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Thermal performance in solar air heater with perforated-winglet-type vortex generator
(2018-08-01)
Skullong, Sompol
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Promthaisong, Pitak
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Pimsarn, Monsak
An experimental and numerical study of turbulent convective heat transfer in a solar air heater duct with winglet-type vortex generators (WVGs) placed on the absorber plate is presented. Air as the test fluid enters the duct having a uniform wall heat-flux applied on the upper wall or the absorber plate with Reynolds number from 4100 to 25,500. Two types of WVGs are introduced: rectangular (RWVG) and trapezoidal (TWVG) WVGs, in order to create multiple vortex flows along the duct. The WVG parameters in the present study include two relative height (B<inf>R</inf> = e/H = 0.2 and 0.48), three longitudinal pitch ratios (P<inf>R</inf> = P<inf>l</inf>/H = 1, 1.5 and 2) and a single attack angle, α = 30°. The experimental result reveals that the RWVG with B<inf>R</inf> = 0.48 and P<inf>R</inf> = 1 provides the highest heat transfer and friction factor at about 7.1 and 109.5 times above the flat duct, respectively while the TWVG with B<inf>R</inf> = 0.2 and P<inf>R</inf> = 1.5 yields the maximum thermal performance around 1.84. Then, to improve the performance by reducing the substantial pressure loss, both the WVGs with B<inf>R</inf> = 0.48 and P<inf>R</inf> = 1.5 are modified to be perforated rectangular and trapezoidal winglet-type vortex generators (P-RWVG and P-TWVG) with four different punched hole/pore diameters (d = 1, 3, 5 and 7 mm) on their central area. The investigation indicates that among the perforated WVGs, the P-RWVG at d = 1 mm yields the highest heat transfer and friction factor up to 6.78 and 84.32 times higher than the smooth duct but the best thermal performance of about 2.01 is found for the P-TWVG with d = 5 mm. To explore the flow and heat transfer pattern, a 3D numerical flow simulation is performed and validated with available measurements where both the numerical and measured results are in good agreement.
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Thermal performance of heat exchanger tube inserted with curved-winglet tapes
(2018-01-25)
Skullong, Sompol
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Jayranaiwachira, Nuthvipa
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Pimsarn, Monsak
The paper deals with the effect of curved-winglet (CW) inserts on thermal and flow behaviors in a constant heat-fluxed tube. A straight tape is used to support the 45° CWs mounted repeatedly on both tape sides to generate two pairs of longitudinal counter-rotating vortices along the test tube in order to assist the chaotic flow mixing and to disrupt the boundary layer leading to faster rate of heat transfer. The airflow and heat transfer behaviors in the tube are examined for Reynolds number (Re) in the range of 4150–25,400. The curved-winglet tape (CWT) parameters involved are the winglet attack angle of 45° three relative winglet heights (b/D = B<inf>R</inf> = 0.1, 0.2 and 0.3) and winglet pitches, (P/D = P<inf>R</inf> = 0.5, 1.0 and 2.0). The investigation reveals that the maximum thermal enhancement factor (TEF) of the CWT is about 1.62 at B<inf>R</inf> = 0.1 and P<inf>R</inf> = 1.0. For further improvement, the CWT at B<inf>R</inf> = 0.1 and P<inf>R</inf> = 1.0 is modified by punching the CW to be the perforated-curved-winglet tape (P-CWT) to reduce the pressure loss. The P-CWT characteristics include five different punched hole diameters (d = 1.0, 1.5, 2.0, 2.5 and 3.0 mm). The experimental results show that TEF of all the P-CWTs is higher than that of the CWT and the maximum TEF of 1.76 higher than the CWT around 9% is found for d = 1.5 mm. To understand the flow pattern and heat transfer mechanism, a three-dimensional CFD investigation is also performed and for validation, the good agreement between numerical and experimental results is found. For experimental data, empirical correlations for Nu, f and TEF for the CWT and P-CWT inserts are also determined.
