Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators

International Communications in Heat and Mass Transfer 50 (2014) 34–43

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Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators☆ Sompol Skullong a,⁎, Sutapat Kwankaomeng b, Chinaruk Thianpong b, Pongjet Promvonge b a b

Department of Mechanical Engineering, Faculty of Engineering at Si Racha, Kasetsart University Si Racha Campus, 199 M.6, Tungsukhla, Si Racha, Chonburi 20230, Thailand Department of Mechanical Engineering, Faculty of Engineering, King Mongkut's Institute of Technology Ladkrabang, Bangkok 10520, Thailand

a r t i c l e

i n f o

Available online 14 November 2013 Keywords: Thermal performance Solar air heater Rib Groove Channel flow Turbulator

a b s t r a c t The paper presents an experimental study on turbulent flow and heat transfer characteristics in a solar air heater channel fitted with combined wavy-rib and groove turbulators. The experiments are performed by controlling the airflow rate to obtain Reynolds numbers in the range of 4000 to 21,000. To produce recirculation flow in the tested channel having a constant heat-flux on the upper wall only, the triangular wavy ribs are placed repeatedly on the tested grooved channel walls. Three test cases of different rib-pitch to channel-height ratios (PR = P/ H = 0.5, 1 and 2) with a single rib-to-channel height ratio (BR = b/H = 0.25) are introduced in the present work. The wavy ribs are placed with the attack angle of 45° relative to main flow direction. There are three types of rib arrangements, namely, rib-groove on the upper wall only, inline rib-groove, and staggered ribinline groove on two principal walls. The experimental result reveals that the combined rib-groove on both the upper and lower walls of the test channel provides the highest heat transfer rate and friction factor in comparison with the smooth channel with/without ribs. However, the ribbed-grooved upper wall at PR = 0.5 yields the highest thermal performance. The combined rib-groove turbulator is found to be considerably higher thermal performance than the groove alone. © 2013 Elsevier Ltd. All rights reserved.

1. Introduction Rib/groove is one of the commonly used passive heat transfer enhancement techniques in single-phase internal flows in a channel solar air heater by placing the rib/groove periodically in the absorber plate. For decades, several engineering techniques have been developed for enhancing the convective heat transfer rate from the channel surface. The turbulators used for the cooling/heating channel or channel solar air heater such as ribs [1], fins [2,3], grooves [4,5] or baffles [6,7] are often encountered in order to increase the convective heat transfer coefficients leading to the compact heat exchanger and increasing the efficiency. The reason of this may be that the use of ribs/grooves completely makes the change of the flow field and thus the distribution of the local heat transfer coefficient. The application of rib-groove into the channel is to provide an interruption of boundary layer development, to increase the heat transfer surface area and to cause enhancement of heat transfer by increasing turbulence intensity or fast fluid mixing. Therefore, more compact and economic heat exchanger with lower operation cost can be obtained. In general, the geometry parameters of ribs in the channel are among the most important factor in the design of channel heat exchangers which effects on both local and

☆ Communicated by W.J. Minkowycz ⁎ Corresponding author. E-mail address: [email protected] (S. Skullong). 0735-1933/$ – see front matter © 2013 Elsevier Ltd. All rights reserved. http://dx.doi.org/10.1016/j.icheatmasstransfer.2013.11.001

