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Experimental study of enhancement of heat transfer and pressure drop in a solar air channel with discretized broken V-pattern baffle
Renewable Energy 101 (2017) 856e872
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Experimental study of enhancement of heat transfer and pressure drop in a solar air channel with discretized broken V-pattern baffle Raj Kumar, Muneesh Sethi, Ranchan Chauhan, Anil Kumar* School of Mechanical and Civil Engineering, Shoolini University, Solan, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2016 Received in revised form 27 August 2016 Accepted 17 September 2016
This article presents an experimental study on heat transfer and friction characteristics of solar air channel fitted with discretized broken V-pattern baffle on the heated plate. The effect of geometrical parameters, predominantly the gap width and gap location has been investigated. The roughened baffle air channel has a width to height ratio, W/H of 10. The relative baffle gap distance, Dd/Lv and relative baffle gap width, gw/Hb has been varied from 0.26 to 0.83 and 0.5e1.5, respectively. Experiments have been carried out for the range of Reynolds number, Re from 3000 to 21,000 with the relative baffle height, Hb/H range of 0.25e0.80, relative baffle pitch, Pb/H range of 0.5e2.5; and angle of attack, aa range of 30 e70 . The optimal values of geometrical parameters of roughness have been obtained and discussed. For Nurs the greatest enhancement of the order of 4.47 times of the corresponding data of the without channel has been obtained. The absolute highest data of thermal hydraulic performance parameter has been found to be greater corresponding to Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5, and aa of 60 . The maximum value of the thermal hydraulic performance parameter was found to be 3.14 for the range of parameters investigated. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Turbulence Thermal Baffle height Baffle distance
1. Introduction The efficiency of compact heat exchangers can be improved by modifying the boundary layer developed on the heated surface. One of the well-known approach of modifying the boundary layer is to break the laminar viscous sub-layer formed on the heat transfer surface by creating rough surface in the form of transverse baffle, angled baffle, V-baffle and perforated baffle etc. Air channel is one of the simplest and extensively used types of heat exchanger in which heat energy is being exchanged between heated wall and air streaming through the system. The major constraint of air channel use is low overall thermal performance due to low heat transfer rate between heated wall and air [1e4]. In order to attain higher thermal performance it is beneficial that the stream at the heat transfer wall should be made turbulent. The baffle roughness has been used extensively for the augmentation of forced convective heat transfer coefficient of air channels. Use of baffle roughness seems to be a useful proposition for improving the local Nurs [5e8]. For detailed descriptions of some experimental investigations on
* Corresponding author. E-mail address: [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.renene.2016.09.033 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
air channel with transverse baffle, inclined baffle, delta baffle, diamond shaped baffle, V-type baffle etc. Karwa and Maheshwari [9] experimentally study Nurs and frs in a SAC with transverse fully perforated baffles and half perforated baffles attached to one of the broad wall. They reported that for fully perforated baffle the improvement in Nurs is 79e169% and 133e274% in case of half perforated baffles. Ozgen et al. [10] reported the thermal performance in a SAC with baffles fitted to the heated wall. Bopche and Tandale [11] reported the wholly developed stream in a roughened SAC with U-shaped pattern baffles. Eiamsa-ard et al. [12] investigated the heat transfer improvement in a SAC with winglet delta twisted tape baffles with different bO and Hb/H. Their studies shows that Nurs and frs data with winglet delta twisted tape were superier as compared to without winglet delta twisted tape. Promvonge et al. [13] mathematically examined the performance of Nurs and frs in square channel attached with 45 inclined baffles with a Re ranging from 100 to 1200. They informed that for the 45 baffle with Hb/H ¼ 0.4 and Re ¼ 1200, Nurs is superior to that of 90 baffle. Promovong [14] experimentally investigated the turbulent forced convection Nurs and frs loss behaviour in a high W/H channel attached with 60 V- shaped baffles. Akpinar et al. [15] experimentally investigate the performance analysis of four types
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Nomenclature Ap Ao Af Cdo Cp Dd Dhd f frs fss gw ht H Hb gw/Hb Hb/H Ka Lt Lv Dd/Lv ma Nu
Surface area of heated plate, m2 Area of orifice, m2 Area of flow, m2 Coefficient of discharge Specific heat of air, J/kg K Gap or broken distance, m Hydraulic diameter of channel, m Friction factor Friction factor of roughened baffle Friction factor without baffle channel Gap or discrete width, m Convective heat transfer coefficient, W/m2K Height of channel, m Height of baffle, m Relative gap width Relative baffle height Thermal Conductivity of air, W/mK Length of test section, m Length of V-type baffle, m Relative baffle gap distance Mass stream rate of air, kg/s Nusselt number
of SAH with different obstacles and without obstacle. They reported that efficiency of SAH depends on the surface geometry of collectors, solar radiation of air stream line. Chompookham et al. [16] experimentally studied the effect of winglet vortex type generators on the Nurs and frs behaviours for a turbulent stream. Bekele and Mishra [17] carried out the experimental studied of the turbulent air stream and heat transfer characteristics of SAC with delta shaped obstacle attached to the upper wall of a channel. Khanoknaiyakarn [18] carried out an experiment to study Nurs and frs by using V-pattern baffles on a broad heated wall of a large W/H channel. The effects of the baffles on Nurs and frs were investigated. Sriromreun et al. [19] reported experimental predictions of the Nurs and frs for a SAC with Z-shaped baffles. Their experiments were performed by controlling the air stream rate to attain Re values in the range of 4400 to 20,400. Thianpong et al. [20] reported the experimental studies of the collector performance of a SAC with twisted rings type baffles. Zhou and Ye [21] carried out the experimental studied of the turbulent air stream and heat transfer characteristics of SAC with delta winglet vortex generator baffles attached to the upper surface of a channel. Chamoli and Thakur [22] conducted an indoor experimental investigation to study Nurs and frs data of air passing through an air channel that was roughened by V-shaped perforated baffles. Bayrak et al. [23] studied the performance valuation of porous baffles introduced SAC by energy and energy method. They reported that the maximum collector efficiency and air temperature increase are attained by SAC with a thickness of 6 mm and ma of 0.025 kg/s while the lowermost data are obtained for the SAC with non-baffle collectors with ma of 0.016 kg/s. Tamna et al. [24] investigated the effect of multiple V-baffle vortex generators to improve Nurs in a channel fitted with 45 BVG with Re ranging from 4000 to 21000, Hb/H ¼ 0.25, Pb/H ¼ 0.5, 1 and 2 and aa equal to 45 respectively. Alam et al. [25] experimentally investigated the effect of Hb/H of 0.4e1.0, Pb/H of 4e12, bO of 5e25%, aa of 60 and Re varies from 2000 to 20,000 on V-shaped perforated blocks SAC with W/H of 10.
Nurs Nus Pb Pb/H (Dp)d (Dp)o Qu Re Tf Ti To Tp U V W SAH SAC
857
Nusselt number of baffle channel Nusselt number of channel without baffle Pitch of baffle channel, m Relative pitch ratio Pressure drop across test section, Pa Pressure drop across orifice plate, Pa Useful heat gain, W Reynolds number Mean bulk air temperature, K Inlet temperature of air, K Outlet temperature of air, K Plate temperature of air, K Mean air velocity, m/s Velocity of air, m/s Width of channel, m Solar air heater Solar air channel
Greek symbols aa Angle of attack, b Ratio of orifice meter to pipe diameter ra Density of air, kg/m3 n Kinematic viscosity of air, m2/s hp Thermo hydraulic performance
They observed that average improvement in Nurs for perforated Vshaped blockage is 33% higher over solid blockages, frs of perforated blockage gets reduced by 32% of the value as compared to solid blockage. Skullong et al. [26] carried out an experimental study on the turbulent flow and heat transfer characteristics in a SAC attached baffles with combined groove baffles. Shin and Kwak [27] studied the effect of the perforation shape for a blockage wall on the Nurs in a stream passage. It was observed that a blockage surface with wider perforation provided a more uniform Nurs and greater thermal performance factor. plate. Table 1 summarizes the experimental investigations of some important rib arrangements investigated by the investigators. Literature review shows that, the transverse baffle shape improve the heat transfer rate by stream separation and creation of vortices on the upward and downward of the baffles and reattachment of stream in inter-baffle spaces. Angling of transverse baffle further enhances the heat transfer on account the movement of vortices along the baffle wall and creation of a secondary stream cell close to the leading end, which outcome in local wall turbulence. V- type baffle of an extended angled baffle benefits in the type of two secondary stream jets as compared to single in case of an angled baffle resulting in still superior heat transfer rate. Making a broken in the baffle is found to improve the heat transfer by disturb the secondary stream and produce advanced level of turbulence in the fluid downward of the baffles. It is hypothesized that discretized broken V-pattern baffle will raise heat transfer rate compared to without broken V-type baffle.
2. Experimental details 2.1. Experimental set-up To study the outcome of discretized broken V-pattern baffle turbulent promoter on the Nurs and frs of air stream an experimental setup was intended and made-up accordance with guidelines suggested in ASHRAE standard [28]. A schematic illustration
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Table 1 Previous experimental investigation of various baffle roughened solar air channel. Sr. Type of baffles No.
Shape of baffles
Major findings
1.
Angled baffles [5]
Augmentation of about 3.16 and 3.56 times in heat transfer and pressure drop respectively has been reported over smooth rectangular channel.
2.
Perforated baffles [9]
The perforated baffles helped in achieving high values of heat transfer and pressure drop of about 3.56 and 4.18 times respectively over smooth rectangular channel.
3.
U-shaped baffles [11]
Augmentation in heat transfer and pressure drop of about 2.82 and 3.72 times has been achieved compared to smooth absorber plate.
4.
Delta shaped baffles [17]
The insertion of delta shaped baffles augmented heat transfer and pressure drop by 3.67 and 4.89 times respectively over smooth rectangular channel.
5.
V-shaped baffles [18]
Heat transfer and pressure drop augmented by about 4.02 and 5.23 times respectively compared to that of smooth absorber surface.
6.
Single V-Perforated shaped baffles [22]
Augmentation of about 3.95 and 5.35 times in heat transfer and pressure drop respectively is reported over smooth rectangular channel.
7.
Perforated V-shaped blockages [25]
Average enhancement in heat transfer is 13% higher over solid baffles while the pressure drop decreased by 18% of the value compared to solid baffles.
8.
Transverse Perforated blocks baffles [27]
Heat transfer augmented by 3.98 and pressure drop by 4.28 times respectively compared to the smooth rectangular channel.
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Table 1 (continued ) Sr. Type of baffles No. 9.
