heat input to collector Qin Ap tg ap I
(33)
Uncertainty in a measurement can be defined as the root sum square of the fixed or bias error (EB) of the instrumentation and random error (ER) observed at various measurements. Bias error is a constant and systematic error in the process. Random error is the repeatability error [23,35,36].
4.3. Thermohydraulic efficiency Solar air heater is operated by using an electrical blower which converts the electrical energy into mechanical energy and forces the air through the heating system. The amount of energy (pumping power e PP) required for this system is vary with the pressure drop due to the expansion (entry) and contraction (exit) of heating systems and frictional losses due to movement air over absorber plate. These factors reduce the system efficiency and this pumping power is considered for evaluate the thermohydraulic efficiency of system [32]. Whereas the energy efficiency does not consider this effect (Eqn. (33)) and it leads in higher than thermohydraulic efficiency.
hTHD ¼
Qu PP Ap tg ap I
(34)
where the pumping is calculated from
PP ¼
FP ¼
FP
(35)
hBlower mDP r
U¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E2B þ E2R
(43)
Uncertainty range of the Reynolds number friction factor and Nusselt number is calculated using the following relation
" #0:5 Dm 2 DDh 2 þ m Dh
DRe ¼ Re Df ¼f f
"
DNu ¼ Nu
DðDPÞ DP
2
DL þ L
2
3DDh þ Dh
DQu ¼ Qu
2
#0:5 2DRe 2 þ Re
" 2 #0:5 Dh 2 DD þ h D
2 DTp 2 Dh 4 DQu 2 ¼ þ þ h Qu Tp
(36)
(44)
"
(45)
(46)
DTf Tf
!2 30:5 5
(47)
#0:5 Dm 2 DTin 2 DTout 2 þ þ m Tin Tout
(48)
4.4. Exergy efficiency of solar air heater Second law (Exergy) analysis is used to represent the quantitatively useful energy. Exergy efficiency (Exergy) of a system is a measure of work potential and is given by Ref. [33].
hII ¼
Exergy Output Exout ¼ Exergy Input Exin
(37)
Exergy output of the SAH is calculated by using the change in enthalpy, entropy and it is given as by Ref. [33].
Exout ¼ m ½DH Ta Ds
(38)
DH ¼ Hout Hin ¼ Cp ðTout Tin Þ
(39)
Ds ¼ sout sin ¼ Cp ln
Tout Pout R ln Tin Pin
(40)
6. Results and discussion This section discusses the performance of the solar air heater by comparing the theoretical and experimental results and discuss in terms of Nusselt number, friction factor, Nusselt number ratio, friction factor ratio, thermal enhancement factor, energy, exergy and thermohydraulic efficiency. Also the discussion is extended by means of studying the effect of solar radiation and wind velocity. 6.1. Validation of experimental results Fig. 4 compares the theoretical and experimental Nusselt number of conventional and modified SAH (type-c). The experimental results of conventional SAH build a good agreement with correlations and the deviations are between ±6%. In modified SAH,
Exergy input from the system is given as [33].
Ta Exin ¼ I t a Ap 1 Ts
(41)
Primary quantities
5. Uncertainty analysis The uncertainty associated with Primary (Thermocouple, solarimeter, manometer, anemometer and hygrometer) and secondary quantities (Reynolds number, Nusselt number and friction factor) are calculated and given in Table 3. Error for the primary quantities is calculated as follows [34].
Error ¼
Accuracy of instrument Minimum value of the out put measured
Table 3 Uncertainty analysis for primary and secondary quantities.
(42)
Sl. No
Instrument
Accuracy
Range
% Error
1 2 3 4 5
Thermocouple Solarimeter Manometer Anemometer Hygrometer
±1 C ±1 W/m2 1 mm ±0.1 m/s ±1
0e100 C 0e2500 W/m2 500 mm 0e15 m/s 0e100
0.25% 2.5% 4% 10% 0.25%
Secondary quantities Sl. No
Parameter
Uncertainty (%)
1 2 3
Reynolds number Nusselt number Friction factor
±2.3 ±4.47 ±3.2
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Fig. 4. Comparison of experimental and theoretical Nusselt number.
Fig. 5. Hourly variation of different parameters in SAH.
weiting correlation is matches more with the experimental results than other two correlations. Deviation between the experimental and weiting Nusselt number varies between the ±8.5%. Whereas the deviation is about ±14.1% for Manglik and Bergles correlations and ±17.8% for Dubrovsky and Vasiliev correlations. Thus the further discussions are carried out with weiting correlation for modified SAH. Temperature of the absorber plate, glass and air for conventional and modified SAH is presented in Fig. 5. The variation
between theoretical and experimental results is within 10% for both the conventional and modified SAH. 6.2. Effect of heat transfer rate and friction factor The consequence on the Nusselt number and friction factor characteristics of SAH with different modifications are presented in Figs. 6 and 7 respectively. Placing the turbulators increases the
Fig. 6. Nusselt number effect on different modifications in SAH.
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Fig. 7. Friction factor effect on different modifications in SAH.
heat transfer rate than conventional SAH. Nusselt number is increase with Reynolds number for all modifications and the Nusselt number ratio follows the opposite pattern. It shows that enhance in Nusselt number is low for modified SAH than conventional SAH which resulted in lower Nusselt number ratio with increase in Reynolds number. In particular, type (f) has higher Nusselt number than others due to the presence of V-type turbulators in concave shape which increases the residence time of air by means of restricting the flow of air. Corresponding highest friction is observed in type -f arrangement as presented in Fig. 7 due to immense resistance to flow than other modifications. Increase in contact surface and maximizing the reverse flow leads to higher friction factor ratio 71 at Re of 11615. 6.3. Effect of the thermal enhancement factor
Fig. 8. Effect of TEF on Reynolds number.
contact surface between the air and absorber surface, resulted in increased Nusselt number. Circumferences of the turbulators generate the turbulence in air flow rate at regular interval throughout the flow field which ensures further enhancement in
Fig. 8 analyzes the behavior of thermal enhancement factor (TEF) in modified SAH. Thermal enhancement factor evaluate the performance of SAH by considering the effect of Nusselt number ratio and friction factor ratio. It is identified that the TEF value of all types of turbulators is above unity and it means that adoption of turbulators provide thermal advantage over conventional SAH. TEF attains high value at lower mass flow rate in all condition, due to the superior value of Nusselt number ratio. Increased thermal performance is recorded with the type (f) arrangement than any other due to the larger interruption in the flow field.
Fig. 9. Reduced temperature parameter on energy and exergy efficiency.
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Fig. 10. Comparison of thermohydraulic and energy efficiency.
6.4. Effect of energy and exergy efficiency
6.6. Heat losses in solar air heater
Physical characteristics of the collector effectiveness can be represented by a curve that indicates efficiency versus the reduced temperature parameters as shown in Fig. 9. Energy efficiency is reduces with increase in temperature parameters due to higher heat loss with increase in solar radiation. Both the energy and exergy efficiency is higher for type (f) modification due to higher heat transfer rate.
