Objective - To develop heat transfer and system performance information, which will assist in the selection and implementation of the best replacements for high-GWP hydrofluorocarbon (HFC) refrigerants.
What is the new technical idea? Most of the currently used HFC refrigerants will be phased out or phased down because of their relatively large global warming potential. Consequently, candidate replacement fluids must be researched and evaluated. The scope of the evaluation includes two-phase heat transfer and cycle measurements that will support the development of a prediction model for a comprehensive and fair comparison of low-GWP refrigerants. For complete characterization of new fluids, fundamental heat transfer and pressure drop measurements will be taken in a two-phase heat transfer apparatus, and cycle performance will be measured in a laboratory heat pump apparatus. The prediction model will be a new cycle simulation-based methodology that will determine refrigerant performance parameters, volumetric capacity, and coefficient of performance while accounting for the refrigerant’s thermodynamic and transfer properties. The new methodology will provide more realistic predictions of low-GWP refrigerant performance than conventional theoretical models based on refrigerant thermodynamic properties alone.
What is the research plan? The research plan encompasses three Products. Within Product 1, the two-phase heat transfer of candidate replacements for high-GWP refrigerants will be determined. In previous project years, the apparatus was used to measure local convective-boiling and pool boiling of several low-GWP candidate refrigerants inside a micro-fin tube and on an enhanced surface, respectively. The test fluids were selected by following the guidance of U.S. refrigerant manufacturers and by theoretically evaluating the performance of 40 potential low-GWP refrigerants as identified by the Air-Conditioning, Heating, and Refrigeration Institute’s (AHRI) Low-GWP Alternative Refrigerants Evaluation Program (AREP). Three more low-GWP refrigerants will be tested during this year’s effort. For each refrigerant, roughly eighty discrete operating points will be recorded to characterize the heat flux and governing temperature difference for the database. The heat transfer measurements for low-GWP refrigerants will broaden the fluid database in the literature, which will be used to extend the validity of an existing NIST heat transfer correlation to low-GWP fluids.
NIST will be working closely with chemical manufacturers that have supplied the test refrigerants. Some of these test refrigerants may be mildly flammable having an ASHRAE 2L designation. Consequently, modifications have been made to the test rigs to safely test the 2L refrigerants.
Product 2 consists of the laboratory characterization of low-GWP refrigerants in a mini-breadboard heat pump (MB-HP). This laboratory apparatus was completely rebuilt and equipped with specialized, variable-area heat exchangers, a variable-speed compressor, and two water chillers to control the temperatures of the heat source and the heat sink. Shakedown-baseline tests with R134a were done followed by tests with the first low-GWP fluid and a second baseline fluid, R410A. The rig will be used to evaluate an additional four new low-GWP refrigerants and mixtures. These data will be used to validate the new CYCLE_D-HX model developed within this project.
The modeling element of this project (Product 3) supports two NIST Standard Reference Databases (SRDs): CYCLE_D and REFLEAK. These programs are under continuous maintenance and are upgraded, as needed, to assist the industry in the transition to new-generation refrigerants. Previous year’s efforts produced updates CYCLE_D Ver. 5.1.1 and REFLEAK Ver. 5.0. Also, developmental work took place on a new cycle model, CYCLE_D-HX, which accounts for both thermodynamic and transport properties of refrigerants. Inclusion of transport phenomena within the heat exchangers enables the analysis and optimization of the fundamental trade-off between the pressure drop penalty and refrigerant heat transfer enhancement with increasing refrigerant mass flux. The Beta version of CYCLE_D-HX has been completed with extensive testing and prepared for distribution by the Office of Data Informatics to become the third (in addition to CYCLE_D and REFLEAK) NIST Standard Reference Database supported by this project.