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Abstract

In this work, the effect of angle of attack $(α_{a})$ of the discrete V-pattern baffle on thermohydraulic performance of rectangular channel has been studied experimentally. The baffle wall was constantly heated and the other three walls of the channel were kept insulated. The experimentations were conducted to collect the data on Nusselt number $(N u_{b})$ and friction factor $(f_{b})$ by varying the Reynolds number (Re) = 3000–21,000 and angle of attack $(α_{a})$ from 30° to 70°, for the kept values of relative baffle height $(H_{b} / H) = 0.50$ , relative pitch ratio $(P_{b} / H) = 1.0$ , relative discrete width $(g_{w} / H_{b}) = 1.5$ and relative discrete distance $(D_{d} / L_{v}) = 0.67$ . As compared to the smooth wall, the V-pattern baffle roughened channel enhances the Nusselt number $(N u_{b})$ and friction factor $(f_{b})$ by 4.2 and 5.9 times, respectively. The present discrete V-pattern baffle shapes with angle of attack $(α_{a})$ of 60° equivalent to flow Reynolds number of 3000 yields the greatest thermohydraulic performance. Discrete V-pattern baffle has improved thermal performance as compared to other baffle shapes’ rectangular channel.

Keywords

Thermal performance heat transfer enhancement friction factor Nusselt number discrete width discrete baffle

Introduction

Energy is the basic ingredient needed to sustain life and development. Social, cultural and economic development of the people is seen to a great extent by the amount of per capita energy consumed and by the potency of the technological means with which it is converted to work.^1,2 Conventionally, energy is being extracted from fossil fuels, large hydroelectric systems and fuel wood. The present energy scenario is biased towards the conventional energy sources, although it has been proven that these are finite in nature, economically out of reach to many growing countries and cause environmental degradation.^3,4 The energy utilization pattern in the developing countries like India projects a scenario, which is also biased towards the energy available in urban areas. The rural areas are deprived of the calibre, energy sources, therefore making a broad gap between requirement and availability of energy.⁵

Rectangular channel is one of the simplest and widely used types of heat exchanger in which heat energy is being exchanged between the heated wall and the air flowing through the system. The major drawback of rectangular channel use is the low overall thermal performance due to low heat transfer rate between the heated wall and the air.^6,7 In order to attain higher thermal performance, it is important that the flow at the heat transfer wall should be turbulent. The baffle roughness has been used widely for the augmentation of forced convective heat transfer coefficient of solar air heaters (SAHs).^8–10 Use of baffle roughness seems to be an attractive proposition for improving the local Nusselt number.¹¹ For detailed descriptions of some experimental investigations on rectangular channel with transverse baffle, inclined baffle, delta baffle, diamond-shaped baffle, V-pattern baffle and so on with different shapes, dimensions and orientations, readers may refer to some of the previous works.^11–35

Molki and Mostoufizadesh¹¹ experimentally studied the Nusselt number and friction factor in a rectangular channel with repeated baffle blockages. The baffles are fitted in a staggered fashion with fixed axial spacing. Yeh et al.¹² experimentally and numerically studied the effect of aspect ratio $(W / H)$ on the collector efficiency of baffle SAHs. Won et al.¹³ reported the effect of angled rib baffles with an angle of attack $(α_{a})$ value of 45°, relative roughness height $(e / D)$ of 0.078, relative pitch ratio (P/e) of 10 and open area ratio $(β_{o})$ of 25% for an Reynolds number $(Re)$ range of 9000–76,000. Tandiroglu¹⁴ experimentally investigated the effect of flow parameters on transient forced convection heat transfer for turbulent flow with baffle shape inserts in a channel. Khanoknaiyakarn et al.¹⁵ carried out an experimental study on Nusselt number and friction factor using V-pattern baffles on one broad heated wall of large aspect ratio. The effects of the baffles on Nusselt number and friction factor were examined. The overall performance of the tested baffled wall showed an increase in the Nusselt number and friction factor induced by the baffles with respect to a smooth channel under similar flow conditions.

