Twisted-Tape Inserts in Swirl Flow Applications
Swirl flow heat transfer has been extensively studied for its ability to improve efficiency in tubular heat exchangers. One of the most researched and widely implemented methods to achieve this is the twisted-tape insert, a simple yet powerful enhancement technique. This article dives into the mechanisms, behind twisted-tape inserts, as documented in Swirl Flow Heat Transfer and Pressure Drop with Twisted-Tape Inserts by Manglik and Bergles.
Why Swirl Flow? Understanding the Need for Heat Transfer Enhancement
Heat exchangers play a critical role in industries ranging from chemical processing and power generation to refrigeration and waste heat recovery. However, in many practical applications, particularly those involving viscous or laminar flows, heat transfer rates can be low due to limited mixing and high thermal resistance.
Swirl flow devices, such as twisted-tape inserts, address this issue by inducing secondary flows, enhancing mixing, and disrupting boundary layers—all leading to increased convective heat transfer.
Twisted-tape inserts improve performance in both single-phase and two-phase flows (boiling and condensation processes). Their widespread applicability is due to the following key benefits:
Higher convective heat transfer coefficients
Greater turbulence even at lower Reynolds numbers
Increased effective heat transfer surface area due to helical flow paths
Potential suppression of critical heat flux (CHF) in boiling applications
However, these improvements come at a cost—namely, an increase in pressure drop due to the obstruction and acceleration of the flow.
How They Work
1. Swirl Flow Induction and Secondary Circulation
One of the defining features of twisted-tape inserts is their ability to generate secondary flow structures, particularly in laminar and transitional regimes.
This swirling motion improves heat transfer by:
Increasing fluid mixing across the tube cross-section
Reducing stagnant thermal boundary layers near the tube walls
Maintaining higher wall temperature gradients for enhanced conduction
2. The Role of Twist Ratio (y = H/d)
The twist ratio (y = H/d), defined as the pitch of one full 180° twist divided by tube diameter, is a key parameter affecting performance:
Lower twist ratios (tighter twists) result in stronger swirl flows, higher heat transfer rates, but also greater pressure drop.
Higher twist ratios (looser twists) reduce the pressure penalty but may not induce sufficient turbulence for meaningful enhancement.
The research suggests an optimal range where heat transfer benefits outweigh pumping losses, typically y = 3–6 for practical industrial applications.
3. Impact on Pressure Drop
Twisted tapes increase pressure drop —by as much as 7.5× compared to a smooth tube. However, this trade-off is often justified by the increase in thermal performance.
Some strategies explored to mitigate pressure drop while retaining heat transfer benefits include:
Using intermittently spaced twisted-tape inserts rather than full-length tapes
Employing perforated or slotted tapes to reduce flow resistance
Combining twisted tapes with other passive or active enhancement techniques
Conclusion
Twisted-tape inserts have stood the test of time as an effective method for enhancing heat transfer across a range of industries. From their origins in 19th-century boilers to their modern applications in energy conservation, refrigeration, and chemical processing, these simple devices continue to provide high-performance solutions for improving heat exchanger efficiency.
Innovations like variable-pitch twisted tapes, perforated designs, and compound enhancement techniques can further optimize their performance. The balance between heat transfer gains and pressure drop remains a key engineering challenge, but with modern computational tools and experimental validation, twisted tapes continue to be a leading passive enhancement method.
twisted-tape inserts can lead to better energy efficiency, cost savings, and system performance.
This post is based on Swirl Flow Heat Transfer and Pressure Drop with Twisted-Tape Inserts by Raj M. Manglik & Arthur E. Bergles.