Experimental study and correlation development for Nusselt number and friction factor for discretized broken V-pattern baffle solar air channel

Accepted Manuscript Experimental study and correlation development for Nusselt number and friction factor for discretized broken V-pattern baffle solar air channel Raj Kumar, Ranchan Chauhan, Muneesh Sethi, Anil Kumar PII: DOI: Reference:

S0894-1777(16)30274-6 http://dx.doi.org/10.1016/j.expthermflusci.2016.10.002 ETF 8888

To appear in:

Experimental Thermal and Fluid Science

Received Date: Revised Date: Accepted Date:

30 June 2016 22 September 2016 2 October 2016

Please cite this article as: R. Kumar, R. Chauhan, M. Sethi, A. Kumar, Experimental study and correlation development for Nusselt number and friction factor for discretized broken V-pattern baffle solar air channel, Experimental Thermal and Fluid Science (2016), doi: http://dx.doi.org/10.1016/j.expthermflusci.2016.10.002

This is a PDF file of an unedited manuscript that has been accepted for publication. As a service to our customers we are providing this early version of the manuscript. The manuscript will undergo copyediting, typesetting, and review of the resulting proof before it is published in its final form. Please note that during the production process errors may be discovered which could affect the content, and all legal disclaimers that apply to the journal pertain.

Experimental study and correlation development for Nusselt number and friction factor for discretized broken V-pattern baffle solar air channel Raj Kumar, Ranchan Chauhan, Muneesh Sethi, Anil Kumar School of Mechanical and Civil Engineering, Shoolini University, Solan, India Abstract The present study examines the augmentation in heat transfer and friction in a flow through solar air channel with discretized broken V-pattern baffle. Experiments have been carried out for system parameter such as a width to height ratio, distance,

of 10, the relative baffle gap

range of 0.26-0.83, relative baffle gap width,

relative baffle height, and angle of attack,

range of 0.5-1.5,

range of 0.25-0.80, relative baffle pitch,

range of 0.5-2.5,

range of 30o-70o. The effect of discretized broken V-pattern baffle has

been investigated for the range of Reynolds number from 3000 to 21000. The maximum enhancement is observed at a

of 0.67,

of 1.0,

of 0.50,

of 1.5, and

of 60°. Discretized broken V-pattern baffle has better thermal hydraulic performance as compared to other baffle shapes investigated by various investigators under similar operating conditions. By the use of the experimental data, correlations for heat transfer and friction characteristics have been developed for solar air channel as function of system parameters of discretized broken V-pattern baffle. Keywords: Energy, turbulence, heat transfer, baffle width, baffle distance, solar air channel Nomenclature Surface area of heated plate, Area of orifice, Area of flow, Coefficient of discharge Specific heat of air, Gap or broken distance, Hydraulic diameter of channel, Friction factor Friction factor of roughened baffle

 Corresponding Author. Anil Kumar, School of Mechanical and Civil Engineering, Shoolini University-India, E-mail addresses: [email protected] (A. Kumar)

1

Friction factor without baffle channel Gap or discrete width, Convective heat transfer coefficient, Height of channel, Height of baffle, Relative gap width Relative baffle height Thermal Conductivity of air, Length of test section, Length of V-type baffle, Relative baffle gap distance Mass stream rate of air, Nusselt number Nusselt number of baffle channel Nusselt number of channel without baffle Pitch of baffle channel, Relative pitch ratio Pressure drop across test section, Pressure drop across orifice plate, Useful heat gain, Reynolds number Mean bulk air temperature, Inlet temperature of air, Outlet temperature of air, Plate temperature of air, Mean air velocity, Velocity of air, Width of channel, Width to height ratio of channel WVGs

Winglet-type vortex generators

CTWP

Curved trapezoidal winglet pair

O-DWT

Oblique delta-winglet twisted tape

S-DWT

Straight delta-winglet twisted tape 2

SAH

Solar air heater

SAC

Solar air channel

Greek symbols Angle of attack, Ratio of orifice meter to pipe diameter Density of air, Kinematic viscosity of air, Thermo hydraulic performance

1. Introduction The sun is the source of life on the earth, but at the same time it is a “free” source of energy for various systems using this resource to power a process. The furthermost benefit of solar energy as compared with other forms of energy is that it is clean and can be abounding without any environmental pollution [1]. Solar energy can be used for various purposes. We quote here as examples three applications: refrigeration, power generation and chemical reactions or metallurgical processes. Solar air channel (SAC) is one of the basic equipment through which solar energy is converted into thermal energy. The main applications of SAC are space heating, seasoning of timber, curing of industrial products and these can also be effectively used for curing/drying of concrete/clay building components [2-3]. SAC is simple in design and requires low maintenance. However, the value of the heat transfer rate between the heated plate and air is low and this result in a lower thermal efficiency [4]. The thermal efficiency of SAC is low because of low value of convective heat transfer coefficient between the flowing air and heated plate (heat transferring surface) due to the formation of thin laminar viscous sub-layer on its heated plate [5]. The efficiency of SAC can be improved by modifying the boundary layer developed on the heated surface. One of the well-known methods of modifying the boundary layer is to break the laminar viscous sub-layer formed on the heat transfer surface by creating artificial roughness in the form of blocks, winglet, dimples and baffle [6-9].

A variety of tabulators have been used to accelerate the heat transfer rate, including ribs, baffles, blocks, winglets and dimples depending upon the requirement needed [7-8]. Large height tabulators, such as baffles, are generally used for high heat transfer rates due to the turbulence in the stream field. However, baffles are also responsible for high pressure drops.

3

Different shaped baffles are typically including used V-shaped, perforated that can be attached and bent away from the heated wall to produce turbulence in the stream field that results in an improved heat transfer [9]. Furthermore, these baffles are modified to improve the thermo-hydraulic performance. Detailed descriptions of numerous experimental and numerical studies on baffles with different shapes, size, and orientations, are discussed herein.

Rajaseenivasan et al. [10] studied the effect of circular and V-shape inserts on of solar air channel with

varied from 6000 to 12,000. It was found that

and turbulators in heated plate and attained utmost data at

and rises with the

of 11,615. Bovand et al.

[11] numerically investigated the effect of porous material on the

and

in a solar air

channel. Handoyo et al. [12] numerically studied the effect of obstacles spacing inserted in a V-corrugated channel of a solar air heater on

and

. The obstacles are delta-shaped

and mounted on bottom plate of the channel. It was observed that

improved from 27.2

when without obstacle used to 94.2 when obstacles used with S/ H = 0.5. The improved 3.46 times. The = 0.5. The

will rises from 0.0316 at without obstacle to 0.628 at ratio S/H

raised 19.9 times.