The effect of wind velocity on energy losses is plotted in Fig. 11. Convective heat transfer rate from glass to ambient ðQc;ga Þ is strongly depends on the wind velocity of the particular day. It can be noted that the other heat losses (radiative heat transfer from ambient to air and bottom heat loss) are not affected by the wind velocity. Also with increase in wind velocity the convective heat loss becomes higher than other heat losses at mid noon.
6.5. Thermohydraulic efficiency of solar air heater
6.7. Effect of solar radiation on temperature
Thermohydraulic efficiency of the SAH is a measure of efficiency due to pumping power is given in Fig. 10. It is observed from thermohydraulic efficiency graph, the pumping power is less for conventional SAH when compared to the modified SAH. However, the outlet temperature and system efficiency of modified SAH is higher than conventional as shown in energy efficiency graph and it shows the benefit of the turbulators. Thermohydraulic efficiency follows the similar path with energy efficiency with a little drop in efficiency due to variation in change in pressure. Average reduction in thermohydraulic efficiency is about 5% compared to energy efficiency, which is acceptable range in the performance of the system.
Response of varying solar radiation on the temperature difference between the outlet and inlet of air is presented in Fig. 12. Both the solar radiation and turbulator modification has considerable effect on the outlet temperature of air. Highest temperature difference of 29 C is achieved with type (f) turbulator at the solar radiation of 900 W/m2. It clearly demonstrates the benefit of the turbulators in SAH. Fig. 13 compares the TEF of present work with previous works in different absorber plate condition [37 e 39]. Using of different type ribs in absorber plate enhances the TEF by considerably. However,
Fig. 11. Heat losses in SAH with time.
Fig. 12. Effect of solar radiation on outlet and inlet air temperature difference.
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Fig. 13. Comparison of TEF on present and previous works.
increasing of geometry complex for turbulence leads to higher performance than previous results. Type (a) has lower TEF than ribbed surfaces due to less number of turbulators and the TEF increases significantly by increasing the number of turbulators (Type f). The ribbed grooves and other ribs used in the previous works are covers the flow height maximum of 25% whereas in present work it covers 50% of the flow height. It leads to higher mixing rate of air throughout the section and higher thermal performance. 7. Conclusion A detailed theoretical and experimental study has been performed to evaluate the capacity of the single pass SAH with six patterns of turbulators in the absorber plate with a wide range of operating conditions. Theoretical results are compared with experimental results and the deviations are within 10%. Nusselt number ratio is found to be greater at lower Reynolds number and decreases by increasing Reynolds number. Friction factor ratio is higher for type-(f) because it point out larger resistance in flow over other modifications and reaches highest of 71 at Re 11615. Thermal enhancement factor proclaims an impressive progress in the type (f) arrangement with top figure of 3.65 at Re of 6200. Energy and exergy efficiencies are decrease with increase in temperature reduces parameter. Presence of turbulators increases the pumping power of the system and reduces the efficiency by 1e5% depends on the modification. Wind velocity has a significant effect on convective heat loss from glass to ambient and it is directly proportional. Circular turbulators integrated with V-type insert yields a supreme outlet air temperature of 66 C at the mass flow rate of 0.016 kg/s. 6 4 zigzag arrangement of circular turbulators with concave shape inserts demonstrates better performance compared to other patterns experimented and also better than conventional model. However, all the patterns expose greater scope than the conventional SAH which proves that introducing turbulators helps in improving the thermal performance of the solar air heater.
D Ex EB ER f H h I k L m Nu P Pr Q R Re s T U V
D
r
Hydraulic diameter, m Exergy Bias Error Random Error Friction factor Enthalpy, J/kg Heat transfer coefficient, W/m2 K Solar radiation, W/m2 Thermal conductivity, W/m K Length of test section, m Mass flow rate, kg/s Nusselt number Pressure, N/m2 Prandtl number Heat transfer rate, W Gas constant, J/kg K Reynolds number Entropy, J/kg K Temperature, C Uncertainity Velocity of air, m/s change Density of air, kg/m3
The authors would like to acknowledge with appreciation, TEQIP e II grant, Thiagarajar College of Engineering, Madurai, India for their partial financial support in this research.
Abbreviations PP Pumping power FP Flow power TEF Thermal enhancement factor SAH Solar Air Heater out Outlet in inlet p absorber plate f fluid g glass cover a ambient s sun c conventional m modified a Absorpitivity h Efficiency t Transmitivity
Nomenclature
References
Acknowledgement
A Cp
Area, m2 Specific het capacity of air, J/kg K
[1] Mohammadi K, Sabzpooshani M. Comprehensive performance evaluation and parametric studies of single pass solar air heater with fins and baffles attached over the absorber plate. Energy 2013;57:741e50.
Please cite this article in press as: Rajaseenivasan T, et al., Comprehensive study on solar air heater with circular and V-type turbulators attached on absorber plate, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.07.020
T. Rajaseenivasan et al. / Energy xxx (2015) 1e11 [2] Karsli S. Performance analysis of new-design solar air collectors for drying applications. Renew Energy 2007;32:1645e60. [3] Krishnananth SS, Murugavel KK. Experimental study on double pass solar air heater with thermal energy storage. J King Saud Univ Eng Sci 2013;25: 135e40. [4] Gupta MK, Kaushik SC. Exergetic performance evaluation and parametric studies of solar air heater. Energy 2008;33:1691e702. [5] Sabzpooshani M, Mohammadi K, Khorasanizadeh H. Exergetic performance evaluation of a single pass baffled solar air Heater. Energy 2014;64:697e706. [6] Singh S, Chander S, Saini JS. Exergy based analysis of solar air heater having discrete V-down rib roughness on absorber plate. Energy 2012;37:749e58. [7] Lanjewar A, Bhagoria JL, Sarviya RM. Heat transfer and friction in solar air heater duct with W-shaped rib roughness on absorber plate. Energy 2011;36: 4531e41. [8] Sun W, Ji J, He W. Influence of channel depth on the performance of solar air heaters. Energy 2010;35:4201e7. [9] El-Sebaii AA, Aboul-Enein S, Ramadan MRI, Shalaby SM, Moharram BM. Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters. Energy 2011;36:1076e86. [10] Tanda G. Performance of solar air heater ducts with different types of ribs on the absorber plate. Energy 2011;36:6651e60. [11] Karwa R, Chitoshiya G. Performance study of solar air heater having v-down discrete ribs on absorber plate. Energy 2013;55:939e55. [12] Aldabbagh LBY, Egelioglu F, Ilkan M. Single and double pass solar air heaters with wire mesh as packing bed. Energy 2010;35:3783e7. [13] Ramadan MRI, El-Sebaii AA, Aboul-Enein S, El-Bialy E. Thermal performance of a packed bed double-pass solar air heater. Energy 2007;32:1524e35. [14] Chaube A, Sahoo PK, Solanki SC. Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater. Renew Energy 2006;31:317e31. [15] Nowzari R, Aldabbagh LBY, Egelioglu F. Single and double pass solar air heaters with partially perforated cover and packed mesh. Energy 2014;73: 694e702. [16] Sara ON, Pekdemir T, Yapici S, Yilmaz M. Heat-transfer enhancement in a channel flow with perforated rectangular blocks. Int J Heat Fluid Flow 2001;22:509e18. [17] Gentry MC, Jacobi AM. Heat transfer enhancement by delta-wing vortex generators on a flat plate vortex interaction with the boundary layer. Exp Therm Fluid Sci 1997;14:231e42. [18] Zhou G, Ye Q. Experimental investigations of thermal and flow characteristics of curved trapezoidal winglet type vortex generators. Appl Therm Eng 2012;37:241e8. [19] Alvarez G, Arce J, Lira L, Heras MR. Thermal performance of an air solar collector with an absorber plate made of recyclable aluminum cans. Sol Energy 2004;77:107e13. [20] Ozgen F, Esen M, Esen H. Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans. Renew Energy 2009;34:2391e8. [21] Paisarn N, Kongtragool B. Theoretical study on heat transfer characteristics and performance of the flat-plate solar air heaters. Int Commun Heat Mass Transf 2003;30(8):1125e36.