overall heat transfer coefficients. In particular, the angled rib, rib blockage ratio (BR = b/H), rib pitch ratio (PR) and rib arrangement are all parameters that influence both the heat transfer coefficient and the overall thermal performance. Several studies have been carried out to investigate the effect of these parameters of ribs on heat transfer and friction loss for two opposite roughened surfaces. Han et al. [8] studied experimentally the heat transfer in a square channel with ribs on two walls for nine different rib configurations for P/b = 10 and b/H = 0.0625. They found that the angled ribs and ‘V’ ribs yield higher heat transfer enhancement than the continuous ribs and the heat transfer rate and the friction factor were highest for the 60° orientation amongst the angled ribs. For heating either only one of the ribbed walls or both of them, or all four channel walls, Han et al. [9] also reported that the former two conditions resulted in an increase in the heat transfer with respect to the latter one. Han and Zhang [10] studied the heat transfer augmentation in a square channel with various broken ribs of b/H = 0.0625 and P/b = 10 on two channel walls. They found that 60° broken ‘V’ ribs provide higher heat transfer at about 4.5 times the smooth channel and perform better than the continuous ribs. By using a real time Laser Holographic Interferometry to measure the local as well as average heat transfer coefficient, Liou and Hwang [11,12] investigated experimentally the performance of square, triangular and semi-circular ribs and found that the square ribs give the best heat transfer performance among them. This is contrary to the experimental result of Ahn [13] indicated that the triangular rib performs better than the square one.

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Nomenclature A AR b BR Cp D e f H h I k L m˙ Nu P ΔP PR Pr Re Q T TEF t U V W w

convection heat transfer area of channel, m2 aspect ratio of channel, (W/H) rib height, m rib blockage ratio, (b/H) specific heat capacity of air, J/kgK hydraulic diameter of channel, m groove depth, m friction factor channel height, m average heat transfer coefficient, W/m2K current, A thermal conductivity of air, W/mK length of test channel, m mass flow rate of air, kg/s Nusselt number, (hD/k) rib pitch spacing (axial length of spacing), m pressure drop, Pa rib pitch to channel height ratio, (P/H) Prandtl number Reynolds number, (UD/ν) heat transfer, W temperature, K thermal performance enhancement factor thickness of rib, m mean air velocity, m/s voltage, volt width of channel, m width of groove, m

Greek letters α attack angle of rib, degree ρ density of air, kg/m3 ν kinematics viscosity, m2/s

Subscripts b bulk 0 smooth channel conv convection i inlet o out pp pumping power s channel surface

Taslim et al. [14] examined the heat transfer behaviors in a ribbed square channel with three b/H ratios (b/H = 0.083, 0.125 and 0.167) and a fixed P/b = 10 using a liquid crystal technique. They reported that the average Nusselt number was increased with the rise in b/H ratio and the best b/H ratio was found to lie between 0.083 and 0.125. Tanda [15] examined the effect of transverse, angled ribs, discrete, angled discrete ribs, V-shaped, V-shaped broken and parallel broken ribs on heat transfer and friction and reported that 90° transverse ribs provided the lowest thermal performance while the 60° parallel broken ribs or 60° V-shaped broken ribs yielded a higher heat transfer augmentation than the 45° parallel broken ribs or 45° V-shaped broken ribs. Parallel angled discrete ribs were seen to be superior to parallel angled full ribs and its 60° discrete ribs performed the highest heat transfer. Promvonge and Thianpong [16] experimentally studied the thermal performance of wedge ribs pointing upstream and downstream, triangular and rectangular ribs with b/H = 0.3 and P/b = 6.67 mounted on the two opposite walls of a channel with AR = 15. They found that

