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Flow and thermal mechanisms in a heat exchanger tube inserted with twisted cross-baffle turbulators
(2017-03-05)
Nanan, K.
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Thianpong, C.
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Pimsarn, M.
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Chuwattanakul, V.
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Eiamsa-ard, S.
The turbulent forced convective heat transfer behaviors in a tube inserted with baffle turbulators (typical straight baffles, straight cross-baffles, straight alternate-baffles, twisted-baffles, alternate twisted-baffles, and twisted cross-baffles) have been experimentally and numerically investigated. The influence of pitch ratio (P/D) from 1.0 to 2.0 and Reynolds numbers (Re) from 6000 to 20,000 are also reported. Among the studied turbulators, the twisted cross-baffles gave the highest heat transfer enhancement and thermal enhancement factor while the straight cross-baffles gave the lowest heat transfer rate and thermal enhancement factor. The twisted-baffles, alternate twisted-baffles, and twisted cross-baffles created high longitudinal vortex/swirl around the tube core. The strongest longitudinal vortex/swirl was achieved by the use of twisted cross-baffles. The recirculation flows were found in the tube fitted with straight baffles (a recirculation flow), straight alternate-baffles (double recirculation flows) and straight cross-baffles (quadruple recirculation flows). The heat transfer rate, friction factor and thermal enhancement factor increased with decreasing pitch ratio (P/D) and increasing Reynolds number (Re). At the optimum condition of the smallest pitch ratio (P/D = 1.0) and the lowest Reynolds number (Re = 6000), the twisted cross-baffles offered the highest thermal enhancement factor of 1.7.
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Heat transfer enhancement by tapered twisted tape inserts
(2015-10-01)
Piriyarungrod, N.
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Eiamsa-ard, S.
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Thianpong, C.
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Pimsarn, M.
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Nanan, K.
The effects of inserted tapered twisted tapes, their taper angle and twist ratio on heat transfer rate, pressure drop and thermal performance factor characteristics have been reported. The experiments were carried out by using the tapered twisted tapes with 4 different taper angles which θ= 0.0° (typical twisted tape), 0.3°, 0.6° and 0.9°. At each taper angle, the tapered twisted tapes were twisted at three different twist ratios (y/W) of 3.5, 4.0 and 4.5. All tapes were tested under turbulent flow regime for Reynolds numbers between 6000 and 20,000. A twist ratio is defined as the ratio of twist length (y) to twisted tape width at the large end (W). The plain tube was also tested for comparison. Heat transfer enhancement and friction loss increased with decreasing taper angle and twist ratio. Thermal performance factor tended to increase with increasing taper angle and decreasing tape twist ratio. For the present range, the tube with the tape with taper angle (θ) of 0.9° and twist ratio (y/W) of 3.5 yielded the maximum thermal performance factor of 1.05 at Reynolds number (Re) of 6000.
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Heat transfer augmentation through the use of wire-rod bundles under constant wall heat flux condition
(2013-11-01)
Nanan, K.
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Pimsarn, M.
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Jedsadaratanachai, W.
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Eiamsa-ard, S.
This article reports an experimental investigation on heat transfer, friction factor and thermal performance characteristics of turbulent flow (6000. ≤. Re≤. 20,000) in heat exchanger tubes with wire-rod bundles as flow turbulators. The experiments were carried out at three different pitch ratios (P/D) of 1.0, 1.5 and 2.0 and three wire-rod number per bundle (N) of 4, 6 and 8. The experimental results show that Nusselt number increases with increasing Reynolds number and wire-rod number per bundle, and decreasing pitch ratio (P/D) of the turbulators. As compared to the results of the tube without wire-rod (the plain tube), heat transfer rate and friction factor are respectively increased in ranges of 3.5 to 68.8% and 156 to 353%, depending on the operating conditions. At the same pumping power, the use of wire-rod turbulators results in thermal performance factor up to 1.02 times of those of the plain tube. In addition, the correlations that developed from the present experimental data for Nusselt number, friction factor and thermal performance factor are also presented. © 2013 Elsevier Ltd.
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Heat transfer enhancement by helically twisted tapes inducing co- and counter-swirl flows
(2013-08-01)
Nanan, K.
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Yongsiri, K.
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Wongcharee, K.
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Thianpong, C.
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Eiamsa-ard, S.
An experimental investigation has been conducted for determining heat transfer enhancement by inserting helically twisted tapes, to induce co- and counter-swirl flows, (the tapes are symbolized as co-HTT and C-HTT, respectively). Tape pitch ratio (p/ D) was varied between 1.0 and 2.0, while tape width ratio (w/D) and twist ratio (y/w) were fixed at 0.2 and 3.0, respectively. The experiments were performed for fully developed turbulent flow with Reynolds number range (Re) between 6000 and 20,000, under uniform wall heat flux condition. At similar conditions, the use of Co-HTT results in lower Nusselt number and friction factor but higher thermal performance factor than that of C-HTT. Nusselt number and friction factor increase with decreasing pitch ratio, while thermal performance factor possesses opposite trend. In addition, the empirical correlations for Nusselt number, friction factor and thermal performance factor as functions of the Reynolds number (Re), Prandtl number (Pr) and tape pitch (p/D), were developed through a multi-variant linear normal regression. © 2013 Elsevier Ltd.