Lin¹⁶ experimentally investigated the local heat transfer in rectangular channel with baffles and analysed the experimental results of heat transfer for baffle designed with different heights and pores in the event of five Reynolds numbers and three heating quantities. Nasiruddin and Siddiqui¹⁷ investigated a significant Nusselt number in a heat exchanger tube by attaching a baffle into flow. Romdhane¹⁸ studied the solar collectors and gave a comparative study on various techniques that favourize and increase the heat transfer coefficient between the caloporting fluid (air) and the absorber, the manner in which the air flows the absorber and the shape of the collector itself and those of the inlets and outlets. It was found that on introduction of suitable baffles in solar air collectors increases the couple efficiency-increase in temperature. The baffles placed in the air channel situated between the insulator and the absorber have the particularity of extending the trajectory of the circulation, to keep the caloporting air constantly in contact with the absorber, and finally to play the role of wings and improving the heat transfer from the absorber to the caloporting air. Nie et al.¹⁹ studied the effects of a baffle on the Nusselt number from the arithmetical simulations of three-dimensional laminar forced convection stream adjacent to rearward-facing step in a rectangular channel.

Promvonge et al.²⁰ experimentally investigated the turbulent forced Nusselt number and friction factor loss behaviour in a high aspect ratio channel fitted with 60-V baffle. Sripattanapipat and Promvonge²¹ numerically investigated the laminar periodic flow and heat transfer in a two-dimensional horizontal channel attached to two transverse staggered diamond-shaped baffles. The computations based on the finite volume method with the SIMPLE algorithm have been introduced for the fluid flow with Reynolds number ranging from 100 to 600. The author studied the effects of different baffles’ tip angles on heat transfer enhancement and pressure loss in the channel and found that the heat transfer enhancement for the 5° diamond-shaped baffle is 6% higher than that of the flat baffle.

Karwa and Maheshwari²² experimentally carried out a study on fully and half-perforated baffles with Reynolds number values ranging from 2700 to 11,150. Half-perforated baffles having a relative pitch ratio (P/e) of 7.2 were found to have the utmost enhancement in performance of approximately 68.66% over a smooth wall for equivalent pumping power. Akpinar and Kocyigit²³ experimentally investigated the performance analysis of four SAHs with different obstacles and without obstacles for two air mass rates of 0.0074 and 0.0052 kg/s and compared the basis of the first and second laws of efficiencies. They reported that the efficiency of SAH depends on the surface geometry of collectors and solar radiation of air flow line. The collector efficiency increased with increasing mass flow rate and decreased as the temperature parameters increased.

Sriromreum et al.²⁴ through experimentally and mathematically studied the effect of the baffle on Nusselt number and friction factor in a rectangular channel having aspect ratio $(W / H) = 10$ fixed with the in-stage and out-phase 45° Z-pattern baffle. Shin and Kwak²⁵ studied the effect of the perforation shape of a blockage wall on the Nusselt number in a flow passage. Five wide, narrow and circular hole geometries were tested. It was observed that a blockage surface with wider perforation provided a more uniform local heat transfer and higher thermal performance factor. Mohammadi and Sabzpooshani²⁶ studied the influence of baffles’ pattern mounted over the absorber plate on the performance of the upward-type single-phase SAH. Bekele et al.²⁷ experimentally studied the effect of delta pattern obstacles mounted on one wall subjected to a constant heat flux for a rectangular channel with an aspect ratio $(W / H) = 10$ . Bekele et al.²⁸ conducted indoor experiments to investigate the effect of delta-type baffles mounted on the heated surface of a rectangular channel with a small aspect ratio of 5.0–1.0. Promvonge and Kwankaomeng²⁹ carried out a numerical analysis to observe the laminar periodic flow and Nusselt number and friction factor in a three-dimensional isothermal wall square channel with 45° staggered angled baffle.

Sara et al.³⁰ investigated the Nusselt number and the consequent friction factor over a flat wall in a rectangular channel flow by mounting perforated rectangular cross-sectional block. Karwa et al.³¹ experimentally studied the effect of Nusselt number in a rectangular channel with perforated or solid baffle mounted on single of the broad walls. Mousavi and Hooman³² experimentally and numerically investigated the laminar fluid flow and heat transfer in the entrance region of a two-dimensional horizontal channel with isothermal walls and with staggered baffles. The results were reported for Reynolds number ranging from 50 to 500 and baffle heights between 0 and 0.75. It was observed that the Reynolds number is influential on the location of the periodically fully developed condition, as well as the blockage ratio. The two parameters affect the development in such a way that increasing any of the two will reschedule the development and subsequently increase the Nusselt number. Since the flow reattachment to the channel wall causes the washing of the wall, it results in grater values of the local Nusselt number.