Priyam and Chand [13] analytically carried out the performance analysis of finned absorber solar air channel. They examined the effect of operating parameters such as parameter viz. fin spacing on

and

and system

. The greatest improvement in thermo hydraulic

efficiency has been found with wavy fin solar air channel with least fin spacing as 35.83% and 17.56% with the smallest

of 0.0138 kg/s and greatest

of 0.0834 kg/s as compared

to without fins SAH. Shin and Kwak [14] measured experimentally the heat transfer coefficients by an improved hue detection based liquid crystal technique in the stream passage with the blockages wall at

ranging from 20,000 to 40,000. They studied five

shapes of holes in order to examine the effect of hole shape on

and

in a rectangular

channel with repeated impingement jet holes. Saim et al. [15] numerically studied the turbulent stream in a rectangular channel with diamond shaped baffles placed on the top and bottom walls. The finite volume method is used to describe the thermo hydraulic behaviour. The highest data of the normalized

was attained when the baffles were tilted five

degrees from the vertical.

4

Skullong et al. [16] experimental investigated the thermal performance of a solar air heater channel with combined wavy-groove and delta-wing vortex generator (WVG) placed on the heated plate having a uniform wall heat-flux. The

based on the

of the channel varied

from 4800 - 23,000. The effect of the combined groove and WVG on the

and

in the

channel was analyzed. Gawande et al., [17] used mathematical modelling and simulations techniques to predict the thermal performance of solar air channel roughened with 20° angled rib. It was observed that the greatest 0.042 and

and

are obtained at

of 7.143,

of

of 15,000. Bayrak et al. [18] 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

of 0.025 kg/s. The lowermost data are obtained for the SAC with non-baffle

collectors with

of 0.016 kg/s.

Sripattanapipat and Promvonge [19] carried out numerical investigation of laminar periodic stream and

in a two dimensional horizontal channel with isothermal walls and staggered

diamond shaped baffles. Zhou and Ye [20] experimentally investigated the performance of a pair of new vortex generators curved trapezoidal winglet (CTW). They compared these CTW with traditional vortex generators rectangular winglet, trapezoidal winglet and delta winglet using dimensionless factors. Nuntadusit et al. [21] experimentally investigated a wind channel with distinct types of cut baffles for

improvement.

Sriromreun et al. [22] reported experimental and numerical predictions of the

and

for a SAC with Z-shaped baffles. Their experiments were performed by controlling the air stream rate to attain

data in the range of 4,400 to 20,400. The Z-baffles inclined to 45˚

relative to the main stream direction are characterized at three

= 0.1, 0.2 and 0.3 and

=1.5, 2 and 3. They found considerable effect of presence of the Z-baffle on the and

over smooth channel. Khanoknaiyakarn et al. [23] carried out an experiment to study and

by using V-pattern baffles on a broad heated wall of a large

effects of the baffles on

and

channel. The

were investigated. Bekele et al. [24] experimentally

investigated the study of the effect of delta-shaped obstacles mounted on the heated surface of an air heater channel with an

of 6:1. In solar air heater channel has a delta-shaped

obstacle with longitudinal pitch (

varied from 3/2 to 11/2, and relative obstacle height

(

) varied from 0.25 to 0.75. Outcome shows the obstacle-mounted channel improve the 5

by 3.6 times over the smooth channel under similar geometrical and stream conditions at Re = 7276.82,

= 3/2, and

= 0.75.

Chompookham et al. [25] investigated a channel to study the effect of combined wedge ribs and winglet-type vortex generators (WVGs) on

and

behaviors for a turbulent air

stream. Both rib types arranged inside the opposite channel walls are in-line and staggered arrays. Two pairs of the WVGs with an angle of 60˚ were attached on the test channel entrance to generate the longitudinal vortex stream through the tested section. Akpinar and Kocyigit [26] experimentally investigate the performance analysis of four types 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. Promvonge and Kwankaomeng [27] carried out numerical investigation to examine laminar stream and heat transfer characteristics in a three dimensional isothermal wall square channel with 45˚ staggered angled baffles with

in the range of 100–1200.

Tzeng et al. [28] carried out experimental determination of local and average heat transfer characteristics in asymmetrically heated sintered porous channels with metallic baffles. Eiamsa-ard et al. [29]. Influence of the oblique delta-winglet twisted tape (O-DWT) and straight delta-winglet twisted tape (S-DWT) arrangements are studied. Bopche and Tandale [30] carried out experimental investigation to study

and

by using artificial roughness

by using U shaped turbulators on the heated surface of an air heater channel over the range of parameters

: 3800–18,000,

= 0.0186–0.03986,

= 6.67–57.14. It was observed

that Roughness pitch strongly affects the stream pattern and hence the performance of the channel. Nie et al. [31] conducted numerical simulations of three dimensional laminar forced convection stream adjacent to backward facing step in rectangular channel to examine effects of the baffle on stream and

distributions.

Karwa and Maheshwari [32] carried out experimental study of

and

in a rectangular

section channel with transverse fully perforated baffles (open area ratio of 46.8%) and half perforated baffles (open area ratio of 26%) affixed to one of the wall. Ary et al. [33] numerically and experimentally studied the effect of a number of inclined perforated baffles on the

and

in the rectangular channel with distinct types of baffles.

6

Dutta and Hossain [34] experimentally investigated the local

and

in a rectangular

channel with inclined solid and perforated baffles. The baffle is mounted to the top heated surface, while the position, orientation, and the form of the other baffle is varied to know the optimum configuration for improved

.

Hu et al. [35] examine the simple-structure MV-SAC with internal baffles. The experimentation outcome shows that the introduction of baffles strengthen the convective heat transfer process and decrease the radiation heat loss, which contributed towards efficiency enhancement. Saini and Saini [36] studied the effect of arc shaped ribs on and

of rectangular channel of solar air heater. Maximum enhancement in

and

as

compared to smooth channel was observed to be 3.6 and 1.75 times respectively. The studies on previous experimental investigations on various baffle shapes solar air channel have been shown in Table 1.

Table 1 Previous experimental investigations on various baffle shapes solar air channel

S.N. Investigators 1.

shape

Principle findings

V-shaped

Enhancement of

baffle

and

was reported to be of order

[4]

3.45

and

respectively

3.89 over

times smooth

channel.

