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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.

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.