Chamoli and Thakur³³ experimentally investigated the effect of geometrical parameters of the V-pattern perforated blocks on Nusselt number and friction factor of rectangular channel. Alam et al.³⁴ experimentally investigated the effect of geometrical parameters of the V-pattern perforated baffle on Nusselt number and friction factor of rectangular channel. Table 1 summarizes the experimental investigations of some important baffle arrangements investigated by the investigators.

Table 1.

Experimental investigations of various baffles’ rectangular channel in previous investigations.

S. No.	Investigators	Principle findings
1.	Angled baffles¹³	3.16 and 3.56 times augmentations in $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth wall rectangular channel
2.	V-shaped baffles¹⁵	3.97 and 5.45 times augmentations in $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth rectangular channel
3.	Perforated baffles²²	3.87 and 4.12 times augmentations $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth rectangular channel
4.	Transverse perforated blocks’ baffles²⁵	3.98 and 4.28 times augmentations in $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth rectangular channel
5.	Delta-shaped baffles²⁸	3.67 and 4.89 times augmentations in $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth rectangular channel
6.	Single V-perforated shaped baffles³³	3.95 and 5.35 times augmentations $N u_{b}$ and $f_{b}$ , respectively, were reported over smooth rectangular channel
7.	Discrete V-pattern baffle (proposed shape)	Literature review shows that V-pattern baffles have better overall thermal performance than other simple angled baffle, transverse baffle and perforated baffle shapes and configurations. It is thought that discrete V-pattern baffle will augment heat transfer compared to without discrete V-pattern baffle and perforated baffle shapes

The literature review shows that the transverse baffle shape enhances the heat transfer by stream separation and creation of vortices on the upwards and downwards of the baffles and reattachment of stream in inter-baffle spaces. The angle of transverse baffle rises the heat transfer more on account of the movement of vortices along the baffle wall and creation of a secondary stream cell close to the leading end, which results in local wall turbulence. V-down pattern baffle of an extended angled baffle benefits in the pattern of two secondary stream jets as compared to single in case of an angled baffle resulting in still upper heat transfer rate. Making a discrete in the inclined baffle is found to augment the heat transfer by disturbing the secondary stream and produce advanced level of turbulence in the fluid downwards of the baffles. It is hypothesized that discrete V-pattern baffle will rise heat transfer compared to without discrete V-pattern baffle. To the best of the author’s knowledge, the open literature does not contain a similar experimental study on a rectangular air channel with discrete V-pattern baffle on the heated plate.

Experimental details

To study the outcome of discrete V-pattern baffle turbulent promoter on the Nusselt number and friction factor of air flow, an experimental set-up was intended and made up in accordance with the guidelines recommended in American Society of Heating, Refrigerating and Air-Conditioning Engineers (ASHRAE) standard.³⁵ A schematic illustration and pictorial view of experimental set-up are shown in Figure 1. The air channel is 2000 mm extended with a stream cross section of $300 mm \times 30 mm$ made up from ply panel of 20-mm thickness. The channel comprises 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/m K to minimize heat loss to the environment. An electric heating element was made up by combining six loops of nichromecable in series and parallel having a size of $1200 mm \times 300 mm$ to supply a kept heat flux of 1000 W/m² to the heated wall which is careful to be sensibly reasonable value of heat energy input for testing rectangular channel. The asbestos sheet is converted with strip of mica to keep the uniform distance between the wires and prevent back heating.

Figure 1.

Systematic diagram of experimental set-up.

A galvanized iron (GI) sheet of 18 SWG size black painted in order to facilitate the heat transfer is used as a heated wall. Discrete V-pattern baffles were pasted on the base of the heated wall by means of epoxy resin. This plate formed the top wall of the channel. The bottom side of the rectangular 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 flow rate of air through rectangular channel. The control valves provided in the lines control the flow. Copper–constantan thermocouples were used for air and absorber plate temperature measurement. Such thermocouples are usually recommended for temperature measurement in the range of 0°C–400°C.³⁶ The thermocouple output is measured by a Digital Micro Voltmeter, connected through a selector switch to designate the output of the thermocouples in degree centigrade. To ascertain the accuracy of temperature measurement, thermocouples have been calibrated under laboratory conditions against a dry block temperature calibrated instant.

