A review of two-phase heat transfer and pressure drop correlations for natural refrigerants in small diameter tubes.

The use of small diameter tubes in heat exchangers (HXs) has significant advantages, which include the increase in heat transfer coefficient, reduction in HX size, material, weight, and refrigerant charge. Due to these desirable characteristics and reduction in environmental impact, natural refrigerants become good candidates to substitute HFCs and HCFCs with the application of small diameter tubes. This paper comprehensively reviews the available two-phase heat transfer and pressure drop correlations for natural refrigerants and small diameter tubes (Dh ≤ 5 mm). In addition, the effects of critical Reynolds number and two-phase flow regimes are discussed in the context of small diameter tube. Overall, two-phase heat transfer and pressure drop correlations for this application interest are not sufficient. This paper provides a guideline to choose correlations among existing models and identifies the research topics worthy of future study.