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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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Heat transfer in round tube with rectangular-winglet vortex generators
(2014-01-01)
Skullong, Sompol
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Promvonge, Pongjet
Effect of 30° rectangular-winglet vortex generators (WVGs) mounted in the central core of a circular tube on convection heat transfer and friction loss is experimentally investigated in the present work. The rectangular-WVGs with two different winglet-height to tube-diameter ratios (called blockage ratio, BR = b/D = 0.1 and 0.2) and three winglet-pitch to tube-diameter ratios (PR=P/D=0.5, 1.0, and 1.5) are introduced. In the experiment, air at ambient condition is passed through the uniform heat-fluxed circular tube for Reynolds numbers (Re) in a range of 5000-24,000. The use of WVGs is to generate longitudinal vortex flows in the tube. The experimental results of heat transfer and pressure loss presented in terms of Nusselt number and friction factor are compared between the inserted and the smooth tubes. It is found that the BR and PR provide a significant effect on the thermal performance of the test tube. The results reveal that at smaller PR, the WVG with BR=0.2 provides the highest heat transfer and friction factor but the one with BR=0.2, PR=1.5 yields the best thermal performance. © (2014) Trans Tech Publications, Switzerland.
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Thermal performance of turbulent flow in a solar air heater channel with rib-groove turbulators
(2014-01-01)
Skullong, Sompol
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Kwankaomeng, Sutapat
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Thianpong, Chinaruk
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Promvonge, Pongjet
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 upperwall only, the triangular wavy ribs are placed repeatedly on the tested grooved channelwalls. 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 lowerwalls 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.
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Thermal behaviors in a solar air heater channel with arc-shaped baffle turbulators
(2014-01-01)
Koolnapadol, Narin
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Kaewkohkiat, Yingyong
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Promvonge, Pongjet
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Eiamsa-Ard, Smith
Influences of arc-shaped baffles turbulators (ASB) on thermal behaviors in a solar air heater channel are studied. In the experiment, the ASB with three pitch ratios (P/e) are placed on the absorber plate (bottom channel wall in this case) to generate stronger turbulence intensity in the channel. The distributions of temperature and local Nusselt number contours in the absorber plate fitted with ASB are observed with thermochromic liquid crystal (TLC) sheet. The results reveal that the heat transfer increases with the decrease in P/e due to longitudinal vortex flow effect. The heat transfer from employing the ASB at P/e = 4.0, 6.0 and 8.0 is higher than the smooth channel around 116%, 120% and 127%, respectively. This indicates that the ASB pitch spacing is optimal at 8 times baffle height in the present work.
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Heat transfer behavior in a square duct with tandem wire coil element insert
(2012-01-01)
Eiamsa-Ard, Smith
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Koolnapadol, Narin
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Promvonge, Pongjet
Effects of insertion of tandem wire coil elements used as turbulator on heat transfer and turbulent flow friction characteristics in a uniform heat-flux square duct are experimentally investigated in this work. The experiment is conducted for turbulent flow with the Reynolds number from 4000 to 25000. The wire coil element is inserted into the duct with a view to generating a swirl flow that assists to wash up the flow trapped in the duct corners and then increase the heat transfer rate of the test duct. Apart from the full-length coil, 1D and 2D length coil elements placed in tandem inside the duct with various free-space lengths are introduced to reduce the friction loss. The results obtained from these wire coil element inserts are also compared with those from the smooth duct. The experimental results reveal that the use of wire coil inserts for the full-length coil, 1D and 2D coil elements with a short free-space length leads to a considerable increase in heat transfer and friction loss over the smooth duct with no insert. The full-length wire coil provides higher heat transfer and friction factor than the tandem wire coil elements under the same operating conditions. Also, performance evaluation criteria to assess the real benefits in using the wire coil insert into the square duct are determined.
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Thermal characteristics in square channel with 45° staggered baffle inserts
(2010-12-06)
Kwankaomeng, Sutapat
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Skullong, Sompot
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Teschareon, Thavee
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Promvonge, Pongjet
The paper presents a study of heat transfer in a heat exchanger channel inserted with staggered inclined baffles. The channel has a square section with uniform wall heat flux conditions. The fluid flow and heat transfer characteristics are presented for Reynolds numbers based on the hydraulic diameter of the channel ranging from 4000 to 25,000. The inclined baffles with an axial pitch equal to three times of channel height and with the attack angle of 45° are mounted in tandem and staggered arrangement on the lower and upper walls of the test channel. Effects of four baffle-to-channel height ratios (e/H = 0.1, 0.2, 0.3 and 0.4) on heat transfer in terms of Nusselt number and pressure loss in the form of friction factor are experimentally investigated. The experimental result shows that the insertion of staggered inclined baffles with the e/H=0.4 provides higher heat transfer and friction factor values than others. This is caused by higher baffle-to-channel height ratios of using e/H = 0.4 interrupting the flow and diverting its direction thus promoting high levels of mixing over others.