11
[22] Manglik RM, Bergles AE. Heat transfer and pressure drop correlations for the rectangular offset strip fin compact heat exchanger. Exp Therm Fluid Sci 1995;10(2):171e80. [23] Yang M, Yang X, Li X, Wang Z, Wang P. Design and optimization of a solar air heater with offset strip fin absorber plate. Appl Energy 2014;113:1349e62. [24] Wieting AR. Empirical correlations for heat transfer and flow friction characteristics of rectangular offset-fin plate-fin heat exchangers. Trans ASME J Heat Transf 1975;97:488e90. [25] Dubrovsky EV, Vasiliev VY. Enhancement of convective heat transfer in rectangular ducts of interrupted surfaces. Int J Heat Mass Transf 1988;31: 807e18. [26] Ong KS. Thermal performance of solar air heaters: mathematical model and solution procedure. Sol Energy 1995;55(2):93e109. [27] Rajaseenivasan T, Murugavel KK. Theoretical and experimental investigation on double basin double slope solar still. Desalination 2013;319:25e32. [28] Singh S, Chander S, Saini JS. Investigations on thermo-hydraulic performance due to flow-attack-angle in V-down rib with gap in a rectangular duct of solar air heater. Appl Energy 2012;97:907e12. [29] Promvonge P, Eiamsa-ard S. Heat transfer in a circular tube fitted with free spacing snail entry and conical nozzle turbulators. Int Commun Heat Mass Transf 2007;34:838e48. [30] Muthusamy C, Vivar M, Skryabin I, Srithar K. Effect of conical cut-out turbulators with internal fins in a circular tube on heat transfer and friction factor. Int Commun Heat Mass Transf 2013;44:64e8. [31] Kongkaitpaiboon V, Nanan K, Eiamsa-ard S. Experimental investigation of heat transfer and turbulent flow friction in a tube fitted with perforated conical rings. Int Commun Heat Mass Transf 2010;37:560e7. [32] Hegazy AA. Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates. Energy Convers Manag 2000;41:1361e78. [33] Oztop HF, Bayrak F, Hepbasli A. Energetic and exergetic aspects of solar air heating (solar collector) systems. Renew Sustain Energy Rev 2013;21:59e83. [34] Velmurugan V, Pandiarajan S, Guruparan P, Harihara Subramanian L, David Prabaharan C, Srithar K. Integrated performance of stepped and single basin solar stills with mini solar pond. Desalination 2009;249:902e9. [35] Jaisankar S, Radhakrishnan TK, Sheeba KN. Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes. Energy 2009;34:1054e64. [36] Moffat RJ. Describing the uncertainties in experimental results. Exp Therm Fluid Sci 1988;1:3e17. [37] Skullong S, Kwankaomeng S, Thianpong C, Promvonge P. Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators. Int Commun Heat Mass Transf 2014;50:34e43. [38] Lanjewar A, Bhagoria JL, Sarviya RM. Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness. Exp Therm Fluid Sci 2011;35:986e95. [39] Promvonge P, Khanoknaiyakarn C, Kwankaomeng S, Thianpong C. Thermal behavior in solar air heater channel fitted with combined rib and deltawingle. Int Commun Heat Mass Transf 2011;38:749e56.
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Contents lists available at ScienceDirect
Energy journal homepage: www.elsevier.com/locate/energy
Comprehensive study on solar air heater with circular and V-type turbulators attached on absorber plate T. Rajaseenivasan, S. Srinivasan, K. Srithar* Department of Mechanical Engineering, Thiagarajar College of Engineering, Madurai 625015, Tamil Nadu, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 8 March 2015 Received in revised form 11 June 2015 Accepted 4 July 2015 Available online xxx
Performance enrichment in a single pass SAH (solar air heater) with circular and V-shape inserts are considered in this paper. Two identical SAH: conventional and modified SAH are tested with different Reynolds number ranging from 6000 to 12,000. Circular and V-shape turbulators are fixed in the absorber plate of modified SAH at six different configurations; inline arrangements with 4 4 (type-a), 5 4 (type-b), 6 4 (type-c) and 6 4 zigzag arrangement of circular inserts (type-d). Experiments are extended by introducing V-type inserts in convex (type-e) and concave shape (type-f) to create additional turbulence motion. Experiment results revealed that the system efficiency increases with Reynolds number and number of turbulators in absorber plate. Air temperature reaches an upper value of 66 C in type-f with the mass flow rate of 57.7 kg/hr. Nusselt number increases with the Reynolds number and reaches the maximum of 210 for type-f turbulators at Reynolds number of 11615. Thermal enhancement factor decreases with increase in Reynolds number for all modifications. First law, thermohydraulic and second law efficiency increases up to 85%, 63% and 45% respectively for type-f at Reynolds number of 11615. Theoretical analysis also carried out and agrees well with experimental results. © 2015 Elsevier Ltd. All rights reserved.
Keywords: Solar air heater Circular turbulator V-shape turbulator Exergy analysis Thermal enhancement factor Performance study
1. Introduction Energy is a crucial driving factor in today's world and plays major role in economic growth and industrialization. Population growth and its material needs increase the demand of energy every year. On the other side, consumption of fossil fuel reduces the available resource and cause to environmental degradation and it creates the awareness towards renewable energy sources. Considering other alternatives, solar energy stands a promising future of renewable energy. Solar energy is free and provides an infinite and eco-friendly reservoir of energy. The easiest way to utilize solar energy is by converting it into thermal energy using solar collectors. Air heating is one of the major solar thermal applications, used for space heating and process heating like laundry, desalination, crop drying and other drying processes. Utilizing of conventional electrical energy for this process will increase the process cost as well as pollute the environment. Using solar energy for air heating will reduce the operational cost of the system, environmental free and reduce the consumption of conventional energy.