35

the in-line wedge rib pointing downstream performed the highest heat transfer but the best thermal performance is the staggered triangular rib. Thianpong et al. [17] again investigated the thermal behaviors of isosceles triangular ribs attached on the two opposite channel walls with AR = 10 and suggested the optimum thermal performance of the staggered ribs could be at about b/H = 0.1 and P/H = 1.0. Extensive literature reviews over hundred references on various rib turbulators were reported by Varun et al. [18] and Han et al. [19]. Momin et al. [20] experimentally studied on heat transfer and flow characteristics of a solar air heater duct fitted with V-shaped ribs for b/D = 0.02–0.034 and the angle of attack (α) = 30° − 90° for a fixed P/b = 10. They found that at α = 60°, the highest Nusselt number and friction factor values obtained by the ribs are, respectively, 2.30 and 2.83 times above the smooth duct. For using combined/compound turbulators, Chompookham et al. [21] and Promvonge et al. [22] experimentally investigated the effect of using winglet vortex generators in common with several ribs on heat transfer and friction characteristics in an uniform heat flux channel and found that the heat transfer rate increases considerably for using both enhancement devices and is about 50–80% of using a single enhancement device. Promvonge et al. [23] examined numerically the laminar heat transfer enhancement in a square channel with 45° inclined baffle on one wall and reported that a single streamwise vortex flow occurs throughout the channel and helps to induce impingement jets on the upper, lower and side walls. Again, Promvonge et al. [24,25] also investigated numerically the laminar flow structure and thermal behaviors in a square channel with 30° or 45° inline baffles on two opposite walls. They found that two streamwise counter-rotating vortex flows appear along the channel and vortex-induced impinging jets occur on the upper, lower and side walls. For a system with only one roughened wall and three smooth walls, several investigations [26–30] have been carried out on rib roughened absorber plates of solar air heaters. Correlations for heat transfer coefficient and friction factor have been developed for such a system. However, the increase in heat transfer is accompanied by an increase in the resistance of fluid flow. From the literature review above, most of the investigations are focused only on the single use of the turbulators or the combined rib/ baffle and winglet, providing a similar flow pattern in the channel. A combination of rib/baffle and groove in the channel has rarely been reported. Thus, the main aim of the present work is to extend the experimental data of using the combined rib-groove on the channel walls for three different rib pitch ratios (PR = 0.5, 1 and 2) at a single rib height, BR = 0.25. Also, three types of mounting wavy ribs on the grooved walls, namely, rib-groove on the upper wall only, inline rib-groove and staggered rib-inline groove on two opposite walls on a high aspect ratio (AR = 10) rectangular channel are introduced. The use of the ribgroove turbulators is expected to create vortex flows throughout the tested channel to stronger mixing of flows between the core and the near-wall regimes leading to higher heat transfer rate in the channel. Experimental results using air as the test fluid for the rib-grooves are presented in turbulent flows in a range of Re from 4000 to 21,000. 2. Experimental setup The experiments were conducted to examine the effect of using combined the wavy-rib and transverse groove on heat transfer and friction characteristics of a channel solar air heater. A schematic diagram of the experimental apparatus is presented in Fig. 1a while the detail of the 45° triangular wavy rib and transverse groove placed on the upper wall only, in-line and staggered rib arrays is displayed in Fig. 1b. Three test cases of different rib-pitch to channel-height ratios (PR = 0.5, 1 and 2) with a single rib-to-channel height ratio (BR = 0.25) are considered in the present work. The form of grooved plates was accomplished by means of wire-EDM (electrical discharge machine) machining with its dimension of 5 mm depth (e), 10 mm width (w) with a single groove

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S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

Fig. 1. (a) Schematic diagram of experimental apparatus and (b) test section with 45° wavy-rib and transverse-groove.

pitch of 40 mm. The 45° wavy-rib (thin-rib) strip is 5 mm high (b) and 0.3 mm thick (t). The wavy-ribs made of a 0.3 mm aluminum strip were attached on the aluminum grooved plates with hot superglue as depicted in Fig. 2. In the experimental setup, a circular pipe was used for connecting a high-pressure blower to a settling tank, which an orifice flow meter was mounted in this pipeline while a rectangular channel including a calm section and a test section was employed following the settling tank. The rectangular duct configuration was characterized by the channel height (H) of 20 mm and the channel width (W) of 200 mm. The overall length of the duct was 2000 mm which included length of the test section (L) of 650 mm with a constant heat-flux on the upper wall or the absorber plate, as can be seen in Fig. 2. In the experiment, a combination of the two phenomena; (1) the re-circulating/reverse flow induced by the grooves and (2) the vortex flows created by the ribs, is supposed to be effective in the vicinity of the absorber plate. In addition, both turbulators are expected to provide stronger mixing of the fluid between the core and the near-wall, thereby enhancing the heat transfer rate.

