A Diddertatiop Presentation

A Dissertation Presentationon

Thermo-Hydraulic Performance of Square Perforated Solar Air

Heater

Guided By: Submitted By:

Dr. B. K. Maheshwari Rahul TripathiAssistant professor M.E. (Thermal Engg.)

DEPARTMENT OF MECHANICAL ENGINEERINGM. B. M. ENGINEERING COLLEGE

FACULTY OF ENGINEERING AND ARCHITECTUREJ. N. V. UNIVERSITY, JODHPUR

June, 2014

April 18, 2023 M.E. Dissertation by Rahul Tripathi 2

Contents Of Dissertation

Performance Enhancement Of Solar Air Heater Using

Baffles

Experimental Setup And Programme

Result And Graphs

Conclusions

References


PERFORMANCE ENHANCEMENT OF SOLAR AIR HEATER USING BAFFLES

Perforated Rectangular Blocks

Asymmetrically Heated Rectangular Duct with

Perforated Baffles

Use of Porous Baffles to Enhance Heat Transfer

Fully perforated ribs

Half perforated turbulence promoters

Fins provided with baffles


Perforated Rectangular Blocks

• Sara et al. (2001) had investigated perforated rectangular cross-sectional.

• Results from the perforated blocks concluded that as the degree of the perforation increased the heat-transfer enhancement capability of the blocks also increased.


A typical variation in the heat transfer with Re for varying degree of the perforated open-area ratio (φ=0.05, 0.10 and 0.15) is shown in Figure.With their experimental studies they have concluded that perforations in the blocks enhance the heat transfer and the enhancement increases with increasing the degree of perforations.

Fig. Variation of with Re (Sara et al, 2001)


Asymmetrically Heated Rectangular Duct with Perforated Baffles

• Maheshwari et al. (2005) has experimentally studied the heat transfer and friction factor of rectangular ducts with baffles (β = 18.4%, 28.4%, 41.7% & 46.8%)

They have concluded that the baffles with the highest open area ratio (46.8%) give the best performance compared with the smooth duct at equal pumping power.


Fig. (a) Schematic of the baffled duct, (b) sketch of a baffle with perforations. (Karwa et al 2005).

The longitudinal section of the duct is shown in Fig.(a) and (b) shows two views of a perforated baffle.


Fig. Thermal performance at equal pumping power. (Karwa et al. 2005)


Use of Porous Baffles to Enhance Heat Transfer

Anand and Ko (2003) have experimentally studied heat transfer enhancement in a rectangular channel by using a porous baffle made up of aluminum foam. Baffles were mounted on bottom and top walls in a staggered fashion. Porous baffles as shown in Fig.

Fig. Aluminium foam structure (Anand and Ko 2003)


The heat transfer enhancement ratio (Nu+) decreases with increase in Reynolds number and increases with increase in pore density.

Fig. Heat transfer enhancement ratio of different pore density. ( Anand and Ko,2003 ).


Fully Perforated Ribs

Hwang and Liou (1995) investigated heat transfer and friction in a rectangular

channel (W/H=4) with symmetrically mounted solid and fully perforated ribs on

parallel broad walls (β=50%, e/H=0.13 and 0.26, δ/e=0.38 and 0.76, Re=10,000–

50,000, p/e=5–20).

Their studies had also concluded that the perforated ribs are thermohydrolically

better than the solid ribs.


Half Perforated Turbulence Promoters

Tanasawa et al. investigated the effect of the half perforated turbulence promoters

symmetrically mounted on two opposite walls on the heat transfer and friction in a

rectangular channel.

They found that surfaces with the half perforated turbulence promoters with

perforations on the lower half of the ribs performed better than those with the solid

type promoters.


Fins Provided with Baffles

Yeh and Chou (1991) experimentally investigate the efficiency of solar air heaters with baffles as shown in Fig. and found considerable improvement in the collector efficiency of solar air heaters with fins in the collector are provided with attached baffles to create air turbulence and an extended heat transfer area and on increasing the density of baffles.

Fig. Baffle attached to absorber plate. (Yeh and Chou, 1991)


Roughness elements of larger height give a high increase in the heat transfer but increase in pressure drop is a serious concern. Hot zones develop in the wake of these elements because of re-circulating flow. This leads to lower heat transfer from these zones; thus an attempt has been made by the designers to overcome this effect by putting perforation in the baffles which increase the heat transfer from these zones and help in reducing the pressure drop across the channel. The perforated elements allow a part of the flow to pass through these perforations and thus the hot zones and form drag are reduced.


