ABSTRACT
The performance of a turbocharger centrifugal compressor is affected by the opening and closing of the engine intake manifold and consequently experiences a pulsating backpressure. This off-design pulsating environment results in internal losses and affects compressor performance. Previous research on the effect of exit pulsation has demonstrated a surge margin improvement at low end torque conditions, however the pulsating effect on instantaneous efficiency is yet unknown; this paper aims to address this lack of knowledge.
Experimental analysis is undertaken on an instrumented centrifugal compressor under compressor exit pulsating conditions. The instrumented compressor has been manufactured to include sensor ports for measurements of static pressure and temperature at internal locations at the impeller, diffuser and volute. The experiments have been conducted on steady flow conditions and pulsating outlet conditions at pulse frequencies of 30, 46.7 and 66Hz. These frequencies are critical engine frequencies that correspond to near surge, peak pressure, and peak efficiency operating points of the turbocharger compressor under study. Instantaneous static pressure amplitudes for operating points and the corresponding hysteresis behaviour have been studied for the volute, diffuser and impeller pulses.
The results show that the pressure amplitude is largest when the flow rate is high and reduces as the flow rate reduces. The efficiency in pulsating conditions starts to decrease as the flow rate increases and it is largest at high flow conditions. The volute is influenced by the pulse amplitude as well as the frequency of pulsation. Within the volute, the resulting large flow fluctuations have internal disturbances which effect the propagation of the pressure waves; near the tongue, instantaneous static pressure measurements show the pressure in that location distinctively does not follow the pressure wave at the outlet. The region near the volute tongue region has been shown to be particularly critical due to its vicinity to the outlet changing (pulsating) boundary condition and reduces the efficiency of the compressor. This initial assessment leads to interesting insights in the response of the compressor to an unsteady pulsating environment thus allowing a better understanding on the real turbo-engine system performance.