The AC power supply was the source of power for the plate-type heater, used for heating the upper plate of the test section only to maintain a uniform heat-flux on the absorber plate. Air as the test fluid, was directed into the systems by a 1.45 kW high-pressure blower. The operating speed of the blower was varied by using an inverter to provide desired airflow rates. The flow rate of air in the systems was measured by a calibrated orifice plate. The pressure across the orifice was measured using inclined manometer. In order to measure temperature distributions on the principal upper wall, twelve thermocouples were fitted to the wall. The thermocouples were mounted in holes drilled from the rear face and centered of the wall with the respective junctions positioned within 2 mm of the inside wall and axial separation was 40 mm apart. To measure the inlet and outlet temperatures, two RTD (Pt100) thermocouples were positioned upstream and downstream of the test channel. The thermocouple voltage outputs were fed into a data acquisition system (Fluke 2650A) and then recorded via a computer. Two static pressure taps were located at the top of the principal channel wall to measure axial pressure drops across the test section,

S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

used to evaluate average friction factor. These were located at the center line of the channel. One of these taps is 50 mm upstream from the leading edge of the test channel and the other is 50 mm downstream from the trailing edge. The pressure drop was measured by a digital differential pressure and a data logger connected to the 2 mm diameter taps and recorded via a laptop. To quantify the uncertainties of measurements the reduced data obtained experimentally were determined. The uncertainty in the data calculation was based on Ref. [31]. The maximum uncertainties of non-dimensional parameters were ±5% for Reynolds number, ±8% for Nusselt number and ±10% for friction. The uncertainty in the axial velocity measurement was estimated to be less than ±7%, and pressure has a corresponding estimated uncertainty of ± 5%, whereas the uncertainty in temperature measurement at the channel wall was about ± 0.5%. 3. Data processing The goal of this experiment is to investigate the Nusselt number in a solar air heater channel using rib and groove turbulators. The independent parameters are Reynolds number and rib-groove arrangements. The Reynolds number based on the channel hydraulic diameter is given by Re ¼ UD=ν

ð1Þ

The local heat transfer coefficients are evaluated from the measured temperatures and heat inputs. With heat added uniformly to fluid (Qair) and the temperature difference of wall and fluid (Tw − Tb), average heat transfer coefficient will be evaluated from the experimental data via the following equations: Q air ¼ Q conv ¼m˙C p ðT o −T i Þ ¼ VI−Q loss Z Q conv ¼ W

37

The thermal enhancement factor, TEF, defined as the ratio of the heat transfer coefficient, h of an augmented surface to that of a smooth surface, h0, at an equal pumping power: TEF ¼

h Nu j ¼ j ¼ h0 pp Nu0 pp



Nu Nu0



f f0

−1=3

:

ð8Þ

4. Results and discussion 4.1. Validation of smooth channel The present experimental results on heat transfer and friction characteristics in a smooth channel are first validated in terms of Nusselt number, (Nu) and friction factor, (f). The Nu and f obtained from the present smooth channel are compared with those from the correlations of Dittus–Boelter, Gnielinski; Blasius and Petukhov found in the literature [32] for turbulent flow in ducts. Correlation of Dittus–Boelter, 0:8

Nu ¼ 0:023Re

0:4

Pr

ð9aÞ

for heating

Correlation of Gnielinski, Nu ¼

ð f =8ÞðRe−1000ÞPr   1 þ 12:7ð f =8Þ1=2 Pr2=3 −1

6

for 3000bReb5  10

Correlation of Blasius, −0:25

f ¼ 0:316Re

for 3000≤Re≤20; 000

ð2Þ

  hx ðT s −T bx Þdx ¼ W ðT sx −T bx ÞhL ¼ Tes −T b Ah:

Thus, Q h ¼  conv  A Tes −T b

ð3Þ

in which, T b ¼ ðT o þ T i Þ=2

ð4Þ

and Tes ¼

X

T s =12:

ð5Þ

The term A is the convective heat transfer area of the upper channel wall whereas Tes is the average surface temperature obtained from local surface temperatures along the axial length of the heated channel. Then, average Nusselt number is written as: Nu ¼

hD : k

ð6Þ

The friction factor is evaluated by: f ¼

2 ΔP ðL=DÞ ρU 2

ð7Þ

where ΔP is a pressure drop across the test section and U is the mean air velocity in the channel. All of thermo-physical properties of the air are determined at the overall bulk air temperature.