Experimental Setup And ProgrammeExperimental investigation of heat transfer and friction characteristics of a

rectangular duct with square perforated baffles on one broad wall which is subjected to solar radiation has been carried out using an outdoor test facility available in department of mechanical engineering. The test facility has been designed according to the guide line of ASHRAE standard 93-1986 for testing of solar collector operating in an open-loop mode.

Instrumentation

Solar Radiation Measurement

A pyranometer (shown in figure ) was used to measure the total short wave

radiation from both sun and sky and a pyrheliometer wad used to measure the

direct normal insulation.


Fig. Pyranometer Solar radiation measuring instrument


Temperature Measurement

In present investigation a Butt-welded 1 mm dia Chromel Alumel bead (K-type) thermocouples having temperature range 0o-1200oC , calibrated against mercury thermometer of 0.1oC least count, was used for the temperature measurement.

Fig. Thermocouple measurement circuit


The thermocouples(3/24 inch diameter “K type” – CR/AL (Compensating)PVC

THERMOCOUPLE Indian Standards IS – 8784 STANDARD. confirmation ECO

SERIES) were provided along the axial center line of the absorber plate in small

holes on the span-wise variation of absorber plate temperature as shown in fig.

Nineteen (19) thermocouple arranged span-wise in the duct, as shown in the fig.

To measure the exit air temperature after the mixing section three thermocouples

were provided as shown in figure.

Inlet temperature of air to the duct was also measured using a thermocouple in

the same manner.


Fig. (a) Thermocouple positions of baffled absorber plate.(b) Thermocouple positions of smooth absorber plate.


Fig. Thermocouple locations on Smooth and Baffled plates in Solar Air Heater.


Air Flow Measurements

For flow measurement an orifice plate (as shown in fig.) and inclined manometers with tilt of 5:1 had been used to achieve the desired accuracy in present investigation.

Fig. Detailed view of orifice plate (Gharai et al, 2011)


Pressure Drop across Collector

The static pressure drop across the solar collector was measured using a differential pressure measuring device. Each side of the device was connected to four externally main folded pressure taps as shown in Fig. The pressure taps consist of 6.4 mm nipples soldered to the duct and centered overt mm diameter holes. The edge of these holes on the inside surface of duct was made sure to be free from burrs and other surface irregularities.

Fig. Schematic representation of the measurement of pressure drop across the solar collector.(ASHRAE Codes ,93-1986)

April 18, 2023 23

Wind VelocityThe wind velocity was measured with an instrument (shown in figure) and

associated readout device that can determine the integrated average wind velocity for each test period accuracy of ± 0.8 m/s (1.8 mph)

Fig Wind velocity measurement device

M.E. Dissertation by Rahul Tripathi


Apparatus and Method of Testing

The experimental test facility, designed and fabricated as per ASHRAE

Standard for testing of solar collectors (ASHRAE Standards, 1986), consists of

300 mm wide parallel ducts with entrance, test, exit and mixing sections, a

blower, control valves, orifice plates and provision for temperature and pressure

drop measurements as shown in Fig. (a) and (b). It works in an open loop mode.The ducts are made of good quality smooth faced plywood and wooden

boards. Each duct is 2880 mm long consisting of 1.64 m long test section, 550

mm long insulated entry and exit sections are installed to reduce any upstream and

downstream effect due to entrance and exit section respectively on the test section.For the turbulent flow regime, ASHRAE Standard recommends entry and

exit lengths of 5(WH) and 2.5(WH), respectively, i.e. 526 mm and 263 mm,

respectively for the duct cross-section employed in the present investigation.The height of both the ducts has been kept fixed at 38.4 mm. The combined

width of ducts (with side walls) is 850 mm where ducts being 300mm wide as

shown in figure.


Fig (a) and (b) Plan and Elevation of Solar Air Heater.


The test section length is 1.64 m resulting in length to hydraulic diameter (D=38.4

mm) ratio of 24.3. Topside of the heated test section carries 12 pieces of 3.25 mm

thick aluminium plate with square perforated baffles on the lower side.

Fig. Sectional view of solar air heater


The sun facing sides of both the absorber plates are smooth and blackened.

