To increase heat transfer efficiency in double-pipe heat exchangers, twisted tape inserts are a commonly utilized enhancement technique. The fluid flow is forced into a spiral motion by inserting a twisted tape inside the inner pipe, upending the laminar boundary layer and increasing turbulence. As a result of the increased turbulence, the fluid mixes better and the heat transfer coefficient rises greatly, improving thermal performance. This CFD simulation is contingent upon the reference paper entitled “ Numerical study on heat transfer enhancement and flow characteristics of double pipe heat exchanger fitted with rectangular cut twisted tape”.

• Reference [1]: Barzegar, Ali, and Dovood Jalali Vahid. “Numerical study on heat transfer enhancement and flow characteristics of double pipe heat exchanger fitted with rectangular cut twisted tape.” Heat and Mass Transfer12 (2019): 3455-3472.

Figure 1: CFD Model of Twisted Tape in Double-pipe Heat Exchanger

Simulation Process

One of the most challenging parts of the present simulation is designing the twisted tape inside the inner tube of the double-pipe heat exchanger. It is modeled using Design Modeler software. Then, a hexagonal grid is produced, resulting in Hybrid grid. The article represents an estimation of water properties by Regression equations in Table 4. Consequently, a User-defined Function is written to employ all the properties dependent on temperature.

Figure 2: Grid generation over Twisted Tape in Double-pipe Heat Exchanger

Post-processing

The Nusselt number analysis provides quantitative evidence of this enhancement, showing an impressive increase from 34.52 in the standard exchanger to 43.16 with the twisted tape installed – a 25% improvement in thermal performance. This substantial gain occurs primarily because the swirling motion continuously brings fresh fluid into contact with the heat transfer surfaces while simultaneously increasing the effective flow path length. Additionally, the velocity contours depicts secondary flow patterns between the tape edges and pipe walls that create localized mixing zones with particularly high heat transfer rates. Despite these benefits, it’s worth noting that this enhanced thermal performance comes with a moderate pressure drop penalty of approximately 18%, which is still favorable compared to other enhancement techniques.

Figure 3: Localized mixing zones around twisted tape inside heat exchanger

The velocity streamline visualization perfectly captures why twisted tape inserts dramatically enhance heat transfer performance in the double-pipe configuration. Looking at the flow patterns, you can clearly see how the fluid transitions from a relatively straight path to complex spiral trajectories with velocities ranging from 0.07 m/s (blue regions) to 0.438 m/s (red zones). What’s fascinating is how the twisted geometry forces fluid particles to follow elongated helical paths, which consequently increases their residence time within the exchanger. Furthermore, this swirling motion repeatedly disrupts the thermal boundary layer that normally insulates pipe walls, leading to significantly improved thermal mixing. The rectangular cuts in the twisted tape create additional flow disturbances that further intensify turbulence, especially in the near-wall regions where heat exchange is most critical.