ð9bÞ

Fig. 2. Test section with (a) in-line and (b) staggered arrays.

ð10aÞ

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Correlation of Petukhov, −2

f ¼ ð0:79 ln Re−1:64Þ

for

6

3000 ≤Re≤5  10

ð10bÞ

Fig. 3a and b shows, respectively, a comparison of Nu and f obtained from the present work with those from correlations of Eqs. (9a)–(9b) and (10a)–(10b). In Fig. 3a, the present smooth channel results are in excellent agreement with the correlations data within ±4% and ±6%, for the Nu correlations of Dittus–Boelter and Gnielinski, respectively. In Fig. 3b, the present smooth channel results are also in excellent agreement within ±4% and ±5%, with the f correlations of Petukhov and of Blasius. 4.2. Effect of rib and groove The present results on heat transfer and flow friction characteristics in the absorber plate of a solar air heater channel with 45° wavy-ribs

mounted repeatedly on the groove plate are presented in terms of Nu and f. The variation of Nu obtained under a turbulent regime with Reynolds number (Re) is presented in Fig. 4. In the figure, the rib-groove turbulators yield a substantial heat transfer enhancement with a similar trend in comparison with the smooth channel and the Nu increases with the increment of Re. This is because the wavy-rib can generate longitudinal vortex flows that help to increase the flow turbulence degree and to transport the central core flow to the near-wall region. Also, the vortex flows can wash up the flow trapped in the groove-corner regions which normally act as ineffectively heat transfer areas, leading to higher heat transfer rate in the channel. It is found that the Nu increases with the rise of Re. Importantly, the inline rib-groove provides the highest heat transfer and the staggered rib and inline groove performs better than the ribbed-grooved upper wall. The heat transfer rate of the combined rib-groove turbulator is found to be much higher than that of the single groove or the inline grooves alone. At PR = 0.5, the increases in Nu for using the in-line, staggered rib-groove and

(a)

(b)

Fig. 3. Validation of (a) Nu and (b) f for smooth channel.

S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

39

Fig. 4. Variation of Nu with Re for various rib-groove turbulators.

ribbed-grooved upper walls are, respectively, in the range of 707– 769%, 672–725% and 642–676% above the smooth channel. The increases in Nu for the inline groove and the groove on one wall are about 168–172% and 154–159% higher than the smooth channel, respectively. The effect of using the rib-groove turbulator on the isothermal pressure drop across the tested channel is presented in Fig. 5. The variation of the pressure drop is shown in terms of f with Re. In the figure, it is apparent that the use of the combined wavy-rib and groove leads to a considerable increase in f over that of the groove alone and the smooth channel. The increase rate in f for the rib-groove turbulators is much higher than that for the smooth channel and is also higher than that in Nu, however. The inline rib-groove yields much higher f than the staggered rib-groove and the rib-groove upper wall. As expected, the f

obtained from the rib and groove is significantly higher than that from the groove alone, especially for the smaller PR, (PR = 0.5). The increase in f of the combined rib-groove is in a range of 14–134 times over the smooth channel, depending on the PR, the array and Re values. The f value of the rib-groove is found to be higher than that of the groove alone around 38–73%. The losses may come from the dissipation of the dynamical pressure, the reverse flow and increasing surface area due to of the presence of the rib-groove turbulator. Significantly, the inline rib-groove with PR = 0.5 provides higher f than that with PR = 1 or 2. At PR = 0.5, the increases in f for the in-line, staggered and rib-groove on one wall are in a range of 93–134%, 86–119% and 57–83% over the smooth channel, respectively. Also, the increases in f for the inline groove and the groove on one wall are about 1.8–2.3%, 1.3–1.6% over the smooth channel, respectively.

