Enhancement of Heat Transfer in Helical Coils via Air Bubbles Injection: A Review

Abstract

This review paper is on Helical coil heat exchangers, which are extensively used in industrial processes, as they have the advantage of compact structure and high heat transfer efficiency. Injection of air bubbles in the working fluid is one promotional method to enhance heat transfer in such systems. The introduction of this technique enables flowing gas–liquid two phase which greatly disrupts thermal boundary layers and improves convection heat transfer. The air bubbles presence has been shown to facilitate fluid mixing and turbulence particularly in curved geometries and hence increase heat transfer coefficients. It is found that the improvement depends on bubble size, injection rate, coil diameter, and flow orientation. Particularly, by deriving optimal bubble flow, energy savings and performance improvement over current best can be accomplished at little to no design modification. Despite that, bubbles remain to be controlled in the distribution, while pressure drops are undesirable. Next, future research will be aimed at optimizing operating parameters and developing models for predicting bubble behavior. As a whole, air bubble injection is a cost effective scalable method of increasing heat transfer in helical coil systems deployed in refrigeration, chemical processing, and heat recovery applications.