Transient Behavior of R744 Vehicle Refrigeration Cycles and the Influence of the Suction Side Accumulator Design

The article presents new insights regarding accumulator behavior and its impact on transient and steady-state system behavior that have come to light during the development process of the R744 refrigeration cycle. A new accumulator model is introduced featuring a special liquid height dependent refrigerant density model as well as the pressure drop related mixing behavior of the oil bleed hole and J-tube inlet mass flow rate. The accumulator model is validated over a wide accumulator operating range. Experiments were carried out on a setup specifically designed for this purpose because literary references to experimental data on accumulator behavior are scarce. Comprehensive measurements show clearly the influence of the mass flow rate, the charge and geometry parameters e.g. the oil bleed hole diameter on the accumulator outlet properties. The measured accumulator vapor outlet quality, which has a major impact on the horizontal cycle process position in the p/h diagram, is well reflected by the calculations based on the new accumulator model. The paper provides an insight into the comparison of simulation versus measurement results for a transient validated refrigeration cycle model. A simulation sensitivity analysis is then carried out on the basis of the validated cycle model, in order to provide information on the accumulator's influence on the transient and steady-state cycle behavior.