Fig. 5. Variation of f with Re for various rib-groove turbulators.

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S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

4.3. Effect of pitch ratio Effects of the rib pitch spacing ratio, PR, on the Nu and f are also presented in Figs. 4 and 5, respectively. In Fig. 4, the combined rib and groove provides a considerable heat transfer rate with a similar trend in comparison with the single groove/smooth channel alone and the Nu tends to increase with reducing the PR. The use of smaller PR interrupts the development of the boundary layer of the fluid flow and increases the turbulence intensity of the flow. It is worth noting that the heat transfer rates for the rib-groove at PR = 0.5 are, respectively, about 17% and 34% higher than those with PR = 1 and 2 for all arrangements. This caused by the PR = 0.5 interrupting the flow and promoting higher levels of turbulence mixing apart from providing higher vortex strength over the others. The influence of the combined rib and groove with different PRs on the isothermal pressure drop across the tested channel is also displayed in Fig. 5. The variation of the pressure drop is shown in the form of f against Re values. In the figure, it is observed that the rib-groove turbulator yields a substantial increase in f over the groove alone and smooth channel with no rib. The f obtained from the PR = 0.5 is approximately 22% and 44% higher than that from the PR = 1 and 2, respectively. Thus, the increase in the PR in order to reduce the f in the channel is not effective in this case due to the similar decrease in both heat transfer and friction loss. 4.4. Performance evaluation The Nusselt number ratio, Nu/Nu0, defined as a ratio of the augmented Nu to the Nu of smooth channel plotted against the Re value is depicted in Fig. 6. In the figure, the Nu/Nu0 tends to decrease slightly with the increment of Re for using the rib-groove turbulator. It is interesting to note that the Nu/Nu0 values of the in-line rib-groove are higher than those of the staggered, the rib-groove on one wall and the groove alone. A close examination reveals that the rib-groove turbulator at PR = 0.5 provides higher Nu/Nu0 values of about 8–11% than the one with PR = 1 or 2 at a given Re value. The average Nu/Nu0 values for the in-line, staggered and rib-groove on one wall are, respectively, around 7.3, 6.9, and 6.5; 6.1, 5.6 and 5.2; and 4.8, 4.5 and 4.2 at the PR = 0.5, 1 and 2. The average Nu/Nu0 values for the inline groove and groove on one wall are, respectively, around 1.7 and 1.55.

