Vapor compression systems (VCS) offer significant benefits as the backbone for next generation aircraft thermal management systems (TMS). For a comparable lift, VCS offer higher system efficiencies, improved load temperature control, and lower transport losses than conventional air cycle systems. However, broad proliferation of VCS for many aircraft applications has been limited primarily due to maintenance and reliability concerns. In an attempt to address these and other VCS system control issues, the Air Force Research Laboratory has established a Vapor Cycle System Research Facility (VCSRF) to explore the practical application of dynamic VCS control methods for next-generation, military aircraft TMS.
The total refrigerant mass contained within the closed refrigeration system (refrigerant charge) is a critical parameter to VCS operational readiness. Too much or too little refrigerant can be detrimental to system performance. Extreme values of refrigerant charge can lead to a loss of evaporator temperature control, loss of high side pressure control, or other potentially catastrophic occurrences. The objective of this work is to examine real-time methods for determination of acceptable refrigerant charge in a prototypical VCS system, as a function of operational points, using only sensors already utilized in the control system (in-situ control sensors). It is envisioned that studies such as these can be used to guide development of a simple in-situ prognostic tool for system state-of-health indication (i.e. “Red Light, Yellow Light, Green Light”), with respect to level of charge, and to enable on-demand maintenance. Additionally, a method for continuous management of refrigerant charge as a means for optimizing system efficiency over a range of dynamic operating points is presented.