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Heat transfer characterization in a tubular heat exchanger with V-shaped rings
(2017-01-05)
Chingtuaythong, Witoon
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Pimsarn, Monsak
This article presents the influence of V-shaped rings (or V-ring) placed in a uniform heat-fluxed tube on heat transfer and flow resistance characteristics. In the current experiment, the V-ring elements with attack angle of 30° are inserted into the tube with four relative ring-pitches (R<inf>P</inf> = P/D = 0.5, 1.0, 1.5 and 2.0) and three ring blockage ratios (R<inf>B</inf> = e/D = 0.1, 0.15 and 0.2). Air as the test fluid flows through the tube for Reynolds number of about 5000–25,000. The experimental results reveal that the V-ring can considerably enhance the heat transfer rate up to 5.8 times above the plain tube whereas the friction factor is up to 82 times. The increase in R<inf>B</inf> leads to higher heat transfer and friction loss while the increment in R<inf>P</inf> provides the reversing trend. The thermal enhancement factor of the V-ring is in the range of 1.36–1.63 where its maximum regarded as the optimum point is at R<inf>B</inf> = 0.1 and R<inf>P</inf> = 1.0. In comparison with other vortex-flow devices, the V-ring yields much higher thermal performance than the published inclined rings, wire-coils and twisted tapes. Nusselt number and friction factor correlations for the V-ring are also determined.
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Heat transfer augmentation in a solar air heater channel with combined winglets and wavy grooves on absorber plate
(2017-01-01)
Skullong, Sompol
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Jayranaiwachira, Nuthvipa
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Pimsarn, Monsak
An investigation on heat transfer characteristics in a solar air heater channel using wavy grooves incorporated with pairs of trapezoidal-winglets (TW) placed on the absorber plate has been experimentally conducted. Air as the test fluid entered the test section having a constant heat-flux on the absorber plate while the Reynolds numbers obtained by controlling the airflow rate is in a range of 4500–22,000. The TW characteristics include three relative winglet-pitches (P<inf>R</inf>) and five relative winglet height or blockage ratios (B<inf>R</inf>) at a single attack angle of 45° whereas the wavy rectangular-groove parameters are three relative groove-pitch lengths (P<inf>R</inf>) similar to the TW case but at a fixed width and height. The investigation shows that the TW together with the groove provides the substantial increase in heat transfer over the smooth channel. The TW alone gives much higher heat transfer but the groove yields considerably lower pressure drop. The combined groove and TW devices at a given B<inf>R</inf>, perform the highest heat transfer and friction factor at smaller P<inf>R</inf> and also provides considerably higher thermal performance than the single device acting alone. At P<inf>R</inf> = 1, the compound device with B<inf>R</inf> = 0.28 offers the highest heat transfer and friction factor while the one with B<inf>R</inf> = 0.24 gives the maximum thermal performance.
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Thermal performance in solar air heater channel with combined wavy-groove and perforated-delta wing vortex generators
(2016-05-05)
Skullong, Sompol
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Pimsarn, Monsak
An experimental investigation on thermal performance improvement in a solar air heater channel with combined wavy-groove and delta-wing vortex generator (WVG) placed on the absorber plate having a uniform wall heat-flux is carried out. The Reynolds number based on the hydraulic diameter of the channel ranges from 4800 to 23,000. The effect of the combined groove and WVG on the heat transfer and pressure drop in the channel in terms of respective Nusselt number and friction factor is examined. Investigated parameters of the WVG mounted on the grooved absorber are three wing porosity area ratios (called porosity ratio, A<inf>h</inf>/A<inf>w</inf> = 0.031, 0.085 and 0.167) and four groove-wing distance to channel-height ratios (g/H = 0.4, 0.5, 0.75 and 1) at a single attack angle (α = 45°). The experimental result reveals that at g/H = 0.5, the smaller A<inf>h</inf>/A<inf>w</inf> provides the highest Nusselt number and friction factor around 6 and 30 times over the smooth channel, respectively, but the optimum thermal performance is at A<inf>h</inf>/A<inf>w</inf> = 0.085 and g/H = 0.5. The combined devices give the thermal performance augmentation at about 37.7-46.3% higher than the groove alone and also at about 1.5-12.5% above the combined groove and non-perforated WVG (without hole).