2.

Fin and baffle

Enhancement of

[5]

and

was reported to be of order 2.45

and

respectively channel.

7

3.24 over

times smooth

3.

Transverse

Enhancement of

dimpled

and

was reported to be of order

baffle [8]

2.49

and

2.98

respectively

times

over

smooth

channel. 4.

Staggered

The

diamond

transfer for 5° diamond shaped

shaped baffle

baffle is around 2.14 times

of

heat

higher than that of smooth

[19]

5.

improvement

baffle.

Rectangular

The baffle with rectangular

cut

zigzag-cut gives the superior

baffle

[21]

thermal 1.84

performance times

over

about smooth

channel. 6.

Z-shaped

They observed that significant

baffle [22]

improvement in

and

with the presence of Z- shaped baffle as compared to without baffle channel. of

and

Enhancement was reported to

be of order 3.45 and 4.55 times respectively

over

smooth

channel. 7.

U- shaped

The maximum improvement in

baffle [30]

and

are of the order of

2.38 and 2.50 respectively.

8

8.

Inclined

Inclined

perforated

baffle

perforated

provides 1.2 to 1.7 times the

baffle [33]

heat transfer improvement with 1.8 to 2.2 times the pressure drop

penalty

over

smooth

channel. 9.

Arc shaped

Enhancement of

baffle [36]

was reported to be of order 2.98

and

and

3.15

respectively

times

over

smooth

channel. 10.

Discretized broken

As per according literature V-

review it was found that, V-

pattern baffle

pattern

baffle

[Proposed

thermal hydraulic performance

shape]

than other rib shapes and configurations.

have

better

It

is

hypothesized that discrete Vpattern baffle will augment heat

transfer

continuous

compared

V-pattern

to

baffle

(without discrete) solar air channel.

It has been reveals from literature that an experimental investigation is required to be carried out on

and

of SAC having absorber plate roughened by formation of discretized

broken V-pattern baffle, because no such type research has been reported in literature on such kind of turbulence promoters. In the current research, an experimental investigation has been reported for analysing

and

of SAC having discretized broken V-pattern baffle

mounted on the heated plate. In order to predict performance of the SAC having such type of roughened heated plate,

and

correlations as a function of system parameters has

been developed by using experimental data. 9

2. Experimental details To study the outcome of discretized broken V-pattern baffle turbulent promoter on the and

of air stream an experimental setup was intended and made-up accordance with

guidelines suggested in ASHRAE standard 93-77 [42]. The air channel is 2000 with a stream cross section of

is made-up from ply panel of 20 mm

thickness. The channel is comprises of inlet section 500 length and an exit section of 300

extended

long, a test section of 1200

length. The complete channel is insulated with 50

thick polystyrene insulation having thermal conductivity of 0.037

to minimise heat

loss to the environment. The data has also been collected for conventional solar air heater channel under similar system and operating conditions for the validation purpose and so as to compare the same with discretized broken V-pattern baffle solar air channel. The schematic diagram of experimental test rig is as shown in Fig. 1.

Fig. 1. Schematic of experimental setup.

The air blower is used to propel the atmospheric air in the air heater channel which passes through the discretized broken V-pattern baffle provided on the plate and then exits at the other end. The air flow in the channel was controlled by means of control valves provided at 10

inlet and outlet of the suction blower. 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 pressure taps were provided at entrance and exit of the test section to measure pressure difference across the test section by micro-manometer. A Galvanised Iron (GI) sheet of 18 SWG size black painted used as a heat transferring surface over which heater was placed to provide constant flux of 1000 W/m2. Discredited broken V-pattern baffle were attached on the base of heated wall by means of epoxy resin. The heater was connected to power supply through a variable transformer (variac) and ammeter to control the power supply and maintain uniform heat flux throughout the experimentation. The temperature of the absorber surface, inlet air and exit air was measured by calibrated copper-constantan thermocouples. The thermocouples were attached to the temperature scanner to display the temperature of the flowing fluid and absorber plate. The rectangular channel is the major part of the experimental test set up. The entry and exit lengths are chosen as per ASHRAE Standards [45] which recommends entry length ≥ 5√WH and exit length ≥ 2.5√WH for the turbulent stream regime. The aspect ratio of the channel is 10.0. Fig. 2 shows the cross-sectional view of the rectangular channel.

Fig. 2. Cross-sectional view of the channel A uniform heat flux is provided by an electric heater, fabricated by combining loops of nichrome wire in series and parallel combination of size

11

located on

top wall of the test section with other sides insulated. A variable transformer is connected to maintain a specific voltage and an ammeter to measure the current flowing through the circuit in order to maintain uniform heat flux of 1000 W/m2. The asbestos sheet is converted with strip of Mica to keep the uniform distance among the wires and prevent back heating. The temperature measurement of air at inlet, outlet and that of absorber plate was carried out by calibrated copper–constantan (T-type) thermocouples. Such thermocouples are usually recommended for temperature measurement in the range of 0-400°C (Benedict, [43]). Total numbers of 26 thermocouples have been mounted on the upper surface of the absorber plate from which two are used for measurement of ambient air temperature, three are used for heated air temperature and the rest of twenty one are used for absorber plate temperature measurement. The thermocouples output are measured by a temperature indicator which indicates the output of the thermocouples in degree centigrade ( ). To determine the accuracy of temperature measurement, thermocouples have been calibrated under laboratory conditions against a dry block temperature calibrated instant. Fig. 3 shows the position of the thermocouples in the rectangular channel.

Fig. 3. Thermocouples position in the rectangular channel The air flow through the rectangular channel was measured by a flange type orifice meter which was fitted in the pipe of 80 mm diameter connected with the plenum and carrying air to the blower. The ratio of orifice diameter to pipe diameter (β) is 0.50. The orifice had a straight edge of 1.5 mm at orifice diameter which was then chamfered by 30˚ angle at the downstream end. U-tube manometer tapes were connected at 60 mm at upstream 12

side and 30 mm at downstream side of the orifice. The length of straight pipe before and after the orifice plate are kept as 720 mm and 500 mm respectively to ensure fully developed flow prior to the orifice. The pressure drop through the test section of the air channel was obtained by a micro-monometer having a least count of 0.01

.

3. Range of parameters SAC has a length equal to 2000 and width equal to 300

while the height of the channel is set equal to 30

. The hydraulic diameter of the channel is equal to 54.54

The baffle parameters are determined by baffle height distance in the baffle length attack

width of the discrete region

, broken

and the angle of

. These parameters have been expressed in the form of dimensionless roughness

parameters, viz., relative baffle height ( distance (

baffle pitch

.

