Abstract
This paper investigates the forced convection condensation heat transfer performance of R134a in circular tubes with three-dimensional conical pin fin structures. Five conical pin fin tubes of different circumferential fin pitch (pc) and longitudinal fin pitch (pl) were fabricated by Selective Laser Melting (SLM) with the aim of enhancing the internal forced convection condensation of R134a. Experiments were performed to characterize the condensation heat transfer coefficients (href) and pressure drops (ΔP) across these enhanced tubes. These experiments were conducted at the refrigerant mass fluxes (mref) of 50 kg/m2·s to 200 kg/m2·s, average vapor qualities (xave) from 0.2 to 0.8 and saturation pressure (Psat) of 13.4 bar. The effects of xave, mref, pc and pl on href and ΔP were determined and the results were compared against a commercial Al tube, a plain tube fabricated by SLM and two SLM fabricated enhanced tubes with dome-shaped fins. It was found that href of the conical pin fin tubes increases with increasing xave and mref and these values are also significantly higher than those of the plain tubes. Both pl and pc were found to significantly affect the href values of the conical pin fin tubes whereas the ΔP values were affected only by the change in pc. An efficiency index (η1) is defined to evaluate the thermal-hydraulic performances of the enhanced tubes. The experimental results show that all the conical pin fin tubes demonstrated higher η1 than the dome-shaped fin tubes. Based on the boundary layer approach, a semi-empirical model is developed to predict the Nusselt numbers of the conical pin fin tubes. Reasonably accurate predictions were achieved with an overall mean absolute error (MAE) of 10.5%.
Published Version
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