Glass plates of 5 mm thickness have been used as cover over the absorber plates at a

height of 60 mm. The top side of entry and exit lengths of each duct is covered with

the plywood.A 100 mm long baffled mixing section is provided to get uniform temperature

of the exit air in the measuring section just after the mixing section. The exit end of

each duct is connected, through a rectangular to circular transition piece, to a 70 mm

ID G.I. pipe with orifice plate assembly.The other end of the each pipe is connected through control valve to the

suction of a 10 HP blower using flexible pipe and a Y-section (not shown in the

figure).50 mm thick thermocole insulation has been provided on the back of the

collector from test section inlet to outlet of twin duct while the transition pieces and

orifice plate assembly pipes (up to the orifice plates) are covered with foam blanket

insulation.All joints are properly sealed with putty.The set-up was installed horizontal (in north-south orientation) on the roof top

at a height of 750 mm on an iron stand.


thermocouples have been fixed with M-seal in the sun facing side of

absorber plate in small diameter holes drilled about 2.5 mm deep at nineteen axial

and span wise locations on the smooth and baffled absorber plates, respectively.


Assembly and Fabrication of the Experimental setup

Below are images for the sequence of assembly and fabrication of solar air heater experimentation.

Fig Wooden Frame of the solar air heater

April 18, 2023 M.E. Dissertation by Rahul Tripathi 30Fig. Valves, Pipe fitings, and insulation used for solar air heater


Fig. Rectangular (300x38.4 mm2) to circular transition piece(70 mm ID), G.I. pipe with orifice plate assembly. The throat diameter of the orifice plate is 38 mm. The other end

of the each pipe is connected through control valve


Fig Finished Solar Air Heater Wooden Frame and pipe fitting


Fig 3.25 mm thick Aluminium sheet used for fabrication of smooth and

baffled absorber plates

Fig. .Final Assembly of transition piece and control valves on solar air heater


Fig. Hand punching machine used for fabrication of baffles


Fig Punching die of square shape.


Fig Assembly of Smooth and Rough Plates on Twin Duct Solar Air Heater.


Fig. Attachment of Thermocouples on absorber plates


Fig Blackened Absorber plates for increased absorptivity

April 18, 2023 M.E. Dissertation by Rahul Tripathi 39Fig. Complete Fabrication of solar air Heater with Glass plate Covers


Experimental Conditions and Procedure

All components of the experimental set up and the instrument have been

checked for proper operation. The blower was then switched on and the joints of the

setup were properly sealed to prevent leakage.Micro-manometer and inclined U-tube manometer were properly leveled.

Blower was switched on and the flow control valve was adjusted to give a

predetermined rate of air flow through the test section. Before the covers were put off, it was ensured that all the thermocouples gave

the same output.All reading was noted under steady state condition, which was assumed to

have been obtained when the plate and air outlet temperature did not deviate over a

10 minute period. The steady state for each run was obtained in about 1 hour and two Reynolds

numbers was investigated throughout the day in the following manner.


Day 1 Day 2 Day 3 Day 4 Day 5 Day 6

Nov.-13 Nov.-14 Nov.-15 Nov.-16 Nov.-17 Nov.-18

β=42.70% β=42.70% β=51.98% β=51.98% β=60.69% β=60.69%

Re=3000and

Re=6000

Re=9000and

Re=12000

Re=3000and

Re=6000

Re=9000and

Re=12000

Re=3000and

Re=6000

Re=9000and

Re=12000

Table : Experimentation Plan


Reynolds number, Re 3034 – 12003

Duct depth, H 38.4 mm

Width of duct, W 300 mm

Hydraulic Diameter, D 68.16 mm

Duct aspect ratio, W/H 7.81

Test section length, L 1640 mm

Test section length to hydraulic diameter ratio,

L/D

24.06

Thickness of baffle, δ 0.643 mm

Baffle height, e 19 mm

Spacing between baffles (pitch), p 134.1 mm

Baffle height-to-duct height ratio, e/H 0.495

Baffle thickness-to-height ratio, δ/e 0.034

Baffle pitch-to-height ratio, p/e 7.06

Open area ratio of perforated baffle, β Type I β = 42.70%

Type II β = 51.98%

Type III β = 60.69%

Table : Experimental conditions and dimensions of baffle and duct


Fig. Details and dimensions of square perforated baffles of open area ratio β=60.69%.