), relative gap width (

), relative baffle pitch

relative gap

). The shape of the discretized broken V- pattern

baffle is shown in Fig. 4. Fig. 5 shows the variation of gap distance in discretized broken Vpattern baffle arrangement. The photographic view of V-pattern baffle roughened plate is shown in Fig. 6. The values of system and operating parameters of this investigation are listed in Table 2.

Table 2 Range of parameters

S.No.

Parameters

Range

1.

3000 to 21000

2.

0.26-0.83

3.

0.5-1.5

4.

0.5-2.5

5.

0.25-0.80

6.

30°-70°

13

Fig. 4. Discretized broken V-pattern baffle.

Fig. 5. Variation of gap distance in discretized broken V-pattern baffle arrangement

14

Fig. 6. Photographic view of discretized broken V-pattern baffle roughened plate.

4. Raw data collection The data collected have been used to compute

,

and

. Relevant expressions for the

computation of the above parameters and some intermediate parameters have been given below. The mean temperature of the plate

The mean bulk air temperature

is the average of all temperatures of the heated plate:

is a simple arithmetic mean of the measured data at the

inlet and the exit temperature of air streaming through the test section:

where The mass stream rate

, of air has been calculated from the pressure drop measurement

through the calibrated orifice meter by using the following formula:

15

The velocity of air

is calculated from the knowledge of

Equivalent hydraulic diameter (

The Reynolds number (

The friction factor (

and the stream as

) is determined by

) of air stream in the channel is intended from

) is determined by using the Darcy equation as

Heat Transfer Coefficient ( ). The heat transfer rate

from absorber to the air is given

by:

The heat transfer coefficient ( ) for the heated test section has been calculated as:

The

can be used to determine the Nusselt number (

16

), which is defined as:

5. Validation of experimental data The value of

and

calculated through experimental outcomes for a smooth channel

have been compared with the outcomes obtained from the Dittus-Boelter equation [Eq.(11)] for the

, and modified Blasius equation [Eq.(12)] for the

.

The

for a smooth channel is given by the Dittus-Boelter equation as:

The

for a smooth channel is given by the modified Blasius equation as:

The comparison of the experimental and estimated outcomes of the

and

as a function of

is shown in Fig. 7. (A) and (B) respectively.

6. Results and discussion In this experimental investigation, the effect of discretized broken V-pattern baffle shape parameters such as;

,

,

,

, and

on heat transfer and friction factor

characteristics has been studied extensively and discussed below.

6.1 Heat transfer and friction factor Fig. 8(A) shows the effect of

on

for discretized broken V-pattern baffled channel. It

has been observed that discretized broken V-pattern baffle yields higher

as compared to

that of smooth channel. Nusselt number increases with increase in

for all cases as

expected. Discretized broken V-pattern baffle shape induces strong secondary streams along the limbs and higher level of mixing and turbulence when jets issuing from broken region of the baffle reattach and mix with the main stream thereby resulting in an increased

17

.

Fig. 7. (A) Comparison of experimental and predicted data of experimental and predicted data of

In order to compare the improvement of the

. (B) Comparison of .

achieved as an outcome of

providing a broken in the V-pattern baffle arrangement, the data of the, the, variant of

of 1.0, and different data of = 1.5,

= 0.50 and

for fixed data of

is given in Fig. 8(B). Fig. 8(B) shows the = 60˚, with 18

at different data of

for a

fixed

of 1.0. It can be seen that the

to 0.67, attains a extreme at a,

increases with increase in

of 0.67 and thereafter it reduces with increase in the

. Fig. 8(B) shows the data of the

for a 60˚ discretized

as a function of

broken V-pattern baffle air channel at different selected , the

from 0.58

is the highest for the

. It can be observed that at any = 0.67 for every data of

Producing broken near the leading edge (say at

.

= 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

. A rise in the data of

say at

= 0.55 signifies shifting of the broken toward trailing edge. This raises the strength of the secondary stream and presented the data of the, and

increases with increase in the for fixed data of the,

= 60˚ and distinct data of

up to 0.67. Fig. 8(C)

of 0.67,

. This figure shows the

of 1.0 and smallest for the,

. The data of

is

of 1.5.

Fig. 8(C) shows the data of the

for a 60˚ discretized

as a function of

broken V-pattern baffle air channel at distinct selected , the

= 0.50

rises with rise in the

up to about 1.0, beyond which it reduces with rise in the greatest for

= 1.5,

is the highest for the

. It can be observed that at any

= 1.0 for all data of

. It appears that as the

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

due to

this stream may not be raised significantly whereas with the reduction of this

to data

lower than 1.0, there may be very little space for stream of the fluid through it which outcomes in low turbulent and hence reduce the improvement of achieve the improvement of

. Thus in order to

, 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(C).

Fig. 8(D) shows the variation of roughness parameters were kept observed that the

with

= 0.67,

rises with rise in

for distinct data of =1.0 as

=1.5 and

for all data of

of 0.50. Fig. 8(D) shows the data of the

19

= 60°. It is

due to increased protrusion

into stream causing more turbulence, thereby, resulting in rise in observed at

. The other

. The highest

as a function of

is for a

60˚ discredited broken V-pattern baffle air channel at different selected observed that at any

, the

is the highest for the

Fig. 8(E) shows the variation of

= 60°. For all

= 0.50 for all data of

as a function of

and fixed data of other channel parameters as, , the greatest data of

=1.0

= 0.50 and

has been observed corresponding to the has been found to occur at the

2.5 for the range of investigations. Fig. 8(E) shows the data of the

is the highest for the

Fig. 8(F) shows the variation of channel parameters as

= 0.67 ,

has been plotted as a function of channel parameters.

with

selected

and fixed data of other = 1.5. In this plot, and fixed data of other =

. Fig. 8(F) shows the data of the

for discredited broken V-pattern baffle air channel at distinct

. It can be observed that at any

each value of

.

, attains a highest data corresponding to

60° and then decreases with further rises in the data of as a function of

= 0.50 and

for some selected data of

rises with rise in

. It can be

= 1.5 for all data of

for distinct data of =1.0,

data of

as a function of

for a 60˚ discredited broken V-pattern baffle air channel at distinct selected , the

.

for distinct data of

= 0.67,

data of 1.5, whereas the smallest data of

observed that at any

. It can be

, the

is the highest for the

= 60° for

.