* Corresponding author. E-mail address: [email protected] (K. Srithar).
Conventional air heaters are typically low efficient due to its high thermal resistance and low heat transfer rate. Heat transfer rate can be improved by creating turbulence in the flow field. Mohammadi and Sabzphoosanim [1] investigated the influence of fins and baffles attached with the absorber plate of single pass SAH and the results revealed that attaching fins and baffles effectively increases the outlet air temperature and efficiency, in comparison to a simple conventional device. Karsli et al. [2] fabricated a single pass flat plate collector for drying applications and concluded that efficiency depends on solar radiation and the surface geometry of solar air collectors. Krishnananth and Murugavel [3] investigated the ability of a double pass SAH to store the heat with paraffin wax and found that the efficiency of SAH integrated with thermal storage medium is higher than the conventional one. Gupta and Kaushik [4] conducted an exergetic performance and parametric studies of a solar air heater. The exergy evaluation criterion routed an optimal value of aspect ratio and duct depth, which depends on mass flow rate. Sabzpooshani et al. [5] studied the exergetic analysis for single pass air heater with baffles and established that, increasing the baffle width, reducing the distance between baffles and increasing the number of fins are effective at low mass flow rates and the inverse trend in higher mass flow rate.
http://dx.doi.org/10.1016/j.energy.2015.07.020 0360-5442/© 2015 Elsevier Ltd. All rights reserved.
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T. Rajaseenivasan et al. / Energy xxx (2015) 1e11
Singh et al. [6] made an exergy analysis on air heater having vdown rib roughness on absorber plate and resulted in better performance compared with conventional air heater under same climatic conditions. Lanjewar et al. [7] experimentally studied the heat transfer and friction factor in duct with w-shaped rib roughness on absorber plate and concluded that thermo-hydraulic performance improves with angle of attack of the flow with maximum at 60 and relative roughness height. Sun et al. [8] studied the influence of duct depth on the performance of solar air heaters and discovered an optimum height of 10 cm for single pass and for double pass, the ratio of upper to down channel height should not be less than 11 cm. Sebaii et al. [9] considered the double pass flat and v-shaped corrugated plate solar air heaters and concluded that v-shaped air heater displays 11e14% more efficiency and better thermo hydraulic performance than the flat plate double pass air heater. Tanda [10] tested the SAH with different types of ribs on absorber plate and concluded that repeated ribs were considered as effective way to improve convective heat transfer and also that all other rib configurations are better than smooth channel air heater. Karwa and Chitoshiya [11] examined the SAH having v-down ribs on absorber plate and found that 12.5e20% more thermal efficiency than the normal smooth duct. Aldabagg et al. [12] reported the performance of single and double pass air heaters with wire mesh as packing bed instead of flat absorber plate and found that efficiency of double pass is 34e45% greater than single pass and also displays that packed bed collector indicates a substantial enhancement in thermal efficiency over the conventional collector. Ramadan et al. [13] stated that double pass SAH with packed bed materials like limestone and gravel exhibits increased thermo hydraulic performance. Chaube et al. [14] studied the different geometries square, rectangular, chamfered, semicircular, circular rib for a Reynolds number range from 3000 to 20,000 and the best thermal performance was found with chamfered ribs. Nowzari et al. [15] considered the single and double pass SAH with partially perforated covers and packed mesh and concluded that efficiency of double pass is always 5e22.7% greater than single pass SAH. Sara et al. [16] investigated the performance of flat plate collector fitted with rectangular and perforated blocks and confirmed that energy gain of about 20% larger than the channel without perforated blocks. Gentry and Jacobi [17] achieved an average of 50e60% heat enhancement in a flat plate collector using delta-wing vortex generators. Zhou and Ye [18] stated that curved trapezoidal winglet has best thermo-hydraulic behavior at fully turbulent flow region. Alvarez et al. [19] reported that SAH with aluminium cans on absorber plate has maximum efficiency of 74%. Ozg et al. [20] experimentally investigated the double flow air heater with aluminium cans and it also leads to improved thermal efficiency than the single flow SAH with aluminum cans fitted on absorber plate. Aforementioned literatures explained that the ability of solar air heater is considerably improved by creating the turbulence effect in flow field. In previous works, fins with baffles, different types of rib (v shape, w shape, square, rectangular, circular, chamfered and semi circular), corrugated plate, wire mesh, packed bed materials, perforated duct, rectangular and solid block, delta wing vortex generator, trapezoidal winglet and aluminium cans were used as insert in SAH. Presence of these turbulators enhances the heat transfer rate in the solar air heater. Thus the present objective of this work is to impact the recasting of a solar air heater with different new configurations namely circular and V - shape turbulators in the flow field to augment its efficiency. Initially, experiments are carried out with circular turbulator in the flow passage as inline arrangement with number of insert ranging as 4 4, 5 4,
6 4 and then number of insert with 6 4 zigzag arrangement for optimized result. To create the additional turbulence in the flow field, two V-type inserts are placed inside the 6 4 zigzag circular turbulators at convex and concave shape. A conventional SAH is simultaneously experimented with modified SAH for comparison purpose. 2. Experimental setup and procedure A graphic view of the conventional single pass SAH is displayed in Fig. 1. Both solar air heaters consist of a sheet metal enclosure (1 mm thickness) with the size of 0.75 0.95 0.1 m3 which makes the test section of SAH. This enclosure is extended as duct in both sides with the reduction in breath from 0.75 m to 0.2 m in 0.3 m length. Window glass of 6 mm thickness used as transparent cover. Black coated steel plate with the thickness of 1.6 mm and area of 0.7 0.9 m2 is used as an absorber plate in both SAH. Thermocol of thickness 5 cm is used as insulation material to reduce the heat loss through the side and bottom. SAH is placed at an inclination of 10 equal to latitude of Madurai to receive possible utmost radiation (Fig. 2). Modified SAH is designed with same features of conventional SAH except providing turbulators (Circular and V-type turbulators) in the absorber plate to create turbulence effect. Circular turbulators are made by hollow steel pipes of 2 inch diameter with 10 cm length. V-type turbulators are made by L-angles with the size of 5 2 0.03 cm. The modifications and number of turbulators are given in Table 1 and in Fig. 3. Air blower (1 hp) is connected with the flow control valve to provide appropriate quantity of air and flow rate is measured by adopting an orifice meter setup. Pressure drop across the test section is measured with the help of U-tube manometer. The temperature of the systems is measured in absorber plate (4 points), glass cover (2 points), inlet and outlet air by using K-type thermocouple. Thermocouples are linked with digital temperature indicator and selector switch arrangement. Solar radiation is measured with the help of solarimeter. All the experiments were carried out from 9 am to 4 pm local time at Thiagarajar College of Engineering, Madurai, India in the month of June to September 2014. Due to varying climatic condition, the experiments are operated at different days and discussion is made for average solar radiation and ambient temperature condition within the deviation of 10% for comparison purpose. 3. Theoretical analysis One dimensional steady state energy balance is formulated for the single pass solar air heater considered in this work. Some assumptions are made to simplify the analysis: (i) There is no air leakage from the heater and negligible edge los; (ii) Air inside the heater does not absorb the solar radiation; (iii) Air has uniform velocity inside the heater; (iv) The air temperature varies in the flow direction only. On the basis of above assumptions, the following energy balance equations are formulated as follows for conventional solar air heater. Energy balance at absorber plate [21]
Iap tg Ap ¼ mp Cp;p
dTp þ qc;pa þ qr;pa þ qloss dt
(1)
Energy balance for fluid medium [21]
qc;pa þ qr;pa þ qc;ga ¼ ma Cp;a
dTa dt
(2)
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3
Fig. 1. Schematic diagram of conventional solar air heater (Dimensions are in cm e Not to scale).