Figure 2(a) shows the photograph of the calibrator. The thermocouple to be calibrated was placed in the calibration bath where a constant temperature is maintained and the responses of the thermocouple and the standard probe were noted with the help of a digital temperature indicator for various pre-set values of the standard probe, and the error between the reading of standard probe and the thermocouple was calculated. If this error is more than certain limit of the calibrator, then the thermocouple is rejected, and if this error is less than the tolerance limit of the calibrator then the thermocouple is accepted. This process was repeated in several steps of increasing as well as decreasing temperature range. The calibration curve is presented in Figure 2(b); temperature scanners have been used to display the temperature of the heated plate and the inlet and outlet of air. The pressure drop through the test section of the rectangular channel was obtained by a micro manometer having a least count of 0.01 mm.

Figure 2.

(a) Photographic view of dry block thermocouple calibrated and (b) calibration curve for thermocouple.

Range of parameters

The rectangular channel has an $L_{t}$ equal to 2000 mm while H is set equal to 30 mm and W is 300 mm; the hydraulic diameter $D_{hd}$ is equal to 54.54 mm. The baffle parameters are determined by baffle height $H_{b}$ , pitch of baffle $P_{b}$ , gap or discrete distance $D_{d}$ , gap or discrete width $g_{w}$ , length of V-pattern baffle $L_{v}$ , angle of attack $α_{a}$ and the shape of the roughness elements.

For an exact roughness type, a family of geometrically alike roughness is possible to recognize by altering angle of attack while maintaining stable $H_{b} / H$ , $P_{b} / H$ , $g_{w} / H_{b}$ and $D_{d} / L_{v}$ . The shape is shown in Figure 3(a) and (b). Table 2 gives the range of the parameters.

Figure 3.

(a) Discrete V-pattern baffle and (b) photographic view of discrete V-pattern baffle.

Table 2.

Range of parameters.

S. No.	Parameters	Range
1	Re	3000–21,000
2	$H_{b} / H$	0.50
3	$P_{b} / H$	1.5
4	$g_{w} / H_{b}$	1.0
5	$D_{d} / L_{v}$	0.67
6	$α_{a}$	30°–70°

Raw data collection

The data collected have been used to compute convective heat transfer coefficient, Nusselt number and friction factor. Relevant expressions for the computation of the above parameters and some intermediate parameters have been given below.

Temperature measured

Weighted average plate air temperature

The mean temperature of the plate is the average of all temperatures of the heated plate

T_{p} = \frac{\sum T_{p i}}{N}

(1)

The mean air temperature $T_{f}$ is a simple arithmetic mean of the measured values at the inlet and the exit temperature of air flowing through the test section

T_{f} = \frac{T_{i} + T_{o}}{2}

(2)

where $T_{o} = (T_{A 2} + T_{A 3} + T_{A 4} + T_{A 5} + T_{A 6}) / 5$ and $T_{i} = T_{A 1}$ .

Mass flow rate measurement

The mass flow rate of air $(m_{a})$ of air has been calculated from the pressure drop measurement through the calibrated orifice meter using the following formula

m_{a} = C_{do} A_{o} {[\frac{2 ρ_{a} {(Δ p)}_{0}}{1 - β^{4}}]}^{0.5}

(3)

where ${(Δ p)}_{0} = 9.81 \cdot {(Δ p)}_{0} \cdot ρ_{a} \cdot m_{a} \cdot \sin θ$ .

Velocity of air through channel

The velocity of air $(V)$ is calculated from the knowledge of mass flow rate of air $(m_{a})$ and the flow as

V = \frac{m_{a}}{ρ_{a} WH}

(4)

Equivalent hydraulic diameter

D_{hd} = \frac{4 \cdot (W \cdot H)}{2 \cdot (W + H)}

(5)

Reynolds number

The Reynolds number $(Re)$ of air flow in the channel is intended from

Re = \frac{V \cdot D_{hd}}{ν}

(6)

Friction factor

The friction factor $(f)$ is determined from the measured value of pressure drop ${(Δ_{p})}_{d}$ across the test section length using the Darcy equation as

f = \frac{2 {(Δ_{p})}_{d} D_{hd}}{4 ρ_{a} L_{t} V^{2}}

(7)

where ${(Δ_{p})}_{d} = 9.81 \cdot {(Δ_{h})}_{d} \cdot D_{hd} \cdot ρ_{a} \cdot m_{a}$ .