Discrete V-pattern baffles (Proposed shape)
Shape of baffles
Major findings The literature survey over the available experimental and analytical studies reported that the V-pattern baffles have elevated overall thermal performance compared to other simple angled, transverse, perforated baffle shapes and configurations. The research gap with discrete V-pattern baffles has been carried out in the present study which augments the convective heat transfer due to insertion of secondary flow regions.
of experimental set up is shown in Fig. 1. The air channel is 2000 mm extended with a stream cross section of 300 mm 30 mm is made-up from ply panel of 20 mm thickness. The channel is comprises of inlet section 500 mm long, a test section of 1200 mm length and an exit section of 300 mm length. The complete channel is insulated with 50 mm thick polystyrene insulation having thermal conductivity of 0.037 W/mK to minimise heat loss to the environment. An electric heating element was made-up by combining six loops of nichrome cable in series and parallel having a size of 1,200 mm 300 mm to supply a kept heat flux 1000 W/m2 to the heated wall which is careful to be sensibly reasonable data of heat energy input for testing rectangular air channel. The asbestos sheet is converted with strip of Mica to keep the uniform distance between the wires and prevent back heating. A Galvanised Iron (GI) sheet of 18 SWG size black painted in order to facilitate the heat transfer is used as a heated wall. Discredited broken V-Pattern baffle were attached on the base of heated wall by means of epoxy resin. This plate formed the top wall of the air channel. The bottom side of the air channel is covered with smooth face using sun mica sheet. A calibrated Orifice meter (having coefficient of discharge 0.62) connected to U-tube manometer using methyl alcohol as manometer fluid was used to measure the mass stream rate of air through rectangular air channel. The control valves provided in the lines control the stream. Copper-constantan thermocouples were used for air and aluminium plate temperature measurement. Such thermocouples are usually recommended for temperature measurement in the
range of 0e400 C (Benedict [29]). The thermocouple output is measured by a Digital Micro Voltmeter, joined through a selector switch to designate the output of the thermocouples in C. To verify the accuracy of temperature measurement, thermocouples have been calibrated under laboratory conditions against a dry block temperature calibrated instant. The thermocouple to be calibrated was located in the calibration bath where kept temperature is maintained and the response of the thermocouple and the standard probe were noted with the help of a digital temperature indicator for various pre-set data of the standard probe, and the error among the reading of standard probe and the thermocouple were measured. If this error is more than certain limit of the calibrator then the thermocouple is rejected and if this error is less than tolerance limit of the calibrator then the thermocouple is accepted. This process was repeated in numerous steps of rising as well as reducing temperature range. Temperature scanners have been used to show the temperature of heated plate and inlet and outlet of air. The pressure drop through the test section of the air channel was obtained by a micro monometer having a least count of 0.01 mm. 2.2. Range of parameters Rectangular air channel has an Lt equal to 2000 mm while the, H is set equal to 30 mm and W is 300 mm, the hydraulic diameter, Dhd is equal to 54.54 mm. The baffle parameters are determined by baffle height Hb, pitch of baffle Pb, broken distance Dd, broken width
Fig. 1. Schematic of experimental setup.
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gw, length of V-type baffle Lv, angle of attack aa and the shape of the roughness elements. These parameters have been expressed in the form of dimensionless roughness parameters, viz., relative gap distance (Dd/Lv), relative gap width (gw/Hb), relative baffle pitch (Pb/H), relative baffle pitch (Pb/H), relative baffle height (Hb/H), and angle of attack (aa). The shape is shown in Figs. 2 and 3 and Table 2 gives the range of parameters.
2.3. Raw data collection The data collected have been used to compute ht, Nu, and f. Relevant expressions for the computation of the above parameters and some intermediate parameters have been given below.
2.3.1. Temperature measured Weighted average plate air temperature: The mean temperature of the plate (Tp) is the average of all temperatures of the heated plate:
P Tp ¼
Tpi N
(1)
The mean bulk air temperature (Tf) is a simple arithmetic mean of the measured values at the inlet and the exit temperature of air streaming through the test section:
Tf ¼
Ti þ To 2
(2)
Fig. 3. Experimental setup and discretized broken V-Pattern baffle roughened plates.
Table 2 Range of parameters. Sr. No.
Parameters
Range
1. 2. 3. 4. 5. 6.
Re Dd/Lv gw/Hb Pb/H Hb/H
3000 to 21000 0.26e0.83 0.5e1.5 0.5e2.5 0.25e0.80 30 e70
aa
2.3.3. Velocity of air through channel (V) The V is calculated from the knowledge of ma and the stream as
ma
where To ¼ (TA2 þ TA3 þ TA4 þ TA5 þ TA6)/5, Ti ¼ TA1.
V¼
2.3.2. Mass stream rate measurement (ma) The ma of air has been calculated from the pressure drop measurement through the calibrated orifice meter by using the following formula:
2.3.4. Equivalent hydraulic diameter (Dhd)
" ma ¼ Cdo Ao
2ra ðDpÞ0 1 b4
ra WH
(4)
#0:5 (3)
Dhd ¼
4:ðW:HÞ 2:ðW þ HÞ
Fig. 2. Discretized broken V-pattern baffle.
(5)
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2.3.5. Reynolds number (Re) The Re of air stream in the channel is intended from
Re ¼
V$Dhd
Nurs ¼
ht Dhd Ka
861
(10)
(6)
n
2.3.6. Friction factor (frs) The frs is determined from the measured value of (Dp)d across the test section length using the Darcy equation as
frs ¼
2 Dp d Dhd
(7)
4ra Lt V 2
2.3.7. Heat transfer coefficient (ht) The heat transfer rate Qu, from absorber to the air is given by:
Qu ¼ ma cp ðT0 Ti Þ
(8)
The ht for the heated test section has been calculated as:
ht ¼
Q u Ap $ Tp Tf
(9)
Fig. 5. Comparison of experimental and predicted values of fss.
2.3.8. Nusselt number (Nurs) The ht can be used to determine the Nurs, which is defined as: Table 3 Range of uncertainty in the measurement of essential parameters. S. No.
Parameters
Error range, %
1. 2. 3. 4. 5. 6. 7. 8.
Mass flow rate (ma) Velocity of air (V) Useful heat gain (Qu) Heat transfer coefficient (ht) Nusselt number (Nurs) Friction Factor (frs) Reynolds Number (Re) Thermo-hydraulic performance parameter (hp)
1.597e2.033 1.653e1.811 2.131e3.267 2.213e3.732 3.378e4.667 1.283e2.331 1.43e3.76 3.675e5.221
Fig. 4. Comparison of experimental and predicted values of Nuss.
Fig. 6. (A) Variation of Nurs with Re at different Dd/Lv. (B) Variation of Nurs with Dd/Lv at different selected Re.
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Nuss and fss as a function of the Re is shown in Figs. 4 and 5 respectively.
2.4. Uncertainties analysis An uncertainty analysis has been carried to estimate the errors involved in experimental data measurement. The uncertainty is estimated based on errors associated with measuring instruments [32]. Table 3 shows the range of uncertainty in the measurement of essential parameters. The details of the analysis are given in Appendix-A.
4. Results and discussion A study was conducted to understand the effect on Nurs and frs of
3. Validation of experimental data The value of Nuss and fss calculated through experimental outcomes for a smooth channel have been compared with the outcomes obtained from the Dittus-Boelter equation [Eq. (11)] for the Nuss, and modified Blasius equation [Eq. (12)] for the fss [6]. The Nuss for a smooth channel is given by the Dittus-Boelter equation as:
Nuss ¼ 0:023Re0:8 Pr 0:4
(11)
The fss for a smooth channel is given by the modified Blasius equation as:
fss ¼ 0:085Re0:25
(12)
The comparison of the experimental and estimated outcomes of
Fig. 7. (A) Variation of Nurs with Re at different gw/Hb. (B) Variation of Nurs with gw/Hb at different selected Re.
Fig. 8. Secondary stream type with discretized broken V-pattern baffle air channel.
Fig. 9. (A) Variation of Nurs with Re at different Hb/H. (B) Variation of Nurs with Hb/H at different selected Re.
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In order to compare the improvement of the Nurs achieved as an outcome of providing a broken in the V-pattern baffle arrangement, the values of the, Nurs for fixed values of the, gw/Hb of 1.0, and distinct values of Dd/Lv is given in Fig. 6(A). Fig. 6(A) shows the variant of and Pb/H ¼ 1.5, Hb/H ¼ 0.50 and aa ¼ 60 , with Re at distinct values of Dd/Lv for a fixed gw/Hb of 1.0. It can be seen that the Nurs rises with rise in Dd/Lv from 0.58 to 0.67, attains an extreme at a, Dd/Lv of 0.67 and thereafter it reduces with rise in the Dd/Lv. Fig. 6(B) shows the values of the Nurs as a function of Dd/Lv for a 60 discretized broken V-pattern baffle air channel at distinct selected Re. It can be observed that at any Dd/Lv, the Nurs is the highest for the Dd/Lv ¼ 0.67 for every value of Re. Producing broken near the leading edge (say at Dd/Lv ¼ 0.26), the strength of the secondary stream may not be sufficient to energize the main stream passing through the broken and this broken distance does not lead to significant rise in Nurs. A rise in the values of Dd/Lv say at Dd/Lv ¼ 0.55
signifies shifting of the broken toward trailing edge. This raises the strength of the secondary stream and Nurs rises with rise in the Dd/ Lv up to 0.67. It appears that up to theses Dd/Lv values, there is significant contribution baffle second part of secondary stream to the Nurs whereas this truncated second part begins to become insignificant as the distance is further increased resulting in a slight reduce in Nurs as the Dd/Lv is raised beyond 0.67. These results broadly agree with previous studies on broken baffle roughened channels [15] [17] [22], and [24,25]. Fig. 7(A) presented the values of the, Nurs for fixed values of the, Dd/Lv of 0.67, Pb/H ¼ 1.5, Hb/H ¼ 0.50 and aa ¼ 60 and distinct values of gw/Hb. This figure shows the Nurs rises with rise in the gw/ Hb up to about 1.0, beyond which it reduces with rise in the gw/Hb. The value of Nurs is greatest for gw/Hb of 1.0 and smallest for the, gw/ Hb of 1.5. Fig. 7(B) shows the values of the Nurs as a function of gw/Hb for a 60 discretized broken V-pattern baffle air channel at distinct selected Re . It can be observed that at any gw/Hb, the Nurs is the highest for the gw/Hb ¼ 1.0 for all value of Re. It appears that as the gw/Hb is raised beyond 1.0, the stream velocities through the broken will reduce, which may not be strong enough to accelerate the stream through the broken and hence the Nurs due to this stream may not be raised significantly whereas with the reduction of this gw/Hb to values lower than 1.0, there may be very little space for stream of the fluid through it which outcomes in low turbulent and
Fig. 10. (A) Variation of Nurs with Re at different Pb/H. (B) Variation of Nurs with Pb/H at different selected Re.
Fig. 11. (A) Variation of Nurs with Re at different aa. (B) Variation of Nurs with aa at different selected Re.
the stream Re and discretized broken V-pattern baffle used to provide roughness for an air channel. In this experimental investigation, effect of discretized broken V-pattern baffle shape parameters such as; Dd/Lv, gw/Hb, Hb/H, Pb/H, and aa on Nurs and frs has been studied extensively and discussed below. 4.1. Heat transfer
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Fig. 12. (A) Variation of frs with Re at different Dd/Lv. (B) Variation of frs with Dd/Lv at different selected Re.
Fig. 13. (A) Variation of frs with Re at different gw/Hb. (B) Variation of frs with gw/Hb at different selected Re.
hence reduce the improvement of Nurs. Thus in order to achieve the improvement of Nurs, the width of the broken should be such that it can rise the velocity of the fluid passing through it in order to create the local turbulence as shown in Fig. 8. These outcomes broadly agree with previous studies on broken roughened channels [15] [17] [22], and [24,25]. Fig. 9(A) shows the variation of Nurs with Re for distinct values of Hb/H. The other roughness parameters were kept Dd/Lv ¼ 0.67, gw/ Hb ¼ 1.0 as Pb/H ¼ 1.5 and aa ¼ 60 . It is observed that the Nurs rises with rise in Hb/H for all values of Re due to increased protrusion into stream causing more turbulence, thereby, resulting in rise in Nurs. The highest Nurs is observed at Hb/H of 0.50. Fig. 9(B) shows the values of the Nurs as a function of Hb/H for a 60 discredited broken V-pattern baffle air channel at different selected Re. It can be observed that at any Hb/H, the Nurs is the highest for the Hb/H ¼ 0.50 for all value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. Fig. 10(A) shows the variation of Nurs as a function of Re for distinct values of Pb/H and fixed values of other channel parameters as, Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0 Hb/H ¼ 0.50 and aa ¼ 60 . For all Re, the greatest value of Nurs has been observed corresponding to the Pb/H value of 1.5, whereas the smallest value of Nurs has been found to occur at the Pb/H value of 2.5 for the range of investigations. Fig. 10(B) shows the values of the Nurs as a function of Pb/H for a 60
discredited broken V-pattern baffle air channel at distinct selected Re. It can be observed that at any Pb/H, the Nurs is the highest for the Pb/H ¼ 1.5 for all value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. Fig. 11(A) shows the variation of Nurs with Re for distinct values of aa and fixed values of other channel parameters as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0, Hb/H ¼ 0.50 and Pb/H ¼ 1.5. In this plot, Nurs has been plotted as a function of aa for some selected values of Re and fixed values of other channel parameters. Nurs rises with rise in aa, attains a highest value corresponding to aa ¼ 60 and then decreases with further rises in the value of aa. Fig. 11(B) shows the values of the Nurs as a function of aa for discredited broken V-pattern baffle air channel at distinct selected Re . It can be observed that at any aa, the Nurs is the highest for the aa ¼ 60 for each value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. 4.2. Friction factor Invariable, use of baffle roughness substantially enhances Nurs from heated surface of air channels however, there occurs a corresponding rise in frictional losses. In this experimental investigation, effect of discretized broken V-pattern baffle parameters such
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Fig. 14. (A) Variation of frs with Re at different Hb/H. (B) Variation of frs with Hb/H at different selected Re.
