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Turbulent heat transfer and pressure loss in a square-duct heat exchanger with inclined-baffle inserts
(2017-12-28)
Koolnapado, Narin
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Hoonpong, Panuwat
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Skullong, Sompol
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Kammul, Pongsak
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Promvonge, Pongjet
Thermal and friction loss characteristics in a square-duct heat exchanger fitted with inclined-baffles are experimentally examined. Air as the test fluid enters the test duct having a uniform surface heat-flux. The baffles are placed repeatedly on both sides of a rectangular centre-cleared tape/frame before diagonally inserting the baffled frame into the test duct to produce longitudinal vortex flows through the test section. Effects of five different relative baffle height or flow blockage ratios (b/H=BR=0.1, 0.2, 0.3, 0.4 and 0.5) on heat transfer, pressure loss and thermal performance in the square duct are investigated for Reynolds number ranging from 4100 to 25,600. The relative baffle pitch or pitch ratio (P/H=PR) and baffle attack angle (α) are fixed at 3.0 and 30°, respectively. The experimental results reveal that the heat transfer and pressure drop in the form of respective Nusselt number (Nu) and friction factor (f) from using the baffle tend to increase with the rise of Reynolds number (Re) and BR. The maximum enhancement in Nu and f has been found to be 4.61 and 63.67 times above the smooth duct, respectively. The thermal enhancement factor (η) is maximum at BR=0.3.
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Enhanced heat transfer in a heat exchanger square-duct with discrete V-finned tape inserts
(2015-03-01)
Noothong, Watcharin
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Suwannapan, Supattarachai
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Thianpong, Chinaruk
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Promvonge, Pongjet
The article presents an experimental and numerical study on thermal performance enhancement in a constant heat-fluxed square-duct inserted diagonally with 45° discrete V-finned tapes (DFT). The experiments were carried out by varying the airflow rate through the tested square duct with DFT inserts for Reynolds number from 4000 to 25000. The effect of the DFT with V-tip pointing upstream at various relative fin heights and pitches on heat transfer and pressure drop characteristics was experimentally investigated. Both the heat transfer and pressure drop were presented in terms of Nusselt number and friction factor respectively. Several V-finned tape characteristics were introduced such as fin- to duct-height ratio or blockage ratio (R<inf>B</inf> = e/H = 0.075, 0.1, 0.15 and 0.2), fin pitch to duct height ratio (R<inf>P</inf> = P/H = 0.5, 1.0, 1.5 and 2.0) and fin attack angle, α = 45°. The experimental results reveal that the heat transfer and friction factor values with DFT inserts increase with the increment of R<inf>B</inf> but the decrease of R<inf>P</inf>. The inserted square-duct at R<inf>B</inf> = 0.2 and R<inf>P</inf> = 0.5 provides the highest heat transfer and friction factor while the one with R<inf>B</inf> = 0.1 and R<inf>P</inf> = 1.5 yields the highest thermal performance. Also, a numerical simulation was conducted to investigate the flow structure and heat transfer mechanism inside the tested duct with DFT inserts.
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Experimental study on heat transfer in square duct with combined twisted-tape and winglet vortex generators
(2014-12-01)
Promvonge, Pongjet
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Suwannapan, Supattarachai
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Pimsarn, Monsak
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Thianpong, Chinaruk
The article presents an experimental investigation on thermal performance enhancement in a constant heat-fluxed square duct fitted with combined twisted-tape and winglet vortex generators. The experiments are carried out for the airflow rate through the tested square duct fitted with both the vortex generators for Reynolds number from 4000 to 30,000. The effect of the combined twisted tape and rectangular winglet inserts on heat transfer and pressure drop presented in terms of respective Nusselt number and friction factor is experimentally investigated. The characteristics of the combined twisted-tape and winglet include two twist ratios (Y=4 and 5), three winglet- to duct-height ratios, (R<inf>B</inf>=0.1, 0.15 and 0.2), four winglet-pitch to tape-width ratios, (R<inf>P</inf>=2, 2.5, 4 and 5) and a single attack angle of winglet, α=30°. The experimental results reveal that the Nusselt number and friction factor for the combined twisted-tape and V-winglet increase with increasing R<inf>B</inf> but decreasing R<inf>P</inf>. The inserted duct at R<inf>B</inf>=0.2, R<inf>P</inf>=2 and Y=4 provides the highest heat transfer rate and friction factor but the one at R<inf>B</inf>=0.1, R<inf>P</inf>=2 and Y=4 yields the highest thermal performance. The application of combined vortex-flow devices gives thermal performance around 17% higher than the twisted tape alone.
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Thermal behaviors in a square duct with U-ribbed tape inserts
(2014-01-01)
Khanoknaiyakarn, Chitakorn
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Skullong, Sompol
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Promvonge, Pongjet
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Jayranaiwachira, Nuthvipa
This paper presents an experimental study on thermal characteristics in a constant heatfluxed square-duct heat exchanger with U-ribbed tape inserts. The experiments are carried out by varying airflow rate for Reynolds number ranging from 4000 to 38,000. The insertion of the Uribbed tape is performed with an axial rib-pitch set to four times duct-height (4H) at a single attack angle, α=45° and the ribbed tape is diagonally inserted in the square duct in order to generate longitudinal vortex flows. Effects of five blockage ratios (e/H=0.1, 0.15, 0.2, 0.25 and 0.3) on heat transfer and friction loss are experimentally investigated. The experimental result shows that the insertion of the U-ribbed tape at e/H=0.3 provides the highest heat transfer and friction factor values but the one at e/H=0.25 yields the highest thermal performance enhancement. © (2014) Trans Tech Publications, Switzerland.