Heat transfer coefficient

The heat transfer rate $(Q_{u})$ from the absorber to the air is given by

Q_{u} = m_{a} c_{p} (T_{0} - T_{i})

(8)

The convective heat transfer coefficient $(h_{t})$ for the heated test section has been calculated as

h_{t} = \frac{Q_{u}}{A_{p} \cdot (T_{p} - T_{f})}

(9)

Nusselt number

The convective heat transfer coefficient $(h_{t})$ can be used to determine the Nusselt number, which is defined as

Nu = \frac{h_{t} D_{hd}}{K_{a}}

(10)

Validation of experimental data

The values of Nusselt number calculated through the experimental outcomes for a smooth channel have been compared with the results obtained from the Gnielinski empirical correlation and Dittus–Boelter equations (equations (11) and (12)) for the Nusselt number.

The Nusselt number for a smooth wall is given by the Gnielinski empirical correlation and Dittus–Boelter equations as

N u_{s} = \frac{(\frac{f}{8}) (Re - 1000) \Pr}{1 + 12.7 {(\frac{f}{8})}^{\frac{1}{2}} (P r^{\frac{2}{3}} - 1)} for 3000 < Re < 10, 000

(11)

N u_{s} = 0.023 R e^{0.8} P r^{0.4} for Re > 10, 000

(12)

where $f = {(0.79 \ln (Re) - 1.64)}^{- 2}$ .

The comparison of the experimental and the estimated outcomes of Nusselt number as a function of the Reynolds number is shown in Figure 4. It is observed that the outcomes obtained from the experimental results are in good agreement with the results obtained from equations (11) and (12). This ensures the correctness of the experimental outcomes obtained from this work. For the absolute percentage, deviation of the Nusselt number $(N u_{s})$ by the experimental results is 3.45% from the values predicted by equations (11) and (12).

Figure 4.

Comparison of experimental and predicted values of Nusselt number.

Uncertainty analysis

An uncertainty analysis has been done to determine the errors occurred in experimental data collection. The uncertainty is determined based on the errors associated with measuring equipment.³⁷ The maximum possible calculated errors in the values of major parameter are shown below:

Friction factor: $f_{b} = 3.46 %$

Heat transfer coefficient: $h_{t} = 5.75 %$

Mass flow rate: $m_{a} = 1.55 %$

Nusselt number: ${Nu}_{b} = 5.47 %$

Reynolds number: $Re = 5.76 %$

Results and discussion

The effect of angle of attack on Nusselt number and friction factor in a discrete V-pattern baffle channel was examined by plotting the baffle Nusselt number and friction factor as a function of Reynolds number. The outcome concerning the discrete V-pattern baffle channel has been compared with those obtained without baffle channel under similar operating conditions in order to find the augmentation of heat transfer.

Heat and fluid flow

The current experimental outcomes on heat transfer and friction factor in a constant heat flux rectangular channel with discrete V-pattern baffle are obtainable in the form of Nusselt number and friction factor in order to understand the outcome of Reynolds number and angle of attack in discrete V-pattern baffle on Nusselt number. The Nusselt numbers have been plotted as a function of angle of attack for the value of Reynolds number as shown in Figure 5. The other values of baffle parameters are kept as $H_{b} / H = 0.50$ , $P_{b} / H = 1.5$ , $g_{w} / H_{b} = 1.0$ and $D_{d} / L_{v} = 0.67$ .

Figure 5.

Effect of Reynolds number on Nusselt number.

The Nusselt numbers obtained under turbulent stream conditions in all cases are represented in Figure 5. The Nusselt number rises with rise in Reynolds number as expected, due to rise in turbulent intensity with streamrise in Reynolds number, which leads to superior heat transfer. This is observed from Figure 5 that flow angle of attack value of 60° yields the greatest heat transfer enhancement. It has been observed that the Nusselt number rises with rise in angle of attack and attains a superior value corresponding to angle of attack value of $60^{\circ}$ in the range of parameters considered. With further rise in the value of angle of attack, the value of Nusselt number reduces. The roughness with angle of attack of 60° shows a higher heat transfer rate and an enhancement of around 4.2 times higher as compared without baffle channel.

