The Role of Twist Rate in Twisted Tape Inserts

In industries ranging from power generation to petrochemicals, efficient heat transfer is crucial for maintaining system performance and energy efficiency. One method that has gained attention for enhancing heat transfer in tubular systems is the use of twisted tape inserts. These simple yet effective devices can significantly improve heat transfer by inducing turbulence in the fluid flow, but how do they work, and what twist ratio is best? This post will delve into the impact of different twist ratios on heat transfer in pipes.

 

What Are Twisted Tape Inserts?

Twisted tape inserts are metal strips twisted along their length and inserted into a pipe. As fluid flows through the pipe, the twisted tape causes the fluid to swirl and mix, which disrupts the boundary layer—the thin layer of fluid near the pipe wall where heat transfer is typically slowest. This increased turbulence enhances the overall heat transfer coefficient, making the system more efficient.

 

Why Twist Ratio Matters

The twist ratio of a tape insert, defined as the length required for the tape to complete one full 360-degree twist relative to the pipe’s diameter, is a critical factor in determining its effectiveness. The twist ratio affects the intensity of the turbulence created in the fluid flow, which in turn influences the heat transfer rate and the friction factor, a measure of the resistance to fluid flow.

 

Key Findings from the Study

A comprehensive study using numerical simulations explored the performance of twisted tape inserts with various twist ratios in a U-loop pipe system. The study considered five different twist ratios: 2.9, 3.0, 3.25, 3.5, and 4.0, under turbulent flow conditions with Reynolds numbers ranging from 5,000 to 25,000. Here’s what the researchers found:

 

  • Optimal Twist Ratio for Heat Transfer: The twist ratio of 3.5 consistently outperformed the others in terms of heat transfer efficiency. It resulted in the highest Nusselt number, a dimensionless parameter that represents the efficiency of heat transfer in the system.

  • Balancing Friction and Efficiency: While smaller twist ratios (e.g., 2.9 and 3.0) created more intense turbulence and, consequently, higher friction, the twist ratio of 3.5 struck a balance by providing substantial heat transfer improvements without causing excessive pressure drops.

  • Effect of Reynolds Number: The study also found that as the Reynolds number increased, the temperature difference across the pipe decreased. This suggests that while higher flow rates can increase heat transfer, the efficiency gains diminish at very high Reynolds numbers.

 

Practical Implications

For industries that rely on heat exchangers, selecting the appropriate twist ratio for twisted tape inserts can lead to significant improvements in system performance. A twist ratio of 3.5 appears to offer the best trade-off between enhancing heat transfer and maintaining manageable levels of friction.

 

Conclusion

Twisted tape inserts are a powerful tool for enhancing heat transfer in tubular systems. By carefully selecting the twist ratio, industries can achieve significant improvements in heat transfer efficiency, leading to better system performance and energy savings.

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* Chowdhury, M. K., Bhuyan, M. M., & Deb, U. K. (2019). Augmentation of heat transfer in pipe flow using plain twisted tape inserts for different twist ratios. Energy and Power Engineering, 11(9), 342-354.