Fig. 15. (A) Variation of frs with Re at different Pb/H. (B) Variation of frs with Pb/H at different selected Re.
as, Dd/Lv, gw/Hb, Hb/H, Pb/H and aa on friction characteristics of the air channel has been studied extensively and discussed below. The variation of frs with Re for distinct values of Dd/Lv and fixed values of other baffle parameters as gw/Hb ¼ 1.0 Hb/H ¼ 0.50 Pb/ H ¼ 1.5 and aa ¼ 60 has been shown in Fig. 12(A). It is seen that the value of frs reduces with rising Re and towards a kept data as expected. The frs rises with rise in the, Dd/Lv of up to 0.67 and reduces with further rise in the Dd/Lv. The plot shows that the highest and lowest values of frs for discretized broken V-pattern baffle air channel occur for the, Dd/Lv of 0.67 and 0.26 respectively. The lesser value of frs for broken on the upstream side is due to weakened strength of secondary stream. Fig. 12(B) shows the values of the frs as a function of Dd/Lv for a 60 discredited broken V-pattern baffle air channel at different selected Re It can be observed that at any Dd/ Lv, the frs is the highest for the Dd/Lv ¼ 0.67 for all value of Re. These outcomes broadly agree with previous studies on broken baffle air channels These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. The variation of frs with Re for different values of gw/Hb and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, Hb/H ¼ 0.50, Pb/ H ¼ 1.5 and aa ¼ 60 has been shown in Fig. 13(A). It has been observed that for all values of gw/Hb, frs reduce with rise in Re. Fig. 13(A) shows that at all Re, the frs rises as gw/Hb is raised from 0.5 to 1.0 and decreases as gw/Hb is raised further. Fig. 13(B) shows the
values of the frs as a function of gw/Hb for a 60 discredited broken V-pattern baffle air channel at distinct selected Re . It can be observed that at gw/Hb, the frs is the highest for the gw/Hb ¼ 1.0 for all value of Re. The air streaming through the broken creates turbulence at the downstream side of the gap. Addition of relative broken width in the baffles induces recirculation loops, which are responsible for greater turbulence and hence higher pressure losses. Strength of secondary stream is weakened in case of gw/Hb of 1.5 as compared to gw/Hb of 0.5, 0.75, 1.0 and 1.25 hence the frs is lower than in other cases. These outcomes broadly agree with previous studies on broken baffle channels. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. The variation of, frs with Re for distinct values of Hb/H have been plotted in Fig. 14(A). The other roughness parameters were kept as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0 Pb/H ¼ 1.5, and aa ¼ 60 . It has been observed from this plot that for a given Hb/H value frs reduces with rise in Re. Fig. 14(A) clearly shows that frs rises with rise in Hb/H and the greatest value of frs correspond to Hb/H value of 0.50. Fig 14(B) shows the values of the frs as a function of Hb/H for a 60 discretized broken V-pattern baffle air channel at distinct selected Re. It can be observed that at Hb/H, the frs is the highest for the Hb/H ¼ 0.80 for all value of Re.
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Fig. 16. (A) Variation of frs with Re at different aa. (B) Variation of frs with aa at different selected Re.
It is due to the fact that with the rise in Hb/H value, baffles protrude more and more into the core stream resulting in rise in turbulence level as well as the frs. These outcomes broadly agree with previous studies on baffle roughened channel. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17], and [24,25]. Fig. 15(A) shows the variation of frs with Re for different values of Pb/H and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, gw/ Hb ¼ 1.0 Hb/H ¼ 0.50 and aa ¼ 60 . It has been observed from Fig. 15(A) that for all values of Pb/H, frs reduces with rise in Re. For, Pb/H value of 2.5 and 1.5 yield the lowest and highest values of frs respectively. Fig. 15(B) shows the values of the frs as a function of Pb/ H for a 60 discretized broken V-pattern baffle air channel at different selected Re. It can be observed that at Pb/H, the frs is the highest for the Pb/H ¼ 1.5 for all value of Re. These outcomes broadly agree with previous studies on roughened channels. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17], [22], and [24,25]. The variation of frs with Re for distinct values of aa and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0, Pb/ H ¼ 1.5 and Hb/H ¼ 0.50 has been shown in Fig. 16(A). It has been observed that for all the values of aa, frs reduces with rise in Re. The smallest and highest value of frs have been obtained corresponding to aa values of 30 and 60 respectively. Fig. 16(B) shows the values of the frs as a function of aa for discretized broken V-pattern baffle
Fig. 17. (A) Variation of hp with Re at different Dd/Lv. (B) Variation of hp with Dd/Lv at different selected Re.
air channel at distinct selected Re. It can be observed that at aa, thefrs is the highest for the aa ¼ 60 for all value of Re. 4.3. Thermo hydraulic performance Study of the Nurs and frs characteristics shows that an improvement in Nurs is in general accompanied with friction power penalty due to a corresponding increase in the frs. Therefore it is essential to decide the geometry that will outcome in maximum improvement in Nurs with minimum frs penalty. In order to achieve this purpose of simultaneous consideration of thermo hydraulic performance, researchers proposed a thermo hydraulic parameter known as efficiency parameter ‘hp’ which evaluates the improvement in Nurs of a roughened air channel compared to that of the smooth channel for the same pumping power requirement and is defined as [30,31]:
hp ¼ ðNurs =Nuss Þ=ðfrs =fss Þ0:33
(13)
A heat transfer improvement device having a data of thermo hydraulic parameter (hp) higher than unity ensures the fruitfulness of using improvement device and therefore, this parameter is usually used to compare the performance of distinct roughness arrangements to prefer the best roughness arrangement among all
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Fig. 18. (A) Variation of hp with Re at different gw/Hb. (B) Variation of hp with gw/Hb at different selected Re.
the feasible combinations. Figs. 17e21 shows the effect of baffle parameters on thermo hydraulic performance parameter(Nurs/Nuss)/(frs/fss)0.33, as function of Re. In Table 4, those values of roughness geometry parameters have been presented for which thermo hydraulic performance parameter (Nurs/Nuss)/(frs/fss)0.33 values have been found to be highest. The highest absolute value of (Nurs/Nuss)/(frs/fss)0.33 has been observed to be 3.14 corresponding to Dd/Lv value of 0.67, gw/ Hb value of 1.0, Hb/H value of 0.50, Pb/H value of 1.5, and aa value of 60 for discretized broken V-pattern baffle air channel. On evaluating the thermal hydraulic performance for all possible sets of roughness parameters one can recognize the roughness shapes which provides maximum enhancement in heat transfer at the cost of least friction penalty. The values of thermal hydraulic performance parameter, hp ¼ (Nurs/Nuss)/(frs/fss)0.33 determined for this shape of broken V-pattern baffle have been compared with the corresponding values for angled baffle [5], perforated baffle [9], U-shaped baffle [11], delta shaped baffle [17], V-perforated baffle [22] and continuous V-shaped baffle [18] as shown in Fig. 22. It can be seen that the broken V-pattern baffle shape results in the better thermal hydraulic performance, hp ¼ (Nurs/Nuss)/(frs/fss)0.33 among all the shapes investigated. On the basis of present experimental investigation found that
867
Fig. 19. (A) Variation of hp with Re at different Hb/H. (B) Variation of hp with Hb/H at different selected Re.
used of broken V-pattern baffle across the width of the plate to augment the heat transfer rate. Because creating of a gap in the Vpattern baffle allows release of fluid belonging to secondary flow and main flow through the gap. The main flow passing through the gap is the developed flow with thicker boundary layer consisting of viscous sub layer. As a result of the presence of gap, the secondary flow along the baffle joins the main flow to accelerate it which energizes the retarded boundary layer flow along the surface. This increases the heat transfer through the gap width area behind the baffle.
5. Conclusions On the basis of experimental analysis of heat transfer, friction factor and thermo hydraulic performance of air channel provided with heated plate having discretized broken V-Pattern baffle shape of artificial roughness, the subsequent conclusions can be drawn from the this work: 1. Providing a discretized broken V-pattern baffles outcomes in substantial improvement in Nurs of air channel the improvement is a strong function of broken width and broken distance. As compared to the without baffle channel the presence of
868
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Fig. 20. (A) Variation of hp with Re at different Pb/H. (B) Variation of hp with Pb/H at different selected Re.
Fig. 21. (A) Variation of hp with Re at different aa. (B) Variation of hp with aa at different selected Re.
discretized broken V-pattern baffle rough surface yields Nurs up to 4.47 times while the frs rises up to 4.59 times in the range of parameters investigated. 2. The values of Nurs and frs rise with rise in Dd/Lv attains a highest value corresponding to Dd/Lv value of 0.67 and with further rise in the value of Dd/Lv, the Nurs and frs are found to decrease. The value of Nurs and frs is greater for a, Dd/Lv of 0.67 and smaller for the, Dd/Lv of 0.26. As a result of creating broken near the leading edge, the strength of the secondary stream may not be sufficient to energize the main stream passing through the broken and Dd/ Lv does not lead to significant rise in Nurs and frs. A rise in the values of Dd/Lv signifies shifting of the broken toward trailing edge. This rise the strength of the secondary stream and Nurs and frs rises with rise in the Dd/Lv up to 0.67. 3. An rise in gw/Hb outcomes in a rise in Nurs and frs attaining the highest value corresponding to gw/Hb of 1.0 and the values of these parameters reduce with further rise of gw/Hb. The value of Nurs and frs is highest for a, gw/Hb of 1.0 and lowest for the gw/Hb of 1.5. An increasing broken stream promotes local turbulence and stream mixing along the broken stream region while the baffle induced secondary stream is maintained in the air channel. Therefore, it may be reasoned that the rise in gw/Hb beyond 1.0 reduces the stream velocities through the broken and hence
the local turbulence. At the same time too small broken width will also not allow sufficient amount of secondary stream fluid to pass through and hence the turbulence level will remain low. 4. The values of Nurs and frs rise with rise in aa and attain the highest values corresponding to aa value of 60 . With further rise in the value of aa beyond 60 , Nurs and frs reduces. The values of Nurs and frs have been found to rise with rise in Pb/H, attaining the highest value at pitch value of 1.5 and then reduce with further rises in Pb/H. The present investigation shows that baffled air channel with Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5, and aa of 60 yields the highest values of thermo hydraulic performance parameter. 5. Discrete V-pattern baffle has also been shown to be thermal hydraulic better in comparison to angled baffle, perforated baffle, U-shaped baffle, delta shaped baffle, V-perforated baffle and continuous (without broken) V-shaped baffle. Because Vshaping of the baffle helps in the formation of two leading ends and a single trailing end as well as two secondary flow cells which promote turbulent mixing and hence increased heat transfer. Creating discrete in the V-pattern baffle allows release of the secondary flow and mix with main flow through discrete. This result in its acceleration, which energizes the retarded boundary layer flow along the surface resulting in the increase
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
869
Table 4 Baffle rough parameters corresponding to maximum hp ¼ (Nurs/Nuss)/(frs/fss)0.33 Roughness parameters
Fixed parameters
Optimum values of (Nurs/Nuss)/(frs/ fss)0.33
Maximum values of hp
gw/Hb Dd/Lv Hb/H Pb/H
Dd/Lv ¼ 0.67, Hb/H ¼ 0.50, Pb/H ¼ 1.5, aa ¼ 60 . gw/Hb ¼ 1.0, Hb/H ¼ 0.50, Pb/H ¼ 1.5, aa ¼ 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Pb/H of 1.5, aa of 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, aa ¼ 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5.
