Virtual Laboratory for Refrigerant Compressors

by Michael M. Cui, The Trane Company, USA

At The Trane Company we have developed a Virtual Laboratory for the design of our centrifugal refrigerant compressors. Using ANSYS CFX computational fluid dynamics software, with a real-gas equation of state for the refrigerant, the Trane Virtual Laboratory provides overall performance and local flow field details for complete compressor stages.

Different from single-component analysis, the Trane Virtual Laboratory simulates the entire compressor, readily providing information on the interactions between the components. This feature has special value for the overall improvement of machines, since it quantifies the impact of design changes of a single part on the performance of the whole compressor. This capacity has proven very useful when we investigated the options to use different diffusers in compressors. The impact on the overall compressor performance was analyzed in detail, with the simulations indicating that the change would lead to different flow fields inside the upstream and downstream components. The individual performance of these components was altered when the diffuser was changed. The information obtained was of great value to designers, guiding the product improvements and helping to avoid unnecessary design iterations.

Transient simulations in the Trane Virtual Laboratory provide further physical insights into compressor performance and flow field unsteadiness, reduction of which is critical for efficiency, vibration and noise levels. Pressure fluctuation distributions inside the impellers and diffusers can be obtained for different compressor designs and loss mechanisms inside the flow field can be studied thoroughly. We are also using CFX-TASCflow to explore large-scale unsteady flow phenomena. Removing these instabilities has the benefit of extending the working range of the compressor.

Vaneless
Vaned
	The different pressure distributions inside a compressor before (top) and after (bottom) changing from vaneless to vaned diffusers	Impeller pressure fluctuation in compressors with vaneless (top) and vaned (bottom) diffusers.

The Trane Virtual Laboratory is an excellent compliment to physical measurements. It is being used as a diagnostic tool to track down the sources of problems, and for research, to identify new physical mechanisms that can provide the direction for product improvement. Before a new compressor is built, we can now optimize the design in detail without investing significant resources making prototypes for each of the different designs. As a result, this technology is allowing Trane to evaluate multiple design options, to optimize compressor performance and to reduce the risk of delay in production.