V-pattern baffle in a solar air channel gives rise to secondary flow jets along the baffle length, which allows the working fluid to travel from leading edge to trailing edge of the baffle. The introduction of a discrete in the V-pattern baffle allows release of secondary flow and main flow through the discrete as shown in Fig.9. The main flow is a developed flow with thicker boundary layer, and due to the presence of viscous sub layers, it leads to a low amount of heat transfer. In fact, the baffles are introduced to break this retarded flow and let it reattach again with the surface to enhance the heat transfer. However, in case of discrete in the V-pattern baffle, the secondary flow along the baffle join the main flow to accelerate it, which energizes the retarded boundary layer flow along the surface. This increases the heat transfer through the discrete width area behind the baffle.

20

Fig. 8. (A) Variation of Variation of (E) Variation of

with

with

(B) Variation of

at distinct

with

with

(D) Variation of

at distinct

(F) Variation of

21

at distinct with with

(C)

at distinct at distinct

.

Fig. 9. Fluid flow pattern in discrete V-pattern baffles.

Fig. 10 (A) shows the effect of ,

on

,

for discretized broken V-pattern baffle (

,

, and

Friction factor is observed to decreases with rise

) and smooth wall channel.

for all cases as expected. It can be seen

that discretized broken V-pattern baffle yield higher pattern baffle. The variation of baffle parameters as

with

=1.0

expected. The the

for distinct data of = 0.50

10(B). It is seen that the data of

and fixed data of other

=1.5 and

= 60° has been shown in Fig.

reduces with increasing

and towards a kept data as

rises with rise in the,

of up to 0.67 and reduces with further rise in

. The plot shows that the highest and lowest data of

pattern baffle air channel occur for the, of

to that of without discretized V-

for discretized broken V-

of 0.67 and 0.26 respectively. The lowest data

for broken on the upstream side is due to weakened strength of secondary stream. Fig.

10(B) shows the data of the

baffle air channel at different selected

It can be observed that at any

highest for the

= 0.67 for all data of

The variation of

with

parameters as

= 0.67,

, the

for different data of = 0.50,

rises as

, the

is the

. and fixed data of other baffle

=1.5 and

10(C). It has been observed that for all data of shows that at all

for a 60˚ discredited broken V-pattern

as a function of

,

= 60° has been shown in Fig. reduce with rise in

. Fig. 10(C)

is raised from 0.5 to 1.0 and decreases as

22

is raised further. Fig. 10(C) shows the data of the

discredited broken V-pattern baffle air channel at distinct selected at

, the

is the highest for the

for a 60˚

as a function of

. It can be observed that

= 1.0 for all data of

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 hence the

of 1.5 as compared to

of 0.5, 0.75, 1.0 and 1.25

is lower than in other cases. These outcomes broadly agree with previous

studies on broken baffle channels. The variation of,

with

for distinct data of

other roughness parameters were kept as

have been plotted in Fig. 10(D). The

= 0.67,

=1.0

It has been observed from this plot that for a given Fig. 10(D) clearly shows that correspond to

data

rises with rise in

=1.5, and

= 60°.

reduces with rise in

.

and the maximum data of

data of 0.50. Fig 10(D) shows the data of the

as a function of

for a 60˚ discretized broken V-pattern baffle air channel at distinct selected

. It can be

observed that at

.

, the

is the highest for the

= 0.80 for all data of

It is due to the fact that with the increase in

data, baffles protrude more and

more into the core stream resulting in rise in turbulence level as well as the

. These

outcomes broadly agree with previous studies on baffle roughened channel. Fig. 10(E) shows the variation of data of other baffle parameters as

with

= 0.67,

for different data of =1.0

been observed from Fig. 10(E) that for all data of

,

reduces with rise in

data of 2.5 and 1.5 yield the lowest and highest data of data of the

= 0.50 and

and fixed = 60°. It has . For,

respectively. Fig. 10(E) shows the

for a 60˚ discretized broken V-pattern baffle air channel

as a function of

at different selected

. It can be observed that at

, the

is the highest for the

=1.5 for all data of

. These outcomes broadly agree with previous studies on roughened

channels. The variation of parameters as

= 0.67,

with

for distinct data of =1.0,

= 1.5 and

Fig. 10(F). It has been observed that for all the data of smallest and highest data of

and fixed data of other baffle = 0.50 has been shown in ,

reduces with rise in

have been obtained corresponding to

respectively. Fig. 10(F) shows the data of the 23

as a function of

. The

data of 30° and 60° for discretized broken

V-pattern baffle air channel at distinct selected the highest for the

= 60˚ for all data of

Fig. 10. (A) Variation of Variation of

with

(E) Variation of

with

, the

is

.

(B) Variation of

at different

with

. It can be observed that at

with

(D) Variation of

at different

(F) Variation of

24

at different with with

(C)

at different at different

.

6.2 Thermal hydraulic performance Study of the

and

characteristics shows that an improvement in

in

general accompanied with friction power penalty due to a corresponding increase in the

.

Therefore it is essential to decide the geometry that will outcomes in highest improvement in with minimum

penalty. In order to achieve this purpose of simultaneous

consideration of thermo hydraulic performance, Lewis [44] proposed a thermo hydraulic parameter known as efficiency parameter ‘

’ which evaluates the improvement in

of a

roughened air channel compared to that of the smooth channel for the same pumping power requirement and is defined as :

A heat transfer improvement device having a data of thermo hydraulic parameter

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 the feasible combinations. Figs. 11(A-E) illustrates the effect of baffles parameters on thermo hydraulic performance parameter

, as a function of

. The highest absolute data of

has been observed to be 3.14 corresponding to 0.67,

data of 1.0,

data of 0.50,

discretized V-pattern baffle solar air channel.

25

data of 1.5, and

data of data of 60o for

Fig. 11. (A) Variation of

with

at different

different

(C) Variation of

with

at different

at different

(E) Variation of

with

at different

(B) Variation of (D) Variation of

with

at

with

.

The values of thermal hydraulic performance parameter determined for the shape of discretized broken V-pattern baffle have been compared with the values determined for Vshaped baffle [4], staggered diamond baffle [19], rectangular cut baffle [21], Z-shaped baffle [22], U-shaped baffle [30], inclined perforated baffle [33], broken arc rib [37], V-ribs with 26

symmetrical gap [38], continuous multiple V-ribs [39], multi V-shaped with gap rib [40], and discrete multi V-rib with staggered rib [41]. It can be seen that from the Table 3 discretized broken V-pattern baffle shape results in highest thermal hydraulic performance among all other baffle shapes investigated.