Fig. 7 presents the variation of the friction factor ratio, f/f0, with the Re value. It can be observed that the f/f0 tends to increase with the rise of Re for all. The in-line rib-groove at smaller PR provides a considerable increase in the f/f0 above the staggered and the rib-groove upper wall under the same operating condition. The inline rib-groove turbulator with PR = 0.5 provides the f/f0 of about 22–44% higher than the one with PR = 1 or 2 at a given Re value. The mean f/f0 values for the inline, staggered rib-groove and the rib-groove on one wall are, respectively, about 121.4, 108.9, and 75.2; 95.3, 85.7 and 31.6; and 67.7, 55.4 and 18.9 for PR = 0.5, 1 and 2 in the range of Re studied. This implies that the larger PR and the rib-groove on one wall can help to reduce the pressure loss considerably. The average f/f0 values for the inline groove and groove on one wall are, respectively, around 2.14 and 1.51. The variation of the thermal enhancement factor (TEF) with the Re values for all turbulators is displayed in Fig. 8. In the figure, the data obtained by the measured Nu and f values are compared at an identical pumping power condition as defined in Eq. (8). It is visible in the figure that the TEF values for the combined rib and groove turbulator generally are found to be above unity and much higher than those for employing a single use of the turbulators. This means that the use of the groove in conjunction with the wavy rib leads to the advantage over that of a single turbulator. The TEF tends to reduce with the increase in Re for all turbulators applied. The rib-groove turbulator on one wall with PR = 0.5 gives the highest TEF at lower Re. This is because of lower flow blockage from using a single rib-groove and thus, the larger BR of the rib should be avoided. It is worth noting that the maximum TEF is found for using the rib-groove on one wall at PR = 0.5. In the current study, the rib-groove on one wall provides higher TEF than the in-line and staggered ones. For the rib-groove on one wall at PR = 0.5, 1 and 2, the maximum TEFs are, respectively, about 1.75, 1.57 and1.55. At a given Re value, the use of PR = 0.5 yields the TEF around 7–10% and 18–20% higher than that of PR = 1 and 2, respectively. The TEF of the rib-groove turbulator is found to be much higher than that of the single groove turbulator around 27–49%. The Nu and f values for the 45° wavy rib and transverse groove are correlated as functions of Reynolds number (Re), Prandtl number (Pr) and rib pitch ratio (PR), and they are formulated as in Eqs. (11)–(16). The plots of the Nu and f for the rib-groove on one wall, inline and staggered arrays, predicted by Eqs. (11)–(16) and measured data are depicted in Fig. 9a and b, respectively. In the figure, the majority of the

Fig. 6. Variation of Nu/Nu0 with Re for various rib-groove turbulators.

S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

41

Fig. 7. Variation of f/f0 with Re for various rib-groove turbulators.

measured data falls within ±8% and ±10%, for the predicted Nu and f, respectively. Correlations for Nu and f of the 45° wavy rib and transverse groove with BR = 0.25 at PR = 0.5, 1 and 2 for Re ranging from 4000 to 21,000 are written as: Correlation for rib-groove on one wall: 0:771

Nu ¼ 0:261Re

−0:056

f ¼ 7:69Re

0:4

Pr

−0:622

ðPR þ 1Þ −1:947

ðPR þ 1Þ

ð11Þ ð12Þ

Correlation for inline rib-groove: 0:746

Nu ¼ 0:375Re

−0:063

f ¼ 9:419Re

0:4

Pr

−0:607

ðPR þ 1Þ

−0:842

ðPR þ 1Þ

ð13Þ ð14Þ

Correlation for staggered rib-inline groove: 0:759

Nu ¼ 0:307Re

−0:078

f ¼ 10:495Re

0:4

Pr

−0:606

ðPR þ 1Þ

−0:983

ðPR þ 1Þ

ð15Þ ð16Þ

5. Conclusions An experimental study has been carried out to investigate airflow friction and heat transfer characteristics in a high aspect ratio solar air heater channel fitted with combined wavy-rib and groove turbulators for turbulent regime, Re of 4000–21,000. The application of the ribgroove at smaller PR causes a much high pressure drop increase, f/ f0 = 14–134, especially for the inline rib-groove, and also provides considerable heat transfer augmentations, Nu/Nu0 = 4.4–7.69, depending

Fig. 8. Variation of TEF with Re for various rib-groove turbulators.

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S. Skullong et al. / International Communications in Heat and Mass Transfer 50 (2014) 34–43

(a)

(b)

Fig. 9. Predicted data of (a) Nu and (b) f versus experimental data.

on PR, rib array and Re. The Nu/Nu0 augmentation tends to decrease slightly with the rise in Re. The combined rib-groove on the absorber plate at smaller PR should be applied instead of using the groove alone to obtain higher heat transfer and thermal performance of about 49–52%, leading to higher performance solar air heater system. The best operating regime for using these compound turbulators is found for the smaller PR at lower Re values. The absorber wall fitted with rib-groove turbulators at PR = 0.5 yields the highest TEF of about 1.75 at lower Re. Acknowledgment The funding of this research from the Thailand Research Fund (TRF) is gratefully acknowledged.

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