qr;pg ¼ Ac hr;pg Tp Tg
(4)
The radiative heat transfer coefficient between the plate to glass is given by Ref. [1].
hr;pg
s Tp2 þ Tg2 T p þTg ¼ 1 1 εp þ εg 1
(5)
The heat loss from absorber plate to the ambient is given by Ref. [1].
qloss ¼ Ac Uloss Tp Tamb
(6)
where the bottom heat loss coefficient can be calculate by Ref. [1].
Uloss ¼
Lins kins
(7)
The convective and radiative heat transfer from glass cover to ambient air is given by Ref. [1]. Fig. 2. Photographic view of modified (left) and conventional (right) solar air heater.
Energy balance at glass cover [21]
Iag Ag þ qr;pg ¼ mg Cp;g
dTg þ qc;gamb þ qr;gsky þ qc;ga dt
(3)
The radiative heat transfer rate between the plate to glass can be computed by Ref. [1].
qc;gamb ¼ Ac hc;gamb Tg Tamb
(8)
qr;gsky ¼ Ac hr;gsky Tg Tsky
(9)
where the convective and radiative heat transfer coefficient from the cover to the atmosphere is as follows [1].
Table 1 Turbulators arrangement and notation in modified solar air heater. S. No
Notation
Arrangement
No. of turbulators (Circular þ V-shape)
1 2 3 4 5 6 7
Conventional Type (a) Type (b) Type (c) Type (d) Type (e) Type (f)
Conventional 4 4 inline 5 4 inline 6 4 inline 6 4 inline with zigzag 6 4 inline with zigzag and convex pattern inside the cylinder 6 4 inline with zigzag and concave pattern inside the cylinder
e 16 20 24 39 39 39
þ þ þ þ þ þ
0 0 0 0 78 78
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Fig. 3. Arrangement of turbulators in absorber plate.
hc;g;amb ¼ 2:8 þ 3V hr;gsky ¼ εg s
Tg2
þ
2 Tsky
(10)
Tsky ¼ Tamb 6
Tg þ Tsky
(11) (12)
The Convection heat transfer rate from absorber plate to air is given by Ref. [1].
qc;pa ¼ Ac hc;pa Tp Ta
(13)
ka Dh
(14)
hc;pa ¼ Nu
The Nusselt number is estimated by following equation for fully developed turbulent flow [1,5].
Nu ¼ 0:0158ðReÞ0:8
Tout ¼ Tout þ dTout
(18)
The experimentally measured values of solar radiation and ambient temperature of the corresponding day are used for evaluating these parameters. The parameters are used in this modeling given in Table 2. The above same procedure is followed for the modified solar air heater expect the convective heat transfer coefficient relation due to the presence of turbulators in absorber plate. Nusselt number of the solar air heater is depends on the surface geometry of collector which affects by the hydraulic diameter of the system. The plate fin heat exchanger offset strip fin array correlations are used in this theoretical analysis due to lack of correlations available for circular pipe turbulators and the turbulators arrangement is almost same as fin heat exchanger offset strip fin [22,23]. There are three correlations used to find out the Nusselt Number and it is compared with the experimental values to validate the result. Wieting correlation [24]
!0:322
(15)
For the first time step, initial temperatures of absorber plate, glass and air are assumed equal to the atmosphere temperature. Different heat transfer coefficients are computed as given in Eqns. (4)e(15) and used to determine the temperatures. The change in absorber plate, glass cover and air is calculated by solving the Eqns. (1)e(3), respectively for the interval of 10 s. For the next time step, the parameter is redefined as
Tp ¼ Tp þ dTp
Tg ¼ Tg þ dTg
(16) (17)
Nu ¼ 0:242
where
l DH;turb
DH;turb ¼
!0:089
t
1
Re0:632 Pr3
DH;turb
2sh sþh
(19)
(20)
Dubrovsky and Vasiliev correlation [25]
Nu ¼ 0:000437
t Dh;turb
!2:6
l Dh;turb
!0:15 Rex
(21)
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5
Table 2 Parameters used for theoretical study [26,27].
x ¼ 2:2
S. No
Thermo physical values used
1 2 3 4 5 6 7 8 9 10 11 12
Area of absorber plate (Ap) Absorptivity of absorber plate (ab) Transmitivity of glass (tg) Specific heat of absorber plate (Cp,p) Specific heat of air (Cp,a) Absorptivity of glass (ag) Specific heat of glass (Cp,g) Density heat of air ðra Þ Thermal conductivity of air (ka) Dynamic viscosity of air ðma Þ Mass of absorber plate (mp) Mass of glass (mg)
t Dh;turb
!0:55
l
!0:02
Dh;turb
(22)
Manglik and Bergles [22]
Nu ¼ 0:6522Re0:4597 a0:1541 d0:1499 g0:0678 i0:1 h 1 þ 5:269 105 a0:504 d0:456 g1:055 Pr1=3
(23)
Pr ¼ 0.7, a ¼ s=h, d ¼ t=l, g ¼ t=s: Where the hydraulic diameter with turbulator is given as [22,23].
Dh;turb ¼
4shl 2ðsl þ hl þ thÞ þ ts
(24)
This theoretically calculated Nusselt number for conventional and modified solar air heater is compared with the experimental values discussed in Section 4.1.1.
0.63 m2 0.95 0.9 473 J/kg C 1.005 þ 0.000066 (Tf 27) 0.0475 800 J/kg C 1.1774e0.00359 (Tf 27) 0.02624 þ 0.0000758 (Tf 27) ½ 1:983 þ 0:00184 ðTf 27Þ 105 4.5 kg 2.7 kg
4.1.2. Friction factor Friction factor ðf Þ is the parameter used to calculate the frictional resistance of the fluid passing through the channel. It is related with the pressure drop ðDPÞ across the channel [7]. Friction factor is inversely proportional to the heat transfer rate.
f ¼
DPD 2rLV 2
(28)
4.1.3. Nusselt number and friction factor ratio Nusselt number ratio ðNur Þ and friction factor ratio ðfr Þ indicates the Nusselt number and friction factor of modified SAH increased than conventional SAH in number of times respectively.
