Untersuchung des transienten Verhaltens von CO2 - PKW - Klimaanlagen mit Niederdrucksammler (English shop)

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In this work, the transient behaviour of an automotive refrigeration system operating with the refrigerant R744 (carbon dioxide) is investigated. A simulation platform was created in which the steady-state and transient models of the individual components as well as of the overall circuit are stored. While physical model structures based on finite volumes are implemented for the heat exchangers and the receivers, the compressor is described predominantly by means of performance maps and the expansion device by means of characteristic curves. Extensive measurements across the operating range relevant to practice are carried out for the validation of the individual components as well as for the reproduction of the circuit behaviour, in order to be able to assess the quality of the models. Given the objective of “optimising controller concepts for refrigerant circuits”, particular attention is paid to ensuring that the component models can be described with a small number of finite volumes, in order to save computing time.

The focus and the essential advance compared with the previous state of knowledge are the findings on receiver behaviour and its steady-state and transient influence on system behaviour. This is where the work begins, presenting the first experimentally obtained findings on the fill-level and mass-flow dependence of accumulator behaviour. These were determined by means of a special measurement setup for a broad operating range. The receiver model type presented here, which in particular incorporates a height-dependent density profile, is validated against the measurement database. It has been shown that the influence of the oil bore in the receiver can be well reproduced by means of the developed models and that it lastingly determines the horizontal position of the cycle in the p/h diagram. Previously known receiver models were not able to take this influence into account with sufficient accuracy. Finally, a simulative sensitivity analysis is carried out which provides information on the accumulator dependence of the circuit behaviour.

Experiments have shown that, in many transient load cases, the hot-gas temperature at the compressor changes considerably more slowly than the remaining state variables in the circuit. For this reason, a dynamic compressor model was created in this work which better describes the transient of the hot-gas temperature.