Table 3 Thermal hydraulic performance parameters compared with previous investigations

Investigators

Configuration

Maximum value of thermal hydraulic performance

Khanoknaiyakarn et al. [4]

V-shaped baffle

3.03

Sripattanapipat and Promvonge [19]

Staggered diamond baffle

1.86

Nuntadusit et al. [21]

Rectangular cut baffle

2.68

Sriromreun [22]

Z-shaped baffle

2.98

Bopche andTandale [30]

U-shaped baffle

1.97

Ary et al.[33]

Inclined perforated baffle

2.79

Hans et al. [37]

broken arc rib

2.08

Maithani and Saini [38]

V-ribs with symmetrical gap

2.38

Hans et al. [39]

Continuous multiple v-ribs

3.37

Kumar et al.[40]

Multi v-shaped with gap rib

3.46

Kumar and Kim [41]

Discrete multi V-rib with

3.53

staggered rib Present study

Discretized broken V-pattern baffle

27

3.14

7. Correlations for heat transfer As discussed earlier that the parameters, namely

and friction factor and

are strong functions of flow and roughness

and the roughness dimensions of

. The functional relationships for

, ,

and

,

,

,

,

and

can therefore be written as

,

,

(14)

,

Experimental data collected, processed and discussed in detail in terms of variation of and

as a function of system and operating parameters in the previous section has been

used to develop these relationships in the form of correlations. In order to determine the functional relationships between

and

, the well known power law relationship

between these parameters for forced convective heat transfer has been utilized, consequently;

Above functional relationship between (

and

can be expressed as:

)=n

where

.

It can be concluded that the parameter ‘ ,

,

,

’ is a function of roughness parameters (

). A composite plot of

vs

,

has been shown in Fig. 12

which resulted in following relationship:

The coefficient and

in Eq. (18) is influenced by parameters i.e.,

,

,

,

,

.

A functional relationship between against the data of

and

has been established by plotting data of

for fixed value of other parameters. It has been

observed that the data yields straight lines with nearly same slope while the data of intercept of each line is different. This also shows that a linear relationship between parameter and

. A composite plot of parameter

in Fig. 13 resulted in following relationship:

or

28

vs

as shown

The coefficient

in Eq. (20) is a function of

determine a functional between the data of ln(

,

and

,

data of ln(

) for different data of

and

. In order to

) have been plotted against

and fixed data of other roughness parameters.

Similarly plots of ln

and ln(

) had been drawn

for different sets of roughness geometry parameters. It was observed that a polynomial function relationship of the form given below exists between

A composite plot of parameters

vs ln(

and ln(

).

) as shown in Fig. 14 resulted in the

following relationship:

or

The data of

and

are selected from the regression analysis done on SIGMA PLOT

software. From the regression analysis curve between

vs ln(

upon the geometry of the regression analysis curve between

). Since it depends vs ln(

) i.e from the

equation

of

the curve Fig. 14 and also

is a function of

hence

depends upon the geometry of the

curve.

where

is anti

Similar procedure was adopted to determine the relationships between parameters namely relative baffle gap distance ( angle of attack (

by plotting respectively

Further composite plots respectively

), relative baffle pitch ( vs ln(

,

and other

and

),

, and

and .

as shown in Figs. 15-17 were

prepared. Corresponding relationships obtained there from are respectively written below: For roughness parameter, relative baffle gap distance ( pitch (

in Eq. (26) and angle of attack (

in Eq. (27).

29

) in Eq. (25), relative baffle

exp(0.043 The final correlation for

can be written in the following form.

exp(0.043

This Eq. (27) represents the correlation for

as function of

and other roughness

parameter.

Fig. 12. Plot of

as a function of

30

for all the experimental data.

Fig. 13. Plot of

as a function of

Fig. 14. Plot of

as a function of

31

.

.

Fig. 15. Plot of as a function of

Fig. 16. Plot of as a function of .

32

Fig. 17. Plot of exp exp(0.043

as a function of

A similar method has been employed to develop a statistical correlation for friction factor (

) on the basis of regression analysis of data obtained from the experimental investigations

in the following form:

Fig.18 shows a comparison between the experimental data of

and those predicted from

the correlation developed in Eq. (27). Around ninety five percent of the data points are observed to lie within

13.5%. It is therefore concluded that the above

reasonably satisfactory for the prediction of the

33

correlation is

for the discretized broken V-pattern

baffle roughened solar air channel. The regression coefficient data for this correlation is 0.97 and average absolute percentage deviation is 4.38. Fig.19 shows the comparison between the experimental data of

and those predicted by the

correlation developed as Eq. (28). It is seen that ninety six percent of data points lie within 13.5% of the predicted data. The regression coefficient is 0.98 and average absolute percentage deviation is 4.12. Hence the correlation is reasonably satisfactory for the prediction of the

and

of roughened channel in the range of parameters investigated.

Fig. 18. Plot of predicted data vs. experimental data of Nusselt number.

34

Fig. 19. Plot of predicted data vs. experimental data of friction factor.

8. Conclusions Based on the experimental investigation, heat transfer and fluid flow characteristics in a solar air channel having discretized V-pattern baffle shapes on the heated wall. The effect of relative baffle height ( (

), relative baffle gap width (

), relative baffle pitch (

), relative baffle gap distance

and angle of attack (

on

and

has been

studied. 

Provided that a discretized broken V-pattern baffles outcomes in substantial improvement in

of solar air channel the improvement is a strong function of

discrete width and discrete distance. 

The data of

and

is more for discretized broken V-pattern baffle than that for

without discretized broken (continuous) V-pattern baffle. A highest enhancement obtained in

and

is 4.78 and 5.64 respectively for discretized broken V-

pattern baffle shape as compared with smooth one. 

The present investigation shows that baffled air channel with of 1.0,

of 0.50,

of 1.5, and

of 0.67,

of 60o yields the highest data of thermal

hydraulic performance parameter. 

Discretized broken V-pattern baffle has also been shown to be thermal hydraulic performance better in comparison to V-shaped baffle, staggered diamond baffle, rectangular cut baffle, Z-shaped baffle, U-shaped baffle and inclined perforated baffle.



Statistical correlations were developed for

and

as function of various

operating and roughness parameters. These correlations were found to predict the data of

and

with reasonable accuracy.