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Heat transfer and turbulent flow friction in a round tube with staggered-winglet perforated-tapes
(2016-04-01)
Skullong, Sompol
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Promvonge, Pongjet
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Thianpong, Chinaruk
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Pimsarn, Monsak
The article deals with thermal and flow resistance characteristics in a tubular heat exchanger fitted with staggered-winglet perforated tapes (WPT). The experiment was conducted in the test tube having a constant wall heat-flux for turbulent airflow, Reynolds number (Re) from 4180 to 26,000. The aim of using the WPT is to generate longitudinal vortex flows to disrupt thermal boundary layer on the tube wall and to provide stronger fluid mixing. The WPT having an winglet inclination angle of 30° was inserted into the test tube at five different winglet blockage ratios (B <inf>R</inf> = 0.1, 0.15, 0.2, 0.25 and 0.3) and three winglet pitch ratios (P <inf>R</inf> = 0.5, 1.0 and 1.5). To find an optimum thermal performance condition, the effect of B <inf>R</inf> and P <inf>R</inf> on the heat transfer and pressure loss due to flow friction in the tube is examined. The experimental results reveal that Nusselt number (Nu) and friction factor (f) for the WPT increase with the increment of B <inf>R</inf> but the reduction of P <inf>R</inf> . The highest thermal enhancement factor (TEF) of 1.71 is achieved by utilizing the WPT with B <inf>R</inf> = 0.15, P <inf>R</inf> = 1.0 at Re = 4180. Compared to staggered-winglet typical non-perforated tape (WTT), the WPT yields the TEF of about 1.2 times higher than the WTT. Correlations of Nu, f and TEF for the WPT and the WTT are also proposed.
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Thermal performance of tubular heat exchanger with multiple twisted-tape inserts
(2015-05-01)
Chokphoemphun, Suriya
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Pimsarn, Monsak
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Thianpong, Chinaruk
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Promvonge, Pongjet
The paper presents an experimental investigation on enhanced heat transfer and pressure loss characteristics by using single, double, triple, and quadruple twisted-tape inserts in a round tube having a uniform heat-fluxed wall. The investigation has been conducted in the heat exchanger tube inserted with various twisted-tape numbers for co- and counter-twist arrangements for the turbulent air flow, Reynolds number (Re) from 5300 to 24000. The typical single twisted-tape inserts at two twist ratios, y/w = 4 and 5, are used as the base case, while the other multiple twisted-tape inserts are at y/w = 4 only. The experimental results of heat transfer and pressure drop in terms of Nusselt number (Nu) and friction factor (f), respectively, reveal that Nu increases with the increment of Re and of twisted-tape number. The values of Nu for the inserted tube are in a range of 1.15-2.12 times that for the plain tube while f is 1.9-4.1 times. The thermal enhancement factor of the inserted tube under similar pumping power is evaluated and found to be above unity except for the single and the double co-twisted tapes. The quadruple counter-twisted tape insert provides the maximum thermal performance.
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Thermal characterization in a circular tube fitted with inclined horseshoe baffles
(2015-01-22)
Promvonge, Pongjet
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Tamna, Sombat
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Pimsarn, Monsak
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Thianpong, Chinaruk
In the present study, the influence of inclined horseshoes baffles placed repeatedly in a tubular heat exchanger on heat transfer rate, friction factor and thermal enhancement factor are experimentally determined. The horseshoe baffle elements with an inclination angle of 20° were inserted periodically into the test tube at three different baffle-pitch ratios (P<inf>R</inf> = 0.5, 1.0 and 2) and -width or blockage ratios (B<inf>R</inf> = 0.1, 0.15 and 0.2). The experiment was conducted in the test tube having a uniform heat-fluxed wall by varying turbulent airflow to obtain Reynolds number in a range of 5300-24,000. The experimental results revealed that the tube fitted with inclined horseshoes baffles provides considerable improvement of the heat transfer rate over the plain tube around 92-208% while the friction factor is increased at about 1.76-6.37 times. To access the real benefits for the inclined horseshoes baffles inserted in plain tube, thermal performance factor is examined and found to be in the range of 1.34-1.92 at which the maximum obtained at P<inf>R</inf> = 0.5 and B<inf>R</inf> = 0.1 is considerably higher than that for published inserted devices. Correlations for Nusselt number and friction factor for the oblique horseshoe-baffled tube are also proposed.

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