1.0 0.67 0.50 1.5 60
3.14 3.14 3.14 3.14 3.14
aa
dy y
" ¼
dy dx d x1 1
2
dy þ dx dx2 2
2
dy þ dx dx3 3
2
dy þ…þ dx dxn n
2 #0:5
Where dx1, dx2, dx3, … dxn are possible error in measurement of x1, d x2, x3, … xn, dy is known as absolute uncertainty and yy is known as relative uncertainty. In the present experiment, important parameters considered for uncertainty analysis are Reynolds number, Heat transfer coefficient, Nusselt number, friction factor. The data of measured parameters are given in Table A1.
Table A1 Measured parameters and their respective data
Fig. 22. Comparison of thermal hydraulic performance with previous investigations.
of the heat transfer through the discrete width are behind the baffles.
6. Future scope The future directions for research can be made on evaluating the thermohydraulic performance of multi discrete regions in a Vpattern baffles. Since, the discretization increases heat transfer due to generation of secondary flow streams, the multi discretization will improve the strength of secondary streams which will add up to the heat transfer from the absorber plate to the air. Also, the multi V- pattern baffles in discrete form can be helpful for increasing heat transfer and thereby the overall performance.
S. No.
Parameter
Data
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Length of test section, Lt Width of the channel, W Height of channel, H Diameter of pipe, DP Diameter of orifice meter, Do Pressure drop across orifice meter, (Dp)o Pressure drop across test section(Dp)d, Atmospheric pressure, Pa Outlet air temperature, To Inlet air temperature, Ti Rise in temperature of air, DT Mean bulk air temperature Tf Mean plate temperature, Tp
1200 mm 300 mm 30 mm 80 mm 36 mm 185 mm 56.2 Pa 97500 25.33 20 5.33 22.66 33
The thermo-physical properties of air have been determined by following standard correlations:
m ¼ 1:81 105
Appendix A. Uncertainties analysis During experimentation, lots of factors come into play which causes deviation in the data of the measured parameters from the actual data. It is essential to investigate this deviation which might occur due to carelessness during experimentation. Uncertainty analysis provides the maximum possible error in numerical digits. It is based on the random sampling during the experimentation. The uncertainty analysis tells us expected accuracy, not the exact accuracy of the system. To evaluate uncertainty involve in this experiment method suggested by Kline and McClintock [40] is used. If the data of any parameter is calculated using certain measured quantities then error in measurement of “y” (parameter) is given as follows.
Cp ¼ 1006
Tf 293
Ka ¼ 0:0257
ra ¼
Tf 293
0:735
0:0155
Tf 293
0:86
97500 Tf 287:045
Uncertainty associated with instruments used in various measurements of parameters in the experiment is given in Table A2.
870
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Table A2 Uncertainty intervals of various measurements S. No.
Measurement
Instrument
Uncertainty
1. 2. 3. 4. 5.
Dimensions of channel Pressure drop across the channel Pressure drop across the orifice-plate Temperature measurement Orifice plate and throat diameter
Vernier caliper Micro-manometer U-tube manometer Copper-constantan thermocouple Vernier caliper
±0.1 mm ±0.1 Pa ±1 mm ±0.1 C ±0.1 mm
1. Uncertainty in Area of absorber plate (Ap).
dDhd 2W 2WH ¼ ðW þ HÞ ðW þ HÞ2 dH
Ap ¼ W Lt " Ap ¼
dAp Ap
dAp Ap
dAp Ap
dAp L dLt t
" ¼
dLt
2
2
þ
Lt "
¼
þ
dW
2 #0:5
dDhd 2 300 2 300 30 ¼ ¼ 1:65289 ð300 þ 30Þ dH ð300 þ 30Þ2 "
2 #0:5
dDhd ¼
þ
0:1 300
2 #0:5
dDhd Dhd
¼ 0:00034359
dDhd Dhd
2. Uncertainty in Area of flow (Af).
dDhd
Af ¼ W H " Af ¼
dAf Af
dAf Af
dAf Af
dAf Af
¼
2
þ
dAf dH dH
2 #0:5
"
dH H
2
þ
dW
¼
dDhd þ dH dH
2 #0:5
2 0:5 þ ddDHhd dH
2ðW HÞðW þ HÞ1 h i0:5 ð1:65289 0:1Þ2 þ ð0:0165289 0:1Þ2 2ð300 30Þð300 þ 30Þ1
4. Uncertainty in Area of orifice meter (Ao)
Ao ¼ D2o 4 Ao
dDo
¼
2pDo 4 "
W
dAo dD dDo o
"
2 #0:5 ¼
pDo 2
2 #0:5 ¼
dDo
pDo dDo 2
pDo dDo Ao 2 dDo 2 0:1 ¼ p2 2 ¼ ¼ 42:96 dAo Do D 4 o
Ao ¼ 0:00334995
dAo
4 ðW HÞ ¼ 2ðWHÞðW þ HÞ2 ¼ 2 ðW HÞ
dDhd ¼ 2ðWHÞð1ÞðW þ HÞ2 dH
2
¼ 0:0030304246
dAo ¼
" #0:5 0:1 2 0:1 2 þ 30 300
h
¼
2 #0:5
3. Uncertainty in measurement of Hydraulic diameter (Dhd)
Dhd
dDhd dW dW
2
p
" #0:5 W dH 2 H dW 2 þ W H W H
¼
¼
Dhd
dAf dW dW
dDhd dW dW
W
2
0:1 1200
dAp dW dW
i
h
i þ ðW þ HÞ1 ð2WÞ
¼ 0:0047
5. Uncertainty in density measurement (ra)
ra ¼
Pa R To "
dra ¼
dra 1 dPa dPa
2
þ
dra 1 dTo dTo
2 #0:5
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
2 dra ¼ 4
1 R To
þ
Pa R To2
!
ra RTo
2
"
dV
dPa
Pa
871
¼ ð0:016241Þ2 þ ð0:00394Þ2 þ
V
!2 30:5 ra RTo dTo 5 Pa
2 #0:5 0:1 2 0:1 þ 300 30
¼ 0:017044 8. Uncertainty in useful heat gain (Qu)
dra ¼ ra
"
dPa
2
dTo
þ
Pa
2 #0:5
Qu ¼ ma cp ðT0 Ti Þ ¼ ma cp DT
To
dQu
Taking Pa ¼ 97500 Pa
dra ¼ ra
"
Qu 2 #0:5
2 0:1 0:1 þ 97500 25:33
6. Uncertainty in mass flow rate measurement (ma)
ma ¼ Cdo Ao
2ra ðDpÞ0 1b
dma ¼
Qu
m
" ¼
dCdo
2
2
þ
Cdo
#0:5
2
Cdo
4
dma þ dA dAo o
2
dma þ dr dra a
2
dAo
2
þ
Ao
dra ra
2
þ
dðDpÞ0 ðDpÞ0
dAp 2 d Qu 2 ¼4 þ þ ht Qu Ap
2 #0:5
2
0:2 þ ð0:0047Þ þ ð0:00394Þ þ 185 2
2
V
ma
¼
dma ma
2
dDTf DTf
þ
0:1 5:33
2 #0:5
!2 30:5 5
2
2
¼ 0:0252017
2 #0:5
dra þ ra
2
þ
dW W
2
þ
dH H
2 #0:5
ht Dhd Ka "
dNurs
¼
dNurs Nurs
dDhd
2 þ
Dhd
dht
2
ht
þ
dKa
2 #0:5
Ka
" ¼ ð0:0030304246Þ2 þ ð0:0252017Þ2 #0:5 0:00001 2 þ 0:0394161 0:02529
11. Uncertainty in Reynolds Number (Re)
Re ¼
ra W H "
2
"
Nurs
7. Uncertainty in measurement of air velocity in channel (V)
dV
0:1 1006:141
2
¼ 0:016241
V¼
2 #0:5
#0:5 0:1 2 ¼ ð0:02481Þ þ ð0:00034359Þ þ 22:66 ht
ðDpÞ0 ¼ DðHÞo sin30 sin90 ¼ 185mm
¼
dDT DT
10. Uncertainty in Nusselt number (Nurs)
m
dht
Nurs ¼
1:5 100
þ
Q Qu ¼ u A DTf p Ap Tp Tf
dht
The uncertainty in (Dp)0, for U-tube manometer is 0.2 mm.
dma
"
1b
¼ 1:5%
"
2
cp
¼ ð0:016241Þ2 þ
ht ¼
The data of
dCdo
ma
dcp
9. Uncertainty in heat transfer coefficient (ht)
2 #0:5 dma þ dðDpÞ0 dðDpÞ0
dma
þ
4
"
dma dC dCdo do
2
#0:5
0:5 ma ¼ Cdo Ao r0:5 a ðDpÞ0
"
dma
Uncertainty in specific heat is. 0.1 So, equation becomes
¼ 3:94 103
dQu "
" ¼
dRe Re
V$Dhd
n
¼
" ¼
dDhd Dhd
ra VDhd m 2
þ
dV V
2
dra þ ra
2
dm þ m
2 #0:5
¼ 0:02481
872
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
2
dRe Re
¼ 4ð0:0030304246Þ2 þ ð0:017044Þ2 þ ð0:00394Þ2 0:001 105 þ 1:87 105
dRe Re
[5]
!2 30:5 5
[6]
[7]
¼ 0:01776
[8] [9]
12. Uncertainty in friction factor (frs)
frs ¼
2 Dp
d
[10]
Dhd
[11]
4ra Lt V 2
2 2 2 2 dfrs 4 dDhd 2 dV dLt dra ¼ þ þ þ þ V frs Dhd Lt ra
dfrs frs
"
¼ ð0:0030304246Þ2 þ ð0:17044Þ2 þ þð0:00394Þ2 þ
dfrs frs
0:1 56:2
!2 30:5 d Dp d 5
Dp
[12]
[13]
d
2
0:1 1200
2 #0:5
[14] [15]
[16]
[17] [18]
¼ 0:01784 [19]
13. Uncertainty in thermo-hydraulic performance parameter (hp)
. hp ¼ ðNurs =Nuss Þ ðfrs =fss Þ0:33
dhp ¼ hp
"
dNurs Nurs
2
þ
dfrs
2 #0:5
[20]
[21]
[22]
frs
i0:5 dhp h ¼ ð0:0394161Þ2 þ ð0:01784Þ2 hp
[23] [24]
[25]
dhp ¼ 0:043265 ¼ 4:3265% hp
[26]
[27]
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Renewable Energy journal homepage: www.elsevier.com/locate/renene
Experimental study of enhancement of heat transfer and pressure drop in a solar air channel with discretized broken V-pattern baffle Raj Kumar, Muneesh Sethi, Ranchan Chauhan, Anil Kumar* School of Mechanical and Civil Engineering, Shoolini University, Solan, India
a r t i c l e i n f o
a b s t r a c t
Article history: Received 1 April 2016 Received in revised form 27 August 2016 Accepted 17 September 2016
This article presents an experimental study on heat transfer and friction characteristics of solar air channel fitted with discretized broken V-pattern baffle on the heated plate. The effect of geometrical parameters, predominantly the gap width and gap location has been investigated. The roughened baffle air channel has a width to height ratio, W/H of 10. The relative baffle gap distance, Dd/Lv and relative baffle gap width, gw/Hb has been varied from 0.26 to 0.83 and 0.5e1.5, respectively. Experiments have been carried out for the range of Reynolds number, Re from 3000 to 21,000 with the relative baffle height, Hb/H range of 0.25e0.80, relative baffle pitch, Pb/H range of 0.5e2.5; and angle of attack, aa range of 30 e70 . The optimal values of geometrical parameters of roughness have been obtained and discussed. For Nurs the greatest enhancement of the order of 4.47 times of the corresponding data of the without channel has been obtained. The absolute highest data of thermal hydraulic performance parameter has been found to be greater corresponding to Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5, and aa of 60 . The maximum value of the thermal hydraulic performance parameter was found to be 3.14 for the range of parameters investigated. © 2016 Elsevier Ltd. All rights reserved.
