Thermohydraulic performance evaluation of heat exchangers equipped with centrally perforated twisted tape: Laminar and turbulent flows

Abstract

Convective heat transfer enhancement in a round tube mounted with a centrally perforated twisted tape (CP-TT) was numerically investigated. Influences of space of cut ratio (s/w = 0.5, 0.7 and 0.9) and twist ratio (y/w = 2.0, 3.0 and 4.0) under laminar and turbulent flow regimes on heat transfer characteristics were determined. Numerical encompassed Reynolds numbers (Re ) from 400 to 2000 for laminar flow and 5000 to 15,000 for turbulent flow. At a given Reynolds number, the tubes with centrally perforated twisted tape (CP-TT) inserts offer higher heat transfer rate than those the plain tube alone. Heat transfer enhancement in a round tube equipped with centrally perforated twisted tape (CP-TT) is strongly dependent on twist ratio (y/w ) and space of cut ratio (s/w ). The results also found that the heat transfer rate (Nu ) and friction factor (f ) increase as twist ratio (y/w ) and space of cut ratio (s/w ) decreases. The thermal enhancement factor (TEF ) increases as space of cut ratio (s/w ) and twist ratio (y/w ) decreases in laminar flow regime while the opposite trend is observed in the turbulent flow regime. Over the studied range, the tube equipped with centrally perforated twisted tape (CP-TT) with s/w = 0.5 and y/w = 2.0 gives the maximum thermal enhancement factor (TEF ) of 8.92 for laminar flow at Re = 2000. In turbulent flow at Re = 5000, the centrally perforated twisted tape (CP-TT) with s/w = 0.9 and y/w = 3.0 yields the maximum thermal enhancement factor (TEF ) of 1.33. In addition, the flow structure, temperature field and local Nusselt number of heat exchanger tubes equipped with centrally perforated twisted tape (CP-TT) are also reported for the clarification of heat transfer and flow topology mechanisms.

Keywords

• heat transfer augmentation
• heat transfer mechanism
• centrally perforated twisted tape
• swirl
• turbulent flow