Abstract
Compressor surge has been investigated and predicted since the early days of turbomachinery research. Experimental testing of turbomachinery applications is still needed to determine whether stable compressor operation is possible in the expected application regime. Measuring compressor maps and operating ranges on hot gas test stands is common. The test benches are designed and optimized to ensure ideal inflow and outflow conditions as well as low measurement uncertainty. Compressor maps are used to match turbocharger and application. However, a shift in surge limit, caused by the piping system or application, can only be adequately addressed with full engine tests. Ideal measurements use the corresponding piston engine in the charged-air system. This can only take place in the development process, when surge detection is unfavorable from an economic perspective. The surge model for turbochargers presented here is an extension of the Greitzer’s surge model, which considers the effect of inlet throttling. Application components, such as air filters, pipe elbows and flow straighteners, reduce pressure in front of the compressor and flow conditions might differ from those in laboratory testing. Experimental results gathered from the hot gas test stand at TU Darmstadt indicate strong variation in surge limit, influenced by inlet throttling. An extension to the surge model is developed to explain the observed phenomena. The model was validated using extensive experimental variations and matches the experienced surge limit shift. Additional measurements with a piston engine downstream of the turbocharger demonstrated the validity of the surge model. The results also show that surge is a system-dependent phenomenon, influenced by compressor aerodynamics and boundary conditions.
Highlights
The onset and prediction of surge in axial and centrifugal compressors has been intensively investigated in recent decades
Tremendous work by Emmons in 1955 [2], performed on axial and centrifugal compressors, showed that it is the compressor aerodynamics that influences the onset of surge
Basic compressor map measurements are in accordance with SAE J1826 [13]
Summary
The onset and prediction of surge in axial and centrifugal compressors has been intensively investigated in recent decades. The suction side of the compressor is assumed to be incompressible; the pressured-sided plenum is compressible. Surge can be avoided with sufficiently small volume sizes The characteristics of both the compressor and the downstream throttle play an important role in the prediction of surge. As there are several resistances in the suction and pressure sided piping of the compressor, Greitzer’s model meet its limits. Components such as the air filter, a set of elbows and flow straighteners are part of the suction side of the centrifugal compressor. The compressor inflow is obstructed by a 90◦ elbow, the surge line shifts to lower mass flow rates independent of the clocking positioning of the bend relative to the compressor housing. This paper relates to all three categories but focuses on the influence of the compression system on surge, with additional information on unsteady exit boundary conditions
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