Keywords: Solar energy Turbulence Thermal Baffle height Baffle distance
1. Introduction The efficiency of compact heat exchangers can be improved by modifying the boundary layer developed on the heated surface. One of the well-known approach of modifying the boundary layer is to break the laminar viscous sub-layer formed on the heat transfer surface by creating rough surface in the form of transverse baffle, angled baffle, V-baffle and perforated baffle etc. Air channel is one of the simplest and extensively used types of heat exchanger in which heat energy is being exchanged between heated wall and air streaming through the system. The major constraint of air channel use is low overall thermal performance due to low heat transfer rate between heated wall and air [1e4]. In order to attain higher thermal performance it is beneficial that the stream at the heat transfer wall should be made turbulent. The baffle roughness has been used extensively for the augmentation of forced convective heat transfer coefficient of air channels. Use of baffle roughness seems to be a useful proposition for improving the local Nurs [5e8]. For detailed descriptions of some experimental investigations on
* Corresponding author. E-mail address: [email protected] (A. Kumar). http://dx.doi.org/10.1016/j.renene.2016.09.033 0960-1481/© 2016 Elsevier Ltd. All rights reserved.
air channel with transverse baffle, inclined baffle, delta baffle, diamond shaped baffle, V-type baffle etc. Karwa and Maheshwari [9] experimentally study Nurs and frs in a SAC with transverse fully perforated baffles and half perforated baffles attached to one of the broad wall. They reported that for fully perforated baffle the improvement in Nurs is 79e169% and 133e274% in case of half perforated baffles. Ozgen et al. [10] reported the thermal performance in a SAC with baffles fitted to the heated wall. Bopche and Tandale [11] reported the wholly developed stream in a roughened SAC with U-shaped pattern baffles. Eiamsa-ard et al. [12] investigated the heat transfer improvement in a SAC with winglet delta twisted tape baffles with different bO and Hb/H. Their studies shows that Nurs and frs data with winglet delta twisted tape were superier as compared to without winglet delta twisted tape. Promvonge et al. [13] mathematically examined the performance of Nurs and frs in square channel attached with 45 inclined baffles with a Re ranging from 100 to 1200. They informed that for the 45 baffle with Hb/H ¼ 0.4 and Re ¼ 1200, Nurs is superior to that of 90 baffle. Promovong [14] experimentally investigated the turbulent forced convection Nurs and frs loss behaviour in a high W/H channel attached with 60 V- shaped baffles. Akpinar et al. [15] experimentally investigate the performance analysis of four types
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Nomenclature Ap Ao Af Cdo Cp Dd Dhd f frs fss gw ht H Hb gw/Hb Hb/H Ka Lt Lv Dd/Lv ma Nu
Surface area of heated plate, m2 Area of orifice, m2 Area of flow, m2 Coefficient of discharge Specific heat of air, J/kg K Gap or broken distance, m Hydraulic diameter of channel, m Friction factor Friction factor of roughened baffle Friction factor without baffle channel Gap or discrete width, m Convective heat transfer coefficient, W/m2K Height of channel, m Height of baffle, m Relative gap width Relative baffle height Thermal Conductivity of air, W/mK Length of test section, m Length of V-type baffle, m Relative baffle gap distance Mass stream rate of air, kg/s Nusselt number
of SAH with different obstacles and without obstacle. They reported that efficiency of SAH depends on the surface geometry of collectors, solar radiation of air stream line. Chompookham et al. [16] experimentally studied the effect of winglet vortex type generators on the Nurs and frs behaviours for a turbulent stream. Bekele and Mishra [17] carried out the experimental studied of the turbulent air stream and heat transfer characteristics of SAC with delta shaped obstacle attached to the upper wall of a channel. Khanoknaiyakarn [18] carried out an experiment to study Nurs and frs by using V-pattern baffles on a broad heated wall of a large W/H channel. The effects of the baffles on Nurs and frs were investigated. Sriromreun et al. [19] reported experimental predictions of the Nurs and frs for a SAC with Z-shaped baffles. Their experiments were performed by controlling the air stream rate to attain Re values in the range of 4400 to 20,400. Thianpong et al. [20] reported the experimental studies of the collector performance of a SAC with twisted rings type baffles. Zhou and Ye [21] carried out the experimental studied of the turbulent air stream and heat transfer characteristics of SAC with delta winglet vortex generator baffles attached to the upper surface of a channel. Chamoli and Thakur [22] conducted an indoor experimental investigation to study Nurs and frs data of air passing through an air channel that was roughened by V-shaped perforated baffles. Bayrak et al. [23] studied the performance valuation of porous baffles introduced SAC by energy and energy method. They reported that the maximum collector efficiency and air temperature increase are attained by SAC with a thickness of 6 mm and ma of 0.025 kg/s while the lowermost data are obtained for the SAC with non-baffle collectors with ma of 0.016 kg/s. Tamna et al. [24] investigated the effect of multiple V-baffle vortex generators to improve Nurs in a channel fitted with 45 BVG with Re ranging from 4000 to 21000, Hb/H ¼ 0.25, Pb/H ¼ 0.5, 1 and 2 and aa equal to 45 respectively. Alam et al. [25] experimentally investigated the effect of Hb/H of 0.4e1.0, Pb/H of 4e12, bO of 5e25%, aa of 60 and Re varies from 2000 to 20,000 on V-shaped perforated blocks SAC with W/H of 10.
Nurs Nus Pb Pb/H (Dp)d (Dp)o Qu Re Tf Ti To Tp U V W SAH SAC
857
Nusselt number of baffle channel Nusselt number of channel without baffle Pitch of baffle channel, m Relative pitch ratio Pressure drop across test section, Pa Pressure drop across orifice plate, Pa Useful heat gain, W Reynolds number Mean bulk air temperature, K Inlet temperature of air, K Outlet temperature of air, K Plate temperature of air, K Mean air velocity, m/s Velocity of air, m/s Width of channel, m Solar air heater Solar air channel
Greek symbols aa Angle of attack, b Ratio of orifice meter to pipe diameter ra Density of air, kg/m3 n Kinematic viscosity of air, m2/s hp Thermo hydraulic performance
They observed that average improvement in Nurs for perforated Vshaped blockage is 33% higher over solid blockages, frs of perforated blockage gets reduced by 32% of the value as compared to solid blockage. Skullong et al. [26] carried out an experimental study on the turbulent flow and heat transfer characteristics in a SAC attached baffles with combined groove baffles. Shin and Kwak [27] studied the effect of the perforation shape for a blockage wall on the Nurs in a stream passage. It was observed that a blockage surface with wider perforation provided a more uniform Nurs and greater thermal performance factor. plate. Table 1 summarizes the experimental investigations of some important rib arrangements investigated by the investigators. Literature review shows that, the transverse baffle shape improve the heat transfer rate by stream separation and creation of vortices on the upward and downward of the baffles and reattachment of stream in inter-baffle spaces. Angling of transverse baffle further enhances the heat transfer on account the movement of vortices along the baffle wall and creation of a secondary stream cell close to the leading end, which outcome in local wall turbulence. V- type baffle of an extended angled baffle benefits in the type of two secondary stream jets as compared to single in case of an angled baffle resulting in still superior heat transfer rate. Making a broken in the baffle is found to improve the heat transfer by disturb the secondary stream and produce advanced level of turbulence in the fluid downward of the baffles. It is hypothesized that discretized broken V-pattern baffle will raise heat transfer rate compared to without broken V-type baffle.
2. Experimental details 2.1. Experimental set-up To study the outcome of discretized broken V-pattern baffle turbulent promoter on the Nurs and frs of air stream an experimental setup was intended and made-up accordance with guidelines suggested in ASHRAE standard [28]. A schematic illustration
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Table 1 Previous experimental investigation of various baffle roughened solar air channel. Sr. Type of baffles No.
Shape of baffles
Major findings
1.
Angled baffles [5]
Augmentation of about 3.16 and 3.56 times in heat transfer and pressure drop respectively has been reported over smooth rectangular channel.
2.
Perforated baffles [9]
The perforated baffles helped in achieving high values of heat transfer and pressure drop of about 3.56 and 4.18 times respectively over smooth rectangular channel.
3.
U-shaped baffles [11]
Augmentation in heat transfer and pressure drop of about 2.82 and 3.72 times has been achieved compared to smooth absorber plate.
4.
Delta shaped baffles [17]
The insertion of delta shaped baffles augmented heat transfer and pressure drop by 3.67 and 4.89 times respectively over smooth rectangular channel.
5.
V-shaped baffles [18]
Heat transfer and pressure drop augmented by about 4.02 and 5.23 times respectively compared to that of smooth absorber surface.
6.
Single V-Perforated shaped baffles [22]
Augmentation of about 3.95 and 5.35 times in heat transfer and pressure drop respectively is reported over smooth rectangular channel.
7.
Perforated V-shaped blockages [25]
Average enhancement in heat transfer is 13% higher over solid baffles while the pressure drop decreased by 18% of the value compared to solid baffles.
8.
Transverse Perforated blocks baffles [27]
Heat transfer augmented by 3.98 and pressure drop by 4.28 times respectively compared to the smooth rectangular channel.
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Table 1 (continued ) Sr. Type of baffles No. 9.