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Heat transfer improvement in a square duct with diagonal inclined ribs
(2014-01-01)
Suwannapan, Supattarachai
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Poomsalood, Ratsak
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Promvonge, Pongjet
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Jedsadaratanachai, Withada
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Limkul, Thitipat
This research presents a numerical study of turbulent periodic flow and heat transfer in three-dimensional isothermal-fluxed square duct with diagonal inclined rib inserted. The fluid flow and heat transfer characteristics are presented for Reynolds numbers in the range of 4000 to 20,000. The computations based on the finite volume method, and the SIMPLE algorithm has been implemented. Effects of rib pitch ratios (0.5 to 2) at a single blockage ratio of 0.2 and attack angle of 60° on heat transfer and friction factor in the duct are examined and their results of the inclined rib are also compared with those of the smooth duct. It is found that the inclined rib provides higher heat transfer rate and friction factor than the smooth duct for all cases. In addition, the decreasing of the pitch ratio leads to the rise in the Nusselt number and friction factor. © (2014) Trans Tech Publications, Switzerland.
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Heat transfer in square duct fitted diagonally with angle-finned tape-Part 1: Experimental study
(2012-05-01)
Promvonge, Pongjet
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Skullong, Sompol
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Kwankaomeng, Sutapat
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Thiangpong, Chinaruk
The paper presents an experimental study on turbulent flow and heat transfer characteristics in a square duct fitted diagonally with 30° angle-finned tapes. The tested duct has a square section and uniform heat-fluxed walls and the flow rate of air used as the test fluid is presented in terms of Reynolds number from 4000 to 23,000. The angle-finned straight tape in the present work is newly invented without previous investigations available. The insertion of the finned tape is performed with three ratios of fin pitch to duct height (PR= P/. H) at the fin attack angle of 30° with respect to the main flow direction. The finned tape inserted diagonally in the duct is expected to generate a longitudinal vortex flow pair through the heated duct. Influences of five fin-to-duct height ratios (BR= b/. H= 0.1-0.3) for each fin pitch on thermal and flow friction characteristics of the inserted duct are investigated. The experimental result shows that at smaller fin pitch spacing, the finned tape with BR= 0.3 provides the highest heat transfer and friction factor but the one with BR= 0.2 and PR= 1.0 yields the best thermal performance. The thermal performance of the newly invented finned tape turbulator is found to be much higher than that of the wire coil/twisted tape turbulator. © 2012 Elsevier Ltd.
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Heat transfer in square duct fitted diagonally with angle-finned tape-Part 2: Numerical study
(2012-05-01)
Promvonge, Pongjet
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Skullong, Sompol
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Kwankaomeng, Sutapat
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Thiangpong, Chinaruk
A numerical work has been conducted to examine turbulent flow and heat transfer characteristics in a three-dimensional isothermal-fluxed square-duct fitted diagonally with 30°-angle finned tapes. The computations are based on the finite volume method with the SIMPLE algorithm implemented. The air flow and heat transfer characteristics in the duct are presented for Reynolds number (Re) in a range of 4000 to 20,000. In the current study, a straight tape with 30°-angled fins mounted repeatedly on both sides is inserted diagonally into the test duct to generate a pair of longitudinal counter-vortices in assisting chaotic flow mixing in the duct including vortex-induced impingement (VI) effect. Effects of fin blockage ratio (BR= b/. H) and pitch ratio (PR= L/. H) on heat transfer and pressure drop behaviors in the duct are investigated and the results of the finned tape insert are compared with available measurements. The computation reveals that predicted results from the finned tape insert are in good agreement with measured data. The study indicates that the vortex flow can help to induce impingement/reattachment flows (VI effect) on the duct walls leading to drastic increase in the heat transfer rate over the duct. The rise of the BR and the reduction of the PR results in the increase in Nusselt number and friction factor values. The maximum thermal performance is found to be 1.95 for using the finned tape at BR= 0.2 and PR= 1 whereas the Nusselt number ratio is about 4.5 at lower Re. © 2012 Elsevier Ltd.
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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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Numerical heat transfer study of turbulent square-duct flow through inline V-shaped discrete ribs
(2011-12-01)
Promvonge, Pongjet
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Changcharoen, Wayo
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Kwankaomeng, Sutapat
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Thianpong, Chinaruk
A numerical work has been conducted to examine turbulent periodic flow and heat transfer characteristics in a three dimensional square-duct with inline 60° V-shaped discrete thin ribs placed on two opposite heated walls. The isothermal-flux condition is applied only to the upper and lower duct walls while the two sidewalls are insulated, similar to internal passage cooling of gas turbine blades. The computations are based on the finite volume method with the SIMPLE algorithm for handling the pressure-velocity coupling. Air is the working fluid with the flow rate in terms of Reynolds numbers ranging from 10,000 to 25,000. The numerical result is validated with available square-rib measured data and found to agree well with measurement. The computation reveals that the ribbed duct flow is fully developed periodic flow and heat transfer profiles at about x/ D=7-11 downstream of the inlet. Effects of different rib height to duct diameter ratios, BR, on thermal characteristics for a periodic ribbed duct flow are investigated. It is found that a pair of counter-rotating vortices (P-vortex) caused by the rib can induce impingement/attachment flows on the walls leading to greater increase in heat transfer over the test duct. In addition, the rise of BR values leads to the increase in heat transfer and friction loss. The maximum thermal performance is around 1.8 for the rib with BR=0.0725 where the heat transfer rate is about 4.0 times above the smooth duct at lower Reynolds number. © 2011 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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