Chair for aerodynamics, dedicated to radial turbomachinery – CASTOR

Castor is organised into five main tasks, addressing the flexibility of radial turbomachinery operating range, how to measure or predict it, and how to design it. The first task (T1) is dedicated to the administrative governance and scientific orientation of the project. Three technical tasks correspond to the four objectives described above. The first two (T2 and T3) are specific to the compressor and the turbine, respectively; the last one (T4) is transversal. Finally, T5 oversees the dissemination of knowledge acquired throughout the project. The technical tasks are:

- T2 [Operating range of centrifugal compressors] is dedicated to the Obj1, which seeks to characterise a novel compressor stage configuration: shrouded impeller with a variable-displacement diffuser, which should allow a significant progression in both performance and operating range. The analysis will be conducted from advanced numerical simulations and from experimental results. Initially, the rotor and the variable geometry system are analysed separately. A doctoral student is planned for each study. Finally, the works are associated, validating this option or proposing different combinations.

- T3 [Non-conventional radial turbines] is dedicated to the Obj2. Its ambition is to review long-established design practices in the context of multi-point specifications and geometric variabilities. Non-conventional configurations (geometries and operating conditions) will be detailed and tested, thus widening the design space of radial turbines. Again, the rotor part and the variable geometry part are treated separately in a first step. The first study is conducted through a PhD thesis. The second will be the subject of a contribution from a post-doctoral researcher. Finally, a third step will attempt to characterise the reversible operation of the radial turbines through measurements in the free-windmilling regime, one of the stage's operating boundaries.

-T4 [Methods] brings together Obj3, which focuses on methods (numerical and experimental), and Obj4, which addresses the issue of design procedures. This task serves as the transmission belt between T2 and T3, connecting academic questions to industrial applications. This task has the natural role of providing methodological recommendations to the industrial environment.

Progress after 18 months:

Task 1:
- Most of the recruitment is ensured, which removes a significant risk from the project;
- the structure of the project is in place, both legally and in terms of management;
- strong interactions between the establishments are established.

Task 2:
- The two planned theses have started, and demonstrate quite satisfactory progress;
- A first rigorous examination of the stability of flattened compressors is available, both by
experimental and digital methods. It shows a lower performance of the shrouded impeller than expected. On the other hand, the behaviour of the stage at the limit of stability is much more
progressive than that of the reference stage. Finally, a detailed study of the diffuser and the inlet recirculation is underway, which presumably sets the operating range;
- A low-order model of a centrifugal compressor stage whose static part comprises variable geometry (input grid and diffuser) is now available and validated from
experimental and numerical data. This model allows for establishing first-order behaviours,
which will be examined throughout the thesis. In particular, two settings for the input grid are enough to obtain the best performance over the entire range. It needs to be verified with more accurate methods. If this conclusion materialises, it offers many concrete perspectives.

Task 3:
- The thesis has started, with a quite satisfactory state of progress;
- the possibility of obtaining good yields outside the classic use range is demonstrated
by means of numerical simulations, and must be demonstrated experimentally;
- a specification compatible with the test bench was identified, and used to produce
non-conventional rotor geometries, by using the results and recommendations of previous research.

Task 4:
- this task is to date the most advanced of the scientific program defined for CASTOR;
- the test benches are operational, and test campaigns of increasing complexity are
scheduled.
- the low-order numerical methods called BFM are successfully adapted to radial turbomachinery;
- this implementation revealed remarkable behaviours regarding the stability of stages. The possible appearance of a recirculation at the hub is theoretically considered by the
method, but never observed in reality. The fact that this configuration presents a better efficiency deserves a thorough examination;
- wall-resolved LES simulations, unprecedented for centrifugal machines, are underway;
- the work carried out in this task irrigates the other components of the project, through
methodological recommendations, which are also formatted for the industrial environment.

The research is underway. The experimental team is about to be reinforced.

Ollivier P., Bousquet Y., & Bayle A., ANALYSIS OF UNSTEADY FLOW STRUCTURE OF A SHROUDED CENTRIFUGAL IMPELLER WITH VANED DIFFUSER, 11th EUROPEAN CONFERENCE FOR AERONAUTICS AND AEROSPACE SCIENCES (EUCASS).

Decarbonization requires some significant evolutions in the architecture of transportation systems. Many subsystems, such as the propulsive unit or the air conditioning systems, are impacted, and the turbomachinery involved in those subsystems needs to be adapted. In the electrification process, the efficient operating range requirement for turbines and compressors must be significantly increased compared with the current state of the art. The chair CASTOR is a research program of cooperation between Liebherr Aerospace, a recognized air conditioning system provider for the aeronautic industry, and ISAE-Supaero, who has acknowledged research activity in the off-design operation of radial turbomachinery. The scientific program is focused on radial turbomachinery. The main objective of the chair is to increase the operability range of centrifugal compressors and radial turbines. Long-established design approaches must be reviewed and updated to the multi-point nature of the new specifications. First, an advanced understanding of the operating range limits is required. The stability limit leading to stall and surge is the main issue on the compressor side and will be tackled. On the turbine side, the possibility offered by unconventional designs will be checked. For both machines, the penalty induced by the tip clearance flow, which dominates the flow for low-dimension stages, will be analyzed, and the flexibility offered by the variable geometry solution will be quantified. The simultaneous study of radial turbines and centrifugal compressors, generally addressed by dedicated scientific communities, is a rare opportunity to transfer generic knowledge from one device to another. It also offers the possibility of mutualizing the methodological work, which will be conducted not only on the experimental ground but also thanks to advanced numerical simulations or low-order modeling.
The industrial chair is also an excellent occasion to structure the thirty-year partnership between Liebherr and ISAE-Suapero and to give a new impulsion to this long-term research framework. A dedicated team of twelve people will conduct the four-year scientific program entangled between the company and the lab. Adapting knowledge to the industrial environment is a major concern of CASTOR. The expected gain in flexibility of the radial turbomachinery will help meet the aircraft electrification requirements. It will also contribute to the development of fuel cells or heat-recovery cycles. It will promote the emergence of new systems architecture, infusing into other industrial fields.