Session: 31-08 Compressor Prediction Methodology

Paper Number: 120925

120925 - Decomposition of Compressible and Thermal Contributions to the Exergy-Balance-Based Analysis of the NASA Rotor 37 

This paper presents the application of a new method allowing to explicitly decompose the compressible and thermal contributions to overall efficiency, aiming at refining the phenomenological performance assessment of turbomachinery flows. It builds up on an approach based on a balance of exergy (which is the potential for mechanical work recovery by means of an adiabatic turbine and a Carnot machine that extract mechanical work while the system evolves towards its thermodynamic dead state). Such a control-volume-based exergy analysis is of particular interest in turbomachinery performance investigations as it quantifies reversible and irreversible losses through exergy outflow and anergy generation respectively, including a direct link to their physical origin.  This is in constrast to common performance investigation approaches relying on total pressure and temperature ratios, where the influence of individual physical phenomena can not be straightforwardly isolated. In the case of a compressor, the exergy balance provides a decomposition of the exergy provided to the fluid into the part  converted to effective thrust, the exergy outflow that can still be converted into mechanical work at the exit of the channel, and the irreversible exergy dissipation (by viscous effects, thermal mixing and shockwaves) inside the turbomachine stage. The mechanical and thermal effects are both considered, thus improving the identification and understanding of the loss sources degrading the performance of the system. In addition, the balance components are derived from a control-volume balance, thus needing no average in their computation.

Such a balance was recently adapted to the study of internal flows in a rotating reference frame, with a decomposition of the overall exergy outflow between mechanical and thermocompressible contributions. 
This decomposition can lead to challenging interpretations, due to the inclusion of both compressible and thermal effects in a single component. In other words, it is not direct to separate the portion of exergy outflow which can be recovered mechanically (e.g. by a turbine-type device) from the one that can only be recovered by using a heat engine (ideally a Carnot machine). This ambiguity is problematic for the performance investigation of an isolated mechanical system providing or extracting exergy, as the thermally-recoverable component is accounted as recoverable in figures of merit while it should be considered as irreversible loss.

This paper adresses this issue by exploiting a new balance decomposition between mechanically- and thermally-recoverable exergy outflows to refine the exergy-balance-based analysis of the NASA rotor 37 test case. Firstly, the theoretical aspect of this new decomposition is discussed in detail, including the expressions, interpretations and figures of merit associated to the different components. Secondly, one operating regime at 98\% of the choke mass flow rate of the rotor 37 is investigated. This will particularly emphasize the trends obtained for the thermally- and mechanically-recoverable components of the overall exergy outflow (respectively impacted by entropy generation and pressure/velocity differences with respect to the system's thermodynamic dead state).  Finally, the analysis of the exergy-based characteristic of the compressor is refined with respect to the one based on the original mechanical/thermocompressible decomposition. In particular, it is found that the impact of the thermally-recoverable exergy outflow on the prediction of the compressor efficiency is non-negligible, as it corresponds to roughly 0.5 points of the compressor rotor stage overall efficiency.

Presenting Author: Ilyès Berhouni ONERA

Presenting Author Biography: Ilyès Berhouni is a research engineer working at ONERA in France. He obtained his Ph.D.at ONERA, which consisted in the development of an exergy-balance-based method for the performance investigation of turbomachinery configurations. In his current position, he works on the development and refinement of performance prediction methods for numerical (i.e. CFD-based) analyses of complex flows.

Authors:

Ilyès Berhouni ONERA
Ilias Petropoulos ONERA
Didier Bailly ONERA