A Study into Refrigeration Cycle Working Fluids using an Air Cycle Machine Environmental Control System

Abstract

This study is the experimental analysis of a fast-jet military aircraft Environmental Control System (ECS) to the variation in Absolute Humidity (AH) of bleed air working fluid. A genuine fast-jet ECS operates within a ground test facility. The thermodynamic performance of the ECS is evaluated with two main metrics, Coefficient of Performance (CoP, a first law efficiency) and cooling capacity (function of exhaust temperature and mass flow rate). The ECS features Low Pressure Water Extraction (LPWE) with the use of a coalescing sock and centrifuge; the operation, efficiency and performance of this component are discussed in depth. The ECS inlet conditions (temperature, pressure and humidity) are typical of flight and atmospheric envelopes of the donor aircraft for all testing. A linear relationship is witnessed between increasing AH and decreasing CoP, while the cooling capacity of the system exhibits a step change in performance based on induced phase change at the Cold Air Unit (CAU) turbine. The lack of visibility regarding working fluid phase change with traditional first law efficiency measures highlights the often misleading nature of this commonly used performance metric. While phase change is a fundamental requirement for water extraction, it is found to be thermodynamically expensive to system capability as the ECS has no mechanism to recover the energy released during the formation of condensate. This is typical of several complex system dynamics and thermodynamic trade-offs not apparent with dry working fluid. A number of time-dependent transient effects of water extractor coalescing sock blockage have been measured and discussed. The most extreme of these is the complete icing of this component causing a degradation in system performance and finally triggering the LPWE pressure release valve; replication of a typical operational problem. The difficulties of accurately modelling these behaviours is discussed and demonstrated to validate the experimental methodology utilised in this paper. It is concluded that an improved system performance is attainable through the accurate control of condensate generation and separation in the high pressure region of the CAU.