Numerical Study of Hydrothermal Enhancement of Two-Phase Flow in a Vertical Pipe by using Modified Vortex Generators

Abstract

The improvement in heat transfer and energy savings in two-phase (gas-liquid) flow in tubes occupies a wide area of research interest. In this work, the effect of modified twisted tape turbulators on heat transfer augmentation in vertical tubes is studied numerically by using the CFD technique. A vortex generator (twisted tape) was used as a passive method to raise the rate of heat transfer. The variants of the water flow rate, air flow rate, heat flux, and twisted ratio of a twisted tape tabulator were considered. This work provides a superficial water Re No. With ranges of 6300–10500, considering three different water and airflow rates. Additionally, the superficial gas Re range was 6000-16000. Three distinct twist ratios (TR) of 7.8, 3.9, and 2.6 were considered for the iron twisted tape to examine the impact of the twist ratio on the tube's thermo-hydraulic characteristics. The test section consisted of an insulated copper vertical tube subjected to three constant heat fluxes. This paper investigated three variants: the friction factor, Nusselt number, and performance evaluation factor. A strong turbulent vortex flow with an increase in secondary flow in the tube's radial direction was significantly intensified in the presence of the twisted tape inserted. Also, an increase in Reynold's number for both water and air and increased heat flux led to a proportional increase in heat transfer while the friction factor decreased. The presence of the modified twisted tape showed an improvement in the heat transfer and the pressure loss reduction when compared with the plain tape for all twisting ratios and for all cases where the highest value of the performance evaluation factor was two at the twisted ratio of 2.6 with an increase percentage of 81%. Conclusively, this investigation concludes that using modified twisted tape can greatly enhance the heat transfer rates and reduce pressure resistance, increasing the performance evaluation factor of heat exchanger systems.