The reason for Nusselt number attaining a superior value corresponding to angle of attack of 60° is the separation of the secondary stream resulting through the presence of the baffle and the displacement of the resulting vortices combining jointly to yield best outcome of angle of attack. Due to this reason, by angling the baffle, the flow can move along the baffle with the fluid entry near the leading end of the baffle and coming out near the trailing end, and subsequently joining the mainstream, producing spanwise rotating secondary stream, which are accountable for the significant spanwise variation in the local Nusselt number. V-pattern of a long angled baffle helps in the formation of two secondary streams as compared to the one in the case of an angled baffle resulting in still superior Nusselt number.

Making discrete in the V-pattern baffle allows the release of the secondary stream which mixes with the mainstream through the discrete as shown in Figure 6. This results in its acceleration, which energizes the retarded boundary layer stream besides the wall resulting in the rise in the Nusselt number through the discrete width area behind the baffle. Figure 7 clearly shows that the maximum and minimum values of Nusselt number for discrete V-pattern baffle air channel occur for the angle of attack of 60° and 30°, respectively.

Figure 6.

Secondary stream pattern for discrete V-pattern baffle.

Figure 7.

Effect of angle of attack on Nusselt number.

Enhancing the Nusselt number by application of baffle roughness will rise the friction factor which boosts the pumping power needs. The variation in friction factor as a function of Reynolds number for different values of angle of attack and kept values of other roughness shape parameters is shown in Figure 8. The value of friction factor rises with an increase in angle of attack and attains an utmost value corresponding to angle of attack value of 60° and decreases with further rise in angle of attack value. In order to elaborate the effect of angle of attack on friction factor, Figure 8 has been replotted as Figure 9. The friction factor rises with rise in angle of attack, attains a maximum value corresponding to angle of attack value of 60° and reduces with further rise in angle of attack value. The least and maximum values of friction factor have been obtained corresponding to angle of attack values of 30° and 60° as compared to other baffle shapes’ rectangular channel.

Figure 8.

Effect of Reynolds number on friction factor.

Figure 9.

Effect of angle of attack on friction factor.

Thermohydraulic performance

The experimental outcomes observed rise in Nusselt number with rise in angle of attack; however, friction factor also rises. The rectangular channel efficiency consequently depends on these two parameters: $N u_{b} / N u_{s}$ and $f_{b} / f_{s}$ . The overall thermal performance parameter was defined as the overall enhancement ratio and expressed as follows^{2,5,7, 20}

η = {\frac{(N u_{b} / N u_{s})}{(f_{b} / f_{s})}}^{0.33}

(13)

Figure 10 shows the overall thermal performance parameter for the rectangular channel with various values of angle of attack for Reynolds number range from 3000 to 21,000. It rises with rises in the angle of attack up to about 60° and then decreases with further rises in the angle of attack at all Reynolds number values. It therefore attained utmost at an angle of attack of about 60°. In order to elaborate the effect of angle of attack on $η = (N u_{b} / N u_{s}) / {(f_{b} / f_{s})}^{0.33}$ , Figure 11 has been replotted as Figure 10. The $η = (N u_{b} / N u_{s}) / {(f_{b} / f_{s})}^{0.33}$ rises with rise in angle of attack, attains a maximum value corresponding to angle of attack value of 60° and reduces with further rise in angle of attack value. The least and maximum values of $η = (N u_{b} / N u_{s}) / {(f_{b} / f_{s})}^{0.33}$ have been obtained corresponding to angle of attack values of 30° and 60°, respectively.

Figure 10.

Effect of angle of attack on thermohydraulic performance with Reynolds number.

Figure 11.

Effect of angle of attack on thermohydraulic performance.

The data of $η = (N u_{b} / N u_{s}) / {(f_{b} / f_{s})}^{0.33}$ determined for the shape of discrete V-pattern baffle have been compared with the data determined for angled baffle,¹³ continuous V-pattern baffle,¹⁵ perforated baffle,²² transverse perforated baffle,²⁵ delta shaped baffle²⁷ and V-pattern perforated baffle.³⁵ It can be observed that from Figure 12. The discrete V-pattern baffle shape results in the most excellent thermohydraulic performance among all the shapes investigated.

Figure 12.

Comparison of various baffle rectangular channels.

