So far, there have only been a few in-depth research studies related to system simulation of unitary air conditioners and heat pumps at off-design conditions. However, it is clear that the current simulation tools and tuning methodologies do not provide accurate predictions at some off-design conditions. This report focuses on improved modeling methods for these situations. Comprehensive experimental and simulation studies are performed in this report to support this effort. Specific accomplishments of this work include identification of improved modeling methods for handling two-phase refrigerant entering the compressor, investigation of the effects of non-uniform distribution of refrigerant flow and air flow, and development of an improved tuning method for charge modeling. The charge tuning method significantly improves simulations in the case of two-phase refrigerant entering the fixed-area expansion device.

An existing system simulation model (ACMODEL) is the tool that is used for this work and multiple improvements have been made in its capabilities. Based on an exhaustive literature search, the most appropriate up-to-date research results have been incorporated into each component model of the system simulation tool. In addition, a flexible heat exchanger modeling scheme has been developed that allows specification of heat exchanger design and circuiting.

Extensive tests with detailed measurements were performed for two R-410A units and one R-407C unit, which included over 200 points in different test series. All the tests have been simulated and analyzed in the report. The simulations were conducted at two levels for each test series, one at component level and the other at system level. The simulations at the component level helped to identify the sources of discrepancies. The simulations at the system level have been useful for identifying inaccuracies due to charge simulation and for validating the improved modeling methods.

The improved ACMODEL is validated using all the existing R-22 data and the new HFC data obtained by Herrick Labs. The latest version of the ORNL (Oak Ridge National Labs) model (Mark VII) is assessed using the new HFC data.