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Experimental Study of the Effects of Inclusion of Rectangular-Cut Twisted Tapes on Heat Transfer and Pressure Drop in a Round Tube
(2019-12-01)
Ruengpayungsak, K.
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Kumar, Manoj
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Chuwattanakul, V.
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Eiamsa-ard, S.
Twisted tape inserts were used as swirl generators in the flow through a tube for enhancing heat transfer. Twisted tapes have commonly been applied in many thermal applications used in industry. Several modified twisted tapes have been investigated for improving the thermal performance of heat exchangers. The effects in tubes with modified rectangular-cut twisted tape (RC-TT) inserts with various free-spacing ratios (s/w = 2.0, 2.5, 3.0, 3.5 and 4.0) and edge–width ratios (t/W = 0.1, 0.15, 0.2 and 0.25) on heat transfer and pressure losses are reported. The RC-TT used in this work had a fixed thickness of 1.2 mm, width of 60 mm, length of 2000 mm and twist ratio (y/W) of 3.0. Experiments were performed over a range of Reynolds numbers, 10,000 ≤ Re ≤ 20,000, using air as a test fluid. A tube with a classical/typical twisted tape was also tested for comparison. In all cases, the Nusselt number tended to increase with the Reynolds number, while the friction factor and thermal performance showed the opposite trend. For the rectangular-cut twisted tape inserts, the friction factor and Nusselt number decreased, while thermal performance increased with increasing free-spacing ratios (s/w) and edge–width ratios (t/W). Furthermore, the maximum thermal performance was achieved at the lowest free-spacing ratio (s/w), edge–width ratio (t/W) and Reynolds number (Re).
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Dynamic heat transfer modeling of a closed refrigerated display cabinet
(2019-10-01)
Chaomuang, Nattawut
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Laguerre, Onrawee
;
Flick, Denis
This study proposes a simplified heat transfer model of a closed refrigerated display cabinet using a zonal approach. The dynamic model was developed and solved with a spectral method to predict the temperature fluctuations occurring with “on/off” compressor regulation. The equivalent load thickness in which the temperature fluctuations were of the same order as those at the load surface was estimated as a function of “on/off” compressor operating frequency. The air temperature fluctuations are dampened whenever the air exchanges heat with solid surfaces (e.g. walls, loads) and with the ambient air (e.g. through the doors). Despite exposure to large air temperature fluctuations (up to 5 °C), the load surface temperature showed only small fluctuations (ΔT<inf>l</inf> ≤ 0.5 °C). Good agreement was found between the predicted and the measured temperature fluctuations. The model can also be applied in order to predict air and load temperature increases during defrost operation.
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Effects of spiral start number and depth ratio of corrugated tube on flow and heat transfer characteristics in turbulent flow region
(2019-08-01)
Kongkaitpaiboon, V.
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Promthaisong, P.
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Chuwattanakul, V.
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Wongcharee, K.
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Eiamsa-ard, S.
Influence of spirally-corrugated tubes on heat transfer (Nu), pressure loss (f) and thermal enhancement factor (TEF) characteristics was numerically investigated. The physical models of spirally-corrugated tubes and also a straight circular tube were built for the comparative study. The predictions were performed for the spirally-corrugated tubes with spiral start numbers (N) of 2, 3, 4 and 5 and depth ratios (DR = e/D) of 0.02, 0.04, 0.06, 0.08, 0.1, 0.12, 0.14 and 0.16 at Reynolds numbers of 5000, 8000, 14000 and 20000. The numerical results show that the spiral corrugated tubes created two types of flow structures: main swirl flows (appear around the core) and secondary swirl flows (appear near tube wall) which facilitate heat transfer between the fluid and the tube walls. The overall heat transfer depends on the competition between two effects: the positive effect of the stronger swirl intensity by increasing start number and the negative effect of the difficulty of fluid transfer into grooves by increasing depth ratio. For the investigated range, the corrugated tubes yield Nusselt numbers and thermal performance factors varying from 0.82 to 2.16 times and 0.66–1.2 times of the straight circular tube, respectively. The highest thermal enhancement factor (TEF) of 1.2 is achieved by using the tube with spiral start number of 4 having moderate depth ratio of 0.08 at Re = 5000.