Nur ¼
fr ¼
Num Nusselt number of improved SAH ¼ Nusselt number of conventional SAH Nuc
fm Friction factor of improved SAH ¼ Friction factor of conventional SAH fc
(29)
(30)
4. Experimental data reduction Performance of solar air heater should evaluate to know the effect of different modifications in absorber plate. For this purpose, experimentally measured values are used and the results are discussed in terms of Nusselt number, friction factor, TEF (thermal enhancement factor), energy efficiency, thermohydraulic and exergy efficiency. Method to estimate the above parameters are discussed below.
4.1.4. Thermal enhancement factor Nusselt number ratio Nur reveals the performance improvement in modified SAH without considering the effect of friction factor. Thermal enhancement factor (TEF) is used to evaluate the capacity of the modified system over conventional by considering constant pumping power [29,30]. This factor is calculated by considering the effect of Nusselt number and friction factor and given as [29,30].
4.1. Heat transfer analysis
4.1.1. Nusselt number The heat transfer rate, Qu to the air from absorber plate can be equated as [7,28].
Qu ¼ mCp ðTout Tin Þ ¼ hc Ap Tp Tf Tf ¼
Tin þ Tout 2
hc D k
c
¼ f Re3
m
(31)
TEF is the ratio between the convective heat transfer coefficient of the modified collector and conventional collector at constant pumping power [30,31].
(25) TEF ¼
ðNur Þ 1
ðfr Þ3
(32)
(26)
From Eqn. (25), experimental heat transfer coefficient can be determined and based on this experimental Nu (Nusselt number) can be calculated from following equation [7].
Nu ¼
f Re3
(27)
4.2. Energy efficiency Energy (First law) efficiency measures the potential of solar air heater in energy conversion process. It is the ratio between useful heat gain to the incident solar radiation on collector area [11]. Better thermal efficiency is an indication of more heat absorption of air.
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hI ¼
mCp ðTout Tin Þ Useful heat gain Qu ¼ ¼ Heat input to collector Qin Ap tg ap I
(33)
Uncertainty in a measurement can be defined as the root sum square of the fixed or bias error (EB) of the instrumentation and random error (ER) observed at various measurements. Bias error is a constant and systematic error in the process. Random error is the repeatability error [23,35,36].
4.3. Thermohydraulic efficiency Solar air heater is operated by using an electrical blower which converts the electrical energy into mechanical energy and forces the air through the heating system. The amount of energy (pumping power e PP) required for this system is vary with the pressure drop due to the expansion (entry) and contraction (exit) of heating systems and frictional losses due to movement air over absorber plate. These factors reduce the system efficiency and this pumping power is considered for evaluate the thermohydraulic efficiency of system [32]. Whereas the energy efficiency does not consider this effect (Eqn. (33)) and it leads in higher than thermohydraulic efficiency.
hTHD ¼
Qu PP Ap tg ap I
(34)
where the pumping is calculated from
PP ¼
FP ¼
FP
(35)
hBlower mDP r
U¼
qffiffiffiffiffiffiffiffiffiffiffiffiffiffiffiffi E2B þ E2R
(43)
Uncertainty range of the Reynolds number friction factor and Nusselt number is calculated using the following relation
" #0:5 Dm 2 DDh 2 þ m Dh
DRe ¼ Re Df ¼f f
"
DNu ¼ Nu
DðDPÞ DP
2
DL þ L
2
3DDh þ Dh
DQu ¼ Qu
2
#0:5 2DRe 2 þ Re
" 2 #0:5 Dh 2 DD þ h D
2 DTp 2 Dh 4 DQu 2 ¼ þ þ h Qu Tp
(36)
(44)
"
(45)
(46)
DTf Tf
!2 30:5 5
(47)
#0:5 Dm 2 DTin 2 DTout 2 þ þ m Tin Tout
(48)
4.4. Exergy efficiency of solar air heater Second law (Exergy) analysis is used to represent the quantitatively useful energy. Exergy efficiency (Exergy) of a system is a measure of work potential and is given by Ref. [33].
hII ¼
Exergy Output Exout ¼ Exergy Input Exin
(37)
Exergy output of the SAH is calculated by using the change in enthalpy, entropy and it is given as by Ref. [33].
Exout ¼ m ½DH Ta Ds
(38)
DH ¼ Hout Hin ¼ Cp ðTout Tin Þ
(39)
Ds ¼ sout sin ¼ Cp ln
Tout Pout R ln Tin Pin
(40)
6. Results and discussion This section discusses the performance of the solar air heater by comparing the theoretical and experimental results and discuss in terms of Nusselt number, friction factor, Nusselt number ratio, friction factor ratio, thermal enhancement factor, energy, exergy and thermohydraulic efficiency. Also the discussion is extended by means of studying the effect of solar radiation and wind velocity. 6.1. Validation of experimental results Fig. 4 compares the theoretical and experimental Nusselt number of conventional and modified SAH (type-c). The experimental results of conventional SAH build a good agreement with correlations and the deviations are between ±6%. In modified SAH,
Exergy input from the system is given as [33].
Ta Exin ¼ I t a Ap 1 Ts
(41)
Primary quantities
5. Uncertainty analysis The uncertainty associated with Primary (Thermocouple, solarimeter, manometer, anemometer and hygrometer) and secondary quantities (Reynolds number, Nusselt number and friction factor) are calculated and given in Table 3. Error for the primary quantities is calculated as follows [34].
Error ¼
Accuracy of instrument Minimum value of the out put measured
Table 3 Uncertainty analysis for primary and secondary quantities.
(42)
Sl. No
Instrument
Accuracy
Range
% Error
1 2 3 4 5
Thermocouple Solarimeter Manometer Anemometer Hygrometer
±1 C ±1 W/m2 1 mm ±0.1 m/s ±1
0e100 C 0e2500 W/m2 500 mm 0e15 m/s 0e100
0.25% 2.5% 4% 10% 0.25%
Secondary quantities Sl. No
Parameter
Uncertainty (%)
1 2 3
Reynolds number Nusselt number Friction factor
±2.3 ±4.47 ±3.2
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7
Fig. 4. Comparison of experimental and theoretical Nusselt number.
Fig. 5. Hourly variation of different parameters in SAH.
weiting correlation is matches more with the experimental results than other two correlations. Deviation between the experimental and weiting Nusselt number varies between the ±8.5%. Whereas the deviation is about ±14.1% for Manglik and Bergles correlations and ±17.8% for Dubrovsky and Vasiliev correlations. Thus the further discussions are carried out with weiting correlation for modified SAH. Temperature of the absorber plate, glass and air for conventional and modified SAH is presented in Fig. 5. The variation
between theoretical and experimental results is within 10% for both the conventional and modified SAH. 6.2. Effect of heat transfer rate and friction factor The consequence on the Nusselt number and friction factor characteristics of SAH with different modifications are presented in Figs. 6 and 7 respectively. Placing the turbulators increases the
Fig. 6. Nusselt number effect on different modifications in SAH.
