Numerical investigation on heat transfer characteristics in corrugated tube with tapered twisted tape insert

Abstract

A numerical investigation has been carried out to study the combined effect of insertion of the tapered twisted tapes inside the helically outward corrugated tubes as a compound method, on the heat transfer augmentation, pressure drop characteristics, flow behavior, and overall performance of a heat exchanger, for turbulent flow. A 3-D computational model has been developed with the SST k-ω turbulence model, from a comparative study among four different turbulence models against standard empirical correlations. In addition, reported literature discretely asserted the model for the twisted tape inserts and the corrugated tubes. In the present study, the turbulent flow, Reynolds number ranging from 3400 to 21000, inside the straight circular tube (SCT), and the helically outward corrugated tubes (HCT) with two different pitch-length-to-tube-diameter ratios (pl/D = 1 and 1.5), and two different height-length-to-tube-diameter ratios (Hl/D = 0.05 and 0.1), inserted with tapered twisted tapes at two different twist ratios (y/w = 3.5 and 5) and three different tapered angles (θ = 0°, 0.3°, and 0.5°), have been investigated for a constant heat flux. It has been found from the present study that the heat transfer rate enhanced with the decrease of twist ratio, and the pressure drop decreased significantly with the increase of tapered angle. In addition, the corrugated tube with lower (pl/D) and (Hl/D), has been exhibited a higher heat transfer rate compared to other investigated tubes. Moreover, an Artificial Neural Network (ANN) model has been developed to predict the Nusselt number, normalized Nusselt number, friction factor, normalized friction factor, and the thermal performance factor for the helically corrugated tubes with the pitch-length-to-tube-diameter ratio of 0-1.5, and the height-length-to-tube-diameter ratio of 0-0.1. The predicted values of the performance factors from the trained artificial neural network (ANN) are in good agreement with the results of the computational investigation, with error bands of 2%, 2%, 1%, 1%, and 1% for the Nusselt number, friction factor, normalized Nusselt number, normalized friction factor, and thermal performance factor, respectively. The modeled (ANN) consequently obtained correlations for Nusselt number, friction factor, and thermal performance factor, which have been exhibited ±4%, ±5%, and ±5% error bands respectively, against the computationally analyzed Nusselt number, friction factor, and thermal performance factor. Finally, the results from the numerical study and the artificial neural network predictions, have revealed that the tapered twisted tape insert (y/w = 3.5 and θ = 0.5°) inserted in the helically corrugated tube (Hl/D = 0.1, pl/D = 1.0) yields better thermal performance factor compared to other investigated cases.