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Numerical study of laminar flow and heat transfer in square channel with 30° inline angled baffle turbulators
(2010-08-01)
Promvonge, Pongjet
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Jedsadaratanachai, Withada
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Kwankaomeng, Sutapat
The article presents a numerical investigation on periodic laminar flow and heat transfer behaviors in a three-dimensional isothermal wall square-channel fitted with 30°-angled baffles on two opposite channel walls. The computations based on the finite volume method with the SIMPLE algorithm have been conducted for the fluid flow in terms of Reynolds numbers ranging from 100 to 2000. To generate a pair of streamwise counter-rotating vortex (P-vortex) flows through the tested channel, the angled baffles with the attack angle of 30° are mounted periodically and inline arrangement on the lower and upper channel walls. Effects of different baffle heights and three pitch ratios on heat transfer and flow behaviors in the channel are examined. It appears that P-vortex flows help to induce impinging flows over the baffle leading end side and the inter-baffle cavity walls resulting in drastic increase in heat transfer rate over the test channel. The computational results reveal that the maximum thermal enhancement factors for the baffle with PR = 1, 1.5 and 2 are found to be about 3.6, 3.8 and 4.0 at BR = 0.2, 0.2 and 0.15, respectively. © 2010 Elsevier Ltd. All rights reserved.
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Heat transfer and pressure drop in a channel with multiple 60° V-baffles
(2010-08-01)
Promvonge, Pongjet
The research work has been conducted to assess turbulent forced convection heat transfer and friction loss behaviors for airflow through a channel fitted with a multiple 60° V-baffle turbulator. Measurements have been carried out for the channel of aspect ratio, AR=10 and height, H=30. mm with three different baffle blockage ratios, (e/. H=0.10, 0.20 and 0.30) and three baffle pitch spacing ratios, (PR=P/. H=1, 2 and 3) while the transverse pitch of the V-baffle is set to 2H and kept constant. The air flow rate is in terms of Reynolds numbers based on the inlet hydraulic diameter of the test channel ranging from 5000 to 25,000. The experimental results show that the V-baffle provides the drastic increase in Nusselt number, friction factor and thermal enhancement factor values over the smooth wall channel due to better flow mixing from the formation of secondary flows induced by vortex flows generated by the V-baffle. In addition, substantial increases in Nusselt number and friction factor values are found for the rise in blockage ratio and/or for the decrease in pitch ratio values. Assessing thermal performance of the V-baffled channel, the use of the V-baffle with PR=1 and e/. H=0.10 leads to maximum thermal enhancement factor of about 1.87 at lower Reynolds number. © 2010 Elsevier Ltd.
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Numerical prediction on laminar heat transfer in square duct with 30° angled baffle on one wall
(2010-08-01)
Kwankaomeng, Sutapat
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Promvonge, Pongjet
A numerical investigation on periodic laminar flow and heat transfer behaviors in a three-dimensional isothermal wall square duct fitted with 30° angled baffles on lower duct wall only is presented. The computations based on a finite volume method with the SIMPLE algorithm have been conducted for the fluid flow in terms of Reynolds numbers ranging from 100 to 2000. The angled baffles with attack angle of 30° are mounted periodically on the lower duct wall to generate a longitudinal vortex flow through the tested duct. Effects of different baffle height and three pitch length ratios on heat transfer and flow characteristics in the duct are investigated. The study shows that the longitudinal vortex flow created by the baffle helps to induce impinging flows over the baffle trailing end sidewall and the inter-baffle cavity wall resulting in drastic increase in heat transfer rate over the test duct. The computational results reveal that the Nusselt number ratio and the maximum thermal enhancement factor values for using the angled baffle are, respectively, found to be about 7.9 and 3.1 at Re=2000, BR=0.3 and PR=1.5. © 2010 Elsevier Ltd.
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Thermal characteristics of turbulent rib-grooved channel flows
(2009-08-01)
Eiamsa-ard, Smith
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Promvonge, Pongjet
Experimental work has been performed to examine the combined effects of rib-grooved turbulators on the turbulent forced convection heat transfer and friction characteristics in a rectangular duct under a uniform heat flux boundary condition. In the experiments, three types of rib-groove arrangements: rectangular-rib and triangular-groove (RR-TG), triangular-rib and rectangular-groove (TR-RG) and triangular-rib with triangular-groove (TR-TG), were examined. Measurements were carried out for the duct of one aspect ratio, AR = W/H = 20 and duct height, H = 9 mm with rib height, e = 3 mm at three pitch ratios, PR = P/e = 6.6, 10 and 13.3. Experiments were conducted for the Reynolds number range of 3000 to 10,000. Influences of rib-groove arrangements on the Nusselt number and friction factor have been discussed and compared with smooth duct results under similar test conditions. Isothermal friction factors were also taken and presented. The obtained results of the smooth duct are in good agreement with the previous studies found in the literature. Experimental results also show that the duct with RR-TG arrangement provides maximum heat transfer rate and friction factor than others. On the other hand, the thermal enhancement index obtained at constant pumping power reveals that the TR-TG provides the highest values for all pitch ratios studied. Finally, correlations for the heat transfer (Nu), friction factor (f) and the enhancement index (η) have been developed as a function of pitch ratio (PR) and Reynolds number (Re). © 2009 Elsevier Ltd. All rights reserved.

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