Fig. Detailed nomenclature and various parameters of baffle


The studies (Han et al., 1978; Han and Park, 1988; Han et al., 1989) show that

the effect of the values of the friction factor and the heat transfer either vanishes or is

only marginal when the aspect ratio is greater than 8. It is to be noted that the solar air

heaters have generally high aspect ratio duct. Keeping the above information in the

mind, the value of the baffle height-to-duct height ratio, (e/H), baffle thickness-to-

height ratio, (δ/e), baffle pitch-to-height ratio, (p/e), and the duct aspect ratio, (W/H)

have been fixed for the present study. Only Open area ratio of perforated baffle, (β),

have been varied and three values were selected, (i.e. 60.69%, 51.98%, and 42.70%).

While the baffles height has been fixed at 19 mm. The airflow rate was varied to

give flow Reynolds number of about 3034 to 12003.


Perforated Baffles

The details of baffles used in this study and other relevant system parameters

are given in previous Tables and figures. In the present study, the baffle height-to-duct

height ratio e/H has been fixed at 0.495, which reduces flow passage blockage effect

and simultaneously the baffles extend sufficiently deeper into the flow into the buffer

zone. The baffle pitch- to- height ratio p/e is 7.06 and the flow Reynolds number study

ranges from 3000-12000.

The ratio of the area of the perforations to the baffle frontal area is known as

open area ratio and is given by

= n a2 / (be)

where n is the number of the holes punched through the baffle and a is the side

of a square hole


Data Reduction

Mass flow rate of air has been determined from pressure drop measurement across the orifice plate using the following relationship:

The heat transfer coefficient for the heated test section was calculated from:

Where the heat transfer rate Qu, to the air is given by

The heat transfer coefficient has been used to determine the Nusselt number using the equation

Where


The Reynolds number was determined from the value of the mass flow rate, m, using the equation:

Where

The friction factor was determined from the measured values of pressure drop,

The thermo physical properties of the air have been taken at the corresponding mean temperature Tm = Tfm or Tmpg. The following relations of

thermo physical properties, obtained by correlating data from NBS (U.S.), have been used:


RESULT AND GRAPHS

Variation of Nusselt Number with Reynolds Number

The Comparative plot of Nusselt number v/s Reynolds number has been shown in Fig. for smooth and baffled duct of different perforations (i.e., β= 60.69%, 51.98%, and 43.70%).

2000 4000 6000 8000 10000 12000 140000

20

40

60

80

100

120

Smooth Plate β=42.70% Perforated Bafflesβ=51.98% Perforated Baffles β=60.69% Perforated Baffles

Reynolds Number (Re)

Nus

selt

Num

ber

(Nu)

Fig Variation of Nusselt Number with Reynolds Number


2000 4000 6000 8000 10000 12000 140000

0.5

1

1.5

2

2.5

3

Smooth Plate

β=42.70% Perforated Baffles




Nus

selt

Num

ber

ratio

(N

u/N

u s)

Variation of Nusselt Number ratio (Nu/Nus) with Reynold Number (Re)

Fig. Plot of Nusselt number ratio versus the Reynolds number

The enhancement in Nusselt number for baffled plate of open area ratio β=42.70% is found highest and is order of 1.36-2.09 times more than smooth plate. It is to be noted that value of Nusselt number is directly related to change in heat transfer coefficient. And eventually we get better heat transfer coefficient for baffled plates as compared to the smooth plate.


Variation of Friction Factor with Reynolds Number

2000 4000 6000 8000 10000 12000 140000

0.01

0.02

0.03

0.04

0.05

0.06

0.07

0.08

0.09

0.1

Smooth β=60.69% Perforated Bafflesβ=51.98% Perforated Baffles β=42.70% Perforated Baffles


Fric

tion

Fact

or (f)

As shown in Fig variation of friction factor has been found in good agreement with theoretical value given by Moody’s Chart

Fig Variation of Friction factor with Reynolds Number


In the present experimentation study there was an enhancement of 1.36-

2.09 times in Nusselt number while friction factor increased 1.91-2.19 times for

the baffle of open area ratio β=42.70%.It is found in the study that friction factor increment for other two baffles

of open area ratio β=51.98% and 60.69% are 1.77-2.10 and 1.49-1.7

respectively.