9. Future Scope The future directions for research can be made on evaluating the thermal hydraulic performance of multiple discrete regions in a V- pattern baffles. Since, the gaps (discrete) increases heat transfer due to generation of secondary flow streams, the multiple gaps will improve the strength of secondary streams which will add up to the heat transfer from the 35

absorber plate to the air. Also, the multiple V- pattern baffles in discrete form can be helpful for increasing heat transfer and thereby the overall performance.

Acknowledgements We sincerely thank the reviewers for constructive criticisms and valuable comments, which were of great help in revising the manuscript.

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 [45] 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.

Where

,

,

, …..

are possible error in measurement of

known as absolute uncertainty and

,

, …..

,

,

is

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 S. No.

Parameter

1.

Length of test section,

2.

Width of the channel,

36

data

3.

Height of channel,

4.

Diameter of pipe,

5.

Diameter of orifice meter,

6.

Pressure drop across orifice meter,

7.

Pressure drop across test section

8.

Atmospheric pressure,

N m2

9.

Outlet air temperature,

C

10.

Inlet air temperature,

11.

Rise in temperature of air,

12.

Mean bulk air temperature

13.

Mean plate temperature,

,

C C C

C

The thermo-physical properties of air have been determined by following standard correlations:

Uncertainty associated with instruments used in various measurements of parameters in the experiment is given in Table A2.

37

Table A2 Uncertainty intervals of various measurements

S. No.

Measurement

Instrument

1.

Dimensions of channel

Vernier caliper

2.

Pressure drop across the channel

Micro-manometer

3.

Pressure drop across the orifice-plate

U-tube manometer

4.

Temperature measurement

Copper-constantan thermocouple (T-type)

5.

Orifice plate and throat diameter

1. Uncertainty in Area of absorber plate (

2. Uncertainty in Area of flow (

Vernier caliper

)

)

38

Uncertainty

3. Uncertainty in measurement of Hydraulic diameter (

39

)

4. Uncertainty in Area of orifice meter (

)

5. Uncertainty in density measurement (

)

Taking

40

6. Uncertainty in mass flow rate measurement (

)

The data of

The uncertainty in

, for U-tube manometer is

7. Uncertainty in measurement of air velocity in channel ( )

41

8. Uncertainty in useful heat gain (

)

Uncertainty in specific heat is So, equation becomes

9. Uncertainty in heat transfer coefficient ( )

10. Uncertainty in Nusselt number (

)

42

11. Uncertainty in Reynolds Number (

12. Uncertainty in friction factor (

)

)

13. Uncertainty in thermo-hydraulic performance parameter (

43

)

Table A3 Range of uncertainty in the measurement of essential parameters S. No.

Parameters

Error range, %

1.

Mass flow rate (

2.

Velocity of air ( )

3.

Useful heat gain (

4.

Heat transfer coefficient ( )

2.213 – 3.732

5.

Nusselt number (

3.378 – 4.667

6.

Friction Factor (

7.

Reynolds Number (

8.

Thermo-hydraulic performance parameter (

1.597 – 2.033

)

1.653 – 1.811 2.131 – 3.267

)

)

1.283 – 2.331

)

1.43 – 3.76

)

) 3.675 – 5.221

As the uncertainty calculation was done on a single test run (constant Reynolds number), the uncertainty analysis for complete test run for single geometry (complete set of Reynolds number) was carried out and outcomes are presented in Table A3 for the experimental data.

References [1] Z. Sen, Solar energy in progress and future research trends, Progress in Energy and Combustion Science 30 (2004) 367–416. [2] S.A. Kalogirou, Solar thermal collectors and applications, Progress in Energy and Combustion Science30 (2004) 231-295. [3] B.N. Prasad, J.S. Saini, Effect of artificial roughness on heat transfer and friction factor in a solar air heater, Solar Energy. 41 (1988) 555-560. [4] S. Khanoknaiyakarn, Kwankaomeng, P. Promvonge, Thermal performance enhancement in solar air heater channel with periodically v-shaped baffles, in: IEEE Explore, International Conference on Utility Exhibition on Power and Energy Systems Issues and 44

Prospectus for Asia, Sep, 2011. [5] H.M. Yeh, W.H. Chou, Efficiency of solar air heaters with baffles, Energy 16 (1991) 983–7. [6] O.N. Sara, T. Pekdemir, S. Yapicic, M. Yilmaz, Thermal performance analysis for solid and perforated blocks attached on a flat surface in a duct flow, Energy Convers Manage 41 (2000) 1019-28. [7] G. Zhou, Q Ye, Experimental investigations of thermal and flow characteristics of curved trapezoidal winglet type vortex generators, Appl Therm Eng 37 (2012) 241-8. [8]

R.P. Saini, J. Verma, Heat transfer and friction factor correlations for a duct having dimple

[9]

shape artificial roughness for solar air heaters, Energy 33 (2008) 1277-87.

T. Alam, R.P. Saini, J.S. Saini, Experimentally investigation on heat transfer enhancement due to V-shaped perforated blocks in a rectangular duct of solar air heater, Energy conversion and management 81(2014) 374-383.

[10] T. Rajaseenivasan, S. Srinivasan, K. Srithar, Comprehensive study on solar air heater with circular and V-type turbulators mounted on absorber plate, Energy 88 (2015) 863873. [11] M. Bovand, S. Rashidi, J.A. Esfahani, Heat transfer enhancement and pressure drop penalty in porous solar heaters: Numerical simulations, Solar Energy 123 (2016) 145– 159. [12] A. Ekadewi, Handoyo, D. Ichsani, Prabowo, Sutardi, Numerical studies on the effect of delta-shaped obstacles’ spacing on the heat transfer and pressure drop in V-corrugated channel of solar air heater, Solar Energy 131 (2016) 47–60. [13] A. Priyam, P. Chand, Thermal and thermo hydraulic performance of wavy finned absorber solar air heater, Solar Energy 130 (2016) 250–259. [14] S. Shin, J.S. Kwak, Effect of Hole Shape on the Heat Transfer in a Rectangular Channel with Perforated Blockage Walls, J. Mechanical Science Technology 22 (2008) 1945– 1951. [15] R. Saim, H. Benzenine, H.F. Oztop, K. A. Salem,

Heat transfer enhancement of

turbulent flow in a channel with diamond shaped baffles, Progress in Computational Fluid Dynamics, 13(2013) 397-406. [16] S. Skullong, P. Promvonge, C. Thianpong, M. Pimsarn, Thermal performance in solar air heater channel with combined wavy-groove and perforated-delta wing vortex generators, Applied Thermal Engineering 100 (2016) 611–620. 45