Conclusion

An experimental analysis has been carried out to examine air flow friction and heat transfer rate in a high aspect ratio $W / H = 10$ fixed with discrete V-pattern baffle for the turbulent stream regime, Reynolds number of 3000–21,000. Making a discrete in V-pattern baffle results in considerable improvement in heat transfer coefficient of air flow channel; the enhancement is a strong function of discrete distance and size. The Nusselt number augmentation tends to rise with the rise in Reynolds number. The Nusselt number of roughened rectangular channel as compared to that of without baffle channel has been found to be increased between 4.2 and 5.9 for the range of parameters investigated. Both the Nusselt number and friction factor are strong functions of angle of attack. The use of the discrete V-pattern baffle with angle of attack of 60° causes a very high Nusselt number and friction factor as compared with other values of angle of attack. The upper most thermohydraulic performance occurs at angle of attack of 60°. Discrete V-pattern baffle has better thermal performance as compared to other baffle shapes’ rectangular channel.

Footnotes

Appendix 1

Academic Editor: Oronzio Manca

Declaration of conflicting interests

The author(s) declared no potential conflicts of interest with respect to the research, authorship, and/or publication of this article.

Funding

The author(s) received no financial support for the research, authorship, and/or publication of this article.

References

Kumar

Sani

Saini

JS.

Experimental investigation on heat transfer and fluid flow characteristics of air flow in a rectangular duct with multi V-shaped rib with gap roughness on the heated plate. Sol Energy 2012; 86: 1733–1749.

Kumar

Kim

MH.

Thermo hydraulic performance of rectangular ducts with different multiple V-rib roughness shapes: a comprehensive review and comparative study. Renew Sust Energ Rev 2016; 54: 635–652.

Kumar

Saini

JS.

Numerical optimization of effective efficiency of a discrete multi V-rib solar air channel. Heat Mass Transfer 2015; 12: 1–15.

Patankar

Liu

Sparrow

EM.

Fully developed flow and heat transfer in ducts having stream wise-periodic variations of cross-sectional area. J Heat Transf 1977; 99: 180–186.

Kumar

Saini

JS.

Heat and fluid flow characteristics of roughened solar air heater duct: a review. Renew Energ 2012; 47: 77–94.

Kumar

Kim

MH.

Convective heat transfer enhancement in solar air channels. Appl Therm Eng 2015; 89: 239–261.

Kumar

Sani

Saini

JS.

Experimental investigations on thermohydraulic performance due to flow- attack- angle in multiple V-ribs with gap in a rectangular duct of solar air heaters. J Sustain Energy Environ 2013; 4: 1–7.

Berner

Durst

McEigot

DM.

Flow around baffles. J Heat Transf 1984; 106: 743–749.

Kumar

Kim

MH.

Effect of roughness width ratios in discrete multi V-rib with staggered rib roughness on overall thermal performance of solar air channel. Sol Energy 2015; 119: 399–414.

10.

Kumar

Sani

Saini

JS.

An experimental investigation of enhanced heat transfer due to a gap in a continuous multiple V-rib arrangement in a solar air channel. J Enhanc Heat Transf 2014; 1: 22–49.

11.

Molki

Mostoufizadesh

RA.

Turbulent heat transfer in rectangular ducts with repeated-baffle blockages. Int J Heat Mass Tran 1989; 32: 1491–1499.

12.

Yeh

Lin

CY.

The influence of collector aspect ratio on the collector efficiency of baffled solar air heaters. Energy 1998; 23: 11–16.

13.

Won

Burgess

Peddicord

. Spatially resolved surface heat transfer for parallel rib turbulators with 45 deg orientations including test surface conduction analysis. J Heat Transf 2004; 126: 193–201.

14.

Tandiroglu

Effect of flow geometry parameters on transient heat transfer for turbulent flow in a circular tube with baffle inserts. Int J Heat Mass Tran 2006; 49: 1559–1567.

15.

Khanoknaiyakarn

Kwankaomeng

Promvonge

Thermal performance enhancement in solar air heater channel with periodically V-shaped baffles. In: International conference on utility exhibition on power and energy systems issues and prospectus for Asia, Pattaya City, Thailand, 28–30 September 2011. New York: IEEE.

16.

Lin

CW.

Experimental study of thermal behavior in a rectangular channel with baffles of pores. Int Commun Heat Mass 2006; 33: 985–992.

17.

