Khwanchit Wongcharee

Thermohydraulic Performance of Heat Exchanger Tube Equipped with Single-, Double-, and Triple-Helical Twisted Tapes

Heat transfer augmentation by dual-helical twisted tapes (D-HTTs) and triple-helical twisted tapes (T-HTTs) is presented in comparison with that by single-helical twisted tape (S-HTT). The effects of tape width ratio (w/D = 0.1, 0.15, and 0.2) on heat transfer, friction factor, and thermal performance are also reported. The experiments were performed using air as a working fluid for Reynolds number between 6000 and 20,000. At similar conditions, the use of D-HTTs and T-HTTs leads to the increase of Nusselt number by respectively 15.6% to 17.6% and 19.5% to 23.4%, the increase of friction factor by respectively 83% to 206% and 143% to 335%, and the decrease of thermal performance factor by 3.9% to 20.3% and 8.3% to 26.2% compared to those obtained from the use of S-HTT/promoter. As the width ratio increases, both Nusselt number and friction factor are considerably increased. However, due to the poorer trade-off between the increases of Nusselt number and friction factor, lower thermal performance factor is consistently obtained at the conditions possessing higher Nusselt number. At similar conditions, S-HTTs consistently give a higher thermal performance factor than D-HTTs and T-HTTs; the maximum thermal performance factors given by S-HTTs with w/D of 0.1, 0.15, and 0.2 are 1.34, 1.31, and 1.29, respectively (at Reynolds number of 6000).

Numerical Investigation of Flow Friction and Heat Transfer in a Channel with Various Shaped Ribs Mounted on Two Opposite Ribbed Walls

Heat transfer and fluid flow characteristics through a channel with two - dimensional transverse ribs are numerically investigated. The effects of different shaped ribs (rectangular rib, triangular rib, cylindrical rib, concave-concave rib, convex-concave rib, long convex-short concave rib and long concave-short concave rib) are examined. The investigation is performed for the ribs mounted on top and bottom walls with rib pitch to height ratio (P/e) of 6.67, in Reynolds number between 3000 and 7000, under constant wall temperature condition. For the present case, the data predicted by SST k-omega turbulence model show better agreement with the experimental data, than those predicted by RNG k-epsilon turbulence model. The mean heat transfer, flow friction, temperature fields and the local heat transfer coefficients as well as the flow structure behaviors are reported. For the ribs considered, the channel with triangular-ribs yields the highest Nusselt number, the one with concave-concave rib provides the highest friction and the one with cylindrical rib show the best thermal enhancement. At the similar condition, the triangular rib gives higher Nusselt number than that provided by the cylindrical one by around 29.3 percent, while the cylindrical rib offers the highest thermal enhancement factor of 1.4 which is higher than that of the cylindrical one by around 1.33 percent while the cylindrical rib offers the highest thermal enhancement factor of 1.33 which is higher than that of the triangular one by around 9.6 percent. In addition, in spite of their high heat transfer, the concave-concave rib and long concave-concave rib yield low thermal performance factors, due to the prominent effect of high friction factor.

INFLUENCE OF NONUNIFORM TWISTED TAPE ON HEAT TRANSFER ENHANCEMENT CHARACTERISTICS

An experimental study of heat transfer and friction factor characteristics in a circular tube fitted by twisted tapes with nonuniform twist ratios is reported. The twisted tapes are used as swirl generators playing roles as heat transfer enhancers. The nonuniform twisted tapes examined in the present work have (1) sequentially increasing twist ratios (SL), (2) sequentially decreasing twist ratios (LS), (3) repeatedly increasing-decreasing twist ratios (SL/SL), (4) repeatedly decreasing-increasing twist ratios (LS/LS), (5) intermittently increasing-decreasing twist ratios (SL→LS), and (6) intermittently decreasing-increasing twist ratios (LS→SL). In addition, three different typical twisted tapes with twist ratios of 3.0, 4.0, and 5.0 (TT) were also evaluated. Apparently, LS yields monotonically increasing swirling intensity and SL decreasing swirling intensity, resulting in lower heat transfer rate and friction factor than the other four nonuniform twisted tapes, which possess extra fluid fluctuation. However, among the tapes tested, the SL/SL offers the maximum thermal performance factor of around 1.03, which corresponds to Nusselt number of around 36% and friction factor of 3.57 times over those of the plain tube.