[17] V.B. Gawande, A.S. Dhoble, D.B. Zodpe, Sunil Chamoli, Analytical approach for evaluation of thermo hydraulic performance of roughened solar air heater, Case Studies in Thermal Engineering 8 (2016) 19–31. [18] F. Bayrak, H.F Oztop, A. Hepbasli, Energy and energy analysis of porous baffles solar air heater for building applications, Energy and buildings 457(2013) 338-345. [19] S. Sripattanapipat, P. Promvonge, Numerical analysis of laminar heat transfer in a channel with diamond shaped baffles, Int Commun Heat Mass Transfer 36 (2009) 32–8. [20] G. Zhou, Q.Ye, Experimental investigations of thermal and flow characteristics of curved trapezoidal winglet type vortex generators, Appl. Therm. Eng. 37(2012) 241248. [21] C. Nuntadusit, I. Piya, M. Wae-hayee, S. Eiamsa-ard, Heat transfer characteristics in a channel fitted with zigzag-cut baffles, Journal of Mechanical Science and Technology 29 (2015) 2547-2554. [22] P. Sriromreun, C. Thianpong, P. Promvonge, Experimental and numerical study on heat transfer enhancement in a duct with Z-shaped baffles, Int. Commun Heat Mass Transfer 39 (2012) 945–952. [23] S. Khanoknaiyakarn, S. Kwankaomeng, P. Promvonge, Thermal performance enhancement in solar air heater duct with periodically V-shaped baffles, IEEE explore, International conference on utility exhibition on power and energy systems issues and prospectus for Asia; 2011. [24] A. Bekele, M. Mishra, S. Dutta, Effects of delta-shaped obstacles on the thermal performance of solar air heater, Advanced Mechanical Engineering (2011) 103502. [25] T. Chompookham, C. Thianpong, S. Kwankaomeng, P. Promvonge. Heat transfer augmentation in a wedge ribbed duct using winglet vortex generators. Int. Commun Heat Mass Transfe 2010;37:163-169. [26] E.K. Akpinar, F. Kocyigit. Energy and energy analysis of a new flat plate solar air heater having different obstacles on absorber plates, Applied Energy 87 (2010) 3438-3450. [27] P. Promovong, S. Kwankaomeng, Periodic laminar flow and heat transfer in a channel with 45° staggered V-baffles, Int Commun Heat Mass Transfer 37(2010) 841-9.

[28] S.C. Tzeng, T.M. Jeng, Y.C.Wang, Experimental study of forced convection in asymmetrically heated sintered porous channels with/without periodic baffles, Int J Heat Mass Transfer 49 (2006) 78–88.

46

[29] S. Eiamsa-ard, K. Wongcharee, P. Eiamsa-ard, C. Thianpong, Heat transfer enhancement in a tube using delta-winglet twisted tape insets, Appl. Therm. Eng 30 (2010) 310-318. [30] S.B. Bopche, M.S. Tandale, Experimental investigation on heat transfer and frictional characteristics of a turbulator roughened solar air heater duct, Int. J Heat Mass Transfer 52 (2009) 2834–2848. [31] J.H. Nie, Y.T. Chen, H.T. Hsieh, Effect of a baffle on separated convection flow adjacent to backward-facing step, International Journal of Thermal Science 48 (2009) 618-625. [32] R. Karwa, B.K. Maheshwari, Heat transfer and friction in an asymmetrically heated rectangular duct with half and fully perforated baffles at different pitches, Int. Commun Heat Mass Transfer 36 (2009) 264–268. [33] B.K.P Ary, M.S. Lee, S.W. Ahn, D.H. Lee The effect of the inclined perforated baffle on heat transfer and flow patterns in the channel. International communications in Heat and mass Transfer. 39 (2012), pp. 1578-1583. [34] P. Dutta and A. Hossain, Internal cooling augmentation in rectangular channel using two inclined baffles, International Journal of Heat and Fluid Flow 26 (2005) 223-232. [35] J. Hu, X. Sun, J. Xu, Z. Li. Numerical analysis of mechanical ventilation solar air collector with internal baffles, Energy and Buildings 62 (2013) 230-238. [36] S.K. Saini, R.P. Saini, Development of correlations for Nusselt number and friction factor for solar air heater with roughened duct having arc-shaped wire as artificial roughness, Sol Energy 82 (2008) 1118–30. [37] V.S. Hans, R.S. Gill, S. Singh, Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with broken arc ribs, Experimental Thermal and Fluid Science, 80 (2017) Pages 77-89. [38] R. Maithani, J.S. Saini, Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with V-ribs with symmetrical gaps, Experimental Thermal and Fluid Science 70 (2016) 220–227. [39] V.S. Hans, R.P. Saini J.S. Saini, Heat transfer and friction factor correlations for a solar air heater duct roughened artificially with multiple v-ribs, Solar Energy 84 (2010) 898– 911. 47

[40] A. Kumar, R.P. Saini, J.S. Saini, Heat transfer and friction factor of solar air heater having duct roughened artificially with discrete multiple v-ribs, Journal of Renewable and Sustainable Energy 4 (2012) 033103.

[41] A. Kumar, M.H. Kim, Effect of roughness width ratios in discrete multi V-rib with staggered rib roughness on overall thermal performance of solar air channel, Solar Energy 119 (2015) 399–414.

[42] ASHRAE Standard 93, Method of Testing to Determine the Thermal Performance of Solar Collectors. Atlanta, GA: American Society of Heating, Refrigeration and Air Conditioning Engineers, 2003. [43] R. P. Benedict, Fundamental of temperature pressure and stream measurements, third edition, Wiley-Interscience Publication, New York 1984. [44] M.J. Lewis, Optimizing the thermohydraulic performance of rough surfaces, International Journal of Heat and Mass Transfer 18 (1975) 1243-1248. [45] S.J.Kline, F.A. Mcclintock, Describing uncertainties in single sample experiments. Mechanical Engineering 75(1953) 3-8.

48

Highlights 

Experimental investigation of discretized broken V-pattern baffle solar air channel has been carried out.



Nusselt number, pressure drop and thermo-hydraulic performance are experimentally investigated.



Optimum value of roughness parameters is determined on the basis of thermal as well hydraulic performance.



Correlations are developed based on heat transfer and pressure drop results.

49