Nasiruddin

Siddiqui

Heat transfer augmentation in a heat exchanger tube using a baffle. Int J Heat Fluid Fl 2001; 28: 318–328.

18.

Romdhane

BS.

The air solar collectors: comparative study, introduction of baffles to favour the heat transfer. Sol Energy 2007; 81: 139–149.

19.

Nie

Chen

Hsieh

HT.

Effect of a baffle on separated convection flow adjacent to backward-facing step. Int J Therm Sci 2009; 48: 618–625.

20.

Promvonge

Jedsodaratanachi

Kwankaomeng

Heat transfer and pressure drop in a channel with multiple 60° V-baffles. Int Commun Heat Mass 2010; 37: 835–840.

21.

Sripattanapipat

Promvonge

Numerically analysis of laminar heat transfer in a channel with diamond shaped baffles. Int Commun Heat Mass 2009; 36: 32–38.

22.

Karwa

Maheshwari

BK.

Heat transfer and friction in an asymmetrically heated rectangular channel with half and fully perforated baffles at different pitches. Int Commun Heat Mass 2009; 36: 264–268.

23.

Akpinar

Kocyigit

Energy analysis of a new flat plate solar air heater having different obstacles on absorber plates. Appl Energ 2010; 87: 3438–3450.

24.

Sriromreum

Thianpong

Promvonge

Experimental and numerical study on heat transfer enhancement in a channel with Z-shaped baffles. Int Commun Heat Mass 2012; 39: 945–952.

25.

Shin

Kwak

JS.

Effect of hole shape on the heat transfer in a rectangular channel with perforated blockage walls. J Mech Sci Technol 2008; 22: 1945–1951.

26.

Mohammadi

Sabzpooshani

Comprehensive performance evaluation and parametric studies of single pass solar air heater with fins and baffles attached over the absorber plate. Energy 2013; 57: 741–750.

27.

Bekele

Mishra

Dutta

Performance characteristics of solar air heater with surface mounted obstacles. Energ Convers Manage 2014; 85: 603–611.

28.

Bekele

Mishra

Dutta

Effects of delta-shaped obstacles on the thermal performance of solar air heater. Adv Mech Eng 2011; 3: 103502.

29.

Promvonge

Kwankaomeng

Periodic laminar flow and heat transfer in a channel with 45° staggered V-baffles. Int Commun Heat Mass 2010; 37: 841–849.

30.

Sara

Pekdemir

Yapici

Heat transfer enhancement in a channel flow with perforated rectangular blocks. Int J Heat Fluid Fl 2001; 22: 509–518.

31.

Karwa

Maheshwari

Karwa

Experimental study of heat transfer enhancement in an asymmetrically heated rectangular duct with perforated baffles. Int Commun Heat Mass 2005; 32: 275–284.

32.

Mousavi

Hooman

Heat and fluid flow in entrance region of a channel with staggered baffles. Energ Convers Manage 2006; 47: 2011–2019.

33.

Chamoli

Thakur

NS.

Correlations for solar air heater duct with V-shaped perforated baffles as roughness elements on absorber plate. Int J Sustain Energ 2013; 35: 1–20.

34.

Alam

Saini

JS.

Experimentally investigation on heat transfer enhancement due to V-shaped perforated blocks in a rectangular duct of solar air heater. Energ Convers Manage 2014; 81: 374–383.

35.

ASHRAE Standard 93:2003. Method of testing to determine the thermal performance of solar collectors.

36.

Benedict

RP.

Fundamental of temperature pressure and flow measurements. 3rd ed.New York: John Wiley & Sons, 1984.

37.

Klein

McClintock

The description of uncertainties in a single sample experiments. Mech Eng 1953; 75: 3–8.

Experimental investigation of effect of flow attack angle on thermohydraulic performance of air flow in a rectangular channel with discrete V-pattern baffle on the heated plate

Abstract

Keywords

Introduction

Experimental details

Range of parameters

Raw data collection

Temperature measured

Weighted average plate air temperature

Mass flow rate measurement

Velocity of air through channel

Equivalent hydraulic diameter

Reynolds number

Friction factor

Heat transfer coefficient

Nusselt number

Validation of experimental data

Uncertainty analysis

Results and discussion

Heat and fluid flow

Thermohydraulic performance

Conclusion

Footnotes

Appendix 1

Declaration of conflicting interests

Funding

References