Experimental investigation of twisted tape-induced mixed convection for optimized thermofluidic performance in transitional flow regimes

Abstract

The current study aims to experimentally investigate the thermohydrodynamics coaction in a solar air heater tube equipped with twisted tape turbulators, specifically focusing on laminar and transitional flow regimes. This study uses air with Reynolds numbers (Rea) from 563 to 10,240. Wall heat flux (q) is uniform on heat exchanger tube. On the tube surface, “2, 3, and 4” kW-m−2 heat fluxes are used. Twisted tapes with twist ratios (y) “3, 4 and 5” were used as passive heat transfer enhancement devices. The results of heat transfer are presented in terms of Nusselt number (Nua) and Colburn j-factor (j) while the pressure drop is presented as friction factor (f). The results revealed changes in the transition boundary. For plain channel being subjected to uniform heat flux of 3 kW-m−2, the transition begins at Rea equals to 2595 and ends at Rea 3833. For twisted tape having twist ratio 3 at 4 kW-m−2 heat flux, the improvement in the heat transfer was highest. Similar results were obtained for f. Four empirical correlations were developed for predicting the Nusselt number (Nua) and friction factor (f), achieving deviations as low as ±0.75 % from experimental data. These correlations provide highly reliable guidance for optimizing heat exchanger design and performance.