Design Considerations in Turbochargers (Part 1 – Incidence)

A turbocharger (TC) has to provide a required pressure ratio for efficient combustion and operation of an internal combustion engine (ICE). The turbocharger consists of a turbine and a compressor sides on the same shaft. The turbine utilizes the energy of exhaust gases while the compressor forces the air into the engine. The compressor with a wide operating range is a strict requirement in the automotive industry because the unit has to operate across all of the ICE regimes.

Even though any compressor has a design point, the ability to operate at low and high mass flows is critical for TC compressors. To satisfy the operating range requirement, a designer tries increasing mass flow at choke and decreasing mass flow at surge. This is quite a challenge. For smaller mass flow rates, the impeller outlet and diffuser should be optimized. The choice of a vaneless diffuser is always justified by increased flow range at the cost of efficiency.

To increase the right-most mass flow limit, a designer optimizes the compressor inlet. The common practice is to design blades with large inlet metal angles. Increase in inlet angles open larger area for the flow to pass. This, in turn, leads to large incidence angles at design point. Therefore, many TC compressors are designed with large positive incidence in the design point. The incidence angle increases for every speedline going toward the surge line. Incidence distribution on a TC compressor map is shown in the figure below. It is equal to +12 deg (with respect to tangent) in the design point.

Fig. 1 Incidence on the TC compressor map
Fig. 1 Incidence on the TC compressor map

Glossary:

Blog - incidenceDesign point: An operating condition where a compressor reaches maximum efficiency

Compressor Map: Pressure versus mass flow characteristic at different rotational speeds and isoefficiency contours

Speedline: Dependence of pressure on mass flow rate for a given shaft speed

Surge: Left-most point on a compressor map for a given shaft speed

Choke: Right-most point on a compressor map for a given shaft speed

Incidence: The difference between inlet flow and metal angles. If an incidence is small, the flow has less resistance to enter the impeller.

Source: http://www.turbobygarrett.com/turbobygarrett/compressor_maps

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