Transient modeling of a flash tank vapor injection heat pump system – Part I: Model development

• A first-principles dynamic model of a two-stage flash tank vapor injection system.
• Separated flow model without interfacial exchange is used for two-phase flows.
• Compressor on/off and fan on/off conditions are accounted for in the model.
• Modular nature of component models allows flexibility in system configuration.
• The same model can be utilized to simulate both cooling and heating mode.

This two-part article explores the dynamic behavior of a flash tank vapor injection heat pump system from a numerical simulation perspective. Part I provides a first-principles model describing the transient heat transfer and flow phenomena of the system with detailed modeling techniques for each component. The vapor injection scroll compressor is analyzed with the internal heat transfer between the refrigerant and metallic parts taken into account. Lumped-parameter models are developed for the flash tank and expansion devices. Heat exchangers are modeled using a finite volume approach and accounting for the complex tube circuitry. The separated flow model without interfacial exchange is utilized for two-phase flows in order to incorporate an appropriate void fraction model so that a more accurate prediction for refrigerant mass distribution can be achieved. The modular nature of the component models allows flexibility in the system configuration. Transient simulations are carried out for start-up and shut-down operations. A detailed comparison of model predictions against experimental data is presented in the companion paper.