Discrete V-pattern baffles (Proposed shape)
Shape of baffles
Major findings The literature survey over the available experimental and analytical studies reported that the V-pattern baffles have elevated overall thermal performance compared to other simple angled, transverse, perforated baffle shapes and configurations. The research gap with discrete V-pattern baffles has been carried out in the present study which augments the convective heat transfer due to insertion of secondary flow regions.
of experimental set up is shown in Fig. 1. The air channel is 2000 mm extended with a stream cross section of 300 mm 30 mm is made-up from ply panel of 20 mm thickness. The channel is comprises of inlet section 500 mm long, a test section of 1200 mm length and an exit section of 300 mm length. The complete channel is insulated with 50 mm thick polystyrene insulation having thermal conductivity of 0.037 W/mK to minimise heat loss to the environment. An electric heating element was made-up by combining six loops of nichrome cable in series and parallel having a size of 1,200 mm 300 mm to supply a kept heat flux 1000 W/m2 to the heated wall which is careful to be sensibly reasonable data of heat energy input for testing rectangular air channel. The asbestos sheet is converted with strip of Mica to keep the uniform distance between the wires and prevent back heating. A Galvanised Iron (GI) sheet of 18 SWG size black painted in order to facilitate the heat transfer is used as a heated wall. Discredited broken V-Pattern baffle were attached on the base of heated wall by means of epoxy resin. This plate formed the top wall of the air channel. The bottom side of the air channel is covered with smooth face using sun mica sheet. A calibrated Orifice meter (having coefficient of discharge 0.62) connected to U-tube manometer using methyl alcohol as manometer fluid was used to measure the mass stream rate of air through rectangular air channel. The control valves provided in the lines control the stream. Copper-constantan thermocouples were used for air and aluminium plate temperature measurement. Such thermocouples are usually recommended for temperature measurement in the
range of 0e400 C (Benedict [29]). The thermocouple output is measured by a Digital Micro Voltmeter, joined through a selector switch to designate the output of the thermocouples in C. To verify the accuracy of temperature measurement, thermocouples have been calibrated under laboratory conditions against a dry block temperature calibrated instant. The thermocouple to be calibrated was located in the calibration bath where kept temperature is maintained and the response of the thermocouple and the standard probe were noted with the help of a digital temperature indicator for various pre-set data of the standard probe, and the error among the reading of standard probe and the thermocouple were measured. If this error is more than certain limit of the calibrator then the thermocouple is rejected and if this error is less than tolerance limit of the calibrator then the thermocouple is accepted. This process was repeated in numerous steps of rising as well as reducing temperature range. Temperature scanners have been used to show the temperature of heated plate and inlet and outlet of air. The pressure drop through the test section of the air channel was obtained by a micro monometer having a least count of 0.01 mm. 2.2. Range of parameters Rectangular air channel has an Lt equal to 2000 mm while the, H is set equal to 30 mm and W is 300 mm, the hydraulic diameter, Dhd is equal to 54.54 mm. The baffle parameters are determined by baffle height Hb, pitch of baffle Pb, broken distance Dd, broken width
Fig. 1. Schematic of experimental setup.
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gw, length of V-type baffle Lv, angle of attack aa and the shape of the roughness elements. These parameters have been expressed in the form of dimensionless roughness parameters, viz., relative gap distance (Dd/Lv), relative gap width (gw/Hb), relative baffle pitch (Pb/H), relative baffle pitch (Pb/H), relative baffle height (Hb/H), and angle of attack (aa). The shape is shown in Figs. 2 and 3 and Table 2 gives the range of parameters.
2.3. Raw data collection The data collected have been used to compute ht, Nu, and f. Relevant expressions for the computation of the above parameters and some intermediate parameters have been given below.
2.3.1. Temperature measured Weighted average plate air temperature: The mean temperature of the plate (Tp) is the average of all temperatures of the heated plate:
P Tp ¼
Tpi N
(1)
The mean bulk air temperature (Tf) is a simple arithmetic mean of the measured values at the inlet and the exit temperature of air streaming through the test section:
Tf ¼
Ti þ To 2
(2)
Fig. 3. Experimental setup and discretized broken V-Pattern baffle roughened plates.
Table 2 Range of parameters. Sr. No.
Parameters
Range
1. 2. 3. 4. 5. 6.
Re Dd/Lv gw/Hb Pb/H Hb/H
3000 to 21000 0.26e0.83 0.5e1.5 0.5e2.5 0.25e0.80 30 e70
aa
2.3.3. Velocity of air through channel (V) The V is calculated from the knowledge of ma and the stream as
ma
where To ¼ (TA2 þ TA3 þ TA4 þ TA5 þ TA6)/5, Ti ¼ TA1.
V¼
2.3.2. Mass stream rate measurement (ma) The ma of air has been calculated from the pressure drop measurement through the calibrated orifice meter by using the following formula:
2.3.4. Equivalent hydraulic diameter (Dhd)
" ma ¼ Cdo Ao
2ra ðDpÞ0 1 b4
ra WH
(4)
#0:5 (3)
Dhd ¼
4:ðW:HÞ 2:ðW þ HÞ
Fig. 2. Discretized broken V-pattern baffle.
(5)
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2.3.5. Reynolds number (Re) The Re of air stream in the channel is intended from
Re ¼
V$Dhd
Nurs ¼
ht Dhd Ka
861
(10)
(6)
n
2.3.6. Friction factor (frs) The frs is determined from the measured value of (Dp)d across the test section length using the Darcy equation as
frs ¼
2 Dp d Dhd
(7)
4ra Lt V 2
2.3.7. Heat transfer coefficient (ht) The heat transfer rate Qu, from absorber to the air is given by:
Qu ¼ ma cp ðT0 Ti Þ
(8)
The ht for the heated test section has been calculated as:
ht ¼
Q u Ap $ Tp Tf
(9)
Fig. 5. Comparison of experimental and predicted values of fss.
2.3.8. Nusselt number (Nurs) The ht can be used to determine the Nurs, which is defined as: Table 3 Range of uncertainty in the measurement of essential parameters. S. No.
Parameters
Error range, %
1. 2. 3. 4. 5. 6. 7. 8.
Mass flow rate (ma) Velocity of air (V) Useful heat gain (Qu) Heat transfer coefficient (ht) Nusselt number (Nurs) Friction Factor (frs) Reynolds Number (Re) Thermo-hydraulic performance parameter (hp)
1.597e2.033 1.653e1.811 2.131e3.267 2.213e3.732 3.378e4.667 1.283e2.331 1.43e3.76 3.675e5.221
Fig. 4. Comparison of experimental and predicted values of Nuss.
Fig. 6. (A) Variation of Nurs with Re at different Dd/Lv. (B) Variation of Nurs with Dd/Lv at different selected Re.
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Nuss and fss as a function of the Re is shown in Figs. 4 and 5 respectively.
2.4. Uncertainties analysis An uncertainty analysis has been carried to estimate the errors involved in experimental data measurement. The uncertainty is estimated based on errors associated with measuring instruments [32]. Table 3 shows the range of uncertainty in the measurement of essential parameters. The details of the analysis are given in Appendix-A.
4. Results and discussion A study was conducted to understand the effect on Nurs and frs of
3. Validation of experimental data The value of Nuss and fss calculated through experimental outcomes for a smooth channel have been compared with the outcomes obtained from the Dittus-Boelter equation [Eq. (11)] for the Nuss, and modified Blasius equation [Eq. (12)] for the fss [6]. The Nuss for a smooth channel is given by the Dittus-Boelter equation as:
Nuss ¼ 0:023Re0:8 Pr 0:4
(11)
The fss for a smooth channel is given by the modified Blasius equation as:
fss ¼ 0:085Re0:25
(12)
The comparison of the experimental and estimated outcomes of
Fig. 7. (A) Variation of Nurs with Re at different gw/Hb. (B) Variation of Nurs with gw/Hb at different selected Re.
Fig. 8. Secondary stream type with discretized broken V-pattern baffle air channel.
Fig. 9. (A) Variation of Nurs with Re at different Hb/H. (B) Variation of Nurs with Hb/H at different selected Re.
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In order to compare the improvement of the Nurs achieved as an outcome of providing a broken in the V-pattern baffle arrangement, the values of the, Nurs for fixed values of the, gw/Hb of 1.0, and distinct values of Dd/Lv is given in Fig. 6(A). Fig. 6(A) shows the variant of and Pb/H ¼ 1.5, Hb/H ¼ 0.50 and aa ¼ 60 , with Re at distinct values of Dd/Lv for a fixed gw/Hb of 1.0. It can be seen that the Nurs rises with rise in Dd/Lv from 0.58 to 0.67, attains an extreme at a, Dd/Lv of 0.67 and thereafter it reduces with rise in the Dd/Lv. Fig. 6(B) shows the values of the Nurs as a function of Dd/Lv for a 60 discretized broken V-pattern baffle air channel at distinct selected Re. It can be observed that at any Dd/Lv, the Nurs is the highest for the Dd/Lv ¼ 0.67 for every value of Re. Producing broken near the leading edge (say at Dd/Lv ¼ 0.26), the strength of the secondary stream may not be sufficient to energize the main stream passing through the broken and this broken distance does not lead to significant rise in Nurs. A rise in the values of Dd/Lv say at Dd/Lv ¼ 0.55
signifies shifting of the broken toward trailing edge. This raises the strength of the secondary stream and Nurs rises with rise in the Dd/ Lv up to 0.67. It appears that up to theses Dd/Lv values, there is significant contribution baffle second part of secondary stream to the Nurs whereas this truncated second part begins to become insignificant as the distance is further increased resulting in a slight reduce in Nurs as the Dd/Lv is raised beyond 0.67. These results broadly agree with previous studies on broken baffle roughened channels [15] [17] [22], and [24,25]. Fig. 7(A) presented the values of the, Nurs for fixed values of the, Dd/Lv of 0.67, Pb/H ¼ 1.5, Hb/H ¼ 0.50 and aa ¼ 60 and distinct values of gw/Hb. This figure shows the Nurs rises with rise in the gw/ Hb up to about 1.0, beyond which it reduces with rise in the gw/Hb. The value of Nurs is greatest for gw/Hb of 1.0 and smallest for the, gw/ Hb of 1.5. Fig. 7(B) shows the values of the Nurs as a function of gw/Hb for a 60 discretized broken V-pattern baffle air channel at distinct selected Re . It can be observed that at any gw/Hb, the Nurs is the highest for the gw/Hb ¼ 1.0 for all value of Re. It appears that as the gw/Hb is raised beyond 1.0, the stream velocities through the broken will reduce, which may not be strong enough to accelerate the stream through the broken and hence the Nurs due to this stream may not be raised significantly whereas with the reduction of this gw/Hb to values lower than 1.0, there may be very little space for stream of the fluid through it which outcomes in low turbulent and
Fig. 10. (A) Variation of Nurs with Re at different Pb/H. (B) Variation of Nurs with Pb/H at different selected Re.
Fig. 11. (A) Variation of Nurs with Re at different aa. (B) Variation of Nurs with aa at different selected Re.
the stream Re and discretized broken V-pattern baffle used to provide roughness for an air channel. In this experimental investigation, effect of discretized broken V-pattern baffle shape parameters such as; Dd/Lv, gw/Hb, Hb/H, Pb/H, and aa on Nurs and frs has been studied extensively and discussed below. 4.1. Heat transfer
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Fig. 12. (A) Variation of frs with Re at different Dd/Lv. (B) Variation of frs with Dd/Lv at different selected Re.
Fig. 13. (A) Variation of frs with Re at different gw/Hb. (B) Variation of frs with gw/Hb at different selected Re.
hence reduce the improvement of Nurs. Thus in order to achieve the improvement of Nurs, the width of the broken should be such that it can rise the velocity of the fluid passing through it in order to create the local turbulence as shown in Fig. 8. These outcomes broadly agree with previous studies on broken roughened channels [15] [17] [22], and [24,25]. Fig. 9(A) shows the variation of Nurs with Re for distinct values of Hb/H. The other roughness parameters were kept Dd/Lv ¼ 0.67, gw/ Hb ¼ 1.0 as Pb/H ¼ 1.5 and aa ¼ 60 . It is observed that the Nurs rises with rise in Hb/H for all values of Re due to increased protrusion into stream causing more turbulence, thereby, resulting in rise in Nurs. The highest Nurs is observed at Hb/H of 0.50. Fig. 9(B) shows the values of the Nurs as a function of Hb/H for a 60 discredited broken V-pattern baffle air channel at different selected Re. It can be observed that at any Hb/H, the Nurs is the highest for the Hb/H ¼ 0.50 for all value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. Fig. 10(A) shows the variation of Nurs as a function of Re for distinct values of Pb/H and fixed values of other channel parameters as, Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0 Hb/H ¼ 0.50 and aa ¼ 60 . For all Re, the greatest value of Nurs has been observed corresponding to the Pb/H value of 1.5, whereas the smallest value of Nurs has been found to occur at the Pb/H value of 2.5 for the range of investigations. Fig. 10(B) shows the values of the Nurs as a function of Pb/H for a 60
discredited broken V-pattern baffle air channel at distinct selected Re. It can be observed that at any Pb/H, the Nurs is the highest for the Pb/H ¼ 1.5 for all value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. Fig. 11(A) shows the variation of Nurs with Re for distinct values of aa and fixed values of other channel parameters as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0, Hb/H ¼ 0.50 and Pb/H ¼ 1.5. In this plot, Nurs has been plotted as a function of aa for some selected values of Re and fixed values of other channel parameters. Nurs rises with rise in aa, attains a highest value corresponding to aa ¼ 60 and then decreases with further rises in the value of aa. Fig. 11(B) shows the values of the Nurs as a function of aa for discredited broken V-pattern baffle air channel at distinct selected Re . It can be observed that at any aa, the Nurs is the highest for the aa ¼ 60 for each value of Re. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. 4.2. Friction factor Invariable, use of baffle roughness substantially enhances Nurs from heated surface of air channels however, there occurs a corresponding rise in frictional losses. In this experimental investigation, effect of discretized broken V-pattern baffle parameters such
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Fig. 14. (A) Variation of frs with Re at different Hb/H. (B) Variation of frs with Hb/H at different selected Re.
