How Does Pulsating Flow Improve Heat Transfer in PTFE Heat Exchangers?

While a heat exchanger is normally run at a steady flow rate, it has been shown that if you add a pulsation (or a rhythm of rapid acceleration and deceleration of flow) you can improve the heat transfer without increasing the total throughput. This counter-intuitive approach is especially noteworthy for PTFE exchangers which are notorious for their low thermal conductivity. Pulsating flow is identified as a possible way to improve heat transfer in such systems. It provides a potential solution to improve the thermal performance of PTFE heat exchangers.

PTFE heat exchangers with pulsating flow mechanism
The pulsating flow avoids the development of a stable thermal boundary layer in the tube, which is a main factor limiting the heat transfer in conventional steady flow systems. The pulsations further mix the fluid by periodically speeding and decelerating the flow. This disturbance of the boundary layer provides for a more effective heat exchange between the fluid and the walls of the PTFE tube.

The pulsing flow was found to improve the heat transfer coefficient with negligible increase of the average flow velocity in laboratory trials. The periodic oscillation improves the ability of the fluid to transmit heat by disrupting the stagnant fluid along the tube wall, which normally acts as an insulating barrier. This disturbance increases the fluid wall interaction, resulting in an improved heat transfer, without requiring larger flow rates.

Mechanisms of Pulsating Flow
During the deceleration phase of pulsatile flow, vortices are shed into the flow, generating turbulence which "scours" the tube wall. These vortices serve to break up the thermal boundary layer so that the fluid may make greater contact with the tube surface and heat transfer is improved.

The core of the fluid, normally hotter, penetrates further into the tube during the acceleration phase. The thermal exchange is improved by the contact of the fresher and warmer fluid with the cooler portions of the tube. This rhythmic motion leads to better mixing which further enhances heat transfer over the PTFE tube surface.

Practical Realisation of Pulsating Flow
This pulsation of the flow can be created by a number of means including a reciprocating pump, a quick acting valve or a fluidic oscillator. All of these approaches introduce periodic oscillations in the flow, which assist in inducing the proper mixing and promote heat transmission.

An important concern is the increased complexity and risk of vibration of pulsing flow systems. The benefits of heat transfer are obvious, but the design of the pulsating system should include the possible mechanical stresses and vibrations that could be introduced into the system especially at high frequencies or in systems with sensitive components.

Effect of pulsating flow on the flow regimes
The influence of pulsing flow is significant in the laminar and transitional flow regimes, where its relative impact on mixing is highest. In these regimes of flow, the flow is less turbulent than in the fully developed turbulent flow and hence the advantages of the pulsations in breaking up the boundary layer are more evident. For higher Reynolds numbers, turbulence already contributes a significant amount of mixing and the influence of pulsing flow on heat transmission is less significant.

Pulsating flow is useful in improving heat transfer at lower Reynolds numbers but the net energy savings need to be analysed carefully. The energy cost of generating the pulsations, particularly where active mechanisms such as reciprocating pumps or quick acting valves are employed, can negate some of the advantages of heat transmission. This trade-off has to be considered when considering if pulsing flow is a viable enhancing strategy for a given application.

Summary
Pulsating flow is an innovative and niche technique to improve heat transfer in PTFE heat exchangers, particularly effective in systems that operate at low Reynolds numbers. Pulsations can enhance the heat transfer coefficient considerably without greater average velocities by disturbing the thermal barrier layer and promoting fluid mixing. But in applying this method, practical concerns, such as the possibility of vibration and the energy necessary to produce the pulsations, must be considered. The thermal engineer has a rich toolbox of pulsating flow and various active and passive enhancing strategies to optimise the PTFE heat exchanger performance.