Heat exchangers are essential devices that are used in many industrial applications, including power generation systems, refrigeration and air conditioning systems. These devices are designed to efficiently transmit heat from one medium to another, leading to significant energy savings and enhanced system reliability.

However, the need for more powerful and efficient industrial systems has caused the application requirements of heat exchangers to be more stringent now more than ever. As a result, scientists and engineers have been studying and modifying existing heat exchanger designs to improve their thermal performance.

The approaches to thermal performance enhancement of heat exchangers can be broadly categorized into passive and active methods. Source: Surasak/Adobe Stock

The approaches to thermal performance enhancement of heat exchangers can be broadly categorized into passive and active methods. Each has its own unique advantages and suitability for a given application requirement.

Passive method for thermal performance enhancement

The passive method enhances heat transfer performance without the use of external energy. Instead, this method involves modifying the surface area and flow channel to achieve enhanced performance.

#1 Surface area augmentation

The heat transfer area available for heat transfer is a key factor that influences the performance of heat exchangers. As a rule, the higher the heat transfer area available for heat transfer, the higher the thermal performance of the heat exchanger.

Engineers typically increase the heat transfer surface area by adding fins or corrugation to the heat exchanger surfaces. Fins are protrusions on the wall of the flow channel that help to increase the surface area and significantly enhance the heat transfer rates.

However, one of the major challenges associated with surface area augmentation using fins is the complexity and high costs associated with manufacturing heat exchanger surfaces with protrusions. Moreover, these protrusions obstruct the flow path of the working fluid, causing pressure losses and increased pumping power demand.

One way of mitigating the high cost associated with this surface area augmentation technique is through the use of inserts. Commonly used inserts for heat exchanger application are twisted tape inserts. However, like the fins, these inserts also cause a pressure drop and increased pumping power demand.

#2 Surface roughness modification

Introducing roughness to the heat exchanger surface disrupts the boundary layer of the working fluid that forms on the heat transfer surface. Normally, this boundary layer acts as a thermal resistance that hinders efficient heat transfer. By introducing roughness to the heat exchanger wall, the thickness of the boundary layer is reduced, and turbulence is also increased, enhancing the heat transfer rate. Techniques such as sandblasting or adding dimples to the surface are common methods of surface roughness modification.

Learn more about boundary layers in fluid dynamics

Active method for thermal performance enhancement

Active enhancement technique requires the use of external energy input to improve the thermal performance of heat exchangers. Approaches used in this method include vibration and electromagnetic fields.

#1 Vibration

Applying vibration to the heat exchanger can significantly enhance heat transfer performance and efficiency. Like the passive enhancement techniques, vibration reduces the boundary layer and introduces turbulence to the flow, which promotes the mixing of the working fluid and enhances heat transfer.

The use of vibration also helps with the reduction of fouling (the accumulation of unwanted material on the heat transfer surfaces). By vibrating the heat exchanger, engineers can continuously shake off or prevent the build-up of these deposits, keeping the surfaces clean and maintaining high heat transfer rates.

#2 Electromagnetic fields

Electromagnetic fields can influence the flow behavior in heat exchangers, especially when the working fluid has electrical or magnetic properties. For instance, applying a magnetic field to a ferrofluid can enhance heat transfer.

Conclusion

The enhancement of heat transfer performance of heat exchangers is an interesting field that is gaining considerable attention. While active and passive enhancement methods are helping to improve this device’s heat transfer performance, the desirable performance of heat exchangers still depends on correctly sizing and specifying them for the required application. Therefore, it is recommended to reach out to heat exchanger manufacturers to discuss application requirements.