Fig. 15. (A) Variation of frs with Re at different Pb/H. (B) Variation of frs with Pb/H at different selected Re.
as, Dd/Lv, gw/Hb, Hb/H, Pb/H and aa on friction characteristics of the air channel has been studied extensively and discussed below. The variation of frs with Re for distinct values of Dd/Lv and fixed values of other baffle parameters as gw/Hb ¼ 1.0 Hb/H ¼ 0.50 Pb/ H ¼ 1.5 and aa ¼ 60 has been shown in Fig. 12(A). It is seen that the value of frs reduces with rising Re and towards a kept data as expected. The frs rises with rise in the, Dd/Lv of up to 0.67 and reduces with further rise in the Dd/Lv. The plot shows that the highest and lowest values of frs for discretized broken V-pattern baffle air channel occur for the, Dd/Lv of 0.67 and 0.26 respectively. The lesser value of frs for broken on the upstream side is due to weakened strength of secondary stream. Fig. 12(B) shows the values of the frs as a function of Dd/Lv for a 60 discredited broken V-pattern baffle air channel at different selected Re It can be observed that at any Dd/ Lv, the frs is the highest for the Dd/Lv ¼ 0.67 for all value of Re. These outcomes broadly agree with previous studies on broken baffle air channels These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. The variation of frs with Re for different values of gw/Hb and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, Hb/H ¼ 0.50, Pb/ H ¼ 1.5 and aa ¼ 60 has been shown in Fig. 13(A). It has been observed that for all values of gw/Hb, frs reduce with rise in Re. Fig. 13(A) shows that at all Re, the frs rises as gw/Hb is raised from 0.5 to 1.0 and decreases as gw/Hb is raised further. Fig. 13(B) shows the
values of the frs as a function of gw/Hb for a 60 discredited broken V-pattern baffle air channel at distinct selected Re . It can be observed that at gw/Hb, the frs is the highest for the gw/Hb ¼ 1.0 for all value of Re. The air streaming through the broken creates turbulence at the downstream side of the gap. Addition of relative broken width in the baffles induces recirculation loops, which are responsible for greater turbulence and hence higher pressure losses. Strength of secondary stream is weakened in case of gw/Hb of 1.5 as compared to gw/Hb of 0.5, 0.75, 1.0 and 1.25 hence the frs is lower than in other cases. These outcomes broadly agree with previous studies on broken baffle channels. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17] [22], and [24,25]. The variation of, frs with Re for distinct values of Hb/H have been plotted in Fig. 14(A). The other roughness parameters were kept as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0 Pb/H ¼ 1.5, and aa ¼ 60 . It has been observed from this plot that for a given Hb/H value frs reduces with rise in Re. Fig. 14(A) clearly shows that frs rises with rise in Hb/H and the greatest value of frs correspond to Hb/H value of 0.50. Fig 14(B) shows the values of the frs as a function of Hb/H for a 60 discretized broken V-pattern baffle air channel at distinct selected Re. It can be observed that at Hb/H, the frs is the highest for the Hb/H ¼ 0.80 for all value of Re.
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Fig. 16. (A) Variation of frs with Re at different aa. (B) Variation of frs with aa at different selected Re.
It is due to the fact that with the rise in Hb/H value, baffles protrude more and more into the core stream resulting in rise in turbulence level as well as the frs. These outcomes broadly agree with previous studies on baffle roughened channel. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17], and [24,25]. Fig. 15(A) shows the variation of frs with Re for different values of Pb/H and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, gw/ Hb ¼ 1.0 Hb/H ¼ 0.50 and aa ¼ 60 . It has been observed from Fig. 15(A) that for all values of Pb/H, frs reduces with rise in Re. For, Pb/H value of 2.5 and 1.5 yield the lowest and highest values of frs respectively. Fig. 15(B) shows the values of the frs as a function of Pb/ H for a 60 discretized broken V-pattern baffle air channel at different selected Re. It can be observed that at Pb/H, the frs is the highest for the Pb/H ¼ 1.5 for all value of Re. These outcomes broadly agree with previous studies on roughened channels. These outcomes broadly agree with previous studies on baffle roughened channel [15] [17], [22], and [24,25]. The variation of frs with Re for distinct values of aa and fixed values of other baffle parameters as Dd/Lv ¼ 0.67, gw/Hb ¼ 1.0, Pb/ H ¼ 1.5 and Hb/H ¼ 0.50 has been shown in Fig. 16(A). It has been observed that for all the values of aa, frs reduces with rise in Re. The smallest and highest value of frs have been obtained corresponding to aa values of 30 and 60 respectively. Fig. 16(B) shows the values of the frs as a function of aa for discretized broken V-pattern baffle
Fig. 17. (A) Variation of hp with Re at different Dd/Lv. (B) Variation of hp with Dd/Lv at different selected Re.
air channel at distinct selected Re. It can be observed that at aa, thefrs is the highest for the aa ¼ 60 for all value of Re. 4.3. Thermo hydraulic performance Study of the Nurs and frs characteristics shows that an improvement in Nurs is in general accompanied with friction power penalty due to a corresponding increase in the frs. Therefore it is essential to decide the geometry that will outcome in maximum improvement in Nurs with minimum frs penalty. In order to achieve this purpose of simultaneous consideration of thermo hydraulic performance, researchers proposed a thermo hydraulic parameter known as efficiency parameter ‘hp’ which evaluates the improvement in Nurs of a roughened air channel compared to that of the smooth channel for the same pumping power requirement and is defined as [30,31]:
hp ¼ ðNurs =Nuss Þ=ðfrs =fss Þ0:33
(13)
A heat transfer improvement device having a data of thermo hydraulic parameter (hp) higher than unity ensures the fruitfulness of using improvement device and therefore, this parameter is usually used to compare the performance of distinct roughness arrangements to prefer the best roughness arrangement among all
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Fig. 18. (A) Variation of hp with Re at different gw/Hb. (B) Variation of hp with gw/Hb at different selected Re.
the feasible combinations. Figs. 17e21 shows the effect of baffle parameters on thermo hydraulic performance parameter(Nurs/Nuss)/(frs/fss)0.33, as function of Re. In Table 4, those values of roughness geometry parameters have been presented for which thermo hydraulic performance parameter (Nurs/Nuss)/(frs/fss)0.33 values have been found to be highest. The highest absolute value of (Nurs/Nuss)/(frs/fss)0.33 has been observed to be 3.14 corresponding to Dd/Lv value of 0.67, gw/ Hb value of 1.0, Hb/H value of 0.50, Pb/H value of 1.5, and aa value of 60 for discretized broken V-pattern baffle air channel. On evaluating the thermal hydraulic performance for all possible sets of roughness parameters one can recognize the roughness shapes which provides maximum enhancement in heat transfer at the cost of least friction penalty. The values of thermal hydraulic performance parameter, hp ¼ (Nurs/Nuss)/(frs/fss)0.33 determined for this shape of broken V-pattern baffle have been compared with the corresponding values for angled baffle [5], perforated baffle [9], U-shaped baffle [11], delta shaped baffle [17], V-perforated baffle [22] and continuous V-shaped baffle [18] as shown in Fig. 22. It can be seen that the broken V-pattern baffle shape results in the better thermal hydraulic performance, hp ¼ (Nurs/Nuss)/(frs/fss)0.33 among all the shapes investigated. On the basis of present experimental investigation found that
867
Fig. 19. (A) Variation of hp with Re at different Hb/H. (B) Variation of hp with Hb/H at different selected Re.
used of broken V-pattern baffle across the width of the plate to augment the heat transfer rate. Because creating of a gap in the Vpattern baffle allows release of fluid belonging to secondary flow and main flow through the gap. The main flow passing through the gap is the developed flow with thicker boundary layer consisting of viscous sub layer. As a result of the presence of gap, the secondary flow along the baffle joins the main flow to accelerate it which energizes the retarded boundary layer flow along the surface. This increases the heat transfer through the gap width area behind the baffle.
5. Conclusions On the basis of experimental analysis of heat transfer, friction factor and thermo hydraulic performance of air channel provided with heated plate having discretized broken V-Pattern baffle shape of artificial roughness, the subsequent conclusions can be drawn from the this work: 1. Providing a discretized broken V-pattern baffles outcomes in substantial improvement in Nurs of air channel the improvement is a strong function of broken width and broken distance. As compared to the without baffle channel the presence of
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R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Fig. 20. (A) Variation of hp with Re at different Pb/H. (B) Variation of hp with Pb/H at different selected Re.
Fig. 21. (A) Variation of hp with Re at different aa. (B) Variation of hp with aa at different selected Re.
discretized broken V-pattern baffle rough surface yields Nurs up to 4.47 times while the frs rises up to 4.59 times in the range of parameters investigated. 2. The values of Nurs and frs rise with rise in Dd/Lv attains a highest value corresponding to Dd/Lv value of 0.67 and with further rise in the value of Dd/Lv, the Nurs and frs are found to decrease. The value of Nurs and frs is greater for a, Dd/Lv of 0.67 and smaller for the, Dd/Lv of 0.26. As a result of creating broken near the leading edge, the strength of the secondary stream may not be sufficient to energize the main stream passing through the broken and Dd/ Lv does not lead to significant rise in Nurs and frs. A rise in the values of Dd/Lv signifies shifting of the broken toward trailing edge. This rise the strength of the secondary stream and Nurs and frs rises with rise in the Dd/Lv up to 0.67. 3. An rise in gw/Hb outcomes in a rise in Nurs and frs attaining the highest value corresponding to gw/Hb of 1.0 and the values of these parameters reduce with further rise of gw/Hb. The value of Nurs and frs is highest for a, gw/Hb of 1.0 and lowest for the gw/Hb of 1.5. An increasing broken stream promotes local turbulence and stream mixing along the broken stream region while the baffle induced secondary stream is maintained in the air channel. Therefore, it may be reasoned that the rise in gw/Hb beyond 1.0 reduces the stream velocities through the broken and hence
the local turbulence. At the same time too small broken width will also not allow sufficient amount of secondary stream fluid to pass through and hence the turbulence level will remain low. 4. The values of Nurs and frs rise with rise in aa and attain the highest values corresponding to aa value of 60 . With further rise in the value of aa beyond 60 , Nurs and frs reduces. The values of Nurs and frs have been found to rise with rise in Pb/H, attaining the highest value at pitch value of 1.5 and then reduce with further rises in Pb/H. The present investigation shows that baffled air channel with Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5, and aa of 60 yields the highest values of thermo hydraulic performance parameter. 5. Discrete V-pattern baffle has also been shown to be thermal hydraulic better in comparison to angled baffle, perforated baffle, U-shaped baffle, delta shaped baffle, V-perforated baffle and continuous (without broken) V-shaped baffle. Because Vshaping of the baffle helps in the formation of two leading ends and a single trailing end as well as two secondary flow cells which promote turbulent mixing and hence increased heat transfer. Creating discrete in the V-pattern baffle allows release of the secondary flow and mix with main flow through discrete. This result in its acceleration, which energizes the retarded boundary layer flow along the surface resulting in the increase
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
869
Table 4 Baffle rough parameters corresponding to maximum hp ¼ (Nurs/Nuss)/(frs/fss)0.33 Roughness parameters
Fixed parameters
Optimum values of (Nurs/Nuss)/(frs/ fss)0.33
Maximum values of hp
gw/Hb Dd/Lv Hb/H Pb/H
Dd/Lv ¼ 0.67, Hb/H ¼ 0.50, Pb/H ¼ 1.5, aa ¼ 60 . gw/Hb ¼ 1.0, Hb/H ¼ 0.50, Pb/H ¼ 1.5, aa ¼ 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Pb/H of 1.5, aa of 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, aa ¼ 60 . Dd/Lv of 0.67, gw/Hb of 1.0, Hb/H of 0.50, Pb/H of 1.5.
