Flow instabilities in centrifugal compressors are one of the most limiting factors to the performance of the system where the compressor is installed (i.e., internal combustion engines, fuel cell, etc.). A better understanding of the surge phenomenon allows to properly set-up the operative region of the turbomachine in order to avoid a decrease in performance and damages to the compressor. Strong vibrations generated during surge conditions combined with high temperature and high rotational speed generate stress on the rotating elements, causing serious damage to the blade structure. Normally to avoid instability and damage to the compressor, the maximum boost pressure is limited with a by-pass system, which reduces the mass flow rate provided to the machine. The aim of the paper is to extensively analyze the compressor behavior in unstable operating conditions in order to improve simulation models that can predict accurately the occurrence of surge and therefore extend the operating range of the compressor.

This work shows the results of a broad experimental investigation performed at the turbocharger test bench of the University of Genoa which focused on the behavior of a small turbocharger centrifugal compressor during unsteady flow conditions and during the transient from stable to unstable operation. A specific circuit adaptable in volume and length was designed to study the effect of different configurations on steady compressor performance with special reference to unstable operation. To identify wave propagation phenomena along the circuit and filling and emptying of the components in the system, instantaneous pressure and mass flow rate signals were collected in different sections located upstream and downstream the turbomachine. In particular, high frequency response transducers were adopted to measure instantaneous pressure signals and instantaneous turbocharger rotational speed. The system was also equipped with a hot-wire anemometric system, which was previously calibrated to accurately measure the mass flow rate in unstable flow conditions and in different operating conditions, i.e. different temperature values. The data post-processing is described in detail highlighting the difficulties related to the measurements of instantaneous fluid-dynamics parameters in the turbocharging circuit under unsteady flow conditions and the solutions adopted for each sensor.

The effect of the compressor operating conditions (i.e., turbocharger rotational speed) and the circuit geometry is studied during the transition from steady to unsteady operation. The analysis focuses on filling and emptying phenomena  as well as wave propagation in the region of zero mass flow and in reverse flow condition to highlight and explain the shape and position of deep surge hysteresis loops around the steady state map. All measured quantities presented in this paper are an ensemble average of a large number of consecutive cycles in deep surge operation.

 

 

 

 

 

 

 


Experimental Investigation in Turbocharger Compressors During Surge Operation

Division / Sponsor:
International Gas Turbine Institute (IGTI)

Paper Proceedings: https://asmedigitalcollection.asme.org/GT/GT2020/volume/84058

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