Boiling and condensing convective transfers of new refrigerants: Synthetic review of the latest correlations

The objective of this report is to list and provide a critical look at the new calculation methods (correlations) of the heat exchange coefficient in the context of two-phase flow (evaporation and condensation) of new refrigerants.

This document is made available exclusively to GRETh members who requested a report on this subject during the 2018 General Assembly.

In this report, we have focused on the fluids which are the most studied and able to be used in the greatest number of installations. In this context, the refrigerants studied in this report belong to the family known as HFOs (Hydrofluoro-Olefins) or HCs (Hydrocarbons). The latter also belong to the family of so-called natural refrigerants which also include water (R718), CO2 (R744) and ammonia NH3 (R717), included in this report.

In general, this report thus makes it possible to provide, after extensive sorting in the databases of scientific publications, methods for calculating heat exchange coefficients in the form of correlations, which allows for simplified use.

The publications selected are those that we have deemed the most relevant on the following main criteria :

• a wide range of validity whether in terms of the fluids tested (often including the usual refrigerants and the "new" refrigerants) ;
• wide ranges of operating conditions and geometries.

In terms of content, beyond the writing of the correlations and their descriptions (construction and domain of validity) and the unique analysis of the publication, points which are obviously included in this report, we have endeavored to make comparisons of the selected correlations with known and recognized correlations. Thus, the results presented in this report, in the form of a graph of the evolution of the transfer coefficient as a function of the vapor mass content, are therefore the fruit of the coding and calculation of the new correlations (as well as the usual correlations) in cases mentioned in the legend of the figures and in the body of the text.

Finally, from a geometrical/technological point of view, we have also tried to provide answers for most of the exchanger technologies used in the evaporator and condenser of a cold production unit. Thus, we have focused on the selection of correlations that can be used for the following technologies :

• conventional smooth circular channels ;
• conventional grooved circular channels ;
• mini and micro channels ;
• corrugated plate heat exchangers (except plate and fin heat exchangers).

We hope that this report will provide you with concrete elements that can be integrated into your heat exchanger sizing codes and/or complete system simulation codes.