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Fig. 7. Friction factor effect on different modifications in SAH.
heat transfer rate than conventional SAH. Nusselt number is increase with Reynolds number for all modifications and the Nusselt number ratio follows the opposite pattern. It shows that enhance in Nusselt number is low for modified SAH than conventional SAH which resulted in lower Nusselt number ratio with increase in Reynolds number. In particular, type (f) has higher Nusselt number than others due to the presence of V-type turbulators in concave shape which increases the residence time of air by means of restricting the flow of air. Corresponding highest friction is observed in type -f arrangement as presented in Fig. 7 due to immense resistance to flow than other modifications. Increase in contact surface and maximizing the reverse flow leads to higher friction factor ratio 71 at Re of 11615. 6.3. Effect of the thermal enhancement factor
Fig. 8. Effect of TEF on Reynolds number.
contact surface between the air and absorber surface, resulted in increased Nusselt number. Circumferences of the turbulators generate the turbulence in air flow rate at regular interval throughout the flow field which ensures further enhancement in
Fig. 8 analyzes the behavior of thermal enhancement factor (TEF) in modified SAH. Thermal enhancement factor evaluate the performance of SAH by considering the effect of Nusselt number ratio and friction factor ratio. It is identified that the TEF value of all types of turbulators is above unity and it means that adoption of turbulators provide thermal advantage over conventional SAH. TEF attains high value at lower mass flow rate in all condition, due to the superior value of Nusselt number ratio. Increased thermal performance is recorded with the type (f) arrangement than any other due to the larger interruption in the flow field.
Fig. 9. Reduced temperature parameter on energy and exergy efficiency.
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Fig. 10. Comparison of thermohydraulic and energy efficiency.
6.4. Effect of energy and exergy efficiency
6.6. Heat losses in solar air heater
Physical characteristics of the collector effectiveness can be represented by a curve that indicates efficiency versus the reduced temperature parameters as shown in Fig. 9. Energy efficiency is reduces with increase in temperature parameters due to higher heat loss with increase in solar radiation. Both the energy and exergy efficiency is higher for type (f) modification due to higher heat transfer rate.
The effect of wind velocity on energy losses is plotted in Fig. 11. Convective heat transfer rate from glass to ambient ðQc;ga Þ is strongly depends on the wind velocity of the particular day. It can be noted that the other heat losses (radiative heat transfer from ambient to air and bottom heat loss) are not affected by the wind velocity. Also with increase in wind velocity the convective heat loss becomes higher than other heat losses at mid noon.
6.5. Thermohydraulic efficiency of solar air heater
6.7. Effect of solar radiation on temperature
Thermohydraulic efficiency of the SAH is a measure of efficiency due to pumping power is given in Fig. 10. It is observed from thermohydraulic efficiency graph, the pumping power is less for conventional SAH when compared to the modified SAH. However, the outlet temperature and system efficiency of modified SAH is higher than conventional as shown in energy efficiency graph and it shows the benefit of the turbulators. Thermohydraulic efficiency follows the similar path with energy efficiency with a little drop in efficiency due to variation in change in pressure. Average reduction in thermohydraulic efficiency is about 5% compared to energy efficiency, which is acceptable range in the performance of the system.
Response of varying solar radiation on the temperature difference between the outlet and inlet of air is presented in Fig. 12. Both the solar radiation and turbulator modification has considerable effect on the outlet temperature of air. Highest temperature difference of 29 C is achieved with type (f) turbulator at the solar radiation of 900 W/m2. It clearly demonstrates the benefit of the turbulators in SAH. Fig. 13 compares the TEF of present work with previous works in different absorber plate condition [37 e 39]. Using of different type ribs in absorber plate enhances the TEF by considerably. However,
Fig. 11. Heat losses in SAH with time.
Fig. 12. Effect of solar radiation on outlet and inlet air temperature difference.
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Fig. 13. Comparison of TEF on present and previous works.
increasing of geometry complex for turbulence leads to higher performance than previous results. Type (a) has lower TEF than ribbed surfaces due to less number of turbulators and the TEF increases significantly by increasing the number of turbulators (Type f). The ribbed grooves and other ribs used in the previous works are covers the flow height maximum of 25% whereas in present work it covers 50% of the flow height. It leads to higher mixing rate of air throughout the section and higher thermal performance. 7. Conclusion A detailed theoretical and experimental study has been performed to evaluate the capacity of the single pass SAH with six patterns of turbulators in the absorber plate with a wide range of operating conditions. Theoretical results are compared with experimental results and the deviations are within 10%. Nusselt number ratio is found to be greater at lower Reynolds number and decreases by increasing Reynolds number. Friction factor ratio is higher for type-(f) because it point out larger resistance in flow over other modifications and reaches highest of 71 at Re 11615. Thermal enhancement factor proclaims an impressive progress in the type (f) arrangement with top figure of 3.65 at Re of 6200. Energy and exergy efficiencies are decrease with increase in temperature reduces parameter. Presence of turbulators increases the pumping power of the system and reduces the efficiency by 1e5% depends on the modification. Wind velocity has a significant effect on convective heat loss from glass to ambient and it is directly proportional. Circular turbulators integrated with V-type insert yields a supreme outlet air temperature of 66 C at the mass flow rate of 0.016 kg/s. 6 4 zigzag arrangement of circular turbulators with concave shape inserts demonstrates better performance compared to other patterns experimented and also better than conventional model. However, all the patterns expose greater scope than the conventional SAH which proves that introducing turbulators helps in improving the thermal performance of the solar air heater.
D Ex EB ER f H h I k L m Nu P Pr Q R Re s T U V
D
r
Hydraulic diameter, m Exergy Bias Error Random Error Friction factor Enthalpy, J/kg Heat transfer coefficient, W/m2 K Solar radiation, W/m2 Thermal conductivity, W/m K Length of test section, m Mass flow rate, kg/s Nusselt number Pressure, N/m2 Prandtl number Heat transfer rate, W Gas constant, J/kg K Reynolds number Entropy, J/kg K Temperature, C Uncertainity Velocity of air, m/s change Density of air, kg/m3
The authors would like to acknowledge with appreciation, TEQIP e II grant, Thiagarajar College of Engineering, Madurai, India for their partial financial support in this research.
Abbreviations PP Pumping power FP Flow power TEF Thermal enhancement factor SAH Solar Air Heater out Outlet in inlet p absorber plate f fluid g glass cover a ambient s sun c conventional m modified a Absorpitivity h Efficiency t Transmitivity
Nomenclature
References
Acknowledgement
A Cp
Area, m2 Specific het capacity of air, J/kg K
[1] Mohammadi K, Sabzpooshani M. Comprehensive performance evaluation and parametric studies of single pass solar air heater with fins and baffles attached over the absorber plate. Energy 2013;57:741e50.