Variation of Temperature Rise with Mass Flow Rate

0.005 0.01 0.015 0.02 0.025 0.03 0.035 0.040

2

4

6

8

10

12

14

16

18

Smooth β=60.69% Perforated Bafflesβ=51.98% Perforated Baffles β=42.70% Perforated Baffles

Mass Flow Rate (m/s)

(To-T

i) T

empe

ratu

re R

ise

(oC

)

From the figure 4.4 if is cleat that Temperature Rise for baffled plates are higher than smooth plates with same value of mass flow rates and solar insolation

Fig Variation of Temperature Rise with Mass Flow Rate


Thermo-Hydraulic Performance Parameter

Lewis [1975] proposed a thermo-hydraulic performance parameter known as efficiency parameter h, which evaluates the enhancement of heat transfer for same pumping power requirement and is defined as,

2000 4000 6000 8000 10000 12000 140000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

Smooth Plate β=60.69% Perforated Baffles β=51.98% Perforated Bafflesβ=42.70% Perforated Baffles Series9


(St/

Sts)

/(f/

fs)1/

3

Fig Thermo-hydraulic Performance Parameter


Thermo-hydraulic Performance Parameter for square perforated baffle

are found greater than unity it is found 1.49, 1.75, 1,86 and 1.87 for baffle of

open area ratio β=60.70% for different Reynolds number range investigated in

the study between 3034 to 12003.


Comparison of Square and Circular Perforated Baffles

2000 4000 6000 8000 10000 12000 140000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2

β=60.69% square perforated baffles β=46.8% circular perforated baffles


Rat

io o

f Nus

selt

Num

ber

(Nu/

Nus

)

Maheshwari et. al, 2005, experimentally investigates the circular perforated baffles of four different open area ratios 18.4%, 28.4%, 41.7% and 46.8%. He found that the baffle of open area ratio of 46.8% give the greatest performance advantage.

Fig. Plot of Nusselt number ratio versus the Reynolds number for circular and square perforated baffles.


They concluded that increasing the open area ratio increase the performance

of the heater but in case of circular perforation it is not possible to increase the open

area ratio more than 46.8%. I opted square perforation and in it, it is possible to

increase the perforation to a limit of 60.69%.In this experimental study it is found that for the square perforated baffle of

open area ratio of 60.69% is the best one. By comparing it with circular perforated

baffle of 46.8% perforation, increment in Nusselt number is more by 21%.


2000 4000 6000 8000 10000 12000 140000

0.2

0.4

0.6

0.8

1

1.2

1.4

1.6

1.8

2β=60.69% Perforated Baffles β=46.8% circular perforated baffles

Reynolds Number

(St/

Sts)

/(f/

fs)1/

3

Fig Comparison of thermo hydraulic performance of 60.69% square perforated baffles and 46.8% circular perforated baffles

By the comparing of thermo hydraulic performance of 60.69% square perforated baffles and 46.8% circular perforated baffles it is found that 60.69% square perforated baffles are thermo hydraulically better than 46.8% circular perforated baffles by 18%


CONCLUSIONSNusselt number enhancement by use of square perforated baffles of different

open area ratios was found to be in the range of 1.36 to 2.09 times to the

corresponding values of smooth plate for the Reynolds Number 3034 to 12003.

Increment in Friction factor (power penalty) was found 1.91-2.19, 1.77-2.10 1.49-1.7

for the baffle of open area ratio β=42.70%, β=51.98% and 60.69% respectively to the

corresponding values of smooth plate for the Reynolds Number 3034 to 12003.Nusselt number increases whereas friction factor decreases with increase of

Reynolds number. Values of friction factor and Nusselt number were highest for

perforated baffle of open area ratio β=42.70% and was lowest for perforated baffle of

open area ratio β=60.69% but . This is due to change in flow characteristics because of

baffles that cause flow separation, reattachments and generation of secondary flow.Thermo-hydraulic Performance Parameter for square perforated baffle are

found greater than unity it is found 1.87, 1.70, and 1.55 for baffle of open area ratio

β=60.69%, 51.98% and 42.70% respectively for different Reynolds number range

investigated in the study between 3034 to 12003.


The ratio of Nusselt number of baffled duct to smooth duct was found in the

range of 1.59 to 1.88 for the Reynolds no. of 3034 to 12003 for square perforated

baffles of open area ratio of 60.69% but for circular perforated baffles of open area

ratio of 46.8% it was reported in the range of 1.45 to 1.55 by Maheshwari et. al,

2005. So it is concluded that square perforated baffles of 60.69% perforation are

better than circular perforated baffles of 46.8% perforation.


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