Experimental Investigation on the Dynamic Response of a Vaned Diffuser Compressor in Innovative Heat-Pump Systems

Abstract Nowadays heat pumps are playing a key role in reducing carbon footprint of heating and cooling systems. In large-scale industrial applications, centrifugal compressors are often used under variable conditions and at low mass flow rate, where instabilities may occur. To investigate these aspects in refrigerant closed-loop systems, Carrier supplied the University of Genoa with a small-size chiller test rig. This rig is equipped with a vaned diffuser high-speed centrifugal compressor driven by a variable-speed motor, enabling experimental investigation in a wide operating range. This experimental activity investigates surge transients in centrifugal compressors by employing a comprehensive approach that integrates pressure measurements using high-frequency piezoresistive pressure transducers with dynamic vibro-acoustic response data. Different signal processing techniques in time and frequency domain were applied to investigate the different system response components, and to separate the overall signal into individual contributions to better identify the rise of anomalous fluid-dynamic conditions. A coherence analysis was also applied and different types of system responses were considered, including the function computation between physically non-homogeneous system responses such as inflow pressure and structural vibrational signals. Additionally, the acquired uniform-time-increment signals were resampled at constant increments of the compressor shaft angle to obtain angle-domain sampled data. This allowed for the computation of synchronously averaged response spectra both in linear and energy modes. Statistical techniques, such as skewness and kurtosis, were also utilized for the detection of signals with strong additional noise, where transient and non-deterministic contents may arise as the compressor approaches unstable conditions. The obtained results provide promising diagnostic and predictive paths to detect compressor instabilities in closed-loop heat pump systems.