1.0 0.67 0.50 1.5 60
3.14 3.14 3.14 3.14 3.14
aa
dy y
" ¼
dy dx d x1 1
2
dy þ dx dx2 2
2
dy þ dx dx3 3
2
dy þ…þ dx dxn n
2 #0:5
Where dx1, dx2, dx3, … dxn are possible error in measurement of x1, d x2, x3, … xn, dy is known as absolute uncertainty and yy is known as relative uncertainty. In the present experiment, important parameters considered for uncertainty analysis are Reynolds number, Heat transfer coefficient, Nusselt number, friction factor. The data of measured parameters are given in Table A1.
Table A1 Measured parameters and their respective data
Fig. 22. Comparison of thermal hydraulic performance with previous investigations.
of the heat transfer through the discrete width are behind the baffles.
6. Future scope The future directions for research can be made on evaluating the thermohydraulic performance of multi discrete regions in a Vpattern baffles. Since, the discretization increases heat transfer due to generation of secondary flow streams, the multi discretization will improve the strength of secondary streams which will add up to the heat transfer from the absorber plate to the air. Also, the multi V- pattern baffles in discrete form can be helpful for increasing heat transfer and thereby the overall performance.
S. No.
Parameter
Data
1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12. 13.
Length of test section, Lt Width of the channel, W Height of channel, H Diameter of pipe, DP Diameter of orifice meter, Do Pressure drop across orifice meter, (Dp)o Pressure drop across test section(Dp)d, Atmospheric pressure, Pa Outlet air temperature, To Inlet air temperature, Ti Rise in temperature of air, DT Mean bulk air temperature Tf Mean plate temperature, Tp
1200 mm 300 mm 30 mm 80 mm 36 mm 185 mm 56.2 Pa 97500 25.33 20 5.33 22.66 33
The thermo-physical properties of air have been determined by following standard correlations:
m ¼ 1:81 105
Appendix A. Uncertainties analysis During experimentation, lots of factors come into play which causes deviation in the data of the measured parameters from the actual data. It is essential to investigate this deviation which might occur due to carelessness during experimentation. Uncertainty analysis provides the maximum possible error in numerical digits. It is based on the random sampling during the experimentation. The uncertainty analysis tells us expected accuracy, not the exact accuracy of the system. To evaluate uncertainty involve in this experiment method suggested by Kline and McClintock [40] is used. If the data of any parameter is calculated using certain measured quantities then error in measurement of “y” (parameter) is given as follows.
Cp ¼ 1006
Tf 293
Ka ¼ 0:0257
ra ¼
Tf 293
0:735
0:0155
Tf 293
0:86
97500 Tf 287:045
Uncertainty associated with instruments used in various measurements of parameters in the experiment is given in Table A2.
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R. Kumar et al. / Renewable Energy 101 (2017) 856e872
Table A2 Uncertainty intervals of various measurements S. No.
Measurement
Instrument
Uncertainty
1. 2. 3. 4. 5.
Dimensions of channel Pressure drop across the channel Pressure drop across the orifice-plate Temperature measurement Orifice plate and throat diameter
Vernier caliper Micro-manometer U-tube manometer Copper-constantan thermocouple Vernier caliper
±0.1 mm ±0.1 Pa ±1 mm ±0.1 C ±0.1 mm
1. Uncertainty in Area of absorber plate (Ap).
dDhd 2W 2WH ¼ ðW þ HÞ ðW þ HÞ2 dH
Ap ¼ W Lt " Ap ¼
dAp Ap
dAp Ap
dAp Ap
dAp L dLt t
" ¼
dLt
2
2
þ
Lt "
¼
þ
dW
2 #0:5
dDhd 2 300 2 300 30 ¼ ¼ 1:65289 ð300 þ 30Þ dH ð300 þ 30Þ2 "
2 #0:5
dDhd ¼
þ
0:1 300
2 #0:5
dDhd Dhd
¼ 0:00034359
dDhd Dhd
2. Uncertainty in Area of flow (Af).
dDhd
Af ¼ W H " Af ¼
dAf Af
dAf Af
dAf Af
dAf Af
¼
2
þ
dAf dH dH
2 #0:5
"
dH H
2
þ
dW
¼
dDhd þ dH dH
2 #0:5
2 0:5 þ ddDHhd dH
2ðW HÞðW þ HÞ1 h i0:5 ð1:65289 0:1Þ2 þ ð0:0165289 0:1Þ2 2ð300 30Þð300 þ 30Þ1
4. Uncertainty in Area of orifice meter (Ao)
Ao ¼ D2o 4 Ao
dDo
¼
2pDo 4 "
W
dAo dD dDo o
"
2 #0:5 ¼
pDo 2
2 #0:5 ¼
dDo
pDo dDo 2
pDo dDo Ao 2 dDo 2 0:1 ¼ p2 2 ¼ ¼ 42:96 dAo Do D 4 o
Ao ¼ 0:00334995
dAo
4 ðW HÞ ¼ 2ðWHÞðW þ HÞ2 ¼ 2 ðW HÞ
dDhd ¼ 2ðWHÞð1ÞðW þ HÞ2 dH
2
¼ 0:0030304246
dAo ¼
" #0:5 0:1 2 0:1 2 þ 30 300
h
¼
2 #0:5
3. Uncertainty in measurement of Hydraulic diameter (Dhd)
Dhd
dDhd dW dW
2
p
" #0:5 W dH 2 H dW 2 þ W H W H
¼
¼
Dhd
dAf dW dW
dDhd dW dW
W
2
0:1 1200
dAp dW dW
i
h
i þ ðW þ HÞ1 ð2WÞ
¼ 0:0047
5. Uncertainty in density measurement (ra)
ra ¼
Pa R To "
dra ¼
dra 1 dPa dPa
2
þ
dra 1 dTo dTo
2 #0:5
R. Kumar et al. / Renewable Energy 101 (2017) 856e872
2 dra ¼ 4
1 R To
þ
Pa R To2
!
ra RTo
2
"
dV
dPa
Pa
871
¼ ð0:016241Þ2 þ ð0:00394Þ2 þ
V
!2 30:5 ra RTo dTo 5 Pa
2 #0:5 0:1 2 0:1 þ 300 30
¼ 0:017044 8. Uncertainty in useful heat gain (Qu)
dra ¼ ra
"
dPa
2
dTo
þ
Pa
2 #0:5
Qu ¼ ma cp ðT0 Ti Þ ¼ ma cp DT
To
dQu
Taking Pa ¼ 97500 Pa
dra ¼ ra
"
Qu 2 #0:5
2 0:1 0:1 þ 97500 25:33
6. Uncertainty in mass flow rate measurement (ma)
ma ¼ Cdo Ao
2ra ðDpÞ0 1b
dma ¼
Qu
m
" ¼
dCdo
2
2
þ
Cdo
#0:5
2
Cdo
4
dma þ dA dAo o
2
dma þ dr dra a
2
dAo
2
þ
Ao
dra ra
2
þ
dðDpÞ0 ðDpÞ0
dAp 2 d Qu 2 ¼4 þ þ ht Qu Ap
2 #0:5
2
0:2 þ ð0:0047Þ þ ð0:00394Þ þ 185 2
2
V
ma
¼
dma ma
2
dDTf DTf
þ
0:1 5:33
2 #0:5
!2 30:5 5
2
2
¼ 0:0252017
2 #0:5
dra þ ra
2
þ
dW W
2
þ
dH H
2 #0:5
ht Dhd Ka "
dNurs
¼
dNurs Nurs
dDhd
2 þ
Dhd
dht
2
ht
þ
dKa
2 #0:5
Ka
" ¼ ð0:0030304246Þ2 þ ð0:0252017Þ2 #0:5 0:00001 2 þ 0:0394161 0:02529
11. Uncertainty in Reynolds Number (Re)
Re ¼
ra W H "
2
"
Nurs
7. Uncertainty in measurement of air velocity in channel (V)
dV
0:1 1006:141
2
¼ 0:016241
V¼
2 #0:5
#0:5 0:1 2 ¼ ð0:02481Þ þ ð0:00034359Þ þ 22:66 ht
ðDpÞ0 ¼ DðHÞo sin30 sin90 ¼ 185mm
¼
dDT DT
10. Uncertainty in Nusselt number (Nurs)
m
dht
Nurs ¼
1:5 100
þ
Q Qu ¼ u A DTf p Ap Tp Tf
dht
The uncertainty in (Dp)0, for U-tube manometer is 0.2 mm.
dma
"
1b
¼ 1:5%
"
2
cp
¼ ð0:016241Þ2 þ
ht ¼
The data of
dCdo
ma
dcp
9. Uncertainty in heat transfer coefficient (ht)
2 #0:5 dma þ dðDpÞ0 dðDpÞ0
dma
þ
4
"
dma dC dCdo do
2
#0:5
0:5 ma ¼ Cdo Ao r0:5 a ðDpÞ0
"
dma
Uncertainty in specific heat is. 0.1 So, equation becomes
¼ 3:94 103
dQu "
" ¼
dRe Re
V$Dhd
n
¼
" ¼
dDhd Dhd
ra VDhd m 2
þ
dV V
2
dra þ ra
2
dm þ m
2 #0:5
¼ 0:02481
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R. Kumar et al. / Renewable Energy 101 (2017) 856e872
2
dRe Re
¼ 4ð0:0030304246Þ2 þ ð0:017044Þ2 þ ð0:00394Þ2 0:001 105 þ 1:87 105
dRe Re
[5]
!2 30:5 5
[6]
[7]
¼ 0:01776
[8] [9]
12. Uncertainty in friction factor (frs)
frs ¼
2 Dp
d
[10]
Dhd
[11]
4ra Lt V 2
2 2 2 2 dfrs 4 dDhd 2 dV dLt dra ¼ þ þ þ þ V frs Dhd Lt ra
dfrs frs
"
¼ ð0:0030304246Þ2 þ ð0:17044Þ2 þ þð0:00394Þ2 þ
dfrs frs
0:1 56:2
!2 30:5 d Dp d 5
Dp
[12]
[13]
d
2
0:1 1200
2 #0:5
[14] [15]
[16]
[17] [18]
¼ 0:01784 [19]
13. Uncertainty in thermo-hydraulic performance parameter (hp)
. hp ¼ ðNurs =Nuss Þ ðfrs =fss Þ0:33
dhp ¼ hp
"
dNurs Nurs
2
þ
dfrs
2 #0:5
[20]
[21]
[22]
frs
i0:5 dhp h ¼ ð0:0394161Þ2 þ ð0:01784Þ2 hp
[23] [24]
[25]
dhp ¼ 0:043265 ¼ 4:3265% hp
[26]
[27]
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