Please cite this article in press as: Rajaseenivasan T, et al., Comprehensive study on solar air heater with circular and V-type turbulators attached on absorber plate, Energy (2015), http://dx.doi.org/10.1016/j.energy.2015.07.020
T. Rajaseenivasan et al. / Energy xxx (2015) 1e11 [2] Karsli S. Performance analysis of new-design solar air collectors for drying applications. Renew Energy 2007;32:1645e60. [3] Krishnananth SS, Murugavel KK. Experimental study on double pass solar air heater with thermal energy storage. J King Saud Univ Eng Sci 2013;25: 135e40. [4] Gupta MK, Kaushik SC. Exergetic performance evaluation and parametric studies of solar air heater. Energy 2008;33:1691e702. [5] Sabzpooshani M, Mohammadi K, Khorasanizadeh H. Exergetic performance evaluation of a single pass baffled solar air Heater. Energy 2014;64:697e706. [6] Singh S, Chander S, Saini JS. Exergy based analysis of solar air heater having discrete V-down rib roughness on absorber plate. Energy 2012;37:749e58. [7] Lanjewar A, Bhagoria JL, Sarviya RM. Heat transfer and friction in solar air heater duct with W-shaped rib roughness on absorber plate. Energy 2011;36: 4531e41. [8] Sun W, Ji J, He W. Influence of channel depth on the performance of solar air heaters. Energy 2010;35:4201e7. [9] El-Sebaii AA, Aboul-Enein S, Ramadan MRI, Shalaby SM, Moharram BM. Investigation of thermal performance of-double pass-flat and v-corrugated plate solar air heaters. Energy 2011;36:1076e86. [10] Tanda G. Performance of solar air heater ducts with different types of ribs on the absorber plate. Energy 2011;36:6651e60. [11] Karwa R, Chitoshiya G. Performance study of solar air heater having v-down discrete ribs on absorber plate. Energy 2013;55:939e55. [12] Aldabbagh LBY, Egelioglu F, Ilkan M. Single and double pass solar air heaters with wire mesh as packing bed. Energy 2010;35:3783e7. [13] Ramadan MRI, El-Sebaii AA, Aboul-Enein S, El-Bialy E. Thermal performance of a packed bed double-pass solar air heater. Energy 2007;32:1524e35. [14] Chaube A, Sahoo PK, Solanki SC. Analysis of heat transfer augmentation and flow characteristics due to rib roughness over absorber plate of a solar air heater. Renew Energy 2006;31:317e31. [15] Nowzari R, Aldabbagh LBY, Egelioglu F. Single and double pass solar air heaters with partially perforated cover and packed mesh. Energy 2014;73: 694e702. [16] Sara ON, Pekdemir T, Yapici S, Yilmaz M. Heat-transfer enhancement in a channel flow with perforated rectangular blocks. Int J Heat Fluid Flow 2001;22:509e18. [17] Gentry MC, Jacobi AM. Heat transfer enhancement by delta-wing vortex generators on a flat plate vortex interaction with the boundary layer. Exp Therm Fluid Sci 1997;14:231e42. [18] Zhou G, Ye Q. Experimental investigations of thermal and flow characteristics of curved trapezoidal winglet type vortex generators. Appl Therm Eng 2012;37:241e8. [19] Alvarez G, Arce J, Lira L, Heras MR. Thermal performance of an air solar collector with an absorber plate made of recyclable aluminum cans. Sol Energy 2004;77:107e13. [20] Ozgen F, Esen M, Esen H. Experimental investigation of thermal performance of a double-flow solar air heater having aluminium cans. Renew Energy 2009;34:2391e8. [21] Paisarn N, Kongtragool B. Theoretical study on heat transfer characteristics and performance of the flat-plate solar air heaters. Int Commun Heat Mass Transf 2003;30(8):1125e36.
11
[22] Manglik RM, Bergles AE. Heat transfer and pressure drop correlations for the rectangular offset strip fin compact heat exchanger. Exp Therm Fluid Sci 1995;10(2):171e80. [23] Yang M, Yang X, Li X, Wang Z, Wang P. Design and optimization of a solar air heater with offset strip fin absorber plate. Appl Energy 2014;113:1349e62. [24] Wieting AR. Empirical correlations for heat transfer and flow friction characteristics of rectangular offset-fin plate-fin heat exchangers. Trans ASME J Heat Transf 1975;97:488e90. [25] Dubrovsky EV, Vasiliev VY. Enhancement of convective heat transfer in rectangular ducts of interrupted surfaces. Int J Heat Mass Transf 1988;31: 807e18. [26] Ong KS. Thermal performance of solar air heaters: mathematical model and solution procedure. Sol Energy 1995;55(2):93e109. [27] Rajaseenivasan T, Murugavel KK. Theoretical and experimental investigation on double basin double slope solar still. Desalination 2013;319:25e32. [28] Singh S, Chander S, Saini JS. Investigations on thermo-hydraulic performance due to flow-attack-angle in V-down rib with gap in a rectangular duct of solar air heater. Appl Energy 2012;97:907e12. [29] Promvonge P, Eiamsa-ard S. Heat transfer in a circular tube fitted with free spacing snail entry and conical nozzle turbulators. Int Commun Heat Mass Transf 2007;34:838e48. [30] Muthusamy C, Vivar M, Skryabin I, Srithar K. Effect of conical cut-out turbulators with internal fins in a circular tube on heat transfer and friction factor. Int Commun Heat Mass Transf 2013;44:64e8. [31] Kongkaitpaiboon V, Nanan K, Eiamsa-ard S. Experimental investigation of heat transfer and turbulent flow friction in a tube fitted with perforated conical rings. Int Commun Heat Mass Transf 2010;37:560e7. [32] Hegazy AA. Thermohydraulic performance of air heating solar collectors with variable width, flat absorber plates. Energy Convers Manag 2000;41:1361e78. [33] Oztop HF, Bayrak F, Hepbasli A. Energetic and exergetic aspects of solar air heating (solar collector) systems. Renew Sustain Energy Rev 2013;21:59e83. [34] Velmurugan V, Pandiarajan S, Guruparan P, Harihara Subramanian L, David Prabaharan C, Srithar K. Integrated performance of stepped and single basin solar stills with mini solar pond. Desalination 2009;249:902e9. [35] Jaisankar S, Radhakrishnan TK, Sheeba KN. Studies on heat transfer and friction factor characteristics of thermosyphon solar water heating system with helical twisted tapes. Energy 2009;34:1054e64. [36] Moffat RJ. Describing the uncertainties in experimental results. Exp Therm Fluid Sci 1988;1:3e17. [37] Skullong S, Kwankaomeng S, Thianpong C, Promvonge P. Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators. Int Commun Heat Mass Transf 2014;50:34e43. [38] Lanjewar A, Bhagoria JL, Sarviya RM. Experimental study of augmented heat transfer and friction in solar air heater with different orientations of W-Rib roughness. Exp Therm Fluid Sci 2011;35:986e95. [39] Promvonge P, Khanoknaiyakarn C, Kwankaomeng S, Thianpong C. Thermal behavior in solar air heater channel fitted with combined rib and deltawingle. Int Commun Heat Mass Transf 2011;38:749e56.
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