Experimental Study of TiO2-Water Nanofluid Flow in Corrugated Tubes Mounted with Semi-Circular Wing Tapes

ABSTRACT Heat transfer of corrugated tubes mounted with semi-circular wing tapes has been experimentally investigated. TiO2-water nanofluids were employed as working fluids. Experiments were conducted in the following ranges: (i) TiO2-water nanofluids with three TiO2 concentrations (0.05 to 0.15% by volume), (ii) semi-circular wing tapes with three wing angles (45 to 75°), and (iii) two wing arrangements, and (4) Reynolds numbers between 8000 and 15,000. The experiment using water as the working fluid in a corrugated tube without tape was also performed, for comparison. Evidently, the heat transfer enhancement increases with increasing wing angle. Moreover, the corrugated tubes combined with semi-circular wing tapes in parallel arrangement show better thermal performance than the ones in counter arrangement. The highest thermal enhancement factor of 1.98 is obtained by the use of the combined devices in parallel pattern at wing angle of 75°, concentration of TiO2-water nanofluid of 0.15% by volume and Reynolds number of 8000.

Influence of Short Helical Tape on Local Convective Heat Transfer and Thermal Performance Characteristics in Turbulent Decaying Swirl Flow

This paper reports the experimental investigation of local convective heat transfer enhancement, flow friction and thermal performance factor behaviors in the tube fitted with the short helical tapes (SHTs) acting as decaying swirl flow generators. The tapes with three different helical tape angles (? = 90°, 135° and 180°) and three different channel numbers (N = 2, 3 and 4 channels) were tested under the uniform wall heat flux condition. The performance of each tape is compared with the performance of the plain tube subject to the same pumping power. The experimental results show that the heat transfer rates and friction factors of the tube with SHTs are respectively in range of 1.15 to 1.9 and 1.49 to 2.31 times of those in the plain, corresponding to thermal performances between 0.98 and 1.46. The correlations for Nusselt number (Nu) as a function of Reynolds number (Re), Prandtl number (Pr), helical tape angle (?) and the number of channel (N) are also developed.

Heat Transfer Enhancement in a Double Pipe Heat Exchanger by Insertion of Propeller-Type Swirl Generators

This paper describes an experimental study of turbulent convective heat transfer and flow friction characteristics in a double tube heat exchanger equipped with propellers (2 blade-type). The propellers are used as the decaying swirl generators in the inner tube. The experiments were performed using the propellers with four different interval lengths (l = 1D, 2D, 3D and 4D where D is diameter of the inner tube), for the Reynolds number ranging from 5000 to 32,000, using water with temperature of 27°C and 70°C as cold and hot working fluids, respectively. The data of the tube equipped with the propellers are reported together with those of the plain tube, for comparison. The obtained results demonstrate that the heat transfer rate in term of Nusselt number (Nu) and friction factor (f) in the tube with propellers are higher than those in the plain tube at the similar operating conditions. This is due to the chaotic mixing and efficient interruption of thermal boundary layer caused by the propellers. In addition, the Nusselt number and friction factor in the tube fitted with the propellers increase as the interval length decreases. Depending on Reynolds number and interval length, Nusselt numbers and friction factors in the tube fitted with the propellers are augmented to 1.95 to 2.3 times and 5.8 to 13.2 times of those in the plain tube. In addition, the correlations of the Nusselt number (Nu) and the friction factor (f) for tube fitted with the propellers are reported and the performance evaluation to access the real benefits of using the turbulators is also determined.Copyright

Heat Transfer Enhancement in a Tube Fitted With Twisted Tape Swirl Generator

Flow friction, heat transfer and thermal performance characteristics in a tube fitted with peripherally-cut twisted tape (PT) have been experimentally investigated. The twist ratio (y/W) of twisted tape was varied from 3 to 5 while the depth of peripheral cut ratio (d/W) and the width of peripheral cut ratio (w/W) were both kept constant at 0.22. The experiments were conducted for the Reynolds number ranging from 5100 to 19,700, using water as a working fluid. The plain tube and the tube equipped with the typical twisted tape (TT) with three different twist ratios (y/W = 3, 4 and 5) were also tested for comparison. The obtained results reveal that the use of PT enhances Nusselt number up to 211% and 138% compared to those of the plain tube and the tube with TT, respectively. It is also found that heat transfer rate increases with decreasing twist ratio. Additionally, the performance evaluation to assess the real benefits in using the PT as heat transfer enhancer has also been determined.Copyright