K. Nanan

Thermohydraulics of Turbulent Flow Through Heat Exchanger Tubes Fitted with Circular-rings and Twisted Tapes

Abstract The influences of circular-ring turbulators (CRT) and twisted tape (TT) swirl generators on the heat transfer enhancement, pressure drop and thermal performance factor characteristics in a round tube are reported. The circular-ring turbulators were individually employed and together with the twisted tape swirl generators in the heated section of the tube. Three different pitch ratios ( l/D = 1.0, 1.5, and 2.0) of the CRT and three different twist ratios ( y/W = 3, 4, and 5) of the TT were introduced. The experiments were conducted using air as the working fluid under a uniform wall heat flux condition, for the Reynolds number between 6000 and 20000. The experimental results reveal that the heat transfer rate, friction factor and thermal performance factor of the combined CRT and TT are considerably higher than those of CRT alone. For the range examined, the increases of mean Nusselt number, friction factor and thermal performance, in the tube equipped with combined devices, respectively, are 25.8%, 82.8% and 6.3% over those in the tube with the CRT alone. The highest thermal performance factor of 1.42 is found for the combined device consisting of the CRT with l/D = 1.0 and TT with y/W = 3. The correlations of the Nusselt number, friction factor and thermal performance factor of the tubes with combined devices are also developed in terms of Reynolds number, Prandtl number, twist ratio and pitch ratio.

Thermal Performance Evaluation of Heat Exchanger Tubes Equipped with Coupling Twisted Tapes

The present work has been conducted to investigate the effect of coupling twisted tapes on heat transfer enhancement in a heat exchanger. The effects of (1) twisted tape orientation (co-coupling twisted tapes or counter-coupling twisted tapes), (2) width ratio, and (3) twist ratio were also examined. Results of coupling twisted tapes were also compared to those of a typical twisted tape. Experimental results showed that the use of counter-coupling twisted tapes resulted in higher heat transfer, friction loss, and thermal performance factor than that of co-coupling twisted tapes and typical twisted tapes. Thermal performance factor increased as twist ratio and Reynolds number decreased while width ratio increased.

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).

Thermal Performance Assessment of Turbulent Flow Through Dimpled Tubes

This paper presents the heat transfer augmentation and friction factor characteristics by means of dimpled tubes. The experiments were conducted using the dimpled tubes with two different dimpled-surface patterns including aligned arrangement (A-A) and staggered arrangement (S-A), each with two pitch ratios (PR = p/Di = 0.6 and 1.0), for Reynolds number ranging from 9800 to 67,000. The experimental results achieved from the dimpled tubes are compared with those obtained from the plain tube. Evidently, the dimpled tubes with both arrangements offer higher heat transfer rates compared to the plain tube and the dimpled tube with staggered arrangement shows an advantage on the basis of heat transfer enhancement over the dimpled tube with aligned arrangement. The increase in heat transfer rate with reducing pitch ratio is due to the higher turbulent intensity imparted to the flow between the dimple surfaces. The mean heat transfer rate offered by the dimpled tube with staggered arrangement (S-A) at the lowest pitch ratio (PR = 0.6), is higher than those provided by the plain tube and the dimpled tube with aligned arrangement (A-A) at the same PR by around 127% and 8%, respectively. The empirical correlations developed in terms of pitch ratio (PR), Prandtl number (Pr) and Reynolds number, are fitted the experimental data within ±8% and ±2% for Nusselt number (Nu) and friction factor (f), respectively. In addition, the thermal performance factors under an equal pumping power constraint of the dimple tubes for both dimpled-surface arrangements are also determined.Copyright