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Heat transfer augmentation in solar receiver heat exchanger with hole-punched wings
(2019-06-05)
Promvonge, Pongjet
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Skullong, Sompol
Thermal characteristics in a solar air heater duct with perforated/punched rectangular wing (P-RW) mounted on the absorber plate have been experimentally examined. Air as the test fluid flowed through the test duct having a constant wall heat-flux with Reynolds number (Re) based on the hydraulic duct diameter from 5300 to 22,600. Two parts on the P-RWs are offered herein. First, the characteristics of forward P-RWs in the tests were three porosity ratios (A<inf>h</inf>/A<inf>w</inf> = 0.12, 0.34 and 0.64) and three relative pitches (R<inf>P</inf> = P/H = 1.0, 1.5 and 2.0) at a single attack angle (α = 60°). The study revealed that the optimal thermal performance is found at R<inf>P</inf> = 1.5 and A<inf>h</inf>/A<inf>w</inf> = 0.34. Second, the optimal P-RWs in the first part was further studied by varying the attack angles, α = 30° 45° and 60° and by rearranging the P-RWs from the forward to the backward P-RWs. The results of the heat transfer rate and flow resistance inside the duct were displayed in terms of Nusselt number (Nu) and friction factor (f), respectively. The experimental results have showed that the P-RWs give the considerable increase in Nu over the flat-plate duct (smooth absorber plate) around 3.91–5.52 times while the increase in f is about 10.37–36.35 times. The highest thermal performance around 2.01 using the backward P-RWs is seen at A<inf>h</inf>/A<inf>w</inf> = 0.34, R<inf>P</inf> = 1.5 and α = 45°. Correlations for Nu and f have also been determined as a function of P-RW parameters.
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Experimental analysis of heat transfer and airflow in a closed refrigerated display cabinet
(2019-03-01)
Chaomuang, Nattawut
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Flick, Denis
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Denis, Alain
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Laguerre, Onrawee
This study presents the experimental investigations on heat transfer and airflow in a closed refrigerated display cabinet. Air and product temperatures and air velocity were measured with thermocouples and a hot-wire anemometer, respectively. Temperature variation in the cabinet depends on the positions. The front areas contributed to higher temperature, whereas the rear areas were at a lower temperature. Benefits of doors were also examined by comparing the results of air and product temperatures with the case without doors. The cabinet with doors provided less temperature heterogeneity (ΔT<inf>max</inf>= 2.1 °C) compared to the case without door (ΔT<inf>max</inf>= 4.9 °C). The maximum air velocity in the air curtain of 0.6 m s−1 was observed at the discharge grille. The horizontal air velocity from the perforated plate was low (<0.2 m s−1) for all shelves. The loading percentage in the cabinet did not significantly affect the airflow rate through the perforated plate.
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Thermal performance improvement in a square channel heat exchanger with various parameters of V-wavy plates
(2019-01-01)
Boonloi, Amnart
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Jedsadaratanachai, Withada
Numerical examinations on flow and heat transfer behaviors in a square channel heat exchanger equipped with various configurations of V-wavy plate are performed. The pitch-to-channel height ratios, wavy height-to-channel height ratios and flow directions of the test section are investigated for the Reynolds number in the range of 100 – 1000 (laminar flow regime). The finite volume method is selected for the present investigation. The results are reported in terms of flow and heat transfer mechanisms in the channel. The thermal performance assessments of the square channel fitted with the V-wavy plate are also concluded. As the numerical results, the V-wavy plate can produce the swirling flow through the heating channel. The swirling flow disturbs the thermal boundary layer on the heat transfer surface that leads to enhance heat transfer rate and thermal efficiency.