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Thermohydraulic characteristics of co/counter swirl flow through a round tube fitted with helical screw tape and twisted tape
(2013-03-01)
Eiamsa-Ard, S.
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Yongsiri, K.
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Nanan, K.
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Thianpong, C.
Influence of co/counter swirl flow on the characteristics of friction, heat transfer and thermal performance factor in heat exchanger tubes has been experimentally investigated. Each twisted tape is inserted into a helical screw tape and then these combined tapes are subsequently equipped in a heat exchanger tube for generating the swirl flow. The combined tapes are arranged in two different forms, namely (i) co-swirl arrangement in which their twist are in the same direction and (ii) counter-swirl arrangement in which their twists directions are in the opposite directions. The experiments using helical tapes alone are also performed for assessment. The helical screw tapes are used at three different tape-twist ratios (PR = p/D = 1.0, 1.5 and 2.0), each with three different tape-width ratios (WR = w/D = 0.15, 0.2 and 0.25), while twisted tapes are used at three different twist ratios (YR = y/W = 3, 4 and 5). The results show the maximum thermal performance factor of 1.49 for helical tape alone with PR=2.0 and WR=0.2. At similar conditions, the heat transfer rate associated with the combined tapes in counter-swirl arrangement is 3.4% and 10% higher than those in co-swirl arrangement and helical tape alone.
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Heat transfer augmentation by helically twisted tapes as swirl and turbulence promoters
(2012-10-01)
Eiamsa-ard, S.
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Yongsiri, K.
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Nanan, K.
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Thianpong, C.
This paper describes heat transfer enhancement attributed to helically twisted tapes (HTTs). Each helically twisted tape was fabricated by twisting a straight tape to form a typical twisted tape then bending the twisted tape into a helical shape. The experiments were performed using HTTs with three twist ratios (y/. W) of 2, 2.5 and 3, three helical pitch ratios (p/. D) of 1, 1.5 and 2 for Reynolds number between 6000 and 20,000. The conventional helical tape (CHT) was also tested for comparison. The obtained results reveal that at similar conditions (y/. W and . p/. D), HTTs give lower Nusselt number and friction but higher thermal performance factor than CHTs. Heat transfer rate and friction factor increase as the tape twist ratio and helical pitch ratio decrease, while the thermal performance shows opposite trend. In the present range, the highest thermal performance factor of 1.29 is achieved by utilizing the tape with the largest twist ratio (y/. W=3) and helical pitch ratio (p/. D=2) at Reynolds number of 6000. © 2012 Elsevier B.V.
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Thermal performance evaluation of heat exchangers fitted with twisted-ring turbulators
(2012-07-01)
Thianpong, C.
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Yongsiri, K.
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Nanan, K.
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Eiamsa-ard, S.
Heat transfer, friction factor and thermal performance characteristics in a tube equipped with twisted-rings (TRs) are experimentally investigated. The experiments were conducted using TRs with three different width ratios (W/D. =. 0.05, 0.1 and 0.15) and three pitch ratios of (p/D. =. 1, 1.5 and 2) for Reynolds numbers ranging from 6000 to 20,000 using air as a test fluid. The typical circular rings (CRs) were also tested for an assessment. The experimental results reveal that most TRs yield lower Nusselt numbers and friction factor than CRs, except at the largest width ratio (W/D. =. 0.15) and the smallest pitch ratio (p/D. =. 1.0). In addition, Nusselt number and friction factor increase as width ratio increases and pitch ratio decreases. However, a maximum thermal performance factor is associated by TRs with the smallest width ratio and pitch ratio. The empirical correlations of the heat transfer (Nu) and friction factor (f) are also included in this paper. © 2012 Elsevier Ltd.
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Heat transfer enhancement in a rectangular channel with twisted tape inserts
(2009-01-01)
Nuthong, W.
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Eiamsa-Ard, S.
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Nanan, K.
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Eiamsa-Ard, P.
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Thianpong, C.
Experimental investigation of heat transfer and friction factor behaviors in a rectangular channel with multiple twisted tapes (MTTs) is carried out. In each run, five MTTs are simultaneously used as the swirl flow generators in a test section. The lower wall of the channel is subjected to uniform heat flux conditions while the upper wall is insulated. Parametric runs are made for Reynolds numbers ranging from 2700 to 9000, the tapes with twist-length to twist-width ratios (y/w) of 3, 4 and 5 while the aspect ratio of AR = W/H = 10 is fixed throughout. Experiments using smooth channel are also performed for comparison. The experimental results reveal that heat transfer in the channels with MTTs at twist ratios (y/w) of 3, 4 and 5 are respectively increased around 55%, 37% and 22% over those in the smooth channel. In addition, the empirical correlations of the Nusselt number and friction factor in the developed flow region for the channels fitted with MTTs are also derived.

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