Heat transfer and fluid flow characteristics in helically coiled tube heat exchanger (HCTHE) using nanofluids: A review

One of the most common methods used to transfer heat is by using various types of heat exchangers. The efficiency of heat exchangers can be improved through active or passive techniques. Active technique involves the use of external forces such as vibration, rotation, electric field, etc. Where else the passive techniques involve fluid additives or modified surface geometries. Helical coiled tubes have been known to exhibit excellent heat transfer characteristics compared to regular straight tubes and for this reason it is widely used in industrial applications. Another known passive technique to enhance heat transfer is by altering the fluid thermal conductivity. Recent studies have shown that nanofluids (nano sized particles suspended in conventional heat transfer fluids) posses superior heat transfer capabilities due to its high thermal conductivity. The amount of research done in this area is fairly new and limited. Most studies done on helical coiled tubes and nanofluids show enhanced heat transfer capabilities and results that challenge conventional theories and limitations on heat transfer devices and fluids. Several important aspects of HCTHE that affect the performance such as geometry, fluid inlet and outlet arrangement, and types of HCTHE are discussed based on the reported findings from experimental and numerical studies. This review also focuses on the vital aspects of nanofluids such as types, properties and heat transfer characteristics and limitations towards the application of nanofluids.