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Influence of geometric parameters of alternate axis twisted baffles on the local heat transfer distribution and pressure drop in a rectangular channel using a transient liquid crystal technique
(2019-01-01)
Phila, Arnut
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Thianpong, Chinaruk
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Eiamsa-Ard, Smith
This paper reports the effects of alternate axis twisted baffle geometric parameters on the heat transfer and flow characteristics within rectangular channels. In our experiments we used modified shapes of alternate axis twisted baffles according to relative pitch ratios (s/w) equal to 2-12 and twist ratios (y/w) equal to 1-5, under conditions where the angle of attack (a) was 90 and the relative blockage height (e/Dh) was at a constant value of 0.095. The results for the Reynolds numbers based on the duct hydraulic diameter ranged from 9000 to 24,000 at a constant Prandtl number, Pr = 0.707, using air as a working fluid. A 0.05 mm thick stainless-steel foil was used as a heater, and a thermochromic liquid crystal technique was used to obtain the local temperature distribution on the heated surfaces. Images were captured in areas with periodic, fully developed regions in the channel. The results show that rectangular channels equipped with alternate axis twisted baffles demonstrated 80%-185% greater heat transfer than rectangular channels with no baffles. Channels with alternate axis twisted baffles at higher twist ratios (y/w) and smaller relative pitch ratios (s/w) showed increased heat transfer, as well as pressure loss within the system, compared with other types of twisted baffles. The thermal enhancement factor of the rectangular channels equipped with alternate axis twisted baffles was higher than that for transverse baffles and smooth channels under similar operating conditions.
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Flow and heat transfer characteristics of air in square channel heat exchanger with c-shaped baffle: A numerical study
(2019-01-01)
Boonloi, Amnart
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Jedsadaratanachai, Withada
The purpose of the present work is to study flow configuration and heat transfer behavior in a square channel heat exchanger equipped with C-shaped baffle. The influences of flow attack angle and baffle size on flow and heat transfer characteristics are considered for the laminar flow regime with the Reynolds number around 100 – 2000. The numerical study with finite volume method is selected for the present investigation. The SIMPLE algorithms is opted to solve the numerical problem. The numerical results are concluded in terms of flow and heat transfer mechanisms in the tested section. The thermal performance analysis; Nusselt number ratio (Nu/Nu0), friction factor (f/f0) and thermal enhancement factor (TEF), are also summarized. The numerical model of the smooth square channel is validated with the correlations on both Nusselt number and friction factor. The numbers of grid cell for the computational domain are also compared. The numerical results reveal that the C-shaped baffle in the tested section leads to the appearance of the thermal boundary layer disturbance on the channel walls that the important cause for heat transfer rate and thermal efficiency enhancements. The maximum TEF of the square duct with C-shaped baffle is around 3.89. In addition, the optimum gap spacing value for the square channel inserted with C-shaped baffle is around 5%.
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Influence of ring size and location on flow topology, heat transfer structure and thermal efficiency in heat exchanger square channel placed with 30-degree inclined square ring
(2019-01-01)
Boonloi, Amnart
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Jedsadaratanachai, Withada
This paper presents the numerical investigations (finite volume method with SIMPLE algorithm) on flow structure, heat transfer behavior and performance assessment in heat exchanger square channel placed with 30o inclined square ring (ISR). The influences of ring size and placement on flow and heat transfer characteristics are considered for laminar flow region with the Reynolds number in the range around 100 – 2000. The purpose for the insertion of the ISR in the square channel is to induce the vortex flow and also increase the turbulent mixing. The numerical result reveals that the ring size and location have effects for the changes of the flow and heat transfer behaviors in the tested section. The present of the ISR in the tested section gives the maximum heat transfer rate around 8.13 times above the smooth square channel. In addition, the optimum thermal performance at similar pumping power or TEF is around 3.10.
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A simplified heat transfer model of a closed refrigerated display cabinet
(2019-01-01)
Chaomuang, Nattawut
;
Laguerre, Onrawee
;
Flick, Denis
A simplified heat transfer model for a closed refrigerated display cabinet was developed based on a zonal approach. The developed model allows the prediction of time-averaged air and load temperatures at various positions in the display cabinet. These predicted values were compared with measured air and load temperatures. Good agreement was found between them with the maximum difference of 0.5°C for every studied position and the overall mean absolute error of 0.2°C. Air and load temperatures vary with positions. High temperature was observed at the front (highest value at the front top) and low at the back (lowest value at the back bottom). Based on the simulation, the cabinet thermal performance can be enhanced by minimizing the size of door gaps.

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